Over 100-bit Integrated All-Optical Memory on a Photonic Crystal Chip is Demonstrated

[From Left to Right] Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Masaya Notomi

Authors: Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Masaya Notomi

Affiliation: NTT Nanophotonics Center and NTT Basic Research Laboratories, NTT Corporation, Atsugi-shi, Kanagawa, Japan.

Random access memory (RAM) is used extensively in a wide variety of instruments. It is based on the bistable operation of electronic transistors and memorizes bit information. Recently, its optical counterpart, optical RAM (o-RAM), has become highly desirable in high-speed network processing, especially for routers, because it is more efficient to manage network information all-optically without power-consuming electric-to-optical (EO) or OE conversions. Various o-RAM devices have been proposed and fabricated, but so far they have been too big, too power-consuming, or too difficult to integrate.

In 2012, we demonstrated ultralow-power and ultra-small o-RAMs using a photonic crystal nanocavity [1]. Photonic crystals enable strong light confinement, so these o-RAMs were able to achieve a huge reduction of footprint and power consumption. However, the size of these o-RAMs was limited to just four bits. In the June 2014 issue of 'Nature Photonics' [2], we reported our successful integration of wavelength-addressable 105-bit o-RAMs. This is the first realization of an integrated o-RAM with more than four bits.

We implemented a 105-bit o-RAM in a small silicon photonic crystal chip less than 1.1 mm long (Fig. 1). This chip contained 128 nanocavities, each of which can serve as an o-RAM. This novel configuration enables wavelength-addressable o-RAM operation.

Figure 1: (a) Electron microscope images of large-scale nanocavity array integrated in a Si photonic crystal. (b) Transmission spectrum of nanocavity array coupled to an in-plane input/output waveguide. 105 cavities can be operated as bistable optical memories. (c) Schematic of o-RAM operation using the bistable output bias power of a nanocavity side-coupled to a bus waveguide. (d) o-RAM operation of 105 cavity modes in the nanocavity array.

First we explain how an optical memory works. It is well-known that a cavity with optical nonlinear medium exhibits optical bistability at a certain condition. In the present device, we make use of this phenomenon for bit memory operation. Figure 1(c) explains operation mechanism. When one injects light into a nonlinear cavity, the output light intensity shows hysteresis with two bistable states. We can switch between the two bistable states by applying an optical pulse or cutting the bias light. This operation can be used as a bit memory where the high output state (OFF) and low output state (ON) correspond to binary information of '0' and '1'. It should be noted that in a photonic crystal nanocavity, the optical nonlinearity effect is greatly enhanced by high quality factor (Q: ~10⁵) and ultra-small mode volume (< 1 (λ/n)³).

In the present study, we were able to achieve bistable o-RAM operation of 105 nanocavities integrated monolithically in a photonic crystal chip as shown in Fig. 1(d). The optical transmission spectrum is shown in Fig. 1(b). Here each dip corresponds to each cavity. Fabricated cavities have different resonant wavelengths with an averaged spacing of 0.23 nm, which was realized by precisely changing the lattice constant of photonic crystal by 0.125 nm. Such high level of nanofabrication accuracy was achieved by cutting-edge electron beam lithography.

In order to realize wavelength-addressable o-RAMs, each cavity should have a sharp resonance and a wide mode spacing to avoid undesired mode overlap. Conventional high quality factor (Q) cavities in a photonic crystal do not have sufficiently wide mode spacing. It is well known that a three-missing-hole defect cavity (so-called L3 cavity) in a photonic crystal has a wide mode spacing, but suffers from relatively low Q [3]. To overcome this problem, here we employed a modified L3 nanocavity as shown in Fig. 2, where we systematically tuned the 6 sets of the holes denoted A-F.

Figure 2: (a) New tuning design of an L3 nanocavity that improves Q over 10 times. (b) Spectrum of the fundamental cavity mode of an L3 nanocavity. The full-width half-maximum (FWHM) line-width (1.5 pm) corresponds to the experimental Q of 1.0 X 10⁶.

This novel design allows ~10 times enhancement of Q to the conventional L3 cavities with keeping the volume of the cavity mode. Not only the theoretical Q but also the experimental Q was enhanced to ~10⁶ which is 10 times higher than previously reported value [3]. The above-mentioned highly-dense wavelength division multiplexing scheme was enabled by this high performance of modified L3 cavities.

Although we employed Si for o-RAMs discussed above, we also employed InGaAsP for constructing multi-bit o-RAMs, which showed even small operation power (~100 nW) [2] owing to the efficient optical nonlinearity.

We are expecting that the present technology will be employed in an all-optical router as we mentioned. In this application, gigantic RAMs are not necessary, but kilobyte-sized RAMs would be sufficient. To realize this, we plan to combine the present wavelength addressing memory integration scheme with the parallel integration scheme. We have already demonstrated the parallel integration of o-RAM with an all-optical addresser in four-bit o-RAM [1]. The appropriate combination of two methods would be sufficient for our target. Of course, high-speed optical RAMs should also play various important roles in all-optical logical processing other than routers. In addition to that, the demonstrated wavelength addressing nanocavity integration technologies for o-RAM can also be applied for other types of devices such as multi-channel all-optical switches [4].

References: 
[1] Kengo Nozaki, Akihiko Shinya, Shinji Matsuo, Yasumasa Suzaki, Toru Segawa, Tomonari Sato, Yoshihiro Kawaguchi, Ryo Takahashi, and Masaya Notomi, “Ultralow-power all-optical RAM based on nanocavities”. Nature Photonics, 6, 248 (2012). Abstract.
[2] Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Koji Takeda, Tomonari Sato, Shinji Matsuo, Hideaki Taniyama, Hisashi Sumikura and Masaya Notomi, “Large-scale integration of wavelength-addressable all-optical memories on a photonic crystal chip”. Nature Photonics, 8, 474 (2014). Abstract.
[3] Yoshihiro Akahane, Takashi Asano, Bong-Shik Song, and Susumu Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal”. Nature, 425, 944 (2003). Abstract.
[4] Kengo Nozaki, Eiichi Kuramochi, Akihiko Shinya, and Masaya Notomi, "25-channel all-optical gate switches realized by integrating silicon photonic crystal nanocavities". Optics Express, 22, 14263 (2014). Abstract.

Milestones in a Continuing Tale of Big and Small : Large Magnitude Squeezed Light at 100 Hz, and a Squeezed 4km Gravitational-wave Detector

Sheon Chua

[Sheon Chua is the recipient of the 2013 GWIC (Gravitational Wave International Committee) Thesis Prize for his PhD thesis “Quantum Enhancement of a 4km Laser Interferometer Gravitational-Wave Detector” (PDF). -- 2Physics.com]


Author: Sheon Chua

Affiliation:

Currently at: Laboratoire Kastler Brossel, University of Pierre and Marie Curie (UPMC), Paris, France.

PhD research performed at: Centre for Gravitational Physics, Australian National University (ANU), Canberra, Australia.

Gravitational-wave sources of astronomical size from our Universe. Gravitational-wave displacement signals expected at one thousandth of the diameter of a single proton. Interferometric instruments with kilometre-long arms. Light ‘squeezed’ on the quantum scale.

The construction and implementation of second-generation laser-interferometric gravitational-wave detectors [1] are rapidly progressing [2], forming a detector network expected to be online over the next few years. These amazing instruments will use state-of-the-art isolation systems, optics, and hundred watt input lasers, and have kilometre-scale arms. For these detectors, the effect of a passing gravitational wave causes a relative displacement change between the interferometer arm end mirrors, which is encoded in the relative phase of the light beams propagating in the arms [3]. The relative displacement sensitivities will be of order of 10^-19 m in the 10 Hz to 10 kHz Fourier frequency band, achieved after monumental efforts in research and development across many fields of physics and engineering.

2Physics articles by past winners of the GWIC Thesis Prize:

Paul Fulda (2012): "Precision Interferometry in a New Shape: Higher-order Laguerre-Gauss Modes for Gravitational Wave Detection"
Rutger van Haasteren (2011): "Pulsar Timing Arrays: Gravitational-wave detectors as big as the Galaxy"
Haixing Miao (2010): "Exploring Macroscopic Quantum Mechanics with Gravitational-wave Detectors"
Holger J. Pletsch (2009): "Deepest All-Sky Surveys for Continuous Gravitational Waves"
Henning Vahlbruch (2008): "Squeezed Light – the first real application starts now"
Keisuke Goda (2007): "Beating the Quantum Limit in Gravitational Wave Detectors"
Yoichi Aso (2006): "Novel Low-Frequency Vibration Isolation Technique for Interferometric Gravitational Wave Detectors"
Rana Adhikari (2003-5)*: "Interferometric Detection of Gravitational Waves : 5 Needed Breakthroughs"
*Note, the gravitational wave thesis prize was started initially by LIGO as a biannual prize, limited to students of the LIGO Scientific Collaboration (LSC). The first award covered the period from 1 July 2003 to 30 June 2005. In 2006, the thesis prize was adopted by GWIC, renamed, converted to an annual prize, and opened to the broader international community.

However, even after these impressive technological efforts, there remain fundamental noise sources that limit measurement sensitivity that arise from the underlying physics of the instrument itself. One such noise source is the quantum nature of light [4], that comes from a non-zero commutation relationship between a light beam’s phase (ϕ) and amplitude (A) [5]. The Heisenberg Uncertainty Principle for this specific pair of quantities is given by ∆ϕ ∆A ≥ 1. Figure 1(a) shows this relation diagrammatically as a noise ‘ball’. As the gravitational-wave signal is encoded in the relative phase, with quantum phase noise Δϕ present we reach a signal-to-noise level where we can no longer distinguish a passing gravitational wave in the measurement. Therefore, the quantum noise of light is a limitation to achievable sensitivity.

However, the Heisenberg Uncertainty Principle relation is multiplicative. This means that one of the uncertainties can be below the quantum level, or ‘squeezed’, if the other uncertainty is above the level, or ‘antisqueezed’. This is illustrated in Figure 1(b), where the overall uncertainty is the same, but the individual uncertainties have been ‘rearranged’. The amount of squeezing has units of decibels [dB], referenced to the unsqueezed quantum noise level amplitude, given by [dB]=20 log₁₀[(anti)squeezed noise / unsqueezed noise].

Figure 1 (a) Quantum noise ‘ball’, showing the even distribution of uncertainty between the two quantities amplitude (A) and phase (ϕ). (b) Squeezed noise, where the uncertainty in one quantity is less than quantum noise, while the other uncertainty is greater than quantum noise.

As an example, if we have 6 dB of squeezing, we mean that the noise is squeezed to about half the value of the quantum noise level, or that the noise is reduced by a factor of 2. It follows that if we inject squeezed light into an interferometer so that it results in the phase uncertainty being reduced, the measurement sensitivity limited by quantum phase noise will be improved.

The tale of squeezed light for enhancing gravitational-wave detectors is now three decades young, with theoretical proposals for injecting squeezed light into interferometers published in the early 1980s [6], a few years before first experimental measurement of squeezing [7] took place. Since then, there has been a steady advancement in techniques and technologies to generate squeezed light within the 10 Hz to 10 kHz detection band [8-10], as well as to implement squeezed light with interferometers [11-14]. The GEO600 detector is now routinely using squeezed light, with ever-increasing timescales and duty cycles [15].

Figure 2: First measurement of greater than 10 dB squeezing across the audio gravitational-wave detection band, with 11.6 dB from 200 Hz and above. The degradation of squeezing level below 100 Hz is due to remaining residual classical noise entering the squeezing detector. Adapted from [16], and includes resolution bandwidth and window information.

The first milestone recently added to this story is the measurement of greater than 10 dB squeezing across the 10 Hz – 10 kHz frequency band [16]. This measurement was achieved by a team at the Australian National University, with valuable input from the Albert Einstein Institute. Figure 2 shows the result, with a maximum of 11.6 dB measured at 100 Hz and above. This was achieved after a detailed study characterizing and minimizing classical noise sources that impacted the squeezing measurement. This result represents the current record for squeezing in the 10 Hz – 10 kHz band, and further demonstrates the availability of large squeezing magnitude applicable for gravitational-wave detector enhancement.

The second milestone recently achieved is realising a squeezed 4 km interferometric gravitational-wave detector [17]. This was an experiment completed on the Enhanced LIGO 4 km interferometer in Washington State USA, performed by scientists from across the LIGO Scientific Collaboration, with LIGO Hanford Observatory, LIGO Massachusetts Institute of Technology, Australian National University and the Albert Einstein Institute being the lead institutions.

Figure 3: Enhanced LIGO interferometer with squeezing. (a) The Reference trace shows the displacement sensitivity of the interferometer without squeezing being injected, while the Squeezing trace shows the interferometer with squeezing injected. (b) Squeezing enhancement in LIGO’s most sensitive frequency band, at a lesser level due to significant contributions from noise sources other than quantum noise. Adapted from [17].

Figure 3(a) shows the interferometer displacement sensitivity curve with and without squeezed light. Up to 2.15 dB of squeezing enhancement is measured in the quantum noise limited regime (above 150 Hz). This is in line with the expected experiment parameters. Furthermore, as shown in Figure 3(b), in the most sensitive band between 150 Hz and 300 Hz, there is enhancement gained by squeezing. This result confirmed the compatibility of squeezing at lower detection frequencies where future gravitational-wave detectors will have their best sensitivity.

Squeezed light is a tool that is now available for, and being used for enhancing interferometric gravitational-wave detectors [15]. Third generation detector designs, such as the Einstein Telescope [18], have squeezed light injection as part of baseline technology. To realise maximum benefit from squeezed light injection, further improvements and refinements are needed, such as for improved parameters for squeezing injection and for minimizing adverse impacts on future detectors with more stringent requirements. This development work continues on as I write. It is safe to say that there are many more milestones to come in this continuing tale of big and small.

This article is a ‘synopsis’ of the squeezed light story and the two milestone results. For an in-depth review of squeezed light, squeezed light technologies and injection experiments up to 2013 (including both of these recent milestones), a Topical Review article is to be published soon [19]. I also recommend the LIGO Magazine, Issue 3 [20], which is focussed on squeezed light.

References:
[1] Advanced LIGO website: www.advancedligo.mit.edu ; Advanced Virgo website: wwwcascina.virgo.infn.it/advirgo ; KAGRA website: gwcenter.icrr.u-tokyo.ac.jp/en/; GEO600 website: www.geo600.org
[2] For example: www.advancedligo.mit.edu/adligo_news.html .
[3] For an expanded introduction to interferometric gravitational-wave detector measurement, I recommend this short video: www.youtube.com/watch?v=RzZgFKoIfQI .
[4] P.R. Saulson, "Fundamentals of interferometric gravitational wave detectors". World Scientific, Singapore (1994).
[5] D.F. Walls and G. Milburn, "Quantum Optics". Springer-Verlag, 2nd edition, Berlin (2008).
[6] Carlton M. Caves, "Quantum-mechanical noise in an interferometer". Physical Review D, 23, 1693 (1981) . Abstract.
[7] R.E. Slusher, L.W. Hollberg, B. Yurke, J.C. Mertz, J.F. Valley, "Observation of Squeezed States Generated by Four-Wave Mixing in an Optical Cavity", Physical Review Letters, 55, 2409 (1985). Abstract.
[8] Kirk McKenzie, Nicolai Grosse, Warwick P. Bowen, Stanley E. Whitcomb, Malcolm B. Gray, David E. McClelland, Ping Koy Lam, "Squeezing in the Audio Gravitational-Wave Detection Band". Physical Review Letters, 93, 161105 (2004). Abstract.
[9] Roman Schnabel and Henning Vahlbruch, "Squeezed Light – the first real application starts now". 2Physics : April 03, 2008.
[10] Tobias Eberle, Sebastian Steinlechner, Jöran Bauchrowitz, Vitus Händchen, Henning Vahlbruch, Moritz Mehmet, Helge Müller-Ebhardt, Roman Schnabel, "Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection", Physical Review Letters, 104, 251102 (2010). Abstract.
[11] Kirk McKenzie, Daniel A. Shaddock, David E. McClelland, Ben C. Buchler, and Ping Koy Lam, "Experimental Demonstration of a Squeezing-Enhanced Power-Recycled Michelson Interferometer for Gravitational Wave Detection", Physical Review Letters, 88, 231102 (2002). Abstract.
[12] Henning Vahlbruch, Simon Chelkowski, Boris Hage, Alexander Franzen, Karsten Danzmann, Roman Schnabel, "Demonstration of a Squeezed-Light-Enhanced Power- and Signal-Recycled Michelson Interferometer", Physical Review Letters, 95 211102 (2005). Abstract.
[13] Keisuke Goda, Alan Weinstein, Nergis Mavalvala, "Beating the Quantum Limit in Gravitational Wave Detectors". 2Physics : May 10, 2008.
[14] Hartmut Grote, Roman Schnabel, Henning Vahlbruch, "A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit". 2Physics : September 25, 2011.
[15] H. Grote, K. Danzmann, K. L. Dooley, R. Schnabel, J. Slutsky, H. Vahlbruch, "First Long-term Application of Squeezed States of Light in a Gravitational-Wave Observatory". Physical Review Letters, 110, 181101 (2013). Abstract.
[16] M S Stefszky, C M Mow-Lowry, S S Y Chua, D A Shaddock, B C Buchler, H Vahlbruch, A Khalaidovski, R Schnabel, P K Lam, D E McClelland, "Balanced Homodyne Detection of Optical Quantum States at Audio-Band Frequencies and Below". Classical and Quantum Gravity, 29 145015 (2012). Abstract.
[17] The LIGO Scientific Collaboration, "Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light". Nature Photonics 7,  613 – 619 (2013). Abstract.
[18] Einstein Telescope: www.et-gw.eu .
[19] S. Chua et al, "Quantum Squeezed Light for Advanced Gravitational-wave Detectors". Classical and  Quantum Gravity Topical Review, accepted for publication (2014).
[20] LIGO Magazine, Issue 3: www.ligo.org/magazine/LIGO-magazine-issue-3.pdf .

Few-layer Black Phosphorus Phototransistors for Fast and Broadband Photodetection

From Left to Right: (Top) Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, (Bottom) Gary A. Steele, Herre S.J. van der Zant and Andres Castellanos-Gomez

Authors:
Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, Gary A. Steele, Herre S.J. van der Zant, Andres Castellanos-Gomez.

Affiliation:
Kavli Institute of Nanoscience, Delft University of Technology, The Netherlands.

Introduction

The isolation of graphene has opened the door for the studying of the large family of layered two-dimensional (2D) materials, driven by the extraordinary properties that these materials show in their single and few-layer form [1-4]. Graphene, a one-atom thick layer of carbon atoms, has shown excellent electrical properties (e.g. mobility in the order of 170 000 cm²/Vs at room temperature) and large breaking strength [5,6]. However, its applicability in low-power field effect transistors (FETs) and optoelectronic devices (e.g. photodetectors) is hampered by its zero bandgap.

This absence of a bandgap has intensified the current research in other 2D materials with an intrinsic bandgap [7]. For instance, silicene, a single layer of silicon atoms, represents a semiconducting analogue to graphene but so far it has only been realized in an ultra-high-vacuum environment, a severe limitation for further studies and applications [8]. Other promising candidates for optoelectronic applications are the members of the transition metal dichalcogenides (TMDCs) material class [9-13]. The large and direct bandgap of their single-layer form provides strong light absorption – a necessary condition for large photoresponse – but operation is limited to part of the visible spectrum. A material with a direct and small bandgap is needed to extend the detection range accessible with 2D materials.

Black phosphorus

Few-layer black phosphorus is a new member of the 2D-materials family. Black phosphorus is a layered allotrope of the element phosphorus and, in bulk, it is a semiconductor with a direct bandgap of 0.35 eV [14]. In its few-layer form, the bandgap is predicted to strongly depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single-layer). Moreover, FETs based on few-layer black-phosphorus show promising electrical properties [15-18], making them an appealing candidate for tunable photodetection from the visible to the infrared part of the spectrum.

Main Results

In our recently published work [19], we characterized the response to light excitation of FETs based on few-layer black phosphorus (thickness ranging from 3nm to 8nm). Figure 1a shows a schematic of the device and of the measurement circuit. Without illumination, the black-phosphorus FETs show ambipolar behavior, as both holes and electrons can be induced in the conducting channel by the gate electric field. The measured mobilities are in the order of 100 cm²/Vs and current on/off ratio in the order of 10³, demonstrating good electrical behavior. Under illumination, we measure a sizable photoresponse to excitation wavelengths from the visible up to 940 nm (see Figure 1b). Figure 1c shows the photocurrent measured for a single pulse of light excitation from which we estimate a rise time of 1 ms, demonstrating broadband and fast photodetection. For comparison, photodetectors based on single-layer molybdenum disulphide (MoS₂) can reach higher responsivities (~ 880 X 10³ mA/W) but their response time is limited to 0.6 sec [20].

Figure 1: (a) Device schematics (b) Source-drain current vs. gate voltage in dark (black solid line), with λ = 940 nm illumination (purple solid line), λ = 640 nm illumination (red solid line) and λ = 532 nm illumination (green solid line). The total incident optical power is 750 μW for all wavelengths. (c) Source-drain current vs. time for a single period of light modulation with a mechanical chopper (different device from panel b).

Future trends and outlook

Taking advantage of the ambipolarity, one could think of electrostatically defining a PN junction in a few-layer black phosphorus flake, as already pioneered in single-layer tungsten diselenide (WSe₂) [9-11]. A PN junction could be used to boost the photoresponse and generate electrical power via the photovoltaic effect. Given the small and direct bandgap of few-layer black-phosphorus, it would be possible to harvest photons also in the near-infrared part of the spectrum.

References:
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, "Two-dimensional gas of massless Dirac fermions in graphene". Nature, 438, 197-200 (2005). Abstract.
[2] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, "Two-dimensional atomic crystals". Proceedings of the National Academy of Sciences of the United States of America, 102, 10451(2005). Full Article.
[3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A. A. Firsov, "Electric Field Effect in Atomically Thin Carbon Films". Science, 306, 666 (2004). Abstract.
[4] Qing Hua Wang, Kourosh Kalantar-Zadeh, Andras Kis, Jonathan N. Coleman, Michael S. Strano, "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides". Nature nanotechnology, 7, 699–712 (2012). Abstract.
[5] L. Wang, I. Meric, P.Y. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L. M. Campos, D.A. Muller, J. Guo, P. Kim, J. Hone, K. L. Shepard, C. R. Dean, "One-Dimensional Electrical Contact to a Two-Dimensional Material". Science, 342, 614-617 (2013). Abstract.
[6] Changgu Lee, Xiaoding Wei, Jeffrey W. Kysar, James Hone, "Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene". Science, 321, 385-388 (2008). Abstract.
[7] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, "Single-layer MoS₂ transistors". Nature Nanotechnology, 6, 147-150 (2011). Abstract.
[8] Patrick Vogt, Paola De Padova, Claudio Quaresima, Jose Avila, Emmanouil Frantzeskakis, Maria Carmen Asensio, Andrea Resta, Bénédicte Ealet, Guy Le Lay, "Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon". Physical Review Letters, 108, 155501 (2012). Abstract.
[9] Britton W. H. Baugher, Hugh O. H. Churchill, Yafang Yang, Pablo Jarillo-Herrero, "Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide". Nature Nanotechnology, 9, 262-267 (2014). Abstract.
[10] Andreas Pospischil, Marco M. Furchi, Thomas Mueller, "Solar-energy conversion and light emission in an atomic monolayer p–n diode". Nature Nanotechnology, 9, 257-261 (2014). Abstract.
[11] Jason S. Ross, Philip Klement, Aaron M. Jones, Nirmal J. Ghimire, Jiaqiang Yan, D. G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David H. Cobden, Xiaodong Xu, "Electrically tunable excitonic light-emitting diodes based on monolayer WSe₂ p–n junctions". Nature Nanotechnology, 9, 268-272 (2014). Abstract.
[12] Zongyou Yin, Hai Li, Hong Li, Lin Jiang, Yumeng Shi, Yinghui Sun, Gang Lu, Qing Zhang, Xiaodong Chen, Hua Zhang, "Single-Layer MoS₂ Phototransistors". ACS Nano, 6, 74-80 (2012). Abstract.
[13] Néstor Perea-López, Ana Laura Elías, Ayse Berkdemir, Andres Castro-Beltran, Humberto R. Gutiérrez, Simin Feng, Ruitao Lv, Takuya Hayashi, Florentino López-Urías, Sujoy Ghosh, Baleeswaraiah Muchharla, Saikat Talapatra, Humberto Terrones, Mauricio Terrones, "Photosensor Device Based on Few-Layered WS₂ Films". Advanced Functional Materials, 23, 5511-5517 (2013). Abstract.
[14] Yuichi Akahama, Shoichi Endo, Shin-ichiro Narita, "Electrical Properties of Black Phosphorus Single Crystals". Journal of the Physical Society of Japan, 52, 2148-2155 (1983). Abstract.
[15] Han Liu, Adam T. Neal, Zhen Zhu, David Tomanek, Peide D. Ye, "Phosphorene: A New 2D Material with High Carrier Mobility". arXiv:1401.4133 [cond-mat.mes-hall] (2014).
[16] Likai Li, Yijun Yu, Guo Jun Ye, Qingqin Ge, Xuedong Ou, Hua Wu, Donglai Feng, Xian Hui Chen, Yuanbo Zhang, "Black phosphorus field-effect transistors". Nature Nanotechnology, 9, 372–377 (2014). Abstract.
[17] Steven P. Koenig, Rostislav A. Doganov, Hennrik Schmidt, A. H. Castro Neto, Barbaros Özyilmaz, "Electric field effect in ultrathin black phosphorus". Applied Physics Letters, 104, 103106 (2014). Abstract.
[18] Han Liu, Adam T. Neal, Zhen Zhu, Zhe Luo, Xianfan Xu, David Tománek, Peide D. Ye, "Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility". ACS Nano, 8, 4033-4041 (2014). Abstract.
[19] Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, Gary A. Steele, Herre S. J. van der Zant, Andres Castellanos-Gomez, "Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors". Nano letters, 14, 3347-3352 (2014). Abstract.
[20] Oriol Lopez-Sanchez, Dominik Lembke, Metin Kayci, Aleksandra Radenovic, Andras Kis, "Ultrasensitive photodetectors based on monolayer MoS₂" Nature Nanotechnology, 8, 497-501 (2013). Abstract.

Realizing Two-Dimensional Optics with Metal Antennas and Graphene Plasmons

(From Left to Right) Pablo Alonso-González, Alexey Nikitin and Rainer Hillenbrand.

Authors: 
Pablo Alonso-González¹, Alexey Nikitin^1,2, Federico Golmar^1,3, Alba Centeno⁴, Amaia Pesquera⁴, Saül Vélez¹, Jianing Chen¹, Gabriele Navickaite⁵, Frank Koppens⁵, Amaia Zurutuza⁴, Félix Casanova^1,2, Luis E. Hueso^1,2, Rainer Hillenbrand^1,2.

Affiliation:
¹CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.
²IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
³I.N.T.I-CONICET and ECyT-UNSAM, San Martín, Bs. As., Argentina.
⁴Graphenea SA, 20018 Donostia-San Sebastián, Spain.
⁵ICFO-Institut de Ciéncies Fotoniques, Mediterranean Technology Park, Barcelona, Spain.

Optical circuits and devices could make signal processing and computing much faster. However, although light is very fast, it needs too much space. In fact, propagating light needs at least the space of half its wavelength, which is much larger than state-of-the-art electronic building blocks in our computers. For that reason, a quest for squeezing light to propagate it through nanoscale materials arises.

Graphene, a single layer of carbon atoms with extraordinary properties, has been proposed as one solution. The wavelength of light captured by a graphene layer can be strongly shortened by a factor of 10 to 100 compared to light propagating in free space [1, 2]. As a consequence, this light propagating along the graphene layer - called graphene plasmon - requires much less space and promises ultra-compact photonic devices [3,4].

Past 2Physics articles by this Group:
July 22, 2012: "Capturing, Tuning and Controlling Light with a Single Sheet of Carbon Atoms"
by Jianing Chen, Michela Badioli, Pablo Alonso-González, Susokin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza, Nicolas Camara, Javier García de Abajo, Rainer Hillenbrand, Frank Koppens

Converting light efficiently into graphene plasmons, however, has been a major challenge. In our recent work [5], we demonstrate that the antenna concept of radio wave technology could be a promising solution. We show that a nanoscale metal rod on graphene (acting as an antenna for light) can capture infrared light and convert it into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable. The excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by geometrically tailoring the antennas. The latter is essential for the development of applications that require focusing and guiding of graphene plasmons.

Fig. 1: Launching graphene plasmons with a gold antenna. The oscillations of the calculated electromagnetic field around the antenna reveal the graphene plasmons.

Based on calculations (Fig. 1), we fabricated gold nanoantennas on graphene provided by Graphenea. We then used the Neaspec near-field microscope to image how infrared graphene plasmons are launched and propagate along the graphene layer. In the experimental near-field images, we observed that indeed electromagnetic waves on the graphene propagate away from the antenna, with a wavelength that is about 30 times smaller than that of the incident light (Fig. 2).

Fig. 2: Top: Topography of a gold nanoantenna on graphene. Bottom: Near-field image showing the fields of the antenna and the graphene plasmons around the antenna. The image was taken at an illumination wavelength of 11.06 μm and shows the real part of the imaged field. The distance between fringes of the same color reveals the graphene plasmon wavelength.

In order to test whether the two-dimensional propagation of light waves along a one-atom-thick carbon layer follow the laws of conventional optics, we tried to focus and refract the waves. For the focusing experiment, we curved the antenna. The images then showed that the graphene plasmons focus away from the antenna, similar to the light beam that is concentrated with a lens or concave mirror.

We also observed that graphene plasmons refract (bend) when they pass through a prism-shaped graphene bilayer (Fig. 3), analogous to the bending of a light beam passing through a glass prism. The big difference is that the graphene prism is only two atoms thick. By measuring the graphene plasmon wavelengths in the bi- and monolayer, λ₁ and λ₂, as well as the propagation angles α₁ and α₂, we could demonstrate that the refraction of graphene plasmons qualitatively follows the fundamental law of refraction (Snell´s law): sin(α₁)/sin(α₂) = λ₁/λ₁.

Fig. 3: (a) Illustration of a graphene bilayer prism next to a gold antenna. (b) Near-field image (taken at an illumination wavelength of 10.20 μm) of graphene plasmons refracting at a graphene bilayer prism. The yellow lines and arrows illustrate the plasmon wavefronts and their refraction.

Intriguingly, the graphene plasmons are refracted because the conductivity in the two-atom-thick prism is larger than in the surrounding one-atom-thick layer. In the future, local conductivity changes in graphene could be generated by simple electronic means, such as gating, allowing for highly efficient electrical control of refraction, among others for steering applications.

Altogether, the experiments show that the fundamental and most important principles of conventional optics also apply for graphene plasmons, in other words, squeezed light propagating along a one-atom-thick layer of carbon atoms. Future developments based on these results could lead to extremely miniaturized optical circuits and devices that could be useful for sensing and computing, among other applications.

References:
[1] Jianing Chen, Michela Badioli, Pablo Alonso-González, Sukosin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza Elorza, Nicolas Camara, F. Javier García de Abajo, Rainer Hillenbrand, Frank H. L. Koppens, "Optical nano-imaging of gate-tunable graphene plasmons". Nature, 487, 77-81 (2012). Abstract. 2Physics Article.
[2] Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, "Gate-tuning of graphene plasmons revealed by infrared nano-imaging". Nature, 487, 82-85 (2012). Abstract. 2Physics Article.
[3] Ashkan Vakil, Nader Engheta, “Transformation optics using graphene”. Science, 332, 1291-1294 (2011). Abstract.
[4] A.N. Grigorenko, M. Polini, K.S. Novoselov, “Graphene plasmonics”, Nature Photonics, 6, 749-758 (2012). Abstract.
[5] P. Alonso-González, A.Y. Nikitin, F. Golmar, A. Centeno, A. Pesquera, S. Vélez, J. Chen, G. Navickaite, F. Koppens, A. Zurutuza, F. Casanova, L.E. Hueso and R. Hillenbrand. “Controlling grapheme plasmons with resonant metal antennas and spatial conductivity patterns”. Science,  344, 1369-1373 (2014). Abstract.

Institutes:

CIC nanoGUNE
The nanoGUNE Cooperative Research Center, located in Donostia-San Sebastian, Basque Country, is a research centre set up with the mission to conduct excellence research into nanoscience and nanotechnology with the aim of increasing the Basque Country’s business competitiveness and economic and social development.

GRAPHENEA S.A.
Graphenea is a pioneer graphene production start-up company founded in 2010 by private investors and CIC nanoGUNE. The company produces and commercializes graphene films by Chemical Vapor Deposition technology and graphene powders by Chemical Exfoliation techniques.

ICFO
ICFO is a young research institution located in Barcelona that aims to advance the very limits of knowledge in Photonics, namely the science and technology of harnessing Light. Its research programs target the global forefront of photonics, and aim to tackle important challenges faced by society at large. ICFO is focused on current and future problems in Health, Energy, Information, Safety, Security and caring for the Environment.

Micropillar Laser Mimics 'Excitability' of Neurons

Left to Right: S. Barbay and F. Selmi

Authors: S. Barbay, F. Selmi

Affiliation:
Laboratoire de Photonique et de Nanostructures, Marcoussis, France.

It is well known in neurophysiology textbooks (see e.g. [12]) that neurons are biological excitable systems possessing an absolute and a relative refractory period. These properties are fundamental for the propagation of nerve impulse and for information processing in the brain. Excitability is a generic property that is also found in biological [8], chemical [11], and optical systems [5]. An excitable system possess a rest state. If perturbed above a certain threshold - the excitable threshold - with a single perturbation, it emits a pulse with a characteristic shape (light pulse in optics, electrical pulse in neurons). If perturbed with two successive pulses above the excitable threshold, it can respond by emitting two identical pulses if the perturbation pulses are well separated temporally. It can emit a unique pulse if the second perturbation occurs too early after the first one : we are then in the absolute refractory period. However, in an intermediate regime, the relative refractory period, it is possible but harder to trigger a second response and this second response is inhibited and has smaller amplitude.

Figure 1: Schematic representation and SEM image of the micropillar laser with embedded saturable absorber. The pillar microcavity is coated with a thick, 2 microns, layer of SiN. The active zone is sandwiched between two aperiodic multilayer mirrors and is composed of a gain and an absorber section made by respectively 2 and 1 quantum wells. The laser emits around 980nm and is 4μm in diameter.

Our system is a special kind of micropillar laser embedding in its center a saturable absorber, and optimized for optical pumping [4]. Semiconductor lasers are interesting systems for the study of excitability [2, 3, 6, 7, 14-19] since they can have a small footprint and have short timescales. They can thus lead to short response times, necessary in view of their utilization in a neuromorphic information processing context. It was also recognized recently that microlasers with saturable absorber can work as leaky integrate-and-fire laser neurons [10] paving the way to fast cognitive computing.

We have shown in [13] that a micropillar laser with saturable absorber is a fast excitable unit with an absolute, and for the first time, a relative refractory period. The response time is of the order of 200ps, similar to the absolute refractory period, and the relative refractory period is about 350ps. This demonstrates that this system behaves analogously to a neuron but with much faster timescales (sub-nanosecond vs millisecond). The existence of a relative refractory period proves that the system keeps memory of its past state. Contrarily to a common belief, the excitable threshold is not a constant of the system and may be controlled either externally, via the bias pumping of the system, or dynamically via the past state memory. These properties can be utilized for neuro-mimetic optical processing of information, for instance by coupling several micropillar lasers together for building logic gates [1] or for neuromorphic processing [9,10].

References:
[1] Adrian Jacobo, Damià Gomila, Manuel A Matías, Pere Colet, "Logical operations with localized structures". New Journal of Physics, 14, 013040 (2012). Abstract.
[2] Stéphane Barland, Oreste Piro, Massimo Giudici, Jorge R. Tredicce, Salvador Balle, "Experimental evidence of van der Pol—Fitzhugh—Nagumo dynamics in semiconductor optical amplifiers". Physical Review E, 68, 036209 (2003). Abstract.
[3] Maia Brunstein, Alejandro M. Yacomotti, Isabel Sagnes, Fabrice Raineri, Laurent Bigot, Ariel Levenson, "Excitability and self-pulsing in a photonic crystal nanocavity". Physical Review A, 85:031803, 2012. Abstract.
[4] T. Elsass, K. Gauthron, G. Beaudoin, I. Sagnes, R. Kuszelewicz, S. Barbay, "Control of cavity solitons and dynamical states in a monolithic vertical cavity laser with saturable absorber". Euro Physics Journal D, 59, 91 (2010). Abstract.
[5] F. Plaza, M. G. Velarde, F. T. Arecchi, S. Boccaletti, M. Ciofini, R. Meucci. "Excitability following an avalanche-collapse process". Europhysics Letters, 38, 85 (1997). Abstract.
[6] M. Giudici, C. Green, G. Giacomelli, U. Nespolo, J. R. Tredicce. "Andronov bifurcation and excitability in semiconductor lasers with optical feedback". Physical Review E, 55, 6414 (1997). Abstract.
[7] D. Goulding, S. P. Hegarty, O. Rasskazov, S. Melnik, M. Hartnett, G. Greene, J. G. McInerney, D. Rachinskii, G. Huyet, "Excitability in a Quantum Dot Semiconductor Laser with Optical Injection". Physical Review Letters, 98, 153903 (2007). Abstract.
[8] J. D. Murray. Mathematical biology. Springer, New York, 1990. 
[9] Wolfgang Maass, Thomas Natschläger, Markram Henry, "Real-Time Computing Without Stable States: A New Framework for Neural Computation Based on Perturbations". Neural Computation, 14, 2531-2560 (2002). Abstract.
[10] M.A. Nahmias, B.J. Shastri, A.N. Tait, P.R. Prucnal, "A Leaky Integrate-and-Fire Laser Neuron for Ultrafast Cognitive Computing". IEEE Journal of Selected Topics in Quantum Electronics, 19(5), 1-12 (2013).
[11] Adolphe Pacault, Patrick Hanusse, Patrick De Kepper, Christian Vidal, Jacques Boissonade. Phenomena in homogeneous chemical systems far from equilibrium. Accounts of Chemical Research, 9(12), 438-445 (1976). Abstract.
[12] D. Randall, W. Burggren, K. French, R. Eckert. Eckert Animal Physiology. W. H. Freeman, 2002. Google Book.
[13] F. Selmi, R. Braive, G. Beaudoin, I. Sagnes, R. Kuszelewicz, S. Barbay, "Relative Refractory Period in an Excitable Semiconductor Laser". Physics Review Letters, 112, 183902 (2014). Abstract.
[14] Stefano Beri, Lilia Mashall, Lendert Gelens, Guy Van der Sande, Gabor Mezosi, Marc Sorel, Jan Danckaert, Guy Verschaffelt, "Excitability in optical systems close to Z₂-symmetry". Physics Letters A, 374, 739 (2010). Abstract.
[15] Sylvain Barbay, Robert Kuszelewicz, Alejandro M. Yacomotti. Excitability in a semiconductor laser with saturable absorber. Opt. Lett., 36(23):4476—4478, 2011. Abstract.
[16] Thomas Van Vaerenbergh, Martin Fiers, Pauline Mechet, Thijs Spuesens, Rajesh Kumar, Geert Morthier, Benjamin Schrauwen, Joni Dambre, Peter Bienstman, "Cascadable excitability in microrings". Optics Express, 20, 20292 (2012). Abstract.
[17] Sebastian Wieczorek, Bernd Krauskopf, Daan Lenstra, "Multipulse Excitability in a Semiconductor Laser with Optical Injection". Physical  Review Letters, 88, 063901 (2002). Abstract.
[18] H. J. Wünsche, O. Brox, M. Radziunas, F. Henneberger, "Excitability of a Semiconductor Laser by a Two-Mode Homoclinic Bifurcation". Physical  Review Letters, 88, 023901 (2001). Abstract.
[19] A. M. Yacomotti, P. Monnier, F. Raineri, B. Ben Bakir, C. Seassal, R. Raj, J. A. Levenson, "Fast Thermo-Optical Excitability in a Two-Dimensional Photonic Crystal". Physical  Review Letters, 97, 143904 (2006). Abstract.

Diffusive-Light Invisibility Cloaking

[From Left to Right] Robert Schittny, Muamer Kadic, Tiemo Bückmann, Martin Wegener

Authors:
Robert Schittny^1,2, Muamer Kadic^1,3, Tiemo Bückmann^1,2, Martin Wegener^1,2,3

Affiliations:
¹Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), Germany, 
²DFG-Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), Germany, 
³Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Germany.

In an invisibility cloak [1–5], light is guided on a detour around an object such that it emerges behind unchanged, thus making the object invisible to an outside observer. An ideal cloak should be macroscopic and work perfectly for any direction, polarization, and wavelength of the incoming light. To make up for the geometrical detour, light has to travel faster inside the cloak than outside, that is, faster than the vacuum speed of light for cloaking in air or vacuum. Furthermore, the absence of wavelength dependence means that energy velocity and phase velocity are strictly equal. However, general relativity forbids energy velocities higher than the vacuum speed of light. Thus, macroscopic, omnidirectional, and broadband invisibility cloaking is fundamentally impossible in air [4, 5]. Consistently, all experimental demonstrations of optical cloaking so far came with a drawback in terms of operation bandwidth, size, or both [6–10].

In contrast to this, more recently, we have demonstrated [11] close to ideal macroscopic and broadband invisibility cloaking in diffusive light scattering media at visible wavelengths.

Past 2Physics articles by this Group:
May 06, 2012: "A Cloak for Elastic Waves in Thin Polymer Plates"
     by Nicolas Stenger, Manfred Wilhelm, Martin Wegener
June 19, 2011:
"3D Polarization-Independent Invisibility Cloak at Visible Wavelengths"
     by Tolga Ergin, Joachim Fischer, Martin Wegener
April 11, 2010: "3D Invisibility Cloaking Device at Optical Wavelengths"
     by Tolga Ergin, Nicolas Stenger, Martin Wegener

Fig. 1 [11] illustrates the principle and results of invisibility cloaking in diffusive media. In such media, many scattering particles are randomly distributed, causing each photon to travel along a random path (see artistic illustration in the magnifying glass in Fig. 1). This effectively slows down light with respect to the vacuum speed of light, making perfect cloaking possible. In contrast to “ballistic” light propagation in vacuum or air as described by Maxwell’s equations, light propagation in such a medium can be described by diffusion of photons [12].

Figure 1 (Ref.[11]): Principle of diffusive-light cloaking. Computer-generated image of an illuminated cuboid diffusive medium with a zero-diffusivity obstacle (left-hand side) and a core-shell cloak (right-hand side). The magnifying glass shows an artistic illustration of a photon’s random walk inside the diffuse medium. The black streamline arrows are simulation results illustrating the photon current around obstacle and cloak. Corresponding measurement results are projected onto the front side of the cuboid volume, showing a diffuse shadow for the obstacle (left-hand side) and its elimination for the cloak (right-hand side). The euro coin illustrates the macroscopic dimensions of the cloak.

If a diffusive medium is illuminated from one side, any object with a different diffusivity inside this medium will cause perturbations of the photon flow. On the left-hand side of Fig. 1, a hollow cylinder with a diffusivity of exactly zero (the “obstacle”) suppresses any photon flow inside and casts a pronounced shadow, reducing the photon current on the downstream side (see black streamline arrows in Fig. 1). To compensate for this, a thin layer with a higher diffusivity than in the surrounding medium is added to the cylinder on the right-hand side of Fig. 1 (the “cloak”). Intuitively, a higher diffusivity (that is, a lower concentration of scattering particles) leads to an effectively higher light propagation speed and thus makes up for the geometrical detour the light has to take on its way around the obstacle. The black streamline arrows show that the photon current behind the cloak is unchanged. In other words, the shadow cast by the obstacle vanishes.

Such a core-shell cloak design can be thought of as the reduction of more complex multilayer designs based on transformation optics [1–3] to just two layers. It is known theoretically [13, 14] to work perfectly in the static case and for spatially constant gradients of the photon density across the cloak. Core-shell cloaks have been demonstrated before in magnetostatics [15], thermodynamics [16, 17], and elastostatics [18], recently even for non-constant gradients [16, 17].

For our experiments, we used a hollow aluminum cylinder as the obstacle, coated with a thin layer of white paint that acted as a diffusive reflector. For the cloaking shell, we coated the cylinder with a thin layer of a transparent silicone doped with dielectric microparticles. Obstacle and cloak are truly macroscopic, as indicated by the euro coin in Fig. 1 for comparison. We realized the diffusive background medium by mixing de-ionized water and white wall-paint. By changing the paint concentration, we could easily vary the surrounding’s diffusivity to find good cloaking performance. Other common examples of diffusive media are clouds, fog, paper or milk.

The samples were submerged in a Plexiglas tank filled with the water-paint mixture. The tank was illuminated from one side with white light coming from a computer monitor; photographs of the other side of the tank were taken with an optical camera. Two of these photographs are projected onto the front side of the cuboid volume shown in Fig. 1. The left-hand side shows the case with just the obstacle inside, exhibiting a pronounced diffuse shadow as expected from the discussion above. This shadow vanishes almost completely on the right-hand side, where the cloak is inside the tank. The yellowish tint of the photographs is caused by partial absorption of blue light in the water-paint mixture. Furthermore, we could trace the small remaining intensity variations for the cloaking case back to a finite absorption of light at the core-shell interface.

While the illustration in Fig. 1 only shows results for homogeneous illumination, we also found excellent cloaking performance using an inhomogeneous line-like illumination pattern (not depicted). Furthermore, we also performed successful experiments with spherical samples (not depicted), proving that our cloak is truly three-dimensional and works for any polarization and any direction of incidence.

References:
[1] J. B. Pendry, D. Schurig, D. R. Smith, "Controlling electromagnetic fields". Science, 312, 1780 (2006). Abstract.
[2] Ulf Leonhardt, "Optical conformal mapping". Science, 312, 1777 (2006). Abstract.
[3] Vladimir M. Shalaev, "Transforming light". Science, 322, 384 (2008). Abstract.
[4] David A. B. Miller, "On perfect cloaking". Optics Express, 14, 12457 (2006). Full Article.
[5] Hila Hashemi, Baile Zhang, J. D. Joannopoulos, Steven G. Johnson, "Delay-bandwidth and delay-loss limitations for cloaking of large objects". Physical Review Letters, 104, 253903 (2010). Abstract.
[6] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, "Metamaterial electromagnetic cloak at microwave frequencies". Science, 314, 977 (2006). Abstract.
[7] R. Liu, C. Ji, J.J. Mock, J.Y. Chin, T.J. Cui, D.R. Smith, "Broadband ground-plane cloak". Science, 323, 366 (2009). Abstract.
[8] Jason Valentine, Jensen Li, Thomas Zentgraf, Guy Bartal, Xiang Zhang, "An optical cloak made of dielectrics". Nature Materials, 8, 568 (2009). Abstract.
[9] Lucas H. Gabrielli, Jaime Cardenas, Carl B. Poitras, Michal Lipson, "Silicon nanostructure cloak operating at optical frequencies". Nature Photonics, 3, 461 (2009). Abstract.
[10] Tolga Ergin, Nicolas Stenger, Patrice Brenner, John B. Pendry, Martin Wegener, "Three-dimensional invisibility cloak at optical wavelengths". Science, 328, 337 (2010). Abstract. 2Physics Article.
[11] Robert Schittny, Muamer Kadic, Tiemo Bückmann, Martin Wegener, "Invisibility Cloaking in a Diffusive Light Scattering Medium". Science, Published Online June 5 (2014). DOI:10.1126/science.1254524.
[12] C. M. Soukoulis, Ed., “Photonic Crystals and Light Localization in the 21st Century”, (Springer, 2001).
[13] Graeme W. Milton, “The Theory of Composites”, (Cambridge Univ. Press, 2002).
[14] Andrea Alù, Nader Engheta, "Achieving transparency with plasmonic and metamaterial coatings". Physical Review E, 72, 016623 (2005). Abstract.
[15] Fedor Gömöry, Mykola Solovyov, Ján Šouc, Carles Navau, Jordi Prat-Camps, Alvaro Sanchez, "Experimental realization of a magnetic cloak". Science, 335, 1466 (2012). Abstract.
[16] Hongyi Xu, Xihang Shi, Fei Gao, Handong Sun, Baile Zhang, "Ultrathin three-dimensional thermal cloak". Physical Review Letters, 112, 054301 (2014). Abstract.
[17] Tiancheng Han, Xue Bai, Dongliang Gao, John T. L. Thong, Baowen Li, Cheng-Wei Qiu, "Experimental demonstration of a bilayer thermal cloak". Physical Review Letters, 112, 054302 (2014). Abstract.
[18] T. Bückmann, M. Thiel, M. Kadic, R. Schittny, M. Wegener, "An elasto-mechanical unfeelability cloak made of pentamode metamaterials". Nature Communications, 5, 4130 (2014). Abstract.

2D Electronic-Vibrational Spectroscopy Technique Provides Unprecedented Look into Photochemical Reactions

From left to Right: Nicholas Lewis, Graham Fleming and Tom Oliver [photo courtesy: Lawrence Berkeley National Laboratory, USA].

From allowing our eyes to see, to enabling green plants to harvest energy from the sun, photochemical reactions – reactions triggered by light – are both ubiquitous and critical to nature. Photochemical reactions also play essential roles in high technology, from the creation of new nanomaterials to the development of more efficient solar energy systems. Using photochemical reactions to our best advantage requires a deep understanding of the interplay between the electrons and atomic nuclei within a molecular system after that system has been excited by light. A major advance towards acquiring this knowledge has been reported by a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley.

Graham Fleming, UC Berkeley’s Vice Chancellor for Research, a faculty senior scientist with Berkeley Lab’s Physical Biosciences Division, and member of the Kavli Energy NanoSciences Institute at Berkeley, led the development of a new experimental technique called two-dimensional electronic-vibrational spectroscopy (2D-EV). By combining the advantages of two well-established spectroscopy technologies – 2D-electronic and 2D-infrared – this technique is the first that can be used to simultaneously monitor electronic and molecular dynamics on a femtosecond (millionth of a billionth of a second) time-scale. The results show how the coupling of electronic states and nuclear vibrations affect the outcome of photochemical reactions.

“We think that 2D-EV, by providing unprecedented details about photochemical reaction dynamics, has the potential to answer many currently inaccessible questions about photochemical and photobiological systems,” says Fleming, a physical chemist and internationally acclaimed leader in spectroscopic studies of events that take place on the femtosecond time-scale. “We anticipate its adoption by leading laboratories across the globe.”

Fleming is the corresponding author of a paper in the Proceedings of the National Academy of Sciences (PNAS) titled “Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy” [1]. Co-authors are Thomas Oliver and Nicholas Lewis, both members of Fleming’s research group.

Fleming and his research group were one of the key developers of 2D electronic spectroscopy (2D-ES), which enables scientists to follow the flow of light-induced excitation energy through molecular systems with femtosecond temporal resolution. Since its introduction in 2007, 2D-ES has become an essential tool for investigating the electronic relaxation and energy transfer dynamics of molecules, molecular systems and nanomaterials following photoexcitation. 2D infrared spectroscopy is the go-to tool for studying nuclear vibrational couplings and ground-state structures of chemical and complex biological systems.

“Combining these two techniques into 2D-EV tells us how photoexcitation affects the coupling of electronic and vibrational degrees of freedom,” says Oliver. “This coupling is essential to understanding how all molecules, molecular systems and nanomaterials function.”

In 2D-EV, a sample is sequentially flashed with three femtosecond pulses of laser light. The first two pulses are visible light that create excited electronic states in the sample. The third pulse is mid-infrared light that probes the vibrational quantum state of the excited system. This unique combination of visible excitation and mid-infrared probe pulses enables researchers to correlate the initial electronic absorption of light with the subsequent evolution of nuclear vibrations.

“2D-EV’s ability to correlate the initial excitation of the electronic–vibrational manifold with the subsequent evolution of high-frequency vibrational modes, which until now have not been explored, opens many potential avenues of fruitful study, especially in systems where electronic–vibrational coupling is important to the functionality of a system,” Fleming says.

As a demonstration, Oliver, Lewis and Fleming used their 2D-EV spectroscopy technique to study the excited-state relaxation dynamics of DCM dye dissolved in a deuterated solvent. DCM is considered a model “push-pull” emitter – meaning it contains both electron donor and acceptor groups – but with a long-standing question as to how it fluoresces back to the ground energy state.

Figure 1: 2D-EV spectral data tells researchers how photoexcitation of a molecular system affects the coupling of electronic and nuclear vibrations that is essential to understanding how the system functions.

“From 2D-EV spectra, we elucidate a ballistic mechanism on the excited state potential energy surface whereby molecules are almost instantaneously projected uphill in energy toward a transition state between locally excited and charge-transfer states before emission,” Oliver says. “The underlying electronic dynamics, which occur on the hundreds of femtoseconds time-scale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as an excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics.”

One example of how 2D-EV might be applied is in the study of rhodopsin, the pigment protein in the retina of the eye that is the primary light detector for vision, and carotenoids, the family of pigment proteins, such as chlorophyll, found in green plants and certain bacteria that absorb light for photosynthesis.

“The nonradiative energy transfer in rhodopsin and carotenoids is thought to involve the breakdown of one of the most widely used approximations of quantum mechanics, the Born-Oppenheimer approximation, which states that since motion of electrons are far faster than nuclei, as represented by vibrational motion, the nuclei respond to changes in electronic states,” Oliver says. “With 2D-EV, we will be able to directly correlate the degrees of electronic and vibrational freedom and track their evolution as a function of time. It’s a chicken and egg kind of problem: Do the electrons or nuclei move first? 2D-EV will give us insight into whether or not the Born-Oppenheimer approximation is still valid in these cases.”

For nanomaterials, 2D-EV should be able to shed much needed light on how the coupling of phonons – atomic soundwaves – with electrons impacts the properties of carbon nanotubes and other nanosystems. 2D-EV can also be used to investigate the barriers to electron transfer between donor and acceptor states in photovoltaic systems.

“We are continuing to refine the 2D-EV technology and make it more widely applicable so that it can be used to study lower frequency motions that are of great scientific interest,” Oliver says.

This research was funded by the DOE Office of Science and the National Science Foundation.

References:
[1] Thomas A. A. Oliver, Nicholas H. C. Lewis, Graham R. Fleming, "Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy". Proceedings of the National Academy of Sciences of the United States of America, published May 16 (2016). doi: 10.1073/pnas.1409207111.

[The text is authored by Lynn Yaris of Lawrence Berkeley National Laoratory]

Light that is Advanced

Paul Lett 
(photo courtesy: University of Maryland at College Park)

Michael Lewis’s bestselling book Flash Boys describes how some brokers, engaging in high frequency trading, exploit fast telecommunications to gain fraction-of-a-second advantage in the buying and selling of stocks. But you don’t need to have billions of dollars riding on this-second securities transactions to appreciate the importance of fast signal processing. From internet to video streaming, we want things fast.

Paul Lett and his colleagues at the Joint Quantum Institute (JQI, jointly operated by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park) specialize in producing modulated beams of light for encoding information. They haven’t found a way to move data faster than c, the speed of light in a vacuum, but in a new experiment they have looked at how light traveling through so called “fast-light” materials does seem to advance faster than c, at least in one limited sense. They report their results (published online May 25, 2014) in the journal 'Nature Photonics'.

Seeing how light can be manipulated in this way requires a look at several key concepts, such as entanglement, mutual information, and anomalous dispersion. At the end we’ll arrive at a forefront result.

Continuous Variable Entanglement :

Much research at JQI is devoted to the processing of quantum information, information coded in the form of qubits. Qubits, in turn are tiny quantum systems---sometimes electrons trapped in a semiconductor, sometimes atoms or ions held in a trap---maintained in a superposition of states. The utility of qubits increases when two or more of them can be yoked into a larger quantum arrangement, a process called entanglement. Two entangled photons are not really sovereign particles but parts of a single quantum entity.

The basis of entanglement is often a discrete variable, such as electron spin (whose value can be up or down) or photon polarization (say, horizontal or vertical). The essence of entanglement is this: while the polarization of each photon is indeterminate until a measurement is made, once you measure the polarization of one of the pair of entangled photons, you automatically know the other photon’s polarization too.

But the mode of entanglement can also be vested in a continuous variable. In Lett’s lab, for instance, two whole light beams can be entangled. Here the operative variable is not polarization but phase (how far along in the cycle of the wave you are) or intensity (how many photons are in the beam). For a light beam, phase and intensity are not discrete (up or down) but continuous in variability.

Quantum Mutual Information:

Biologists examining the un-seamed strands of DNA can (courtesy of the correlated nature of nucleic acid constituents) deduce the sequence of bases along one strand by examining the sequence of the other strand. So it is with entangled beams. A slight fluctuation of the instantaneous intensity of one beam (such fluctuations are inevitable because of the Heisenberg uncertainty principle) will be matched by a comparable fluctuation in the other beam.

Lett and his colleagues make entangled beams in a process called four-wave mixing. A laser beam (pump beam) enters a vapor-filled cell. Here two photons from the pump beam are converted into two daughter photons proceeding onwards with different energies and directions. These photons constitute beams in their own right, one called the probe beam, the other called the conjugate beam. Both of these beams are too weak to measure directly. Instead each beam enters a beam splitter (yellow disk in the drawing below) where its light can be combined with light from a local oscillator (which also serves as a phase reference). The ensuing interference patterns provide aggregate phase or intensity information for the two beams.

When the beam entanglement is perfect, the mutual correlation is 1. When studying the intensity fluctuations of one beam tells you nothing about those of the other beam, then the mutual correlation is 0.

Fast-Light Material:

In a famous experiment, Isaac Newton showed how incoming sunlight split apart into a spectrum of colors when it passed through a prism. The degree of wavelength-dependent dispersion for a material that causes this splitting of colors is referred to as its index of refraction.

In most materials the index is larger than 1. For plain window glass, it is about 1.4; for water it is 1.33 for visible light, and gradually increases as the frequency of the light goes up. At much higher frequency (equivalent to shorter wavelength), though, the index can change its value abruptly and go down. For glass, that occurs at ultraviolet wavelengths so you don’t ordinarily see this “anomalous dispersion” effect. In a warm vapor of rubidium atoms, however, (and especially when modified with laser light) the effect can occur at infrared wavelengths, and here is where the JQI experiment looks.

Figure 1: Experimental setup for studying fast light. Pump beams (purple) create correlated probe (turquoise) and conjugate (gold) beams. Each of these beams is aimed at a beam splitter (yellow disks). A local oscillator (LO) also sends a laser beam into each of the beam splitters. The resulting interference pattern---registered in a spectrum analyzer, SA---for the probe and conjugate arms are compared [Image courtesy: Paul Lett, JQI]

In Figure 1  notice that the conjugate beam is sent through a second cell, filled with rubidium vapor. Here the beam is subject to dispersion. The JQI experiment aims to study how the entanglement of this conjugate beam with the probe beam (subject to no dispersion) holds up.

When the refraction is “normal”---that is, when index of refraction causes ordinary dispersion---the light signal is slowed in comparison with the beam which doesn’t undergo dispersion. For this set of conditions, the cell is referred to as a “slow-light” material. When, however, the frequency is just right, the conjugate beam will undergo anomalous dispersion. When the different frequency components that constitute a pulse or intensity fluctuation reformulate themselves as they emerge from the cell, they will now be just slightly ahead of a pulse that hadn’t gone through the cell. (To make a proper measurement of delay one needs two entangled beams---beams whose fluctuations are related.)

Causality:

No, the JQI researchers are not saying that any information is traveling faster than c. Figure 2 shows that the peak for the mutual information for the fast-light-material is indeed ahead of the comparable peaks for an unscattered beam or for a beam emerging from a slow-light material. It turns out that the cost of achieving anomalous dispersion at all has been that additional gain (amplification) is needed, and this amplification imposes noise onto the signal.

Figure 2. The mutual information of the two beams (how much we know about one beam if we know the fluctuation of the other beam) peaks at different times depending on whether the conjugate beam passes through a fast-light medium (red), a slow-light medium (green), or no medium at all (black) [Image courtesy: Paul Lett, JQI].

This inherent limitation in extracting useful information from an incoming light beam is even more pronounced with beams containing (on average) one or less-than-one photon. Such dilute beams are desirable in many quantum experiments where measurement control or the storage or delay of quantum information is important.

“We did these experiments not to try to violate causality, said Paul Lett, “but because we wanted to see the fundamental way that quantum noise “enforces” causality, and working near the limits of quantum noise also lets us examine the somewhat surprising differences between slow and fast light materials when it comes to the transport of information.”

Reference:
[1] Jeremy B. Clark, Ryan T. Glasser, Quentin Glorieux, Ulrich Vogl, Tian Li, Kevin M. Jones, Paul D. Lett, "Quantum mutual information of an entangled state propagating through a fast-light medium". Nature Photonics, published online May 25th (2014). doi:10.1038/nphoton.2014.112.