Graphene Tuned Terahertz Lasers

Thomas Folland (left) and Subhasish Chakraborty

Authors: Subhasish Chakraborty¹, Owen Marshall^1,2, Thomas Folland¹, Yong-Jin Kim², Alexander Grigorenko², Konstantin Novoselov²

Affiliation:
¹School of Electrical and Electronic Engineering, University of Manchester, UK.
²School of Physics and Astronomy, University of Manchester, UK.

Terahertz light falls between microwave and the infrared in the electromagnetic spectrum, and has the useful property that it can pass through many non-metallic materials, including packaging and clothing. Furthermore, a wealth of material information can be obtained using terahertz waves. As such practical terahertz sources and detectors could be used for security scanning, quality control and wireless communication [1]. High power terahertz waves can be generated by using quantum cascade lasers. These semiconductor chips contain multiple stacked quantum wells, carefully designed so that the conduction band is split into multiple energy levels in which terahertz light is generated [2].

Controlling quantum cascade lasers so that they produce tuneable, single frequency light is particularly difficult. In our earlier work we achieved all-electrical tuning in a terahertz laser over a discrete set of frequencies [3]. We did this by integrating a specially designed photonic lattice into the gold waveguide of the laser, which we dubbed an ‘aperiodic lattice laser’. Such lasers are compact (~1 cm²), powerful (~10 mW of power), and capable of working at a range of terahertz frequencies. However, to be a practical THz source, these lasers need to show continuous tuning over a wide range of THz frequencies.

Figure 1. Graphene integrated laser, showing (from bottom to top) the QCL (Quantum Cascade Laser) chip, the gold grating, the graphene layer, the polymer electrolyte and the top gate.

Enters graphene, a single atomic layer of carbon atoms. Graphene interacts strongly with terahertz waves [4] forming surface plasmons, coupled oscillations between light and the electrons in a material. In graphene the properties of these surface plasmons can be controlled by applying a gate potential to the surface of the device (similar to field effect transistors). This allows graphene to control the properties of a range of optical devices, such as modulators [5]. In our article [6] we integrate graphene into a terahertz quantum cascade laser, allowing us to use tuneable surface plasmons to control the frequency of laser light. We did this by depositing graphene and a polymer electrolyte gate on top of an aperiodic lattice laser (see figure 1). The aperiodic lattice consists of a series sub-wavelength slits, inside which graphene surface plasmons could be excited. When no gate potential was applied to the graphene, a surface plasmon formed inside the slit to suppress light scattering from the lattice. Conversely when a gate potential was applied to the graphene, the surface plasmon wavelength became larger than the slit width, and scattering was enhanced from the lattice. The effect of this change in scattering strength is to change the laser emission frequency, as defined by the properties of the lattice (see figure 2).

Figure 2. Tuning laser emission using a graphene. The figure shows the simulated electric field in each lattice site, and the laser emission spectrum before and after the introduction of a gate potential.

This result shows that graphene plasmons can be excited and used to tune terahertz lasers. Although currently a proof of concept, it is in principle possible to individually gate each slit, allowing us to engineer the laser output. Such control would make terahertz lasers significantly more appealing for real world applications. Furthermore, although the design approach used in our work is most suitable for terahertz lasers, they could in principle be applied to more conventional lasers in the infrared. This would offer an entirely different way of controlling the properties of laser systems.

References:
[1] A. Giles Davies, Andrew D. Burnett, Wenhui Fan, Edmund H. Linfield, John E. Cunningham, "Terahertz spectroscopy of explosives and drugs", Materials Today, 11, 18–26 (2008). Full PDF.
[2] Rüdeger Köhler, Alessandro Tredicucci, Fabio Beltram, Harvey E. Beere, Edmund H. Linfield, A. Giles Davies, David A. Ritchie, Rita C. Iotti, Fausto Rossi, "Terahertz semiconductor-heterostructure laser", Nature, 417, 156–9 (2002). Abstract.
[3] Subhasish Chakraborty, Owen Marshall, Chen Wei Hsin, Md. Khairuzzaman, Harvey Beere, David Ritchie, "Discrete mode tuning in terahertz quantum cascade lasers", Optics Express, 20, B306–14 (2012). Abstract.
[4] Alessandro Tredicucci, Miriam Serena Vitiello, "Device Concepts for Graphene-Based Terahertz Photonics", IEEE Journal for Selected Topics in Quantum Electronics, 20, 130–138 (2014). Full Article.
[5] A. N. Grigorenko, M. Polini, K. S. Novoselov, "Graphene plasmonics", Nature Photonics, 6, 749–758 (2012). Abstract.
[6] S. Chakraborty, O. P. Marshall, T. G. Folland, Y.-J. Kim, A. N. Grigorenko, K. S. Novoselov, "Gain modulation by graphene plasmons in aperiodic lattice lasers", Science, 351, 246–248 (2016). Abstract.

All-Optical Reconstruction of Crystal Band Structure

Giulio Vampa

Paul Corkum

Authors: Giulio Vampa¹, Thomas Hammond¹, Nicolas Thiré², Bruno Schmidt², François Légaré², Chris McDonald¹, Thomas Brabec¹, Dennis Klug³,
Paul Corkum^1,3

Affiliation:
¹Department of Physics, University of Ottawa, Ontario, Canada,
²INRS-EMT, Varennes, Québec, Canada,
³National Research Council of Canada, Ottawa, Ontario, Canada.

The electronic band structure of solids determines their key properties, for example electrical conductivity and optical absorption. The energy bands are conventionally measured with Angle Resolved Photoemission Spectroscopy (ARPES), in which the kinetic energy and the momentum of electrons ionized by highly energetic photons are simultaneously measured [1]. The working principle is sketched in the left of Fig. 1. ARPES has been an incredibly successful method to probe conventional and less conventional materials, such as superconductors [2], graphene [3] and topological insulators [4].

However, electrons are not always accessible. Only high vacuum must oppose their propagation from the surface of the sample to the detector. This prevents measuring the band structure of the bulk, or of matter under extreme pressures, where the sample is enclosed in a diamond anvil cell, or in high magnetic fields, where electrons are deviated. Further, many surface chemical reactions that happen at ambient conditions, such as catalysis, are difficult to study with ARPES.

Figure 1: Comparison between photoemission and high harmonic generation. In photoemission experiments, an incoming photon (purple arrow) causes emission of an electron (black vertical arrow) with kinetic energy (K1 and K2) proportional to the initial binding energy (E1 and E2). In high harmonic generation, an electron first tunnels (marked by circled 1) to the conduction band. The electron-hole pair is then accelerated (circled 2) and recombines (circled 3) emitting a harmonic photon in the process. Electrons and holes are denoted by filled and empty blue circles, respectively. Two trajectories are identified by the wiggled solid black arrows, corresponding to different times of creation and recollision of the electron-hole pair.

In our recent letter [5], we report on an optical method to determine the band structure of solids. Because the technique does not detect electrons, it does not need high vacuum. The technique requires focusing a strong femtosecond laser pulse (1 fs = 10^-15 s), with a peak field strength of ~0.1-1 V/Ang – comparable to the force binding electrons to their atoms – with a central wavelength in the mid-infrared spectral region (therefore < 1 eV photon energy) inside the bulk of the solid sample to generate several new frequencies at odd multiples of the mid-infrared one. The process, known as High Harmonic Generation [6], proceeds through three steps that are depicted in the right of Fig. 1. First, when the field is close to a maximum, electrons tunnel from the top of the valence to the bottom of the conduction band, creating an electron-hole pair with zero initial crystal momentum (circled “1” in Fig. 1). Then, the laser field accelerates the electron-hole pair to high crystal momentum (it can reach the edge of the Brillouin Zone!) - marked “2” in Fig. 1 – in a way that depends on the shape of their respective bands. Then, at a specific time in the laser cycle, the electron can recombine with the hole, emitting a high harmonic photon in the process with energy equal to the band gap at the momentum of recombination (marked “3” in Fig. 1).

The process is highly coherent: ionization, propagation and recombination are perfectly synchronized with the laser filed. This allows to link each harmonic photon energy to a specific trajectory of the electron-hole pair, which is ultimately determined by when it was created about the peak of the laser field. Therefore, knowledge of the trajectory is sufficient to determine the momentum at recollision, the only ingredient required to determined the band structure – together with the harmonic photon energy. The technique relies on a second laser pulse, superimposed to the first, at twice the mid-infrared frequency, to perturb – and therefore measure – the trajectory.

In a simulated experiment, we are able to fully reconstruct the momentum-dependent band gap of a target model solid, chosen to approximate the band structure of a ZnO crystal. The technique requires measuring many harmonic orders, a task at the moment beyond the capabilities of our experimental setup. In a real experiment we conducted, we are able to measure only 20% of the Brillouin Zone. Even with this limited experimental data, we determine that the split-off valence band on ZnO is the one mostly contributing to tunnelling of electrons to the conduction band.

In conclusion, rather than using weak incoherent fields of high energy photons – as in photoemission experiments – we use intense coherent light of low energy photons to create, accelerate and probe electron-hole pairs in solids. The information about the energy of the bands in which the electron and the hole move is imprinted on a high harmonic photon upon their recombination. Like other spectroscopy techniques based on high harmonic generation in gases [7], the method has intrinsic sub-laser cycle temporal resolution. This could be used to track band structure modifications following laser excitation [8].

References:
[1] Andrea Damascelli, Zahid Hussain, Zhi-Xun Shen, "Angle-resolved photoemission studies of the cuprate superconductors", Review of Modern Physics, 75, 473 (2003). Abstract.
[2] A. Lanzara, P.V. Bogdanov, X.J. Zhou, S.A. Kellar, D.L. Feng, E.D. Lu, T. Yoshida, H. Eisaki, A. Fujimori, K. Kishio, J.-I. Shimoyama, T. Noda, S. Uchida, Z. Hussain, Z.-X. Shen, "Evidence for ubiquitous strong electron–phonon coupling in high-temperature superconductors", Nature, 412, 510 (2001). Abstract.
[3] Søren Ulstrup, Jens Christian Johannsen, Federico Cilento, Jill A. Miwa, Alberto Crepaldi, Michele Zacchigna, Cephise Cacho, Richard Chapman, Emma Springate, Samir Mammadov, Felix Fromm, Christian Raidel, Thomas Seyller, Fulvio Parmigiani, Marco Grioni, Phil D. C. King, Philip Hofmann, "Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene", Physical Review Letters, 112, 257401 (2014). Abstract.
[4] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z.K. Liu, S.-K. Mo, X.L. Qi, H.J. Zhang, D.H. Lu, X. Dai, Z. Fang, S.C. Zhang, I.R. Fisher, Z. Hussain, Z.-X. Shen, "Experimental Realization of a Three-Dimensional Topological Insulator, Bi₂Te₃", Science 325, 178 (2009). Abstract.
[5] G. Vampa, T.J. Hammond, N. Thiré, B.E. Schmidt, F. Légaré, C.R. McDonald, T. Brabec, D.D. Klug, P.B. Corkum, "All-Optical Reconstruction of Crystal Band Structure", Physical Review Letters, 115, 193603 (2015). Abstract.
[6] Shambhu Ghimire, Anthony D. DiChiara, Emily Sistrunk, Pierre Agostini, Louis F. DiMauro, David A. Reis, "Observation of high-order harmonic generation in a bulk crystal", Nature Physics, 7, 138 (2011). Abstract.
[7] S. Baker, J.S. Robinson, C.A. Haworth, H. Teng, R.A. Smith, C.C. Chirilă, M. Lein, J.W.G. Tisch, J.P. Marangos, "Probing Proton Dynamics in Molecules on an Attosecond Time Scale", Science, 312, 424 (2006). Abstract.
[8] E. N. Glezer, Y. Siegal, L. Huang, E. Mazur, "Laser-induced band-gap collapse in GaAs", Physical Review B, 51, 6959 (1995). Abstract.

Topological Defect Lasers

From left to right: Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Hui Cao

Authors: Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Hui Cao

Affiliation: Department of Applied Physics, Yale University, New Haven, Connecticut, USA.

Topological defects have been widely studied in liquid crystals and colloids. One important application of topological defects to photonics is the manipulation of the orbital angular momentum of optical beams. However, they have not been incorporated into lasers. Inspired by the topological defects existing in nematic liquid crystal, we recently designed and fabricated a new type of laser – a topological defect laser [1].

Past 2Physics articles by this group:
April 03, 2011: "Time-reversed Lasing and Coherent Control of Absorption"
 by A. Douglas Stone, Yidong Chong and Hui Cao.

One essential component of a laser is a cavity that confines light. A microscale cavity can be made within a photonic crystal, e.g., a two-dimensional membrane containing a periodic array of circular air holes, as shown in Fig. 1(a). Light, of wavelength comparable to the lattice constant, may be confined in the central square region as a result of high reflectivity from the surrounding photonic crystal walls. Figure 1(b) shows a spatial field profile of a confined mode, and Fig. 1(c) is the spatial map of its energy flow [2].

Figure 1: Introducing topological defect to a photonic crystal with anisotropic unit cell. (a,c) A square lattice of air holes embedded in a dielectric membrane. In (a), the holes have circular cross-section. The 4×4 holes are removed from the center to from a cavity that confines light by high reflection from the surrounding photonic crystal walls. The lattice constant is a = 220nm, and the hole radius is 65nm. To introduce topological defect in (c), the air holes are deformed to ellipses. The major axis of each ellipse is rotated to an angle φ = kθ + c from the horizontal axis. θ is the polar angle of the center position of the ellipse. k = 1 and c = π/4. (b,d) Calculated spatial distribution of the magnetic field for a confined optical mode in (a,b). The wavelength of the mode is 880nm. The topological defect rotates the mode in (d). (c,e) The spatial map of the Poynting vector for the modes in (b,d). Each arrow points in the direction of local energy flux, and its length is proportional to the amplitude of the flux. In the photonic crystal structure, the energy flows out of the central defect region in all four directions. In the topological defect structure, the optical flux circulates in the central region.

To introduce the topological defect to this micro-cavity, we deform the air hole shape from circle to ellipse, and rotate the orientation of individual ellipses to form a vortex-like topological defect, as seen in Fig. 1(d). Light can still be confined in the central region, but the spatial field profile of confined mode is twisted as shown in Fig. 1(e). Figure 1(f) reveals an even more dramatic change in the energy flow: a tightly confined circulating flux pattern arises at the center, in contrast to the outward energy flow in Fig. 1(c). Such change is attributed to the spatial variation of the ellipse orientation in the topological defect structure, which creates the swirling vortex of light [2].

Experimentally we fabricated such structure in a free-standing gallium-arsenide membrane by molecular beam epitaxy, electron-beam lithography, reactive ion etching and wet chemical etching. Figure 2 is the scanning electron micrograph of a fabricated sample. The 190nm-thick membrane contains three layer of quantum dots made of indium arsenide. When pumped by an external laser, the quantum dots are excited to upper states. They subsequently decay to lower states by emitting photons to the confined modes such as the one shown in Fig. 1. When the amplification rate exceeds the leakage rate through the walls, lasing action starts. The laser light swirls around in a vortex.

Fig. 2: Scanning-electron micrograph (SEM) of a topological defect laser fabricated in a GaAs membrane. (a) Top-view SEM image of a square lattice of 32 × 32 air holes with elliptical shape. The ellipticity is ε = 1.4. The lattice constant is a = 220 nm. The air filling fraction is 0.27. At the array center, 4 × 4 air holes are removed. (b) Magnified SEM of a section in (a), highlighted by the gray rectangle. (c) Tilt-view SEM image showing the free-standing GaAs-membrane. Scale-bars in (b) and (c) represent a length of 500 nm.

The power-flow vortex persists beyond the membrane. Our calculation shows that the evanescent field above the top surface of the membrane possess circulating energy flows. These may transfer angular momentum to particles or molecules in the vicinity of the topological defect structure. Thus the optical vortex of the lasing mode may potentially be used for on-chip nanoparticle manipulation. Further experimental study is required to optimize the topological defect laser and exploit the unique potential for applications in areas such as microfluidics for particle sorting and separation. Finally, this work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of a lasing mode, enabling the manipulation of its field pattern and energy flow landscape.

References:
[1] Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Mikhael I. Guy, Glenn S Solomon, Hui Cao, “Topological defect lasers”, Journal of Optics, 18, 014005 (2015). Full Article.
[2] Seng Fatt Liew, Sebastian Knitter, Wen Xiong, Hui Cao, “Photonic crystals with topological defects”, Physical Review A, 91, 023811 (2015). Abstract.

Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption

Ihar Faniayeu (left) and Viktar Asadchy

Authors: Ihar Faniayeu^1,2, Viktar Asadchy^2,3, Younes Ra'di³, Sergey Khakhomov², Igor Semchenko², Sergey Tretyakov³

Affiliation:
¹Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan,
²Department of General Physics, Francisk Skorina Gomel State University, Gomel, Belarus,
³Department of Radio Science and Engineering, Aalto University, Aalto, Finland.

People these days exhibit strong desire to control their surroundings, which -- in addition to tangible objects -- involves electromagnetic radiation omnipresent as radio waves, heat, and light. In this regard, the latest trend is to use electromagnetic metamaterials for transforming flow and absorption of electromagnetic waves. This trend has opened up new possibilities in imaging, telecommunications, signal processing, environmental sensing, medicine and other areas of science and technology. While metamaterial-based absorbers are typically tailored to exhibit efficient absorption at the desired resonance frequency, they are usually not transparent at other non-operative frequencies, and may exhibit strong unwanted back-reflections [1], which limit their functionality. Here we demonstrate for the first time a metamaterial-based absorber which uses 3D architecture without an opaque ground plane, which leads to complete off-resonance transparency, as is illustrated schematically in Fig. 1 (a) and (b).

Figure 1(a): Schematic design and working principle of metamaterial absorber.

Figure 1(b): Transmission, reflection and absorption spectra of the structure illustrate its perfect absorbance (R=1) at the resonance, and complete transparency (T=1) away from it.

Our work published in Physical Review X [2], presents both theoretical concept and experimental realization of such invisible metamaterial absorber for the microwave range. Conceptually, the structure consists of a periodic array of right- and left-handed single- and double-turn helices made of lossy metal, embedded in a dielectric. Balanced periodic arrangement of these bi-anisotropic elements leads to extremely broadband resonant response, which can be utilized in transmission arrays and absorbers [3]. In the case of absorber, strong resistive losses occurring in the metal transforms the absorbed electromagnetic field energy into heat. This concept is quite general and is therefore applicable in the entire electromagnetic spectrum. Practical demonstration of such absorber uses thin chromium-nickel wire helices embedded in a plastic foam sheet as shown in Fig. 2. As expected, strong absorption resonance is seen around 3 GHz frequency (see Fig. 3), whereas reflection remains low in the entire measured range.

Figure 2(a): Fabricated absorbers of single-turn helices comprising 480 elements embedded in plastic foam.

Figure 2(b): Fabricated absorbers of double-turn helices comprising 324 elements embedded in plastic foam.

Figure 3: Measured and numerically simulated reflection, transmission, and absorption coefficients for the fabricated metasurfaces with (a) single- and (b) double-turn helical inclusions. Experimental data is shown by points, the solid lines are guides to the eye, and the numerically simulated data are shown by dashed lines.

This concept and its experimental verification suggest that it is possible to realize metamaterial-based absorbers having significant advantages over other existing designs. We stress here that off-resonance transparency of the single absorber layer allows realization of multilayer structures where layers operate at different frequencies simultaneously without cross-talk, thus drastically expanding functionality of the device. The structure is tunable by changing its unit cell size. Terahertz and even visible wavelength range can be reached, provided that electromagnetic dispersion of the metal is taken into account and fabrication technique allowing realization of downscaled lattice is available. It is expected that currently available nanofabrication techniques, such as 3D printing and direct laser writing lithography will allow practical realization of such metamaterial structures, thus making a further step toward more versatile tailoring of electromagnetic radiation.

We thank Prof. Vygantas Mizeikis for helpful discussions and support in this article.

References:
[1] Claire M. Watts, Xianliang Liu, Willie J. Padilla, "Metamaterial Electromagnetic Wave Absorbers", Advanced Materials, 24, OP98 (2012). Abstract.
[2] V.S. Asadchy, I.A. Faniayeu, Y. Ra’di, S.A. Khakhomov, I.V. Semchenko, S.A. Tretyakov, "Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption", Physical Review X, 5, 031005 (2015). Abstract.
[3] V.S. Asadchy, I.A. Faniayeu, Y. Ra'di, I.V. Semchenko, S.A. Khakhomov, "Optimal arrangement of smooth helices in uniaxial 2D-arrays", Advanced Electromagnetic Materials in Microwaves and Optics (Metamaterials), 7th International Congress, pp. 244–246 (16-21 September, 2013). Abstract.

High-Resolution Optical Spectroscopy Using Multimode Interference in a Compact Tapered Fibre

Noel H. Wan (left) and Dirk Englund

Authors: Noel H. Wan and Dirk Englund

Affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA.

Link to Quantum Photonics Laboratory >>

Optical spectroscopy is a powerful technique that has found widespread application in fundamental science and in fields ranging from biochemical sensing to optical communication. In recent years, there has been strong interest in developing compact spectrometers for portable applications. A typical spectrometer uses a dispersive element such as a prism or a grating to separate the different constituent colors or wavelengths of light, each of which is channeled into separate detectors. Accordingly, a spectrometer can better distinguish different wavelengths of light if the distance between the dispersive optic and the detectors is increased. High-resolution spectrometers tend to be bulky, and miniature spectrometers based on the traditional design have so far not overcome the trade-off between footprint and performance.

Past 2Physics article by Dirk Englund:
April 05, 2015: "Efficient Photon Collection from a Nitrogen Vacancy Center in a Circular Bullseye Grating in Diamond" by Luozhou Li, Edward Chen, Dirk Englund.

In our work, we show a spectroscopy technique that circumvents the traditional spectrometer principle, leading to a millimeter-long fiber spectrometer that is simultaneously compact and has high spectral resolutions [1]. A waveguide such as an optical fiber allows only specific spatial distributions of light, or modes, as given by Maxwell’s equations. When multiple modes are launched into an optical fiber, they constructively and destructively interfere, resulting in highly random but specific intensity patterns (Figure 1). Additionally, by tapering the optical fiber, we were able excite different modes along the fiber, thereby further increasing the spectral resolution over that of an un-tapered, straight waveguide. More importantly, the taper geometry facilitates the detection and measurement of the multimode interference signals.

Figure 1: Click on the figure to view with higher resolution. (a) Illustration of the tapered fiber multimode interference (TFMMI) spectrometer. (b) Measured interference signals of different colors or wavelengths of light.

As modes of different wavelengths do not interfere, the interference can be regarded as the fingerprints of their respective wavelengths. By first mapping every wavelength of light to its fingerprint, the device can then algorithmically solve the inverse problem of determining the composition of an input light. Using our tapered fiber spectrometer, we were able to distinguish wavelengths separated by 40 picometers and 10 picometers in the visible and near-infrared regimes, respectively.

Figure 2: Click on the figure to view with higher resolution. (a) We determined the spectral resolution of the TFMMI spectrometer by resolving two closely spaced spectra. The device clearly resolved two wavelengths at 637.96 nm and 638.00 nm but did not fully resolve input wavelengths of 637.00 nm and 637.02 nm, bounding the resolution to 40 pm at these wavelengths. (b) The TFMMI device reconstructed a continuous signal with good agreement with that obtained from a commercial spectrometer.

With an optical fiber as our multimode waveguide, we were able to perform spectroscopy on light with wavelengths ranging from 500 nanometers to 1600 nanometers but in principle, our design can be implemented in a large variety of waveguides for any operational regimes. By suitably choosing the material, for example, compact spectrometers for absorption spectroscopy in the mid-infrared can be achieved. The proposed spectrometer can be realized on-chip in volume production through photolithography or printing and may also lead to the development of designer spectrometers for fundamental science, integrated photonics and lab-on-a-chip applications.

Reference: 
[1] Noel H. Wan, Fan Meng, Ren-Jye Shiue, Edward H. Chen, Tim Schröder, Dirk Englund, “High-resolution optical spectroscopy using multimode interference in a compact tapered fibre”, Nature Communications, 6, 7762 (2015). Abstract.

Universal Linear Optics

The Bristol team : (from left to right) Anthony Laing, Chris Sparrow, Jacques Carolan and Christopher Harrold.

Authors: Christopher Harrold¹ and Chris Sparrow^1,2

Affiliation:
¹Centre for Quantum Photonics, H. H. Wills Physics Laboratory, and Department of Electrical and Electronic Engineering, University of Bristol, UK.
²Department of Physics, Imperial College London, UK.

In our recent work [1], we demonstrate a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of the circuit.

Since 1897, when it was shown by the German mathematician Adolf Hurwitz, it has been known that any unitary matrix can be built up from smaller 2 X 2 matrices [2]. Then in 1994 Reck et al. showed that any arbitrary unitary can be realised via some physical process [3]. In particular, they showed that if we use a certain arrangement of reconfigurable linear optical elements, we can perform any unitary matrix.

This network can be implemented as a sequence of ≈ m²/2 two-mode primitives with two free parameters. These primitives are built out of variable reflectivity beam splitters and phase shifters. The beam splitters are comprised of a phase shifter sandwiched between two directional couplers, known as a Mach-Zehnder interferometer (MZI). By tuning the phase inside the interferometer we can arbitrarily split the light between the two outputs of the interferometer. A phase shifter placed on the output of the interferometer allows us to tune the phase of the resulting state. The phase shifters are resistive heaters patterned above a small region of waveguide. A change in temperature changes the local refractive index in the waveguide with respect to another and thus realises a phase shift. Cascading these primitive units into the triangular arrangement, together with full control over these phase shifters allows us to configure any linear optical unitary on six modes.

Our device was fabricated by Japanese company Nippon Telegraph and Telephone (NTT), the world’s leading telecommunications company. The six-mode universal system consists of 15 MZIs with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters. Once calibrated, we could implement any unitary we wanted and switch between experiments that typically would have taken months to build in a matter of seconds.

Figure 1: A fully reconfigurable optical chip. The device allows us to implement any quantum information protocol in linear optics on up to 6 modes.

We realised heralded quantum logic gates [4] designed to be scalable for the circuit model of quantum computation (QC) for the first time in integrated photonics. We also implemented a heralded entangling gate and used measurements in different bases to verify the successful generation of an entangled state. Entangling gates such as the one implemented here are an essential requirement for one of the most promising routes to photonic QC, measurement based quantum computing [5, 6]. Not only were we able to perform these protocols, but importantly we were able to report experimental results with high fidelity.

We then turned our attention to a completely disparate aspect of linear optics and configured our device in order to perform boson sampling. Boson sampling is a computational task with one objective: sampling from a probability distribution governed by the output statistics of photons injected into a fixed linear optical network [7]. It provides a rapid route to demonstrate the intrinsic exponential advantage a quantum computer has over a classical one. We implemented 100 instances of the boson sampling problem, injecting up to three photons into our circuit and reproduced the expected theoretical probability distributions with exceptional fidelity. We also investigated techniques used to verify the successful implementation of Boson Sampling by implementing a family of unitaries known as the quantum complex Hadamards [8]. These sorts of techniques will be crucial in the future when the circuits and number of photons used mean that classical verification will be intractable even for the most powerful classical computers [9, 10]. Crucially, the reconfigurable circuit allows us to carry out Boson sampling and the verification protocols all on the same device.

The chip’s ability to implement arbitrary circuits promises to replace a multitude of existing prototype systems as well as providing a platform for those yet to be discovered. The results presented in [1] required ~ 1000 configurations of the device and point the way towards applications across quantum technologies.

References:
[1] Jacques Carolan, Christopher Harrold, Chris Sparrow, Enrique Martín-López, Nicholas J. Russell, Joshua W. Silverstone, Peter J. Shadbolt, Nobuyuki Matsuda, Manabu Oguma, Mikitaka Itoh, Graham D. Marshall, Mark G. Thompson, Jonathan C. F. Matthews, Toshikazu Hashimoto, Jeremy L. O’Brien, Anthony Laing, "Universal linear optics", Science, 349, 711 (2015). Abstract.
[2] A. Hurwitz, "über die Erzeugung der Invarianten durch Integration", Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse, p.71 (1897). Full Article.
[3] Michael Reck, Anton Zeilinger, Herbert J. Bernstein, Philip Bertani, "Experimental realization of any discrete unitary operator", Physical Review Letters, 73, 58 (1994). Abstract.
[4] E. Knill, R. Laflamme, G.J. Milburn, "A scheme for efficient quantum computation with linear optics", Nature, 409, 46 (2001). Abstract.

[5] Robert Raussendorf, Hans J. Briegel, "A One-Way Quantum Computer", Physical Review Letters, 86, 
5188 (2001). Abstract.

[6] Daniel E. Browne, Terry Rudolph, "Resource-Efficient Linear Optical Quantum Computation", Physical Review Letters, 95, 010501 (2005). Abstract.

[7] Scott Aaronson, Alex Arkhipov, "The computational complexity of linear optics" in Proceedings of the ACM symposium on Theory of Computing, pp.333-342 (ACM, New York, 2011), Abstract.
[8] Anthony Laing, Thomas Lawson, Enrique Martín López, Jeremy L. O’Brien, "Observation of Quantum Interference as a Function of Berry’s Phase in a Complex Hadamard Optical Network", Physical Review Letters, 108, 260505 (2012). Abstract.
[9] Jacques Carolan, Jasmin D. A. Meinecke, Peter J. Shadbolt, Nicholas J. Russell, Nur Ismail, Kerstin Wörhoff, Terry Rudolph, Mark G. Thompson, Jeremy L. O'Brien, Jonathan C. F. Matthews, Anthony Laing, "On the experimental verification of quantum complexity in linear optics", Nature Photonics, 8, 621 (2014). Abstract.
[10] Malte Christopher Tichy, Markus Tiersch, Fernando de Melo, Florian Mintert, Andreas Buchleitner, "Zero-Transmission Law for Multiport Beam Splitters", Physical Review Letters, 104, 220405 (2010). Abstract.

Computational High-Resolution Optical Imaging of the Living Human Retina

Some authors of the paper in Nature Photonics (reference [3]). From left to right: Yuan-Zhi Liu, Fredrick A. South, and Stephen A. Boppart.

Authors: Yuan-Zhi Liu, Fredrick A. South, Stephen A. Boppart

Affiliation:
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.

Link to Biophotonics Imaging Laboratory >>

Eye imaging is very important not only because the vision is the primary human sense but also because the eye is the direct viewing window of the central nervous system. Acquiring the detailed information of the individual cells, blood vessels, and nerves at the retina of the living human eye could enable earlier diagnosis and better treatment for degenerative eye and neurological diseases. Optical coherence tomography, OCT, is a medical imaging technique analogous to ultrasound imaging, but has a higher resolution because it uses light rather than sound. Based on low coherence interferometry, OCT decouples the transverse and axial resolution, and can reject the out-of-focus background to achieve the depth-resolved image. Since the first application in eye imaging, OCT has become the standard of eye care for diagnosing and tracking disease, such as glaucoma and age-related macular degeneration [1].

The presence of ocular aberrations in the eye, however, limits cellular resolution tomography of the human retina. Combined with OCT, hardware-based adaptive optics (HAO) has pushed the limits of eye imaging, enabling diffraction-limited imaging of previously unresolvable structures, such as the three-dimensional visualization of photoreceptor distributions and individual nerve fiber bundles in the living human retina [2]. However, the sophisticated hardware and controlling software of HAO adds considerable complexity and cost to the standard OCT system, limiting the number of researchers and medical professionals who could benefit from the technology.

In our recent publication [3], we developed a phase stabilization technique and fully-automated computational adaptive optics (CAO) approach in conjunction with a standard en face optical coherence tomography (OCT) setup to computationally correct strong optical aberrations even in a highly-dynamic sample such as the human eye. This is the first time clear images of individual human photoreceptors, both near and far from the fovea, have been obtained in the living retina without the need of HAO.

Figure 1: (click on the image to view with higher resolution). Cone photoreceptors in the foveal region of the retina. (a) SLO image with the location of the photoreceptor mosaic outlined. (b) En face OCT mosaic before aberration correction. (c) The same OCT mosaic after automated correction. The insets are zoomed by 2X.

In HAO, a wavefront modulation device, e.g. deformable mirror, is used to physically modulate the pupil function to correct wavefront aberrations. CAO is a post-processing technique based on the complex signal from the interferometric measurement of OCT. The digital aberration correction filter is applied to the computational pupil plane, which is calculated from the Fourier transform of the focal plane image. The frequent and unavoidable motion of eye can severely interrupt the post-processing of the interference signal. In combination with the high-speed image acquisition from an en face OCT setup, the phase stabilization algorithm corrects even the sub-wavelength motion by comparing the phase differences of the adjacent frames. Afterward, a computational technique simulating the operation of the hardware Shack-Hartmann wavefront sensor is used to correct large optical aberrations which revealed individual photoreceptor cells. A technique named guide-star-based CAO is then applied to utilize the individual photoreceptors to sense and correct smaller, but still significant, aberrations.

CAO opens a new way for high-resolution imaging in ophthalmology. In addition to lower cost and simpler system configuration than HAO, the post-processing nature of CAO also allows more flexible aberration correction. Consider spatially-variant aberrations as an example. HAO needs to sense the aberrations in each small volume and then execute each correction while imaging the subject. By using CAO, researcher/doctors can shorten the image acquisition time and apply multiple aberration corrections afterward. Released from the physical limitations of the deformable mirror, e.g. the number and the stroke range of actuator, CAO has the potential to correct higher order and larger amplitude aberrations. Because CAO manipulates the complex interference signal, in principle better image quality can be achieved by applying not only the phase but also the amplitude corrections.

References:
[1] D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, "Optical coherence tomography", Science, 254 (5035), 1178–1181 (1991). Abstract.
[2] Yan Zhang, Barry Cense, Jungtae Rha, Ravi S. Jonnal, Weihua Gao, Robert J. Zawadzki, John S. Werner, Steve Jones, Scot Olivier, Donald T. Miller, "High-speed volumetric imaging of cone photoreceptors with adaptive optics spectral-domain optical coherence tomography", Optics Express, 14, 4380-4394 (2006). Abstract.
[3] Nathan D. Shemonski, Fredrick A. South, Yuan-Zhi Liu, Steven G. Adie, P. Scott Carney, Stephen A. Boppart, "Computational high-resolution optical imaging of the living human retina", Nature Photonics, 9, 440-443 (2015). Abstract.

Bending Electric Discharges with Lasers

Matteo Clerici






Authors: Matteo Clerici^1,2, Yi Hu^1,3, Philippe Lassonde¹, Carles Milián⁴, Arnaud Couairon⁴, 
Demetrios N. Christodoulides⁵, Zhigang Chen^3,6, Luca Razzari¹, François Vidal¹, François Légaré¹, Daniele Faccio², Roberto Morandotti^1,7



Affiliations:
¹Institut National de la Recherche Scientifique–Énergie Matériaux Télécommunications, Varennes, Québec J3X 1S2, Canada.
²School of Engineering and Physical Sciences, Scottish Universities Physics Alliance, Heriot-Watt University, Edinburgh, UK.
³The MOE Key Laboratory of Weak Light Nonlinear Photonics, School of Physics and TEDA Applied Physics School, Nankai University, Tianjin, China.
⁴Centre de Physique Théorique CNRS, École Polytechnique, Palaiseau, France.
⁵College of Optics, Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, Florida, USA.
⁶Department of Physics and Astronomy, San Francisco State University, California, USA.
⁷Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China.

Introduction:

Electric discharges are part of our everyday life. On a large scale, lightning is one of the most spectacular atmospheric phenomena, while on a much smaller scale -- cars, for instance -- rely on electric discharges for igniting the fuel. Small-scale controlled discharges find relevant industrial applications, e.g. in cutting and milling, and are also investigated as a mean to control gas flows and pollution.

Since the seventies, it has been realized that intense laser pulses may trigger and guide electric discharges [1], and ground breaking experimental results started to appear few years later (see e.g. [2–4] and references therein). From these successes, scientists have theoretically and experimentally investigated further on the laser guiding effect, also stimulated by the possibility of applying this concept to lightning control and protection [5].

While at standard air temperature and pressure conditions an electric field of nearly 34 kV/cm is required to start an electric arc between two electrodes, this threshold decreases with decreasing gas density. An intense laser focused in air ionizes the gas and locally increases the temperature. To this initial stage, a localized decrease in the density follows, which allows the discharge to start at lower than a standard electric field. This is the dominant effect responsible for the laser triggering and guiding mechanism [6,7].

Curved and healing beams:

In last few years researchers have demonstrated that the intense peak of laser pulses may travel in free space along complex trajectories, instead of the well-known straight line [8]. To enable this new degree of control the laser wavefront must be manipulated with proper optical elements. For instance, to generate curved laser beams a cubic phase modulation is required, and can be obtained either by designing a specific phase mask or by exploiting the aberrations of standard lenses. The beam generated this way is named Airy beam and the generalization of its behavior recently resulted in a vast available literature. Now, curved beams that follow an almost circular path or more complex trajectories can be obtained [9,10].

The research into laser shaping also brought to attention the ability of particular beams to self-heal, i.e. to recreate their intense part even after being partially obstructed. This happens, for instance, for Airy beams, and also for Bessel beams, another class of laser beams obtained by focusing a standard laser with a conical lens, instead of an ordinary one.

Bending the currents:

In the present work, recently published on Science Advances [11], we have combined the concept of laser guided electric discharge with the possibilities allowed by the recent results on beam shaping, and we have shown how electric sparks can be guided along a curved path avoiding an obstacle (see Fig. 1). Furthermore we have demonstrated that the self-healing property of laser beams allows the discharge to go back on track also when the obstacle cannot be avoided.

In our work we have employed a high-voltage generator able to deliver a potential difference of up to 35 kV applied to two wire electrodes suspended in air. At a distance of 5 cm no discharge occurs since the applied field is, at most, 5 times lower than the air breakdown field.

Figure 1. Sketch of the experiment. A phase mask and a lens reshape the laser beam into an Airy beam, which bend along the propagation and generate plasma on a parabolic shape. The region of air heated by the plasma has a reduced density, hence allowing the discharge to occur and guiding it along its curved path, overcoming an obstacle that is in the line of sight of the two electrodes. The inset on the top-right shows the actual measurement, acquired with a photo camera (top view). The obstacle is an opaque glass.

We then focus a shaped, intense laser pulse (intensity > 10¹² W/cm²) between the electrodes, ionizing the air and therefore heating the gas. We compared different shapes: a Gaussian beam (the most common beam shape), a Bessel beam, an Airy beam, and an S-shaped beam, obtained inserting in the laser path i) a standard lens, ii) an axicon (conical lens), iii) a tilted cylindrical lens system and iv) a phase mask, respectively. This way we have seen that electric discharge occurs only when the laser pulse ignites the plasma and the discharge path indeed follows the trajectory of the shaped laser pulse. For the Gaussian beam we observe a quite erratic behavior, consequence of the poor spatial localization of the heated gas; the discharge appear more deterministic when employing a Bessel beam, due to the high spatial localization of the Bessel peak. Finally, the discharge is clearly curved when an Airy beam is employed, as shown in the inset of Fig. 1. This allows conducting the charge from one electrode to the other while avoiding an obstacle that is placed in the direct line of sight.

Current self-healing:

Bessel beams and Airy beams share the ability to regenerate after hitting on an obstacle. This is mainly due to the fact that the energy responsible for the build-up of the intense laser peak flows from a reservoir that is off-axis with respect to the intense portion of the beam.

We have shown that the beam self-healing allows guiding the electric discharge along a predefined trajectory even in case of an obstacle blocking part of the beam. This is shown in Fig. 2 for the case of a Bessel beam. The laser propagates from left to right and is focused by the conical lens visible in the left of the picture. A tiny plasma plume develops over few centimeters and regenerates after hitting the obstacle (see inset in Fig.2). When the potential difference is applied to the electrodes, a discharge occurs following the path defined by the laser plasma. It performs a jump in a non-controlled way, right after the obstacle, and then goes back on track as soon as the laser plasma reappears (see Fig. 2).

Figure 2. Self-healing and electric discharges. We show that Bessel beam induced plasma reappears after the beam is obstructed by an obstacle, due to the Bessel beam self-healing property (see inset, the laser propagates from left to right). This effect finds a straightforward application in guiding electric currents: even if the dielectric obstacle cannot be avoided, the current can be delivered on target following the predefined path.

Conclusions:

We have shown that shaping of laser beams allows extensive control of the path over which electric currents can be guided in air. Our results have shown that a reduction of up to a factor 10 in the breakdown voltage can be obtained with the proper laser beam, and this opens the possibility to realize complex discharge path and extreme curvatures that will be investigated in the future and may find applications e.g., in the machining industry.

Acknowledgments:
R.M. and F.L. gratefully acknowledge Ministère de l’Enseignement Supérieur, de la Recherche, de la Science et de la Technologie (MERST), the Natural Sciences and Engineering Research Council of Canada (NSERC) and Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) for funding this work. M.C. acknowledges the support from the People Program (Marie Curie Actions) of the European Union’s Seventh Framework Program (FP7/2007-2013) under REA grant agreement no. 299522. The work of D.N.C. was supported by the Air Force Office of Scientific Research (AFOSR, MURI grant no. FA9550-10-1-0561), and of Z.C. by the AFOSR and National Science Foundation (NSF). D.F. acknowledges financial support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement no. 306559, and the Engineering and Physical Sciences Research Council (EPSRC, UK, grant EP/M006514/1). A.C. and C.M. acknowledge financial support from the French Direction Générale de l’Armement (DGA). Experiments were carried out at the Advanced Laser Light Source (ALLS) facility located at the Institut National de la Recherche Scientifique.

References:
[1] David W. Koopman, T.D. Wilkerson, "Channeling of an ionizing electrical streamer by a laser beam", Journal of Applied Physics, 42, 1883–1886 (1971). Abstract.
[2] X.M. Zhao, J.C. Diels, "Femtosecond pulses to divert lightning", Laser Focus World, 29, 113 (1993). 
[3] M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, J.-P. Wolf, "Triggering and guiding megavolt discharges by use of laser-induced ionized filaments", Optics Letters, 27, 772–774 (2002). Abstract.
[4] D. Comtois, C. Y. Chien, A. Desparois, F. Génin, G. Jarry, T. W. Johnston, J.-C. Kieffer, B. La Fontaine, F. Martin, R. Mawassi, H. Pépin, F.A.M. Rizk, F. Vidal, P. Couture, H. P. Mercure, C. Potvin, A. Bondiou-Clergerie, I. Gallimberti, "Triggering and guiding leader discharges using a plasma channel created by an ultrashort laser pulse", Applied Physics Letters, 76, 819–821 (2000). Abstract.
[5] Jérôme Kasparian, Roland Ackermann, Yves-Bernard André, Grégoire Méchain, Guillaume Méjean, Bernard Prade, Philipp Rohwetter, Estelle Salmon, Kamil Stelmaszczyk, Jin Yu, André Mysyrowicz, Roland Sauerbrey, Ludger Wöste, Jean-Pierre Wolf, "Electric events synchronized with laser filaments in thunderclouds", Optics Express, 16, 5757–5763 (2008). Abstract.
[6] F. Vidal, D. Comtois, Ching-Yuan Chien, Alain Desparois, B. La Fontaine, T.W. Johnston, J. Kieffer, Hubert P. Mercure, H. Pepin,  F.A. Rizk, "Modeling the triggering of streamers in air by ultrashort laser pulses", IEEE Transaction Plasma Science 28, 418–433 (2000). Abstract.
[7] S. Tzortzakis, B. Prade, M. Franco, A. Mysyrowicz, S. Hüller, P. Mora, "Femtosecond laser-guided electric discharge in air", Physical Review E, 64, 057401 (2001). Abstract.
[8] G.A. Siviloglou, J. Broky, A. Dogariu, D.N. Christodoulides, "Observation of accelerating Airy beams", Physical Review Letters, 99, 213901 (2007). Abstract.
[9] F. Courvoisier, A. Mathis, L. Froehly, R. Giust, L. Furfaro, P.A. Lacourt, M. Jacquot, J.M. Dudley, "Sending femtosecond pulses in circles: highly nonparaxial accelerating beams", Optics Letters, 37, 1736–1738 (2012). Abstract.
[10] Ido Kaminer, Rivka Bekenstein, Jonathan Nemirovsky, Mordechai Segev, "Nondiffracting accelerating wave packets of Maxwell’s equations", Physical Review Letters, 108, 163901 (2012). Abstract.
[11] Matteo Clerici, Yi Hu, Philippe Lassonde, Carles Milián, Arnaud Couairon, Demetrios N. Christodoulides, Zhigang Chen, Luca Razzari, François Vidal, François Légaré, Daniele Faccio, Roberto Morandotti, "Laser-assisted guiding of electric discharges around objects", Science Advances, 1, e1400111 (2015). Article.

A Quantum Gas Microscope for Fermionic Atoms

The Fermi gas microscope group: (from left) graduate students Katherine Lawrence and Melih Okan, postdoc Thomas Lompe, graduate student Matt Nichols, Professor Martin Zwierlein, and graduate student Lawrence Cheuk. Photo credit: Jose-Luis Olivares/MIT.

Authors: Lawrence Cheuk and Martin Zwierlein

Affiliation: 
Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

Link to Ultracold Quantum Gases Group >>

What do electrons, protons, neutrons and even quarks have in common? They all are fermions, particles with half-integer spin. Unlike their bosonic counterparts, integer spin particles, fermions cannot occupy one and the same quantum state. This simple fact leads to the structure of our elements, where electrons have to avoid each other and occupy different orbits around the atomic nucleus, or at least differ in their spin orientation.

When many fermions interact strongly with each other, they can form complex matter with exotic properties, from atomic nuclei to solid state materials, to distant neutron stars. Their collective behavior leads to diverse phenomena such as the structure of the elements, high-temperature superconductivity and colossal magneto-resistance.

Yet our understanding of strongly-interacting Fermi systems is limited. In recent years, ultracold atomic Fermi gases have emerged as a pristine platform to study many-fermion systems. In particular, fermionic atoms trapped in an optical lattice formed by standing waves of light can simulate the physics of electrons in a crystalline solid, shedding light on novel physical phenomena in materials with strong electron correlations.

Yet our understanding of strongly-interacting Fermi systems is limited. In recent years, ultracold atomic Fermi gases have emerged as a pristine platform to study many-fermion systems. In particular, fermionic atoms trapped in an optical lattice formed by standing waves of light can simulate the physics of electrons in a crystalline solid, shedding light on novel physical phenomena in materials with strong electron correlations.

In the present work, recently published in Physical Review Letters [3], we have realized quantum gas microscope that images ultracold fermionic ⁴⁰K atoms with single-lattice-site resolution. Similar results have also been achieved at about the same time by researchers at University of Strathclyde and Harvard University [4,5].

Figure Caption: Fermionic ⁴⁰K atoms in a 2D optical lattice with 541nm spacing imaged using Raman sideband cooling. Image taken from [3].

In our experiment, we prepare a two-dimensional layer of ⁴⁰K atoms via laser cooling and forced evaporation. The atoms are then trapped in an optical lattice formed by retro-reflected laser beams, which form a standing wave with 541nm spacing. In order to resolve atoms with single-lattice-site resolution, we utilize a novel setup that incorporates a solid immersion lens into the vacuum window. This allows an enhancement in the numerical aperture, leading to higher resolution and enhanced light collection. In addition, optical aberrations that arise from a planar window are minimized in this setup.

In order to detect the atoms, we perform fluorescence imaging while simultaneously cooling the atoms. To make the atoms fluoresce, they are illuminated with near-resonant light. However, as the atoms emit photons, they experience heating from the recoil of photons. As the atoms are heated up, they hop between lattice sites and can even hop out of the lattice. In order to faithfully measure the occupation of the lattice sites, one must therefore eliminate the heating that arises when atoms fluoresce. We accomplish this via a technique known as Raman sideband cooling.

Raman sideband cooling, a technique first demonstrated in the 1990s, selectively transfers atoms from high-energy states to lower energy states via a two-photon Raman process. Atoms that are already in the lowest energy state, however, remain “dark” to the Raman light. By collecting photons that are scattered during this cooling process, we extract the position of the atoms while cooling the atoms. Hopping and atom loss are thus avoided. Furthermore, we have found that even after imaging the atoms with Raman sideband cooling, the atoms are predominantly in the lowest energy state. This invites the possibility of assembling low-entropy many-fermion states atom by atom.

The advent of fermion microscope will allow new studies of many-fermion systems in optical lattices, such as measurement of high order correlations and detection of magnetic ordering. Such studies could shed light on the behavior of other fermions, in particular, electrons. This may one day advance our understanding of the diverse phenomena that arise in complex solid-state systems.

References:
[1] Waseem S. Bakr, Jonathon I. Gillen, Amy Peng, Simon Fölling, Markus Greiner, "A quantum gas microscope for detecting single atoms in a Hubbard-regime optical lattice", Nature, 462, 74 (2009). Abstract.
[2] Jacob F. Sherson, Christof Weitenberg, Manuel Endres, Marc Cheneau, Immanuel Bloch, Stefan Kuhr, "Single-atom-resolved fluorescence imaging of an atomic Mott insulator", Nature, 467, 68 (2010). Abstract.
[3] Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, Martin W. Zwierlein, "Quantum-Gas Microscope for Fermionic Atoms", Physical Review Letters, 114, 193001 (2015). Abstract.
[4] Maxwell F. Parsons, Florian Huber, Anton Mazurenko, Christie S. Chiu, Widagdo Setiawan, Katherine Wooley-Brown, Sebastian Blatt, Markus Greiner, "Site-Resolved Imaging of Fermionic ⁶Li in an Optical Lattice", Physical Review Letters, 114, 213002 (2015). Abstract.
[5] Elmar Haller, James Hudson, Andrew Kelly, Dylan A. Cotta, Bruno Peaudecerf, Graham D. Bruce, Stefan Kuhr, "Single-atom imaging of fermions in a quantum-gas microscope", arXiv:1503.02005v2 [cond-mat.quant-gas] (2015).

Metamaterial Shrinks Integrated-Photonics Devices

Rajesh Menon

Author: Rajesh Menon

Affiliation: Department of Electrical & Computer Engineering, University of Utah, USA.

Integrated electronics is the driving force behind the information revolution of the last 6 decades. A similar revolution is happening in photonics, where devices that manipulate the flow of light (or photons) are being miniaturized and integrated. The main challenge for integrated photonics is that the wavelength of light is far larger than the equivalent wavelength of electrons. This is the main reason that devices fundamental to integrated electronics are significantly smaller than those used in integrated photonics. Furthermore, no one had come up with a way to design devices close to this limit for integrated photonics.

We recently solved this problem by first coming up with a new design algorithm and then experimentally verifying that our devices work as intended [1-3]. One crucial advantage of our method is that our fabrication process is completely compatible with the very mature processes already developed for silicon electronics. This means that we can exploit the vast existing manufacturing infrastructure to enable integrated photonics.

In our recent publication, we demonstrated the smallest polarization beam-splitter to date [1]. This device (shown in the figure below) has 1 input and 2 outputs. The 2 outputs correspond to the 2 linear polarization states of light. The device is designed to take either polarization of light (or both) as the input and separate the 2 polarizations into the 2 outputs. We input light into our device one polarization at a time and measured the transmission efficiency into the correct output. This allowed us to verify that the device performs as designed. This is analogous to separating two channels of communication (for example, a video stream from PBS and another from Netflix). Previously such separation would have required time and power-consuming electronics or if photonics devices were used, they would have been much larger (so much harder to integrate onto a chip).

Figure 1: (a) Scanning-electron micrograph of fabricated polarization beamsplitter. Simulated intensity distribution at (b) TE polarization and (c) TM polarization showing the separation of the beams.

In the big picture, our research has the potential to maintain Moore's law for photonics. By enabling integrated photonics devices to be much smaller (in fact, close to their theoretical limits), we allow the integration of more devices in the same area (which increases functionality) and also enable the devices to communicate faster (since they are closer together; light has to travel shorter distances). Finally, by packing more devices into the same chip, one also exploits economies of scale to reduce the cost per chip (similar to what has happened in electronics). The practical impact for customers is that one can expect to drastically reduce power consumption and enable faster communications and computing. Data centers today consume over 2% of the total global electricity. Reducing power consumption in data centers and other electronics can go a long way to reduce our CO2 emissions and stem global climate change.

Our vision is to create a library of ultra-compact devices (including beamsplitters, but also other devices) that can then be all connected together in a variety of different ways to enable both optical computing and communications. The first devices were fabricated at a University. Next, we need to fabricate these in a standard process at a company, and then provide this library of devices to designers and hopefully, unleash their creativity. We believe that these devices will usher in unpredictable, but unbelievably exciting applications.

References :
[1] Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “An integrated-nanophotonic polarization beamsplitter with 2.4 × 2.4 μm² footprint”, Nature Photonics, 9, 378-382 (2015). Abstract.
[2] Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “Integrated metamaterials for efficient, compact free-space-to-waveguide coupling”, Optics Express, 22, 27175-27182 (2014). Abstract.
[3] Bing Shen, Randy Polson, Rajesh Menon, “Integrated digital metamaterials enables ultra-compact optical diodes”, Optics Express, 23, 10847-10855 (2015). Abstract.

Spiral Electric Fields Imposed on Laser Beam Creates Spiral Complex, Surface Micro-structures

Jinglie Ouyang, the Ph.D student of the Laser Group at Liverpool, who carried out this research led by Dr. Walter Perrie and Dr. Olivier Allegre. 


Author: Walter Perrie

Affiliation: Laser Group, University of Liverpool, UK.

Scientists at the University of Liverpool have generated new polarisation states of light and imposed these on an ultrafast laser beam producing 10 picosecond (1ps = 10^-12s) laser pulses [1]. Linear polarisation states are familiar in physics where the electric field is uni-directional in space across the laser beam and output intensity often a “Gaussian” mode with an intensity maximum at centre. Much less familiar polarisation states are, for example, Radial and Azimuthal polarisations which are vector fields in which the electric field direction varies spatially in a fixed plane with radially pointed vectors over 0-360° (Radial polarisation) and the orthogonal state (Azimuthal polarisation) where the field vectors consist of concentric circles. Such states have an intensity and polarisation singularity at their centre and so have ring intensity distributions.

By creating superpositions of Radial and Azimuthal polarisation states, the resulting laser electric fields were logarithmic spirals, a natural spiral (first described by Descartes and admired by Bernoulli) describing, for example the spiral arms of galaxies. The electric field at a given point is given by E(r, φ) = a e^{k φ} where a and k are constants.

Ph.D student Jinglie Ouyang and colleagues (led by Dr. W.Perrie and Dr.O.J Allegre) used these states to imprint Laser Induced Periodic Surface Structures (LIPSS) to create beautiful spiral grooved structures with 1 μm pitch on polished metals for the first time [1]. These Plasmon structures develop orthogonal to the local electric field component and so elucidate the incident electric field distribution unambiguously. The spiral states are created by rotating an incident linear polarised laser beam on a specially nano-structured waveplate which generates Radial, Azimuthal and superpostion states of polarisation resulting in the spirals.

Interestingly, these spiral fields were predicted theoretically by Professor Franco Gori of the University degli Studi Roma Tre in 2001 [2] and not observed until now. The above figure shows two of these spiral structures ablated on stainless steel with incident angles to the waveplate axis of 22.5° and 45° respectively with theoretical fits (red curves) which agree well.

The scientists also added Optical Angular momentum (OAM) to these states by twisting the wavefronts so that each photon carries a z-component of angular momentum, L_z = h/2π per photon and focusing of these beams created a near Gaussian beam intensity distribution with circular polarisation (carrying spin angular momentum L_z = h/2π per photon) at the centre. This is an example of Orbital to Spin angular momentum conversion and creates even more complex microstructures.

Relevant Papers:
[1] J. Ouyang, W. Perrie, O. J. Allegre, T. Heil, Y. Jin, E. Fearon, D. Eckford, S. P. Edwardson, G. Dearden, “Tailored Optical vector fields for ultrashort-pulse laser induced complex surface Plasmon structuring”. Optics Express, 23, 12562-12572 (2015). Abstract.
[2] Franco Gori, "Polarisation basis for vortex beams”. Journal of the Optical Society of America A 18, 1612 (2001). Abstract.
[3] Qiwen Zhan, “Cylindrical Vector Beams from mathematical concepts to applications”. Advances in Optics and Photonics, 1(1), 1-57 (2009). Abstract.
[4] Martynas Beresna, Mindaugas Gecevičius, Peter G. Kazansky, Titas Gertus, “Radially polarised optical converter created by femtosecond laser nanostructuring of glass”, Applied Physics Letters, 98, 201101 (2011). Abstract.
[5] Y.Jin, O. J. Allegre, W. Perrie, K. Abrams, J. Ouyang, E. Fearon, S. Edwardson, G.Dearden, “Dynamic modulation of spatially stuctured polarisation fields for real time control of ultrafast laser –material interactions”. Optics Express, 21, 25333 (2013). Abstract.