Differing Isomeric Responses used as a Barometer for Organic Materials in the Cosmos

From left to right: Wren Montgomery, Jonathan S. Watson, Mark A. Sephton

Authors: Wren Montgomery, Jonathan S. Watson, Mark A. Sephton

Affiliation: Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College, London, UK

We showed that a family of organic molecules having the same atomic composition but different structures have substantially different responses to the application of high pressure. This is the first potential cosmic barometer capable of recording high pressure events throughout the history of the universe.

Organic matter is common throughout the universe. It is found in interstellar and circumstellar clouds, asteroids, meteorites, comets, planets and satellites [1,2]. Taking a cue from terrestrial geochemistry, where isomer distributions of aromatic hydrocarbons can be used to determine formation environments (primarily temperature), we were intrigued by the possibility of a similar indicator for astronomical environments.

In order to reach high pressures in the laboratory, we used the diamond anvil cell (Figure 1). In this device, two modified brilliant-cut gem quality diamonds are mounted culet-to-culet and gently pressed against each other. The broad transparent window (UV to Far Infrared) of the diamond anvils means a variety of techniques can be used for in-situ measurements.

Figure 1. (a) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements parallel to the synchrotron beam. Not to scale. (b) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements in the plane of the sample. Not to scale. (c) Microphotograph of 1,7-dimethylnaphthalene loaded into the diamond anvil cell. The ruby is the sphere toward the center of the sample chamber ( © 2014. The American Astronomical Society).

In our recent work [3], we used synchrotron source FTIR spectroscopy in the diamond anvil cell (Figure 1) at the Swiss Light Source and SOLEIL Synchrotron (France) to collect FTIR spectra for 5 isomers of dimethylnaphthalene (DMN). DMNs were chosen because this type of organic structure has been suggested as the cause of some unidentified infrared bands [4] and is present in meteorites [5].

By plotting peak center against pressure, we determined the relative effects of pressure on the isomers (Figure 2). Overall, the greatest changes occurred in 1,8-DMN, possibly due to the greater void space in the crystal at ambient conditions. Meanwhile, 1,5-DMN shows very little change with the application of high pressure, contrary to the previously known instability at ambient conditions [6].

Figure 2. Compression and decompression peak fit data for 1,8-, 1,5-, 1,7-, 2,6-, and 2,7-dimethylnaphthalene. Each data point represents the center of a peak associated with that material at a single pressure ( © 2014. The American Astronomical Society).

Overall, the responses of dimethylnaphthalenes to pressure conform to those observed in response to heat. The response of 1,5-DMN to pressure is contrary to those changes brought about by high temperature and reveals that although the molecular consequences of heating and pressurizing are often the same, the two mechanisms are very distinct and can generate opposing effects.

This suggests that measurement of infrared spectra of organic samples from the cosmos, either on samples in the laboratory or using remote instruments on Rovers or Landers can be used to quantify the thermodynamic history of the universe.

References:
[1] Th. Henning, and F. Salama, “Carbon in the Universe”. Science, 282, 2204 (1998). Abstract.
[2] Scott A. Sandford, "Terrestrial Analysis of the Organic Component of Comet Dust".  Annual Review of Analytical Chemistry, 1, 549 (2008). Full Article.
[3] Wren Montgomery, Jonathan S. Watson, Mark A. Sephton, “An Organic Cosmo-barometer: Distinct Pressure and Temperature Effects for Methyl Substituted Polycyclic Aromatic Hydrocarbons”. The Astrophysical Journal, 784, 98 (2014). Abstract.
[4] Jun Shan, Masako Suton, L. C. Lee, “3.3 micron emission from ultraviolet excitation of some aromatic molecules”. The Astrophysical Journal, 383, 459 (1991). Abstract.
[5] M. A. Sephton, “Pyrolysis and mass spectrometry studies of meteoritic organic matter”. Mass Spectrometry Reviews, 31, 560-569 (2012). Abstract.
[6] Chick C. Wilson, “Locked-in methyl groups in 1,5-dimethylnaphthalene close to the melting point”. Chemical Communications, 1281-1282 (1997). Abstract.

Reconfigurable Acoustic Metamaterials: From Random To Periodic In A Split Second

Mihai Caleap (left) and Bruce Drinkwater (right)

Authors: Mihai Caleap and Bruce Drinkwater

Affiliation:
Faculty of Engineering, University of Bristol, United Kingdom.

Using an acoustic metadevice that can influence the acoustic space and can control any of the ways in which waves travel, we have demonstrated, for the first time, that it is possible to dynamically alter the geometry of a three-dimensional colloidal crystal in real time [1].

The colloidal crystals designed in our study, called metamaterials, are artificially structured materials that extend the properties of existing naturally occurring materials and compounds.

The reconfigurable metamaterial is assembled from microspheres in aqueous solution, trapped with acoustic radiation forces. The acoustic radiation force is governed by an energy landscape, determined by an applied high amplitude acoustic standing wave field, in which particles move swiftly to energy minima. This creates a colloidal crystal of several millilitres in volume with spheres arranged in an orthorhombic lattice in which the acoustic wavelength is used to control the lattice spacing.

Dynamically reconfigurable metamaterials based devices with optical or acoustic wavelengths from tens of microns to tens of centimetres could have a wide range of applications. In optics it could lead to new beam deflectors or filters for terahertz imaging and in acoustics it might be possible to create acoustic barriers that can be optimised depending on the changing nature of the incident sound. Further applications in reconfigurable cloaks and lenses are also now conceivable. The reconfigurable acoustic assembly method developed in this study is an important step as it has clear advantages over other possible approaches, for example optical trapping and self-assembly. In particular, acoustic assembly is scalable with wavelength from microns to metres. The method will work on a vast range of materials, such as nearly all solid-fluid combinations, it will also enable almost any geometry to be assembled and it is cheap and easy to integrate with other systems.

Figure 1: Acoustic metadevice capable of manipulating the acoustic space and controlling the propagation of waves (Image credit: Mihai Caleap, University of Bristol, copyright © 2014)

This first realization of an acoustic metamaterial that is reconfigurable in real-time represents a new and versatile means by which wave propagation phenomena can be controlled. The approach will allow the design and synthesis of a wide range of new colloidal crystals based on micrometre-sized particles arranged in crystal lattices. Using our acoustic assembly technique, low-density acoustic colloids could be conceived to form quasi-crystalline structures [2], which would otherwise be difficult by the self-assembly technique. The range of structures that can be constructed is limited only by the ability to produce the appropriate acoustic potential energy landscape. The technique can also provide a platform for performing bio-assays and cell-assays in a non-contact way using acoustic or magnetic standing waves.

References:
[1] Mihai Caleap and Bruce W. Drinkwater, "Acoustically trapped colloidal crystals that are reconfigurable in real time". Proceedings of the National Academy of Sciences of USA, 111 (17), 6226–6230 (2014). Full Article.
[2] Jules Mikhael, Johannes Roth, Laurent Helden, Clemens Bechinger Mikhael, "Archimedean-like tiling on decagonal quasicrystalline surfaces". Nature, 454, 501–504 (2008). Abstract.

“True” Stopping of Light – A New Regime for Nanophotonics

Kosmas L Tsakmakidis (left), Ortwin Hess (right)

Authors: Kosmas L Tsakmakidis¹, Ortwin Hess²

Affiliation:
¹NSF Nano-scale Science and Engineering Center (NSEC), University of California -- Berkeley, USA, 
²Blackett Laboratory, Department of Physics, Imperial College, London, UK.

1. Introduction

The enormous speed of light allows us to observe distant galaxies and to communicate in real time at remote points on the surface of the earth. At smaller frequencies, light passes through macroscopic solid objects almost completely unhindered, interacting weakly with matter. Reducing its velocity down to zero is of fundamental scientific interest [1-3] that could usher in a host of important photonic applications, some of which are hitherto fundamentally inaccessible. These include cavity-free, low-threshold nanolasers [4,5], novel solar-cell designs for efficient harvesting of light [6,7], nanoscale quantum information processing [8], as well as enhanced biomolecular sensing [9].

Until now complete stopping of light pulses, leading to their localisation in a specific region of space, in solid structures and at ambient conditions, has been hampered by fundamental difficulties. Ultraslow light requires strong group-index (n_g) resonances but the increased (compared with gases) damping at normal conditions invariably broadens and weakens such resonances. Certain resourceful schemes, such as electromagnetically induced transparency (EIT) [10], can exploit very narrow absorption dips arising from quantum interference effects to drastically slow-down light in ultracold atoms (where damping is minimised) but their solid-state implementations [11,12] usually decelerate light by up to a factor of a few hundreds. Furthermore, at the zero-group-velocity (zero-υ_g) point of an EIT scheme a light pulse usually relinquishes its photonic character by coherently mapping its optical quantum states to stationary electronic (spin) excitations [13].

Light is 'stored' in this way [14] but our ultimate goal of observing and harnessing photons at a zero-υ_g point is not satisfied ‒ even under extreme conditions entailing vacuum operation and ultralow temperatures. Periodic nanophotonic structures offer another route for slowing-down light, essentially via periodic back-reflections by a lattice of scatterers having sizes comparable to the wavelength [15]. Here, it is structural disorder that fundamentally limits the attainment of light stopping. Tiny nanometer-scale fluctuations destroy the (theoretically assumed) 'perfect' periodicity, leading to a 'smearing out' effect in the attained group indices at the band edges [16,17]. Practically this results in slowing-down factors that normally do not exceed a few hundreds.

Here we report on a solid-state configuration leveraging media with negative electromagnetic parameters [18,19] whereby an arbitrary number of light pulses can be stopped, remaining well localised ‒ without diffusing ‒ at predefined spatial locations despite the absence of any barriers in front or behind the pulses. Our work addresses two fundamental problems: First, how the detrimental role of decoherence mechanisms (losses and surface roughness) in the light-stopping ability of a solid-state nanophotonic structure can be overcome; and, second, how light stopping can be accomplished without overshooting the optical losses at the zero-υ_g point. The lifetime of the stopped light pulses in our nanoplasmonic structure (Fig.1) decreases only moderately (by a factor of ~ 1.5) compared with the normal (n_g ≤ 10) nanoplasmonic light regime, allowing for a broad range of applications in this field. Further, we present a realistic design and computational demonstration of a negative-refractive-index optical metamaterial in the fabric of which we insert a (four-level) gain medium (laser dyes), such that the metamaterial enters the full lasing regime. We, thus, demonstrate an ultra-thin, ultra-fast nanolaser, operating at visible wavelengths (710 nm) and with modulation bandwidth close to 1 THz ‒ orders of magnitude faster than typical modulation speeds of conventional lasers.

2. Stopping and localisation of light pulses in nanoplasmonic metamaterials

We have identified two light-stopping structures: The first is made of a negative-refractive-index core layer surrounded by air (illustrated schematically in Fig. 1), while the second is made of a Si layer sandwiched by two (plasmonic) ITO layers (see Fig. 1) [20]. While the negative-index structure clearly showed how we may access the zero group-velocity (zero-υ_g) points, it was still found to be incomplete in the following respects: First, the (negative) effective refractive index of the metamaterial slab may exhibit fluctuations, even at microwave frequencies where homogenization works best. Second, the group-velocity dispersion at the zero-υ_g point should be minimised to avoid late-time broadening (diffusion) of the stopped pulses. Third, the configuration should also be immune to the presence of surface roughness.

Figure 1: Schematic illustration of the two configurations used for obtaining complex-ω stopping of light. The first is an air/metamaterial/air slab heterostructure, with the middle metamaterial layer (yellow) having a negative refractive index at microwave wavelengths. An upper prism (not shown here) is used to in-couple spatially separated incident light beams (white) into the middle layer, exciting the complex-ω mode at its zero-υ_g point at different spatial locations along the waveguide. Rather than being guided along the structure, the excited pulses (in colors) remain stopped, decaying with time at their injection points. The second structure, operating at telecommunication wavelengths, is made of a Si slab bounded by two ITO layers, with the upper one (right) having a finite thickness. Here, the complex-ω mode is weakly leaky at its zero-υ_g point and, hence, there is no need to use a prism. The complex-ω pulses in this structure remain stopped and strongly localised, without diffusing and broadening at late times.

A structure that manages to comply with the above requirements, exhibiting complete and dispersion-free stopping of light pulses, is reported in Fig. 1 (schematically) and Figs. 2(b)-(d). We used a silicon slab of thickness 290 nm, bounded by a low-loss plasmonic material (indium tin oxide, ITO). The upper ITO cladding layer, through which the injection of the light pulses into the middle Si layer was performed, had a finite thickness of 500 nm. At near-infrared wavelengths (1.55 µm) and for p-polarized light the (real part of the) permittivity ε of the ITO layers is negative [21], thereby providing the required light-deceleration and stopping mechanism ‒ similarly to the negative-index structure [22]. We expect that in the β = 0 (quasistatic) region the bands of the supported modes become flat (υ_g = 0) [23]; therefore, to minimise group-velocity dispersion and to make the desired band as flat as possible at a finite-β zero-υ_g point, this point must be brought as close as possible to the β = 0 region. Indeed, we have designed the above ITO/Si/ITO heterostructure such that it accomplishes this goal, supporting a bulk complex-ω mode [20,24] with a zero-υ_g point on the right-hand side of the air light-line. In addition to low group-velocity dispersion and preservation of the light-stopping point (in the presence of losses), this weakly leaky mode features two further favourable characteristics: First, since β was designed to be finite but small at the zero-υ_g point, the effective index n_eff = β/k₀ of the mode therein turns out to be smaller than unity: n_eff = 0.39 < 1. Consequently, the wavelength λ_eff inside the structure became larger than the free-space one (λ₀ = 1.55 µm), which further diminished the effect of nm-scale surface roughness. Second, since the mode at the zero-υ_g point is (weakly) leaky there was no need to use a prism to excite it from air, thereby considerably alleviating the realisation of the scheme.

Figure 2(b) illustrates the time-evolution of four light pulses evanescently coupled into the above plasmonic heterostructure. The incident free-space wavelength was λ₀ = 1.55 µm (ω ≈ 1.2 PHz) and the angle of incidence was θ = 72.5_o, both selected such that precisely the zero-υ_g point was hit. We see from Fig. 2(b) that the four light pulses are stopped and strongly localised, decaying with time, without propagating and broadening. The e^-1-lifetime of each pulse, calculated by analytically solving the pertinent dispersion transcendental equation, was found to be 28.4 fs, while the time taken until the pulses are completely absorbed was approximately 131 fs, in agreement with the FDTD calculations of Fig. 2(b). When the angle of incidence of an exciting pulse is such that a non-zero-υ_g point is hit, a pulse injected into the heterostructure propagates away from its starting point, decaying but also dispersing (broadening) with time (Fig. 2(c)), as expected. Importantly, while the deceleration factor (group index) from the non-zero-υ_g point (n_g^B ≈ 6.6) to the zero-υ_g one (n_g^A → ∞) increases dramatically, the temporal losses increase by no more than a factor of two (from 2.4×10¹³ s^-1 to 3.7×10¹³ s^-1). This crucial aspect, together with the fact that the lifetimes at the stopping point (28.4 fs and 131 fs) are typical nanoplasmonic ones, suggests that a wealth of applications within this field should also be possible in the stopped-light regime. Note further from Fig. 2(b) that, since the herein observed localisation is not a cavity- or disorder-aided one, the optical pulses can be injected and stopped at any point along the structure, as well as be brought tighter together ‒ up to the diffraction limit.

Figure 2: (To view higher resolution click on the image) Full-wave FDTD calculations of the time-evolution of one or more pulses injected inside the light-stopping configurations of Fig. 1. In all cases the results are recorded on the vertical yz plane (translucent) cutting through the middle layer. (a) Shown are eight pulses in-coupled into the negative-index heterostructure at its zero-υ_g point. All pulses remain stopped for more than 6 ns, decaying with time at their initial positions. (b) Shown here are four pulses in-coupled into the plasmonic heterostructure at its zero-υ_g point. The pulses remain stopped and localized, without broadening, for more than 130 fs. (c) When a zero-υ_g point is hit, the pulse in-coupled into the plasmonic structure moves away from its initial point, both decaying and broadening with time. (d) The pulses in 'b' can also be brought closer together ‒ up to the diffraction limit ‒ since the structure is completely uniform, and the localization is not aided by disorder or a cavity-like action.

Finally, we have examined the effect of realistic levels of surface roughness on the light-stopping ability of the plasmonic heterostructure. To this end, we introduced random variations in the permittivities within a thin layer of ±1 nm around the two Si/ITO interfaces. We then recorded the time evolution of the centre of energy and full-width at half-maximum (FWHM) of a pulse in-coupled close to the complex-ω zero-υ_g point.

We have found [20] that the presence of roughness does not destroy the localisation (stopping and absence of broadening), since both the centre of energy and the FWHM (perturbed by the random surface landscape) oscillate ‒ with a very small amplitude ‒ around the values they had in the absence of roughness. From these results, we measured a group index of ~ 1.5×10⁷! Hence, although the pulses experience dissipative losses (from ITO), radiative losses (being weakly leaky) and surface roughness at the two Si/ITO interfaces, they remain robustly stopped and localised, with a movement of their center-of-energy being almost indiscernible. This robustness to nm-scale surface roughness arises from the fact that, unlike periodic structures, here the wavelength inside the structure at the zero-υg point is λ_eff = λ₀/n_eff = 3974 nm ‒ thousands of times larger than the surface-roughness amplitude, and therefore essentially insensitive to it.

The simplicity of the final design, together with the gigantic enhancement of the group refractive index and the minor increase of optical losses, make the structure appealing for a host of nanoplasmonic applications. Particularly, the enhancement of the density-of-states in the region around a flat band is a key parameter for enabling high-efficiency solar cells [25], as well as for giving rise to strong interactions between quantum emitters and plasmonic nanostructures [8] ‒ the cornerstone of strong optical nonlinearity. We anticipate that this might be extendable potentially even at the single-photon level, allowing for nonlinear quantum-optical logic operations on nanoscopic scales.

3. Ultrafast nanolasing in active nanoplasmonic metamaterials

Since the lifetime of our stopped-light excitations are of the order of 0.1 ps (owing to the presence of dissipative losses), we may use an optical gain medium to compensate for the losses. In general, the tight light localisation on truly nanoscopic dimensions ‒ well below the diffraction limit for visible light ‒ that nanoplasmonic metamaterials offer enhances the interaction of light with mater, paving the way for a multitude of classical and quantum nano-optics applications. However, metal optics suffers from inherent dissipative losses, which have persistently hampered many of the envisaged applications. We have recently shown that gain-enhanced nanoplasmonic metamaterials can overcome these hindrances and lead to novel nanophotonic components and devices.

Dissipative losses in nanoplasmonics arise from the interaction of the incident photons with the quasi-free conduction electrons of the metals, thereby constituting a rather inherent feature of the responses of metal-based nano-devices. For truly subwavelength plasmonic structures these losses follow universal laws, i.e. they do not depend on the particular geometric configuration but only on the deployed (usually noble) metal. Typical damping rates are thus of the order of Γ ~ 100 ps^-1, requiring gain coefficients Γ/c ~ 10³ - 10⁴ s^-1 to compensate for the losses. Meanwhile, the extreme control of light in optical metamaterials paves the way towards nano-enabled functionalities right at the materials level. But these need to be controlled dynamically, on-demand and in real time. Both of the aforementioned challenges can be met by enhancing metamaterials with gain, incorporated right into their fabric.

In the following, we briefly illustrate – on the basis of full-wave simulations – that high gain densities in a realistic nanoplasmonic metamaterial result in the compensation of, both, dissipative and radiative losses leading into the full lasing regime, where bright and dark lasing states dynamically compete giving rise to ultrafast nonlinear responses.

Figure 3: Electric-field amplitude profiles and charge distributions on the metal films inside the double-fishnet unit cell for (a) the bright mode at λ_bright =717.25 nm and (b) the dark mode at λ_dark = 731.8 nm. The direction of the electric field in the x-z plane is indicated by the white arrows. (c) Total average energy density (black area) and frequency-filtered contributions at λ_pump = 680 nm (green dashed line), λ_bright (white full line) and λ_dark (yellow dash-dotted line).

Figures 3(a) & (b) depict an example of an active (gain-enhanced) nanoplasmonic metamaterial comprised of two thin silver nanofilms, periodically perforated with holes, and laser dyes inserted between the films [4, 5]. The structure has been designed such that there is an optimum coupling of the plasmonic excitations to the gain molecules, ensuring efficient harnessing of the gain. An intense pump pulse of 2 ps duration (λpump = 680 nm) inverts the gain medium and dynamically creates a 3D occupation inversion profile closely matching the spatial distribution of the electric field at the probe wavelength (λprobe = 710 nm). After 7 ps, a weak broadband pulse of duration 12 fs probes the active structure, and its far-field spectrum is recorded at the two sides of the planar metamaterial. Detailed calculations have shown that in the regime of full loss compensation the real part of the effective refractive index of the nanostructure becomes more negative compared to the passive case. By prolonging the duration of the probe pulse such that the energy inside the nano-fishnet became constant with time, we have also ascertained that there can be net outflux of optical energy through a volume encompasing the metamaterial, i.e. there is more energy radiated away from the volume than energy incident on the volume. Hence, our quantum plasmonic amplifier can operate not only transiently but also in a steady-state mode.

Such active nanostructures can function as powerful light sources, either coherent (nanolasers) or incoherent (diodes), delivering intense optical power within ultrasmall volumes or ultrathin flat surfaces. We have shown that when the gain supplied by the active medium embedded within a quasi-2D active metamaterial is sufficient to overcome dissipative and radiative losses, the structure can indeed function as a coherent emitter of surface plasmons over the whole ultrathin 2D area, well below the diffraction limit for visible light. Both, bright and dark plasmonic lasing states can be generated, coupling either strongly or weakly to the continuum. Which one eventually dominates can be controlled by the design of the metametaterial. The dynamic, nonlinear competition between such lasing states is illustrated in Figure 3(c) where one may discern the initial build-up of the bright-mode energy (external and internal; red), followed by the damped-amplitude, ps-period relaxation oscillations, and then steady-state emission that is interrupted (at around 50 ps) by the instability of the dark mode (only internal; yellow) until again steady-state emission is reached.

Stopped-light plasmonic nanostructures, together with gain-enhanced optical metamaterials, constitute an exciting new frontier in nano-photonics and nano-science, and are precursors towards active, integrated quantum nano-optics. Reaching the full-light-stopping regime and bringing gain at the nanoscale is anticipated to improve the performance of a host of active nano-components, such as electro-optic modulators and light sources, but also passive ones, such as plasmonic waveguides or sensors featuring intensified plasmonic hotspots for single-emitter spectroscopy.

References:
[1] Robert W. Boyd, Daniel J. Gauthier, "Slow and fast light". Progress in Optics, 43, 497–530 (2002). Abstract.
[2]  Léon Brillouin, "Wave Propagation and Group Velocity" (Academic, 1960).
[3] P. W. Milonni, "Fast Light, Slow Light, and Left-Handed Light" (Institute of Physics, 2005).
[4] O. Hess, J. B. Pendry, S. A. Maier, R. F. Oulton, J. M. Hamm, K. L. Tsakmakidis, "Active nanoplasmonic metamaterials". Nature Materials, 11, 573–584 (2012). Abstract.
[5] Sebastian Wuestner, Joachim M. Hamm, Andreas Pusch, Fabian Renn, Kosmas L. Tsakmakidis, Ortwin Hess, "Control and dynamic competition of bright and dark lasing states in active nanoplasmonic metamaterials". Physical Review, B 85, 201406 (R) (2012). Abstract.
[6] Alexandre Aubry, Dang Yuan Lei, Antonio I. Fernández-Domínguez, Yannick Sonnefraud, Stefan A. Maier, J. B. Pendry, "Plasmonic light-harvesting devices over the whole visible spectrum". Nano Letters, 10, 2574–2579 (2010). Abstract.
[7] Min Seok Jang, Harry Atwater, "Plasmonic rainbow trapping structures for light localization and spectrum splitting". Physical Review Letters, 107, 207401 (2011). Abstract.
[8] Zubin Jacob, Vladimir M. Shalaev, "Plasmonics goes quantum". Science 334, 463‒464 (2011). Abstract.
[9] Na Liu, Mario Hentschel, Thomas Weiss, A. Paul Alivisatos, Harald Giessen, "Three-dimensional plasmon ruler". Science 332, 1407‒1410 (2011). Abstract.
[10] Lene Vestergaard Hau, S. E. Harris, Zachary Dutton, Cyrus H. Behroozi, "Light speed reduction to 17 metres per second in an ultracold atomic gas". Nature 397, 594–598 (1999). Abstract.
[11] Shuang Zhang, Dentcho A. Genov, Yuan Wang, Ming Liu, Xiang Zhang, "Plasmon-induced transparency in metamaterials". Physical Review Letters, 101, 047401 (2008). Abstract.
[12] N. Papasimakis, V. A. Fedotov, N. I. Zheludev, S. L. Prosvirnin, "Metamaterial analog of electromagnetically induced transparency". Physical Review Letters, 101, 253903 (2008). Abstract.
[13] M. D. Lukin, A. Imamoğlu, "Controlling photons using electromagnetically induced transparency". Nature, 413, 273–276 (2001). Abstract.
[14] Chien Liu, Zachary Dutton, Cyrus H. Behroozi, Lene Vestergaard Hau, "Observation of coherent optical information storage in an atomic medium using halted light pulses". Nature, 409, 490–493 (2001). Abstract.
[15] Toshihiko Baba, "Slow light in photonic crystals". Nature Photonics, 2, 465–473 (2008). Abstract.
[16] Shayan Mookherjea, Andrew Oh, "Effect of disorder on slow light velocity in optical slow-wave structures". Optics Letters, 32, 289–291 (2007). Abstract.
[17] R. J. P. Engelen, D. Mori, T. Baba, L. Kuipers, "Two regimes of slow-light losses revealed by adiabatic reduction of group velocity". Physical Review Letters, 101, 103901 (2008). Abstract.
[18] Tie Jun Cui, David Smith, Ruopeng Liu (Editors), "Metamaterials: Theory, Design, and Applications" (Springer, 2009).
[19] Edmund I. Kirby, Joachim M. Hamm, Tim W. Pickering, Kosmas L. Tsakmakidis, Ortwin Hess, "Evanescent gain for slow and stopped light in negative refractive index heterostructures". Physical Review B, 84, 041103(R) (2011). Abstract.
[20] Kosmas L. Tsakmakidis, Tim W. Pickering, Joachim M. Hamm, A. Freddie Page, Ortwin Hess, "Completely stopped and dispersionless light in plasmonic waveguides". Physical Review Letters, 112, 167401 (2014). Abstract.
[21] Alexandra Boltasseva, Harry A. Atwater, "Low-Loss Plasmonic Metamaterials". Science, 331, 290‒291 (2011). Abstract. 2Physics Article.
[22] Kosmas L. Tsakmakidis, Allan D. Boardman, Ortwin Hess, " ‘Trapped rainbow’ storage of light in metamaterials". Nature, 450, 397–401 (2007). Abstract.
[23] J. B. Pendry, "A chiral route to negative refraction". Science, 306, 1353‒1355 (2004). Abstract.
[24] Alexandre Archambault, Mondher Besbes, and Jean-Jacques Greffet, "Superlens in the time-domain". Physical Review Letters, 109, 097405 (2012). Abstract.
[25] Dennis M. Callahan, Jeremy N. Munday, Harry A. Atwater, "Solar cell light trapping beyond the ray optic limit". Nano Letters, 12, 214 (2012). Abstract.

Detecting Radio Waves with a Laser by means of a Nanomechanical Transducer

Albert Schliesser (left) and Eugene S. Polzik (right)


Authors: 
Albert Schliesser and Eugene S. Polzik


Affiliation: Niels Bohr Institute, Copenhagen University, Denmark.




Radio- and microwave frequency (RF) electric signals are the workhorse for countless applications in Science and Technology: cell phones transmit voice and data through them, medical doctors use them in magnetic resonance imagers to learn about the composition of biological tissue, and radio astronomers draw conclusions on the origin and structure of our universe by scrutinizing the microwave radiation that permeates it. Often, it would be desirable to recover extremely small signals, from objects with a weaker contrast, or further away, than what electronic RF signal receivers permit today: if the signals are too small, they get drowned in the noise added by the receiver.

Past 2Physics articles by Eugene S. Polzik:
August 18, 2013: "Deterministic Quantum Teleportation"
by Christine A. Muschik and Eugene S. Polzik

Recently, we have taken a completely new approach at detecting such signals (Figure 1, and Reference [1]), following the proposal [2]. Instead of using an electronic amplifier, we have applied the RF signal of interest to a simple LC-resonance circuit. The latter contains a special capacitor: one of its electrodes is a 100nm-thin, metallized nanomechanical membrane that hovers only a few microns above the other electrode. It is consequently rather susceptible to the electrostatic forces that arise from an RF voltage dropped at the capacitor. Due to the quadratic dependence of this force on voltage, the mechanical response can be further enhanced by a dc voltages bias. In that manner, membrane deflections on the order of a micron can be achieved already with signal voltages below a millivolt at the input of the LC circuit, provided the signal frequency lies close to the ~1 MHz resonance frequency of the membrane, and the LC circuit tuned to it.

Figure 1: Artist’s impression of the electro-optomechanical transducer. Radio wave signals (green, on the left) are guided to an on-chip capacitor. Above the capacitor’s bottom gold electrodes, a nanomechanical membrane, some 100 nm thick, and coated with a thin layer of aluminum is suspended, at a distance of only a few micron. Together they form a capacitor, and the resulting electrostatic forces deflect the membrane if an RF signal is applied. This deflection is read out with very high sensitivity by a laser beam (red) reflected from the membrane.

Now we combine this electromechanical device with a laser interferometer: by reflecting a laser beam from the capacitor’s membrane, and measuring the phase shift imparted on the laser beam if the membrane is displaced, we can most accurately determine the position of the membrane. Indeed, this measurement is only limited by the quantum noise of the light, which allows us to determine the displacement of the membrane at the level of 1 femtometer after one second of averaging time—that is the size of an atomic nucleus. As a result, by looking at the laser interferometer’s signal, we measure the voltage applied to circuit, and the precision of the optical measurement allows us to do so with very high sensitivity.

However, the optical readout noise is not the only mechanism that can degrade the quality of signal reception. One of the attractive features of our “electrooptomechanical transducer” is that it operates at room temperature. We have carefully studied the dependence of the noise of the transducer on the frequency and impedance of the signal source. One particularly interesting result we have found is that the thermal noise of the membrane at room temperature — its random agitation caused essentially by the Brownian motion of the atoms that constitute it — can be made negligible in most cases. The effective temperature of the membrane is the room temperature reduced by a cooperativity parameter C [3], which quantifies the strength of the electromechanical coupling, and reached values of up to 6800 in our work. Furthermore, any noise intrinsic to the LC resonance circuit can limit the sensitivity. After careful shielding of the inductor (L) against magnetic pickup, its Johnson noise was the dominant source of noise. It left us a total transducer noise at the level of (800 pV)²/Hz. All other sources of noise — from the light and the membrane — added up to about a percent of that value. By using electronic component of LC with lower loss and/or temperature, this number could be further improved. This allows us to dramatically surpass the performance of conventional electronic amplifiers, which, for now, is at a similar level.

At present our transducer operates around the LC frequency of about 1 MHz equal to the mechanical resonance frequency of the membrane. Switching from the dc bias voltage to an ac bias voltage [4] should allow for tuning the device to any desirable frequency in the MHz to GHz range.

Looking at this work from a more general perspective, it combines, for the first time, the functionalities of electro- and optomechanical systems, both of which have separately experienced extremely rapid progress over the last decade [3]. One of the most remarkable feats accomplished in these still young research communities is the ability to control the mechanical dynamics at the quantum level by means of electromagnetic fields, either microwave [4,5] or optical [6,7]. Merging these platforms into a single device may thus enable a quantum-coherent interface between microwave and optical photons — a tool urgently needed if superconducting quantum computers are to be connected through an optical fiber network. Efforts along these lines are now undertaken in many groups worldwide [8,9].

This work was done in collaboration with T. Bagci, A. Simonsen, E. Zeuthen, J. Appel, A. Sorensen and K. Usami at the Niels Bohr Institute, Copenhagen, Denmark; S. Schmid, and L. G. Villanueva at the Danish Technical University, Kongens Lyngby, Denmark; and J. M. Taylor at the Joint Quantum Institute/NIST, College Park, USA.

References:
[1] T. Bagci, A. Simonsen, S. Schmid, L. G. Villanueva, E. Zeuthen, J. Appel, J. M. Taylor, A. Sørensen, K. Usami, A. Schliesser, E. S. Polzik, "Optical detection of radio waves through a nanomechanical transducer". Nature 507, 81 (2014). Abstract.
[2] J. M. Taylor, A. S. Sorensen, C. M. Marcus, E. S. Polzik, "Laser Cooling and Optical Detection of Excitations in a LC Electrical Circuit". Physical Review Letters, 107, 273601 (2011). Abstract.
[3] Markus Aspelmeyer, Tobias J. Kippenberg, Florian Marquardt, "Cavity Optomechanics" (2013). arXiv:1303.0733 [cond-mat.mes-hall].
[4] J. D. Teufel, T. Donner, Dale Li, J. W. Harlow, M. S. Allman, K. Cicak, A. J. Sirois, J. D. Whittaker, K. W. Lehnert, R. W. Simmonds, "Sideband cooling of micromechanical motion to the quantum ground state". Nature, 475, 359 (2011). Abstract.
[5] A. D. O’Connell, M. Hofheinz, M. Ansmann, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, John M. Martinis, A. N. Cleland, "Quantum ground state and single-phonon control of a mechanical resonator". Nature, 464, 697 (2010). Abstract.
[6] Jasper Chan, T. P. Mayer Alegre, Amir H. Safavi-Naeini, Jeff T. Hill, Alex Krause, Simon Gröblacher, Markus Aspelmeyer, Oskar Painter, "Laser cooling of a nanomechanical oscillator into its quantum ground state". Nature, 478, 89 (2011). Abstract.
[7] E. Verhagen, S. Deleglise, S. Weis, A. Schliesser, T. J. Kippenberg, "Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode". Nature, 482, 63 (2012). Abstract.
[8] R. W. Andrews, R. W. Peterson, T. P. Purdy, K. Cicak, R. W. Simmonds, C. A. Regal, K. W. Lehnert, "Bidirectional and efficient conversion between microwave and optical light". Nature Physics, 10, 321 (2014). Abstract.
[9] Joerg Bochmann, Amit Vainsencher, David D. Awschalom, Andrew N. Cleland, "Nanomechanical coupling between microwave and optical photons". Nature Physics, 9, 712 (2013). Abstract.