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Physical Review A

Photon localization in a nematic liquid crystal

J. P. McClymer, H. M. Shehadeh

Published Mar 6, 2009• 1 citation
Publisher

Abstract

Sufficiently strong scattering can cause light propagation to stop, not due to absorption, but due to interference effects, thus localizing light. Efforts to observe localization in three dimensions with visible light have focused on high-scattering-disordered nanometer-sized powders with the unwanted side effect of strong absorption. The strong absorption mimics the signature of localization---exponential decay of transmitted intensity---complicating the interpretation of results. Nematic liquid crystals are more effective at scattering light and can have low absorption. We report experimental observation of strong photon localization in a nematic liquid crystal in which the effects of absorption are minimal.

DOI
10.1103/physreva.79.031802

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Sufficiently strong scattering can cause light propagation to stop, not due to absorption, but due to interference effects, thus localizing light. Efforts to observe localization in three dimensions with visible light have focused on high-scattering-disordered nanometer-sized powders with the unwanted side effect of strong absorption. The strong absorption mimics the signature of localization---exponential decay of transmitted intensity---complicating the interpretation of results. Nematic liquid crystals are more effective at scattering light and can have low absorption. We report experimental observation of strong photon localization in a nematic liquid crystal in which the effects of absorption are minimal.
The absorption of light is one of the most fundamental processes in condensed-matter physics and optics. Here we investigate under which conditions laser light is absorbed by a crystalline material as an electromagnetic wave with delocalized properties or rather as photons that cause discrete, localized, nanometer-sized consequences. We excite the first-order phase transition of vanadium dioxide with laser pulses of sufficient frequency to overcome the band gap but with insufficient pulse energy to overcome the latent heat. According to Maxwell's equations and Bloch theory, no transition should occur, because nowhere in the material is enough energy. Nevertheless, we observe with ultrafast electron diffraction a disordered crystal geometry with nanometer-sized spots of switched material that grow and diminish with time. The amount of localized spots matches approximately to the number of photons in the absorbed laser wave. Two optical experiments substantiate this phenomenon, and simulations reproduce all measurements results. We discuss whether crystals defects, temperature, or a genuine wavefunction collapse can explain the discovered phenomenon. Practically, the reported absorption mechanism enables local consequences at substantially higher energy than average and provides insight into symmetry breaks and non-thermal fluctuations within complex materials.
In recent years there has been great interest in controlling the speed of propagation of electromagnetic waves. In gases and crystals, coherent techniques have been applied to alter the speed of light without changing the physical or chemical structure of the medium. Also, light transmitted by highly disordered solids has exhibited signatures of Anderson localization, indicating the existence of a regime of “stopped” light that is mediated by random elastic scattering. However, to date, light has not been generated in a random medium as a pointlike excitation that is fixed in space from the outset. Here we report experimental evidence for the electrical generation and confinement of light within nanosized volumes of a random dielectric scattering medium in which a population inversion has been established, and discuss the properties of these novel light sources.
We present here our results on using liquid crystals in experiments with nonclassical light sources: (1) single-photon sources exhibiting antibunching (separation of all photons in time), which are key components for secure quantum communication systems, and (2) entangled photon source with photons exhibiting quantum interference in a Hong-Ou- Mandel interferometer. In the first part, cholesteric liquid crystal hosts were used to create definite circular polarization of antibunched photons emitted by nanocrystal quantum dots. If the photon has unknown polarization, filtering it through a polarizer to produce the desired polarization for quantum key distribution with bits based on polarization states of photons will reduce by half the efficiency of a quantum cryptography system. In the first part, we also provide our results on observation of a circular polarized microcavity resonance in nanocrystal quantum dot fluorescence in a 1-D chiral photonic bandgap cholesteric liquid crystal microcavity. In the second part of this paper with indistinguishable, time-entangled photons, we demonstrate our experimental results on simulating quantum-mechanical barrier tunnelling phenomena. A Hong-Ou-Mandel dip (quantum interference effect) is shifted when a phase change was introduced on the way of one of entangled photons in pair (one arm of the interferometer) by inserting in this arm an electrically controlled planar-aligned nematic liquid crystal layer between two prisms in the conditions close to a frustrated total internal reflection. By applying different AC-voltages to the planar-aligned nematic layer and changing its refractive index, we can obtain various conditions for incident photon propagation – from total reflection to total transmission. Measuring changes of tunnelling times of photon through this structure with femtosecond resolution permitted us to answer some unresolved questions in quantum-mechanical barrier tunnelling phenomena.
Decelerating and stopping light is fundamental for optical processing, high performance sensor technologies and digital signal treatment, many of these applications relying on the ability of controlling the amplitude and phase of coherent light pulses. In this context, slow-light has been achieved by various methods, as coupling light into resonant media, Brillouin scattering in optical fibers, beam coupling in photorefractive and liquid crystal media or engineered dispersion in photonic crystals. Here, we present a different mechanism for slowing and storing light, which is based on photo-isomerization induced transparency of azo-dye molecules hosted in a chiral liquid crystal structure. Sharp spectral features of the medium absorption/dispersion, and the long population lifetime of the dye metastable state, enable the storage of light pulses with a significant retrieval after times much longer than the medium response time.
We present here our results on using liquid crystals (LCs) in experiments with nonclassical light sources: (1) single-photon sources exhibiting antibunching (separation of all photons in time), which are key components for secure quantum communication systems and (2) entangled photon source with photons exhibiting quantum interference in a Hong–Ou–Mandel interferometer. In the first part, both nematic and cholesteric liquid crystal (CLC) hosts were used to create definite linear or circular polarization of antibunched photons emitted by different types of single emitters (dye molecules, nanocrystal quantum dots (NQDs), nanodiamonds with color centers, etc.). If the photon has unknown polarization, filtering it through a polarizer to produce the desired polarization for quantum key distribution with bits based on polarization states of photons will reduce by half the efficiency of a quantum cryptography system. In the first part, we also provide our results on observation of a circular polarized microcavity resonance in NQD fluorescence in a 1-D chiral photonic bandgap CLC microcavity. In the second part of this paper with indistinguishable, time-entangled photons, we demonstrate our experimental results on simulating quantum mechanical barrier tunneling phenomena. A Hong–Ou–Mandel dip (quantum interference effect) is shifted when a phase change was introduced on the way of one of entangled photons in pair (one arm of the interferometer) by inserting in this arm an electrically controlled planar-aligned nematic LC layer between two prisms in the conditions close to a frustrated total internal reflection. By applying different AC-voltages to the planar-aligned nematic layer and changing its refractive index, we can obtain various conditions for incident photon propagation – from total reflection to total transmission. Measuring changes of tunneling times of photon through this structure with femtosecond resolution permitted us to answer some unresolved questions in quantum mechanical barrier tunneling phenomena.