Svetlana G. Lukishova

Room temperature single-photon Source:Single-dye molecule fluorescence in Liquid Crystal host

We report on new approaches toward an implementation of an efficient, room temperature, deterministically polarized, single-photon source (SPS) on demand-a key hardware element for quantum information and quantum communication. Operation of a room temperature SPS is demonstrated via photon antibunching in the fluorescence from single terrylene-dye molecules embedded in a cholesteric liquid crystal host. Using oxygen-depleted liquid crystal hosts, dye-bleaching was avoided over the course of more than 1 h of continuous 532-nm excitation. Liquid crystal hosts (including liquid crystal oligomers/polymers) permit further increase of the efficiency of the source: 1) by aligning the dye molecules along a direction preferable for maximum excitation efficiency; 2) by tuning a one-dimensional (1-D) photonic-band-gap microcavity of planar-aligned cholesteric (chiral nematic) liquid crystal layer to the dye fluorescence band.

Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host

Microcavity resonance is demonstrated in nanocrystal quantum dot fluorescence in a one-dimensional (1D) chiral photonic bandgap cholesteric-liquid crystal host under cw excitation. The resonance demonstrates coupling between quantum dot fluorescence and the cholesteric microcavity. Observed at a band edge of a photonic stop band, this resonance has circular polarization due to microcavity chirality with 4.9 times intensity enhancement in comparison with polarization of the opposite handedness. The circular-polarization dissymmetry factor g(e) of this resonance is ~1.3. We also demonstrate photon antibunching of a single quantum dot in a similar glassy cholesteric microcavity. These results are important in cholesteric-laser research, in which so far only dyes were used, as well as for room-temperature single-photon source applications.

Organic photonic bandgap microcavities doped with semiconductor nanocrystals for room-temperature on-demand single-photon sources

We report the first experimental observation of fluorescence from single semiconductor nanocrystals (colloidal quantum dots) in microcavities. In these room-temperature experiments we observed photon antibunching from single CdSe nanocrystals doped into a chiral one-dimensional photonic bandgap liquid-crystal microcavity. The chirality resulted in high-purity, circular polarization of definite handedness of the emitted single photons. We also report the fabrication of chiral microcavities for telecom wavelengths doped with PbSe nanocrystals as well as a solution-processed-polymer microcavity with a defect layer doped with CdSe nanocrystals between two distributed Bragg reflectors. These systems with their low host fluorescence background are attractive for on-demand single-photon sources for quantum information and communication.

Robust organic lasers comprising glassy-cholesteric pentafluorene doped with a red-emitting oligofluorene

Doped with a red-emitting oligofluorene, fluid and glassy cholesteric liquid crystal (CLC) films are characterized by similar lasing thresholds and efficiencies. With picosecond excitations the output from a glassy CLC laser is temporally stable, but that from a fluid CLC laser decays with time. The difference in stability is attributable to external perturbations on supramolecular structure in the fluid but not the solid state, such as heating through optical pumping, light-induced pitch dilation, and laser-induced flow.

Room-temperature single photon sources with definite circular and linear polarizations

We report experimental results of two room-temperature single photon sources with definite polarization based on emitters embedded in either cholesteric or nematic liquid crystal hosts. In the first case, a cholesteric 1-D photonic bandgap microcavity provides circular polarization of definite handedness of single photons from single colloidal semiconductor quantum dots (nanocrystals). In these experiments, the spectral position of the quantum dot fluorescence maximum is at the bandedge of a photonic bandgap structure. The host does not destroy fluorescence antibunching of single emitters. In the second case, photons with definite linear polarization are obtained from single dye molecules doped in a planar-aligned nematic liquid crystal host. The combination of sources with definite linear and circular polarization states of single photons can be used in a practical implementation of the BB84 quantum key distribution protocol.

Dye-doped cholesteric-liquid-crystal room-temperature single-photon source*

Fluorescence antibunching from single terrylene molecules embedded in a cholesteric-liquid-crystal host is used to demonstrate operation of a room-temperature single-photon source. One-dimensional (1-D) photonicband-gap microcavities in planar-aligned cholesteric liquid crystals with band gaps from visible to near-infrared spectral regions are fabricated. Liquid-crystal hosts (including liquid crystal oligomers and polymers) increase the source efficiency, firstly, by aligning the dye molecules along the direction preferable for maximum excitation efficiency (deterministic molecular alignment provides deterministically polarized output photons), secondly, by tuning the 1-D photonic-band-gap microcavity to the dye fluorescence band and thirdly, by protecting the dye molecules from quenchers, such as oxygen. In our present experiments, using oxygen-depleted liquid-crystal hosts, dye bleaching is avoided for periods exceeding one hour of continuous 532 nm excitation.

NONLINEAR OPTICAL RESPONSE OF CYANOBIPHENYL LIQUID CRYSTALS TO HIGH-POWER, NANOSECOND LASER RADIATION

Results from investigations are summarized into: (1) transient refractive and absorptive (two-photon) nonlinearities at 0.532 mb y theZ-scan method, and (2) reflective nonlinearity in the near-IR, of linearly nonabsorbing cyanobiphenyl liquid crystals under nanosecond laser irradiation. (1) For isotropic liquid crystals at the several-nanosecond time scale and several tens-micrometers beam-waist-diameter, transient molecular-reorientation and thermal/density refractive nonlinearities compete in changing the sign of the total transient refractive nonlinearity. For the dierent, given pulse durations, the influence of coupled thermal and density eects on nonlinear refraction depends, through buildup time, on the beam-waist diameter. Nonlinear absorption coecients depend on the incident intensity. For planar nematic layers, cumulative eects in heating (and in refractive nonlinearity) were observed even at low, 2{10 Hz pulse repetition rate. These results are useful for optical power limiting applications, and for intensity and beam-quality sensors of pulsed, high-power lasers. (2) Reflective nonlinearity of chiral-nematic (cholesteric) mirrors near selective reflection conditions for circular polarized light at =1 :064 m was studied both under free space irradiation and inside a laser resonator. Specially chosen experimental irradiation conditions make it possible to attribute the observed changing of reflectivity to athermal helix unwinding by the optical eld. The results can nd applications in laser-resonator mirrors, Q-switches and soft apertures for beam-prole formation, and also in showing the limits of use cholesteric optical elements in high-power laser beams.

Room temperature source of single photons of definite polarization

A definite polarization in fluorescence from single emitters (dye molecules) at room temperature is demonstrated. A planar-aligned, nematic liquid-crystal host provides definite alignment of single dye molecules in a preferred direction. Well-defined polarized fluorescence from single emitters (single photon source) is important for applications in photonic quantum information. Polarized single-photon sources based on single emitters, for example, are key hardware elements both for absolutely secure quantum communication and quantum computation systems.

Liquid Crystals Under Two Extremes: (1) High-Power Laser Irradiation, and (2) Single-Photon Level

This paper describes some of my results on liquid crystal investigations under unconventional, incident light powers: (1) under high-power laser irradiation both in free space and inside laser resonators, and (2) in single-photon source applications for quantum information technology. Several effects under high-power, nanosecond laser irradiation are outlined: athermal helical pitch dilation and unwinding of cholesteric mirrors, showing the limits for using them in laser physics; some pitfalls in measurements of thermal-density refractive nonlinearity and the first observation of thermal lens effects in liquid crystals under several nanosecond, low-pulse-repetition rate (2–10 Hz) laser irradiation in the presence of two-photon absorption; feedback-free kaleidoscope of patterns (hexagons, stripes, etc.) in dye-doped liquid crystals. At the single-photon level, definite linear and circular polarizations of single (antibunched) photons for quantum communications were obtained using single-emitter fluorescence in planar-aligned nematic and cholesteric hosts. Circular polarized cholesteric microcavity resonances were also observed under cw-excitation. In addition, using near-field optical microscopy and AFM, 2D-hexagonal arrays made of cholesteric oligomers were investigated. With progress of this technology, similar arrays can be used for fluorescence control of single emitters.

Single-Photon Source for Quantum Information Based on Single Dye Molecule Fluorescence in Liquid Crystal Host

This paper describes a new application for liquid crystals: quantum information technology. A deterministically polarized single-photon source that efficiently produces photons exhibiting antibunching is a pivotal hardware element in absolutely secure quantum communication. Planar-aligned nematic liquid crystal hosts deterministically align the single dye molecules which produce deterministically polarized single (antibunched) photons. In addition, 1-D photonic bandgap cholesteric liquid crystals will increase single-photon source efficiency. The experiments and challenges in the observation of deterministically polarized fluorescence from single dye molecules in planar-aligned glassy nematic-liquid-crystal oligomer as well as photon antibunching in glassy cholesteric oligomer are described for the first time.