Justin M. Winkler

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.

On the properties of single-mode optical resonators

We study the quality factor of single-mode optical whispering gallery mode resonators using finite element method simulations, with a particular focus on the photonic belt resonator geometry. We experimentally observe a large difference between the quality factors of TM and TE modes in such resonators. Examining radiative losses, we conclude that the TM fundamental mode of single-mode resonators can have geometry related radiative losses caused by mode hybridization and coupling that limits their achievable quality factor. However, TE modes are free from mode hybridization radiative losses. This leads to much higher achievable Q factors for TE modes, only limited by fabrication and material quality. We experimentally observed photonic belt resonator quality factors on the order of one billion for TE modes, higher than in any other single mode optical resonator of similar dimensions.

Quantum Dot Fluorescence in Photonic Bandgap Glassy Cholesteric Liquid Crystal Structures: Microcavity Resonance under CW-Excitation, Antibunching and Decay Time

Nanocrystal quantum dot (NQD) fluorescence in 1-D glassy cholesteric liquid crystal host is investigated: (1) Microcavity resonance is obtained under cw-excitation demonstrating coupling between NQD fluorescence and a cholesteric microcavity. Observed at a band edge of a photonic stopband, this resonance has circular polarization due to microcavity chirality with 4.9 times intensity enhancement in comparison with polarization of the opposite handedness. (2) Photon antibunching of a single NQD in a similar microcavity was observed. (3) Fluorescence decay time constants were measured at different excitation powers. These results are important in developing cholesteric lasers and single-photon sources for secure quantum communication.

Nanocrystal fluorescence in photonic bandgap microcavities and plasmonic nanoantennas

Results are presented here towards robust room-temperature single-photon sources based on fluorescence in nanocrystals: colloidal quantum dots, color-center diamonds and doped with trivalent rare-earth ions (TR3+). We used cholesteric chiral photonic bandgap and Bragg-reflector microcavities for single emitter fluorescence enhancement. We also developed plasmonic bowtie nanoantennas and 2D-Si-photonic bandgap microcavities.

Resonance in quantum dot fluorescence on a band-edge of a 1-D photonic bandgap cholesteric structure under cw-laser excitation

Microcavity resonance is demonstrated in nanocrystal quantum dot fluorescence in a 1-D chiral photonic bandgap cholesteric liquid crystal host. The resonance demonstrates coupling between quantum dot fluorescence and the cholesteric microcavity. Observed at a band edge of a photonic stopband, 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 ge 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 under pulsed excitation were used, as well as for room-temperature single-photon source applications.

Room-temperature single-photon sources with definite circular and linear polarizations based on single-emitter fluorescence in liquid crystal hosts

Definite circular and linear polarizations of room-temperature single-photon sources, which can serve as polarization bases for quantum key distribution, are produced by doping planar-aligned liquid crystal hosts with single fluorescence emitters. Chiral 1-D photonic bandgap microcavities for a single handedness of circularly polarized light were prepared from both monomeric and oligomeric cholesteric liquid crystals. Fluorescent emitters, such as nanocrystal quantum dots, nitrogen vacancy color centers in nanodiamonds, and rare-earth ions in nanocrystals, were doped into these microcavity structures and used to produce circularly polarized fluorescence of definite handedness. Additionally, we observed circularly polarized resonances in the spectrum of nanocrystal quantum dot fluorescence at the edge of the cholesteric microcavitys photonic stopband. For this polarization we obtained a ~4.9 enhancement of intensity compared to the polarization of the opposite handedness that propagates without photonic bandgap microcavity effects. Such a resonance is indicative of coupling of quantum dot fluorescence to the cholesteric microcavity mode. We have also used planar-aligned nematic liquid crystal hosts to align DiI dye molecules doped into the host, thereby providing a single-photon source of linear polarization of definite direction. Antibunching is demonstrated for fluorescence of nanocrystal quantum dots, nitrogen vacancy color centers, and dye molecules in these liquid crystal structures.

Room-Temperature Single-Photon Sources with Definite Circular and Linear Polarizations

In this thesis I present experimental demonstrations of room-temperature, singlephoton sources with definite linear and circular polarizations. Definite photon polarization increases the efficiency of quantum communication systems. In contrast with cryogenic-temperature single-photon sources based on epitaxial quantum dots requiring expensive MBE and nanofabrication, my method utilizes a mature liquid crystal technology, which I made consistent with single-emitter fluorescence microscopy. The structures I have prepared are planar-aligned cholesteric liquid crystals forming 1-D photonic bandgaps for circularly-polarized light, which were used to achieve definite circularly-polarized fluorescence of single emitters doped in this environment. I also used planar-aligned nematic liquid crystals to align single molecules with linear dipole moments and achieved definite linearly-polarized fluorescence. I used single nanocrystal quantum dots, single nanodiamond color-centers, rare-earth-doped nanocrystals, and single terrylene and DiIC18(3) dye molecules as emitters. For nanocrystal quantum dots I observed circular polarization dissymmetry factors as large as ge = −1.6. In addition, I observed circularly-polarized resonances in the fluorescence of emitters within a cholesteric microcavity, with cavity quality factors of up to Q ∼ 250. I also showed that the fluorescence of DiIC18(3) dye molecules in planar-aligned nematic cells exhibits definite linear polarization, with a degree of polarization of ρ = −0.58 ± 0.03. Distributed Bragg reflectors form another type of microcavity that can be used to realize a single-photon source. I characterized the fluorescence from nanocrystal quantum dots doped in the defect layers of such microcavites, both organic and inorganic. Finally, to demonstrate the single-photon properties of single-emitter-doped cholesteric and nematic liquid crystal structures and distributed Bragg reflector

Single-photon experiments with liquid crystals for quantum science and quantum engineering applications

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.

Room-temperature single-photon sources based on nanocrystal fluorescence in photonic/plasmonic nanostructures

Results are presented here towards robust room-temperature SPSs based on fluorescence in nanocrystals: colloidal quantum dots, color-center diamonds and doped with trivalent rare-earth ions (TR3+). We used cholesteric chiral photonic bandgap and Bragg-reflector microcavities for single emitter fluorescence enhancement. We also developed plasmonic bowtie nanoantennas and 2D-Si-photonic bandgap microcavities. The paper also provides short outlines of other technologies for room-temperature single-photon sources.

Room-Temperature Single Photon Source: Nanocrystals in Photonic Bandgap Microcavities

Single-photon sources based on nanocrystals in 1-D photonic bandgap microcavities (cholesteric liquid crystal and Bragg reflectors) are reported. Studied emitters include nanocrystal quantum dots, color centers in nanodiamonds, and trivalent rare-earth ions in nanocrystals.