Optical vector field rotation and switching with near-unity transmission by fully developed chiral photonic crystals.

State-of-the-art nanostructured chiral photonic crystals (CPCs), metamaterials, and metasurfaces have shown giant optical rotatory power but are generally passive and beset with large optical losses and with inadequate performance due to limited size/interaction length and narrow operation bandwidth. In this work, we demonstrate by detailed theoretical modeling and experiments that a fully developed CPC, one for which the number of unit cells N is high enough that it acquires the full potentials of an ideal (N → ∞) crystal, will overcome the aforementioned limitations, leading to a new generation of versatile high-performance polarization manipulation optics. Such high-N CPCs are realized by field-assisted self-assembly of cholesteric liquid crystals to unprecedented thicknesses not possible with any other means. Characterization studies show that high-N CPCs exhibit broad transmission maxima accompanied by giant rotatory power, thereby enabling large (>π) polarization rotation with near-unity transmission over a large operation bandwidth. Polarization rotation is demonstrated to be independent of input polarization orientation and applies equally well on continuous-wave or ultrafast (picosecond to femtosecond) pulsed lasers of simple or complex (radial, azimuthal) vector fields. Liquid crystal-based CPCs also allow very wide tuning of the operation spectral range and dynamic polarization switching and control possibilities by virtue of several stimuli-induced index or birefringence changing mechanisms.

Reconfiguration of three-dimensional liquid-crystalline photonic crystals by electrostriction.

Natural self-assembled three-dimensional photonic crystals such as blue-phase liquid crystals typically assume cubic lattice structures. Nonetheless, blue-phase liquid crystals with distinct crystal symmetries and thus band structures will be advantageous for optical applications. Here we use repetitive electrical pulses to reconfigure blue-phase liquid crystals into stable orthorhombic and tetragonal lattices. This approach, termed repetitively applied field, allows the system to relax between each pulse, gradually transforming the initial cubic lattice into various intermediate metastable states until a stable non-cubic crystal is achieved. We show that this technique is suitable for engineering non-cubic lattices with tailored photonic bandgaps, associated dispersion and band structure across the entire visible spectrum in blue-phase liquid crystals with distinct composition and initial crystal orientation. These field-free blue-phase liquid crystals exhibit large electro-optic responses and can be polymer-stabilized to have a wide operating temperature range and submillisecond response speed, which are promising properties for information display, electro-optics, nonlinear optics, microlasers and biosensing applications.

Extraordinary polarization rotation of vector beams with high-period-number chiral photonic crystals.

We demonstrate by theory and experiments that well-aligned cholesteric liquid crystals that function as 1D chiral photonic crystals (CPCs) having extraordinarily large-period-numbers N (N=d/Λ; d: thickness, Λ: grating period) exceeding 500 possess many optical properties that are impossible with conventional thin CPCs. Even far from the circular Bragg resonance, these CPCs are capable of simultaneously high transmission and large broadband polarization rotation of vector beams; the polarization rotation is independent of relative orientation of the input beam polarization vectors, and a good degree of linear polarization of the output beam can be maintained.

Ultrafast switching of optical singularity eigenstates with compact integrable liquid crystal structures.

By using the strong nonlinear effect and ultrafast electronic response of cholesteric liquid crystals (CLC), ultrafast all optical switching between polarization vortex and phase vortex is realized in a system combining CLC and q-plate. The experimental result shows that switching with high modulation depth can be accomplished in less than 1 picosecond. Furthermore, CLC and q-plates will enable compact integrated devices with sub-mm thicknesses.

High-contrast switching and high-efficiency extracting for spontaneous emission based on tunable gap surface plasmon.

Controlling spontaneous emission at optical scale lies in the heart of ultracompact quantum photonic devices, such as on-chip single photon sources, nanolasers and nanophotonic detectors. However, achiving a large modulation of fluorescence intensity and guiding the emitted photons into low-loss nanophotonic structures remain rather challenging issue. Here, using the liquid crystal-tuned gap surface plasmon, we theoretically demonstrate both a high-contrast switching of the spontaneous emission and high-efficiency extraction of the photons with a specially-designed tunable surface plasmon nanostructures. Through varying the refractive index of liquid crystal, the local electromagnetic field of the gap surface plasmon can be greatly modulated, thereby leading to the swithching of the spontaneous emission of the emitter placed at the nanoscale gap. By optimizing the material and geometrical parameters, the total decay rate can be changed from 103γ0 to 8750γ0, [γ0 is the spontaneous emission rate in vacuum] with the contrast ratio of 85. Further more, in the design also enables propagation of the emitted photons along the low-loss phase-matched nanofibers with a collection efficiency of more than 40%. The proposal provides a novel mechanism for simultaneously switching and extracting the spontaneous emitted photons in hybrid photonic nanostructures, propelling the implementation in on-chip tunable quantum devices.

Large three-dimensional photonic crystals based on monocrystalline liquid crystal blue phases.

Although there have been intense efforts to fabricate large three-dimensional photonic crystals in order to realize their full potential, the technologies developed so far are still beset with various material processing and cost issues. Conventional top-down fabrications are costly and time-consuming, whereas natural self-assembly and bottom-up fabrications often result in high defect density and limited dimensions. Here we report the fabrication of extraordinarily large monocrystalline photonic crystals by controlling the self-assembly processes which occur in unique phases of liquid crystals that exhibit three-dimensional photonic-crystalline properties called liquid-crystal blue phases. In particular, we have developed a gradient-temperature technique that enables three-dimensional photonic crystals to grow to lateral dimensions of ~1 cm (~30,000 of unit cells) and thickness of ~100 µm (~ 300 unit cells). These giant single crystals exhibit extraordinarily sharp photonic bandgaps with high reflectivity, long-range periodicity in all dimensions and well-defined lattice orientation.Conventional fabrication approaches for large-size three-dimensional photonic crystals are problematic. By properly controlling the self-assembly processes, the authors report the fabrication of monocrystalline blue phase liquid crystals that exhibit three-dimensional photonic-crystalline properties.

Tunable enhanced spontaneous emission in plasmonic waveguide cladded with liquid crystal and low-index metamaterial.

The control and enhancement of the spontaneous emission (SE) of emitters embedded in subwavelength structures are fundamentally interesting and of practical interest. For example, in plasmonic lasers and on-chip single photon sources, a large SE rate and the active modulation of SE over a very broad spectral band are highly desired functionalities. In this paper, we demonstrate by an explicit theoretical calculation that a plasmonic waveguide cladded with liquid crystals (LCs) and low-index metamaterials can give rise to an enhancement in the intrinsic SE rate γ0 of more than two orders of magnitude. Furthermore, by varying the refractive index of the LC cladding, thereby changing the density of states of the surface plasmons, the enhanced SE rate can be modulated over a very large range, e.g., from 131γ0 to 327γ0. In general, the modulation range increases with the anisotropy in the refractive index of the LC, while for a fixed range of modulation, the SE rate is larger with lower cladding indices. These results for active modulation and enhanced SE may find application in enabling low-threshold plasmonic nanolasers and tunable on-chip single photon sources.

High efficiency holographic Bragg grating with optically prolonged memory.

In this paper, we show that photosensitive azo-dye doped Blue-phase liquid crystals (BPLC) formed by natural molecular self-assembly are capable of high diffraction efficiency holographic recording with memory that can be prolonged from few seconds to several minutes by uniform illumination with the reference beam. Operating in the Bragg regime, we have observed 50 times improvement in the grating diffraction efficiency and shorter recording time compared to previous investigations. The enabling mechanism is BPLC's lattice distortion and index modulation caused by the action of light on the azo-dopant; upon photo-excitation, the azo-molecules undergo transformation from the oblong-shaped Trans-state to the bent-shaped Cis-state, imparting disorder and also cause the surrounding BPLC molecules to undergo coupled flow &reorientation leading to lattice distortion and index modulation. We also showed that the same mechanism at work here that facilitates lattice distortion can be used to frustrate free relaxation of the lattice distortion, thereby prolonging the lifetime of the written grating, provided the reference beam is kept on after recording. Due to the ease in BPLC fabrication and the availability of azo-dopants with photosensitivity throughout the entire visible spectrum, one can optimize the controlling material and optical parameters to obtain even better performance.

All-optical transistor- and diode-action and logic gates based on anisotropic nonlinear responsive liquid crystal.

In this paper, we show that anisotropic photosensitive nematic liquid crystals (PNLC) made by incorporating anisotropic absorbing dyes are promising candidates for constructing all-optical elements by virtue of the extraordinarily large optical nonlinearity of the nematic host. In particular, we have demonstrated several room-temperature 'prototype' PNLC-based all-optical devices such as optical diode, optical transistor and all primary logic gate operations (OR, AND, NOT) based on such optical transistor. Owing to the anisotropic absorption property and the optical activity of the twist alignment nematic cell, spatially non-reciprocal transmission response can be obtained within a sizeable optical isolation region of ~210 mW. Exploiting the same mechanisms, a tri-terminal configuration as an all-optical analogue of a bipolar junction transistor is fabricated. Its ability to be switched by an optical field enables us to realize an all-optical transistor and demonstrate cascadability, signal fan-out, logic restoration, and various logical gate operations such as OR, AND and NOT. Due to the possibility of synthesizing anisotropic dyes and wide ranging choice of liquid crystals nonlinear optical mechanisms, these all-optical operations can be optimized to have much lower thresholds and faster response speeds. The demonstrated capabilities of these devices have shown great potential in all-optical control system and photonic integrated circuits.

Ultrafast pulse compression, stretching-and-recompression using cholesteric liquid crystals.

We have experimentally demonstrated the feasibility of direct compression, or stretching and recompression of laser pulses in a very wide temporal time scale spanning 10's fs to ~1 ps time with sub-mm thick cholesteric liquid crystal (CLC) cells. The mechanisms at work here are the strong dispersion at the photonic band-edges and nonlinear phase modulation associated with the non-resonant ultrafast molecular electronic optical nonlinearity. The observed pulse compression limit, spectral characteristics and intensity dependence of the compression are in good agreement with theoretical expectations and simulations based on a coupled-mode propagation model. Owing to the large degree of freedom to engineer the wavelength locations of CLC photonic bandgap and band-edges, these self-action all-optical processes can be realized with ultrafast lasers pulses in a very wide spectral region from the visible to near infrared, with potential applications in compact ultrafast photonic modulation devices/platforms.

Observation of photorefractive effects in blue-phase liquid crystal containing fullerene-C60.

Photorefractive effects manifested in two beam coupling and side diffractions are observed in fullerene-C60 doped blue-phase liquid crystals (BPLC-C60) upon application of a DC bias field. The mechanism at work here is attributed to BPLC lattice distortion by the combined DC (Edc)+ photorefractive space-charge (Ephoto) fields, in addition to the DC + optical field induced effects reported in previous studies of dye-doped system. The first order diffraction efficiency of ∼2×10-3 and beam coupling gain of over 2% are observed in a 55 µm thick sample with input laser beam power of 5 mW at an applied DC voltage of 160 V. The effective nonlinear index coefficient n2 of BPLC-C60 is measured to be on the order of 10-2 cm2/W, which is slightly lower than their NLC counterparts. Owing to the isotropy of BPLC optical properties, these effects can be observed with more relaxed requirements on the laser polarizations, directions of incidence, and sample orientations.

Dynamical studies of the mechanisms for optical nonlinearities of methyl-red dye doped blue phase liquid crystals.

Dynamical grating diffraction experiments and reflection/transmission polarization spectroscopy have been conducted on azo-dye doped Blue-Phase Liquid Crystal (BPLC) to investigate the mechanisms responsible for laser induced refractive index changes. The underlying mechanisms for the transient grating diffraction components are attributed to thermal indexing and lattice distortion, whereas the persistent component is due to lattice distortion/expansion caused by laser excited dye molecule isomerization. These mechanisms were distinguishable by their response dynamics and gave rise to the observed reflection spectra and photonic bandgap shift, polarization dependency and optical activity. Some preliminary studies have demonstrated the feasibility of using these mechanisms for coherent holographic and direct image writing operations.

Lasing properties of a cholesteric liquid crystal containing aggregation-induced-emission material.

We demonstrate band edge lasing action from a cholesteric liquid crystal (CLC) containing an aggregation-induced-emission (AIE) dye as gain material. AIE materials do not suffer aggregation-caused quenching, have strong resistance to photobleaching, and can show large Stokes shift. The amplified spontaneous emission (ASE) and lasing emission of the dye-doped CLC cell have been characterized, the lasing threshold has been estimated, and its resistance to photobleaching has been measured. AIE materials with their unique properties are especially suitable for acting as gain materials in liquid crystal lasers where defect structures lower the threshold for nanoscale aggregation effects. Our studies have shown that such AIE-dye-doped CLC is capable of lasing action with unusually large Stokes shift at moderate threshold with strong resistance to photobleaching.

DC-field-assisted grating formation and nonlinear diffractions in methyl-red dye-doped blue phase liquid crystals.

We report the observation of enhanced nonlinear optical responses of methyl-red-doped blue-phase liquid crystals by application of a DC field. We have observed strong multi-order nonlinear grating diffractions characterized by a nonlinear index coefficient n(2)∼0.5 cm(2)/W using unfocused CW laser power of ∼1 mW and a DC field of a few V/µm. The underlying mechanisms are crystalline lattice and director axis reorientations by torques exerted by the DC field and photo-excited dye molecules.

Enhanced optical tuning of modified-geometry resonators clad in blue phase liquid crystals.

Active optical tuning of silicon racetrack resonators clad in dye-doped blue phase liquid crystals (BPLCs) is experimentally demonstrated. An adiabatic racetrack resonator geometry that allows for enhanced tuning is presented and analyzed. The resonance shift of an unmodified geometry racetrack is Δλ=0.7 nm, while an adiabatic racetrack achieves a Δλ=1.23 nm resonance shift because of a greater mode overlap with the cladding. The calculated refractive index change of the BPLC is Δn=0.0041 for both geometries.

Passive Temperature Stabilization of Silicon Photonic Devices Using Liquid Crystals.

In this work we explore the negative thermo-optic properties of liquid crystal claddings for passive temperature stabilization of silicon photonic integrated circuits. Photonic circuits are playing an increasing role in communications and computing, but they suffer from temperature dependent performance variation. Most existing techniques aimed at compensation of thermal effects rely on power hungry Joule heating. We show that integrating a liquid crystal cladding helps to minimize the effects of a temperature dependent drift. The advantage of liquid crystals lies in their high negative thermo-optic coefficients in addition to low absorption at the infrared wavelengths.

Direct femtosecond pulse compression with miniature-sized Bragg cholesteric liquid crystal.

Direct compression of femtosecond optical pulses from a Ti:sapphire laser oscillator was realized with a cholesteric liquid crystal acting as a nonlinear 1D periodic Bragg grating. With a 6 µm thick sample, the pulse duration could be compressed from 100 to 48 fs. Coupled-mode equations for forward and backward waves were employed to simulate the dynamics therein, and good agreement between theory and experiment was obtained.

Optical tuning of silicon photonic structures with nematic liquid crystal claddings.

An analysis of and experimental demonstration of active optical tuning of silicon strip waveguides with methyl red doped nematic liquid crystal claddings is presented. Under low-power irradiation by polarized light, the reorientation of the nematic, the resulting index change, and phase shift produce a tuning range of 5.6 nm for the microresonator resonances.

Blue-phase liquid crystal cored optical fiber array with photonic bandgaps and nonlinear transmission properties.

Blue-phase liquid crystal (BPLC) is introduced into the pores of capillary arrays to fabricate fiber arrays. Owing to the photonic-crystals like properties of BPLC, these fiber arrays exhibit temperature dependent photonic bandgaps in the visible spectrum. With the cores maintained in isotropic as well as the Blue phases, the fiber arrays allow high quality image transmission when inserted in the focal plane of a 1x telescope. Nonlinear transmission and optical limiting action on a cw white-light continuum laser is also observed and is attributed to laser induced self-defocusing and propagation modes changing effects caused by some finite absorption of the broadband laser at the short wavelength regime. These nonlinear and other known electro-optical properties of BPLC, in conjunction with their fabrication ease make these fiber arrays highly promising for imaging, electro-optical or all-optical modulation, switching and passive optical limiting applications.

Polarization-independent dual-band infrared perfect absorber based on a metal-dielectric-metal elliptical nanodisk array.

We have designed and fabricated a dual-band plasmonic absorber in the near-infrared by employing a three-layer structure comprised of an elliptical nanodisk array on top of thin dielectric and metallic films. finite difference time domain (FDTD) simulations indicate that absorption efficiencies greater than 99% can be achieved for both resonance frequencies at normal incidence and the tunable range of the resonant frequency was modeled up to 700 nm by varying the dimensions of the three-layer, elliptical nanodisk array. The symmetry in our two-dimensional nanodisk array eliminates any polarization dependence within the structure, and the near-perfect absorption efficiency is only slightly affected by large incidence angles up to 50 degrees. Experimental measurements demonstrate good agreement with our simulation results.