Traditional neural networks (NNs) require substantial computing resources and energy. Photonics-based systems offer faster and more energy-efficient solutions. However, implementing nonlinear activation functions in photonics has been challenging due to the need for high-power optical sources and extended interaction lengths. The proposed solution uses structural nonlinearity, creating nonlinear output patterns with low energy use and simple digital NN training. The research develops a reconfigurable material platform using liquid crystal/polymer composite (LCPC) and metasurface to control scattering potentials dynamically. These results show that the LCPC’s phase distribution can be reliably controlled, enabling reconfiguration and repetition of scattering responses, which is crucial for advancing photonics-based neuromorphic computing.
Phased Array Antennas (PAAs) and Reconfigurable Intelligent Surfaces (RISs) have garnered substantial attention for their capacity to astutely manipulate the wireless propagation environment, establishing them as a pivotal technology for enhancing capacity and coverage in 6G communication networks. High-performance phase control unit is the key element for PAA/RIS. This talk presents the design and measurement results of a reflective liquid-crystal (LC) phase shifter in microwave range.
We present an optical learning framework based on reconfigurable liquid crystal/polymer composite (LC/PC) scattering media. Our design leverages multiple scatterings between the LC/PC sample and a spatial light modulator (or a digital mirror device) that encodes the input data to realize random nonlinear optical mapping in a tunable manner. The system is applied to several learning tasks to demonstrate its capabilities. The reconfigurability of the LC/PC can optimize the learning performance, and enable photonic ensemble learning, which further improves the overall performance.
A review of recent advances in molecular assembly of liquid crystalline chiral photonic crystals is presented, particularly on fabrication of extraordinarily thick [>2 mm] cholesteric liquid crystals (CLC), and large-areal size monocrystalline Blue-phase liquid crystals (BPLC). CLC’s as 1-D chiral photonic crystals possess large ultrafast optical nonlinearity suitable for self-compression, polarization switching and modulation of complex vector beams. These super thick and highly nonlinear CLC’s are estimated to be capable of all optical switching [π- phase shift] at sub-picosecond (~0.1 x 10-12 s) speed with low threshold energy fluence of ~ 0.25 μJ/cm2. Large areal size monocrystalline BPLC’s will preserve the phase front uniformity in free-space image processing and yield better efficiency, resolution, and image/signal qualities than their typical polycrystalline counterparts.
We report a room-temperature dual-frequency field assembly technique that is capable of fabricating large-areal size (~cm2 or larger), well-aligned cholesteric liquid crystals to thicknesses up to 2.2 mm, corresponding to period number N (thickness/index grating period) of nearly10,000 in the visible spectral regime. The method employs successive application of low- and high- frequency electric field on a thick cell of CLC starting mixture containing a nematic constituent of negative anisotropy. The low-frequency field creates conductive hydrodynamical instabilities that mash the mixture to a state with completely randomized orientation of the cholesteric helices; the next application of a high-frequency field at a field strength below the dielectric hydrodynamic instability reorient all the helices into uniform standing helices. Such extraordinarily thick chiral photonic crystals exhibit many never-before-realized chiral photonic properties such as giant rotation of optical polarization with high transmission (low scattering loss), polarization switching and ultrafast pulse modulation capabilities for visible to mid-infrared lasers, in addition to dynamic tunability by electrical, thermal, or optical means.
An electrically tunable achromatic polarization rotator has been developed based on the hybrid splay-twist (HST) and hybrid-aligned super twist (HAST) liquid crystal. The continuous angular rotation, and achromatic operation across the entire visible spectrum. The tuning range of the polarization rotator is up to 90° or to 180° and the degree of linear polarization (DOLP) remains. Based on the HST-LC, the multi-functional smart glass is realized with light field, dimming and scattering control. This work provides possibilities in the design of optical systems and spatially polarization multiplexing elements. The designed smart glass provides novelties in smart (green) architecture.
By virtue of their constituents’ soft matter attributes and easy susceptibilities to applied fields, liquid crystalline chiral photonic crystals exhibit highly tunable linear and nonlinear optical properties that allow applications throughout a very wide spectrum. In particular, nearly-mm thick 1-D chiral photonic crystals fabricated with cholesteric liquid crystal are shown to be capable of polarization rotation and switching of complex laser vector fields with response times that can be as fast as sub-picoseconds, and operating wavelength ranging from the visible to near- and mid-IR range. [Ref.: Nature Comm. 8, Article number: 727 (2017); PNAS 2021, 118 (16)].
Under an applied electric field, BPLC lattice undergoes with complex reconfiguration dynamics and exhibit meta-stability and new crystal symmetries. Detailed theoretical considerations and experimental results with such Repetitively Applied Field (RAF)Technique for transforming BPLC crystalline lattice structures from cubic to final stable configuration with orthorhombic or tetragonal symmetry will be presented.
An electrically tunable achromatic polarization rotator has been developed based on the hybrid splay-twist (HST) liquid crystal. The proposed polarization rotator is advantageous over the conventional ones owing to the thin thickness (sub-100μm), continuous angular rotation, and achromatic operation across the entire visible spectrum. The tuning range of the polarization rotator is up to 90° via a simple electric field application; meanwhile, the degree of linear polarization (DOLP) remains. The rotation angle can be expanded to 180° by a tandem-cell geometry. The work will offer possibilities in the design of various optical systems and spatially polarization multiplexing elements.
We present the results from recent studies of field-induced reconfiguration of defect network in Blue phase liquid crystals leading to the formation of new stable lattice structures from their natural self-assembled cubic form [Nat. Materials https://www.nature.com/articles/s41563-019-0512-3]. The dynamical evolution of the defect network and reorientation provide new insights into the underlying mechanisms and roles played by various factors, especially the form of applied field for efficient lattice transformation. Recent studies with optical field derived from CW or short-pulsed lasers further demonstrate the possibility of direct reconfiguration/reorientation of the bulk crystals.
Smart window is the films/glasses having the incredible feature of controlling heat, privacy, and getting their transmission properties changed from opaque to translucent to transparent under the influence of voltage. The flow of light and heat between indoor and outdoor is controllable, providing human life more convenient and comfortable. Smart window technique such as Polymer dispersed liquid crystal (PDLC), Electrochromic glass (EC) and Suspended particle devices (SPD) accounted for 90% of the market. However, these techniques can be only operated at a single function. In order to achieve true energy conservation and versatility, it is highly desirable but challenging to design a single device capable of simultaneously modifying haze and tint as well as exhibiting multi-stability. In this presentation, I will talk about the recent technology and development of multi-functional LC smart windows.
Blue phase liquid crystals (BPLC) are self-assembled 3D photonic crystals with high dynamic tunability. We review our recent progress on dynamic tuning of BPLC’s photonic band gap, crystal symmetry and orientation by electric field and light. With negative dielectric anisotropy, electrically induced switching between [110]- and [200]-oriented BPI states can be achieved through a transient flow state induced by electro-hydrodynamic instabilities. In dielectrically positive polymer-stabilized BPLCs, applying a DC electric field will induce a stretch of the lattice along the field direction, resulting in a bandgap red-shift of over 200 nm. Such a field-induced stretch can also induce chirped lattice spacing along the field axis in samples fabricated under a different photopolymerization condition, leading to an effective expansion of the photonic bandgap. With chiral azo molecular switch doped in polymer-free BPLCs, reversible lattice dilation and phase transition from simple cubic to body-centered cubic symmetry can be induced by light. These techniques bring about many new possibilities of blue-phase photonic crystals for photonic applications.
Cholesteric liquid crystals (CLC) has the advantage of large nonlinearity and fast response. This brings great convenience to its applications in ultrafast nonlinear photonic. Here, several kinds of nonlinear phenomenon and applications are reported, such as optical soliton, modulation instability, optical diode effect and pulse compression of cascade CLC samples.
We have succeeded in fabricating unusually thick (up to ~ 550 microns), well aligned cholesteric liquid crystals that possess low scattering loss, large operating temperature range and well-defined photonic bandgap in the visible - near infrared regime. These CLC’s possess sufficiently large ultrafast (sub-picosecond) electronic optical nonlinearity needed for direct compression, stretching and recompression of femtoseconds-picoseconds laser pulses without additional optics, as demonstrated by theory and experiments. Despite such world-record setting thickness, these CLC’s are extremely compact in comparison to other state-of-the-art materials/devices used for similar operations. They are therefore highly promising for miniaturization and reduced complexity of photonic platform/systems for ultrafast pulse modulations.
Liquid crystals materials have the advantage of having a large nonlinear coefficient, but the response time is slow, normally up to several minisecond. This makes it is hard to apply in ultra fast optical devices. Recently, fentosecond (fs) nonlinear effect in choleteric liquid crystals is reported, nonlinear coefficient in the scale of 10−12 cm2∕W is achieved. Base on this effect, in this work, fentosecond pulse compression technique in a miniature choleteric liquid crystal is demonstrated1,2.
Cholesteric liquid crystals (CLC) is a kind of 1-dimensional phontonic structure with helical periodic. In a 10 μm thick CLC, femtosecond pulse with 100 fs is compressed to about 50 fs. CLC sample in planar texture with 500μm thick cell gap is further fabricated. In this sample, femtosecond pulse with 847 fs can be compressed to 286 fs.
Due to the strong dispersion at the edge of photonic band gap, femtosecond pulse stretching and compensation can be achieve. In this experiment, laser pulse with duration 90 fs is stretched to above 2 picosecond in the first CLC sample and re-compressed to 120 fs in the second sample. Such technique might be applied in chirp pulse amplification.
In conclusion, we report ultra fast nonlinear effect in cholesteric liquid crystals. Due to the strong dispersion and nonlinearity of CLC, femtosecond pulse manipulating devices can be achieved in the scale of micrometer.
We have investigated the mechanisms responsible for nonlinear optical processes occurring in azobenzene-doped blue phase liquid crystals (BPLC), which exhibit two thermodynamically stable BPs: BPI and BPII. In coherent two wave-mixing experiments, a slow (minutes) and a fast (few milliseconds) side diffractions are observed. The underlying mechanisms were disclosed by monitoring the dynamics of grating formation and relaxation as well as by some supplementary experiments. We found the photothermal indexing and dye/LC intermolecular torque leading to lattice distortion to be the dominant mechanisms for the observed nonlinear response in BPLC. Moreover, the response time of the nonlinear optical process varied with operating phase. The rise time of the thermal indexing process was in good agreement with our findings on the temperature dependence of BP refractive index: τ(ISO) > τ(BPI) > τ(BPII). The relaxation time of the torque-induced lattice distortion was analogue to its electrostriction counterpart: τ'(BPI) > τ'(BPII). In a separate experiment, lattice swelling with selective reflection of <110> direction changed from green to red was also observed. This was attributable to the isomerization-induced change in cholesteric pitch, which directly affects the lattice spacing. The phenomenon was confirmed by measuring the optical rotatory power of the BPLC.
Blue phase liquid crystals (BPLCs) are self-assembled 3D photonic crystals exhibiting high susceptibility to external stimuli. Two methods for the photonic bandgap tuning of BPs were demonstrated in this work. Introducing a chiral azobenzene into a cholesteric liquid crystal could formulate a photoresponsive BPLC. Under violet irradiation, the azo dye experiences trans-cis isomerization, which leads to lattice swelling as well as phase transition in different stages of the process. Ultrawide reversible tuning of the BP photonic bandgap from ultraviolet to near infrared has been achieved. The tuning is reversible and nonvolatile. We will then demonstract the electric field-induced bandgap tuning in polymer-stabilized BPLCs. Under different BPLCs material preparation conditions, both red-shift and broadening of the photonic bandgaps have been achieved respectively. The stop band can be shifted over 100 nm. The bandwidth can be expanded from ~ 30 nm to ~ 250 nm covering nearly the full visible range. It is believed that the developed approaches could strongly promote the use of BPLC in photonic applications.
We report on the investigation of random lasing in blue phase liquid crystals. Multiple scattering and interference effects arising from disordered platelet texture as well as index mismatch between polymer and mesogen contribute the optical feedbacks towards laser action. In pure blue phase liquid crystals, the random laser can be switched between the coherent and incoherent types by executing distinct heating/cooling cycles; and, the randomness of lasing wavelengths can be determined by the platelet size, which can be set by controlling the cooling rate. After the blue phase liquid crystals are polymer-stabilized, coherent random lasing may occur in both the blue phase with an extended temperature interval and the isotropic liquid state; also, the selected modes are constant from one pulse to another. Additionally, if the laser dye is sensitive to temperature, the excitation threshold and the emission spectrum could be altered via thermal control.
This study investigates an optically switchable band gap of photonic crystal that is based on an azobenzene-doped liquid
crystal blue phase. The trans-cis photoisomerization of azobenzene deforms the cubic unit cell of the blue phase and
shifts the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with different
wavelengths light. The crystal structure is verified using Kossel diffraction diagram. An optically addressable blue phase
display, based on Bragg reflection from the photonic band gap, is also demonstrated. The tunable ranges are around red,
green and blue wavelengths and exhibit a bright saturated color.
This study presents a simple but accurate method for measuring the helical twisting power of chiral doped liquid crystals
using axially symmetrical photo-alignment in azo dye-doped liquid crystal films. As reported in our previous paper1, the
discontinuous twisting effect produces a disclination line in
photo-aligned axially symmetrical liquid crystal films, which
can be applied for use as a polarization converter. The pitch and helical twisting power can be obtained by measuring the
rotation angle of the disclination line in chrial doped liquid crystal. This method is independent of cell gap and provides
an error below 0.5%.
Multi-wavelength lasing in a dye-doped cholesteric liquid crystal (CLC) cell is demonstrated. By adding oversaturated
chiral dopant, the multi-photonic band CLC structure can be obtained with non-uniform chiral solubility. Under
appropriate excitation, multi-wavelength lasing can be achieved with a multi-photonic band edge CLC structure. The
number of lasings can be controlled under various temperature processes. Nine wavelength CLC lasings were observed
simultaneously. The wavelength range covers around 600-675nm.
We demonstrated transmissive- and reflective-type Fresnel lenses based on dye-doped liquid crystal using
photoalignment technique. The former is a polarization-independent and electrically tunable. The maximum diffraction
efficiency reaches 37%, which approaches the theoretical limit ~ 41 %. Such a lens functions as a half-wave plate, and
this feature could be well preserved under the applied voltage. The reflective-type Fresnel lens is based on dye-doped
cholesteric liquid crystals (DDCLC). The formed lens persists without any external disturbance, and its focusing
efficiency, analyzed using circularly polarized light, is ~ 23.7 %, which almost equals the measured diffraction efficiency
of the used Fresnel-zone-plate mask (~ 25.6 %). The lens is thermally erasable, and rewritable. Notably, both of the
transmissive- and reflective-type Fresnel lenses are switchable between focusing and defocusing states, upon application
of a voltage. In addition, these devices are simple to fabricate, and have fast switching responses between focusing and
defocusing state.
This investigation researches and developes two spatial filters based on dye-doped liquid crystal films. One is made
with a dye-doped liquid crystal (DDLC) film, and is polarization controllable. The other is transflective, and is fabricated
with an azo dye doped cholesteric liquid crystal (DDCLC) film. The fabrication of the former type relies on the fact that
the various intensities of the diffracted orders are responsible for various changes of the polarization state induced by the
photo-aligned DDLC film. Particular spatial orders in the Fourier optical signal process can be filtered using an analyzer
placed behind the sample by controlling the polarization state of the diffracted orders. The latter is based on the
photoisomerization effect in a DDCLC film with a concomitant lowering of phase transition temperature from a
cholesteric to an isotropic phase (TCh-I). The fabrication relies on the fact that the various intensities of the diffracted
orders are responsible for various degree of transparency induced by the photoisomerized DDCLC film. Particular spatial
orders in the Fourier optical signal process can be filtered to trans- or reflect- part at the same time. Simulations are also
performed for the two-type spatial filters, and the results agree closely with experimental data.
This investigation establishes two spatial filters based on dye-doped liquid crystal films. One is made with a
dye-doped liquid crystal (DDLC) film, and is polarization controllable. The other is transflective, and is fabricated with
an azo dye doped cholesteric liquid crystal (DDCLC) film. The fabrication of the former type relies on the fact that the
various intensities of the diffracted orders are responsible for various changes of the polarization state induced by the
photo-aligned DDLC film. Particular spatial orders in the Fourier optical signal process can be filtered using an analyzer
placed behind the sample by controlling the polarization state of the diffracted orders. The latter is based on the
photoisomerization effect in a DDCLC film with a concomitant lowering of phase transition temperature from a
cholesteric to an isotropic phase (TCh-I). The fabrication relies on the fact that the various intensities of the diffracted
orders are responsible for various degree of transparency induced by the photoisomerized DDCLC film. Particular spatial
orders in the Fourier optical signal process can be filtered to trans- or reflect- part at the same time. Simulations are also
performed for the two-type spatial filters, and the results agree closely with experimental data.
This work demonstrates the feasibility of using polymer-dispersed liquid crystal (PDLC) films as electrically switchable spatial filters in the optical signal process. The fabrication relies on the fact that the size of the LC droplet formed in a PDLC film is inversely proportional to the intensity of curing. Controlling the driving voltage on the PDLC sample can filter particular spatial frequencies in the Fourier optical signal process. A simulation is also performed, and the results are highly consistent with those of experiments.
This letter examines a planar cholesteric cell (CLC) doped with a collocation of two laser dyes as a one-dimensional photonic crystal. Adding the photo-tunable chiral material-AzoB allows the CLC photonic crystal can be lased at the band edges of the photonic band gap with a tuning range of over 100nm. Tuning is performed by irradiating the chiral AzoB material with UV light so that it undergoes trans-cis isomerization in the CLC film. The tuning range is the visible region from 563nm to 667nm. Moreover, the tuning is reversible.
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