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Liquid crystals (LCs) have very attractive properties that the response can be controlled by the external stimulus such as electric field and temperature. Terahertz (THz) devices based on LC have wide applications. But the development of high-performance LC THz functional devices is still in its infancy. We firstly develop a large birefringence LC material in THz range. Then broadband tunable transmissive driven with porous graphene and reflective THz waveplates based on this LC are proposed. Furthermore, graphene-assisted high efficiency tunable THz metamaterial absorber is demonstrated. A magnetically and electrically polarization-tunable THz emitter that integrates a ferromagnetic heterostructure and the large-birefringence LC is also demonstrated. Last but not least, we use a temperaturesupersensitive cholesteric liquid crystal (CLC) to not only measure the beam profiles but also detect the powers of THz waves generated from a nonlinear crystal pumped by a table-top laser, which is visible, cost-effective, portable and robust at room temperature. These LC based THz devices can be used in applications such as THz imaging, biological sensing, and inspection.

Mesoscopic hierarchical superstructures bridge the micro and macro worlds and play vital roles in natural materials. To mimic hierarchical organization in nature, one promising strategy is the convergence of top-down microfabrication and bottom-up self-assembly. Much efforts have been devoted to this field, but till now, the precise realization and rational control of large-area perfect hierarchical superstructures is still challenging. On the other hand, Smectic liquid crystals (SLCs) are formed by flexible molecular layers of constant thickness. If the nanometer-thin smectic layers can be manipulated in an origami manner, a Japanese art that constructs various 3D objects via folding pieces of papers, brand new hierarchical superstructures possessing exceptional features then could be realized. In this work, the smectic layer origami is accomplished via preprogrammed photoalignment. The principle is rooted in the anisotropy of molecular interactions at interfaces, which makes the preset patterned alignment favoring a certain layer bending of adjacent SLCs, and subsequently dominating the configuration of entire family of smectic layers. Thanks to the excellent flexibility of photoaligning, the unit geometry (shape, size and orientation) as well as the clustering characteristic (lattice symmetry) of fragmented TFCDs can be rationally designed and freely manipulated over square centimeters. The obtained fragmented toric superstructures break the rotational symmetry while maintain the radially gradient director field, enabling metasurface-like direction-determined-diffraction. We believe this work is an important step toward extending the fundamental understanding of self-assembled soft materials and enhancing the construction of possible hierarchical superstructures. It may inspire extra possibilities in advanced functional materials and fantastic photonic applications.

Utilizing the spin degree of freedom breaks new ground for designing photonic devices. Seeking out a suitable platform for flexible steering of photonic spin states is a critical task. In this work, we demonstrate a versatile Liquid crystal (LC) based platform for manipulating photonic spin and orbital states. Owing to the photoalignment technique, the local and fine tuning of the LC medium is effectively implemented to form various anisotropic microstructures. The light-matter interaction in the corresponding medium is tailored to control the evolution of photonic spin states. The physical mechanism of such a system is investigated, and the corresponding dynamical equation is obtained. The high flexibility endows the LC-based photonic system with great value to be used for Hamiltonian engineering. As an illustration, the optical analogue of intrinsic spin Hall effect (SHE) in electronic systems is presented. The pseudospins of photons are driven to split by the anisotropic effective magnetic field arising from the inhomogeneous spin-orbit interaction (SOI) in the twisting microstructures. In virtue of the designability of the LC-based platform, the form of the interaction Hamiltonian is regulated to present diverse PSHE phenomena, which is hard to be realized in the solid electronic systems. Some representative samples are prepared for experimental observation, and the results are in good agreement with theoretical predictions. We believe the tunable LC system may shed new light on future photonic quantum applications.

We investigate the influence of profile of one-dimensional (1D) Ag gratings on the enhancement factor (EF) of surface-enhanced Raman scattering (SERS). An optimized duty ratio of 1D Ag grating is found, and the SERS EF is experimentally obtained on the order of ~ 10 4 , while the finite-difference time-domain simulation shows that the SERS EF can be as high as ~ 10 6 . We ascribe the discrepancy between the simulated and the experimental results mainly to the fluctuation of Ag grating structure, which is confirmed by the topography measurement using scanning electron microscopy and atomic force microscopy.

In recent years, complex optical fields with spatially inhomogeneous phases, polarizations and optical singularities have drawn many research interests. Many novel effects have been predicted and demonstrated for light beams with these unconventional states in both linear and nonlinear optics regimes. Although local optical phase could be controlled directly or through hologram structures in isotropic materials such as glasses, optical anisotropy is still required for manipulating polarization states and wavelengths. The anisotropy could be either intrinsic such as in crystals/liquid crystals (LCs) or the induced birefringence from dielectric or metallic structures. In this talk, we will briefly review some of our attempts in tailoring complex optical fields via anisotropic microstructures. We developed a micro-photo-patterning system that could generate complex micro-images then further guides the arbitrary local LC directors. Due to the electro-optically (EO) tunable anisotropy of LC, various reconfigurable complex optical fields such as optical vortices (OVs), multiplexed OVs, OV array, Airy beams and vector beams are obtained. Different LC modes such as homogeneous alignment nematic, hybrid alignment nematic and even blue phase LCs are adopted to optimize the static and dynamic beam characteristics depending on application circumstances. We are also trying to extend our approaches to new wavelength bands, such as mid-infrared and even THz ranges. Some preliminary results are obtained. In addition, based on our recently developed local poling techniques for ferroelectric crystals, we will also discuss and demonstrate the nonlinear complex optical field conversion in Lithium Niobate wafers with patterned ferroelectric domain structures.

With a unique 3D geometry, a broadband polarizer and single-polarization resonator are demonstrated by wrapping a microfiber on a specialized rod with a hybrid polymer-metal-dielectric nanostructure, respectively. The polarizer has an extinction ratio of more than 20 dB over the spectra range of 450 nm. The resonator has a Q-factor of ~ 80,000 with excellent suppressing of polarization noise. By functionalizing the rod surface with nanoscale silver film and tuning the coil geometry, performances can be improved by optimizing those parameters. Such kind of miniature singlepolarization microfiber and resonator are impossible to be realized by conventional fabrication process, and has the potential in gyro and current sensors.

A serial of LC gratings are fabricated mainly based on photoalignment, which include (1) Nematic LC grating with alternating 90° twisted nematic (TN) regions and homogeneous alignment (PA). Both 1D and 2D diffraction gratings are demonstrated by periodic photoalignment of sulfonic azo-dye (SD1) films with a linearly polarized light beam. (2) A polarization independent of 1D/2D LC gratings with alternate orthogonal homogeneously aligned regions. No polarizer is employed. (3) A polarizer-free submillisecond response grating employing dual-frequency LC (DFLC) together with patterned hybrid aligned nematic (HAN) structures. To obtain instantly controllable LC microstructures rather than simple gratings, a digital micro-mirror device (DMD) based a micro-lithography system is developed. It may generate arbitrary micro-images on photoalignment layers. Besides normal phase gratings, more complex 2D patterns including quasicrystal structure are demonstrated, which give us more freedom to develop microstructured LC based photonic devices.

We propose optical frequency comb generation in a monolithic micro-ring resonator. Being different from the previously reported nonlinear optical frequency combs, our scheme is based on more efficient quadratic frequency conversion rather than the third-order nonlinearity. To overcome the phase mismatch, a partly poled nonlinear ring is employed. Cascading second harmonic generation and parametric down conversion processes thus are realized through quasi-phase matching (QPM). Coupling equations are used to describe the related nonlinear interactions among different whispering-gallery modes, showing some interesting characteristics that are different from conventional QPM technology.

Condensed matter systems are potential candidates to realize the integration of quantum information circuits. Surface phonon polariton (SPhP) is a special propagation mode in condensed matter systems. We present an investigation on the entanglement of SPhP modes. The entangled pairs are generated from entangled photons injected to the system. Quantum performances of entangled SPhPs are investigated by using the interaction Hamiltonian and the perturbation theory. The wave mechanics approach is taken to describe the coupling process as a comparison. Finally, the correlation of system is examined. A whole set of descriptions of SPhP entanglement thus are presented.

There are two steps to obtain as small as possible fiber Bragg gratings. First, it is to taper the fiber and reduce its diameter. A subwavelength-scale microfiber (MF) is the basic element of miniature fiber devices and sub-systems. Then it is to reduce the grating length. For short fiber grating, strong refractive index modulations are necessary. Strong refractive index modulations can be obtained inducing surface corrugation by alternating layers of different materials, one of which can be air. Several techniques have been proposed for the fabrication of surface-corrugated fiber gratings, including photorefractive inscription using etching, femtosecond lasers, and focused ion beam (FIB). So far, FIB is the most flexible and powerful tool for patterning, cross- sectioning or functionalizing a subwavelength circular MF due to its small and controllable spot size and high beam current density. In past two years, a number of ultra-compact surface corrugated microfiber Bragg gratings (SCMGs) have been successfully fabricated by FIB milling. The length of FIB milled SCGMs can be as small as tens of micrometers. In addition, there are several novel proposals on SCMG including wrapping a microfiber on a microstructure rod or put a microfiber on a surface-corrugated planar grating. In this paper, we will introduce recent advances in these ultra-small SCMGs and their characteristics and applications.

We experimentally demonstrate a compact microfiber Bragg grating (MFBG) force sensor that offers greater sensitivity under load. The FBG is fabricated by focused ion beam (FIB) milling a microfiber tapered from nonphotosensitive standard single-mode fiber. It is only ~58μm in length. The force sensitivity of the MFBG is as high as ~4133nm/N around the resonant wavelength of 1550nm, which is three orders of magnitude larger than that recorded in FBGs inscribed in untapered fibers.

Some of our recent progress on liquid crystal (LC) gratings, from nematic to blue phase, is reviewed in this invited talk. The first kind of grating is fabricated by periodically adjusting the LC directors to form alternate micro phase retarders and polarization rotators in a cell placed between crossed polarizers. The second one is demonstrated by means of photoalignment technique with alternate orthogonal homogeneously-aligned domains. To improve the response time of the gratings, several approaches are also proposed by using dual-frequency addressed nematic LC, ferroelectric LC and blue phase LC, which shows great performance including high transmittance, polarization independency and submillisecond response. At last, to obtain other controllable LC microstructures rather than simple 1D/2D gratings, we develop a micro-lithography system with a digital micro-mirror device as dynamic mask forms. It may instantly generate arbitrary micro-images on photoalignment layers and further guides the LC molecule orientations. Besides normal phase gratings, more complex patterns such as quasicrystal structures are demonstrated. Some new applications such as tunable multiport optical switching and vector beam generations are expected.

We fabricate a miniature tapered photonic crystal fiber (PCF) interferometer with enhanced sensitivity by a new acid microdroplets etching method. This method, without elongating the PCF, moving and re-fixing the device during etching and measuring refractive index sensitivity, is very simple, cost-efficient and highly stable over time. We investigate the refractive index sensing properties with different PCF diameters both theoretically and experimentally. The size decreases and the sensitivity increases an order of magnitude after etching the PCF. If we can optimize the etching process, we can fabricate more uniformly and thinly tapered PCF interferometer with higher sensitivity (∼ 100 times) theoretically in the future.

We report a miniaturized fiber probe inline reflective interferometer (FPIRI) sensor, with a several μ-micro-notch cavity fabricated for highly sensitive refractive index measurement. Its sensitivity in liquid is ~100 nm/RIU (refractive index unit) near the wavelength of 1550 nm with a high extinction ratio. This probe sensor is very compact, stable, and cheap, offering great potentials for detecting inside sub-wavelength particles or biocells.

A two-wavelength fiber laser with two π-equivalent phase shifts (EPSs) is studied and fabricated. The π-EPS is obtained through extending one sampling period by 50% in a fiber sampled Bragg grating (SBG). To get a two-wavelength fiber laser lasing in the -1st channel, two π-EPSs are introduced in the position of L/3 and 2L/3 in the SBG, where L is the total length of the SBG. A stable 22-pm-spaced dual wavelength DFB fiber laser is achieved in the experiment.

Dynamic DWDM channel blocking and equalizing is proposed based on liquid crystal (LC) and bulk grating technology. A dispersion compensation and enhancement unit is employed so that low center-frequency shift small footprint could be achieved at the same time. The inter-channel crosstalk is also studied. With the optimized design of dispersion unit and LC module, a 32-channel, 100-GHz spacing sample is demonstrated with the insertion loss (IL) of 5.5 dB and 45dB extinction ratio. Application of our device to reconfigurable optical add-and-drop (ROADM) network is discussed

We report the study of chromatic dispersion ripples in birefringent crystal-based optical interleavers. The simulation indicates that for a 99.8% transmittance AR coating surface, the chromatic dispersion ripples caused by the Rhomb etalon effect could be as high as ±30 ps/nm. The experimental results are in agreement with the simulation results. We also demonstrate a practical approach to eliminate the chromatic dispersion ripples to be coupled into the output collimators. The overall interleaver chromatic dispersion, of less than 15 ps/nm within the passband, is demonstrated.

A variable optical attenuator (VOA) at λ=1.55 μm using a sheared polymer network liquid crystal (SPNLC) is demonstrated. SPNLC is fabricated by mixing ~15 wt % photo-polymerizable monomer in a LC host. To polymerize the SPNLC cell, a two-step UV curing process was adopted. Before shearing, the cell scatters light strongly, but after shearing the cell becomes highly transparent in the near IR region. Using an E7-based SPNLC, the attenuation of our VOA is rather insensitive to wavelength over the ITU C-band. The rise time and decay time were measured to be 35 μs and 205 μs, respectively, at room temperature. Such a response time is at least one order magnitude faster than the state-of-the-art nematic competitors. Comparing with other polymer-stabilized liquid crystals, the SPNLC exhibits a lower driving voltage and negligible light scattering loss in the IR spectral region. A reflection type, polarization-independent VOA with ~240 μs response time and -32 dB dynamic range was demonstrated at room temperature and 35 V rms voltage.

Optical limiting is an essential concept for nonlinear optics and its applications. Recently, the nonlinear response in a two-dimensional (2D) superlattice (a periodic structure with its refractive index modulated in two dimensions) was studied for its potential applications in new optical devices. The available results are only related to the transition between a low transmission state and a high transmission state in the allowed bands. Up to date, optical limiting is not explored in 2D case with one incident wave. In this paper, a novel structure, which is composed of two dissimilar superlattices, is proposed for the optical limiting. The novel structure is named hetero- superlattice-junction structure in this paper. In terms of real-space-matching concept, optical limiting is demonstrated in a 2D nonlinear hetero-superlattice-junction structure, while this phenomena is proved unavailable in a 2D nonlinear superlattice.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new multifunctional active medium: optical superlattice Nd:MgO:LiNbO3, i.e., LiNbO3 crystal with periodic ferroelectric domain structures, was grown in our laboratory. The absorption and fluorescence spectra of a sample with the modulation period of 7.5 micrometer were carefully measured and show some differences from the results of common Nd:MgO:LiNbO3 with monodomain structure. Third-order quasi-phase-matched second harmonic generation was also demonstrated to characterize the nonlinear optical properties of the sample. These results indicate that the optical superlattice LiNbO3 has great potentials for self-frequency-doubling operation using the maximum nonlinear coefficient d33 of LiNbO3 and high gain pi polarization at the same time. Pumped by a pulsed dye laser, self-frequency-doubling green light was observed in a cavity with great loss.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical superlattice LiNbO3 crystals with modulation periods were grown by Czochralski method. The generation of femtosecond pulses from violet to blue has been demonstrated in optical superlattice LiNbO3 crystals using third- order quasi-phase-matching. The output power of 9.0 mW at 390 nm, 11.5 mW at 402 nm, and 10.4 mW at 421 nm has been obtained. The dependence of the output power and the acceptance bandwidth on crystal length has been measured and theoretically discussed.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.