Overcoming chromatic aberrations is a vital concern in imaging systems in order to facilitate full-color and hyperspectral imaging. By contrast, large dispersion holds opportunities for spectroscopy and tomography. Combining both functions into a single component will significantly enhance its versatility. A strategy is proposed to delicately integrate two lenses with a static resonant phase and a switchable geometric phase separately. The former is a metasurface lens with a linear phase dispersion. The latter is composed of liquid crystals (LCs) with space-variant orientations with a phase profile that is frequency independent. By this means, a broadband achromatic focusing from 0.9 to 1.4 THz is revealed. When a saturated bias is applied on LCs, the geometric phase modulation vanishes, leaving only the resonant phase of the metalens. Correspondingly, the device changes from achromatic to dispersive. Furthermore, a metadeflector with tunable dispersion is demonstrated to verify the universality of the proposed method. Our work may pave a way toward active metaoptics, promoting various imaging applications.
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.
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.
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.
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.