Meng-Qiang Cai

Subwavelength multiple focal spots produced by tight focusing the patterned vector optical fields

We numerically and experimentally explored generation and regulation of subwavelength multiple focal spots produced by tight focusing patterned vector optical fields (PVOFs). We presented a modified Richard-Wolf diffraction integration method suitable for the tight focusing of the PVOFs. By tailoring the spatial geometry and the polarization distributions of the PVOFs, simulations show that the diverse spatial configurations of subwavelength multiple focal spots can be achieved. To verify our idea, we experimentally generated the theoretically calculated examples of femtosecond PVOFs, then tightly focused them on the surface of the crystalline silicon wafers, and finally characterized the morphologies of modified surfaces. The SEM (scanning electronic microscopy) images confirmed that the experimental results are in good agreement with the simulations. Based on the diverse controlling degrees of freedom of PVOFs, the resultant subwavelength focal fields are flexible and powerful in parallel processing, optical manipulation and so on.

Hyperbolic-symmetry vector fields

We present and construct a new kind of orthogonal coordinate system, hyperbolic coordinate system. We present and design a new kind of local linearly polarized vector fields, which is defined as the hyperbolic-symmetry vector fields because the points with the same polarization form a series of hyperbolae. We experimentally demonstrate the generation of such a kind of hyperbolic-symmetry vector optical fields. In particular, we also study the modified hyperbolic-symmetry vector optical fields with the twofold and fourfold symmetric states of polarization when introducing the mirror symmetry. The tight focusing behaviors of these vector fields are also investigated. In addition, we also fabricate micro-structures on the K9 glass surfaces by several tightly focused (modified) hyperbolic-symmetry vector fields patterns, which demonstrate that the simulated tightly focused fields are in good agreement with the fabricated micro-structures.

Dielectric broadband meta-vector-polarizers based on nematic liquid crystal

Polarizer, as an indispensable optical element, has been widely used in various optical systems, which splits a beam into two beams of orthogonal linear polarizations. Due to the appearance of the vector optical field (VOF) and its unique properties, the vector polarizer also meets the requirement in a wide variety of applications. Here we present and demonstrate the realization of dielectric broadband vector polarizer, being in fact a liquid-crystal-based space-variant uniaxial crystal (LC-SV-UAC), which is made by a wedged thin cell adhered with space-variant photo-aligned dichroic dye films and filled with the nematic liquid crystal with a thickness of tens of microns. The vector polarizer works based on the birefringent mechanism to spatially separate the orthogonally polarized ordinary and extraordinary beams. The vector polarizers via a LC-SV-UAC have the advantages of mass-production and easy fabrication of large-size and complex structures. In particular, the high-performance broadband vector pola...

Microstructures fabricated by dynamically controlled femtosecond patterned vector optical fields

We have presented and demonstrated a method for the fabrication of various complicated microstructures based on dynamically controlled patterned vector optical fields (PVOFs). We design and generate dynamic PVOFs by loading patterned holograms displayed on the spatial light modulator and moving traces of focuses with different patterns. We experimentally fabricate the various microstructures in z-cut lithium niobate plates. The method we present has some benefits such as no motion of the fabricated samples and high efficiency due to its parallel feature. Moreover, our approach is able to fabricate three-dimensional microstructures.

Controlling optical field collapse by elliptical symmetry hybrid polarization structure

Engineering the diversity of the spatial structure of polarization states offers a new approach to produce controllable and designable filament patterns. Here we predict theoretically and demonstrate experimentally the novel collapsing behaviors of the optical fields by using the elliptical symmetry hybrid polarization structure in a self-focusing Kerr medium. The results reveal that the distribution of the hybrid polarization states, the spin angular momentum (SAM) gradient, and the collapsing patterns could be manipulated by changing the eccentricity, the topological charge, and the initial phase of the optical field. Owing to the synergy of the hybrid polarization states and its spatial symmetry, the collapsing behaviors are controllable, and have the robust feature insensitive to the random noise. Our idea may offer an alternative route to produce the controllable and robust multiple filamentation in other nonlinear systems, thereby facilitating the development of additional surprising applications.

High-efficiency and flexible generation of vector vortex optical fields by a reflective phase-only spatial light modulator

The scheme for generating vector optical fields should have not only high efficiency but also flexibility for satisfying the requirements of various applications. However, in general, high efficiency and flexibility are not compatible. Here we present and experimentally demonstrate a solution to directly, flexibly, and efficiently generate vector vortex optical fields (VVOFs) with a reflective phase-only liquid crystal spatial light modulator (LC-SLM) based on optical birefringence of liquid crystal molecules. To generate the VVOFs, this approach needs in principle only a half-wave plate, an LC-SLM, and a quarter-wave plate. This approach has some advantages, including a simple experimental setup, good flexibility, and high efficiency, making the approach very promising in some applications when higher power is need. This approach has a generation efficiency of 44.0%, which is much higher than the 1.1% of the common path interferometric approach.

Control of femtosecond multi-filamentation in glass by designable patterned optical fields

We present a scheme for realizing femtosecond multi-filamentation with designable quantity and locations of filaments, based on the control of multi-focal spots formed by patterned optical fields (POFs) composed of multiple individual optical fields (IOFs). A computer-controlled spatial light modulator is used to engineer the POFs. In particular, we introduce a blazed phase grating in any IOF, which increases a degree of freedom, making the engineering of multi-focal spots becomes more flexible. We achieve experimentally the aim controlling femtosecond multi-filamentation in a K9 glass. Our scheme has great flexibility and convenience in controlling the multi-filamentation in quantity and locations of filaments and strength of interaction between filaments.