Ling-Jun Kong

Taming the Collapse of Optical Fields

Field collapse, which occurs in various nonlinear systems, has attracted much attention, owing to its universality, complexity, and applicability. A great challenge and expectation is to achieve the controllable and designable collapsing pattern. Here we predict theoretically and demonstrate experimentally the novel collapsing behaviors of the vector optical fields in a self-focusing Kerr medium. Surprisingly, the results reveal that the collapse of the vector optical field is controllable and designable by engineering the distribution of hybrid states of polarization, and has the robust feature insensitive to the random noise. Our idea has its significance which it opens a new window for manipulating the optical field and the different kinds of field, and then facilitates to push the related researches.

Elliptic-symmetry vector optical fields

We present in principle and demonstrate experimentally a new kind of vector fields: elliptic-symmetry vector optical fields. This is a significant development in vector fields, as this breaks the cylindrical symmetry and enriches the family of vector fields. Due to the presence of an additional degrees of freedom, which is the interval between the foci in the elliptic coordinate system, the elliptic-symmetry vector fields are more flexible than the cylindrical vector fields for controlling the spatial structure of polarization and for engineering the focusing fields. The elliptic-symmetry vector fields can find many specific applications from optical trapping to optical machining and so on.

Generalized Poincaré sphere.

We present a generalized Poincaré sphere (G sphere) and generalized Stokes parameters (G parameters), as a geometric representation, which unifies the descriptors of a variety of vector fields. Unlike the standard Poincaré sphere, the radial dimension in the G sphere is not used to describe the partially polarized field. The G sphere is constructed by extending the basic Jones vector bases to the general vector bases with the continuously changeable ellipticity (spin angular momentum, SAM) and the higher dimensional orbital angular momentum (OAM). The north and south poles of different spherical shells in the G sphere represent the pair of different orthogonal vector basis with different ellipticity (SAM) and the opposite OAM. The higher-order Poincaré spheres are just the two special spherical shells of the G sphere. We present a quite flexible scheme, which can generate all the vector fields described in the G sphere.

Security enhancement of double-random phase encryption by iterative algorithm

We propose an approach to enhance the security of optical encryption based on double-random phase encryption in a 4f system. The phase key in the input plane of the 4f system is generated by the Yang–Gu algorithm to control the phase of the encrypted information in the output plane of the 4f system, until the phase in the output plane converges to a predesigned distribution. Only the amplitude of the encrypted information must be recorded as a ciphertext. The information, which needs to be transmitted, is greatly reduced. We can decrypt the ciphertext with the aid of the predesigned phase distribution and the phase key in the Fourier plane. Our approach can resist various attacks.

An efficient and robust scheme for controlling the states of polarization in a Sagnac interferometric configuration

We present a convenient, efficient, and robust scheme for controlling the states of polarization and then generating vector fields using a closed-loop Sagnac interferometric configuration. A geometric phase introduced by the wave plates is used to control the phase shift between the two counterpropagating orthogonally linearly polarized fields. A space-variant phase plate substitutes for a spatial light modulator as a space-variant phase device. We have demonstrated experimentally that this scheme has an efficiency beyond 83% converting the input traditional linearly polarized laser into the vector fields. This scheme should also be efficient and reliable for creating the ultrashort-pulsed, high-power, and single-photon vector sources.

Young’s two-slit interference of vector light fields

We explore the peculiar interference behaviors of the vector fields in the Youngs two-slit configuration. The interference patterns have a chessboard structure in the middle region and depend on the topological charge and the initial phase of the input vector field. The results have potential applications such as characterizing the topological properties of the arbitrary vector fields.

Unveiling stability of multiple filamentation caused by axial symmetry breaking of polarization

Femtosecond laser filamentation is generally initialized from unpredictable symmetry breaking caused by random noise, causing it to be barely controlled. However, it is always anticipated for stable and controllable filamentation. We present and demonstrate the idea that hybridly polarized vector fields with axial symmetry broken polarization, associated with a pair of orthogonally linearly polarized vortices carrying the opposite-handed orbital angular momenta, could achieve controllable and robust multiple filamentation. Here, our motivation is to unveil the underlying physics behind such controllable and robust multiple filamentation. The symmetry breaking should first be actively controllable and then be able to effectively inhibit random noise. Robust multiple filamentation is inseparable from the fact that the phases between the multiple filaments are always locked. In contrast, uncontrollable multiple filamentation is always accompanied with loss of phase, i.e., the multiple filaments become incoherent to each other. Our results may offer a suggestion for achieving controllable and robust multiple filamentation in other systems.

Ghost Imaging with High Visibility Using Classical Light Source

We experimentally demonstrate a novel ghost imaging experiment utilizing a classical light source, capable of resolving objects with a high visibility. The experimental results show that our scheme can indeed realize ghost imaging with high visibility for a relatively complicated object composed of three near-ellipse-shaped holes with different dimensions. In our experiment, the largest hole is ~36 times of the smallest one in area. Each of the three holes exhibits high-visibility in excess of 80%. The high visibility and high spatial-resolution advantages of this technique could have applications in remote sensing.

Recording and reconstruction of vector fields in a Fe-doped LiNbO 3 crystal

We propose a flexible method to record and reconstruct vector fields with space-variant polarization distribution in c-cut Fe-doped LiNbO3, based on photorefractive two-wave mixing. To our knowledge, this is the first approach for the reconstruction of vector fields without using the photoinduced anisotropy of the recording material.

Spatial-Variant Geometric Phase of Hybrid-Polarized Vector Optical Fields

We investigate a novel spatial geometric phase of hybrid-polarized vector fields consisting of linear, elliptical and circular polarizations by Youngs two-slit interferometer instead of the widely used Mach–Zehnder interferometer. This spatial geometric phase can be manipulated by engineering the spatial configuration of hybrid polarizations, and is directly related to the topological charge, the local states of polarization and the rotational symmetry of hybrid-polarized vector optical fields. The unique feature of geometric phase has implications in quantum information science as well as other physical systems such as electron vortex beams.