Chenghou Tu

Two-dimensional microstructures induced by femtosecond vector light fields on silicon

We have fabricated the complicated two-dimensional subwave-length microstructures induced by the femtosecond vector light fields on silicon. The fabricated microstructures have the interval between two ripples in microstructures to be around 670-690 nm and the depth of the grooves to be about 300 nm when the pulse fluence of 0.26 J/cm2 is slightly higher than the ablated threshold of 0.2 J/cm2 for silicon under the irradiation of 100 pulses. The ripples are always perpendicular to the direction of the locally linear polarization. The designable spatial structure of polarization of the femtosecond vector light field can be used to manipulate the fabricated microstructure.

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.

Femtosecond Laser Processing by Using Patterned Vector Optical Fields

We present and demonstrate an approach for femtosecond laser processing by using patterned vector optical fields (PVOFs) composed of multiple individual vector optical fields. The PVOFs can be flexibly engineered due to the diversity of individual vector optical fields in spatial arrangement and distribution of states of polarization, and it is easily created with the aid of a spatial light modulator. The focused PVOFs will certainly result in various interference patterns, which are then used to fabricate multi-microholes with various patterns on silicon. The present approach can be expanded to fabricate three-dimensional microstructures based on two-photon polymerization.

Second harmonic generation of periodically poled potassium titanyl phosphate waveguide using femtosecond laser pulses

The authors have presented in this paper the fabrication and characterization of double line written type waveguides in c-cut periodically poled potassium titanyl phosphate crystals. The waveguides were fabricated by using a femtosecond laser, and were utilized for second harmonic generation at 1064 nm. Our experiments have shown that single mode propagation was observed at optimal waveguide width of 14.5 microm. And a conversion efficiency of 39.6% can be achieved.

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.

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.