Xiangsu Zhang

Artistic effect and application of moiré patterns in security holograms

Using a special holographic technique, moiré patterns can be recorded with a horizontal parallax, thus producing a fascinating artistic effect. With special characteristics, holographic moiré patterns can be used as secret codes in security holograms for different security purposes. Aremarkable anticounterfeit effect has been achieved by using this kind of security hologram to protect famous high-quality products. The technique used for recording moiré patterns is described, and sample holograms are shown.

Fabrication of three-dimensional photonic crystals with two-beam holographic lithography

A holographic technique used to fabricate three-dimensional photonic crystals with a two-beam interference method is presented. In the optical setup of fabrication one beam is incident on the recording plate in the direction of the plate normal and the other beam with an angle to the normal. Three exposures were taken. Between each exposure, the recording plate was rotated 120 degrees on axis until three exposures were completed. Good three-dimensional lattice structures have been obtained. Theoretical analysis, computer simulations, and experimental results are presented.

Fabrication of off-axis holographic Fresnel lens used as multiplexer/demultiplexer in optical communications

Off-axis holographic Fresnel lenses were designed and fabricated, with the aim of using them as multiplexer/demultiplexer in optical networks. The reduction of third-order aberrations was the main consideration. The theoretical analysis indicates that third-order aberration can be effectively reduced if the condition tanαC/tanαR = -λ2/λ1 is applied in geometry design and long object distance is selected. The experimental results verify the theoretical analysis.

An Adaptive Secure Watermarking Scheme for Images in Spatial Domain Using Fresnel Transform

A new adaptive blind watermarking scheme with the watermarks encrypted by Fresnel transform in spatial domain is presented. By utilizing fuzzy c-means clustering (FCM) technique and human visual system (HVS), the watermark can be adaptively embedded according to block classification. To keep imperceptibility and robustness, a novel iterative embedding algorithm is adopted to change the to-be-embedded pixel-values. The experimental results demonstrate that the proposed scheme provides good robustness to withstand different kinds of common attacks.

Adaptive watermarking scheme using a gray-level computer generated hologram

We present a new adaptive blind watermarking scheme using a gray-level computer generated hologram in the discrete wavelet transform domain. By utilizing an improved fuzzy clustering technique and human visual system , the watermark can be adaptively embedded according to block classification. To keep imperceptibility and robustness, a novel iterative embedding algorithm is adopted to change the to-be-embedded coefficients. Compared with the standard Fuzzy c-means (FCM) clustering, the suggested improved FCM (IFCM) converges more quickly and can avoid local optimum effectively. The experimental results demonstrate that the proposed scheme provides good robustness to withstand different kinds of common attack. Compared with other methods, the proposed method has the distinct advantage of better robustness to a JPEG compression attack.

Fabrication of holographic Fresnel lens used as a solar concentrator

One of the more intriguing of solar collection techniques, holographic concentration, has been addressed in many ways, but it has so far met with only limited success. The major attraction of holography is that unlike conventional concentrators, the holographic concentrator can track the sun across the sky while remaining completely stationary. A simple system using holographic Fresnel lenses (HFLs) was proposed in this paper. We present the theoretical analysis and the technique for fabricating HFLs used for solar concentrator. Several HFLs fabricated using corresponding recording geometries, resulted in a combined device that collects light at any angle, with each lens responsible for gathering sunlight for a certain portion of the day. Three HFLs with different fringe pattern were achieved, which operates in the morning, at noon and in the afternoon, respectively. The primary experimental results of the concentrator were presented, which effectively concentrates the visible spectrum over a 60 degrees daily angular variation. Several advantages of the holographic concentrator we obtained include longer lifetime, low cost, and the small size of a flat piece of film compared to a mirror or lens. The dispersive characteristics applied to photovoltaic power generation would offer a novel type of generator with high conversion efficiencies.

One-in-hundreds-out laser splitting generated by diffraction of single holographic optical element

A technique using a holographic optical element to split one incident laser beam into hundreds is proposed. The holographic optical element is fabricated with hexagonal packed lattice structure using 4-beam interference method. When the element is illuminated by a single laser beam with normal incidence, hundreds of beams are generated by diffraction. The element has the potential to be used as the device for interconnection and clock distribution in optical and electronic systems.

Fabrication of 2D photonic crystals with micron to sub-micron hexagonal lattices using single-exposure holographic technique

A single-exposure holographic technique for fabricating 2-dimensional photonic crystals (PCs) with hexagonal lattices is presented. A specially made holographic optical element (HOE), which consists of three holographic gratings, is used to generate three interference beams simultaneously under the illumination of a single laser beam. Theoretical analysis indicates that by selecting appropriate grating period and illuminating wavelength, the hexagonal lattice patterns formed by the interference of the three beams can have the lattice constants in the range from micron to sub-micron. Moreover, to obtain uniform lattice pattern with high contrast, the gratings in the HOE must have proper grating shape and depth. In the experiment, hexagonal lattices with lattice constants from 5 μm to 620 nm were fabricated in photoresist using the method. The patterned photoresist layers can be used as the masks to generate PCs in semiconductor materials through dry or wet etchings.

Fabrication of blazed holographic gratings with two-step technique

A two-step technique of fabricating blazed holographic grating is presented. The grating fabrication includes two exposures and two developments. In the first step, two collimated laser beams with symmetrical incidence about the normal of the holographic plate generate a grating with quasi-sinusoidal groove profile after the first development. In the second step only one collimated laser beam irradiates on the plate with a selected incident angle, resulting in a change of groove profile into quasi-triangular after the second development. A blazed holographic grating with 65% relative diffraction efficiency under non-auto-collimation was obtained in the experiment. The scanning electron micrographs clearly show the quasi-triangular groove profile of the grating. The factors that affect the groove profile are discussed.

Influence of fabrication parameters on groove profile of blazed gratings made with two-step holographic technique

The two-step holographic technique of fabricating blazed gratings has been successfully applied to common positive photoresist. Influence of three fabrication parameters in the second step on groove profile of blazed gratings was studied experimentally. The three fabrication parameters are exposure, incident angle of laser beam and development time. The exposure distribution on the cross-section of one groove stripe of grating was modeled. Results of experiment and modeling with various exposures reveal that the technique is effective only when the symmetrical contour lines of exposure in the first step fade out with increasing second exposure. With other parameters fixed, experiments were also carried out with the beam incident angle varying from 10° to 80° and with the development time varying from 1 s to 9 s. Blazed gratings with quasi-triangular are obtained when incident angle reaches 40° and when development time reaches 5 s. Scanning electron micrographs of the results are presented. Blazed gratings with observed blaze angles of 20° to 50° were obtained by changing the incident angle. Efficiencies of different diffraction orders for gratings with various incident angles are also given.