The reflection efficiency of material surface can be reduced by fabricated sub-micron periodic structures. Part of the light energy will propagate along the surface of the material as guided wave, thereby, the interaction between material and light is increased and the light absorption efficiency in visible light stealth material is improved. In this paper, two-dimensional (2D) relief periodic structure with 300nm was fabricated by holographic lithography. Test results show that the reflection efficiency of the material surface can be reduce after fabricated 2D periodic structures. However, because of the presence of diffraction orders, the zero order transmission diffraction efficiency is reduced in short wavelength band. Through rational design of the duty cycle and etching depth, the diffraction efficiency of reflection can be reduced, and then, the optical coupling efficiency of the material can be improved and the visible light stealth properties of the material can be improved too.
Optical thin film can be used for invisible cloak. As a kind of low-dimension photonic crystal, it is a candidate for metamaterial
with designed Σ and μ. As a coating, it is convenient to be stacked to mimic continuous changing of electromagnetic
media. Anti-reflection film is suitable for matching coating between layers of media.
According to the holographic theory, the influence factors in fabricating photonic crystal templates in photoresist such as the arrangement of interference beam, exposure and developing process and the polarization direction adjustment were discussed in this paper. The fabrication process was simulated by computer program and the optimization parameters of fabricating three-dimensional photonic crystal templates in photoresist were presented.
According to computer-generate holographic theory, a diffraction optics element used for holographic sight is designed
by modified off-axis reference beam coding method. This diffraction optical element is composed by an off-axis Fresnel
holographic lens. The spherical wave light from LD is parallel by this element. The light filter and compensation
function of diffraction grating can also be replaced by this element. The fabrication of this diffraction optics element is
not limited by the recording optic system. Mirrors and lenses are replaced by this element so that the reproducing optical
path is simplified and the stability of the device is improved. A matlab program is used to simulate the hologram.
Theoretical simulation results show that the precision of sight can be improved with the virtue of low aberration and high
matching to holographic sighting window.
Rigorous coupled-wave analysis is used to calculate the diffraction efficiency of THZ spectrum gratings. The ratios of
grating periods and wavelengths and the ratios of grating depths and wavelengths of different type gratings are analyzed
in this paper. Calculated results show that if the ratios of grating periods and wavelengths approximately equal to 1 and
the grating depths greater than1.5, high diffractive efficiency of TE and TM polarization of symmetrical shape gratings
will be got.
In this paper, the technique recently proposed by the authors for the detection of structural local damage in large size
linear structures is extended to explore the detection of local damage in some complex nonlinear structures. The
technique is based on substructural approach in which a complex nonlinear structure is decomposed into substructures
Interaction effect between adjacent substructures is accounted by considering the interaction forces at substructural
interfaces as the 'unknown inputs' to the substructures. An algorithm utilizing the classical Kalman extended estimator
and the recursive least squares estimation for the unknown inputs is proposed to identify structural parameters at element
level and the 'unknown inputs' to the substructure. Two cases that measurements at the substructure interfaces are
available or not available are considered. Structural local damage is estimated from the change of structural parameters,
such as the degradation of the stiffness, at element level. The technique enables distributed identification of local damage
in complex nonlinear structures utilizing only a limited number of measured acceleration responses. Performance of
proposed technique is illustrated by a numerical example of detecting local damage in a multi-story hysteretic building. It
is shown that the proposed technique can be used for detecting structural local damage in some complex nonlinear
structures.
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.
A simple optical system for generating large-size hexagonal lattice used as the mask for photonic crystal LED
fabrication is proposed. The key element in the system is a holographic diffractive optical element consisting of three
phase gratings made with holographic means. Under the illumination of a single plane wave, three plane waves can be
generated and interfere to form a uniform hexagonal pattern. Theoretical analysis demonstrates that equal intensity of
the three plane waves can be obtained if groove depth of the gratings is about 400 nm, which can be achieved by
controlling the development time in grating processing. Mask on LED with lattice area up to 15 cm2 has been obtained
with the method.
A technique for fabricating optimized holographic diffractive optical element (HDOE) used for fabricating photonic crystals is proposed in this paper. The HDOE consists of three identical transmission gratings symmetrically placed around the center with 120° between each other. Usually the polarization of the diffracted beam of a grating will be changed from that of the incident beam depending on the orientation of the grating. As the orientations of the three gratings in a HDOE are different, the polarizations of the diffracted beams and their diffraction efficiencies are different, resulting in low contrast of the interference pattern. This problem can be solved by controlling the parameters in grating fabrication. Rigorous coupled-wave analysis is used to calculate the influence of grating depth and polarization angle on diffraction efficiency and polarization of diffracted beams. Calculated results demonstrate that under 458 nm illumination, if the three sinusoidal gratings in a HDOE have 0.729 μm period and 0.33 μm depth, the polarization and diffraction efficiencies of the three first-order diffracted beams will be equal. The experimental results are presented to prove the practicability of the technique.
A simple holographic 3D photonic crystal fabrication system is presented. The key element in the system is a HDOE made with holographic approach. The HDOE can generate four beams with single illuminating beam to form a 3D interference pattern. The incident angle of the outer beams and beam intensity ratio of the central beam to the outer beam can be adjusted in HDOE fabrication process. Computer simulation demonstrates that to obtain properly connected lattice structure in photoresist, beam intensity ratio and exposure dosage must be considered simultaneously. Experimental result has verified the effectiveness of the 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.
Photonic crystal template can be fabricated by holographic lithography in photoresist. A method for enhancing the quality of holographic photonic crystal template is mentioned in this paper. Because light energy is absorbed by photoresist while propagating through the material, the exposure decreases with the depth of the photoresist layer. In order to compensate for the light energy losing in exposure process, the recording plate is flipped to let the interference beams incident on the side of glass substrate instead of the surface of the material. During the development, the etching is proceeding quickly into the lower part of the material while the structure on the surface is maintained. Experimentally, face centered cubic lattice structures were fabricated in the AZ-4620 positive photoresist using an Ar+ laser with wavelength of 458 nm. Good 3D lattice structures have been obtained using this method. Theoretical analysis, computer simulation and experimental results obtained with the interference beams incident on different sides of the photoresist plate are presented in the paper.
A novel holographic technique for fabricating three-dimensional photonic crystals (PhCs) by two-beam interference is presented in this paper. The optical setup in this method is much simpler and more flexible compared with other multi-beam interference methods, and large and uniform PhCs are easier to be obtained. In PhCs' fabrication, two coherent laser beams interfere and generate a set of two-dimensional interference fringes, which are recorded on a plate of photosensitive material. One laser beam is incident on the plate in normal direction and the other beam with an angle to the normal. Then with the laser beams maintained in a fixed relative position, the plate is rotated by 120 degrees about an axis through the center of the plate and a second recording is made. This procedure is repeated one more time, producing finally a superposition of three sets of interference fringes at angles of 120 degree to each other. After the chemical treatment, a three-dimensional PhC is fabricated in the material with a particular lattice structure that depends on the detail of the basic interference pattern. PhC with fcc lattice structure was fabricated in the experiment with the angle of 38.9 degrees between the two interference beams, verifying the effectiveness of the technique.
The parameters used for recording 3D rainbow holograms are analyzed theoretically. The analysis indicates that the object distance in recording Fresnel hologram is a key parameter for diminishing field curvature and image blur due to wavelength spread and extended source size. To obtain sharp image under extended illumination, the object distance must be reasonably large. The experimental results verify the theoretical analysis.
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