Label-free biosensor based on the dynamic distributed feedback (DFB) laser emission is proposed. The sensitivity of the
sensor is substantially realized by the refractive index of the cladding layer that influences the effective refractive index
(neff) of the whole waveguide structure. The designed multilayer structure consists of the substrate (n=1.51), the low
refractive index mesoporous silica film (n=1.1~1.3), the dye-doped gain layer (n=1.58) and the high refractive index
TiO2 (n=2.1) thin film. We used the finite difference time domain (FDTD) algorithm to simulate the influence of the
mesoporous silica film and the TiO2 layer to the neff of the fundamental transverse electric (TE0) mode and the sensitivity
in the DFB structure. It was found that the increase of refractive index of the mesoporous silica and the thickness of TiO2
layer can slightly increase the neff of the structure. And the sensitivity of the sensor can be enhanced not only by the introduction of mesoporous silica, but also by a thicker TiO2 layer.
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.
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.
Multiplexer/demultiplexer for WDM system based on off-axis holographic Fresnel lens was fabricated. The principle and fabricating technology of the demultiplexer with the central working wavelength of 1310 nm and the channel spacing of 25 nm were described. 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=-λC/λO is applied in geometry design and long object distance is selected. The experiment method and results were presented in this paper, and results verified the theoretical analysis.
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.
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.
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.
The novel method of using holographic optical element (HOE) to realize optical clock distribution for optical interconnection is presented. Polarization holographic technique is adopted to fabricate the HOEs. 4-fan-out clock distribution has been obtained. Methods for obtaining multiple fan-out clock distribution using polarization holographic technique is proposed.
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