JaChing Wang

Real-time holography for optical processing using photorefractive crystals

In this paper, we describe different schemes of phase conjugation during formation of the self-organized hexagonal spot array in the photorefractive crystal KNbO3. Both forward and backward phase-conjugated beams are observed in the two-beam coupling geometry. We present also theoretical description of this multiplexed phase conjugation, introducing concept of the scattering wave-vector diagrams that visualize contributions from the transmission and reflection gratings.

Investigations of charge transfer and photosensitivity in single- and double-doped LiNbO 3 single crystals: an optical-electron paramagnetic resonance study: I

Electron paramagnetic resonance (EPR) for specially doped LiNbO3 crystals was determined. Study of the charge transfer and the photosensitivity in these crystals determined the ions, which participate on the charge transfer processes. In doubly doped crystals with Fe:Cr and Fe:Ce the decay of the EPR signal was less than the decay time in LiNbO3:Fe:Mn. some inherited impurities in these crystals like Mn and Fe were detected. The effect of the inherited impurities on the decay and growth of the EPR signal was estimated by comparing the results with homegrown LiNbO3 crystals.

Novel two-photon optical limiting materials

A detailed study was conducted for two-photon absorption of a selected group of new derivatives of cyclohexanone/piperidone with regard to their molecular structure and the feasibility of using these derivatives as nonlinear optical absorbers for optical limiting. Derivatives of cyclohexanone/piperidone have a cross-section of two-photon absorption of the order of 3000 x 10-50cm4s/photon and compare well against known two-photon absorbers. This makes them promising nonlinear optical absorbers and optical limiters. Their important feature is that the energy is absorbed in the infrared region, where the protection is needed for remote IR sensors and night-vision systems. It is also important that the energy of the dissipated infrared radiation is converted mostly into fluorescence but not into heat that can damage optical limiter.

Visualization of refractive index modulation by optical channeling

We discuss manifestation of the channeling effect in the visualization of hexagonal patterns in KNbO3 and phase gratings in LiNbO3. We have shown that visualization of the domain patterns, discovered earlier in the subharmonic beam in the photorefractive BSO crystal, may be explained by the channeling effect. A wave-packet description allows us to explain backward movement of domains due to the specific dispersion law for the space-charge waves. An analogy is discussed with domain formation in the charge-density-waves, known in quasi-1D conductors. Non-linear phase shift of the couples waves may lead to discomensuration.

Optical Channeling For Radial Holographic Grating Recording In Chalcogenide Glassy Semiconductors Films and Photo-thermo-plastic films

We discuss phenomena of the optical photons and charged particle channeling in the periodic structures. While particle (as protons) channeling is widely used for the characterization of defects in crystals, channeling of photons is less known. We have demonstrated feasibility of optical channeling method for copying of phase radial grating on the chalcogenide semiconductor glass film and photo-thermoplastic films (PTPF). Chalcogenide glassy semiconductors (CGS) as a medium for recording of optical information have some advantages such as the possibility of achieving a higher resolution power, stability, and a high photosensitivity. We report about recording of the radial phase grating in the doped As-S-Se (CGS). Radial grating was recorded by making copy from the master phase grating placed in the near-field zone and exposure to the CW green (λ=532 nm) low power (P=100 mW) solid-state laser or incoherent UV source. The exposure time has been varied from 15 to 30 min. The recording process could be explained by optical channeling. This phenomenon gives us an opportunity to create phase radial grating using coherent and incoherent illumination.

Electro-optic modulators based on high-temperature polyimides

We report on the synthesis of high-temperature electro-optic polyimides and the design of the electro-optic modulators based on these materials. Recently developed electro-optic polyimide has a glass transition temperature of 461°C. It can be processed in a thin light-guiding film form a precursor at a temperature of 160°C. DIfferent novel designs of electro-optic modulators are being considered. A single-arm double-mode interferometer configuration is proposed for a thin-film transverse electro-optic waveguide modulator with low driving voltage. The driving voltage could be less than 5V that makes it compatible with conventional silicon-based electronics. We also consider miniature longitudinal modulators integrated with fiber optics in Fabry-Perot configuration. The advantage of the approach is the use of a small size area of the electro-optic film where high optical quality cna be easily achieved. The approach is being testd for polyimides as well as for single-crystal organic films wtih high electro-optic coefficients.

Development of holographic optical elements based on LiNbO3 photorefractive crystals and As-S-Se chalcogenide films

We have studied holographic grating recording in Fe-doped photorefractive crystals LiNbO3 (LN) and in doped As-S- Se chalcogenide glassy semiconductor (CGS) films. Transmission gratings in (alpha) -cut crystals of LN are efficient enough to demonstrate effect of optical channeling. Volume gratings recorded in LN crystals may be used as a parallel array of the planar waveguides. For the CGS slanted grating geometry was tested, aiming on creation of asymmetric blazed gratings. Asymmetry of non-Bragg diffraction orders was observed and its was 12 times enhanced by a gold coating. For the first time reflection volume grating were recorded in 2 mm thick CGS by green solid state laser ((lambda) equals 532 nm, P equals 100 mW).

Fabrication of MEMS using liquid jet dispensing technique

We describe a simple yet efficient technique of delineation of various polymer MEMS structures using computer driven liquid jet dispenser. A syringe-type dispenser with a replaceable needle is filled with a polymer solution and mounted on a vertical translation stage. The substrate is mounted on two-coordinate horizontal translation stage. Additional vertical translation arm is connected to the plunger of the syringe. Vertical translation provides positioning of the needle at a certain height over the substrate. Delineation is accomplished when the syringe plunger, being pushed down by the vertical translation arm, dispenses the liquid on the substrate at a programmable flow rate simultaneously with horizontal motion of the substrate with respect to the needle. Horizontal motion is done along a programmable trajectory at a programmable sped. Comparing to conventional methods, the proposed device consumes three order of magnitude less amount of material for the fabrication of similar MEMS structures. In addition, we describe a supplement to the liquid delineation system in the form of a microscope objective head connected to a noncoherent UV source with a multimode optical fiber. The head can serve as a tool for UV curing of the polymer material deposited with the jet dispenser as well as a separate UV writing tool. In conclusion, we describe the calculation technique for the distribution of the electric field in electrically or optically controlled ferroelectric polymer MEMS fabricated by the proposed technique.

Charge transfer and the photosensitivity in single- and double-doped LiNbO3 single crystals; an optical-electron paramagnetic resonance study: II

Transition metals in particular increase the photorefractive sensitivity. We have reported in part I of the observed enhancement for the nonlinearity of these crystals due to the single and doubly dopants. In this part, we tried to answer some of the questions about the enhancement of the photorefractive sensitivity due to gamma irradiation. The crystals were irradiated by gamma ray and the EPR spectra were obtained. A new EPR center was observed at 3420G in Mn:Fe:LiNbO3 crystals. The Degenerate four wave mixing experiment was performed for these crystals and we have observed that the intensity of the PC signal from Ni:LiNbO3 crystal was increased by almost 30 percent. By applying the Ar+ laser in situ the EPR signal was decreased and the center was bleached after almost nine minutes in the case of LiNbO3:Ni and four minutes in the case of LiNbO3:Fe:Mn crystal. An interpretation for this phenomenon will be given on the light of Kuktarev model.

Analysis of twelve specially doped lithium niobate crystals

Twelve lithium niobate crystals doped with Ce, Co, Cr, Cu, Fe, Mn, Ni, Rh, Tb, Fe:Ce, Fe:Cr, and Fe:Mn have been grown and tested. The transmission spectra and dark conductivity of the crystals are presented. Holograms have been written in each of the crystals with wavelengths from 457 nm to 671 nm. The quality and stability of the holograms vary dramatically among the crystals. The properties of the holograms and a comparison of sensitivities between the crystals are discussed.