G. Ouyang

Fabrication and characterization of polymer based spatial light modulators

We report on device properties of tunable spatial light modulators for high-resolution optical applications by a novel fabrication process. Thin polydimethylsiloxane (PDMS) films (4ìm-13ìm) were sandwiched between a flexible gold film(50nm) and a rigid substrate with a comb-like electrode either by compression molding or spin coating. By applying voltage between the upper gold film and underlying electrode, the initial plane PDMS surface changes into a form of grating. Far-field scattering pattern with high order light components was observed by illumination at the continuously reflective gold film with laser beam. Characterization was done by measuring the grating profile of the PDMS and the response time. The PDMS deformation was demonstrated to increase with driving voltage. The deformation for 6ìm thick PDMS is measured around 100nm when driving voltage is applied as 230V. Modeling and simulation of the modulator electro-mechanical behavior was done for varies structure design. The simulation results showed fair agreement with the experimental results. The response time, which defines how fast the PDMS response to the applied voltage, was measured as a function of the driving voltage. The measured rise time is around 1 micorseconds and the fall time is around 0.2 microseconds.

Polymer-based multiple diffraction modulator for speckle reduction

We report a polymer based multiple diffraction modulator, in which PDMS (polydimethylsiloxane) is utilized as the actuation material, for speckle reduction. The properties of the PDMS are characterized based on its response time and deformability, which are the key properties concerned in this work. The structure dependent properties of PDMS are discussed. Using the described technique, the PDMS satisfy the system demand. The modulator is used to create real-time diffraction patterns by dynamic gratings formed by flexible PDMS. The diffracted light passes through a diffuser, which is placed after the modulator, and induces speckle patterns on the screen. Speckle-reduction is achieved by adding the time-varying speckle patterns in the integration time of the detector. It is observed that using the modulator which has two gratings, the speckle contrast ratio reaches to 50%, which shows fair agreement with the simulation.

Laser speckle reduction using a dynamic polymer-based diffraction grating spatial phase modulator

Speckle is the constructive and destructive interference pattern observed from an optically rough surface when a highly coherent laser light is used as the illumination source. In this article, we present some methods to reduce the undesired laser speckle in laser display projector and show measurement results indicating reduction in speckle contrast obtained by using a dynamical flexible polymer-based diffraction grating phase modulator. Light is diffracted into multiple orders by the dynamic polymer diffraction grating. This diffracted light, after collection and homogenization, is used as the illumination source for displaying picture information. Due to the time-varying phase of the diffraction grating, the diffraction pattern changes thus creating time-varying speckle patterns on the rough screen. The time-varying speckle is captured by a CCD camera with appropriate integration time to mimic the averaging action of the human eye. Thus the time-integrated speckle is minimized by the intensity averaging of many independent speckle patterns. Different configurations of the diffraction grating along with random phase plate and beam combining lens were explored to ascertain the corresponding reduction in speckle contrast and its dependence on the time-period of the driving voltage of the diffraction grating.

Enhanced electro-mechanical performance of TiO 2 nano-particle modified polydimethylsiloxane (PDMS) as electroactive polymers

This paper reports a polymer nano-composite with enhanced electro-mechanical performance by mixing TiO2 nano-particles into polydimethylsiloxane (PDMS) matrix. The nano-composites with TiO2 concentration up to 30 wt% were synthesized by in-situ polymerization. High energy ball milling and surfactant polyethylene glycol was used to reduce the agglomeration and ensure a stable dispersion. The properties of nano-composites, i.e. transmittance, elastic modulus, response time and dielectric constant, can be tuned by controlling the TiO2 concentration. The nano-composites were applied in tunable gratings and showed reduced driving voltage and response time, comparing with traditional electroactive polymers (EAPs).

Simulation of laser speckle reduction by using an array of diffraction gratings

An array of diffraction gratings and a Random Phase Plate (RPP) are used to suppress laser speckle effect. Dynamic diffraction spots are generated on the surface of the RPP, after which the scattering lights are perceived by a detector. Speckle Contrast Ratio (CR) and Number of Independent Speckle Patterns (NISP) with different gratings rotation orientations (θ), gratings frequencies (grooves per millimeter: f), diameters of laser beam (D), and distances between the array of diffraction gratings and the RPP (Z) are calculated based on ZEMAX simulations, and an optimized model is proposed.

Surface and electromechanical properties of polydimethylsiloxane films prepared by a modified molding technique

In this paper, we report surface and electromechanical properties of polydimethylsiloxane (PDMS) films prepared by a modified molding technique. The film with thickness = 4 μm was deposited on oxidized silicon wafer by the modified molding technique and corresponding elastic modulus of the bulk PDMS materials is equal to 36 kPa. Surface morphology of the film was examined by optical interferometer and Scanning Electron Microscope (SEM). Its roughness is ~ 10 nm and wave-liked structures were locally observed on the film surface. In order to investigate electromechanical performance of the film, sandwich structured multilayer films Au (patterned electrodes, 150 nm) /PDMS (4 μm) /Au (top un-patterned electrode, 50 nm) were deposited on the oxidized silicon wafer. The displacement ratios of the PDMS film parallel to electric fields were characterized from 0 V/μm to 52.5 V/μm and maximum value of the displacement ratio reaches ~ 5.0 %. Meanwhile, the rising and falling response time of the PDMS films are reported as a function of applied electric fields. Since the good light reflection of top gold electrode and displacement ratio of the sandwich-structured films, the multilayer films device might be a proper candidate for the application of spatial light modulator.

Speckle reduction in laser displays using a MEMS tandem modulator

This paper reports a novel idea of using a tandem grating modulator for speckle reduction in laser displays. The modulator was designed based on finite-element method simulations, fabricated using micromachining technology, and characterized for speckle reduction. Two types of modulator were fabricated with different shape of gratings. Experiments showed that these modulators reduced the speckle contrast to 0.57 and 0.37, respectively. The results agreed with theoretical analysis. With some modification of this modulator, the speckle noise can be reduced to 0.07 according to the theory. The despeckle modulator is flexible for system implementation and can be a promising component in display system with coherent light source to restrain the speckle noise.

Surface modification and wettability of silicone PDMS film

In this work, silicone PDMS (polydimethylsiloxane) films were deposited on 100 mm silicon wafer by spin-coating technique and its surface is modified by plasma CHF3 and O2 treatment in a vacuum chamber, respectively. Wettable surface is obtained through subsequent plasma O2 treatment. Scanning Electron Microscope (SEM) and Energy dispersive x-ray spectroscopy (EDX) indicate that the plasma CHF3 treatment induces the formation of island-like nano-structures. The chemical composition of the island-like nano-structures is silicon-rich in near surface region. The surface roughness increases from 9 nm to 230 nm as plasma treatment time increases from 0 to 40 minutes. The wettability of the PDMS surface is strongly dependent on its roughness and the time of plasma O2 treatment. A typical contact angle of 5 µL water droplet after aging 25 hours at room temperature is about 35 ° on the PDMS surface with roughness 230 nm.