Jenq-Yang Chang

Enhanced light trapping based on guided mode resonance effect for thin-film silicon solar cells with two filling-factor gratings

An approach of enhanced light-trapping in a thin-film silicon solar cell by adding a two-filling-factor asymmetric binary grating on it is proposed for the wavelength of near-infrared. Such a grating-on-thin-film structure forms a guided-mode resonance notch filter to couple energy diffracted from an incident wave to a leakage mode of the guided layer in the solar cell. The resonance wave coupled between two-filling-factor gratings would laterally extend the optical power and induce multiple bounces within the active layer. The resonance effect traps light in the cell enhancing its absorption probability. A dynamic light-trapping behaviour in solar cells is observed. A photon dwelling time is proposed for the first time to quantify the light-trapping effect. Moreover, the light absorption probability is also quantified. As compared the grating-on-thin-film structure with the one of planar silicon thin film, simulation results reveal that it is 3-fold enhancement in the light absorption within a spectral range of 920-1040 nm. Moreover, such an enhancement can be maintained even the incident angle of near-IR broadband light wave varies up to +/-40 degrees.

Homogenized LED-illumination using microlens arrays for a pocket-sized projector.

We present an LED-based ultra-mini DMD projector with a size of 75 mm x 67 mm x 42 mm. A compact homogenizer consisting of a double-side microlens array and two condensers was proposed to reduce the size of the pocket-sized projector. The homogenizer not only allowed for a reduction in the total track length of the system, but also reduced the angle of the rays emitted from the LED with the micro field lens array. The double-side well-aligned 124 x 146 microlens array was fabricated using backside alignment and hot embossing techniques. The microlens array was square-arranged and the fill-factor was extremely high. The uniformity and total throughput of this projector were higher than those of the current pocket-sized projectors. Moreover, the optical performances of the projector such as color difference and the LED alignment tolerance were also measured and discussed.

High efficiency pocket-size projector with a compact projection lens and a light emitting diode-based light source system

We present a light emitting diode (LED)-based ultramini digital micromirror device projector with a size of 75 mm x 67 mm x 42 mm and a weight of 338 g. The LED illuminator inside this projector makes it possible to achieve a volume of 18 cm(3) by using a dichroic filter and a collimating lens. The illumination system consists of high uniformity of 93% through a microlens array as a homogenizer. A total internal reflection prism is also used to reduce the size of both the illumination system and the telecentric projection lens. A projection lens system with an ultrasmall track of 42 mm, including a high modulation transfer function value of 0.4 at 46.2 line pairs/mm, an optical distortion of only 0.25 %, and a television distortion of 0.01%, is designed. Through the above superior specification, we can produce a 20 in. (51 cm) color display comparable in brightness to a laptop with a contrast of 3700:1. The device is compact and suitable for personal use.

Laser emission from GaN photonic crystals

In this study, photonic crystals have been designed, fabricated, and characterized in GaN bulk materials. The energy dependent measurement showed that the emission peak width can be significantly reduced as the pumping pulse energy was larger than 0.7μJ at room temperature. The mode at the wavelength of 371nm emitted from the defect due to the structure disorder unintentionally introduced during the fabrication process of the GaN photonic crystals can be obtained.

Bulk-micromachined optical filter based on guided-mode resonance in silicon-nitride membrane

In this paper, a single-layer guided-mode resonance (GMR) filter based on a free-standing silicon-nitride membrane suspended on a silicon substrate is achieved by using bulk-micromachining technology. Both of grating and waveguide structures without a lower-cladding layer, i.e., substrate, are fabricated simultaneously on a silicon-nitride membrane. The device can be used as a transmission bandstop filter with the advantages of simple structure, high efficiency, and feasibility to integrate with other optoelectronic elements into a microsystem chip. The design consideration, fabrication procedures, and measured spectral response are shown in this paper. Moreover, by stacking two proposed devices, /spl Delta//spl lambda/ of the stopband at a transmission below 10% is 5.06 nm.

Portable digital micromirror device projector using a prism

A newly designed ultrasmall total internal reflection prism with a size of 29 mm x 22 mm x 24 mm and weight of 19.5 g is proposed for use in a pocket-sized Digital Micromirror Device projector. The entire projector, including an arc lamp illumination, relay, and projection system, has a height of 48 mm and a footprint of 80 mm x 132 mm. By using an overdriving f/2.0 projection lens, the geometric efficiency of the projection system, eta(geo-pro), can be enhanced from 80% to 92%. Although, at the same time, the contrast decreased from 1200:1 to 500:1, this can be enhanced using an off-axis stop. By tuning the position of the stop, the contrast can be as high as 3700:1 for a eta(geo-pro) equal to 90%. Using what we believe to be a novel prism design, we can get a very compact optical system with a high efficiency and good contrast ratio.

Silicon-based transmissive diffractive optical element

A silicon nitride (SiNx) membrane diffractive optical element (DOE) designed to exhibit beam-splitting and focusing behavior at visible wavelengths has been fabricated and tested. Since the fabrication process is based on silicon micromachining technology, the DOE is easily integrated with a laser diode chip and a photodiode chip on a silicon substrate to function as the hologram-laser-photodiode unit for use in the pickup head of a CD or DVD system. The SiNx film is deposited with low-pressure chemical-vapor deposition and the free-standing membrane is formed by KOH etching. The transmissive DOE showed a high diffraction efficiency (>20% for a binary-phase-level element). The experimental evaluation was in good agreement with the designed and modeled predictions.

InGaN gallium nitride light-emitting diodes with reflective electrode pads and textured gallium-doped ZnO contact layer

We demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts and textured Ga-doped ZnO (GZO) contact layer to serve as the n- and p-type electrode pads, respectively. Compared with the conventional LEDs with flat surface and Cr/Au metal contacts, the nonalloyed Ag/Cr/Au contacts used in the present experimental LEDs play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads. Enhancement of light output power observed from the experimental LEDs is also due to the textured GZO layer that can disperse the angular distribution of photons at the GZO/air interface. With an injection current of 20 mA, the output power of experimental LEDs can be improved markedly by a magnitude of 30% compared with conventional GaN-based LEDs.

Spatial distribution of absorption in plasmonic thin film solar cells

The spatial dependence of absorption in a structured thin film solar cell is investigated through the rigorous coupled-wave analysis method. The investigated structure allows strong localized surface plasmon and surface plasmon polaritons, simultaneously. The absorptance of silver and amorphous silicon can be separately accounted for by calculating the time-averaged energy dissipation although only the absorption of amorphous silicon contributes to the photocurrent. In our studied case, the metallic material absorbs around 15%-20% of the total impinging sunlight while the active layer absorbs only approximately 50%.

Realization of free-space optical pickup head with stacked Si-based phase elements

We have developed a novel stacked silicon-based microoptical system, which is optical-on-axis and transmissible in both visible and infrared ranges. By using the new microoptical system techniques, we fabricated a miniaturized optical pickup head module. This optical pickup head consisted of a 650-nm laser diode, a 45/spl deg/ silicon reflector, a grating, a holographic optical element, and some aspherical Fresnel lenses. These optical phase elements fabricated on a SiN/sub x/ membrane were suspended on Si chips. Each element was then stacked by chip bonding. We could obtain a circular focusing spot on the optical disc as small as 3.1 /spl mu/m.