Ya-Lun Tsai

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.

Enhancing the resonance quality factor in membrane-type resonant grating waveguides

In this Letter, we present a method of reducing the spectral width of guided-mode resonance (GMR) in air-bridged resonant grating-waveguide structures to enhance the Q factor. The posttreatment of adding a dielectric film to the bottom of the membrane to manipulate the resonance behavior is practicable. The introduced underlayer is shown to be capable of effectively reducing the coupling and enhancing the resonant Q factor. The proposed method provides an effective means of adjusting the resonance property without varying the original GMR structure. The results also imply that TM resonance is more feasible for achieving narrow resonance and potentially in sensing applications, because it has higher sensitivity than TE resonance.

Sensitive metal layer assisted guided mode resonance biosensor with a spectrum inversed response and strong asymmetric resonance field distribution

In this paper, a metal layer assisted guide mode resonance (MaGMR) device with high sensitivity is proposed for bioanalytical applications and its functioning is experimentally proved. We find that the reflection spectra present a unique inversed response. The resonance mechanism is also discussed. Numerical calculation results indicate that the high sensitivity performance of MaGMR comes from the strongly asymmetric resonance modal profile and low propagation angle inside the waveguide. There is a one-fold enhancement of the evanescent wave in the analytes region compared to typical GMR. According to the experimental results, the proposed MaGMR achieved a bulk sensitivity of 376.78 nm/RIU in fundamental TM mode resonating at 0.809 μm with the first diffraction angle. Experiment results show a 264.78% enhancement in the sensitivity compared to that of the typical GMR sensor in the same resonance conditions of TM mode.

Propagation loss reduction of photonic crystal slab waveguides by microspheres.

Dielectric microspheres are theoretically studied to reduce the propagation loss of Si-based photonic crystal slab waveguides. Two-dimensional photonic crystal formed by etched air hole can act as a template for microsphere sedimentation. The analytical results show that the transmission of the photonic crystal slab waveguides with microspheres can be enhanced to be around twice that without microspheres.

Photonic Crystal Cavity With Double Heterostructure in GaN Bulk

In this study, the photonic crystal cavity has been designed, fabricated, and characterized in GaN bulk materials with the double heterostructure, which can provide high Q-factor. The cavity is characterized by optical pumping. The resonant mode is observed at the wavelength of 362 nm. The threshold of excitation power is found to be 0.9 mW, corresponding to the power density of 12.7 kW/ cm2 . The Q-factor of the cavity is measured to be as high as 104.

Optical confinement using a doughnut waveguide

This paper examines the photonic band gap of periodically arranged doughnuts (torus). Using the proposed structure as a hollow waveguide, it is possible to obtain the complete bandgap and optical confinement. The diameter of the outputted beam from the waveguide can be the same order of magnitude as the wavelength.

Silicon-based micro and subwavelength optical elements and applications

In this paper, silicon-based micro and subwavelength optical elements based on a free-standing silicon nitride (SiNx) membrane are achieved. These elements, including gratings, microlenses, and holographic optical elements (HOEs), are designed and used within the visible and infrared regions. These devices can be used as collimators, reflectors, and wavelength-dependent filters with advantages of simple structure, high efficiency and feasibility to integrate with other elements into a micro-system chip. In order to demonstrate the advantage of micro-optics of free-standing SiNx membrane type in integration, a miniaturized optical pickup head module based on a stacked micro-optical system is developed. This module consisted of a laser diode, a reflector, a grating, a holographic optical element, and some aspherical Fresnel lenses. The novel microoptical system can overcome the problems encountered in other microoptical systems such as off-axis aberration, lower optical efficiency or durability, integration and even in fabrication. A focal spot with a FWHM diameter of 3.3 μm is obtained while the diffraction limited full-width at half-maximum (FWHM) is 0.7 μm. To extend the advantage of micro-optics of free-standing SiNx membrane, the subwavelength optical elements base on guided-mode resonance is also developed. With various Si-based structures, the filter possesses numerous properties such as variable bandwidths, low sideband, flattop, and etc. They are also applied as biosensors to detect the hybridization process of bio reaction for their high sensitivity. The results show that micro and subwavelength optical elements fabricated on Si-based material will be a candidate for emerging silicon micro-photonics.

Genetic algorithms optimization of photonic crystal fibers for half diffraction angle reduction of output beam

In this work, we optimize the structure of the photonic crystal fibers by using genetic algorithms to provide strong light confinement in fiber and small half diffraction angle of output beam. Furthermore, this article shows the potentials of this study, such as optimizing three purposes at the same time and the arbitrary structure design is achieved. We report two optimized results obtained by different optimization conditions. The results show that the half diffraction angle of the output beam of the photonic crystal fibers can be reduced.

Guided-mode resonance device constructed with membrane structure: theoretical analysis and experimental demonstration

In this paper, the guide-mode resonance (GMR) devices based on a suspended membrane structure is designed and experimentally demonstrated. The presented membrane structure possesses a simple structure for resonance excitation and is capable of improving the spectral response. The results of resonance excitation, improving the sideband and low oscillatory spectrum are presented. Due to the utilization of silicon-based materials, the proposed filter is also potential candidates to be integrated with other optoelectronic devices for further applications.

Effective medium based on two-dimensional photonic crystals for index-confinement waveguide application

In this paper, the design of effective microprism based on the subwavelength periodic lattices is proposed. The microprism is realized by using a two-dimensional photonic crystal (PhC) structure with a periodic lattice of air-holes. In order to behave as a homogeneous and isotropic microprism, the PhC structure with a hexagonal lattice should be operated in the low frequency. By monolithically integrating the effective microprism in the bending area of an optical waveguide, its wavefront of eigenmode could be tilted correctly to suppress the radiation loss in wide-angle bent waveguides. In order to demonstrate the feasibility of proposed microprism for low-index-contrast waveguides, an example of bent waveguide with the eigenmode nearly compatible to the single mode fiber is adopted to design the PhC microprism. The transmission efficiency as high as 92% for the proposed structure with the bending angle of 12.96° and the bending radius of 89.09 μm is achieved.