Chyong-Hua Chen

Wide-field-of-view narrow-band spectral filters based on photonic crystal nanocavities

We describe a novel approach to implementing wide-field-of-view narrow-band spectral filters, using an array of resonant nanocavities consisting of periodic defects in a two-dimensional three-material photonic-crystal nanostructure. We analyze the transmissivity of this type of filter for a range of wavelengths and in-plane incidence angles as a function of the defects refractive index, the number of layers in the photonic-crystal reflectors, and the period of the defects and find that this structure diminishes the angular sensitivity of the resonance condition relative to that of a standard multilayer filter.

Compact and integrated TM-pass waveguide polarizer.

A novel integrated TM-pass waveguide polarizer with a subwavelength-wide slot is introduced and theoretically analyzed. With a proper design of the slot, the waveguide can be used as a single polarization waveguide to guide only TM polarization modes of the light signal. With 26 microm length of this TM-pass polarizer, our computer simulations predict the insertion loss of 0.54 dB for the TM polarization mode with the extinction ratio of 20.3 dB at the wavelength of 1.55 microm.

Photosensitive quantum dot composites and their applications in optical structures

Commercially available CdSe∕ZnS and PbSe colloidal semiconductor quantum dots were employed to produce both an electron beam sensitive poly(methyl methacrylate) (PMMA)-quantum-dot (QD) positive composite via a prepolymerization processing and an electron beam and ultraviolet (UV) light sensitive SU-8-QD negative composite via a direct dispersion procedure. Compared to the QDs in the original colloidal solutions, the photoluminescence of the composites shifts to shorter wavelength due to the oxidation of the surfaces of the QDs. Using the QD composites, optical integrated circuits such as grating and waveguide structures were fabricated by direct electron beam writing and UV optical lithography. The characterization results show promising applications in optoelectronics for the QD composites.

Resonant-cavity-enhanced p-i-n photodiode with a broad quantum-efficiency spectrum by use of an anomalous-dispersion mirror

A resonant-cavity-enhanced photodiode with broad filter transmittance and high quantum efficiency was numerically designed and analyzed, fabricated, and validated experimentally. We show theoretically that the quantum-efficiency spectrum broadens because of anomalous dispersion of the reflection phase of a mirror in the device and describe conditions that allow maximal flatness of the transmitted spectrum to be achieved. To demonstrate the concepts we design, fabricate, and characterize a backilluminated In0.47Ga0.53As-based p-i-n photodiode upon a InP substrate. Experimental measurements of the fabricated devices demonstrate a peak quantum efficiency of 0.80 at 1550 nm and a FWHM of transmittance of 35.96 nm.

Design of optimized dispersive resonant cavities for nonlinear wave mixing.

Dispersive mirrors can be designed to create cavities that resonate at set multiple frequencies while simultaneously meeting the conditions for efficient nonlinear wave mixing. We analyze the conditions that such a cavity design must meet and the free parameters that can be used for optimization. Using numerical methods, we show the benefit in conversion efficiency attained with multiple resonances, and draw conclusions concerning the design parameters. As a specific example, we consider parametric downconversion in a triply-resonant cavity.

Wide-field-of-view GaAs/AlxOy one-dimensional photonic crystal filter.

The design, fabrication, and characterization of a one-dimensional photonic crystal optical filter that has a relatively narrow, flat-topped passband within a wide stop band and small angular sensitivity is presented. The filter is based on a one-dimensional photonic crystal structure that has multiple defects, facilitating simultaneous minimization of the angular sensitivity and optimization of the passbands characteristics. We use epitaxially grown and selectively oxidized GaAs/AlxOy multilayers to achieve a high-index-contrast material system and incorporate the experimentally determined optical and material properties into the design of the device. A flat-topped bandpass filter with a bandwidth of 65 nm and a wide field of view of 50 degrees is experimentally characterized and compared with the design predictions.

Nanophotonics: materials and devices

Optical technology plays an increasingly important role in numerous applications areas, including communications, information processing, and data storage. However, as optical technology develops, it is evident that there is a growing need to develop reliable photonic integration technologies. This will include the development of passive as well as active optical components that can be integrated into functional optical circuits and systems, including filters, switching fabrics that can be controlled either electrically or optically, optical sources, detectors, amplifiers, etc. We explore the unique capabilities and advantages of nanotechnology in developing next generation integrated photonic chips. Our long-range goal is to develop a range of photonic nanostructures including artificially birefringent and resonant devices, photonic crystals, and photonic crystals with defects to tailor spectral filters, and nanostructures for spatial field localization to enhance optical nonlinearities, to facilitate on-chip system integration through compatible materials and fabrication processes. The design of artificial nanostructured materials, PCs and integrated photonic systems is one of the most challenging tasks as it not only involves the accurate solution of electromagnetic optics equations, but also the need to incorporate the material and quantum physics equations. Near-field interactions in artificial nanostructured materials provide a variety of functionalities useful for optical systems integration. Furthermore, near-field optical devices facilitate miniaturization, and simultaneously enhance multifunctionality, greatly increasing the functional complexity per unit volume of the photonic system. Finally and most importantly, nanophotonics may enable easier integration with other nanotechnologies: electronics, magnetics, mechanics, chemistry, and biology.

PMMA quantum dots composites and their applications

Commercially available colloidal semiconductor quantum dots are employed to produce an electron beam sensitive PMMA-quantum-dot (QD) positive composite via pre-polymerization. In PMMA-QD composite, the QDs are stabilized in rapidly formed oligomer matrices to prevent cluster, and the complete polymerization of the PMMA-QD composite is achieved by commonly used polymerization. The properties of the composites are measured and compared with the QDs in original colloidal solution. Patterning of the composites by direct write electron beam and UV optical lithography shows its promising applications in optoelectronics.

Nanophotonics for optoelectronic system integration

This study explores the unique capabilities and advantages of nanotechnology in developing next generation integrated photonic chips. The described approach includes design, modeling and simulations of example components and devices, their nanofabrication, followed by validation via characterization and testing of the fabricated devices. The latter exploits the recently constructed near field complex amplitude imaging tool. The long-range goal is to develop a range of photonic nanostructures- including artificially birefringent and resonant devices, photonic crystals with defects to tailor spectral filters, metallodielectric composites, and nanostructures for spatial field localization to enhance optical nonlinearities- to facilitate on-chip system integration through compatible materials and fabrication processes.

Design, fabrication and characterization of narrowband angularly-insensitive resonant cavity filter

We describe the design, fabrication, and characterization of resonant filters having concurrently a wide range of acceptance angles and a narrow wavelength passband, based on one-dimensional photonic crystals (PC) with defects. A filter having these characteristics would be extremely useful, for example, in a free-space optical communication system among mobile nodes, where the relative bearing of the transmitter is unknown. However, the design of such a structure requires careful design, as these two characteristics are not readily compatible. In the following, we discuss our design approach to implement the wide-field-of-view narrowband resonant filter, describe the fabrication procedure in the GaAs/AlAs material system including oxidation to achieve the high refractive index contrast needed for good performance, and present the structural and optical characterization results for the finished device. The objective is to design a wide-field-of-view narrow band wavelength filter using a multilayer 1-D PC structure with one or more defects. The primary defect in the resonant cavity is realized with the highest possible index of refraction, to minimize the variation of the cavity phase with changes in the incidence angle. Since the angular dependence of the incidence angle cannot be completely eliminated in an isotropic 1-D structure, the ideal filter performance characteristics would be a perfectly square bandpass filter with a bandwidth that can be specified to match the specific application. To approximate this type of performance, we use an asymmetric secondary resonant PC cavity in place of each mirror surrounding the primary cavity, resulting in a resonant 1-D PC multilayer structure having three defects. This approach permits control of the spectral reflectivity and phase of each mirror, facilitating control of the transmission spectrum of the device as a whole.