Chia-Ho Tsai

Engineering space-variant inhomogeneous media for polarization control

Novel devices for converting a linear polarization state to radial or azimuthal polarization states are realized by use of space-variant inhomogeneous media on a subwavelength scale. The two designs presented use form birefringence to locally transform the polarization state. The devices are fabricated in a GaAs substrate for operation in the far-infrared wavelength range. The experimental characterization is in good agreement with the designs, demonstrating high conversion efficiency.

Near-infrared demonstration of computer-generated holograms implemented by using subwavelength gratings with space-variant orientation

We provide an experimental demonstration of novel form-birefringent computer-generated holograms at wavelengths of 1.55 and 10.6 microm. These novel devices utilize a 2-D array of cells that can be fabricated with a single lithographic step. Each cell contains a subwavelength binary grating whose orientation controls the desired continuous phase profile within the cell.

Implementation of a graded-index medium by use of subwavelength structures with graded fill factor

We present a novel configuration for the implementation of subwavelength-based graded-index devices. The proposed concept is based on the etching of one-dimensional subwavelength gratings into a high-index slab waveguide to achieve the desired effective index distribution. A graded-index profile can be achieved by gradually modifying the duty ratio of the grating along the horizontal axis, while the beam is confined in the vertical direction by the slab waveguide. On the basis of this concept, novel graded-index lenses and waveguides are both proposed and characterized numerically by use of finite-difference time-domain and finite-element analysis. The proposed devices can be used for guiding, imaging, optical signal processing, mode matching, coupling, and other applications while offering the intrinsic advantages of on-chip integration such as miniaturization, eliminating the need to align each component separately, and compatibility with standard microfabrication techniques for manufacturability.

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.

Design, fabrication and characterization of subwavelength computer-generated holograms for spot array generation

We report the analysis, design, fabrication and experimental characterization of novel subwavelength computer-generated holograms that produce uniform symmetric spot array. We distinguish between a polarization-sensitive and polarization-insensitive far-field reconstruction and show that a linearly polarized incident illumination is required in the former case in order to generate a symmetric reconstruction. The polarization-insensitive case generates a symmetric response independent of the illumination polarization. We show that this response is equivalent to that of a scalar-based computer-generated hologram but with an additional, independent, term that describes the undiffracted zeroth order. These findings simplify the design and optimization of form birefringent computer-generated holograms (F-BCGH) significantly. We present experimental results that verify our analysis and highlight the advantage of these novel elements over scalar-designed elements.

Form-birefringence structure fabrication in GaAs by use of SU-8 as a dry-etching mask

A thin layer of a SU-8 submicrometer pattern produced by holographic lithography was directly used as the dry-etching mask in a chemically assisted ion-beam-etching system. With optimized etching parameters, etching selectivity of 7:1 was achieved together with a smooth vertical profile. As an application, a half-wavelength retardation plate for a 1.55-microm wavelength was produced and evaluated.

Semiconductor circular ring lasers fabricated with the cryo-etching technique

GaAs-AlGaAs multiple quantum well semiconductor circular ring lasers with a /spl sigma/ shape were fabricated by using the deep UV laser-assisted cryo-etching technique. Most of the fabricated lasers had external quantum efficiencies of more than 18% which mere higher than similar devices previously reported. The modal spacing observed from the resonance spectrum near threshold was always several times that corresponding to the circular oscillation of the ring cavity. The observed modal spacing was quite consistent with the theoretical result based on a coupled-cavity model.

Form-birefringent space-variant inhomogeneous medium element for shaping point-spread functions.

We experimentally characterize the properties of an element that generates a doughnutlike point-spread function by converting the linearly polarized incident field to radially or azimuthally polarized light utilizing space-variant inhomogeneous medium (SVIM) form-birefringent subwavelength structures. To fabricate the high-aspect-ratio SVIM structures, we developed a chemically assisted ion-beam-etching process that permits control of the fabricated form-birefringent structure profile to optimize the effect of birefringence and the impedance mismatch on the substrate-air interface. Fabricated elements perform efficient polarization conversion for incident angles as large as 30 degrees, where the extinction ratio is found to be better than 4.5. The intensity distribution in the far field shows that our SVIM device generates a doughnut point-spread function that may prove useful for various applications.

Form birefringent retardation plates in GaAs substrates: design, fabrication, and characterization

We discuss various practical points in the design, fabrication and characterization of form birefringent retardation plates in GaAs. The role of the substrate in the device performance is presented, together with the importance of using anti-reflection coatings. Also, we discuss the specific case of metallic reflection gratings in GaAs substrates and the resulting enhanced retardation. Finally we present the results of thermal tuning of a nominally half-wave subwavelength retardation plate.

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.