Biomedical subjects

Chia-Ho Tsai

Publications and source records attributed to Chia-Ho Tsai.

5 recordsLinked to original sources

PubMed2006

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.

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PubMed2005

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.

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PubMed2005

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.

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PubMed2005

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.

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PubMed2004

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.

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