Chi-Young Hwang

Anamorphic optical transformation of an amplitude spatial light modulator to a complex spatial light modulator with square pixels [Invited]

A method is proposed for the construction of a square pixel complex spatial light modulator (SLM) from a commercial oblong full-high-definition (full-HD) amplitude SLM using an anamorphic optical filter. In the proposed scheme, one half-band of the optical Fourier transform of the amplitude-only spatial light field is rejected in the optical Fourier plane and the other half-band is reformatted to be an effective complex SLM with square pixels. This has an advantage in the viewing window plane since the shape of the viewing window becomes square and more ideal for observers who watch the hologram contents through it. For optimal transformation, the amplitude computer generated hologram encoding scheme was developed. Mathematical modeling of the proposed system is described herein, and it was experimentally demonstrated that the effective complex SLM displays complex holographic three-dimensional images with a clear depth discrimination effect.

Photo-Stable Transparent Nonvolatile Memory Thin-Film Transistors Using In–Ga–Zn–O Channel and ZnO Charge-Trap Layers

We propose fully transparent charge-trap-type memory thin-film transistors (TFTs) with a top-gate structure composed of ZnO charge-trap and In-Ga-Zn-O active channel layers on glass substrates. Nonvolatile memory characteristics, such as a wide memory window, high-speed programming, and stable retention characteristics, were confirmed even under the illumination of red, green, blue, and white lights. The photo-stable characteristics of the proposed memory TFTs originate from the robust bias illumination stress stabilities of the employed gate-stack structure. The turn-ON voltages (VON) did not fluctuate with the variations of the bias polarity and wavelength of the illumination stresses. The maximum shift of VON observed was as small as -0.4 V at a negative bias stress of -20 V under a blue wavelength after 600 s.

Fast focus estimation using frequency analysis in digital holography

A novel fast frequency-based method to estimate the focus distance of digital hologram for a single object is proposed. The focus distance is computed by analyzing the distribution of intersections of smoothed-rays. The smoothed-rays are determined by the directions of energy flow which are computed from local spatial frequency spectrum based on the windowed Fourier transform. So our method uses only the intrinsic frequency information of the optical field on the hologram and therefore does not require any sequential numerical reconstructions and focus detection techniques of conventional photography, both of which are the essential parts in previous methods. To show the effectiveness of our method, numerical results and analysis are presented as well.

Switchable subwavelength plasmonic structures with phase-change materials for reflection-type active metasurfaces in the visible region

In this work, a switchable plasmonic structure is proposed for reflection-type spatial light modulation in the visible range with subwavelength resolution. This structure is based on a metallic grating in which each resonant cavity couples the incident light into a gap surface plasmon mode and then reflects the light modulated in the cavity. By incorporating an ultrathin layer of the phase-change material Ge2Sb2Te5 at the entrance of the cavity, the optical modulation characteristic of the structure can be switched between two modes. Numerical investigations are conducted to verify the proposed structure, with the focused analysis of two common types of binary modulations: amplitude-only and phase-only modulations.

Reflection-type spatial amplitude modulation of visible light based on a sub-wavelength plasmonic absorber

We present a method for reflection-type spatial amplitude modulation using a sub-wavelength plasmonic absorber structure that can operate in the visible region. We utilize a pixelated array of absorbing elements based on a two-dimensional sub-wavelength metal grating, and the reflectance of each pixel is controlled by simple structural modification. For the purpose of validation, numerical simulations were performed on an amplitude modulation hologram fabricated using our method.

Stepwise angular spectrum method for curved surface diffraction.

We present a method to calculate wave propagation between arbitrary curved surfaces using a staircase approximation approach. The entire curved surface is divided into multiple subregions and each curved subregion is approximated by a piecewise flat subplane allowing the application of conventional diffraction theory. In addition, in order to reflect the local curvature of each subregion, we apply the phase compensation technique. Analytical expressions are derived based on the angular spectrum method and numerical studies are conducted to validate our method.

Development of high-resolution active matrix spatial light modulator

Abstract. We developed a high-resolution active matrix spatial light modulator on a glass substrate. To integrate a switching device on the glass substrate, we designed a high-performance oxide thin-film transistor with a minimum channel length of 1  μm and a maximum processing temperature of 380°C. To drive a large number of data lines, we used multiple source drivers and data drivers as well. For an optical modulation, we optimized a liquid crystal of a high anisotropic refractive index of 0.25 with a cell gap of 2.5  μm, which was effectively operated until pixel pitch is 1.6  μm. Hologram was successfully reconstructed by fabricated SLM with 7-μm pixel pitch. For the other approach for a high-resolution spatial light modulator, we tested a phase change material of Ge2Sb2Te5 [GST]. The variation of refractive index between a polycrystalline phase and an amorphous phase of the GST film is used for a hologram reconstruction. By optimizing the underlying oxide thickness, we can show a color hologram without color filters.

Nitrogen Doping Effect for Improving Operation Reliability of Phase Modulator Using Ge2Sb2Te5 Thin Film for Hologram Image Implementation

A phase modulation device was proposed for the implementation of hologram image for display applications. A Ge2Sb2Te5 (GST) film as thin as 7 nm was prepared between the ITO films to form the cavities corresponding a unit pixel. Nitrogen was incorporated into the GST for improving the thermal stability of the GST active region. The effects of the nitrogen doping on the physical properties of GST was investigated with the variations in doping amounts. The nitrogen incorporation was found to reduce the surface micro-roughness and to improve the thermal stability of the GST even after the crystallization by effectively suppressing the excessive grain growth. As results, the number of repeatable operations for the fabricated phase modulation device was evidently improved from 10 to 69 cycles when a 2.7-at% nitrogen was doped into the GST.

Off-axis complex hologram encoding method for holographic display with amplitude-only modulation

We investigate an off-axis complex hologram encoding method for amplitude-only modulation on the basis of the principle of off-axis holography. The encoding method is analytically derived and formulated in Fresnel approximation. The hologram is reconstructed by using a holographic display with narrow viewing window and we experimentally verify the encoding method.

Spherically-arranged piecewise planar hologram for capturing a diffracted object wave field in 360 degree

We present a new method to record and reconstruct a diffracted object wave field in all directions. For this, we are going to use spherically-arranged holograms instead of a single spherical hologram. Our spherically-arranged holograms are constructed to store all components of plane waves propagating in all directions. One can use the well-known efficient FFT-based diffraction formulae such as Fresnel transform and angular spectrum method in generation and reconstruction of our spherically-arranged holograms. It is possible to synthesize a new hologram with an arbitrary position and orientation without the geometry of the object. Numerical experiments are presented to show the effectiveness of our method.