Shin-Woong Park

Quasi-retroreflection from corner cubes with refractive free-form surfaces

Quasi-retroreflection from corner-cube structures with a refractive free-form surface is studied. It is shown that adjustment of the structural parameters of the free-form surface allows control of quasi-retroreflection. Quasi-retroreflection corner-cube array sheets with specified quasi-retroreflection angle are modeled, and their quasi-retroreflection characteristics are analyzed.

Polycrystalline CdZnTe thick films for low energy x-ray: system evaluation.

The X-ray response of polycrystalline-CdZnTe was measured by signal-to-noise (S/N) analysis. The CdZnTe material has optimal properties in a solid-state X-ray detector, and much research has focused on single crystal CdZnTe with a small-sized, silicon readout device. However, it would be difficult to apply CdTe or CdZnTe single crystal to large area, flat panel detectors, such as those used for radiography and mammography. As an alternative of single crystal CdZnTe, we have grown thick, polycrystalline CdZnTe films of high resistivity (>5times109 Ohm cm) using the thermal evaporation method on carbon substrate. A high signal-to-noise value has a direct impact on the performance of CdZnTe X-ray detectors. Important image parameters, such as dynamic range and detective quantum efficiency, rely on the signal and noise characteristics of the system. In this paper, we analyzed the properties of the X-ray detector and obtained images of the X-ray detector using the data acquisition system. The X-ray detector used the Cd1-xZnxTe (x=0.04), which used carbon substrate and gold as the electrode. The detector design is planar and 32 mmtimes10 mm in size, and it has a 1.75mmtimes1mm pixel electrode size and a detector thickness of 150 mum

Transmission-type photonic crystal structures for color filters

A transmission-type structure based on woodpile photonic crystal layers is proposed for use in color filters. Selective bandpass filters for red, green, and blue wavelength bands are constructed using optimally designed multilayered woodpile photonic crystals. The R/G/B color filtering for a wide range of incidence angles of light is demonstrated numerically, and the operation principle and design method are described.

Iterative method for optimal design of flat-spectral-response arrayed waveguide gratings

A novel iterative projection-type optimal design algorithm of arrayed waveguide gratings (AWGs) with a flat spectral response is proposed based on the Fourier optics model of AWG. The enhancement of the spectral-response flatness of the AWG is demonstrated, with an analysis on the trade-off relationship between band flatness and crosstalk.

Truncated corner cubes with near-perfect retroreflection efficiency

By isolating a finite effective volume from a conventional triangular pyramid corner cube, we obtained truncated corner cube structures with greatly enhanced retroreflection efficiency. We explore an optimal truncated corner cube with near 100% retroreflection efficiency based on the expectation that the traveling paths of the optical rays can be localized in the finite effective volume of the structure, and, as a result, truncated corner cubes with perfect efficiency can be produced. As a case study, the retroreflection efficiency of a commercialized 3M truncated corner cube sample is evaluated. Furthermore, it is shown with numerical verification that a truncated corner cube array sheet with near-perfect retroreflection efficiency can be produced.

Noise performance of the charge sensitive amplifier for photodetection

An integrated circuit for charge sensitive amplifier (CSA) was developed for X-ray detection. The circuit has been optimized for reading signals from silicon strip detectors with few pF input capacitance for low power and low noise. Theoretical calculations and optimizations are presented and compared with experimental results. Noise as low as 237 ENC [electrons] was obtained including a silicon detector of 1.3 pF and 0.386 pA of leakage. The power consumption was less than 100 W. Other circuit parameters were also obtained from experiments. The circuit was fabricated in standard 0.25 um CMOS technology and the circuit occupies an area of 4 mm times 4 mm.

Through-focus scanning optical microscopy with the Fourier modal method

We propose a Fourier modal method (FMM) based through-focus scanning optical microscopy (TSOM) featuring sub-nano scale measurement tolerance. TSOM is very recently conceptualized non-destructive optical metrology technique just at the beginning stage of research. Nowadays the reliability and feasibility of TSOM concept is subject to controversy. We experimentally demonstrate stable nano-scale metrology of the FMM-based TSOM for the verification of the TSOM metrology and provide a numerical tool for true nano-meter scale TSOM through devising the FMM based TSOM scheme. By considering the illumination light parameters of incidence angle, polarization, degree of coherence, illumination numerical aperture, and collection numerical apertures in the FMM modeling of TSOM image acquisition, we reach precise agreement between the calculated and experimentally measured TSOM images. The essential elements of the FMM based TSOM for achieving high-level consistency are elucidated.

Characteristics of complex light modulation through an amplitude-phase double-layer spatial light modulator

The complex modulation characteristics of a light field through an amplitude-phase double-layer spatial light modulator are analyzed based on the wave-optic numerical model, and the structural conditions for the optimal double-layer complex modulation structure are investigated. The relationships of interlayer distance, pixel size, and complex light modulation performance are analyzed. The main finding of this study is that the optimal interlayer distance for the double-layer structure can be found at the Talbot effect condition. For validating the practical usefulness of our findings, a high quality reconstruction of the complex computer-generated holograms and the robustness of the angular tolerance of the complex modulation at the Talbot interlayer distance are numerically demonstrated.

Noise Performance of Readout Electronics for Photodetector

A 16 channel Integrated circuit charge sensitive preamplifier (CSA) has been developed for an X-ray detector module that uses a 16 linear arrays of photodetector. The photodiode is made of 280 um thickness, 30 MOmega shunt resistivity, the measured junction capacitance 200 pF. The main performances of the CSA for photodetector are the following: the input equivalent noise charge is 300 e- rms at peaking time 200 ns, the highest gain is 10 mV/fC, the peaking time is adjustable between 50 ns and 300 us by external passive components. The noise simulation yield ENC values of 50 e- and 250 e- for thermal and 1/f noise, respectively. The design was fabricated in standard 0.35 um CMOS technology and the circuit occupies an area of 4times4 mm2 and dissipates 2 mW per channel from a 3.3 V single power supply.

Development of the translating and rotating volume computed tomography (TRVCT)

We describe a novel system named TRVCT (Translating and Rotating Volume Computed Tomography), developed for computed tomography image from large object with simple method and low price. Tomogram images can be acquired when the object is translating and rotating simultaneously with vertical linear array detector. This method is different from the normal X-ray CT completely. We used fan-beam X-ray, and the direction of the detector and rotating axis are in parallel. Because a hundred or thousand tomograms with Z-axis from just one scanning, it has excellent Z-axis resolution and has an advantage that can improve the resolution in X-Y plane with changing translating speed and frequency of data acquisition. There is no ring artifact that is generated frequently in the third generation CT scanner. So, we can have high resolution tomograms from this TRVCT system. The TRVCT can be used to acquire images for large object like tire, engine, or whole car, and it can remove the scattering from X-ray for high resolution images.