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Dive into the research topics where Hee Ju Choi is active.

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Featured researches published by Hee Ju Choi.


Optics Express | 2010

Design of highly transparent glasses with broadband antireflective subwavelength structures

Young Min Song; Hee Ju Choi; Jae Su Yu; Yong Tak Lee

We present a design optimization of highly transparent glasses with broadband antireflective subwavelength structures (SWS) based on the theoretical calculation using a rigorous coupled wave analysis method. It is found that optical transmission characteristics of SWS integrated glasses are governed mainly by the zero-order condition considering multiple internal reflections but not external reflection. By utilizing parabola-shaped SWS on both sides of the glasses with a period of 200 nm and a height of 200 nm, an average transmittance of 99.58% is achieved over a whole range of visible wavelength. Transmission band shrinkage effects of the SWS integrated glass are also observed with increasing the incident angle of light.


Optics Express | 2012

Large aperture asymmetric Fabry Perot modulator based on asymmetric tandem quantum well for low voltage operation

Byung Hoon Na; Gun Wu Ju; Hee Ju Choi; Yong Chul Cho; Yong-hwa Park; Yong Tak Lee

Large aperture image modulators used as demodulator in receiver path are an important component for the use in three dimensional (3D) image sensing. For practical applications, low voltage operation and high modulation performance are the key requirements for modulators. Here, we propose an asymmetric Fabry-Perot modulator (AFPM) with asymmetric tandem quantum wells (ATQWs) for 3D image sensing. By using ATQWs for the AFPM design, the device operated at -4.25V, and the operating voltage was significantly lower by about 23% compared to -5.5V of a conventional AFPM with 8nm thick multiple QW with a single QW thickness (SQWs), while achieving high reflectivity modulation in excess of 50%. The performance of the fabricated devices is in good agreement with theoretical calculations. The pixelated device shows a high modulation speed of 21.8 MHz over a large aperture and good uniformity. These results show that AFPM with ATQWs is a good candidate as an optical image modulator for 3D image sensing applications.


Journal of Materials Chemistry | 2015

A single-material graded refractive index layer for improving the efficiency of III–V triple-junction solar cells

Chan Il Yeo; Hee Ju Choi; Young Min Song; Seok Jin Kang; Yong Tak Lee

We present a single-material titanium oxide (TiO2) bi-layer antireflection coating (ARC) produced using oblique angle deposition for improving the power conversion efficiency (PCE) of III–V compound semiconductor triple-junction (TJ) solar cells. Experimental demonstration of the porous TiO2 layers and optical modelling by rigorous coupled-wave analysis indicate that bi-layer TiO2 could be used as universal ARC for all known TJ stacks with various material combinations. The optimum TiO2 bi-layer ARC produced on a Ga0.5In0.5P/GaAs/Ge TJ solar cell exhibits considerably enhanced PCE of 31.6%, which is higher than that of the cells without ARC (23.8%) and with TiO2 single-layer ARC (29.1%). Incident angle-dependent device characteristics of the fabricated TJ solar cells are also discussed.


Journal of Micro-nanolithography Mems and Moems | 2013

Three-dimensional imaging using fast micromachined electro-absorptive shutter

Yong-hwa Park; Yong-chul Cho; Jang-woo You; Chang-Young Park; Heesun Yoon; Sang-hun Lee; Jong-oh Kwon; Seung-Wan Lee; Byung Hoon Na; Gun Wu Ju; Hee Ju Choi; Yong Tak Lee

Abstract. A 20-MHz switching high-speed light-modulating device for three-dimensional (3-D) image capturing and its system prototype are presented. For 3-D image capturing, the system utilizes a time-of-flight (TOF) principle by means of a 20-MHz high-speed micromachined electro-absorptive modulator, the so-called optical shutter. The high-speed modulation is obtained by utilizing the electro-absorption mechanism of the multilayer structure, which has an optical resonance cavity and light-absorption epilayers grown by metal organic chemical vapor deposition process. The optical shutter device is specially designed to have small resistor–capacitor–time constant to get the high-speed modulation. The optical shutter is positioned in front of a standard high-resolution complementary metal oxide semiconductor image sensor. The optical shutter modulates the incoming infrared image to acquire the depth image. The suggested novel optical shutter device enables capturing of a full high resolution-depth image, which has been limited to video graphics array (VGA) by previous depth-capturing technologies. The suggested 3-D image sensing device can have a crucial impact on 3-D–related business such as 3-D cameras, gesture recognition, user interfaces, and 3-D displays. This paper presents micro-opto-electro-mechanical systems-based optical shutter design, fabrication, characterization, 3-D camera system prototype, and image evaluation.


Optics Express | 2013

Coupled tandem cavities based electro-absorption modulator with asymmetric tandem quantum well for high modulation performance at low driving voltage

Byung Hoon Na; Gun Wu Ju; Hee Ju Choi; Soo Kyung Lee; Sooraj Ravindran; Yong Chul Cho; Yong-hwa Park; Chang Young Park; Yong Tak Lee

We propose and demonstrate a new electro-absorption modulator (EAM) based on coupled tandem cavities (CTC) having asymmetric tandem quantum well (ATQW) structure with separated electrode configuration to achieve large transmittance change over a broad spectral range at low driving voltage for high definition (HD) 3D imaging applications. Our theoretical calculations show that CTC with ATQW structure can provide large transmittance change over a wide spectral range at low driving voltage. By introducing separated electrode configuration, the fabricated EAM having CTC with ATQW structure shows a large transmittance change over 50%, almost three times larger spectral bandwidth compared to that of EAM having single cavity with a single thickness quantum well without significantly increasing the applied voltage. In addition, the CTC with ATQW structure also shows high speed modulation up to 28 MHz for the device having a large area of 2 mm x 0.5 mm. This high transmittance change, large spectral bandwidth and low voltage operation over a large device area for the EAM having CTC with ATQW demonstrates their huge potential as an optical image modulator for HD 3D imaging applications.


Optics Express | 2012

Wide spectral bandwidth electro-absorption modulator using coupled micro-cavity with asymmetric tandem quantum well

Byung Hoon Na; Gun Wu Ju; Hee Ju Choi; Yong Chul Cho; Yong-hwa Park; Chang Young Park; Yong Tak Lee

For reliable three dimensional (3D) imaging system, it is necessary for the optical shutter to have a wide spectral bandwidth operation and enhanced modulation depth. We propose an electro-absorption modulator (EAM) based on coupled Fabry-Perot cavities with micro-cavity (CCMC) which uses asymmetric tandem quantum wells (ATQWs) to obtain improved spectral bandwidth and enhanced modulation depth. Several modulator designs are investigated to obtain improved modulation performance such as wider spectral bandwidth and enhanced modulation depth. It was found that among all the studied modulator geometries, CCMC structure with ATQWs provides the widest spectral bandwidth of 9.6nm and high modulation depth in excess of 50% at -24V, which is good agreement with theoretical calculations. These results suggest that EAM has excellent potential as optical shutter for 3D imaging application.


Optics Express | 2016

4 channel × 10 Gb/s bidirectional optical subassembly using silicon optical bench with precise passive optical alignment.

Eun Kyu Kang; Yong Woo Lee; Sooraj Ravindran; Jun Ki Lee; Hee Ju Choi; Gun Wu Ju; Jung Wook Min; Young Min Song; Ik-Bu Sohn; Yong Tak Lee

We demonstrate an advanced structure for optical interconnect consisting of 4 channel × 10 Gb/s bidirectional optical subassembly (BOSA) formed using silicon optical bench (SiOB) with tapered fiber guiding holes (TFGHs) for precise and passive optical alignment of vertical-cavity surface-emitting laser (VCSEL)-to-multi mode fiber (MMF) and MMF-to-photodiode (PD). The co-planar waveguide (CPW) transmission line (Tline) was formed on the backside of silicon substrate to reduce the insertion loss of electrical data signal. The 4 channel VCSEL and PD array are attached at the end of CPW Tline using a flip-chip bonder and solder pad. The 12-channel ribbon fiber is simply inserted into the TFGHs of SiOB and is passively aligned to the VCSEL and PD in which no additional coupling optics are required. The fabricated BOSA shows high coupling efficiency and good performance with the clearly open eye patterns and a very low bit error rate of less than 10-12 order at a data rate of 10 Gb/s with a PRBS pattern of 231-1.


IEEE Photonics Technology Letters | 2015

RCEPD With Enhanced Light Absorption by Crown-Shaped Quantum Well

Gun Wu Ju; Byung Hoon Na; Hee Ju Choi; Kwang Wook Park; Young Min Song; Yong Tak Lee

A high-performance resonant cavity enhanced photodetector (RCEPD) is developed by introducing an InGaAs/GaAs crown-shaped quantum well (CSQW) structure. In calculation, the absorption coefficient of the proposed CSQW structure is significantly enhanced by 47.8% compared with that of the conventional QW without increasing the electric field due to the large overlap of electron/hole-wave functions. To verify the feasibility of our proposed QW structure, we fabricate RCEPDs with a designed CSQW and the conventional QW structure. The fabricated CSQW-RCEPD exhibits a maximum quantum efficiency of 45.4%, an improvement of 36.2% in comparison with the conventional RCEPD. Moreover, the spectral bandwidth is 4.7 nm in the CSQW-RCEPD, which is in good agreement with the calculated result. The RCEPD with enhanced light absorption using a CSQW structure is highly promising for optical interconnect and sensing applications.


MOEMS and Miniaturized Systems XIII | 2014

A robust design and fabrication of micromachined electro-absorptive optical modulator for 3D imaging

Yong-hwa Park; Yong-chul Cho; Jang-woo You; Chang-Young Park; Heesun Yoon; Sang-hun Lee; Byung Hoon Na; Gun Wu Ju; Hee Ju Choi; Yong Tak Lee

A time-of-flight (TOF) based three dimensional (3D) image capturing system and its enhanced optical modulating device are presented. The 3D image capturing system includes 850nm IR emitter (typically compact Laser diodes) and high speed image modulator, so called optical shutter. The optical shutter consists of multi-layered optical resonance cavity and electro-absorptive layers. The optical shutter is a solid-state controllable filter which modulates the IR image to extract the phase delay due to TOF of the emitting IR light. This presentation especially addresses robustness issues and solutions when operated under practical environments such as ambient temperature variation and existence of strong ambient light (e.g. outdoors). The wavelength of laser diode varies substantially depending on the ambient temperature, which degrades the modulation efficiency. To get a robust operation, the bandwidth of transmittance of the optical shutter is drastically improved with a novel coupled Fabry-Perot resonance cavity design to come up with the wavelength variation of the laser diode. Also, to suppress the interference of solar irradiance to IR source signal, a novel driving scheme is applied, in which IR light and optical shutter modulation duties are timely localized, i. e. ‘bursted’. Suggested novel optical shutter design and burst driving scheme enable capturing of a full HD resolution of depth image under the realistic usage environments, which so far tackle the commercialization of TOF cameras. Design, fabrication, and evaluation of the optical shutter; and, 3D capturing system prototype, image test results are presented.


Optics Letters | 2016

Shape-controllable, bottom-up fabrication of microlens using oblique angle deposition.

Hee Ju Choi; Eun Kyu Kang; Gun Wu Ju; Young Min Song; Yong Tak Lee

This Letter reports a novel method for the simple fabrication of microlens arrays with a controlled shape and diameter on glass substrates. Multilayer stacks of silicon dioxide deposited by oblique angle deposition with hole mask patterns enable microlens formation. Precise control of mask height and distance, as well as oblique angle steps between deposited layers, supports the controllability of microlens geometry. The fabricated microlens arrays with designed geometry exhibit uniform optical properties.

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Yong Tak Lee

Gwangju Institute of Science and Technology

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Gun Wu Ju

Gwangju Institute of Science and Technology

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Byung Hoon Na

Gwangju Institute of Science and Technology

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Young Min Song

Gwangju Institute of Science and Technology

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Soo Kyung Lee

Gwangju Institute of Science and Technology

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Eun Kyu Kang

Gwangju Institute of Science and Technology

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