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182 results on '"Cho, Kyung-Sang"'

1. Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

Author

Shi, Jiaojian, Gao, Frank Y., Zhang, Zhuquan, Utzat, Hendrik, Barotov, Ulugbek, Farahvash, Ardavan, Han, Jinchi, Deschamps, Jude, Baik, Chan-Wook, Cho, Kyung Sang, Bulović, Vladimir, Willard, Adam P., Baldini, Edoardo, Gedik, Nuh, Bawendi, Moungi G., and Nelson, Keith A.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science, Physics - Chemical Physics, Physics - Optics
Abstract

Continuous and concerted development of colloidal quantum-dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect which persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and non-radiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired non-radiative decay, potentially in a variety of luminescent materials for future device applications., Comment: 28 pages, 4 figures

Published
2021
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6. Direction control of colloidal quantum dot emission using dielectric metasurfaces

Author

Park Yeonsang, Kim Hyochul, Lee Jeong-Yub, Ko Woong, Bae Kideock, and Cho Kyung-Sang

Subjects
asymmetric nanopost array, colloidal quantum dot, dielectric metasurface, light-emitting diode, resonant cavity, tio2 dry etching, Physics, QC1-999
Abstract

Owing to the recent developments of dielectric metasurfaces, their applications have been expanding from those pertaining to the thickness shrinkage of passive optical elements, such as lenses, polarizers, and quarter-wave plates, to applications pertaining to their integration with active optical devices, such as vertical-cavity surface-emitting lasers. Even though directional lasing and beam shaping of laser emission have been successfully demonstrated, the integration of metasurfaces with random light sources, such as light-emitting diodes, is limited because of function and efficiency issues attributed to the fact that metasurfaces are basically based on the resonance property of the nanostructure. To control the direction of emission from colloidal quantum dots, we present a dielectric metasurface deflector composed of two asymmetric TiO2 nanoposts. TiO2 deflector arrays were fabricated with a dry etching method that is adaptive to mass production and integrated with a colloidal quantum dot resonant cavity formed by sandwiching two distributed Bragg reflectors. To ensure the deflection ability of the fabricated sample, we measured the photoluminescence and far-field patterns of emission from the resonant cavity. From the obtained results, we demonstrated that the colloidal quantum dot emission transmitted through our deflector arrays was deflected by 18°, and the efficiency of deflection was 71% with respect to the emission from the resonant cavity. This integration of dielectric metasurfaces with a resonant cavity shows the possibility of expanding the application of visible metasurfaces in active devices and may help to develop next-generation active devices with novel functions.

Published
2020
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8. Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals

Author

Harbold, Jeffrey M., Wise, Frank W., Du, Hui, Krauss, Todd D., Cho, Kyung-Sang, and Murray, Chris B.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

The relaxation of strongly-confined electrons and holes between 1P and 1S levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed.

Published
2004
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15. Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

Author

Massachusetts Institute of Technology. Department of Chemistry, Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology. Department of Physics, Shi, Jiaojian, Gao, Frank Y, Zhang, Zhuquan, Utzat, Hendrik, Barotov, Ulugbek, Farahvash, Ardavan, Han, Jinchi, Deschamps, Jude, Baik, Chan-Wook, Cho, Kyung Sang, Bulović, Vladimir, Willard, Adam P, Baldini, Edoardo, Gedik, Nuh, Bawendi, Moungi G, Nelson, Keith A, Massachusetts Institute of Technology. Department of Chemistry, Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology. Department of Physics, Shi, Jiaojian, Gao, Frank Y, Zhang, Zhuquan, Utzat, Hendrik, Barotov, Ulugbek, Farahvash, Ardavan, Han, Jinchi, Deschamps, Jude, Baik, Chan-Wook, Cho, Kyung Sang, Bulović, Vladimir, Willard, Adam P, Baldini, Edoardo, Gedik, Nuh, Bawendi, Moungi G, and Nelson, Keith A

Abstract

The continuous and concerted development of colloidal quantum dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect that persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and nonradiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired nonradiative decay, potentially in a variety of luminescent materials for future device applications.

Published
2022

16. Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

Author

Shi, Jiaojian, primary, Gao, Frank Y., additional, Zhang, Zhuquan, additional, Utzat, Hendrik, additional, Barotov, Ulugbek, additional, Farahvash, Ardavan, additional, Han, Jinchi, additional, Deschamps, Jude, additional, Baik, Chan-Wook, additional, Cho, Kyung Sang, additional, Bulović, Vladimir, additional, Willard, Adam P., additional, Baldini, Edoardo, additional, Gedik, Nuh, additional, Bawendi, Moungi G., additional, and Nelson, Keith A., additional

Published
2022
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35. Metasurface electrode light emitting diodes with planar light control

Author

Park, Yeonsang, primary, Kim, Jineun, additional, Cho, Kyung-Sang, additional, Kim, Hyochul, additional, Lee, Min-kyung, additional, Lee, Jae-soong, additional, Kim, Un Jeong, additional, Hwang, Sung Woo, additional, Brongersma, Mark L., additional, Roh, Young-Geun, additional, and Park, Q-Han, additional

Published
2017
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39. Color-selective Photodetection from Intermediate Colloidal Quantum Dots Buried in Amorphous-oxide Semiconductors

Author

Cho, Kyung-Sang, Heo, Keun, Baik, Chan-Wook, Choi, Jun Young, Jeong, Heejeong, Hwang, Sungwoo, Lee, Sang Yeol, Cho, Kyung-Sang, Heo, Keun, Baik, Chan-Wook, Choi, Jun Young, Jeong, Heejeong, Hwang, Sungwoo, and Lee, Sang Yeol

Abstract

We report color-selective photodetection from intermediate, monolayered, quantum dots buried in between amorphous-oxide semiconductors. The proposed active channel in phototransistors is a hybrid configuration of oxide-quantum dot-oxide layers, where the gate-tunable electrical property of silicon-doped, indium-zinc-oxide layers is incorporated with the color-selective properties of quantum dots. A remarkably high detectivity (8.1 x 10(13) Jones) is obtained, along with three major findings: fast charge separation in monolayered quantum dots; efficient charge transport through high-mobility oxide layers (20 cm(2) V-1 s(-1)); and gate-tunable drain-current modulation. Particularly, the fast charge separation rate of 3.3 ns(-1) measured with time-resolved photoluminescence is attributed to the intermediate quantum dots buried in oxide layers. These results facilitate the realization of efficient color-selective detection exhibiting a photoconductive gain of 10(7), obtained using a room-temperature deposition of oxide layers and a solution process of quantum dots. This work offers promising opportunities in emerging applications for color detection with sensitivity, transparency, and flexibility.

Published
2017

41. Single-Mode Lasing from a Monolithic Microcavity with Few-Monolayer-Thick Quantum Dot Films

Author

Kim, Hyochul, Cho, Kyung-Sang, Jeong, Heejeong, Kim, Jineun, Lee, Changwon, Koh, Weonkyu, Roh, Young-Geun, Hwang, Sungwoo, Park, Yeonsang, Kim, Hyochul, Cho, Kyung-Sang, Jeong, Heejeong, Kim, Jineun, Lee, Changwon, Koh, Weonkyu, Roh, Young-Geun, Hwang, Sungwoo, and Park, Yeonsang

Abstract

We monolithically fabricated vertical cavity lasers with densely packed colloidal quantum dot films and demonstrated single-mode lasing operation using nanosecond optical excitation. Due to the accurate spectral and spatial alignment of the cavity optical modes and the quantum dot gains, in addition to the high quality factors of our devices, we observed lasing from a colloidal quantum dot gain medium only 35 nm thick (6 or 7 monolayers thick) with a threshold of 20 mJ/cm2. We fabricated a laser with more numerous quantum dot layers as well, which exhibited a lower lasing threshold of 9 mJ/cm2 due to the enhanced modal gain. This work represents an important step toward fabricating monolithic colloidal quantum dot lasers for practical photonic devices.

Published
2016

45. Exciton Recombination, Energy-, and Charge Transfer in Single- and Multilayer Quantum-Dot Films on Silver Plasmonic Resonators

Author

Shin, Taeho, primary, Cho, Kyung-Sang, additional, Yun, Dong-Jin, additional, Kim, Jinwoo, additional, Li, Xiang-Shu, additional, Moon, Eui-Seong, additional, Baik, Chan-Wook, additional, Il Kim, Sun, additional, Kim, Miyoung, additional, Choi, Jun Hee, additional, Park, Gyeong-Su, additional, Shin, Jai-Kwang, additional, Hwang, Sungwoo, additional, and Jung, Tae-Sung, additional

Published
2016
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46. Wet‐Transfer of Freestanding Dense Colloidal Quantum Dot Films and Their Photonic Device Application.

Author

Han, Changhyun, Jung, Hyunho, Lee, Jongho, Lee, Myungjae, Park, Yeonsang, Cho, Kyung‐Sang, and Jeon, Heonsu

Subjects
SEMICONDUCTOR nanocrystals, QUANTUM dots, COLLOIDAL crystals, METHYL methacrylate, POLARITONS, SURFACE structure, SURFACE morphology
Abstract

A method of wet‐transferring a densely packed colloidal quantum dot (CQD) film is reported herein. Layers of poly(vinyl alcohol) (sacrificial layer) and poly(methyl methacrylate) (temporary mechanical support) are spin‐coated below and above the CQD film, respectively; the two layers are eventually removed, resulting in a single CQD film on an alien substrate. CQD films as large as 1 cm × 1 cm are transferred to any substrate, including flexible and prepatterned ones, with very little mechanical or optical damage. The wet‐transfer method can be repeated to form a CQD heterostructure. A CQD distributed feedback (DFB) laser is also demonstrated by transferring a thick CdSe‐based CQD layer (for both optical gain and waveguiding) on top of a surface grating structure. The resultant CQD structure exhibits a single‐mode DFB lasing action, with a high spontaneous emission factor (β ≈ 10−3), owing to a strong vertical mode confinement and smooth surface morphologies on both sides of the CQD waveguide layer. [ABSTRACT FROM AUTHOR]

Published
2018
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