Abstract-Computed terahertz near-field mapping of molecular resonances of lactose stereo-isomer impurities with sub-attomole sensitivity

Kiwon Moon, Youngwoong Do, Hongkyu Park, Jeonghoi Kim, Hyuna Kang, Gyuseok Lee, Jin-Ha Lim, Jin-Woo Kim,  Haewook Han

https://www.nature.com/articles/s41598-019-53366-0

Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints. However, its realization has been impeded by low spatial and spectral resolutions and lack of theoretical models to quantitatively analyze near-field imaging. Here, we show that THz scattering-type scanning near-field optical microscopy (THz s-SNOM) with a theoretical model can quantitatively measure and image such low-energy molecular interactions, permitting computed spectroscopic near-field mapping of THz molecular resonance spectra. Using crystalline-lactose stereo-isomer (anomer) mixtures (i.e., α-lactose (≥95%, w/w) and β-lactose (≤4%, w/w)), THz s-SNOM resolved local intermolecular vibrations of both anomers with enhanced spatial and spectral resolutions, yielding strong resonances to decipher conformational fingerprint of the trace β-anomer impurity. Its estimated sensitivity was ~0.147 attomoles in ~8 × 10−4 μm3 interaction volume. Our THz s-SNOM platform offers a new path for ultrasensitive molecular fingerprinting of complex mixtures of biomolecules or organic crystals with markedly enhanced spatio-spectral resolutions. This could open up significant possibilities of THz technology in many fields, including biology, chemistry and condensed matter physics as well as semiconductor industries where accurate quantitative mappings of trace isomer impurities are critical but still challenging.

Abstract-Sub-cycle Control of Optical Response by Using a Terahertz Excitonic Dressed State

Gyuseok Lee, Inhee Maeng, Chul Kang, Myoung-Kyu Oh, Chul-Sik Kee

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-26-10-13677&id=389598

Optically tunable, strong polarization-dependent transmission of terahertz pulses through aligned Ag nanowires on a Si substrate is demonstrated. Terahertz pulses primarily pass through the Ag nanowires and the transmittance is weakly dependent on the angle between the direction of polarization of the terahertz pulse and the direction of nanowire alignment. However, the transmission of a terahertz pulse through optically excited materials strongly depends on the polarization direction. The extinction ratio increases as the power of the pumping laser increases. The enhanced polarization dependency is explained by the redistribution of photocarriers, which accelerates the sintering effect along the direction of alignment of the Ag nanowires. The photocarrier redistribution effect is examined by the enhancement of terahertz emission from the sample. Oblique metal nanowires on Si could be utilized for designing optically tunable terahertz polarization modulators.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Strong polarization-dependent terahertz modulation of aligned Ag nanowires on Si substrate

Gyuseok Lee, Inhee Maeng, Chul Kang, Myoung-Kyu Oh, and Chul-Sik Kee

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-26-10-13677&id=389598

Optically tunable, strong polarization-dependent transmission of terahertz pulses through aligned Ag nanowires on a Si substrate is demonstrated. Terahertz pulses primarily pass through the Ag nanowires and the transmittance is weakly dependent on the angle between the direction of polarization of the terahertz pulse and the direction of nanowire alignment. However, the transmission of a terahertz pulse through optically excited materials strongly depends on the polarization direction. The extinction ratio increases as the power of the pumping laser increases. The enhanced polarization dependency is explained by the redistribution of photocarriers, which accelerates the sintering effect along the direction of alignment of the Ag nanowires. The photocarrier redistribution effect is examined by the enhancement of terahertz emission from the sample. Oblique metal nanowires on Si could be utilized for designing optically tunable terahertz polarization modulators.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-High-efficiency optical terahertz modulation of aligned Ag nanowires on a Si substrate

Gyuseok Lee, Inhee Maeng, Chul Kang,   Myoung-Kyu Oh,  Chul-Sik Kee,

https://aip.scitation.org/doi/abs/10.1063/1.5008485

High-efficiency optical modulation of a terahertz pulse transmitted through aligned silver nanowires on a silicon substrate is demonstrated. Without optical excitation, the terahertz pulses mostly pass through the silver nanowires. However, an optically excited sample significantly modulates the transmittance compared with an excited silicon substrate. The enhanced modulation efficiency is explained by the redistribution effect of photo-carriers due to the nanowires. The simple structure of metal nanowires on a semiconductor substrate could be useful in implementing optically tunable terahertz wave modulators.

Abstract-Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field Microscopy

Kiwon Moon , Hongkyu Park , Jeonghoi Kim ,Youngwoong Do , Soonsung Lee , Gyuseok Lee ,Hyeona Kang , and Haewook Han *

Department of Electrical and Computer Engineering,POSTECH, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, Korea

Nano Lett., Article ASAP

DOI: 10.1021/nl503998v

Publication Date (Web): December 1, 2014

Copyright © 2014 American Chemical Society

*E-mail: hhan@postech.ac.kr.

http://pubs.acs.org/doi/abs/10.1021/nl503998v?src=recsys

Combined with terahertz (THz) time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides subsurface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the subsurface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ∼λ/3300, at 1 THz, while the AFM only provides a flat surface topography.

Abstract-Sub-surface nanoimaging by broadband terahertz pulse near-field microscopy

Kiwon Moon , Hongkyu Park , Jeonghoi Kim , Youngwoong Do , Soonsung Lee , Gyuseok Lee , Hyeona Kang , and Haewook Han

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl503998v

Publication Date (Web): December 1, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl503998v?journalCode=nalefd

Combined with THz time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides sub-surface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the sub-surface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ~λ/3300, at 1 THz, while the AFM only provides a flat surface topography.