Gyuseok Lee

Subsurface nanoimaging by broadband terahertz pulse near-field microscopy.

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.

Quantitative coherent scattering spectra in apertureless terahertz pulse near-field microscopes

We present quantitative coherent measurements of scattering pulses and spectra in terahertz apertureless near-field microscopes. Broadband near-field image contrasts for both amplitude and phase spectra are measured directly from time-domain scattering signals with an unprecedentedly high single-scan signal-to-noise ratio (∼48 dB), with approach curves for both short (<200 nm) and long (up to 82 μm) ranges. By using the line dipole image method, we obtain quantitative broadband THz imaging contrasts with nanoscale resolution.

Terahertz time-domain spectroscopy of anisotropic complex conductivity tensors in silicon nanowire films

The effective complex conductivity tensor of a highly anisotropic, vertically aligned silicon nanowire film was measured by terahertz time-domain spectroscopy. The silicon nanowires were fabricated on a p-type silicon substrate by metal-assisted chemical etching, which resulted in a film with uniaxially anisotropic optical properties. The measured terahertz transverse and longitudinal conductivity values were in excellent agreement with the results of calculations based on the Drude-Smith and Lorentz models, respectively.

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

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.

High-precision THz Dielectric Spectroscopy of Tris-HCl Buffer

Tris-HCl buffer solution is extensively used in biochemistry and molecular biology to maintain a stable pH for biomolecules such as nucleic acids and proteins. Here we report on the high-precision THz dielectric spectroscopy of a 10 mM Tris-HCl buffer. Using a double Debye model, including conductivity of ionic species, we measured the complex dielectric functions of Tris-HCl buffer. The fast relaxation time of water molecules in Tris-HCl buffer is ~20% longer than that in pure water while the slow relaxation time changes little. This means that the reorientation dynamics of Tris-HCl buffer with such a low Tris concentration is quite different from that of pure water.