Inhee Maeng

Gate-Controlled Nonlinear Conductivity of Dirac Fermion in Graphene Field-Effect Transistors Measured by Terahertz Time-Domain Spectroscopy

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.

Nanoparticle-enabled terahertz imaging for cancer diagnosis.

This paper demonstrates the principle of the nanoparticle-contrast-agent-enabled terahertz imaging (CATHI) technique, which yields a dramatic sensitivity of the differential signal from cancer cells with nanoparticles. The terahertz (THz) reflection signal increased beam by 20% in the cancer cells with nanoparticles of gold nano-rods (GNRs) upon their irradiation with a infrared (IR) laser, due to the temperature rise of water in cancer cells by surface plasma ploritons. In the differential mode, the THz signal from the cancer cells with GNRs was 30 times higher than that from the cancer cells without GNRs. As the high sensitivity is achieved by the surface plasmon resonance through IR laser irradiation, the resolution of the CATHI technique can be as good as a few microns and THz endoscopy becomes more feasible.

Molecular imaging with terahertz waves

A novel terahertz molecular imaging technique using nanoparticle probes is discussed in terms of sensitivity, resolution, and quantification. The in-vivo diagnostic results of cancers are also presented as an example.

Terahertz electrical and optical characteristics of double-walled carbon nanotubes and their comparison with single-walled carbon nanotubes

The electrical and optical properties of double-walled carbon nanotubes (DWNTs) have been characterized and compared with those of single-walled carbon nanotubes (SWNTs) utilizing terahertz time-domain spectroscopy. The power absorption and the complex refractive indices of DWNTs are smaller than those of SWNTs. The conductivity of DWNTs was also observed to be smaller. The experimental results have been fitted with the Bruggman effective medium approximations, which has yielded the transport parameters of DWNTs such as plasma frequency, damping rate, etc.

Measurement of carrier concentration captured by InAs∕GaAs quantum dots using terahertz time-domain spectroscopy

The authors investigated the carrier dynamics of n-type modulation-doped InAs∕GaAs quantum dots (QDs) using terahertz time-domain spectroscopy to estimate the total number of electrons captured by the QDs. The terahertz power absorption of the sample with QDs was less than that of the sample without QDs. This is attributed to the fact that the carriers are confined in the QDs. The experiment results were fitted into the Drude model and the number of electrons captured by QDs was determined through the difference in the numbers of free electrons of the samples with and without QDs.

Tailoring the Spectra of Terahertz Emission from CdTe and ZnTe Electro-Optic Crystals

We compare the spectral amplitude of terahertz pulses generated using difference-frequency mixing, via the second-order nonlinear process in CdTe and ZnTe, with numerical model calculations using the nonlinear Maxwell equation. We have found that the spectral details of the generated terahertz pulses from CdTe are well explained by considering the effects of optical and terahertz absorptions, diffraction, and the frequency dependence of nonlinear coefficients. This method allows us to design tailored compound materials with improved versatility for generating specific terahertz pulse shapes.

Ultrafast zero balance of the oscillator-strength sum rule in graphene.

Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation, responsible for optoelectronic operation such as graphene-based phototransistor or solar-energy harvesting devices.

Nanoparticle contrast agents for Terahertz medical imaging

We show in this study that nanoparticle composites can be used as contrast agents to enhance the sensitivity of terahertz medical imaging. The terahertz reflection signal increases by nearly 50 % for the sample with nanoparticles upon near infrared laser beam irradiation. This is due to the hyperthermia effect induced by surface plasma polaritons. This technique facilitates cancer diagnosis with a micron resolution.

High-sensitivity terahertz imaging technique using nanoparticle probes for medical applications

We present the principle and in vivo experimental results of high sensitivity terahertz imaging technique which enables the target specific sensing of cancers and the molecular imaging of drug delivery.

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.