Chul Kang

Terahertz conductivity of anisotropic single walled carbon nanotube films

Absorption and dispersion of singlewalled carbon nanotube films were measured using an optoelectronic THz beam system for THz time-domain spectroscopy. The anisotropically aligned nanotube films were prepared through simple mechanical squeezing with a bar coater. The angle-dependent absorption and dispersion values were then measured. Results indicate that the index of refraction decreases with increasing frequency (0.1–0.8 THz), whereas the real conductivity increases with increasing frequency. The real conductivity measured is not congruent with the simple Drude model, but it follows a Maxwell–Garnett model, where the nanotubes are embedded in a dielectric host.

Optical and electrical properties of preferentially anisotropic single-walled carbon-nanotube films in terahertz region

The absorption and dispersion of aligned single-walled carbon-nanotube films were measured from 0.2 to 2.0 THz using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation. The real conductivity increased rapidly with increasing frequency up to 0.45 THz and decreased at a high-frequency range. The Maxwell–Garnett model, where the nanotubes were embedded in a dielectric host, fit the results of this study with the Drude–Lorentz model for nanotube network. We have observed the transverse phonon mode of 2.4 THz propagating along the c direction. This suggested that the carbon nanotube network is composed of metallic and semiconducting nanotubes embedded in an air dielectric host.

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.

Geometry-dependent terahertz emission of silicon nanowires

THz emission was observed from the vertically aligned silicon nanowire (Si NW) arrays, upon the excitation using a fs Ti-sapphire laser pulse (800 nm). The Si NWs (length = 0.3 approximately 9 microm) were synthesized by the chemical etching of n-type silicon substrates. The THz emission exhibits significant length dependence; the intensity increases sharply up to a length of 3 mum and then almost saturates. Their efficient THz emission is attributed to strong local field enhancement by coherent surface plasmons, with distinctive geometry dependence.

Organic conjugated material-based broadband terahertz wave modulators

A simple and efficient broadband terahertz (THz) wave modulator based on an organic conjugated material thin film, 200-nm thick organic copper phthalocyanine (CuPc) film, deposited on a Si wafer was proposed. External laser beams significantly decrease the transmittance of THz pulses through the CuPc film over all frequency of the pulse. Modulation efficiency reaches as high as 55% under a cw-laser beam of 80 mW.

Terahertz near-field enhancement in narrow rectangular apertures on metal film

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A huge far-field transmission enhancement is observed in samples of various widths ranging from 10 mum to 1.8 mum which correspond to only an order of lambda/100. Theoretical calculations based on vector diffraction theory indicates 230 times near-field enhancement in case of the 1.8 mum wide rectangular aperture. Transmission measurement through the single rectangular aperture shows that the shape resonance, not the periodicity, is mainly responsible for the transmission enhancement and the corresponding field enhancement.

Frequency-dependent optical constants and conductivities of hydrogen-functionalized single-walled carbon nanotubes

The frequency-dependent optical constants and electrical conductivities of hydrogen-functionalized single-walled carbon nanotubes (SWNTs) have been measured from the 0.2 to 1.5 THz region using a terahertz time domain spectroscopy. The indices of refraction and electrical conductivities of the sample after hydrogen functionalization were smaller than those of the sample before hydrogen functionalization. The experimental results were fitted using the Maxwell–Garnett model, and a reduction of plasma frequency was observed. This can be attributed to the fact that the hydrogen functionalization has reduced the number of free carriers with the bonding change from sp2 to sp3.

Strongly Enhanced THz Emission caused by Localized Surface Charges in Semiconducting Germanium Nanowires

A principal cause of THz emission in semiconductor nanostructures is deeply involved with geometry, which stimulates the utilization of indirect bandgap semiconductors for THz applications. To date, applications for optoelectronic devices, such as emitters and detectors, using THz radiation have focused only on direct bandgap materials. This paper reports the first observation of strongly enhanced THz emission from Germanium nanowires (Ge NWs). The origin of THz generation from Ge NWs can be interpreted using two terms: high photoexcited electron-hole carriers (Δn) and strong built-in electric field (Eb) at the wire surface based on the relation . The first is related to the extensive surface area needed to trigger an irradiated photon due to high aspect ratio. The second corresponds to the variation of Fermi-level determined by confined surface charges. Moreover, the carrier dynamics of optically excited electrons and holes give rise to phonon emission according to the THz region.

Highly efficient terahertz wave modulators by photo-excitation of organics/silicon bilayers

Using hybrid bilayer systems comprising a molecular organic semiconductor and silicon, we achieve optically controllable active terahertz (THz) modulators that exhibit extremely high modulation efficiencies. A modulation efficiency of 98% is achieved from thermally annealed C60/silicon bilayers, due to the rapid photo-induced electron transfer from the excited states of the silicon onto the C60 layer. Furthermore, we demonstrate the broadband modulation of THz waves. The cut-off condition of the system that is determined by the formation of efficient charge separation by the photo-excitation is highly variable, changing the system from insulating to metallic. The phenomenon enables an extremely high modulation bandwidth and rates of electromagnetic waves of interest. The realization of near-perfect modulation efficiency in THz frequencies opens up the possibilities of utilizing active modulators for THz spectroscopy and communications.

A diameter-selective chiral separation of single-wall carbon nanotubes using nitronium lons

We propose a method for a diameter-selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) from semiconducting (s-) ones. Our separation technique is capable of 100% separation of semiconducting and metallic nanotubes for small diameter nanotubes. We dispersed SWCNT powder by sonication in a mixed solution of tetramethylene sulfone and chloroform, where nitronium ions were well disolved. Positively charged nitronium ions were intercalated into nanotube bundles, where the intercalation was promoted also by the counter ions. Nitronium ions selectively attacked the sidewall of m-SWCNTs due to the abundant presence of electron density at the Fermi level, thus yielding stronger binding energy compared to the counterpart s-SWCNTs. The s-SWCNTs were left on the filter after filtration, whereas m-SWCNTs were perfectly destroyed by nitronium ions and drained away as amorphous carbons. This preferable adsorption became obscured for nanotubes with diameters greater than 1.1 nm. The effectiveness of removing m-SWCNTs was confirmed by the transmission electron microscope observations, x-ray photoemission spectra, resonant Raman spectra, and absorption spectra.