Hyung Keun Yoo

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.

Simultaneous forward and backward terahertz generations in periodically poled stoichiometric LiTaO3 crystal using femtosecond pulses

Forward and counterpropagated backward terahertz generations were demonstrated via difference frequency generation using a femtosecond laser. By cooling the crystal to reduce terahertz losses, we achieved the terahertz generations of 1.36 and 0.65THz. Temperature-dependent measurements showed gradual intensity increase of the terahertz pulse and red shift of the center frequency as the temperature decrease from 255to143K, although there were insignificant decreases of the spectral bandwidth.

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.

Terahertz modulation on angle-dependent photoexcitation in organic-inorganic hybrid structures

The characteristics of terahertz (THz) modulation in organic copper phthalocyanine thin films deposited on a Si wafer were investigated by angle-dependent photoexcitation. We reveal that the efficiency of THz modulation reflects not only the angle-dependent reflectivity of the organic thin films that undergo a change of complex refractive index due to photoexcitation but also the laser-induced birefringence induced by a charge density grating in the direction normal to the polarization of the excitation beams.

Transmittances of Terahertz Pulses through Organic Copper Phthalocyanine Films on Si under Optical Carrier Excitation

Transmittances of terahertz (THz) pulses through organic copper phthalocyanine (CuPc) films on Si were investigated under optical carrier excitation. As the external laser power increases, the difference between transmitted energies of THz pulses along the forward (CuPc/Si) and backward (Si/CuPc) directions increases. The transmitted energy in the backward direction is larger than six times that in the forward direction under a laser beam of 300 mW. The big difference between the transmitted energies was explained in terms of the density of photocarriers injected into the CuPc film and carrier transport characteristics correlated with the degree of disorder of CuPc molecules.

Surface-enhanced Raman scattering detection of toluene and dichlorobenzene vapors using 1-propanethiol-linked Ag nanoparticles

Abstract. We have detected the surface-enhanced Raman scattering (SERS) signals of toluene and 1,2-dichlorobenzene (ODCB) vapors at parts per million concentrations using 1-propanethiol-linked Ag nanoparticles and a fiber optics–coupled Raman spectrograph. Calibration curves were constructed for each detector by plotting Raman band intensity ratios of toluene and ODCB with respect to that of 1-propanethiol as a function of vapor concentration. The detectors showed effective vapor sensing ranges of 0.6 to 600 and 10 to 600 ppm for toluene and ODCB, respectively, following vapor adsorption to the SERS hot spots. We provide evidence for the facility of the SERS detection system for instantaneously discriminating and quantifying respective vapors from a composite vapor.

Characteristics of carrier injection on polarization-dependent photoexcitation in copper phthalocyanine thin films

Abstract. The carrier concentration injected from a silicon substrate to a copper phthalocyanine thin film was found to depend on the incidence polarization of the photoexciting beam. The modulation efficiency of terahertz transmission due to transverse-magnetic (TM)-polarized excitation is distinctly higher than that due to transverse-electric (TE)-polarized excitation. Underlying this difference is the enhancement of carrier injection when the TM-polarized light is more transmitted through the surface of organic thin films than the TE-polarization light.

Temperature Dependent Terahertz Generation at Periodically Poled Stoichiometric Lithium Tantalate Crystal Using Femtosecond Laser Pulses

Coherent tunable terahertz generation was demonstrated in periodically poled stoichiometric lithium tantalate crystal via difference frequency generation of femtosecond laser pulses. Simultaneous forward and backward terahertz radiations were obtained around 1.35 and 0.63 THz, respectively at low temperature. By cooling the crystal to reduce losses caused by phonon absorptions, the generated THz bandwidth was as narrow as 23GHz at the center frequency of 0.63 THz. The measurement result of temperature-dependent showed gradual intensity increase of the generated terahertz pulse and red shift of the center frequency as the temperature decrease from 291 to 143 K, but insignificant reduction of the spectral bandwidth. Furthermore, the stoichiometric crystal was very suitable for the suppression of THz loss at low temperature compared to the congruent

Monopole resonators in planar plasmonic metamaterials.

LiNbO_3

Effects of uncertain phase-matching wave vectors of rotating fan-out type poled LiNbO 3 on THz generation

crystal.