Browsing by Author "Ku, S. A."

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A wide range THz pulses generation in ZnTe crystals
(2012)
Ku, S. A.
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Tu, C. -M.
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Luo, C. W.
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Wu, K. H.
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Yabushita, A.
The THz generation in ZnTe crystals by widely varying pumping fluences is reported. Some abnormal nonlinear effects were observed including the evolution of THz spectra and the phase shift of temporal waveforms. Moreover, the THz efficiency can enhance again at high pumping fluence due to the saturation of free carrier.
Comment on "GaSe1-xSx and GaSe1-xTex thick crystals for broadband terahertz pulses generation" [Appl. Phys. Lett. 99, 081105 (2011)]
(2012-03-26)
Ku, S. A.
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Luo, C. W.
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Andreev, Yu M.
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Lanskii, Grigory
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電子物理學系
GaSe1-xSx Crystals for Teraherz Frequency Range
(2009)
Atuchin, Victor V.
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Andreev, Yury M.
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Sarkisov, Sergei Yu.
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Morozov, A. N.
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Luo, C. -W.
GaSe1-xSx single crystals has been grown by Bridgman-Stockbarger technique. Structural quality has been evaluated with TEM observation. Terahertz time-domain spectroscopy has been applied for measurements of the frequency-dependent optical constant of doped GaSe crystals as a function of s-content.
Pulsed terahertz radiation due to coherent phonon-polariton excitation in < 110 > ZnTe crystal
(2012-11-01)
Tu, C. M.
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Ku, S. A.
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Chu, W. C.
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Luo, C. W.
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Chen, J. C.
Pulsed terahertz (THz) radiation, generated through optical rectification (OR) by exciting < 110 > ZnTe crystal with ultrafast optical pulses, typically consists of only a few-cycles of electromagnetic field oscillations with a duration about a couple of picoseconds. However, it is possible, under appropriate conditions, to generate a long damped oscillation tail (LDOT) following the main cycles. The LDOT can last tens of picoseconds and its Fourier transform shows a higher and narrower frequency peak than that of the main pulse. We have demonstrated that the generation of the LDOT depends on both the duration of the optical pulse and its central wavelength. Furthermore, we have also performed theoretical calculations based upon the OR effect coupled with the phonon-polariton mode of ZnTe and obtained theoretical THz waveforms in good agreement with our experimental observation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764917]
Saturation of the free carrier absorption in ZnTe crystals
(2013-06-17)
Ku, S. A.
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Tu, C. M.
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Chu, W. -C.
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Luo, C. W.
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Wu, K. H.
This study systematically investigates the influence of free carriers on the generation of THz in ZnTe crystals, over a wide range of pumping fluences. As the pumping fluence is increased (< 6.36 mJ/cm(2)), the concentration of free carriers gradually increases and the THz output power is saturated, as clearly demonstrated by the time delay in the THz temporal waveforms, the changes in the THz spectral weight and the red-shift in the PL spectra. For high pumping fluences (> 6.36 mJ/cm(2)), spectacularly, there is a significant quadratic increase in the THz output power when the pumping fluence is increased, as well as at low pumping fluences of < 0.58 mJ/cm(2), because of the saturation of free carriers. (C) 2013 Optical Society of America
Snapshots of Dirac Fermions near the Dirac Point in Topological Insulators
(2013-12-01)
Luo, C. W.
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Wang, H. J.
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Ku, S. A.
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Chen, H. -J.
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Yeh, T. T.
The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to 4 explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.
Study of GaSe1-xSx Properties for Terahertz Applications
(2009)
Sarkisov, S. Yu.
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Andreev, Yu. M.
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Atuchin, V. V.
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Morozov, A. N.
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Kazakov, A. V.
For prospective nonlinear crystals GaSe1-xSx experimental study of second harmonic generation processes of Er3+:YSGG-, CO2- and CO-lasers has been performed, control experiment on difference frequency generation of two lines Of CO2-laser has been realized, the transmission spectra and dispersion of complex refractive indices in terahertz range have been measured.
Ultrafast dynamics in topological insulators
(2013)
Luo, C. W.
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Chen, H. -J.
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Wang, H. J.
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Ku, S. A.
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Wu, K. H.
Ultrafast dynamics of carriers and phonons in topological insulators CuxBi2Se3-y (x= 0, 0.1, 0.125, y= 0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (Delta R/R), which is assigned to the coherent optical phonon (A(1g)(1)). According to the red shift of A(1g)(1) phonon frequency, the Cu atoms in CuxBi2Se3 crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in Delta R/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.