Browsing by Author "Tu, C. M."

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Dynamics of phonons in Sr3Ir4Sn13: an experimental study by ultrafast spectroscopy measurements
(2016-07-26)
Luo, C. W.
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Cheng, P. C.
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Tu, C. M.
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Kuo, C. N.
;
Wang, C. M.
We report a study of ultrafast dynamics of photoexcited electrons and phonons in Sr3Ir4Sn13 using dual-color transient reflectivity change (Delta R/R) measurements. Time resolved optical spectroscopy of collective excitations reveal the marked features near its structural phase transition temperature T* similar or equal to 147 K. Two distinctive oscillatory timescales in Delta R/R have been clearly resolved. The rapid THz-range oscillations are attributed to the dynamics of the optical phonons which strongly correlate to the structural phase transition. The slow GHz-range oscillatory phenomenon which only occurs below about 150 K is associated with the dynamic response of the longitudinal-acoustic phonons. These low-energy phonons show a softening feature on approaching the transition temperature, also indicating a strong relevance to the structural phase transition. The information that we demonstrated would provide a deeper understanding of the structural phase transition in Sr3Ir4Sn13.
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.
;
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