Browsing by Author "Liu, TA"

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A new category of semiconductor nonlinear mirrors for ultrafast pulse generation and switching
(2001)
Pan, CL
;
Liu, TA
;
Shieh, JM
;
Huang, KF
;
光電工程學系
Correlation between defect concentration and carrier lifetime of GaAs grown by molecular beam epitaxy at different temperatures
(2001-11-01)
Lin, GR
;
Liu, TA
;
Pan, CL
;
光電工程學系
;
Department of Photonics
A pump-probe study of GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs) is performed. Ultrashort carrier lifetimes of the as-grown LT-GaAs increase from <0.13 ps (measurement limitation) to 1.8 +/-0.2 ps as the growth temperature is increased from 200 degreesC to 320 degreesC. The carrier lifetime was found to be approximately inversely proportional to the antisite defect concentration. This trend is found to be in reasonably good agreement with the Schokley-Read-Hall model. The decreasing trend in the amplitudes of continuous-wave and transient reflectivities (DeltaR/R) as a function of the growth temperature for the LT-GaAs is explained as an induced absorption caused by dense arsenic antisite defects. The sign of the transient DeltaR/R reversed for LT-GaAs grown at 200 degreesC. This is tentatively attributed to the band gap renormalization effect.
Dark current and trailing-edge suppression in ultrafast photoconductive switches and terahertz spiral antennas fabricated on multienergy arsenic-ion-implanted GaAs
(2005-07-01)
Liu, TA
;
Lin, GR
;
Lee, YC
;
Wang, SC
;
Tani, M
We report ultrafast (similar to 2.7 ps, instrument limited) switching responses of a multienergy- implanted GaAs:As' photoconductive switches (PCSs) with suppressed trailing edge and reduced dark. current. This material is highly resistive with dark current as low as 0.94 mu A at a bias of 40 V. The carrier mobility of the former is similar to 590 cm(2)/V s, resulting in a small-signal optical responsivity of similar to 2 mA/W. Pumped at 100 mW and biased at 80 V, the multienergy-implanted GaAs: As+ PCS exhibits peak response (0.35 V) comparable to the best result of single-energy-implanted ones. The improvement on photoconductive response is crucial for the generation of shorter terahertz emission pulses from spiral antennas fabricated on multienergy-implanted GaAs:As+ (0.8 ps) than single-energy-implanted GaAs (1.2 ps), with the central frequency blueshifted to 0.2 THz (from 0.15 THz) and the spectral bandwidth broadened to 0.18 THz (from 0.11 THz). (c) 2005 American Institute of Physics.
Efficient terahertz radiation generation from a bulk InAs mirror as an intracavity terahertz radiation emitter
(2000-04-15)
Liu, ZL
;
Ono, S
;
Ohtake, H
;
Sarukura, N
;
Liu, TA
Terahertz (THz) radiation is generated efficiently from a bulk InAs mirror with a shallow incidence angle inside the cavity of a femtosecond, mode-locked Ti:sapphire laser self-started by a strained saturable Bragg reflector A magnetic field is also applied to the InAs mirror to enhance THz radiation.
Frequency resolved optical gating studies of strained saturable Bragg reflector: Anomalous dispersion near resonance absorption of the exciton resonance
(2005-09-01)
Lee, CK
;
Liu, TA
;
Huang, KF
;
Pan, CL
;
電子物理學系
A strained saturable Bragg reflector (SSBR) for passive mode-locking of Ti:sapphire lasers was investigated by the frequency resolved optical gating (FROG) technique. Incident pulses of several wavelengths and with zero, positive, or negative chirp were employed. A considerable pulse shortening near the excitonic resonance of the strained quantum well was observed and attributed to anomalous dispersion due to resonance absorption. On the long wavelength side of exciton resonance, however, the chirp of the reflected pulse showed a weak wavelength dependence attributed to the material dispersion of SSBR in pulse broadening was obtained.
Generation of THz radiation from resonant absorption in strained multiple quantum wells in a magnetic field
(2001-07-01)
Liu, TA
;
Huang, KF
;
Pan, CL
;
Ono, S
;
Ohtake, H
Optically excited THz-radiation from ten molecular-beam epitaxy (MBE)-grown strained multiple quantum wells (MQWs) on a (100)-oriented semi-insulating GaAs substrate is studied in a 1-T magnetic field. Wavelength-dependent THz-radiation clearly exhibits a resonance behavior at the excitation wavelength near 830 nm, the peak wavelength of photoluminescence (PL). We also observe and explain an emission spectrum from the MQW with slightly enhanced higher-frequency components compared to that of bulk GaAs. In the thin MQW sample, the THz emission is still significant. This indicates the possibility of designing MQWs as efficient THz emitters in the future.
Generation of THz-radiation from resonant absorption in strained multiple-quantum-well in a magnetic field
(2001)
Liu, TA
;
Pan, CL
;
Huang, KF
;
Ono, S
;
Ohtake, H
Wavelength dependent THz-radiation from strained multiple-quantum-well by a 1-T magnetic field had shown the clear resonance behavior near the Photo-luminescence Peak. Comparisons with bulk Semi-insulating GaAs will also be presented.
High average power mode locked Ti : Sapphire laser with intracavity continuous-wave amplifier and strained saturable Bragg reflector
(1999-10-01)
Liu, TA
;
Huang, KF
;
Pan, CL
;
Liu, ZL
;
Ohtake, H
We demonstrate a new scheme for the generation of high average power femtosecond pulses by incorporating an intracavity amplifier and a strained saturable Bragg reflector with low saturation fluence for self-starting mode locking. When the Ti:sapphire oscillator and intracavity amplifier are pumped at 10W and 15W, respectively, the average output power is as high as 1.62 W. The pulse duration is about 145 fs, and the peak power reaches 160 kW at a 68 MHz repetition rate: The pulse-formation time of 400 mu s is sufficiently short to sustain stable mode locking.
Multi-energy arsenic-ion-implanted GaAs photoconductors for ultrafast switching and THz Generation
(2000)
Pan, CL
;
Liu, TA
;
Lin, GR
;
Tani, M
;
光電工程學系
Neuroimmunological evidence and enzymatic characterization of a novel dehydroepiandrosterone sulfotransferase in Drosophila brain
(2006-03-07)
Liu, TA
;
Yang, YS
;
生物科技學系
;
Department of Biological Science and Technology
Optical nonlinearity and ultrafast-carrier dynamics of a strained quantum-well saturable Bragg reflector
(2003-09-01)
Liu, TA
;
Shieh, JM
;
Huang, KF
;
Pan, CL
;
電子物理學系
We present observations of the wavelength-dependent time-resolved differential reflection in the multiple-strained-quantum-well saturable Bragg reflector (SSBR). It is shown that the large optical nonlinearity of transient reflectivity (DeltaR/R) of similar to4% for a pumping wavelength of 755 nm sharply reversed to -0.6% at 760 nm. The relationships of fast- and slow-carrier lifetimes with pumping wavelength are also discussed. We conclude that the strain-induced piezoelectric field and free-electron absorption play a dominant role in determining the optical nonlinearity and its carrier dynamics. This is the first time that wavelength-dependent time-resolved reflectivity in SSBR has been presented in detail.
Piezoelectric-transducer-based optoelectronic frequency synchronizer for control of pulse delay in a femtosecond passively mode-locked Ti : sapphire laser
(2003-05-20)
Lin, GR
;
Chang, YC
;
Liu, TA
;
Pan, CL
;
光電工程學系
We propose a piezoelectric transducer- (PZT-) based optoelectronic frequency synchronizer to control simultaneously changes in the repetition rate, the relative pulse delay, and the phase noise of a passively mode-locked femtosecond Ti:sapphire laser with an intracavity saturable Bragg reflector absorber with respect to an electronic frequency reference. An optoelectronic phase-locked-loop-based PZT feedback controller with a proportional, integral, and differential (PID) circuit and a tunable voltage regulator is designed to achieve frequency synchronization, phase-noise suppression, and delay-time tuning. When the controlling voltage is tuned from -2.6 to 2.6 V, the maximum pulse-delay range, tuning slope, and tuning resolution of the laser pulse-train are 11.3 ns, 2.3 ps/mV, and 1.2 ps, respectively. Setting the gain constant of the PID circuit at 10 or larger causes the delay-time tuning function to be linearly proportional to the controlling voltage. In the delay-time tuning mode the uncorrelated single-side-band phase-noise density of the frequency-synchronized laser is approximately -120 dBc/Hz at an offset frequency of 5 kHz, which is only 7 dBc/Hz; higher than that of the electrical frequency reference. The proposed system also supports linear, continuous switching, and programmable control of the delay time of Ti:sapphire laser pulses when they are frequency synchronized to external reference clocks. (C) 2003 Optical Society of America.
Terahertz field detection beyond 30 THz by proton-bombarded InP photoconductive antennas
(2005)
Liu, TA
;
Tani, M
;
Nakajima, M
;
Hangyo, M
;
Sakai, K
We show that photoconductive antennas fabricated on proton-bombarded InP (InP:H+) substrates with a proton dosage of 1x10(15) ions/cm(2) are promising for THz detection.
THz emission characteristics of dipole antennas fabricated on multi-energy arsenic-ionimplanted GaAs and semi-insulating GaAs
(2003)
Liu, TA
;
Tani, M
;
Lin, GR
;
Pan, CL
;
光電工程學系
We compare the performance of THz dipole antennas fabricated on Multi-energy arsenic-ion-implanted GaAs (multi-GaAs:As+) and Semi-insulating (S. I.) GaAs. High damage threshold biasing (> 60 kV/cm) and large saturating optical pumping power (> 30 mW) for multi-GaAs:As+ based devices are reported.
THz emission characteristics of photoconductive antennas with different gap size fabricated on arsenic-ion-implanted GaAs
(2001)
Liu, TA
;
Tani, M
;
Lin, GR
;
Pan, CL
;
光電工程學系
Significant difference in temporal and spectral characteristics of THz radiation emitted by large- (1 mm) and small- (5mum) aperture dipole antennas fabricated on arsenic-ion-implanted GaAs and undoped semi-insulating GaAs is reported and attributed to the geometry of the antenna.
THz radiation emission properties of multienergy arsenic-ion-implanted GaAs and semi-insulating GaAs based photoconductive antennas
(2003-03-01)
Liu, TA
;
Tani, M
;
Pan, CL
;
光電工程學系
;
Department of Photonics
We compare the performance of THz photoconductive (PC) emitter antennas fabricated on multienergy arsenic ion implanted GaAs (multi-GaAs:As+) and semi-insulating GaAs. High damage threshold biasing (>60 kV/cm) and large saturation optical-pumping power (similar to20 mW) for multi-GaAs:As+ based PC antennas are reported. Carrier mobility in the As ion implanted layer of GaAs:As+ was estimated to be about 150 cm(2)/V/s, which was comparable to that of. low,temperature GaAs. (C) 2003 American Institute of Physics.
THz radiation from intracavity saturable Bragg reflector in magnetic field with self-started mode-locking by strained saturable Bragg reflector
(1999-11-15)
Liu, TA
;
Huang, KF
;
Pan, CL
;
Liu, ZL
;
Ono, S
We demonstrate a new configuration for intracavity generation of THz radiation. A magnetic-field-biased saturable Bragg reflector (SBR) located inside the femtosecond laser cavity is the emitter, while a strained saturable Bragg reflector (SSBR) achieves self-started mode-locking without focusing. The calibrated power of the emitted THz radiation is estimated to be approximately 45 nW with a peak frequency at 0.72 THz and width of approximately 0.7 THz under a 0.88 T magnetic field. The quadratic dependence of THz-radiation power by the SBR on the magnetic field is also observed for the first time.
Ultrabroadband terahertz field detection by photoconductive antennas based on multi-energy arsenic-ion-implanted GaAs and semi-insulating GaAs
(2003-08-18)
Liu, TA
;
Tani, M
;
Nakajima, M
;
Hangyo, M
;
Pan, CL
The photoconductive antennas based on multi-energy arsenic-ion-implanted GaAs and semi-insulating GaAs are demonstrated to have a useful bandwidth beyond 20 THz for a gating laser pulse width of 15 fs. The bandwidth and signal-to-noise ratio are compared with those of reference photoconductive antennas based on low-temperature grown GaAs. (C) 2003 American Institute of Physics.
Ultrabroadband terahertz field detection by proton-bombarded InP photoconductive antennas
(2004-06-28)
Liu, TA
;
Tani, M
;
Nakajima, M
;
Hangyo, M
;
Sakai, K
Photoconductive ( PC) antennas fabricated on InP bombarded with 180 keV protons of different dosages (InP:H+) all exhibit a useful bandwidth of about 30 THz, comparable to that of the LT-GaAs PC antenna. The peak signal current of the best InP: H+ device (dosage of 10(15) ions/cm(2)) is slightly higher than that of the LT-GaAs one, while the signal-to-noise ratio (SNR) of the former is about half of that of the latter due to lower resistivity. This suggests that InP: H+ can be a good substrate for THz PC antennas with proper annealing and/or implantation recipe. (C) 2004 Optical Society of America.
Wireless audio and burst communication link with directly modulated THz photoconductive antenna
(2005-12-12)
Liu, TA
;
Lin, GR
;
Chang, YC
;
Pan, CL
;
光電工程學系
We demonstrate transmission of audio and burst signals through a prototype THz analog communication link employing laser-gated low-temperature-grown GaAs dipole antenna as THz emitter and receiver. The transmission distance is about 100 cm. By using a direct voltage modulation format, we successfully demodulated a burst signal with a rising time of 41 mu s. The corresponding modulating bandwidth achieved was 23 kHz in this first experiment. Noise analysis reveals a 10% power fluctuation in the received signal with on-off extinction ratio of greater than 1000. The transmission of a six-channel analog and burst audio signal with least distortion is also demonstrated. We further demonstrate the fidelity of the transmission of a melody through the THz link with and without any amplification.