Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy （THz-TDS）.T...Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy （THz-TDS）.The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material,thus,THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale.We report on the observed THz emission waveforms emitted from GaAs by using a novel technology,the time domain THz electro-optic （EO） sampling,which has a bipolar feature,i.e.,an initial positive peak and a subsequent negative dip that arises from its velocity overshoot.The initial positive peak has been interpreted as electron acceleration in the bottom of Γ valley in GaAs,where electrons have a light effective mass.The subsequent negative dip has been attributed to intervalley transfer from Γ to X and L valleys.Furthermore,the power dissipation spectra of the bulk GaAs in THz range are also investigated by using the Fourier transformation of the time domain THz traces.From the power dissipation spectra,the cutoff frequency for negative power dissipation （i.e.,gain） under step electric field in the bulk GaAs can also be obtained.The cutoff frequency for the gain gradually increases with increasing electric fields up to 50 kV/cm and achieves saturation at approximately 1 THz at 300 K.Furthermore,based on the temperature dependence of the cutoff frequency,we find that this cutoff frequency is governed by the energy relaxation process of electrons from L to Γ valley via successive optical phonon emission.展开更多
An overview of the major techniques to generate and detect THz radiation so far,especially the major approaches to generate and detect coherent ultra-short THz pulses using ultra-short pulsed laser,has been presented....An overview of the major techniques to generate and detect THz radiation so far,especially the major approaches to generate and detect coherent ultra-short THz pulses using ultra-short pulsed laser,has been presented.And also,this paper,in particularly,focuses on broadband THz spectroscopy and addresses on a number of issues relevant to generation and detection of broadband pulsed THz radiation as well as broadband time-domain THz spectroscopy （THz-TDS） with the help of ultra-short pulsed laser.The time-domain waveforms of coherent ultra-short THz pulses from photoconductive antenna excited by femtosecond laser with different pulse durations and their corresponding Fourier-transformed spectra have been obtained via the numerical simulation of ultrafast dynamics between femtosecond laser pulse and photoconductive material.The origins of fringes modulated on the top of broadband amplitude spectrum,which is measured by electric-optic detector based on thin nonlinear crystal and extracted by fast Fourier transformation,have been analyzed and the major solutions to get rid of these fringes are discussed.展开更多
基金supported by the Nanotechnology Special Project of Science and Technology Commission of Shanghai (No. 1052nm07100)the Ministry of Education Doctoral Fund of New Teachers of China (No. 20093120120007)the National Natural Science Foundation of China (No. 61007059)
文摘Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy （THz-TDS）.The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material,thus,THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale.We report on the observed THz emission waveforms emitted from GaAs by using a novel technology,the time domain THz electro-optic （EO） sampling,which has a bipolar feature,i.e.,an initial positive peak and a subsequent negative dip that arises from its velocity overshoot.The initial positive peak has been interpreted as electron acceleration in the bottom of Γ valley in GaAs,where electrons have a light effective mass.The subsequent negative dip has been attributed to intervalley transfer from Γ to X and L valleys.Furthermore,the power dissipation spectra of the bulk GaAs in THz range are also investigated by using the Fourier transformation of the time domain THz traces.From the power dissipation spectra,the cutoff frequency for negative power dissipation （i.e.,gain） under step electric field in the bulk GaAs can also be obtained.The cutoff frequency for the gain gradually increases with increasing electric fields up to 50 kV/cm and achieves saturation at approximately 1 THz at 300 K.Furthermore,based on the temperature dependence of the cutoff frequency,we find that this cutoff frequency is governed by the energy relaxation process of electrons from L to Γ valley via successive optical phonon emission.
基金supported by the National Basic Research Program of China (No. 2007CB310405)the "Hundreds of Talents Programs" of the Chinese Academy of Sciences (No. J08-029)+1 种基金the Innovative Project of the Chinese Academy of Sciences (No. YYYJ-1123)the CAS/SAFEA International Partnership Program for Creative Research Teams
文摘An overview of the major techniques to generate and detect THz radiation so far,especially the major approaches to generate and detect coherent ultra-short THz pulses using ultra-short pulsed laser,has been presented.And also,this paper,in particularly,focuses on broadband THz spectroscopy and addresses on a number of issues relevant to generation and detection of broadband pulsed THz radiation as well as broadband time-domain THz spectroscopy （THz-TDS） with the help of ultra-short pulsed laser.The time-domain waveforms of coherent ultra-short THz pulses from photoconductive antenna excited by femtosecond laser with different pulse durations and their corresponding Fourier-transformed spectra have been obtained via the numerical simulation of ultrafast dynamics between femtosecond laser pulse and photoconductive material.The origins of fringes modulated on the top of broadband amplitude spectrum,which is measured by electric-optic detector based on thin nonlinear crystal and extracted by fast Fourier transformation,have been analyzed and the major solutions to get rid of these fringes are discussed.
