Bin Jin

Ultrafast high-field carrier transport in GaAs measured by femtosecond pump-terahertz probe spectroscopy

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail for various pump powers and at electric fields up to 15kV∕cm. The pump-induced attenuation of terahertz transmission reduces obviously at high field, and the carrier relaxation time is also found to be correlated with photoinjected carrier density and electric field. These effects can be fully explained in terms of the carrier intervalley scattering and the surface states filling in GaAs, which may influence the carrier recombination process. Moreover, the carrier screening effect at high pump powers is also discussed.

Optical property and spectroscopy studies on the explosive 2,4,6-trinitro-1,3,5-trihydroxybenzene in the terahertz range

Using the terahertz time-domain spectroscopy, the complex refraction index and dielectric function of the pure 2,4,6-trinitro-1,3,5-trihydroxybenzene (TNPG) are calculated with the obtained parameters of the pure polyethylene (PE) and a mixture of TNPG and PE based on the Bruggeman effective medium theory. The theoretical investigation consistent with the experimental data in the vibration spectra of TNPG is presented in the range of 0.2–2.5THz. The results reveal that the two absorption features identified as the fingerprint of TNPG in our studied range are mainly dominated by the intramolecular collective vibration modes.

Terahertz Electromagnetic Response for Metamaterial

Recently, more and more groups are concentrating on learning surface plasma on metal split with sub-wavelength of terahertz. There is a new material which is made of two materials named metamaterial. It is combined with semiconductor and metal materials. Metal parts are made with subwavelength microstructure. Abnormal phenomenon occurred when it is excited by an intense light, showing an enhancement transmission of terahertz wave. In this paper, we design a planar structure composed of gallium arsenide (substrate) and copper (metal). Copper array is plating on a GaAs wafer on order of micron with photoetching technology. The thickness of GaAs and copper layer are 650 and 0.5 micros individually. THz incidence has an enhanced transmission like surface plasmonic resonance with terahertz time-domain system when another exciting lighted on, which made current carriers in photoconductor. This resonance arises from an inductor-capacitor circuit resonance. Comparison is performed between results with and without excitation. The angle of sample plane with terahertz polarization and incidence can affect this electromagnetic response in some degree. Its transmittance with back-incidence is much stronger than that with front-incidence at certain frequencies. This kind of artificial structure has potential abilities in terahertz devices in the future. It is helpful to develop terahertz filter, polarizing film, beam reflector, phase retarder and so on.

Carrier dynamics of doped silicon measured by femtosecond pump-terahertz probe spectroscopy

The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p- Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the relative change of terahertz transmission of p-Si at low pump power is slightly lower than that of n-Si, due to the lower carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At high pump power, the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model, showing that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of semiconductors.

Ultrafast carrier dynamics and terahertz conductivity of photoexcited GaAs under electric field

Ultrafast carrier dynamics in semiconductors has attracted much attention due to the application in high speed devices. Compared to the conventional experimental techniques, such as the time-resolved optical transmission technique and the all-optical pump-probe spectroscopy, the optical pump-terahertz probe spectroscopy has a plethora of advantages to provide the ability to temporally resolve phenomena at the fundamental timescales of carrier motion. The distinct advantage of OPTP is being able to directly measure the photo-induced changes in the photoconductivity, which contains the information of carrier density and mobility, with a temporal resolution of sub-picosecond. The ultrafast carrier dynamics and transient terahertz photoconductivity in semi-insulating GaAs have been investigated under electric field by using optical pump-terahertz probe technique with an unchanged pump power irradiating on the GaAs surface. One-dimensional pump scan at the maximum value of the THz pulse under electric fields of 0 kV/cm, 6 kV/cm, and 15 kV/cm, respectively. The measurements indicate that the terahertz transmission change induced by the pump pulses at high electric field is smaller than that without electric field. It is obvious that the threshold value of E, which begins to enhance the transmission, is about 3 - 4 kV/cm. We attribute this phenomenon to carrier scattering into the L valley or even X valley, which leads to a drop in carrier mobilities due to the large effective masses in those satellite valleys. The calculated transient photoconductivities fit well with the Drude-Smith model, which attributes the negative imaginary conductivity to the backward scattering of electrons. The negative value of c1 in our fitting implies that a fraction, but not all, of the backward scattering is a result of the electron reflecting from surfaces. It could also result from a Coulombic scattering between carriers. Due to the low mobilities of electrons in the L valley, the average mobility of all electrons will decrease under high E. These fitting results are consistent with our intervalley scattering model. Our investigation suggests that the OPTP technique is a very promising method for detecting the ultrafast dynamics in those materials.

The ultrafast photoconductive characteristics of GaAs bulk investigated by optical-pump terahertz probe spectroscopy

The ultrafast photoconductive characteristics of GaAs bulk were investigated by the optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. In our experiment, a significant decrease of the terahertz transmittance has been observed when the time delay between the optical pump pulse and the terahertz probe pulse was adjusted. It can be concluded that the electronic states of the GaAs became metallic, when the optical excitation occurred on the surface of the GaAs material. Since we expect only the free carrier response in the terahertz range, the decrease of the transmittance is obviously assigned to the appearance of the high electronic conductivity due to the increasing free carriers. Furthermore, it was found that transmittance of the terahertz radiation decreased with the increase of the optical pump power due to the more optical generated carriers under the higher power. We can also find that the carrier recombination time of the GaAs became shorter while the power of the optical pump was lowered.