Yulei Shi

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.

Study of L-shaped resonators at terahertz frequencies

In experimental and theoretical studies of terahertz subwavelength “L”-shaped structure, we proposed an unusual-mode current resonance responsible for low-frequency characteristic dip in transmission spectra. Comparing spectral properties of our designed simplified structures with that of split-ring resonators, we attribute this unusual mode to the resonance coupling and splitting under the broken symmetry of the structure. Furthermore, it is found that this phenomenon is also related to not only the spacing between horizontal and vertical sides but also their relative position in “L” structure, showing the weakened coupling and spectral splitting with the increased distance and structure symmetry.

Ultrafast optical modulation of terahertz metamaterials

We have characterized the ultrafast dynamical properties of split ring resonators by utilizing optical pump–terahertz probe spectroscopy on a subpicosecond timescale. The experimental results show that the pump-induced relative changes in terahertz peak transmission, as well as the dynamical transmission spectra, are different when the terahertz electric vector is perpendicular and parallel to the gap of the split ring resonator. It is found that the pump-induced photogenerated carriers play two opposite roles in SRRs, showing one is to absorb terahertz to reduce the transmission in the substrate and the other is to weaken the resonant absorption to enhance the transmission. Our numerical simulations also prove that the pump pulse significantly influences the split ring resonator currents. We also show that the blue-shifts of the resonances are due to the change of optical constants in the photoexcited semiconductor substrate.

2D signature for detection and identification of drugs

The method of spectral dynamics analysis (SDA-method) is used for obtaining the2D THz signature of drugs. This signature is used for the detection and identification of drugs with similar Fourier spectra by transmitted THz signal. We discuss the efficiency of SDA method for the identification problem of pure methamphetamine (MA), methylenedioxyamphetamine (MDA), 3, 4-methylenedioxymethamphetamine (MDMA) and Ketamine.

Quantitative Analysis for Monitoring Formulation of Lubricating Oil Using Terahertz Time-Domain Transmission Spectroscopy

The quantitative analysis of zinc isopropyl-isooctyl-dithiophosphate (T204) mixed with lube base oil from Korea with viscosity index 70 (T204-Korea70) is presented by using terahertz time-domain spectroscopy (THz-TDS). Compared with the middle-infrared spectra of zinc n-butyl-isooctyl-dithiophosphate (T202) and T204, THz spectra of T202 and T204 show the weak broad absorption bands. Then, the absorption coefficients of the T204-Korea70 system follow Beers law at the concentration from 0.124 to 4.024%. The experimental absorption spectra of T204-Korea70 agree with the calculated ones based on the standard absorption coefficients of T204 and Korea70. The quantitative analysis enables a strategy to monitor the formulation of lubricating oil in real time.

Out-of-plane resonances in terahertz photonic crystal slabs modulated by optical pumping

This paper describes detailed optical-pump-terahertz-probe studies of two-dimensional photonic crystal slabs for propagation perpendicular to the slabs. When the slabs are excited by an 800 nm pump pulse and the effect of shielding by photocarriers is removed, we find that the decaying tail in the transmitted terahertz radiation is strikingly enhanced. The photocarriers weaken guided resonances, but they also greatly enhance the excitation efficiency of guided resonances and the ability of the guided resonances to transfer energy back to the radiation field. This increases the resonance-assisted contribution to transmitted field. The photoinduced resonant extremes agree well with the Fano model.

Detection and identification of selected alcohols using terahertz time-domain spectroscopy

The optical property and spectroscopy of selected kinds of alcohols are studied based on the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.5-2.5 THz. The B3LYP method in conjunction with the 6-31G (d) basis set was used to optimize the geometric structure of samples and simulate the alcohols spectrum. No absorption peak occurs in THz wave band based on our simulation, suggesting that the present experimental absorption peaks resulted from the intermolecular interaction.

Anomalous transmission of terahertz waves in arrays of double-ring resonators induced by a 400 nm pump pulse

In this letter, we describe detailed femtosecond-pump–terahertz-probe studies of the electromagnetic response of arrays of double-ring and complementary double-ring resonators. A surprising result is that an increase in the terahertz transmission is observed when a double-ring structure is excited by a 400 nm pump pulse. To explain this anomalous transmission, we propose a mechanism in which the photogenerated carriers play two competing roles. In other cases, for instance, when the wavelength of the pump pulse is 800 nm or in the case of a complementary double-ring structure, the normal transmission can also be understood in terms of this mechanism.

Lateral photovoltage of B-doped ZnO thin films induced by 10.6 mu m CO(2) laser

B-doped ZnO thin films were observed to have a lateral laser-induced photovoltaic effect: the saturation value varied very linearly with the 10.6 mu m constant laser spot position between the electrodes on the ZnO surface. It was found that the temperature gradient in the direction of electron transfer (along the film surface) due to the laser spot causes this photovoltage signal to be linearly dependent on the position of the laser spot in this isotropic system. This linearity is expected to make ZnO a candidate for position-sensitive photodetectors.

Collective vibrational spectra of a- and y-glycine studied by terahertz and raman spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption and dispersion of polycrystalline α - and γ -glycine in the spectral region 0.5-3.0 THz. The spectra exhibit distinct features in these two crystalline phases. The observed far-infrared responses are attributed to intermolecular vibrational modes mediated by hydrogen bonds. We also measure the Raman spectra of the polycrystalline and dissolved glycine in the frequency range 28-3900 cm-1. The results show that all the vibrational modes below 200 cm-1 are nonlocalized but are of a collective (phonon-like) nature. Furthermore, the temperature dependence of the Raman spectra of α -glycine agrees with the anharmonicity mechanism of the vibrational potentials.