The transmission property of microwave through a kind of metamaterial is investigated experimentally.Such metamaterials are fabricated with a structure of double sets of square holes:subwavelength ones,and small ones whose sizes and spacing are smaller at least one order of magnitude than those of the subwavelength holes.The experimental results show that the peak power of the measured transmission spectra is dependent on the structure parameters of the small holes.The physical origin to create this phenomenon is that the designer surface plasmons sustained by the small holes affect on the transmission property of the microwave passing through the subwavelength holes.The results are promising for proposing some techniques for optoelectronic devices in terahertz and microwave regime.
The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz^0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN：Mg crystal is approximately in a range of 22 cm- 1_35 cm- 1 in a frequency range of 0.2 THz-0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH- absorption in this crystal.
The influence of the amplitude ratio between the two THz pulses on two-dimension THz spectroscopy(2DTS)has been studied theoretically via a classical method in which the expressions for the second-order nonlinearity were derived using perturbation approach,and the THz pulses were not treated as a delta function.Three types of nonlinear sources i.e.,anharmonicity,nonlinear damping,and nonlinear coupling,are considered in a single mode system.The simulation results demonstrated that the amplitude ratio had a notable influence on the 2DTSs,and different sources have different influences.This study is promising for guiding future experiments.
Jiangsheng HuJinsong LiuHuquan LiKejia WangZhengang YangShenglie Wang
The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.
The nonlinear radiation responses of two different n-doped bulk semiconductors: indium antimonide(In Sb) and indium arsenide(In As) in an intense terahertz(THz) field are studied by using the method of ensemble Monte Carlo(EMC)at room temperature. The results show that the radiations of two materials generate about 2-THz periodic regular spectrum distributions under a high field of 100 k V/cm at 1-THz center frequency. The center frequencies are enhanced to about 7 THz in In Sb, and only 5 THz in In As, respectively. The electron valley occupancy and the percentage of new electrons excited by impact ionization are also calculated. We find that the band nonparabolicity and impact ionization promote the generation of nonlinear high frequency radiation, while intervalley scattering has the opposite effect. Moreover, the impact ionization dominates in In Sb, while impact ionization and intervalley scattering work together in In As. These characteristics have potential applications in up-convension of THz wave and THz nonlinear frequency multiplication field.
Using a well-developed transient photocurrent model, we combine theoretical and numerical studies on the production of terahertz(THz) radiation in laser-induced air plasma. Air molecules are excited and then ionized by the laser-pulses mixing the fundamental and second harmonic fields. As a result, a non-vanishing directional photoelectron current emerges, radiating the THz pulse wave. We mainly discuss the influences of optical wavelength on THz radiation in five di?erent cases. Results show that the fundamental-pulse width(50 fs), energy, and relative phase between two pulses are invariant. When the laser wavelength(from 400 nm to 1.25 μm) becomes longer, the emitted THz fields are significantly enhanced, with a linear dependence on the optical wavelength.
