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Effect of cross-focusing of two Laser beams on THz radiation in graphite nanoparticles with density ripple
摘要： The present work explores the study of terahertz (THz) wave generation by the cross-focusing of two Gaussian laser beams in a bulk medium containing graphite nanoparticles (NPs). This system acts as a nonlinear medium behaving like plasma. The ripple is applied on the density of NPs to increase the conversion efficiency. Using the perturbative technique, a wave equation is derived which describes the nonlinear interaction of the laser beam with NPs. The nonlinear current at THz frequency arises due to the ponderomotive force exerted on the electrons of graphite NPs by the incident laser beams in the presence of density ripple. Numerical results show that the normalized amplitude of the generated THz wave increases by 3.5 times with the consideration of the phenomenon of cross-focusing. The results show the enhancement in the efficiency of generated THz wave on application of density ripple, due to the fulfillment of the phase matching condition.
关键词： plasma,cross-focusing,nanoparticles,density ripple,THz radiation
更新于2025-09-11 14:15:04
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Spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks
摘要： A superconducting integrated receiver (SIR) comprises all of the elements needed for heterodyne detection on a single chip. Recently, the SIR was successfully implemented for the first spectral measurements of terahertz (THz) radiation emitted from intrinsic Josephson junction stacks (BSCCO mesa) at frequencies up to 750 GHz; a linewidth below 10 MHz has been recorded in the high bias regime. In this report the results of the spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks are summarized; recent results of spectrometric gas detection using THz radiation from a BSCCO mesa are presented.
关键词： BSCCO mesa,spectral measurements,heterodyne detection,intrinsic Josephson junction stacks,THz radiation
更新于2025-09-10 09:29:36
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Terahertz waves radiated from two plasma filaments with opposite directions
摘要： The interaction and the coupling between the plasma filaments introduce some ultrafast phenomena, which not only reveal deep physical mechanisms, but also provide clues to develop the plasma filament-based applications. In this paper, the terahertz (THz) radiation from two plasma filaments with opposite directions, which are produced by focusing two counter propagating femtosecond laser pulses with a pair of off-axis-parabolic mirrors, is measured using electro-optic sampling detection method. The experimental results show that the pre-plasma weakens the THz radiation in the cases of with and without an external biased field. The lifetime of the plasma filament is estimated using the time evolution of the THz peaks. It is found that the THz radiation keeps weaker even when one pump pulse is 300 ps before the other one, which indicates that the lifetime of the plasma filament is longer than 300 ps. In addition, the THz pulses have similar shapes at different time delays between two pump laser pulses. Its underlying physical mechanisms are discussed as well.
关键词： THz radiation,filaments,opposite directions
更新于2025-09-09 09:28:46
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Generation of THz radiation by photoconductive antennas on based thin films InGaAs and InGaAs/InAlAs.
摘要： Epitaxial low-temperature grown (LT) semiconductor arsenides (Al, Ga, In)As are widely used as materials for photoconductive antennas (PCA) generators and detectors of pulsed radiation in the terahertz (THz) frequency range [1–3]. It is the combination of subpicosecond carrier lifetime, relatively high mobility and high resistivity that makes LT-materials suitable for PCA applications. Lately, InGaAs has been investigated as a potential candidate for THz-PCA photoconductive material due to room-temperature band gap of 0.74 eV, which allows for 1.56 μm optical excitation with Er3+ fiber laser femtosecond pulses [4–6]. The low substrate temperatures result in a non-stoichiometric growth with the incorporation of excess arsenic in the crystal structure. The most common non-stoichiometry-related point defects in LT-arsenides are arsenic antisites with concentrations in the range 1017–1019 cm-3 depending on the substrate temperature and arsenic overpressure [7–10]. Antisite-related defect band in the semiconductor energy bandgap play a significant role in carrier dynamics. Fast non-radiative recombination of photogenerated electrons and holes through antisite centers results in sub-picosecond carrier lifetimes in LT-materials at optimized growth and annealing conditions [11, 12]. It is generally agreed that main traps of photo-excited electrons are ionized antisite defects [13–15]. A possible approach to increase the resistivity of LT-InGaAs is to employ LT-InGaAs/InAlAs superlattices [6,13,16,17]. LT-InAlAs layers have a higher dark resistivity as compared to LT-InGaAs and exhibit deep trap states that are situated energetically below the antisite defect levels of adjacent InGaAs layers that results in a reduction of residual carrier concentration. The Fig.1 shows the amplitude of THz radiation in time domain. It is seen that the signal from an InGaAs /InAlAs-based structure is 5-6 times higher due to a higher bias voltage, which is possible (without sample breakdown) due to higher sample resistance and lower dark current. Fig.2 shows a comparison of the Fourier amplitude according to the materials of the antennas LT-InGaAs/InAlAs and LT-InGaAs. It is seen that the spectrum of the LT-InGaAs / InAlAs sample is slightly wider in the range from 0.1 THz to 0.6 THz than that of the LT-InGaAs sample. We explain this effect by the difference between the characteristic relaxation times of electrons in the transition from the conduction band to the antisites. We determined the characteristic times of electron relaxation by the 'pump-probe' spectroscopy method. Fig.3 shows the dependence of the normalized transmission in time domain for the samples of LT-InGaAs and LT-InGaAs / InAlAs.We used 2-exponentional model for description experimental curves. On figures τ1 is an electron capture time (capture by charge AsGa defects) [18,19], τ2 is a recombination time of captured electrons and holes [17]. Due to the experimental fact that the characteristic relaxation times for the LT-InGaAs / InAlAs sample are less than for the LT-InGaAs, we observed the difference in the spectra for these samples. Summing up, it was found that THz generation is about 5-6 times more efficient in the case of LT-InGaAs/InAlAs superlattice than LT-InGaAs generation. It is found that due to the shorter electron relaxation time in the superlattice, the spectrum of these samples is wider in the range of 0.1-0.6 THz.
关键词： photoconductive antennas,InGaAs,THz radiation,terahertz frequency range,InGaAs/InAlAs,LT-materials
更新于2025-09-04 15:30:14