Widely Frequency-Tunable Terahertz Wave Generation and Spectroscopic Application

The THz-wave generator based on the resonance of phonon-polaritons in GaP enabled THz spectral measurements of organic molecules like DNA/RNA-related molecules in wide frequency range (0.6–5.8 THz). High spectral purity of the generated THz-wave enables even the detection of defect structures of organic compounds, and will give a new tool for molecular sciences. We demonstrated THz imaging of a liver cancer. The THz-wave generator can be made small-size, light-weight system.     

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.     

Beat Wave Generation of Plasma Waves for Particle Acceleration

The space-time evolution of beat wave generation is studied analytically and numerically. Electromagnetic cascading, collisional damping and relativistic frequency shift of the beat plasmon are taken into account in the model. In particular, detuning and dispersion effects are investigated. The achievable plasmon amplitude depends strongly on the collisional damping. At low electron temperatures, the induced beat wave follows the laser pulse and decays rapidly behind it. At high electron temperatures, amplitude modulation appears and an intense slowly decaying plasmon wake can be excited. The wake formation is controllable by varying the pulse length or by detuning the driver slightly off resonance. The amount of electromagnetic cascading is proportional to the plasmon amplitude and the propagation distance of the pulse. The EM spectra offer excellent diagnostics for beat wave experiments, because plasmon amplitude variations are directly reflected in them.     

Wave Generation in a Warm Magnetized Multi‐Component Plasma

The resonant interaction between three waves propagating perpendicularly to an external magnetic field in a plasma is considered. We present the explicit expressions for the three wave coupling coefficients of a warm multi‐component plasma. The results of previous work on the generation of THz radiation by laser plasma interaction are significantly improved. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation of Plasma Wave at Pump-Wave Frequency and Second Harmonic Generation at Ultrarelativistic Laser Power

In this paper, the influence of ponderomotive and relativistic nonlinearities on the filamentation of an ultraintense laser pulse is investigated in three dimensions within the paraxial ray approximations. Generation of electron plasma wave (EPW) structure at pump-wave frequency and the second harmonic generation in these filamentary structures are reported. The generation of the plasma wave is due to intensity gradient (in the transverse direction of the laser beam in filamentary structure) and density gradient (due to ponderomotive-force effect). For typical laser–plasma parameters: The $hbox{laser intensity} = 2.5timesbreak 10^{20} hbox{W/cm}^{2}$; the $hbox{particles density} = 1.9 times 10^{19} hbox{cm}^{-3}$; and it is found that the maximum intensity of EPW is in the range of $2.0 times 10^{13} hbox{W}/hbox{cm}^{2}$. Interaction of the plasma wave with the incident laser beam leads to second harmonic generation, and the yield comes out to be $approx! 2.1 times 10^{-7}$.      

太赫兹波频域的GaAs材料特性

利用返波振荡器(BWO)系统对不同电阻率的GaAs在太赫兹波段的透射谱进行了测试,并对太赫兹波透射谱进行了计算和分析,得到不同电阻率的GaAs材料在太赫兹波0.23～0.375 THz波段范围的折射率、吸收系数和介电常数实部和虚部等光学特性参数。实验结果表明,不同电阻率的GaAs在整个测试太赫兹波频段内的折射率,吸收系数,介电常数实部和虚部均随着电阻率的增加而增加。高电阻率的GaAs材料对太赫兹波的吸收很小,其最小吸收系数可达到3.87×10-4 cm-1。同时也表明采用返波振荡器系统研究分析GaAs在太赫兹波段特性是可行的。研究不同电阻率的GaAs在太赫兹波段的光学特性,对设计高效率太赫兹波天线具有重要意义。     

激光能量对气体等离子体产生太赫兹波的影响

双色激光脉冲激励气体等离子体产生太赫兹波是得到高强度宽频带太赫兹波的重要方法,本文利用光电流模型研究了该方法中激光能量对产生太赫兹波的影响。理论计算表明,太赫兹波随激光能量的增大而增强,而太赫兹波的频谱结构不受激光能量的影响。分析了双色激光能量影响太赫兹波强度的原因,并利用自由电子浓度和电子电流密度诠释了该影响的内在物理机制。该研究为提高太赫兹辐射强度提供了一种有效的途径。     

Surface Wave Generation in a Solid State Plasma by Relativistic Electron Beams

The linear and nonlinear theory of the surface wave generation in solid-state plasma by a relativistic electron beam propagating in a gaseous plasma is treated. The proposed scheme is in better agreement with the conditions of real experiment. Besides, the neutralization of beam charge is possible and the oscillation frequency and its stability increase, etc. A nonlinear analysis permits to obtain the estimation of efficiency of the beam interaction with the surface wave and a number of other characteristics of the system.     

Short Terahertz Pulse Generation from a Dispersion Compensated Modelocked Semiconductor Laser

Dispersion compensation is vital for the generation of ultrashort and single cycle pulses from modelocked lasers across the electromagnetic spectrum. It is typically based on addition of an extra dispersive element to the laser cavity that introduces a chromatic dispersion opposite to that of the gain medium. To date, however, no dispersion compensation schemes have been successfully applied to terahertz (THz) quantum cascade lasers for short and stable pulse generation in the THz range. In this work, a monolithic on‐chip compensation scheme is realized for a modelocked QCL, permitting THz pulses to be considerably shortened from 16ps to 4ps. This is based on the realization of a small coupled cavity resonator that acts as an ‘off resonance’ Gires‐Tournois interferometer (GTI), permitting large THz spectral bandwidths to be compensated. This novel application of a GTI opens up a direct and simple route to sub‐picosecond and single cycle pulses in the THz range from a compact semiconductor source.     

亚波长金属块阵列中太赫兹波的传输特性研究

利用太赫兹时域光谱技术,研究了亚波长金属块阵列的太赫兹透射光谱特性及金属阵列结构的周期、金属块尺寸等因素对太赫兹透射特性的影响.结合时域有限差分方法,对实验结果进行了数值模拟,并分析了影响太赫兹透射的因素.结果表明:亚波长金属块阵列结构中,THz波的透射极小的位置由金属块的尺寸和周期决定,其透射谷的半高宽随其周期的增大而减小;透射峰值的位置由阵列的周期结构决定,其频率随周期的增加而减小;亚波长金属块结构的透射极小来源于金属块表面局域化电场等离子体的本征频率反射,该局域化电场与金属块结构密切相关,通过改变金属块结构,可以改变其表面电场分布与局域化.研究结果为研制太赫兹波段带阻滤波器提供了有益的参考.     

亚波长金属块阵列中太赫兹波的传输特性研究

利用太赫兹时域光谱技术,研究了亚波长金属块阵列的太赫兹透射光谱特性及金属阵列结构的周期、金属块尺寸等因素对太赫兹透射特性的影响.结合时域有限差分方法,对实验结果进行了数值模拟,并分析了影响太赫兹透射的因素.结果表明:亚波长金属块阵列结构中,THz波的透射极小的位置由金属块的尺寸和周期决定,其透射谷的半高宽随其周期的增大而减小;透射峰值的位置由阵列的周期结构决定,其频率随周期的增加而减小;亚波长金属块结构的透射极小来源于金属块表面局域化电场等离子体的本征频率反射,该局域化电场与金属块结构密切相关,通过改变金属块结构,可以改变其表面电场分布与局域化.研究结果为研制太赫兹波段带阻滤波器提供了有益的参考.     

Breaking of the Axial Symmetry of Terahertz Radiation from Single-Color Filament Plasma

JETP Letters - Two-dimensional distribution patterns of terahertz radiation generated in a laser single-color filament plasma are measured at several frequencies. In the low-frequency region...     

Recent Advance in Terahertz Wave and Material Basis

The unexplored terahertz (THz) region involves important phenomena of both fundamental and applied natures. Examples include phonon interactions, rotational transitions, and intermolecular dynamics. Frequency tunable high-power THz wave generation has been successfully achieved utilizing lattice resonance of LiNbO₃ and GaP crystals, respectively. Semiconductor devices utilizing the electron tunneling effect have also been developed.     

Terahertz Radiation from a Longitudinal Electric Field Biased Femtosecond Filament in Air

The terahertz(THz) temporal waveform and spectrum from a longitudinal electrically biased femtosecond filament is studied experimentally. The initial direction of the electron motion inside the unbiased filament plasma is deduced from the transformation of the THz temporal waveform with applied fields of opposite polarities.Furthermore, a spectrum shift to lower frequency of the THz spectrum is observed in the presence of a biased field. It agrees well with theoretical predictions.     

半导体中相干亚毫米波的产生和传播

研究了超短光脉冲在ＧａＡｓ表面激发相干亚毫米波振幅随入射角变化的规律，前后向辐射比值随入射角度变化的规律，实验测量了ＧａＡｓ表面镀上极薄金属薄膜后产生的相干亚毫米波振幅对薄膜厚度的依赖关系，利用光生载子偶极辐射的理论模型成功地解释了实验得到的角度依赖关系和相干亚毫米波的偏振特性。     

A Material for Difference-Frequency Generation of Terahertz Radiation

Optics and Spectroscopy - The possibility of efficient difference-frequency generation of terahertz radiation in a metamaterial representing a structure consisting of alternating layers of a...     

Generation of Terahertz Radiation by Multicolor Ionizing Pulses

JETP Letters - Analytical formulas have been obtained for the optimal waveform of an ionizing pulse and corresponding maximum density of residual terahertz currents. The found analytical solutions...     

Manipulating Nonlinear Photocurrent from Interlayer Coupling in Bilayer Metamaterials for Polarized Terahertz Generation

Polarized terahertz (THz) wave generation is of great significance for chiral and anisotropic sensing applications. However, how to manipulate amplitude, polarization, and ellipticity of the THz generation is still a fundamental challenge. Herein, polarized THz wave generation is achieved from a bilayer metamaterial consisting of T-shaped structure (TSS) and split resonator rings (SRRs) by combining Maxwell and hydrodynamic equations. The elliptically polarized THz wave can be synthetized directly from horizontally and vertically polarized THz components due to the orthogonal nonlinear photocurrents along the arm-directions of TSS and SRRs, respectively. Besides, the ellipticity and the orientation angle of the THz polarization ellipse can be modulated by the twist angle between the SRRs and TSS layers. The maximum ellipticity can reach 0.34 while the orientation angle is tunable from −0.45 to 0.48π by tuning the twist angle. This work proposes an interlayer coupling method for the polarized THz sources based on metamaterials in potential circular dichroism and chiral sensing applications.     

Surface Plasma Wave Solitons

It is shown that nonlinear electron plasma surface waves are modulationally unstable and can propagate as solitons on a cold electron plasma-vacuum interface.