Application of Terahertz Field Enhancement Effect in Metal Microstructures

Applications of high-field terahertz pulses are attractive in physics and terahertz technology. In this study, two applications related to high-intensity terahertz pulses are demonstrated. The field enhancement effect by subwavelength metallic microstructures is utilized for terahertz excitation measurement. The spin precession dynamics in magnetic materials was induced by a terahertz magnetic field. Spin precession was amplified by one order of magnitude in amplitude by the enhanced magnetic terahertz field in orthoferrite ErFeO₃ with metal microstructures. The induced spin dynamics was analyzed and explained by LLG-LCR model. Moreover, a detection method for terahertz pulses was developed using a cholesteric liquid crystal at room temperature without any electronic devices. The beam profile of terahertz pulses was visualized and compared to other methods such as the knife edge method using pyroelectric detector and micro-bolometer array. The liquid crystal terahertz imager is very simple and has good applicability as a portable terahertz-sensing card.     

The Polarization and Efficiency of the Terahertz Pulses Generated by Tilted-Pulse-Front Pumping Scheme

Optical rectification of laser pulses in LiNbO₃ by tilted-pulse-front pumping(TPFP) is a powerful way to generate terahertz(THz) pulses. However, comprehensive theoretical analysis is still lack. In this work, we first established and presented a detailed theoretical model for TPFP scheme, which then was used to analyze the pump beam polarization dependent terahertz pulses generated by this scheme. The terahertz pulses polarization and generation efficiency for various pump beam polarization angle were investigated by using nonlinear susceptibility tensor of LiNbO₃ crystal. The results indicate that one can change the polarization state of the terahertz pulse by changing the pump beam polarization.     

Characteristics of a visible modulation instability fiber laser

Cross-phase modulation has been harnessed to construct a modulation instability laser operating at 514.5 nm. The laser output consists of orthogonally polarized upshifted and downshifted sideband pulses separated in wavelength by 2.1 nm from the pump. The laser configuration has been found to generate a much more stable source of terahertz modulated light pulses than the single-pass configuration previously used to demonstrate modulation instability in the visible. The extra stability arises from the feedback, which provides a seed for the modulation, and hence a reduction in the pump power necessary compared with the single-pass configuration, which in turn reduces the Raman gain, and the asymmetry of the sideband amplitudes. The sideband pulses can be combined to generate a modulated envelope of 4-THz oscillations with a duration of 10-15 ps     

强双折射色散缓变光纤中偏振方向矢量调制不稳定性

利用光脉冲在非线性双折射光纤中传播时所遵守的相干耦合非线性薛定谔方程,研究了强双折射色散缓变光纤中偏振方向与双折射轴成任意角度时,在反常色散区和正常色散区所产生的调制不稳定性.结果表明：在反常色散区对于任意的偏振角和任意输入功率的光脉冲都产生调制不稳定性,正常色散区存在着不稳定区域和稳定区域,对应不同的不稳定区域输入临界功率不同,脉冲有不同的增益谱.并且,给出了调制不稳定性增益谱随传输距离和偏振角的变化关系.     

升余弦变迹布拉格光纤光栅中的调制不稳定性

利用相干耦合非线性薛定谔方程,研究了激光脉冲在升余弦变迹布拉格光纤光栅中传输时,在反常色散区和正常色散区所产生的调制不稳定性。结果表明在反常色散区和正常色散区都能产生调制不稳定性;在反常色散区,当输入功率达到一定数值时,产生明显的规律性增益谱;在正常色散区,在产生调制不稳定性功率区域,调制不稳定性存在并从给定值一直持续到无穷;在反常色散区和正常色散区,增益谱都受到升余弦变迹函数的制约。     

保偏阶跃光纤中矢量调制不稳定性

利用光脉冲在非线性双折射光纤中传播时所遵守的相干耦合非线性薛定谔方程，研究了保偏色散阶跃光纤（SWDF）中偏振方向与双折射轴成任意角度时、在反常色散区和正常色散区所产生的调制不稳定性。结果表明：在反常色散区，对于任意的偏振角和任意输入功率的光脉冲都产生调制不稳定性；在正常区散区，存在不稳定区域和稳定区域，对应不同的不稳定区域输入临界功率不同，脉冲有不同的增益谱，给出了调制不稳定性增益谱随传输距离和偏振角的变化关系。     

Modulation instability in erbium-doped fiber amplifiers

The onset of modulation instability in erbium-doped fiber amplifiers (EDFAs) is studied through a stability analysis of the underlying nonlinear Schrodinger equation. The existence of gain in EDFAs lowers the threshold for modulation instability considerably compared with the case of undoped fibers. Modulation instability generates multiple pulses when a single pulse is amplified. It can also create multiple subpulses in mode-locked fiber lasers, a feature observed experimentally. Numerical simulations show that EDFAs can convert a continuous-wave optical signal into a train of high-repetition rate femtosecond pulses     

基于双温耦合模型的太赫兹晶体损伤阈值分析

飞秒激光通过非线性光学整流效应产生太赫兹(THz)波时,THz波转换效率会随着飞秒激光功 率的增大而明显提高。然而,飞秒激光功率过高会造成非线性晶体损伤,进而影响THz波的 产生及输出, 因而对飞秒激光作用下非线性晶体的损伤阈值进行研究具有重要意义。本文在经典的双温模 型基础上,引 入电子激发、载流子吸收等电离过程的影响,建立了飞秒激光作用下非线性THz晶体损伤 阈值的预估模 型。采用有限差分法,数值模拟了飞秒激光辐照下THz晶体的温度场变化,并据此对 晶体损伤阈值进行 预估。在此基础上,分析比较了LiNbO₃、ZnTe和ZnSe 3种THz晶体的损伤 阈值随激光脉宽的变化规 律。结果表明,晶体的禁带宽度和比热容越大 ,晶体的损伤 阈值就越大;LiNbO₃晶体因其具有更高的损伤阈值,在产生高功率THz波方面 具有更大的优势。     

基于氧化钼薄膜的光控太赫兹传输特性研究

本文采用热蒸发法制备了沉积在硅片上的200nm 氧化钼(MoO₃)的太赫兹调制薄膜。 在室温条件下,通过傅里叶光谱仪和太赫兹时域光谱系统(THz-TDS)技术,研究了MoO₃薄膜在不同激励光功率下的太赫兹传输特性。提出了薄膜重要光学参数的提取模型,利用透 射式太赫兹时域光谱技术测量了薄膜的时域信号,分别计算了太赫兹波在薄膜中的透过率及 调制效率及薄膜的复介电常数变化。结果表明,在980 nm激光器条件 下,随着激光器功率 的提高,MoO₃薄膜的太赫兹调制深度逐渐增加。在激光功率为266 mW时,在0.26 THz处 透过率达到最低为61%,调制效率(Modulation factor)达到最高为10%。通过分析MoO₃薄膜 的复介电常数及载流子密度变化,得出了激发生成的载流子浓度的提高导致介电常数的改变 , 增强了薄膜的导电性,从而减低了太赫兹波在薄膜中的透过率的结论。为MoO₃薄膜应用在 太赫兹波段调制领域提供了实验数据。     

低双折射光纤的调制不稳定性

利用连续波在光纤中传播时满足的非线性薛定谔耦合方程,研究了CW波在低双折射光纤的正、反常色散区产生的调制不稳定性。结果表明:调制不稳定性的特征由入射功率和偏振不稳定性的阈值功率的关系决定,增益谱的峰值随着入射功率的变化,大小和位置也发生变化。     

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO₃ crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO₃ crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.     

Ultra-compact 1 × 8 Channel terahertz Wave Power Splitter

Relying on 1?×?2 photonic crystal waveguide and photonic crystal resonator, a compact eight-channel terahertz wave power splitter is proposed. The mechanism of such a device is further theoretically analyzed and numerically investigated with the aid of the plane wave expansion method and the finite-difference time-domain method. With an appropriate design, the proposed power splitter can split the input terahertz wave energy equally into eight output ports at the frequency of 0.667 THz. Furthermore, the total size of the present device is of 4.33 mm?×?3.74 mm. Due to its small size, the multi-channel terahertz wave power splitter has practical applications in the terahertz wave integrated circuit fields.     

Damage Threshold of Ni Thin Film by Terahertz Pulses

Damage in a thin nickel film irradiated by subpicosecond pulses of terahertz (THz) radiation in the range of 1–3 THz at electric-field strengths up to 20 MV/cm at the center of a focal spot is observed. The damage threshold fluence is evaluated for single-pulse experiments. The damage pattern induced by multiple THz pulses has the appearance of a complex periodic structure in the form of elongated channels of metal film discontinuity that are perpendicular to the in-plane electric field direction of THz radiation.     

气体等离子体产生太赫兹波的特性研究

基于光电流模型,对不同偏振情况的双色飞秒激光脉冲聚焦产生的气体等离子体中辐射出的太赫兹波特性进行了研究。根据光电流理论,气体分子被电离释放出的自由电子在非对称的激光场的作用下运动形成电子电流,产生在太赫兹波段的辐射。研究结果表明,太赫兹辐射的偏振特性与强度和入射双色激光的偏振特性紧密相关,仅当双色脉冲均为线偏振时,辐射出的太赫兹波才为线偏振,且强度受到双色脉冲偏振方向的夹角的影响;而对于实验中经过倍频晶体后变成椭圆偏振的基频光,太赫兹强度与倍频晶体的具体放置情况有很大关系,并且产生的太赫兹为椭圆偏振。     

单模光纤中四阶色散导致的调制不稳定性

利用修正的非线性Ｓｃｈｒｏｄｉｎｇｅｒ方程研究了单模光纤在最小群速度色散波长附近的调制不稳定性。从理论上给出了一个新的由光纤四阶色散导致的调制不稳定区域，新区域的最大调制增长率和调制范围与光中的初始入射功率和四阶色散有关，光纤自陆峭效应和Ｒａｍａｎ延勺时调制不稳定区域的最大调制增长率和调制范围都有影响，自肖效应对新区域的影响较大且存在一个决定新区域出现与否的阈值。Ｒａｍａｎ延迟效应对调制不稳定必的     

Multi-Cycle Terahertz Emission from <Emphasis Type="Italic">β</Emphasis>-Barium Borate

We report the observation of strong narrowband terahertz emission at 10.6 THz from the common nonlinear second harmonic generation crystal β-barium borate crystal excited by 40 fs laser pulses at a 800 nm center wavelength. The emitted THz field amplitude is linear with incident pump energy with no saturation observed up to 2 mJ of pump pulse energy, and exhibits a crystal azimuthal angular dependence consistent with an optical rectification process. The narrowband emission can be explained by the large dispersion caused by optical phonons in the vicinity of the emission energy.     

Infrared field induced Gunn oscillations in GaAs

Experimental results are presented which show that intense i.r. laser radiation can enhance intervalley carrier transfer and induce Gunn oscillations in GaAs by direct carrier heating. Nanosecond pulses from a mode-locked CO₂ laser at 10.6 μm wavelength have been used to trigger the instability. The results suggest applications in direct optical monitoring of fast hot carrier dynamics and high speed optical modulation of Gunn microwave sources.     

Analysis of Linewidth Tunable Terahertz Wave Generation in Periodically Poled Lithium Niobate

A new scheme of optical rectification of femtosecond laser pulses in a periodically poled lithium niobate crystal, which generates high energy and linewidth tunable multicycle terahertz (THz) pulses, is analyzed. The developed simple theoretical model allows investigating the generated THz spectrum and its dependence on spot size of the pump beam. It is shown that the transformation of THz radiation from narrowband to broadband is possible by simply reducing the pump beam size. The temporal waveform and energy of the multicycle THz pulses were calculated as well. It is shown that THz energy is inversely proportional to the pump beam size r _y , whereas energy spectral density is independent of r _y . The efficiency of optical-to-THz energy conversion for pump pulse energy of 1?mJ is estimated to be 0.8?×?10^?4. The possibility of tuning the generation frequency by changing the direction of the pump beam propagation is analyzed.     

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO₂ laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A⁺ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation.     

A Superconductor THz Modulator Based on Vortex Flux Flow

A terahertz modulator based on the Type-II superconductor flux flow oscillator has been proposed. Analytical calculations are presented and the effects of intrinsic and extrinsic parameters such as disorder strength of crystal, penetration depth, frequency, and amplitude of the modulated current on the radiation power spectrum have been studied. The proposed structure also exhibits a mixer-like behavior, in the sense that its output harmonics range from the washboard frequency up to the superconductor gap frequency, so the input signal is practically mixed with the washboard frequency and its harmonics. The modulation index for each harmonic of this modulator has also been investigated. This well-featured modulator has a potential to be used in next-generation terahertz integrated transceivers.