Spectroscopic Study on Ultrafast Carrier Dynamics and Terahertz Amplified Stimulated Emission in Optically Pumped Graphene

This paper reviews recent advances in spectroscopic study on ultrafast carrier dynamics and terahertz (THz) stimulated emission in optically pumped graphene. The gapless and linear energy spectra of electrons and holes in graphene can lead to nontrivial features such as negative dynamic conductivity in the THz spectral range, which may lead to the development of new types of THz lasers. First, the non-equilibrium carrier relaxation/recombination dynamics is formulated to show how photoexcited carriers equilibrate their energy and temperature via carrier-carrier and carrier-phonon scatterings and in what photon energies and in what time duration the dynamic conductivity can take negative values as functions of temperature, pumping photon energy/intensity, and carrier relaxation rates. Second, we conduct time-domain spectroscopic studies using an optical pump and a terahertz probe with an optical probe technique at room temperature and show that graphene sheets amplify an incoming terahertz field. Two different types of samples are prepared for the measurement; one is an exfoliated monolayer graphene on SiO₂/Si substrate and the other is a heteroepitaxially grown non-Bernal stacked multilayer graphene on a 3C-SiC/Si epi-wafer.     

Optically Pumped Midinfrared Laser With Simultaneous Dual-Wavelength Emission

We demonstrate optically pumped semiconductor lasers that are capable of simultaneously emitting at two different midinfrared wavelengths. The wavelengths can be independently chosen and designed into the heterostructure. The epitaxial III–V antimonide structure employs two sets of type-II quantum wells in a waveguide that is partitioned by a thin electrical barrier that is transparent to the pump radiation. Two-color devices emitting at wavelengths as far apart as $sim$4.0 and $sim$5.4 $mu$m are reported.      

光抽运石墨烯太赫兹负动态电导率的理论研究

石墨烯特殊的零带隙能带结构和载流子弛豫特性,在研究太赫兹辐射源相干放大领域引起广泛关注。考虑带内和带间跃迁对电导率的贡献,研究了光抽运单层和多层石墨烯中非平衡二维电子-空穴系统的动态电导率特性。结果表明,在足够强的光抽运下,石墨烯中的粒子数反转能够使得动态电导率的实部在太赫兹频段内出现负值,这使基于石墨烯的太赫兹放大或受激辐射源成为可能。同时,通过研究动量弛豫时间、温度、层数、光强对石墨烯的负动态电导率的影响表明,石墨烯多层结构的动态电导率最小值的绝对值更大,作为太赫兹激光器的激活介质更具优势。     

基于D2O的光抽运腔式太赫兹波激光谱线竞争

从半经典密度矩阵理论出发,建立了光抽运太赫兹波谱线竞争的双三能级分子系统模型,理论推导了抽运信号吸收系数以及太赫兹波信号增益系数的数学表达式,采用迭代法数值计算了CO2激光-9P(32)抽运重水(D2O)气体分子腔式太赫兹激光66μm和116μm两条谱线之间的竞争。给出了抽运功率、工作气压及激光腔长等工作条件下谱线竞争的一般规律。     

Intersubband Absorption in Terahertz Lasers Based on Optically Pumped Quantum Well Structures

We study the effects of microscopic dynamics of electrons on the intersubband optical absorption in terahertz lasers based on optically pumped quantum wells (THz-OPQW). The ensemble Monte Carlo method is used to include different electron scattering mechanisms in the calculation, so that the dependence of intersubband absorption lineshape on microscopic dynamics of electrons can be easily investigated. We find that electron-electron scattering is the dominant factor to determine the absorption linewidth. The real distributions of hot electrons are included to study the temporal changes of the spectra lineshape. Our findings show that the dependence of spectral lineshape on electron population essentially results from the electron scattering and the non-Fermi electron energy distribution.     

Study of Optimal Cavity Parameter in Optically Pumped D2O Gas Terahertz Laser

Heavy water gas (D₂O gas) which owns special structure property, can generate terahertz radiation by optically pumping technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO₂ laser 9R (22) output line (λ?=?9.26 μm) acted as pumping source, D₂O gas molecules were operating medium, the expressions of pumping absorption coefficient G _p and Terahertz signal gain coefficient G _s were deduced. It was shown that the gain of Terahertz signal was related with the energy-level parameters of operating molecules and some operating parameters of the Terahertz laser cavity, mainly including cavity length. By means of iteration method, the output power density of Terahertz pulse signal was calculated numerically. Changing the parameter of cavity length and keeping others steady, the relationship curve between the output power intensity (Is) of Terahertz pulse laser and the operating cavity length (L) was obtained. The curve showed that the power intensity (Is) increased with cavity length (L) in a certain range, but decreased when the length (L) exceeded some value because of the absorption effect, and there was an optimal cavity length for the highest output power. We used a grating tuned TEA-CO₂ laser as pumping power and a sample tube of variable length in 70–160 cm as terahertz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other, and it is helpful for miniaturizing terahertz laser volume to make it practical.     

Stimulated Terahertz Emission of Bismuth Donors in Uniaxially Strained Silicon under Optical Intracenter Excitation

Semiconductors - The results of the experimental observation of stimulated terahertz emission under optical intracenter excitation of uniaxially strained bismuth-doped silicon are presented....     

超小型光泵远红外激光研究

在求解NH_3分子三能级系统密度矩阵的基础上,用迭代法计算了长为10cm—1.5cm的5支超小型光泵远红外激光器(OPFIRL)的输出激光能量密度和频谱特性.实验研制的5支超小型OPFIRL成功地产生了远红外激光.研究表明:纵向腔效应和横向反馈效应对超小型OPFIRL激光输出有积极作用,对最佳工作气体压强和频谱特性有重要影响.     

Frequency Tuning of Terahertz Stimulated Emission under the Intracenter Optical Excitation of Uniaxially Stressed Si:Bi

Semiconductors - The results of experimental and theoretical investigations of terahertz-emission-spectrum tuning by means of uniaxial stress of a silicon crystal doped with shallow bismuth donors...     

光抽运双光子激光的全量子理论

本文采用均匀加宽的四能级原子模型与光泵双光子激光相互作用,其系统具有双光子损耗机制。利用全量子理论推导各原子算符的朗之万方程,进而研究了稳态光子数、频率、线宽及激光运转过程。     

光子晶体中原子的受激辐射(英文)

受激辐射机制的研究是光子晶体激光器设计中的一个基本问题。采用全量子理论 分析光子晶体中单个二能级原子的受激辐射。讨论的原子能级间跃迁频率在光子带隙内且接 近带隙边缘。在光子晶体中初态光场影响着系统的行为,激发态粒子数布居出现了有趣的现 象：反束缚、周期振荡和准周期振荡。受激辐射的性质明显与初始态的光场和原子激发态能级 相对光子带隙的位置有关。     

Discovery of Optically Pumped Far-infrared Laser Emissions from Formic Acid and its Isotopologues

An optically pumped molecular laser system with a transverse excitation scheme has been used to generate far-infrared radiation. Over 100 laser emissions with wavelengths up to 1.03 mm have been detected with this system using formic acid and several of its isotopologues. This includes ten new laser lines which are reported with their operating pressure, power, polarization with respect to the CO₂ pump laser, and wavelength measured to an uncertainty of ± 0.5 μm.     

Spectral Characteristics of Raman Emissions from Miniature Pulsed Optically Pumped Far Infrared Laser

A miniature FIR laser with a sample-tube in 10 or 20cm length were constructed, and the laser's cavity consisted of a pair of metallic meshes, with NH3 molecules as active medium pumped by a TEA-CO2 laser, a series of FIR laser lines were observed. By taking account of the properties of the Fabry-Perot cavity and the gain characteristics of Raman transitions in the miniature optically pumped FIR laser, the spectral characteristics of the FIR laser were studied.     

Optically- and Electrically-Stimulated Terahertz Radiation Emission from Indium Nitride

Indium nitride is a novel narrow band gap semiconductor. The material is a potential strong source of terahertz frequency electromagnetic radiation with applications in time-domain terahertz spectroscopy and imaging systems. This article reviews recent experimental research on terahertz emission from the binary compound semiconductor indium nitride excited by near-infrared laser beams or microseconds electrical pulses. Advantages of indium nitride as terahertz radiation source material are discussed. It is demonstrated that different mechanisms contribute to the emission of terahertz radiation from indium nitride. The emission of up to 2.4 μW of THz radiation power is observed when InN is excited with near-infrared femtosecond laser pulses at an average power of 1 W.     

小型光泵远红外激光的放大过程

利用三能级系统的密度矩阵方程，通过迭代法计算了小型光泵ＮＨ３分子远红外激光器（ｍｉ－ｎｉ－ＮＨ３－ＯＰＦＩＲＬ）的介质增益Ｇｓ沿激光管的变化规律以及远红外信号光强Ｉｓ沿激光管的放大过程。计算表明激光管有激活区和饱和区，对一定管长的ｍｉｎｉ－ＮＨ３－ＯＰＦＩＲＬ，在一定范围内工作气压的提高，主要使Ｇｓ的峰值增大，同时使激活区缩短，考虑散射损耗和自吸收效应后，Ｇｓ峰值的提高对总的输出光强贡献不大。而泵浦光强Ｉｐ０的增大，有利于激光管内的介质沿光轴方向均匀激活，从而提高总的输出光强。另外，信号频偏对ｍｉｎｉ－ＮＨ３－ＯＰＦＩＲＬ的激光放大过程有较大的影响     

平面Fabry-Perot有机微腔的光泵特性

采用聚乙烯基咔唑poly(9-vinylcarbazole)(PVK)、8-hydroxyquinoline aluminium (Alq3)掺杂4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran(DCM)的聚合物薄膜作为增益介质,金属Ag层和分布布拉格反射镜(DBR)作为上、下两个反射镜,制作了光泵浦的平面Fabry-Perot有机微腔.采用YAG激光器的三倍频光作为泵浦源,研究了光泵浦平面Fabry-Perot有机微腔的光特性,观测到位于583.72 nm处的谐振峰,这与F-P微腔理论符合得较好,并发现其半高全宽(FWHM)能达到2.608 nm,谱线发生了明显的窄化.     

Study of Lasing Action in a Pulsed Optically Pumped D<Subscript>2</Subscript>O Gas Terahertz Laser

A modified three-level laser kinetics model for a pulsed high-power optically pumped gas terahertz laser is introduced and used to model the lasing kinetics process of a gas terahertz laser system. We, for the first time to our knowledge, investigated the time evolution dynamics process of the pump intensity, population distribution among the energy levels, pump and THz signal gain coefficient, and the THz laser intensity within the pulsed D₂O gas THz laser. High-power THz pulse with peak power of about 7.4 kW and pulse width of 145 ns at wavelength of 385 μm were obtained in the simulation, using an incident pump pulse with peak power of 2.2 MW and pulse width of 110 ns. THz pulse delay of 40 ns and pulse broadening of 35 ns were quantitatively analyzed. In addition, the experimental results for the pulse profile, pulse width, pulse broadening, pulse energy, and peak power are in agreement with the theoretical simulation results.     

New Optically Pumped FIR Laser Lines and Frequency Measurements from CH3OD

This work reports the discovery of 45 new optically pumped far-infrared (FIR) laser lines from CH₃OD in the range 42.6 μm to 207.2 μm. A highly efficient CO₂ laser was used as the pump laser for an Optically-pumped Fabry-Perot FIR laser. The frequencies of most of the new lines were measured in the range 1.7 to 6.4 THz. Twenty-eight CO₂ lines were used as pump lines; nineteen of these had frequencies lower than those previously used to pump CH₃OD in a FIR laser. Sixteen of the new FIR lines had absorption transitions which belong to the OD bending vibrational mode.     

Spectral Characteristics of Miniature Two-Longitudinal-Mode Optically Pumped FIR Laser

With the assumption that the pump source was two-longitudinal-mode, the semiclassical density matrix equation of a three-energy-level system was solved. By means of iteration method, the spectral characteristics of miniature two-longitudinal-mode optically pumped NH₃ far-infrared cavity laser, which was pumped by CO₂-9R(16) line, were calculated numerically. The influences of operating parameters on laser spectrum were studied intensively.     

Frequency Measurements of Optically Pumped Laser Emissions from the CHD2OH Methanol Isotope

Journal of Infrared, Millimeter, and Terahertz Waves - A three‐laser heterodyne system was used to measure the frequencies of eleven optically pumped laser emissions from CHD2OH in the...