太赫兹半导体激光光频梳研究进展

光频梳由一系列等间距、高稳定性的频率线组成.由于具有超高频率稳定性和超低相位噪声,光频梳在精密光谱测量、成像、通信等领域具有重要应用.在太赫兹波段,基于半导体的电抽运太赫兹量子级联激光器具有大功率输出、宽频率覆盖范围等特点,是产生太赫兹光频梳的理想载体.本文主要介绍基于太赫兹半导体量子级联激光器光频梳的研究进展,详细列举了自由运行、主动稳频和被动稳频模式下产生光频梳的方法.双光梳光谱可以克服传统太赫兹光谱仪需要机械扫描系统而难以实现实时光谱检测的难题,是光频梳应用的主要方向.在光频梳基础之上,本文还介绍了采用两个太赫兹量子级联激光器产生双光梳的方法和应用.     

Ge_xSi_(1-x)/Si超晶格中周期不均匀对折叠声学声子拉曼线宽的影响

本文报导了在 Ge_Si_(1-x)/Si 超晶格中观察到了超晶格周期不均匀导致的折叠声学声子拉曼谱线展宽,这种谱线展宽与折叠指数(m)有关,折叠声学声子的谱线越宽,m 相同的折叠声学声子具有相同的线宽。     

基于二硫化钼的可调一维声子晶体

声子晶体所具有的负折射率、局域缺陷态与弹性波带隙等特性,使其在声学隐身、声学波导以及减震降噪等方向展现了巨大的潜力.同时,二硫化钼优异的电学和力学性能使其成为制备纳米机电器件的理想材料.将单层二硫化钼转移到预先图案化的周期性结构上,可以制备出纳米尺度的声子晶体器件.本文设计了一种通过将单层二硫化钼转移贴合在预先制备的周期性沟槽阵列上,形成一维声子晶体的方案.有限元分析表明,这种声子晶体在MHz范围存在声子能带结构,可以实现对声波传播的控制.我们可以通过改变结构参数,或者通过改变施加在栅电极上的电压,对能带进行调控.这种结构为开发基于二维材料的纳米尺度的声子晶体器件提供了可能性.     

用超快电子衍射技术研究Al薄膜的超快动力学行为

超快电子衍射（UED）技术因其同时具有亚皮秒的时间分辨和亚毫埃的空间分辨能力,成为研究物质瞬态结构变化,特别是研究晶格材料超快动力学的有力工具.应用国内首台自行研制的UED系统,我们实时测量了超快激光脉冲激发下,20 nm金属Al多晶薄膜产生的相干声子和晶格热运动.实验结果显示,在晶格热运动加剧的同时,热应力的作用使晶格产生了相干振荡,并最终膨胀达到新的平衡位置.实验中测得的振荡周期以及晶格上升的温度与理论计算的结果符合较好,展示了UED技术在超快晶格动力学研究方面的广阔应用前景 关键词： 超快电子衍射 相干声子 晶格热运动     

Fe/Si薄膜中相干声学声子的光激发研究

本文通过抽运-探测技术, 利用飞秒激光脉冲激发并探测了Fe/Si薄膜中的高频相干声学声子. 通过经典的阻尼谐振函数, 对声学声子的动力学行为进行了拟合. 实验及拟合结果表明, 该声学声子的共振频率约为0.25 THz, 其退相时间约为12 ps, 且都与激发光的波长和能量密度无关. 声学声子的振幅随着激发光能量密度的增加而线性地增强. 临界参数12_τe-ph/T约为0.6, 表明相干声学声子的驱动力主要来源于电子热应力的贡献. 最后, 结合薄膜的厚度和质量密度, 可以得到室温下垂直于该Fe/Si薄膜表面(out of plane) 的弹性常数C_⊥约为283 GPa.     

声子晶体和声学超构材料

声子晶体和声学超构材料进一步拓展了自然界中声学材料的弹性波性质.这种人工的复合结构材料,由于其周期结构的布拉格散射和局域共振特性,使得其具有奇异的色散特征,在某些频段具有负的有效弹性参数,带来了许多新颖的声学传播效应,例如声子带隙效应、负折射效应、超棱镜效应、超透镜效应、异常透射效应、异常隔声效应等.与此同时,在声子晶体和声学超构材料表面,一类具有亚波长特性的声表面倏逝波也引起了人们的关注,研究其激发、传播、耦合的过程对揭示声子晶体和声学超构材料的奇异声传播效应的物理本质具有重要意义.声子晶体和声学超构材料作为一类新型的人工声学结构材料,在隔声、防振、热控制以及新型声学器件研发等方面具有巨大的应用前景.文章综述了近十几年来国际国内关于声子晶体和声学超构材料的研究进展,并对其未来的研究发展方向做一评述.     

电子动力学及相干辐射的强场调控与阿秒探测——强场与阿秒物理领域中的重大课题研究进展

电子动力学及相干辐射的强场调控与阿秒探测是强场物理与阿秒物理领域中的重大课题。通过同步探测阿秒辐射和太赫兹辐射,文章作者首次实现了阿秒精度的太赫兹产生动力学的探测与控制,表明阿秒物理与太赫兹技术的结合有助于深入理解强场驱动下太赫兹产生机制和电子再散射动力学,展示了利用双色场控制电子波包相干相位,实现超快物理过程强场调控的可能。文章作者所提出的精确刻画太赫兹时域瞬时电场方案,有助于推动极化敏感的太赫兹谱学研究。可以预期,阿秒脉冲与太赫兹源技术不会局限于原子分子物理领域。实现阿秒物理与太赫兹技术之间的互为抽运与探测,将会极大地推动化学、材料科学、凝聚态物理等领域的高时空分辨的超快动力学探测。     

科学家研制出新型太赫兹半导体激光器

正据加州大学洛杉矶分校官网报道,该校科研人员利用新方法制造出太赫兹频率下工作的半导体激光器。这一突破或将带来可用于太空探索、军事和执法等领域的新型强大激光器。科研人员设计出的超材料表面既可以放大太赫兹波,又可以反射太赫兹波。在电磁波谱中,太赫兹的频率范围位于微波和红外线之间。太赫兹波可以在不损伤被检测物质的前提下对塑料、服装、半导体和艺术品等进行材料分析,还可以用于分析星体的形成和     

黄昆对物理学的贡献

黄昆是晶格动力学的奠基人和权威,声子物理学科开拓者。他是多声子光跃迁和多声子无辐射跃迁理论学科的开创者,是“极化激元”概念的最早阐述者。他的姓氏也与晶格振动长波唯象方程、X光漫散射理论联系在一起。在多年离开科研第一线后,黄昆70岁还建立了半导体超晶格光学声子的“黄－朱模型”。本文简要介绍黄昆先生在科学研究领域的几项主要贡献。     

声单向操控研究进展

电子二极管的发明标志着现代电子学的诞生, 在整个人类社会中引起了科技的深刻变革. 声波是一种具有非常悠久的研究历史的经典波, 却始终被认为仅具有对称的传播形式. 若能制造出可像电子二极管控制电流般实现声波单向导通的声学器件, 显然将对整个声学研究领域产生重大影响, 具有重要的科学意义及应用价值. 第一个基于非线性媒质与声子晶体的声二极管利用非线性突破声学互易原理的局限, 首次实现了将声能流限制在单一方向上的声整流效应. 针对非线性系统转换效率低下的固有缺陷, 在线性体系内围绕声单向传播这个重要科学问题开展了一系列理论和实验研究, 设计与制备了多种具有特殊结构和性能的线性声学单向结构, 在器件的效率、带宽及尺寸方面产生了突破. 在声二极管研究的基础上, 第一个可以像电子三极管操控电流般对声流进行操控与放大的声三极管理论模型也被提出. 本文介绍了声单向传播这一新兴且富有蓬勃生机的研究领域中的主要进展.     

Terahertz absorption by electrons and confined acoustic phonons in free-standing quantum wells

The acoustic phonon confinement in a free-standing quantum well (FSQW) results in an acoustic phonon energy quantization. Typical quantization energies are in the terahertz frequency range. Free electrons may absorb electromagnetic waves in this frequency range if they emit or absorb acoustic phonons. Therefore, the terahertz absorption reveals the characteristic features of the acoustic phonon spectrum in free-standing structures. We have calculated the absorption coefficient of an electromagnetic wave by free electrons in a FSQW in the terahertz frequency range. We took into account a time dependent electric field, an exact form of the acoustic phonon spectrum and eigenmodes, and electron interactions with confined acoustic phonons through the deformation potential. We demonstrate numerical results for GaAs FSQW of width 100 Å at low lattice temperatures in the frequency range 0.1-1 THz. The absorption coefficient exhibits several structures at frequencies corresponding to the lowest acoustic phonon modes. These features occur due to absorption of photons by electrons, which is accompanied by the emission of corresponding acoustic phonons.     

The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually matches one of the phonon modes, typically theq = 0 optical mode. These oscillations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent phonon signal can be used as a probe of the surface depletion field and (2) multiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced and acoustic modes can now be excited.     

Coherent generation of acoustic phonons in an optical microcavity

Ultrafast coherent generation of acoustic phonons is studied in a semiconductor optical microcavity. The confinement of the light pulse amplifies both the generation and the detection of phonons. In addition, the standing wave character of the photon field modifies the generation and detection phonon bandwidth. Coherent generation experiments in an acoustic nanocavity embedded in an optical microcavity are reported as a function of laser energy and incidence angle to evidence the separate role of the optical and exciton resonances. Amplified signals and phonon spectra modified by the optical confinement are demonstrated.     

Acoustic phonon emission from a weakly coupled superlattice under vertical electron transport: observation of phonon resonance

We report measurements of acoustic phonon emission from a weakly coupled AlAs/GaAs superlattice (SL) under vertical electron transport. The phonons were detected using superconducting bolometers. A peak (resonance) was observed in emission parallel to the SL growth axis when the electrical energy drop per SL period matched the energy of the first SL mini-Brillouin zone-center phonon mode. This peak was mirrored by an increase of the differential conductance of the SL. These results are evidence for stimulated emission of terahertz phonons as previously predicted theoretically and suggest that such a SL may form the basis of a SASER (sound amplification by stimulated emission of radiation) device.     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Phonon-induced optical superlattice

We demonstrate the formation of a dynamic optical superlattice through the modulation of a semiconductor microcavity by stimulated acoustic phonons. The high coherent phonon population produces a folded optical dispersion relation with well-defined energy gaps and renormalized energy levels, which are accessed using reflection and diffraction experiments.     

Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.     

差频可调谐太赫兹技术的新进展

太赫兹技术在最近30年来得到快速发展, 并在医学、生物、农业、材料、安检、通信、天文等领域得到广泛应用. 从太赫兹源的频谱特性可以分为窄带(单频)太赫兹源和宽带太赫兹源. 从频谱技术方面来说, 相干的宽带和窄带太赫兹谱是一种互补性关系, 具有各自的技术特点和应用范围. 宽带太赫兹谱可以用于快速获取较宽频谱范围的分子振转谱, 实现混合特征谱的快速检测或成像. 窄带太赫兹源具有很好的光谱灵敏度和分辨率, 适用于太赫兹抽运-探测、分子振转能级谱精细结构分辨 以及太赫兹远程探测和成像. 因此研制具有可调谐的高峰值功率的窄带太赫兹源是适用于探测和识别分子振转能级指纹谱的应用需求, 而差频技术是获得高功率和宽调谐窄带太赫兹源最重要的技术之一. 为了突出该技术的最新进展, 本综述引证论文仅仅限于近5 年来基于差频技术产生太赫兹波的研究进展, 分为光学激光差频源和量子级联激光器差频源两大部分. 对于光学激光差频源, 分别对目前文献报道的各种双波长差频源和太赫兹产生用的非线性晶体进行分类介绍, 并给出所采用的技术和实验结果; 对于量子级联激光器差频源, 分别介绍了量子级联激光器中的差频产生技术和波长调谐技术的最新进展. 量子级联激光器差频太赫兹源是目前实现量子级联激光器在太赫兹波段室温运转的惟一技术, 是实现小型化、窄带宽调谐和室温运转太赫兹源的新发展领域, 值得关注.     

Generation and detection of very high frequency acoustic phonons in diamond

Generation and detection of very high frequency acoustic phonons in diamond is reported. We generate phonons at a frequency of 28 THz by defect-induced one-phonon absorption of CO₂ laser radiation and observe, after pulsed excitation, phonon decay products in the frequency range from 1 THz to 7 THz. For detection vibronic sideband spectroscopy is used. We find strongly frequency dependent lifetimes for frequencies above 4 THz which we attribute to spontaneous phonon decay.Dedicated to K. Dransfeld on occasion of his 60th birthday     

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.