Raman scattering by folded acoustic phonons in InGaN/GaN superlattices

We use Raman scattering to investigate the folded longitudinal acoustic (LA) phonons in a series of In_xGa_1−xN/GaN superlattices (SLs) grown by molecular beam epitaxy with different compositions (15% < x < 38%) and SL periods (from 8 to 20 nm). A novel, ultralow wavenumber filtering module, which provides access to ultralow wavenumber Raman modes on single‐grating spectrometers, has been used to perform the Raman measurements. Zone‐folding effects are observed, and the wavenumber behavior of the folded LA modes is well reproduced with a linear dispersion for the folded LA modes as predicted by elastic continuum theory. We employ the wavenumber of the doublets to evaluate the period of the SLs. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of the optical gap in novel superconductors by coherent THz radiation

We show how coherent synchrotron radiation (CSR) allows one to measure the slight increase in the reflectivity of a BCS superconductor as it is cooled below T_c. In fact, by CSR coupled to a conventional interferometric apparatus, one can obtain a signal-to-noise ratio 10³ in the sub-THz range. We apply this technique to the measurement of the optical gap in boron-doped diamond and in CaAlSi, a superconductor isostructural to MgB₂. In the latter compound we are also able to determine a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis.     

铁磁异质结构中的超快自旋流调制实现相干太赫兹辐射

利用飞秒激光脉冲在生长于二氧化硅衬底上的W/CoFeB/Pt和Ta/CoFeB/Pt两类铁磁/非磁性金属异质结构中实现高效、宽带的相干THz脉冲辐射.实验中, THz脉冲的相位随外加磁场的反转而反转,表明THz辐射与样品的磁有序密切相关.为了考察三层膜结构THz辐射的物理机制,分别研究了构成三层膜结构的双层异质结构(包括CoFeB/W, CoFeB/Pt和CoFeB/Ta)的THz辐射.实验结果都与逆自旋霍尔效应相符合, W/CoFeB/Pt和Ta/CoFeB/Pt三层膜结构所辐射的THz强度优于同等激发功率下的ZnTe (厚度0.5 mm)晶体.此外,还研究了两款异质结构和ZnTe的THz辐射强度与激发光脉冲能量密度的关系,发现Ta/CoFeB/Pt的饱和能量密度略大于W/CoFeB/Pt的饱和能量密度,表明自旋电子在Ta/CoFeB/Pt中的界面积累效应相对较小.     

Generation of 0.1 THz coherent synchrotron radiation with compact S-band linac at AIST

The 0.1 THz coherent synchrotron radiation (CSR) was successfully generated in the 90° bending magnet of the compact S-band linac with the achromatic arc section using the ultra-short electron bunch which has the energy of 40 MeV, the bunch charge of about 1nc and the bunch length less than 1 ps (rms). The electron bunch compression of 1 nC electron bunch was achieved less than 1 ps (rms) by controlling the Q-magnets in the achromatic arc section as the bunch length was measured by the rms bunch length monitor.     

Observation of THz coherent transition radiation from single-bunch beam at KURRI-LINAC as an intense pulsed light source

A single-bunch beam has been generated using a developed high-speed avalanche-type pulser in KURRI-LINAC in order to lift restrictions of the spectral resolution in the spectroscopic study and the delay time in the time-resolved measurement. Both of the rise and fall times of the developed pulser are 110 ps. The observation of CTR has confirmed the single-bunch beam. The degree of impurity of single bunch has been estimated to be 1.5% by the analysis of the interferogram.     

Coherent phonon dynamics in short-period InAs/GaSb superlattices

We have performed ultrafast pump-probe spectroscopy studies on a series of InAs/GaSb-based short-period superlattice (SL) samples with periods ranging from 46 Å to 71 Å. We observe two types of oscillations in the differential reflectivity with fast (∼$\sim$1–2 ps) and slow (∼$\sim$24 ps) periods. The period of the fast oscillations changes with the SL period and can be explained as coherent acoustic phonons generated from carriers photoexcited within the SL. This mode provides an alternative method for determining the SL period. The period of the slow mode depends on the wavelength of the probe pulse and can be understood as a propagating coherent phonon wavepacket modulating the reflectivity of the probe pulse as it travels from the surface into the sample.     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

可变椭圆极化波荡器相干太赫兹辐射的模拟计算

中国科学院上海应用物理研究所正在研制的相干太赫兹辐射装置是一台利用飞秒电子束团通过可变椭圆极化波荡器产生高亮度水平线极化光、垂直线极化光或各种椭圆极化光的强相干太赫兹辐射光源.本文描述了这种相干太赫兹辐射的光通量密度空间分布、光谱、极化特性和相干性等的计算方法和模拟计算结果.计算结果表明,对于能量为20MeV、发射度为15πmm·mrad、束团长度为200fs、电荷量为50pC的电子束团,在距离0.6m长的波荡器中心1m处可获得光通量密度峰值达2×10⁹photons/mm²/0.1%bw量级的太赫兹水平线极化光或圆极化光     

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith-Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.     

THz Coherent Vavilov-Cherenkov Radiation in a Special Three-Mirror Cavity

In the modern science and technology a compact and having enough output power terahertz radiation source working in room temperature have earned great attention. This paper is devoted to utilize electron bunches stimulate Vavilov-Cherenkov Radiation (VCR) in a special three-mirror quasi-optical cavity to generate coherent THz waves. This novel three-mirror quasi-optical resonant cavity has the coaxial field pattern which enables field establish in this cavity effectively. The analytical theory of the radiation exited by a train of electron bunches in the special kind of three-mirror cavity has been carried out and the coherent VCR has been achieved by the computer simulation. All those shows that this method can be used to establish useful THz radiation source by the normal electron gun and the commonly used microwave devices.     

一种新型THz显微探测技术

由于太赫兹(THz)电磁辐射的波长较长，探测的空间分辨率受到衍射极限的限制.设计了一种 新型的THz显微探测技术，使探测晶体紧贴在薄产生晶体上，激发光采用紧聚焦的方式不但 把空间分辨率提高到光学量级，还克服了典型近场探测方法中存在的低通光量、高通滤波的 缺点.详细介绍了这一新型探测器件的基本结构和显著的探测特性. 关键词： THz 近场 聚焦     

Compact narrow-band THz radiation source based on photocathode rf gun

Narrow-band THz coherent Cherenkov radiation can be driven by a subpicosecond electron bunch trav- eling along the axis of a hollow cylindrical dielectric-lined waveguide. We present a scheme of compact THz radiation source based on the photocathode rf gun. On the basis of our analytic result, the subpicosecond electron bunch with high charge （800 pC） can be generated directly in the photocathode rf gun. According to the analytical and simulated results, a narrow emission spectrum peaked at 0.24 THz with 2 megawatt （MW） peak power is expected to gain in the proposed scheme （the length of the facility is about 1.2 m）.     

Compact narrow-band THz radiation source based on photocathode rf gun

Narrow-band THz coherent Cherenkov radiation can be driven by a subpicosecond electron bunch traveling along the axis of a hollow cylindrical dielectric-lined waveguide. We present a scheme of compact THz radiation source based on the photocathode rf gun. On the basis of our analytic result, the subpicosecond electron bunch with high charge (800 pC) can be generated directly in the photocathode rf gun. According to the analytical and simulated results, a narrow emission spectrum peaked at 0.24 THz with 2 megawatt (MW) peak power is expected to gain in the proposed scheme (the length of the facility is about 1.2 m).     

The effect of two-photon absorption and optical excitation area on the generation of THz radiation

It was found experimentally that the THz radiation throughput based on optical rectification is strongly related to the optical excitation size in a ZeTe emitter. The factors affecting the THz radiation throughput are investigated in detail both experimentally and theoretically. By taking into account optical rectification, diffraction and two-photon absorption effects, a theoretical model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the theoretical results and the experimental data under high excitation power. Both theoretical and experimental results demonstrate that there is a trade-off between these three effects for THz radiation, and that in order to increase the THz radiation throughput based on optical rectification, one should choose suitable excitation size under a given pump power.     

高重复频率相干衍射辐射超宽谱太赫兹源

研究了一种新型相干衍射辐射（CDR）太赫兹源,该太赫兹源基于中国工程物理研究院自由电子激光相干强太赫兹源（FEL-THz）的加速器电子束束流,具有MHz量级重复频率。理论分析、数值计算和PIC模拟证明了此太赫兹源时间长度可达到ps量级,中心频率在100~400 GHz可调,截止频率位于1~2 THz,脉冲峰值功率可达10 kW量级,平均功率可达到W量级,并且功率随束流流强成平方正比关系。     

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In_xGa_1−xN QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.     

Coherent optical phonons in alexandrite crystal studied by Optical Kerr Effect spectroscopy

Herewith, we report on the first observation of coherent phonons in alexandrite crystal obtained by means of the femtosecond Optical Kerr Effect spectroscopy. We have managed to observe dynamics of the Raman‐active modes with high resolution and to find their lifetimes in the temperature range between 263 K and 373 K. We have used the obtained temperature dependence of phonon lifetimes and frequencies for calculation of the anharmonic coupling constants. On the basis of our results, we have also calculated the widths of homogeneously broadened Raman lines. Our results prove that, in case of transparent samples, this experimental technique is a very good alternative to the commonly used transient reflection method. Copyright © 2013 John Wiley & Sons, Ltd.