基于非线性光学差频及参量效应的太赫兹源

基于非线性光学技术的THz源具有其独特的性能和优点,将基于非线性光学差频原理和光学参量效应,从理论上研究并分析THz波与抽运光、闲频光及相位匹配角之间的关系,得到THz波输出的条件和范围,并设计出宽波段连续可调的THz源。以调QNd∶YAG激光器和光学参量振荡器(OPO)作为抽运源,以GaSe和MgO∶LiNbO3晶体作为差频非线性晶体,根据相位匹配理论及光学参量效应,搭建两套THz波产生系统。其中,基于光学参量效应的THz辐射源有效地产生出THz信号。     

Localization-Delocalization Transition in Disordered One-Dimensional Exciton-Polariton System

The transition from the delocalized to the localized state has been investigated in a quasi-onedimensional exciton-polariton system excited nonresonantly in GaAs-based microcavity wire with disordered potential. The photoexcited polariton condensate has been found to spread along the wire with а velocity exceeding 1 μm/ps. The propagation along the wire is provided by high energy polaritons. The LP localization length decreases with decreasing blue shift of LPs in the excited spot. The polariton condensate returns to the Bose glass state when the blue shift of the LP resonance at the excitation spot decreases below the critical level that depends on the potential disorder.     

Structural peculiarities of 4H-SiC irradiated by Bi ions

X-ray diffraction, photoluminescence, micro-cathodoluminescence, and scanning and transmission electron spectroscopy were used to study the 710 MeV Bi ion irradiation effect in the fluence range of 1.4 × 10⁹−5 × 10¹⁰ cm⁻² on the structural and optical characteristics of pure high-resistivity n-type 4H-SiC epitaxial layers grown by chemical vapor deposition. It was established that the distribution of structural damage along the ion trajectory follows the computed profile of radiation defects formed in elastic collisions. The high-density ionization effect on the material characteristics has not been found under the irradiation conditions used. Optical methods revealed a wide spectrum of radiation-induced defects, with some of them contributing to the recombination process. The damaged 4H-SiC crystal lattice party recovers after annealing at 500°C. The text was submitted by the authors in English.     

Energy- and angular distributions of argon neutrals and their influence on etching profiles

In a parallel plate 13.56 MHz reactor energetic argon neutrals are detected by sampling neutrals through a 100 μm orifice in the cathode. Experimental neutral energy distributions are compared with Monte Carlo simulations. In the simulation charge exchange collisions, elastic scattering and multiple collisions of neutrals are taken into account. Experimental and theoretical energy distributions are in good agreement indicating that multiple scattering determines the energy distributions. With the calculated neutral angular distribution a three dimensional etch profile is simulated.     

Influence of collisions on radiative saturation and Lamb dip formation in CO2molecular lasers

Saturated gain profile measurements in CO2at pressures where a Lamb dip appears in the laser intensity reveal that individual rotation-vibration transitions saturate with a strong Doppler component in addition to the Lorentzian holes expected for a Doppler broadened line. This mixed inhomogeneous-homogeneous behavior represents cross-relaxation effects induced by collisions that change the molecule's velocity. A rate equation formulation of collision effects elucidates the role of elastic and rotational thermalizing collisions on saturation and Lamb dip formation in molecular lasers.     

Room-Temperature Exceptional-Point-Driven Polariton Lasing from Perovskite Metasurface

Excitons in lead bromide perovskites exhibit high binding energy and high oscillator strength, allowing for a strong light-matter coupling regime in the perovskite-based cavities localizing photons at the nanoscale. This opens up the way for the realization of exciton-polariton Bose–Einstein condensation and polariton lasing at room temperature – the inversion-free low-threshold stimulated emission. However, polariton lasing in perovskite planar photon cavities without Bragg mirrors has not yet been observed and proved experimentally. In this study, perovskite metasurface is employed, fabricated with nanoimprint lithography, supporting so-called exceptional points to demonstrate the room-temperature polariton lasing. The exceptional points in exciton-polariton dispersion of the metasurface appear upon optically pumping in the nonlinear regime in the spectral vicinity of a symmetry-protected bound state in the continuum providing high mode confinement with the enhanced local density of states beneficial for polariton condensation. The observed lasing emission possesses high directivity with a divergence angle of 1° over one axis. The employed nanoimprinting approach for solution-processable large-scale polariton lasers is compatible with various planar photonic platforms suitable for on-chip integration.     

基于光学方法的太赫兹辐射源

太赫兹波技术在物理、化学、生命科学等基础研究学科,以及医学成像、安全检查、产品检测、空间通信、武器制导等应用学科都具有重要的研究价值和应用前景,而太赫兹辐射源正是太赫兹技术发展的关键部分。概述了基于光学方法产生太赫兹辐射的几种常用方法,着重叙述了利用非线性光学差频技术和基于横向晶格振动光学模受激电磁耦子散射过程的太赫兹参量振荡技术工作原理,以及目前的研究状况,并对这两种方法产生太赫兹波辐射源未来的发展方向进行了展望。     

Analysis of the optogalvanic effect in discharge plasmas using rate equations in a modified Schottky formalism

A rate analysis approach is used to evaluate an example of the recently observed optogalvanic effect in discharge plasmas. The coupling of level populations through radiative transitions and electron collisions is solved in a modified Schottky approximation for a glow discharge. The effects of radiation trapping, superelastic collisions, and buffer-gas pressures are treated in the analysis. The utilization of this effect for isotope separation is discussed.     

近红外波长下COMSOL软件实现新型耦合器的设计

一直以来条状表面等离子波导被认为是实现高集成光路的有效器件,首先采用经典的Drude模型对条状波导中SPP激发与传播特性完成了分析。针对可见光到近红外入射波长的条件完成了条状SPP波导的模场分布的研究,得出,当金属的厚度不变宽度越大,电磁场分布就越集中在条状波导的两侧;当金属条状的宽度不变,厚度越大时电磁场分布会越集中在金属内;入射波长越长会使得金属条状周围电场的集中越小,且还会导致信道间的干扰。利用得到的条状波导特性进而设计了一种新型的耦合器,器件设计结果表明:条状SPP波导在有限传输距离上能量完全转移只发生一次;波长较长时,场的集中度减小,耦合增强;条状SPP波导耦合器可以实现1310 nm和1550 nm的光波分复用。这一研究对光子器件的发展有一定的理论和实际意义。     

In Situ Study of Nanostructure and Electrical Resistance of Nanocluster Films Irradiated with Ion Beams

An in situ study is reported on the structural evolution in nanocluster films under He⁺ ion irradiation using an advanced helium ion microscope. The films consist of loosely interconnected nanoclusters of magnetite or iron‐magnetite (Fe‐Fe₃O₄) core‐shells. The nanostructure is observed to undergo dramatic changes under ion‐beam irradiation, featuring grain growth, phase transition, particle aggregation, and formation of nanowire‐like network and nanopores. Studies based on ion irradiation, thermal annealing and electron irradiation have indicated that the major structural evolution is activated by elastic nuclear collisions, while both electronic and thermal processes can play a significant role once the evolution starts. The electrical resistance of the Fe‐Fe₃O₄ films measured in situ exhibits a super‐exponential decay with dose. The behavior suggests that the nanocluster films possess an intrinsic merit for development of an advanced online monitor for fast neutron radiation with both high detection sensitivity and long‐term applicability, which can enhance safety measures in many nuclear operations.     

Study of the optical transmission enhancement properties in a one-dimensional Ag film with different single slit groove structures

Numerical simulation has been performed on the optical transmission enhancement properties of a one-dimensional Ag film single slit structure with grooves. The results show that the position, depth and number of the grooves have great influence on the optical transmission, and surface plasmon polariton and resonance mode are the primary factors. The maximal extinction ratio of 35.8 dB is achieved in the single slit structure by adjusting groove depth.     

一种人工表面等离激元的小型化毫米波天线

设计了一款基于人工表面等离子体激元(Spoof Surface Plasma Polariton,SSPP)的小型化高效率定向辐射天线.在介质基板表面印刷了共面波导(CPW)、波矢匹配结构和SSPP传输线三种结构,其中波矢匹配结构与共面波导部分相结合,更好地完成阻抗匹配.此外,因人工表面等离子体激元可以将电场约束在天线...     

Enhanced Delayed Fluorescence in Tetracene Crystals by Strong Light‐Matter Coupling

An enhanced delayed fluorescence is demonstrated experimentally in tetracene single crystals strongly coupled to optical modes in open cavities formed by arrays of plasmonic nanoparticles. Hybridization of singlet excitons with collective plasmonic resonances in the arrays leads to the splitting of the material dispersion into a lower and an upper polariton band. This splitting significantly modifies the dynamics of the photoexcited tetracene crystal, resulting in an increase of the delayed fluorescence by a factor of 4. The enhanced delayed fluorescence is attributed to the emergence of an additional radiative decay channel, where the lower polariton band harvests long‐lived triplet states. There is also an increase in total emission, which is wavelength dependent, and can be explained by the direct emission from the lower polariton band, the more efficient light out‐coupling, and the enhancement of the excitation intensity. The observed enhanced fluorescence opens the possibility of efficient radiative triplet harvesting in open optical cavities, to improve the performance of organic light emitting diodes.     

The Investigation of Plasmon-Longitudinal Optical Phonon Coupling and Surface Polariton Modes in Donor Doped GaAs-Al0.33Ga0.67As Multi Quantum Wells

Infrared Fourier transform spectroscopy has been used to investigate phonon, plasmon, surface polariton and plasma-longitudinal optical phonon coupling in highly donor doped multi quantum wells (GaAs/Al_0.33Ga_0.67As) and direct band gap n- type Al_XGa_1-XAs thin layer on GaAs substrate. Using different samples with different concentration of free carriers. The dispersion equation of coupling modes have been calculated by using the condition which the dielectric functions of samples are zero for longitudinal coupled modes and experimental papameters which have been obtained from the best fit p-polarized oblique incidence far infrared reflection spectra. In MQW samples, the free carriers confined to the well and carriers are quasi two dimensional. So, plasmon- LO phonon coupling occur in the well (GaAs). In n- type Al_XGa_1-XAs thin layer, the coupled modes consist of three branches of the high, intermediate and low frequency modes. Their frequencies depend on both concentration and alloy composition. To analyses the surface polariton modes we carry out attenuated total reflection (ATR) measurements. In order to support our assignment the magnetic field profiles and surface polariton dispersion curves have been calculated.     

The thermal structure of the ionosphere

From a large number of rocket, satellite, and ground-based experiments since 1959 it is known that the electron and ion gases of the middle and upper ionosphere are substantially hotter than the neutral atmosphere. At low and midgeomagnetic latitudes the principal heating agent for the ionospheric plasma lies in the excess kinetic energy given to photoelectrons arising from the ionization of the atmospheric gases by solar ultraviolet radiation. Although the photoelectrons lose most of their kinetic energy in the excitation of atomic and molecular gases, a significant amount of energy is given to the ambient Maxwellian electron gas, increasing its temperature above that of the neutral gases. The ion gases, in contrast, appear to be heated almost entirely through the elastic collisions with ambient electrons so that the ion temperature generally lies between the electron and neutral temperatures. The calculation of theoretical temperature profiles has developed into a moderately sophisticated process with a fair degree of correspondence between predicted and observed values for undisturbed geophysical conditions. Current research emphasizes the global aspects of plasma temperatures and the connection between ionospheric and magnetospheric phenomena. However, many of the observed diurnal and seasonal variations in both electron and ion temperatures depend upon the couplings between the neutral and ionized atmospheres, and a complete understanding of all aspects of the ionospheric thermal balance is not possible at the present time.     

Photoluminescence excitation spectroscopy of the dense exciton gasinvolved in the lasing transition in ZnSe epitaxial layers

The lasing transition in ZnSe epitaxial layers has been investigated at 77 K. The lowest lasing threshold (I_th) was achieved when the layers were resonantly excited at the photon energy of the exciton level. It was found that the exciton level at the excitation intensity just above the I_th was red-shifted by about 16 meV compared with the free exciton line (2.792 eV) under weak excitation condition. The energy difference between the exciton line and the lasing peak was about 19 meV at I=I_th and increased with increasing excitation intensity up to I=8×I_th. This suggests that the stimulated emission occurs due to the inelastic exciton-exciton scattering process at this temperature     

ECR等离子体沉积系统中等离子体特性的模拟

本文建立了一个物理模型，编写了数值程序，模拟了微波电子回旋共振（ＥＣＲ）等离子体流的特性。假定等离子体中的电子服从玻尔兹曼关系，离子在从共振区流向衬底的过程中与中性气体发生电荷交换和弹性碰撞。根据等离子体区应满足的电荷准中性条件，采用蒙特卡罗方法可计算出自恰的等离子体电位及离子密度分布。从而分析了磁场形态和气压对等离子体密度空间分布，入射到衬底上的离子通量和平均能量的影响，结果表明：磁场形态和气压都可用来独立控制等离子体流的性能，这对于使用该种等离子体源进行薄膜沉积和刻蚀有一定的理论指导意义。     

汽相外延ZnSe单晶的激光发射(英文)

Much work has been done on the laser emission from ZnSe under electron beam pumping. In this paper, the stimulated emission from ZnSe epilayer has been investigated under a nitrogen laser excitation. The ZnSe epilayers were grown by vapor phase epitaxy on (100) GaAs substrates. Optically pumped lasers were fabricated from the ZnSe epilayers by cleaving the ZnSe/GaAs along the (100) cleavage planes into rectangles with widths 100-600μm, and found to operate in the blue portion of the visible spectrum at 77K. The threshold of laser emission was determined to be about 1MW/cm2 from the dependence of the total light output on excitation intensity. The exciton-exciton inelastic collision process is the dominant laser emission mechanism in the ZnSe laser at 77K.     

Vibrationally Assisted Polariton‐Relaxation Processes in Strongly Coupled Organic‐Semiconductor Microcavities

If a semiconductor with an electronic transition that approximates a two‐level system is placed within an optical cavity, strong coupling can occur between the confined photons and the semiconductor excitons. This coupling can result in the formation of cavity polariton states that are a coherent superposition of light and matter. If the material in the cavity is an organic semiconductor, it has been predicted that interactions between Frenkel excitons, polaritons, and molecular vibrational modes will have a profound role in defining the overall relaxation dynamics of the system. Here, using temperature‐dependent spectroscopy on a microcavity containing a J‐ aggregated cyanine dye, it is shown that a spectrum of localized vibrational modes (identified by Raman scattering) enhances the population of certain polaritonic modes by acting as an energy‐loss channel to the excitons as they undergo scattering. Our work demonstrates that simultaneous control of the optical properties of a cavity and the vibrational structure of a molecular dye could promote the efficient population of k = 0 polariton states, from which lasing and other cooperative phenomena may occur.     

Investigation of a Polariton Condensate in Micropillars in a High Magnetic Field

The photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high-Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.