Pulsed photoacoustic technique to study nonlinear processes in liquids: Results in toluene

Pulsed photoacoustic measurements have been carried out in toluene at 532 nm wavelength using a Q-switched frequency doubled Nd:YAG laser. The variation of photoacoustic signal amplitude with incident laser power indicates that at lower laser powers one photon absorption takes place at this wavelength while a clear two photon absorption occurs in this liquid at higher laser powers. The studies made here demonstrate that pulsed photoacoustic technique is simple and effective for the investigation of multiphoton processes in liquids.     

CdSe/ZnS核-壳结构量子点的非线性光学吸收

利用开孔Z扫描技术研究了吸收峰分别为553 nm和503 nm的两种尺寸CdSe/ZnS核-壳结构量子点溶液的非线性吸收性质.对于532 nm,6 ns激光脉冲,两种材料均表现出饱和吸收向反饱和吸收转化的现象.数值模拟结果表明：当吸收峰波长大于激光波长时,饱和吸收过程由快、慢两种机制组成,分别对应基态载流子被激发至不同的激发态,而强光下的反饱和吸收与快过程相关;当吸收峰波长小于激光波长时,饱和吸收主要由快过程机制引起,强光下的反饱和吸收源自激发态吸收和双光子吸收.我们的研究结果表明半导体量子点是研制光开关和光限制器件的理想候选材料.     

基于低功率分布反馈激光器和数字锁相技术的高灵敏光声气体传感器

光声信号强度与光功率成正比,然而,高功率激光光源存在功耗高、驱动控制电路复杂、低成本高质量的光源可选择范围窄等缺点,此类光源多集中在>6 μm波段,难以实现对基频吸收带位于2～6 μm波段的分子进行有效探测。而且,基于商用驱动控制仪器的光声气体传感器体积较大,不能满足多点连续移动监测工作的需要。利用输出功率为5.2 mW的分布反馈、带间级联激光器(ICL)和基于石英音叉的光声光谱探测方法,在3～4 μm波段实现了nmol·mol-1水平气体分子浓度测量。使用的ICL靶向乙烷(C₂H₆)基频吸收带的强吸收线2 996.88 cm-1。通过使用自主研制的数字锁相放大器及数字激光驱动控制方法,结合波长调制光谱技术,实现了高灵敏检测,同时,有效减小了系统体积并简化了数据获取和处理过程。首先,结合系统原理结构,顺次介绍了设计方案以及光、电等模块的设计细节。分析了目标气体及其临近干扰气体吸收谱线的模拟情况,以及不同气压对谱线展宽及重叠干扰的影响,最终确定系统工作气压为200 Torr。然后,通过对100～1 000 nmol·mol-1共6种浓度C₂H₆进行单周期光谱扫描测试分析,推断系统最低检测下限(MDL)<100 nmol·mol-1。对上述各浓度样品分别进行~10 min二次谐波(2f)信号峰值提取测试,系统线性性能良好,相关度为0.999 65,同时,明确了气体浓度与2f信号峰值的对应关系。最后,通过对氮气连续1小时测试得出系统噪声为~0.347 V,由此估算信噪比和系统灵敏度分别为~28.56和~40 nmol·mol-1。介绍的新型中红外C₂H₆传感器不仅实现了nmol·mol-1级测量,而且,使用自主研制的数字驱动和锁相放大器有效减小了系统体积,弥补了使用商用仪器占用体积大的不足,为将来实现小型化、移动式测量的目标奠定了一定基础。此外,对于功率消耗无限制的其他应用,可通过进一步完善和改进锁相和前置放大等模块的性能以及使用输出功率更高的光源进一步提高传感器灵敏度,并应用于更多场景。     

基于光声光谱技术的1.5 μm波段 NH3吸收谱线测量与分析

NH₃的检测具有广泛的应用,采用光声法检测NH₃是当前研究的热点,而确定NH₃的吸收谱线则是实现光声法检测NH₃的前提。采用外腔可调谐半导体激光器构造了光声气体检测系统,检测了NH₃的近红外吸收谱线,获得了常温常压下NH₃在1 515~1 532 nm范围内的吸收光谱。实验确认了NH₃在1 515.2,1 516.0,1 518.0,1 519.9,1 522.4,1 527.0,1 531.7 nm处存在强吸收。HITRAN 2004光谱数据库在近红外1.5 μm波段NH₃的吸收谱线数据未见报道,该结果为研究光声法检测NH₃提供了更多可选择的吸收谱线。     

单光子波长调制吸收光谱用于1.5 μm激光器的波长锁定

提出了一种kHz调制频率量级的单光子调制技术,实现对激光器的实时锁定. 通过探测乙炔气体的单光子吸收光谱,对离散的单光子响应脉冲进行锁定放大, 获得激光器频率锁定的鉴频曲线.将1.5 μm分布反馈半导体激光器的输出频率稳定在乙炔气体 ν₁ + ν ₃带P5e支吸收峰上,在175 s内典型激光频率起伏小于25 MHz. 这种基于单光子波长高频调制吸收光谱的稳频技术消除了低频段较高背景的噪声, 可应用于单光子量级的量子保密通讯系统以及光学波分复用系统.     

The SNR improvement for quartz-enhanced photoacoustic spectroscopy using wavelength calibration and fiber reflector

Wavelength calibration technique combined with a fiber reflector was used to improve the signal to noise ratio (SNR) of quartz-enhanced photoacoustic spectroscopy (QEPAS). A distributed feedback laser diode (DFB-LD), driven by sawtooth wave and high frequency sinusoidal wave, was used to excite the second harmonic signal of a quartz tuning fork (QTF) through laser-gas molecular interaction. Two collimators conducted the laser alignment through the spacing gap of QTF forks. Central wavelength of the DFB-LD was locked to the target gas absorption center by identifying the second harmonic signal maximum and applying calibration feedback on the driving current. The gas absorption center calibration and gas concentration measurements are conducted at a specific interval. The SNR of the photoacoustic signal was further acoustically enhanced by using a pair of on-beam acoustic resonators through increasing the photo-acoustic conversion efficient, and optically enhanced by using a fiber reflector to improve the laser power for photoacoustic signal excitation. The experimental results show that the SNR in wavelength calibration mode is 15 times higher than the conventional wavelength scanning mode and QEPAS signal with fiber reflector is 1.37 times stronger compared with that without a fiber reflector.     

Optical power limiting and transmitting properties of cadmium iodide single crystals: Temperature dependence

Optical limiting properties of the single crystals of cadmium iodide are investigated using ns laser pulses. It is found that the transmissions in the crystals increase with increasing temperature. However, they limit the transmissions at high input powers. The limiting power is found to be higher at higher temperature. From the measured transmission data, the photon absorption coefficients are estimated. The temperature dependence of the coefficients shows a decrease in magnitude with increasing temperature. This might be due to the temperature-dependent bandgap shift of the material. The results demonstrate that the cadmium iodide single crystals are promising materials for applications in optical power limiting devices.     

灵敏的基于分布反馈式半导体激光波长调制光声光谱

利用室温下单模运行的近红外半导体二极管激光,报导了波长调制共振光声光谱结合二次谐波探测技术.实验系统应用到乙炔探测,在1个标准大气压和3毫瓦平均光功率以及3毫秒锁相积分时间条件下其探测灵敏度可达10ppm(体积比),归一化到激光功率和系统带宽最小可探测吸收为4.0×10-8Wcm-1/Hz,并且实验中发现系统最佳压力响应值在2.66×104Pa附近.本实验装置可有效的消除光声光谱系统中常见的窗片和光声腔壁吸收入射光而引起的背景噪声.此外,相对于其他方法我们描述的基于半导体激光共振光声光谱具有很大的优点,为进一步发展便利、实用、便携式环境监测仪器奠定了坚实的基础.     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

Pulsed optoacoustic detection of multiple photon excitation in molecules

Multiple photon excitation, saturation, and linear absorption of SF₆-argon mixtures when irradiated by a high power CO₂ TEA laser is investigated using a pulsed optoacoustic technique. At low intensities the expected linear dependence of the absorption on laser intensity is observed. At intermediate intensities the absorption exhibits a square root dependence on the incident laser intensity, a dependence which is typical for saturation of an inhomogeneously broadened absorption. At even higher intensities, the absorption shows an intensity dependence typical of multiple photon excitation. The laser intensity was varied between 0.016 kW/cm² and 5 MW/cm², values lower than that needed to produce multiple photon dissociation of SF₆. Increasing the collision frequency of the absorbing molecules with an inert buffer gas is observed to quench the multiple photon excitation.     

空腔靶能量吸收,转换特性实验研究

本文介绍空腔靶设计的物理思想、及能量吸收特性、X光转换特性和堵腔特性的实验研究方法,给出了实验观察到的一系列物理现象,通过对现象的分析而得出空腔靶的能量吸收和X光转换明显优于平面靶;利用相对孔径较大的聚焦透镜打空腔靶有利于改善靶的能量吸收和转换特性的结论。     

基于中红外分布反馈量子级联激光器的光声光谱技术用于痕量甲烷气体检测

由于工业监控和环境检测的需要,甲烷气体检测日益得到人们的关注。研究了基于中红外分布反馈量子级联激光器(DFB-QCL)的光声光谱技术,并应用于痕量甲烷的检测。自主研发的DFB-QCL室温工作时的激射波长在7.6μm附近,覆盖了甲烷的特征吸收谱线1 316.83cm-1。待测甲烷气体充入亥姆霍兹光声谐振腔中,DFB-QCL的工作频率为234Hz、室温脉冲工作时峰值功率为80mW。中红外光经过甲烷吸收后,产生的声波信号经麦克风检测,由锁相放大器对信号进行采集并输入计算机进行处理。按信噪比为1计算,得到甲烷的探测极限为189nmol.mol-1。     

Two photon absorption in Mn-doped ZnSe quantum dots

The nonresonant third order nonlinear optical properties of three different sized Mn²⁺-doped ZnSe quantum dots (QDs) are investigated. The nonlinear absorption is measured at 532 nm using 35 picosecond laser pulses in an open-aperture Z-scan setup. Two photon absorption (2PA) cross-section in ZnSe QDs is found to be three orders of magnitude higher than its bulk value. These nanostructures show size dependent nonlinear absorption coefficients. It is found that the 2PA further enhances with decrease in size of the QD and is twice that of its undoped counterpart due to change in the local electric field.     

Perfect photon absorption based on the optical parametric process

The perfect photon absorption is studied in a cavity quantum electrodynamics(CQED) system, in which an optical parameter amplifier(OPA) is coupled to the cavity mode. This makes it possible to control the optical phase to realize the perfect photon absorption. It is found that in the presence of one and two injected fields, the perfect photon absorption is present in these two cases and can be controlled by adjusting the parametric phase. Moreover, different from the previous predictions of perfect photon absorption in atomic CQED systems, the perfect photon absorption can be changed significantly by the relative phase. Our work provides a new platform to use the parametric processes to make an available way to control the behaviors of photons and to take advantage of the optical phase to achieve the perfect photon absorption.     

基于QCL的红外吸收光谱技术的研究进展

量子级联激光器作为一种新型的单极型半导体激光器,其峰值发射波长处于中红外波段（2.5~25 μm）,具有功率高、线宽窄、响应速率快等传统半导体激光器所没有的独特优势,且具有较高的探测灵敏度,非常适合中红外波段的气体分子的检测。可广泛应用于大气痕量气体、呼吸气体、燃烧气体、生化气体、机动车尾气、工业废气以及农药残留气体等低浓度气体的检测。因此,利用量子级联激光器对气体分子进行探测在非侵入式医学诊断、环境监测以及工农业生产等领域都具有十分重要的意义。自20世纪末量子级联激光器发明以来,室温激光器的性能得到了长足的进步,也出现了多种结构形式的量子级联激光器。这也使得量子级联激光器红外吸收光谱技术得到了很大的发展。事实上,很多光谱技术在量子级联激光器发明之前就已经得到了发展和应用,而利用量子级联激光器作为光源则在很大程度上扩展了可探测波段,也在一定程度上提高了探测极限。这其中就包括了直接吸收光谱技术、波长调制技术、腔衰荡光谱技术、腔增强吸收光谱技术以及光声光谱等。综述了国内外量子级联激光器进行红外吸收光谱技术的研究现状和发展趋势,分析了量子级联激光器红外吸收光谱技术在发展过程中所遇到的瓶颈以及后期得到的解决方案,比较详细地介绍了各种方法的原理、应用,并指出了在吸收光谱测量中的优缺点,同时对外场痕量气体探测作了简要总结。最后,对量子级联激光器红外吸收光谱技术在未来痕量气体探测上的应用和发展进行了展望,指出随着红外吸收光谱技术的快速发展,这些方法可以得到更有效的改进和发展,进而朝着高灵敏度、高集成度以及高时效方向发展。     

Absorption line profile recovery based on TDLS and MEMS micro-mirror for photoacoustic gas sensing

A novel and efficient absorption line recovery technique is presented. A micro-electromechanical systems (MEMS) mirror driven by an electrothermal actuator is used to generate laser intensity modulation through the mirror reflection. Tunable diode laser spectroscopy (TDLS) and photoacoustic spectroscopy (PAS) are used to recover the target absorption line profile which is compared with the theoretical Voigt profile. The target gas is 0.01% acetylene (C2H2) in a nitrogen host gas. The laser diode wavelength is swept across the P17 absorption line of acetylene at 1535.4 nm by a current ramp, and an erbium-doped fibre amplifier (EDFA) is used to enhance the optical intensity and increase the signal-to-noise ratio (SNR). A SNR of about 35 is obtained with 100 mW laser power from the EDFA. Good agreement is achieved between the experimental results and the theoretical simulation for the P17 absorption line profile.     

Study of sequential two photon absorption and conduction band electrons absorption in ZnSe

Using the Z-scan technique and pump–probe technique with 130 fs laser pulses at 800 nm, we verify that an intraband one-photon absorption follows the interband two photon absorption. Particularly, we find that there is an intraband relaxation interspersed between these two absorption processes for some of the conduction band electrons but not all of them. In this study, we measure the interband two photon absorption coefficient and the absorptive cross sections associated with both excitation pathways within the conduction band. In addition, we estimate the time for relaxation of electrons within the conduction band to be 250 fs.     

Multiphoton Processes at the Induced Inverse Bremsstrahlung in Dense Plasmas

Electron energy absorption in laser irradiated plasma is determined by sum of rates of multiphoton induced absorption and emission. These rates contain squares of the Bessel functions. It was shown that in moderate laser field terms with absorption and emission of several photons are large, but cancel exactly each to other. Therefore we should take into account terms with absorption and emission of only one laser photon (first order perturbation theory). (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two‐photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q‐switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.     

GaSe晶体的双光子吸收对太赫兹输出的影响

根据光整流效应,利用超快激光脉冲泵浦GaSe晶体实现了0.2~2.5 THz的宽带太赫兹辐射输出。禁带中的电子在两个800 nm光子的作用下激发到导带中形成自由载流子,进而吸收所产生的太赫兹辐射,最终导致太赫兹的输出随泵浦功率的增加而趋于饱和。为了研究双光子吸收对太赫兹输出的影响,测量了800 nm处的GaSe晶体的双光子吸收系数,结果为 0.165 cm/GW。通过对太赫兹输出实验数据的拟合,得到GaSe晶体中自由载流子对太赫兹输出的吸收截面为1×10^-15 cm²。本文的研究结果可用于优化GaSe晶体在强激光泵浦下的太赫兹转换效率。