Numerical and experimental investigations of dependence of photoacoustic signals from gold nanoparticles on the optical properties

Gold nanoparticles (AuNPs) are used as a contrast agent of the photoacoustic (PA) imaging. The efficiency of AuNPs has been discussed with the absorption cross section. However, the effects of the scattering of the light by AuNPs and surrounding medium on the PA signal from AuNPs have not been discussed. The PA signals from the aqueous solution of AuNPs were examined in the numerical simulation and the experiment. In the numerical simulation, the absorption and scattering cross sections of spherical and polyhedral AuNPs were calculated by Mie theory and discrete dipole approximation. Monte Carlo simulation calculated the absorbed light energy in the aqueous solution of AuNPs. Based on the PA wave equation, the PA signals were simulated. In the experiment, the PA signal from the aqueous solution of AuNP was measured by use of a piezoelectric film and a Q-switched Nd:YAG laser operated at 532 nm. The results of the numerical simulation and the experiment agreed well. In the numerical simulation and the experiment, a single Au nanocube with 50-nm edge generated the peak value of the PA signal significantly. It was approximately 350 times and twice as large as the peak values of the spherical AuNPs with 10- and 50-nm diameters, respectively. The peak value of the PA signal depended on both the absorption and scattering coefficients of the AuNPs and the surrounding medium. The peak value increased with the scattering coefficient in a quadratic manner. The character of the temporal profile of the PA signal such as full width at half maximum depended on the scattering coefficient of the AuNPs.     

Femtosecond laser-induced nanostructure-covered large-scale waves on metals

Through femtosecond (fs) laser pulse irradiation (pulse duration: 65 fs, central wavelength: 800 nm, and repetition rate: 250 Hz), we investigate the morphological evolution of fs laser-induced periodic surface structure on Au and Pt, called a nanostructure-covered large-scale wave (NC-LSW) with a period of tens of microns, densely covered by iterating stripe patterns of nanostructures and microstructures. We show that the surface morphology of NC-LSW crucially depends on the fluence of the laser, the number of irradiating pulses, and the incident beam angle. Our experimental observations allow us to establish a three-step model for the NC-LSW formation: the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference between the incident light and the scattered field, and nonuniform energy deposition due to shielding by the peaks of LSW.     

A Dual-Readout Method for Biothiols Detection Based on the NSET of Nitrogen-Doped Carbon Quantum Dots–Au Nanoparticles System

In this paper, a rapid, simple and highly sensitive method with dual-readout (colorimetric and fluorometric) based on the nanometal surface energy transfer (NSET) between nitrogen-doped carbon quantum dots (NCQDs) and gold nanoparticles (AuNPs) for detection of biothiols is described. Highly luminescent NCQDs were prepared via a simple one-step hydrothermal method by applying sucrose and glycine as carbon and nitrogen sources. The results showed the obtained NCQDs had an average particle diameter of 5 nm and highly luminescent. The maximum emission wavelength was 438 nm with an excitation wavelength of 360 nm. In this system, NCQDs and AuNPs were respectively treated as energy donors and energy acceptors, which enable the nanometal surface energy transfer (NSET) from the NCQDs to the AuNPs, quenching the fluorescence. However, biothiols was used as a competitor in the NSET by the strongly Au-S bonding to release NCQDs from the Au surface, which subsequently produces fluorescent signal recovery and the red-to-purple color change quickly. This probe showed rapid response, high selectivity and sensitivity for biothiols with dual colorimetric and fluorescent turn-on signal changes. The low detection limit was calculated as 20 nM by using L-cysteine acted as target melocules. The method was also successfully applied to the determination of biothiols in human serum samples, and the results were satisfying.     

等离子体闪光法识别薄膜损伤的误判消除方法

传统的等离子体闪光法,是根据探测器是否接收到来自薄膜样片周围发射的闪光信号,对薄膜是否发生损伤进行评判,这样的评判方法极易把空气与薄膜的等离子体闪光混淆而发生误判。为了消除这种误判,提出通过比较空气和薄膜各自的等离子体闪光的点燃时间,利用两者时间上的差异,实现对传统等离子体闪光法误判现象的消除方法。为了验证新方法的可靠性,借助于多光子吸收和级联电离理论,建立了空气等离体子体点燃时间的计算模型,根据薄膜与激光的相互作用原理建立了薄膜被击穿时的等离子体点燃时间计算模型,利用建立的模型仿真计算了空气和薄膜的等离子体闪光点燃时间分别为1.856和7.843 ns;搭建实验装置以实现对传统等离子体闪光法的更新,在装置中的不同位置设置三个光电探测器分别采集入射激光信号、空气和薄膜等离子体闪光信号,采集入射激光信号的光电探测器置于聚焦透镜的侧面,另外两个探测器位于薄膜样片周围且左右对称放置,分别用于采集薄膜的等离子体闪光信号和空气的等离子体闪光信号,所有光电探测器采集的信号转换为电信号后同步传输至示波器,以入射激光信号为基准信号,其与空气和薄膜等离子体闪光信号的起始时刻之差,分别为空气和薄膜等离子体闪光点燃时间。脉宽为10 ns、波长为1 064 nm的Nd∶YAG脉冲激光以0.015 cm的聚焦光斑半径、82.4 mJ的入射能量作用于光学厚度为λ/4、直径为20 mm的单层Al₂O₃薄膜样片上后,采集上述激光作用条件下的各路信号,经处理后得到的空气和薄膜的等离子体闪光点燃时间测试值分别为2.7和7.8 ns;理论计算和实验测试结果表明,空气的点燃时间总是小于薄膜的点燃时间,二者有很好的一致性。说明当强激光作用于单层Al₂O₃薄膜表面时,空气等离子体闪光先于薄膜等离子体闪光发生。基于空气和薄膜等离子体闪光点燃时间上的这种差异,利用闪光信号时间上的差别就可准确分辨出薄膜是否发生损伤,从而获得识别薄膜损伤与否的判据,这种从时间差异上识别薄膜等离子体闪光损伤的新方法,无论从理论上还是实验上均为传统等离子体闪光法误判现象的消除提供了技术基础。     

3.1 W laser-diode-end-pumped composite Nd : YVO4 self-Raman laser at 1176 nm

We report a laser-diode-end-pumped acousto-optical Q-switched double-end diffusion-bonded Nd?:?YVO₄ self-Raman laser at 1176 nm. The maximum average output power at the first-Stokes wavelength of 1176 nm was obtained to be 3.1 W at the incident pump power of 25 W and the repetition rate of 90 kHz, with the corresponding optical conversion efficiency of 12.4%. The shortest pulse width, the maximum pulse energy and the highest peak power were measured to be 5 ns, 42 μJ and 7.5 kW, respectively.     

Background gas collisional effects on expanding fs and ns laser ablation plumes

The collisional effects of a background gas on expanding ultrafast and short pulse laser ablation plumes were investigated by varying background pressure from vacuum to atmospheric pressure levels. For producing Cu ablation plumes, either 40 fs, 800 nm pulses from a Ti: Sapphire laser or 6 ns, 1,064 nm pulses from a Nd:YAG laser were used. The role of background pressure on plume hydrodynamics, spectral emission features, absolute line intensities, signal to background ratios and ablation craters was studied. Though the signal intensities were found to be maximum near to atmospheric pressure levels, the optimum signal to background ratios are observed ~20–50 Torr for both ns and fs laser ablation plumes. The differences in laser–target and laser–plasma couplings between ns and fs lasers were found to be more engraved in the crater morphologies and plasma hydrodynamic expansion features.     

激光对金属铜微孔加工的热力学过程研究

采用纳秒激光脉冲对铜金属进行了打孔实验,对微孔形貌进行了观察并对其热力学过程进行了相应的分析。研究表明,微孔的形貌是由凹坑和周围隆起组成,坑深随着脉冲能量的增加而增加。热力学分析表明,激光辐照金属打孔需要两个基本条件：一是激光脉冲能量的沉积,使金属材料发生熔化、汽化以及电离等相变,使得材料更容易去除;激光等离子体作为二次热源会更有效把激光脉冲能量耦合到金属表面;二是激光等离子体的冲击波效应,这种效应会把发生相变的材料有效及时排出,从而有效形成微孔。     

Simulation of rippled structure adjustments based on localized transient electron dynamics control by femtosecond laser pulse trains

This study investigates the effects of pulse energy distributions on subwavelength ripple structures (the ablation shapes and subwavelength ripples) using the plasma model with the consideration of laser particle–wave duality. In the case studies, the laser pulse (800 nm, 50 fs) trains consist of double pulses within a train with the energy ratios of 1:2, 1:1, and 2:1. Localized transient electron densities, material optical properties, and surface plasmon generation are strongly affected by the energy distributions. Hence, the adjustment of the ablation shape and subwavelength ripples can be achieved based on localized transient electron dynamics control during femtosecond laser pulse train processing of dielectrics. The simulation results show that better, more uniform structures, in terms of ablation shapes and subwavelength ripples, can be easily formed at a lower fluence or subpulse energy ratio of 1:1 with a fixed fluence. It is also found that pulse trains at a 1:1 energy ratio are preferred for drilling high-aspect-ratio microholes or microchannels.     

Generation of a high-temporal contrast ultrafast laser pulse near 1,053 nm through stimulated Raman frequency shift

A high-temporal contrast femtosecond Stokes pulse near 1,053 nm is obtained simply without polarizer extinction ratio limitation based on the stimulated Raman frequency shift process in ethanol with an 800-nm femtosecond Ti:sapphire laser as a pump source. By optimizing the incident pump pulse chirp and the ethanol Raman cell length, a clean Stokes pulse near 1,053 nm with a maximum energy of 0.24 mJ is obtained with ~7.5 % conversion efficiency and 0.8 % (rms) energy fluctuation. Compared with the incident pump pulse, the temporal contrast of the Stokes pulse is improved by at least approximately three orders of magnitude.     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.     

基于半解析自洽理论研究相对论激光脉冲驱动下阿秒X射线源的产生

发展了一种描述相对论激光脉冲与稠密等离子体相互作用产生阿秒X射线源的半解析自洽理论.该理论模型不仅可以获得等离子体界面的振荡轨迹、振荡面电场和磁场等物理参数,而且能够精确计算出激光脉冲驱动下阿秒X射线源的频谱,结果与粒子模拟程序一致.理论计算结果表明阿秒X射线源的辐射特性与等离子体界面随时演化过程相关,在周期量级激光场驱动下等离子体界面振荡振幅呈现中心不对称,通过改变激光场的载波包络相位实现对等离子体界面振荡的控制,获得准单阿秒X射线源.     

Photo-emission studies from Zn cathodes under plasma phase

In this paper, we report investigations of the electron emission from pure Zn cathodes irradiated by UV laser pulses of 23 ns (full-width at half-maximum) at a wavelength of 248 nm (5 eV). The metal cathodes were tested in a vacuum photodiode chamber at 10^?5 Pa. They were irradiated at normal incidence and the anode–cathode distance was set at 3 mm. The maximum applied accelerating voltage was 18 kV, limited by the electrical breakdown of the photodiode gap. Under the above experimental conditions, a maximum applied electric field of 6 MV/m resulted. In the saturation regime, the measured quantum efficiency value increased with the accelerating voltage due to the plasma formation. The highest output current was achieved with 14 mJ laser energy, 18 kV accelerating voltage and its value was 12 A, corresponding to a global quantum efficiency (GQE) approximately of 1×10^?4. The temporal quantum efficiency was 1.0×10^?4 at the laser pulse onset time and 1.4×10^?4 at the pulse tail. We calculated the target temperature at the maximum laser energy. Its value allowed us to obtain output pulses of the same laser temporal profile. Tests performed with a lower laser photon energy (4.02 eV) demonstrated a GQE of two orders of magnitude lower.     

空气中激光等离子体通道的三次谐波光谱特性研究

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12 mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道, 并在前 向观测到谱线半峰全宽（FWHM）为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时 ,通过改变可编程声光色散滤波器（AOPDF）光谱调制的位置（Hole position）,三次谐波的光谱也发生频移。     

Large area laser surface micro/nanopatterning by contact microsphere lens arrays

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning.     

环境气体对激光诱导Al等离子体光谱的影响

调QNd:YAG脉冲激光(波长1.06μm,脉冲宽度10ns,能量42mj/pulse)烧蚀平面Al靶,在垂直靶面方向,利用光学多道分析系统(OMA),测量了激光诱导产生的等离子体发射光谱分别在环境气体为Air,N2和Ar,气压范围分别在10-2Torr,152Torr,304Torr,532Torr和760Torr下的空间分布。实验表明,随着环境气体压强的增大,光谱的空间分布被压缩,且谱线强度的峰值向靶面方向移动;在相同的气压下,Ar环境下产生的光谱强度明显大于在Air和N2环境下产生的光谱强度;在气压为152Torr的不同环境气体下,均发现光谱在离靶面很近的地方(约0.5mm～1.0mm)减小,甚至消失;在Ar环境气体下,谱线396.1nmAlⅠ的空间分布随着与靶面距离的增大,谱线位置向短波方向有约0.03nm(对应OMA探头的一个二极管阵列)的移动,而谱线394.4nmAlⅠ的移动不太明显。     

Double-line terawatt OPCPA laser system for exciting beat wave oscillations

A double-line terawatt beat laser (BEAT) is developed for exciting beat wave oscillations. BEAT consists of two oscillators and an amplification system including optical parametric chirped-pulse amplification (OPCPA) in which two individual pulses with wavelength separations of 10–35 nm are amplified, recompressed, and focused as a single beam. The recompressed pulse trace shows that a 150-fs pulse duration full width at half maximum was modulated at a beating period of 72 fs. This beating period matches a resonant excitation of plasma wave with an electron density of 2.5 × 10¹⁸ cm^?3, resulting in excitation of a beat wave in hydrogen plasma with wave amplitude of 15 GV/m. The multiple beating oscillations can amplify the plasma wave and improve its structure. This scheme would be ideal for stabilizing the plasma wave strength in the plasma cavity and for realizing a practical laser plasma accelerator.     

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase(CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional(2D) particle-in-cell(PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.     

Passively Q-switched Tm,Ho:LuLiF4 laser with near constant pulse energy and duration

A compact diode-pumped passively Q-switched Tm,Ho:LuLiF₄ laser with a Cr:ZnS saturable absorber is demonstrated for the first time. The maximum average output power of 74 mW is achieved at the absorbed pump power of 1.5 W, and the threshold power is only 0.68 W. The laser operates at the wavelength of 2,055 nm and produces pulses with near constant duration of 1.2 μs and pulse energy of 13 μJ at the pulse repetition frequency from 1 to 5.2 kHz. The stable long-pulse laser can be used in laser lidar systems for accurate wind velocity measurements.     

Tunable Q-switched ytterbium-doped fibre laser by using zinc oxide as saturable absorber

This paper demonstrates the use of a zinc oxide (ZnO) thin film in a 1-μm ring laser cavity as a saturable absorber to successfully generate Q-switching pulses. The tunability of the laser pulses is achieved by integrating a tunable bandpass filter (TBPF) in an ytterbium-doped laser cavity that results in 9.4 nm of tuning range, which wavelength is from 1040.70 nm to 1050.1 nm. The peak energy in the pulse which is 1.47 nJ was measured together with a minimum pulse width of 2.4 μs. In addition, the repetition rate increases from 25.77 to 45.94 kHz as the pump power level being increased from 103.1 to 175.1 mW. The results obtained in this experiment demonstrated consistent results and stable throughout the experiment. Therefore, ZnO thin film is considered as a good candidate in 1-μm pulsed laser applications.     

Thermal decomposition study of nitropyrazoles based on time-resolved pulsed photoacoustic spectroscopy

The paper reports the study of thermal decomposition of 3(5)-nitropyrozole, 4-nitropyrazole and 1-nitropyrazole using time-resolved pulsed photoacoustic spectroscopy technique for the first time. The second harmonic, i.e., λ = 532 nm, 7 ns pulse width at 10 Hz repetition rate, obtained from Q-switched Nd: YAG laser has been employed. In addition, we have studied the effect of pressure, temperature and incident laser energy on the thermal decomposition mechanism of nitropyrazoles. The strength of photoacoustic signal provides significant information about the multistep thermal decomposition mechanism of 3(5)-nitropyrazole and 4-nitropyrazole, while 1-nitropyrazole follows the path of direct decomposition based on single reaction, which is also verified by the thermal gravimetric–differential thermal analysis (TG–DTA) technique. We have also experimentally demonstrated the presence of free NO₂ before the melting point, which is due to the thermal dissociation of solid nitropyrazole compounds.