有机分子激发态非线性吸收的瞬态特性研究

提出了一种通过测量单脉冲光的输入与输出波形来研究激发态非线性吸收的新方法，用这种方法研究了纳秒脉作用下类卟啉镉溶液与甲基橙溶液的三重激发态的非线性吸收特性，前者为反饱和吸收现象，后者是由饱和吸收转化为反饱和吸收的新现象，实验结果验证了理论模拟和正确性。     

Coherence Time Measurement of Picosecond Pulses Using Reverse Saturable Absorption in Lead Phthalocyanine

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Rapid synthesize of gold nanoparticles by microwave irradiation method and its application as an optical limiting material

We present rapid synthesis of gold nanoparticles by microwave irradiation method. Sample with average particle size 7.7 nm is obtained from TEM. Linear and nonlinear optical studies of the prepared samples are discussed. Reverse saturable absorption (RSA) at longitudinal surface plasmon resonance (SPR) in gold nanoparticles (Au NPs) have been observed using Z-scan and transient absorption techniques with 532 nm laser pulses. Such RSA behavior makes Au NPs an ideal candidate for optical limiting applications.     

Nonlinear optical properties of [（CH3）4N]Au（dmit）2 using Z-scan technique

The third-order optical nonlinearities of [（CH3）4N]Au（dmit）2 （dmit = 4,5-dithiolate-1,3-dithiole-2-thione） at 532 nm and 1064 nm are investigated using the Z-scan technique with pulses of picoseconds duration. The Z-scan spectra reveal a strong nonlinear absorption （reverse saturable absorption） and a negative nonlinear refraction at 532 nm. No nonlinear absorption is observed at 1064 nm. The molecular second-order hyperpolarizability γ for the [（CH3）4N]Au（dmit）2 molecule at 532nm is estimated to be as high as （2.1 ±0.1） × 10^-31 esu, which is nearly three times larger than that at 1064 nm. The mechanism responsible for the difference between the results is analysed. Nonlinear transmission measurements suggest that this material has potential applications in optical limiting.     

OPTICAL LIMITING EFFECT IN TWO PHTHALOCYANINES OBSERVED BY PICOSECOND PULSED LASER

Optical limiting (OL) properties of two phthalocyanines were investigated by using picosecond laser pulses at 532nm. The relative ratios k of the absorption cross section of the first singlet excited state to that of the ground state were approximately obtained by the analysis of the experimental results in which the reverse saturable absorption model of the three-energy-level scheme was employed. A significant comparison with fullerene C₆₀ was presented for OL. The OL mechanisms have been analysed in detail.     

Reverse saturable absorption and optical limiting in indanthrone dyes

Fluence dependence of transmission in three dyes, viz. indanthrone, dichloroindanthrone and violanthrone, is reported at 532 nm as well as at 1.06 µm. These dyes show optical limiting at both these wavelengths for nanosecond pulses. The limiting is due to the combined effect of reverse saturable absorption from the triplet state and thermal defocussing in the dye solutions.     

Saturable and reverse saturable absorption in aqueous silver nanoparticles at off-resonant wavelength

Silver nano-colloid was prepared by chemical reduction method and its nonlinear absorption properties were investigated by using open aperture z-scan experiment with nanosecond laser pulses operating at 532?nm. Interestingly, a switch over from saturable absorption to reverse saturable absorption was observed when the input intensity is increased from 28.1 to 175.8?MW/cm². The underlying mechanism responsible for the observed switching behaviour is the interplay between ground state plasmon band bleaching and excited state absorption. Theoretical fitting was done by using a model in which nonlinear absorption coefficient as well as saturation intensity are incorporated.     

Optical limiting action of C60 doped poly(dimethylacetylendicarboxylate)

The optical limiting action of poly(dimethylacetylendicarboxylate) polymer doped with fullerene C₆₀ has been investigated under irradiation with 10 ns laser pulses at 532 nm. The optical limiting measurements were performed at four different dopant concentrations. The threshold limiting fluence at 0.3 J/cm² was observed at high doping concentrations, with transmission of about 55%. An explanation based on the combination of two-photon absorption and reverse saturable absorption was proposed for its nonlinear optical absorption behavior.     

Experimental investigation of optical limiting and thermal lensing in toluene solutions of C70

Optical limiting and thermal lensing studies are carried out in C₇₀-toluene solutions. The measurements are performed using 9-ns pulses generated from a frequency-doubled Nd:YAG laser at 532 nm. Optical limiting studies in fullerene molecules lead to the conclusion that reverse saturable absorption is the major mechanism for limiting. Analysis of thermal lensing measurements showed a quadratic dependence of thermal lens signal on incident laser energy, which also supports the view that optical limiting in C₇₀ arises due to sequential two-photon absorption via excited triplet state (reverse saturable absorption).     

Wavelength dependent resonant nonlinearities in a standard saturable absorber IR26 on picosecond time scale

Nonlinear optical properties of a standard dye IR26 have been studied by using the Z-scan technique to decipher the difference in the mechanism of nonlinear absorption on picosecond time scale at two wavelengths i.e. at 1064 nm and 532 nm. A prominent contribution of nonlinear absorption is observed in the Z-scan profiles at 1064 nm. The dye exhibits the mechanism of self-defocusing at 1064 nm in contrast to that of self-focusing at 532 nm. While the two photon absorption has been found to be the dominant mechanism of reverse saturable absorption at 1064 nm, the mechanism of excited state absorption is operating at 532 nm. Additionally, the optical phase conjugate geometry of degenerate four wave mixing (DFWM) technique has been used to measure the third order nonlinear susceptibility values at 532 nm to compare with those obtained from the Z-scan profiles.     

Nonresonant and Resonant Nonlinear Absorption of CdSe-Based Nanoplatelets

We present a comprehensive understanding of the nonlinear absorption characteristics of CdSe-based nanoplatelets(NPLs) synthesized by the solution-phase method and the colloidal atomic layer deposition approach through Z-scan techniques at 532 nm with picosecond pulses. The CdSe NPLs exhibit strong two-photon induced free carrier absorption(effective three-photon absorption) upon the nonresonant excitation, resulting in a remarkable optical limiting behavior with the limiting threshold of approximately 75 GW/cm~2. A nonlinear optical switching from saturable absorption(SA) to reverse saturable absorption(RSA) with increasing the laser intensity is observed when coating CdSe NPLs with a monolayer of CdS shell to realize the resonant absorption. The SA behavior originates from the ground state bleaching and the RSA behavior is attributed to the free carrier absorption.These findings explicitly demonstrate the potential applications of CdSe-based NPLs in nonlinear optoelectronics such as optical limiting devices, optical pulse compressors and optical switching devices.     

Third-order nonlinear optical response of silicon nanostructures dispersed in organic solvent under 1064 nm and 532 nm laser excitations

Silicon nanostructures are dispersed into an organic solvent and the third order optical properties of the system are studied by z-scan technique under 1064 nm and 532 nm excitations. The experiment results show that the silicon nanostructures exhibit self-focus and saturable absorption with both excitation wavelengths. Nonlinear absorption results suggest that a new optical bleaching band exists under 532 nm excitation, and a two-step mechanism is tentatively put forward to explain the saturable absorption under 1064 nm excitation.     

飞秒激光所致金纳米粒子析出的玻璃非线性吸收

依据Z-scan技术,使用波长532nm的纳秒脉冲,研究了通过聚焦的飞秒脉冲诱导并辅以热处理得到的金纳米粒子析出的玻璃的非线性吸收.观察到金纳米粒子析出的玻璃具有饱和吸收特性.根据局域场效应,对实验结果拟合,得到在接近表面等离子体共振激发情况下,金纳米粒子三阶极化率虚部分别为Imχ(3)m=57×10-7esu.玻璃样品中金纳米粒子的非线性响应主要起源于热电子贡献. 关键词： 带内跃迁 带间跃迁 热电子贡献     

Optical and nonlinear optical properties of copper nanocomposite glasses annealed near the glass softening temperature

Copper nanocomposite glasses have been prepared by the ion-exchange method, and annealed at different temperatures up to and above the glass softening temperature. The absorption spectra, fluorescence spectra, and nonlinear optical transmission of the samples at 532 nm for nanosecond laser pulses, have been investigated. The optical and nonlinear optical properties of the glasses are found to be distinctly different below and above the glass softening temperature. For instance, thermal annealing up to the glass softening temperature makes the samples behave like saturable absorbers, while annealing at higher temperatures makes them behave like optical limiters. Such flexibility in controlling the optical nonlinearity in these materials makes them potential candidates for photonic applications.     

Optical limiting response by embedding copper phthalocyanine into polymer host

The multilayer film of PMMA containing mononuclear octakis(mercaptopropylisobutyl-POSS) substituted phthalocyaninato-copper (CuPc) was obtained by spin-coating on quartz substrate. The nonlinear absorption and optical limiting (OL) performance of CuPc have been described using the open-aperture Z-scan technique. The measurements were performed using collimated 4 ns pulses generated from a frequency-doubled Nd:YAG laser at 532 nm wavelength. The results indicate that CuPc/PMMA composite exhibits a much larger nonlinear absorption coefficient, a lower saturable fluence and a lower absorption cross-section ratio for optical limiting when compared to the same CuPc molecules in solution. No evidence of film fatigue or degradation was observed in the CuPc/PMMA film after numerous scans at varying laser intensity. This material is a good candidate for optical limiting applications.     

强非线性吸收下Z扫描衍射模型及实验研究

根据光的衍射理论,建立了非线性光学介质对高斯光束的开孔Z 扫描理论模型.数值计算表明,当相移比值Γ≤0.15,出射光功率随入射光功率的增加出现振荡衰减现象;当Γ>0.15,在输入光强低于某一个值时,输出随输入的增加而线性增加;当输入光强超过该值后,输出功率不再增加甚至趋于零.实验采用Nd∶YAG脉冲激光,在波长为532 nm,脉宽8 ns条件下,对含3.5%和7.0%碳纳米管的聚苯胺/碳纳米管复合体薄膜（PANI/MWCNT）的非线性吸收特性进行了研究.计算表明,该类化合物的激发态吸收截面积远大于其基态吸收截面积,具有很强的反饱和吸收.     

酞菁铜非共振反饱和吸收的稳态特性

酞菁铜溶液在波长532nm调Q激光作用下,实验获得反饱和吸收特性.在稳态条件下,用速率方程理论推得分别描述薄样品和厚样品的反饱和吸收特性的两个解析式,理论分析与实验结果一致.     

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20?J/cm²). Experiments with picosecond pulses at 355?nm are carried out for all five metals and in addition at 532?nm, and 1064?nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355?nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.     

Short pulse Z -scan investigations of optical nonlinearities of a novel organometallic complex: [(C2H5)4N]2[Cu(dmit)2] at 532 and 1064?nm

A novel dmit organometallic complex: [(C₂H₅)₄N]₂[Cu(dmit)₂] (dmit^2-=1,3-dithiole-2-thione-4,5-dithiolate), abbreviated as EtCu, was synthesized. Afterwards its optical nonlinearities in acetone solution at 532 nm and 1064 nm were studied by the Z-scan technique with laser pulses of picosecond duration. The two-photon absorption at 1064 nm and the saturable absorption at 532 nm were observed. The Z-scan curves also revealed that EtCu sample solutions exhibited self-defocusing effects at both wavelengths. The origins were analyzed for the differences between the results. All the outcomes suggest that this material is potential for nonlinear optical device applications. PACS 42.65.An; 42.65.Hw; 42.65.Jx; 81.20.Ka; 78.30.Jw     

Enhanced optical limiting performance of a novel molybdenum complex of fullerene

Reverse saturable absorption of a novel molybdenum complex of fullerene (η²-C₆₀) Mo(CO)₂(o-phen)(DBM)·2C₆H₆·C₅H₁₂ has been investigated under irradiation of 10 ns laser pulses at 532 nm. An enhancement of the optical limiting behavior was observed in comparison with C₆₀. An explanation based on the enhanced triplet-state absorption caused by the intra-molecular charge transfer was predicted. The relationship between the clamped laser fluence and low-intensity transmissivity, or the concentration of the solution, was also investigated and a linear dependence was revealed.