单脉冲纳秒激光诱导硅表面微结构

利用Nd:YAG纳秒脉冲激光(波长532 nm)在空气中对单晶硅表面进行单脉冲辐照,研究了激光能量密度和光斑面积变化对微结构的影响。通过场发射扫描电子显微镜和原子力显微镜(AFM)对样品表征,并对纳秒激光辐照硅的热力学过程进行分析。结果显示：当脉冲激光的能量密度接近硅的熔融阈值且光斑直径小于8 m时,形成尖峰微结构;随着能量密度或光斑面积增大,尖峰结构消失,形成边缘隆起和弹坑微结构。通过流体动力学模型得到微结构形貌的解析解,模拟得到的微结构形貌与实验测得的AFM数据一致。研究表明微结构的形成主要是由于表面张力引起的熔融硅流动。表面张力与表面温度和表面活性剂的质量浓度有关。温度梯度引起的热毛细流作用和表面活性剂浓度引起的毛细作用共同影响下形成尖峰、边缘隆起和弹坑微结构。     

基于脉冲复制的单脉冲激光对比度测量

为测量超短单脉冲激光的时间波形以及对比度信息,基于三阶强度相关原理,结合光脉冲复制技术,提出了基于脉冲复制的测量方法。对测量原理进行了详尽的理论分析。利用分步傅里叶和龙格库塔数值计算方法对测量方案做了模拟验证。基于脉冲复制的测量法能同时进行多窗口测量。通过拼接测量时间窗口,可以有效解决测量分辨率与测量时间窗口不能兼顾的问题,同时实现大时间窗口和高分辨率测量。通过将主脉冲与预脉冲分离到不同的测量窗口,避免了梯度衰减片的使用,且具有高对比度测量能力。     

基于脉冲复制的单脉冲激光对比度测量

为测量超短单脉冲激光的时间波形以及对比度信息,基于三阶强度相关原理,结合光脉冲复制技术,提出了基于脉冲复制的测量方法。对测量原理进行了详尽的理论分析。利用分步傅里叶和龙格库塔数值计算方法对测量方案做了模拟验证。基于脉冲复制的测量法能同时进行多窗口测量。通过拼接测量时间窗口,可以有效解决测量分辨率与测量时间窗口不能兼顾的问题,同时实现大时间窗口和高分辨率测量。通过将主脉冲与预脉冲分离到不同的测量窗口,避免了梯度衰减片的使用,且具有高对比度测量能力。     

Selection of CO laser single nanosecond pulse by electro-optic CdTe shutter

To select a single laser pulse from a train of nanosecond pulses emitted by a mode-locked CO laser with wavelengths from ∼5 to 6 μm, an electro-optic shutter based on CdTe Pockels cell was developed. A contrast between the selected pulse and unselected laser emission was equal to 20 and twice as much decreased in a CO laser power amplifier. To increase the contrast, a feasibility of applying a narrow-gap semiconductor as a saturable absorber is discussed.     

Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses

The nonlinear optical properties of an azo-based dye were investigated using Z-scan technique employing 38 ps pulses at 532 and 1064 nm, and 6 ns laser pulses at 532 nm. Large nonlinear absorption and nonlinear refraction were observed at both ps and ns 532 nm in the azoic dye. When excited at ps 1064 nm, this dye displayed a large two-photon absorption cross-section (σ₂=1810 GM). Meanwhile, the optical nonlinearity mechanism was discussed in terms of molecular structure, excitation wavelength, and pulse width.     

Time-resolved current response of a nanosecond laser pulse illuminated STM tip

Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM) tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth 35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated. At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification. Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

Microdot pattern of multiple organic molecules prepared by laser photopolymerization process with a nanosecond pulsed laser

A microdot pattern of multiple organic molecules was prepared. Fluorescent molecules, which absorb visible light, were added to a commercial ultraviolet photopolymerization polymer solution and the polymer solution was polymerized by irradiating with nanosecond pulsed visible laser light. The size and the shape of the polymerized dots differed with the irradiation condition and the kind of the added molecules. Based on these results, a microdot pattern of multiple organic molecules was prepared by repeating a micropatterning process with the molecules. PACS 82.50.Hp; 81.65.Cf; 81.05.Lg     

Early stage optical emission in nanosecond laser ablation

This paper describes the results of an experimental and theoretical investigation of the physical origin of the visible continuum emission usually observed in the early stages of nanosecond laser ablation of solid materials. It has been suggested, but not confirmed that the continuum is due to radiative recombination and bremsstrahlung emission. Time and space-resolved emission spectroscopy with an absolutely calibrated spectrometer was used to study the spectral emission in laser ablation of zinc in vacuum at 4.1 J?cm^?2 using a 8 ns, 1064 nm laser pulse. By modelling the spectral emission with a spectral synthesis code, it has been shown that the continuum emission is primarily due to bound-bound transitions between strongly Stark broadened energy levels. Similarly, it can be concluded that the optical absorption is primarily due to bound-bound transitions.     

纳秒激光烧蚀铝材料的二维数值模拟

为了探索纳秒脉冲强激光与材料的相互作用机理,建立了二维数值模型,利用有限差分法对纳秒激光脉冲烧蚀金属铝的温度场进行了数值模拟.通过对比不同脉宽、光斑和能量下激光引起的温度场随时间的演化,发现脉冲的前期温度升高比后期快.等温图显示中心温度升高最快,烧蚀轮廓与激光束形状相似,烧蚀深度达1—5 μm.脉宽越长,烧蚀越窄和越深,光斑越大,烧蚀越宽和越浅.数值研究表明,1)激光的脉冲形状、脉宽和功率密度直接影响烧蚀的形状和深度,2)激光功率密度在10⁹ W/cm²量级烧蚀     

Angular filtering by volume Bragg grating in photothermorefractive glass for nanosecond laser pulse

The two-dimensional angular filter based on volume Bragg gratings in photothermorefractive glass for a nanosecond(ns) laser pulse is demonstrated. The experimental results show that the near-field beam quality of the laser pulse was effectively improved. The near-field modulation and contrast ratio were improved by 1.75 and4.48 times, respectively. The power spectral density curves showed that the spatial frequencies more than0.9 mm~(-1)in the x direction and 1.2 mm~(-1)in the y direction were effectively suppressed.     

Controllable photoinduced optical attenuation in a single-mode optical fiber by irradiation of a femtosecond pulse laser

Novel optical attenuation fibers were fabricated by the irradiation of a focused infrared femtosecond pulsed laser onto the core of a silica glass single-mode optical fiber. Optical attenuation at a wavelength of 1.55 microm proportionally increased with increasing numbers of irradiation points and was controllable under laser irradiation conditions. The single-mode property of the waveguide and the mode-field diameter of the optical fiber were maintained after irradiation of the femtosecond laser. It is suggested that the attenuation results from optical scattering at photoinduced spots formed inside the fiber core.     

Reduction of pulse-to-pulse fluctuation in laser pulse energy using the optical Kerr effect

An all-optical stabilization method of laser pulse energy is proposed using the optical Kerr effect (OKE). The method uses the OKE induced by a portion of the laser pulse as a power controller. The decrease (increase) in the throughput of the optical setup for OKE compensates for the increase (decrease) in pulse energy, thereby stabilizing the pulse-to-pulse fluctuation in pulse energy. The validity of this principle was proven by experiments with a femtosecond laser.     

纳秒紫外重复脉冲激光烧蚀单晶硅的热力学过程研究

采用波长为355 nm的纳秒紫外重复脉冲激光对单晶硅片进行了盲孔加工实验, 观测了随脉冲增加激光烧蚀硅片的外观形貌和盲孔孔深、孔径的变化规律, 并对紫外激光辐照硅片的热力学过程进行了分析. 研究结果表明:紫外激光加工硅盲孔是基于热、力效应共同作用的结果, 热效应会使得硅材料熔化、气化甚至发生电离产生激光等离子体,为材料的去除提供条件;激光等离子体冲击波以及高温气态物向外膨胀会对熔化材料产生压力致使其向外喷射,为重复脉冲的进一步烧蚀提供了条件;力效应主要沿着激光传输的方向,垂直于硅表面,使得去除部位主要集中在孔的深度方向,达到较高的孔径比,实验观察孔径比可达8:1;此外,激光等离子体的产生也阻止了激光对靶面的作用,加之随孔深的增加激光发生散焦,使得烧蚀深度有一定的限制,实验观察烧蚀脉冲个数在前100个时加工效率较高.     

Stable nanosecond pulse generation from a graphene-based passively Q-switched Yb-doped fiber laser

We demonstrate stable 70?ns pulse generation from a Yb-doped fiber laser passively Q-switched by a graphene-based saturable absorber mirror in a short linear cavity. The maximum output power was 12?mW and the highest single pulse energy was 46?nJ. The repetition rate of the fiber laser can be widely tuned from 140 to 257?kHz along with the increase of the pump power. To the best of our knowledge, this is the first report for passively Q-switched sub-100-ns pulse operation of a graphene-based saturable absorber in a Yb-doped fiber laser.     

Characterization of organic material micro-structures transferred by laser in nanosecond and picosecond regimes

The laser-induced forward transfer technique has been performed on thin layers of conducting organic materials for applications in plastic micro-electronics.This process is a promising alternative for fabrication of organic electronic components on flexible supports when usual techniques, such like ink-jet printing, cannot be considered. For example, when the organic material has no solubility properties or when complex architectures are needed.Experiments on the influence of pulse duration (nanosecond and picosecond) and wavelength on a large range of fluences have been proceeded using different lasers. An optimization of the process has been carried out by inserting a thin layer of absorbing metallic material between the substrate and the organic film. The advantage of this technique is to preserve organic layers from being damaged by thermal and photochemical effects during the interaction.The morphology and thickness of the deposit have been investigated by optical and scanning electronic microscopy. This experimental study is supplemented by electrical characterization of the deposits.     

Surface delamination of polyimide using 355-nm nanosecond pulse laser

In this paper, we investigate the phenomena of the surface delamination of polyimide (PI) using a 355-nm nanosecond laser. Under the threshold fluence of the ablation, the surface starts to delaminate in a layer-by-layer with nanoscale thickness. As the number of laser pulses with low fluence increases, much more surface delamination occurs. Regarding the chemical characteristics from XPS and XRD measurements, such delamination is due to the layer-stacking structure, a unique molecular bonding structure of the PI wherein the boundaries of the layers become separated and delaminated by the laser.     

Energy transmissivity of high-power nanosecond laser pulse focused on glass

The experimental results show that if the pulse energy is low, the damage does not occur and the corresponding transmissivity is 100%. But with the increase of the pulse energy, the damage occurs and the transmissivity decreases to between 100% and 50%, equal to 50% and lower than 50%.The concept of the cut-off time (T_cut) is proposed to give the physics explanation on it, and by taking the free electron density (FED) and the laser-induced damage threshold (LIDT), mathematical simulation of the change law of the T_cut and the corresponding energy transmissivity with the laser energy has been done, respectively. The analysis results indicate that the former definition method can predict all the situations successfully, but the second one cannot predict the situation of the transmissivity being between 100% and 50%, the reasons of this difference also be analyzed.     

Spatial coherence of laser light propagating in an optical fibre

The modulus of the complex degree of spatial coherence at the exit face of an optical fibre is determined for a quasi-monochromatic, spatially coherent source. It is found that the contribution of the cross-terms between many modes to the formulus of pointwise correlation is negligible for a highly multimode fibre in which even and odd modes are equally excited, although care must be taken when even or odd modes only are selectively excited in the waveguide. Experimental results obtained by using a Koesters prism in the wavefront reversing interferometer are also presented for a graded-index multimode fibre and a quasi-single mode fibre. Modal contents of the propagating field in the optical fibre can be successfully determined, provided that the coherence time of a quasi-monochromatic source and the path difference of the wavefront reversing system are knowna priori.     

Fabrication of planar gratings by direct ablation using an ultrashort pulse laser in a common optical path configuration

Planar gratings have wide applications and to date, many methods for the fabrication of gratings have been reported. Ultrashort pulse lasers have been used for the machining of gratings primarily because they allow direct ablation and the manufacturing of sub-wavelength structures. In this paper, we present a novel direct ablation technique for the fabrication of planar gratings which makes use of the interference of ultrashort pulses in a common optical path configuration. This technique of grating fabrication not only simplifies the optical setup, but also immunizes the system to extraneous and inherent vibrations, thus enabling the manufacturing of planar gratings of good edge acuity. We have successfully fabricated planar gratings on a copper substrate. Received: 6 November 2001 / Accepted: 4 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +65/77-904-674, E-mail: mvenkata@ntu.edu.sg     

30 W output of short pulse duration nanosecond green laser generated by a hybrid fiber-bulk MOPA system

We reported a 30 W average output power 532 nm green laser with 5 ns pulsed duration working at 80–100 kHz generated by a hybrid MOPA system. The hybrid MOPA system consisted of a fiber amplifier and two solid-state amplifiers producing 77 W 1064 nm at pulse repetition rates of 80–100 kHz. The IR-to-green optical conversion efficiency was 44.7% at the working point of 100 kHz. The beam quality of green laser was measured better than M ² < 1.6 in both orthogonal directions. Nanosecond 532 nm lasers with short pulsed duration could be practicable in silicon-based material processing in the Photovoltaic industry.