Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

飞秒激光在6H SiC晶体表面制备纳米微结构

激光诱导周期性纳米微结构在多种材料包括电介质、半导体、金属和聚合物中观察到。研究了800nm和400nm飞秒激光垂直聚焦于6H SiC晶体表面制备纳米微结构。实验观察到800nm和400nm线偏光照射样品表面分别得到周期为150nm和80nm的干涉条纹,800nm圆偏振激光单独照射样品表面得到粒径约100nm的纳米颗粒。偏振相互垂直的800nm和400nm激光同时照射晶体得到粒径约100nm的纳米颗粒阵列,该纳米阵列的方向随400nm激光强度增加而向400nm偏振方向偏转。利用二次谐波的观点对以上纳米结构的形成给出了解释。     

Laser Fabrication of Nanostructures on a Pure Aluminum Surface in Air

We report on the fabrication of surface nanoparticles and micro/nanograting structures on bulk pure aluminum in air using a 150 fs, 775 nm femtosecond laser. We investigate the size of the generated surface nanoparticles under irradiation with different femtosecond laser pulses. Smaller nanoparticles can be induced by a larger number of laser pulses and a lower laser fluence. In addition, we observe the formation of micro/nanogratings when the laser focus is scanned across a pure aluminum surface in air. We obtain micro- and nano-grating composite structures on a pure aluminum surface by adjusting the laser fluence and scan velocity. Femtosecond laser surface ablation of bulk pure aluminum in air is potentially a promising technique for efficient fabrication of surface nanostructures.     

Frequency doubling and Fourier domain shaping the output of a femtosecond optical parametric amplifier: easy access to tuneable femtosecond pulse shapes in the deep ultraviolet

Tuneable, shaped, ultraviolet (UV) femtosecond laser pulses are produced by shaping and frequency doubling the output of a commercial optical parametric amplifier (OPA). A reflective mode, folded, pulse shaping assembly employing a spatial light modulator (SLM) shapes femtosecond pulses in the visible region of the spectrum. The shaped visible light pulses are frequency doubled to generate phase- and amplitude-shaped, ultrashort light pulses in the deep ultraviolet. This approach benefits from a simple experimental setup and the potential for tuning the central frequency of the shaped ultraviolet waveform. A number of pulse shapes have been synthesised and characterised using cross-correlation frequency resolved optical gating (XFROG). This pulse shaping method can be employed for coherent control experiments in the ultraviolet region of the spectrum where many organic molecules have strong absorption bands. D.S.N. Parker and A.D.G. Nunn contributed equally to this work.     

Peculiar properties of nanogranular Ni<Subscript>x</Subscript>Si<Subscript>100-x</Subscript> magnetic films obtained by ultrashort pulsed laser deposition

The use of ultrashort laser pulses enables the deposition of films composed of mono-component nanoparticles exhibiting similar shape and size. Films made of nickel (Ni) and silicon (Si) nanoparticles have been produced and investigated in view of the expected interesting properties, resulting from the uniform distribution of metallic, magnetic Ni particles among semiconductive, non-magnetic Si particles. The morphology of the deposited nanoparticles and the related magnetic and magneto-transport characteristics of the films have been studied for different Ni contents, evidencing properties deriving from the peculiar deposition technique and in particular the important role of the free volume inclusions and the particles tendency to not coalesce. PACS 75.75.+a; 81.07.-b; 81.16.Mk; 73.50.Jt     

Production of picosecond, kilojoule, and petawatt laser pulses via Raman amplification of nanosecond pulses

Raman amplification in plasma has been promoted as a means of compressing picosecond optical laser pulses to femtosecond duration to explore the intensity frontier. Here we show for the first time that it can be used, with equal success, to compress laser pulses from nanosecond to picosecond duration. Simulations show up to 60% energy transfer from pump pulse to probe pulse, implying that multikilojoule ultraviolet petawatt laser pulses can be produced using this scheme. This has important consequences for the demonstration of fast-ignition inertial confinement fusion.     

基于氧化镁晶体中级联四波混频过程的紫外飞秒光脉冲产生

紫外波段飞秒激光脉冲是研究超快化学和超快物理相关过程的重要工具,实现波长可调谐的宽带紫外飞秒光脉冲将有助于推动超快动力学及相关领域的研究.本文报道了以两束400 nm的飞秒光脉冲作为级联四波混频的抽运源,在氧化镁晶体中产生9阶频率上转换和5阶频率下转换边带信号的实验结果.边带波长范围从350 nm到450 nm连续可调谐,这些边带信号的发散角和波长与级联四波混频理论预测结果吻合.紫外边带相对于入射光的整体转化效率约为1.2%.同时,高阶边带的光谱形状呈现高斯型,其谱宽理论上支持傅里叶转换极限脉宽为20—50 fs.本文展示了一种高效产生波长可连续调谐的紫外飞秒光脉冲的便捷方法,为基于紫外超短脉冲的相关研究提供了有效工具.     

飞秒激光制备彩色镍

 采用飞秒脉冲激光在空气条件下扫描处理镍片表面,制备了彩色镍。用扫描电子显微镜、紫外-可见-近红外分光光度计、拉曼光谱仪等对其进行了表征。扫描电子显微镜测量显示,彩色镍表面出现了纳米激光诱导周期表面结构,结构周期为480～510 nm;紫外-可见-近红外分光光度计测量表明,彩色镍对波长为200～2 200 nm的光的反射率大大降低;拉曼谱表明样品表面形成了一定含量的氧化镍。     

Characterization of the third-harmonic radiation generated by intense laser self-formed filaments propagating in air

We perform laboratory experiments to study ultraviolet radiation generated by intense self-formed laser filaments produced by propagating high-power femtosecond laser pulses in air. The laser used in the experiment is a 0.5 TW Ti:sapphire system with the center wavelength at 800 nm. The observed ultraviolet emission occurs in the form of the third harmonic and frequency-upshifted radiation from the fundamental. We present direct characterization of the generated harmonic and frequency-upshifted radiation, including transverse imaging and spatially resolved spectral measurements.     

Generation and amplification of dual-color femtosecond pulses by a noncollinear optical parametric up-conversion process

Noncollinear optical parametric up-conversion generation and amplification are realized in a thick β-barium borate (BBO) crystal, and a couple of visible femtosecond up-conversion laser pulses can be achieved by a femtosecond pulse at 800 nm as the pump sources. The theoretical and experimental results indicate that there exist phase-matching conditions for dual-color noncollinear parametric up-conversion generation and amplification, and their wavelengths can be tuned by rotating the BBO crystal. This parametric up-conversion generation and amplification can be attributed to three and five-wave mixing in a thick BBO crystal, and it shows the potential application on optical parametric chirped pulse amplification (OPCPA) to generate multi-color ultraviolet or visible femtosecond laser pulses pumped directly by femtosecond fundamental laser pulses without frequency-doubling or tripling.     

Synthesis and physico-chemical characteristics of nanosized particles produced by laser ablation of a nickel target in water

Ablation of Ni targets in water by laser impact (532 nm, 40 mJ/pulse, 10 Hz and 8 ns duration) focused on massive samples (2 mm diameter) generates colloids with fine nanoparticles. The amount of metal released in the solution (measured by mass loss of the target or ICP) was found to increase first linearly with time, but slower after 8 min of impact. The size distribution of the nanoparticles thus produced was measured (by TEM) to be in the range 3–5.3 nm in diameter, with a tendency for the size to be smaller for larger number of laser shots. Actually, nickel oxide nanoparticles were produced, rather than nickel nanoparticles as it was shown by HRTEM. XPS photoemission measurements evidenced the presence of nickel oxide species on the crater of the nickel sample surface after laser ablation.     

Beam properties of fully optimized,table-top,coherent source at 30 nm

We present results on development and experimental implementation of a 1-kHz, coherent extreme ultraviolet (XUV) radia- tion source based on high-order harmonic generation of the femtosecond, near-infrared laser pulses produced by the titanium-doped sapphire laser system (35 fs, 1.2 mJ, 810 nm) at the Institute of Physics AS CR / PALS Centre. The source comprises a low-density static gas cell filled with a conversion medium, typically argon. The comprehensive optimization of the XUV harmonic source has been performed with respect to major parameters such as gas pressure in the cell, cell length, position of the focus of the driving laser field with respect to the gas cell position, size of the driving near-infrared laser beam, chirp of the femtosecond pulse, and the focal length of the lens deployed in the experimental setup. Harmonic spectra were recorded using an XUV transmission grating spectrometer developed specifically for this purpose. Detailed characterization of the XUV source has been performed including measurement of the XUV beam profile, M² parameter of the beam, absolute energy, and spatial coherence.     

Generation of high-density electrons based on plasma grating induced Bragg diffraction in air

Efficient nonlinear Bragg diffraction was observed as an intense infrared femtosecond pulse was focused on a plasma grating induced by interference between two ultraviolet femtosecond laser pulses in air. The preformed electrons inside the plasma grating were accelerated by subsequent intense infrared laser pulses, inducing further collisional ionization and significantly enhancing the local electron density.     

Laser ablation of silver in different liquids: Optical and nonlinear optical properties of silver nanoparticles

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared using the method of laser ablation in various liquids at wavelengths of 397, 532, and 795 nm with laser pulses of different duration are studied. An analysis of the dimensional and spectral characteristics of the silver nanoparticles revealed a time dynamics of the nanoparticle size distribution in solutions. It is shown that thermal self-defocusing is observed for the case of nanosecond or shorter pulses generated with a high repetition rate. For picosecond and femtosecond pulses with a low repetition rate, the effects of self-focusing (γ = 3 × 10^?13 cm² W^?1) and saturated absorption (β = ?1.5 × 10^?9 cm W^?1) were observed in the solutions under study. The third-order nonlinear susceptibility of the silver nanoparticles was found to be 5 × 10^?8 esu at a wavelength of 397 nm.     

Coherent control of acoustic vibrations in metal nanoparticles and thin films with sequences of femtosecond pulses: Harmonic-oscillator model

A harmonic oscillator model is used to demonstrate the possibility of coherent control of acoustic vibrations of metal nanoparticles and thin films with sequences of femtosecond laser pulses. When the interval between the pulses in such a sequence is chosen equal to the oscillation period of the expansion mode of a nanoscale system, the relevant acoustic vibrations can be excited in a resonant and selective way. Sequences of femtosecond pulses with picosecond time intervals between the pulses are shown to be ideally suited for a resonant excitation and coherent control of acoustic modes of silver nanoparticles.     

Formation of solar absorber surface on nickel with femtosecond laser irradiation

Through femtosecond (fs) laser pulse irradiation, we produce two-dimensional quasiperiodic arrays of nanostructure-covered conical microstructures (NC-CMs) on Ni. We also find that a significant amount of nickel oxide covers NC-CMs owing to the interaction of fs laser pulses with Ni in ambient air. We show that, by controlling the fluence of laser, the reflectance of the fs laser-treated Ni surface can change dramatically in the infrared but the surface still has a high absorptance at UV and visible wavelengths. Because of this unique spectral reflectance, the fs laser-treated Ni surface is well suited for use as a solar absorber surface.     

飞秒激光诱导有机薄膜自组装微结构

通过改变辐照激光脉冲数、激光的平均功率、显微镜物镜倍数/数值孔径等研究微皇冠结构形成与加工条件的关系,并借鉴激光熔池形成和液滴溅射的模型对其进行理论分析。当激光脉冲数达到一定数量后,烧蚀区域会出现微皇冠结构。烧蚀区域尺寸的改变是由于形成的液体区域中热毛细作用和化学毛细作用共同所致;微皇冠结构溅射个数的改变是由于形成的气体因多光子吸收体积迅速膨胀所致。     

飞秒激光作用下金掺杂硅酸盐玻璃的光致晶化研究

研究了金掺杂硅酸盐玻璃在飞秒激光辐照和热处理作用下的光致晶化行为，分析探讨了其机理和激光辐照条件的影响.吸收光谱测试表明玻璃内部析出金纳米颗粒. 金纳米颗粒掺杂玻璃在近共振纳秒脉冲作用下显示了强的光限幅效应, 且改变飞秒激光诱导参数可改变该复合玻璃的光学非线性. 关键词： 光致晶化 飞秒激光 金纳米颗粒 硅酸盐玻璃 光限幅     

Evolution of terahertz conductivity in ZnSe nanocrystal investigated with optical-pump terahertz-probe spectroscopy

The unique optical properties of nanoparticles are highly sensitive in respect to particle shapes, sizes, and localization on a sample. This demands for a fully controlled fabrication process. The use of femtosecond laser pulses to generate and transfer nanoparticles from a bulk target towards a collector substrate is a promising approach. This process allows a controlled fabrication of spherical nanoparticles with a very smooth surface. Several process parameters can be varied to achieve the desired nanoparticle characteristics. In this paper, the influence of two of these parameters, i.e. the applied pulse energy and the laser beam shape, on the generation of Si nanoparticles from a bulk Si target are studied in detail. By changing the laser intensity distribution on the target surface one can influence the dynamics of molten material inducing its flow to the edges or to the center of the focal spot. Due to this dynamics of molten material, a single femtosecond laser pulse with a Gaussian beam shape generates multiple spherical nanoparticles from a bulk Si target. The statistical properties of this process, with respect to number of generated nanoparticles and laser pulse energy are investigated. We demonstrate for the first time that a ring-shaped intensity distribution on the target surface results in the generation of a single silicon nanoparticle with a controllable size. Furthermore, the generated silicon nanoparticles presented in this paper show strong electric and magnetic dipole resonances in the visible and near-infrared spectral range. Theoretical simulations as well as optical scattering measurements of single silicon nanoparticles are discussed and compared.     

Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.