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.     

Robust plasmonic substrates

We report on resonant infrared laser ablation of polystyrene using single 8 ps pulses at a wavelength of 3.31 μm generated by a MgO:PPLN optical parametric amplifier pumped by a Nd:YLF laser. We determined the single-pulse ablation threshold to be 0.46 J/cm², about a factor of five smaller than in previous free-electron-laser studies. Time-resolved imaging of the laser–target interaction reveals that the detailed dynamics of the ablation process begin with thermal expansion of a large volume of hot material from which a less dense plume of polymeric material evaporates. This plume disappears on a time scale of 0.75 μs and the hot polymer material recedes back into the crater from which it was expelled. Subsequently, and on a much longer time scale, structural alterations in the ablation crater continue to evolve for at least another millisecond. Our results suggest that single picosecond pulses are effective for the ablation of polymers and exhibit dynamics similar to those observed in studies using a free-electron laser.     

1GHz梳状波调制1.3μmDFB半导体激光器皮秒超短光脉冲产生的实验研究

本文介绍一种采用高重复率梳状波电流调制1.3μm分布反馈(DFB)半导体激光器产生皮秒超短光脉冲的技术。采用1GHz重复率尖脉冲电流直接调制1.3μm半导体激光器产生了最短15ps、输出平均功率为4.5mW的超短光脉冲。     

Tapered semiconductor amplifiers for optical frequency combs in the near infrared

A tapered semiconductor amplifier is injection seeded by a femtosecond optical frequency comb at 780 nm from a mode-locked Ti:sapphire laser. Energy gains of more than 17 dB(12 dB) are obtained for 1 mW(20 mW) of average input power when the input pulses are stretched into the picosecond range. A spectral window of supercontinuum light generated in a photonic fiber has also been amplified. Interferometric measurements show sub-Hertz linewidths for a heterodyne beat between the input and amplified comb components, yielding no detectable phase-noise degradation under amplification. These amplifiers can be used to boost the infrared power in f-to-2f interferometers used to determine the carrier-to-envelope offset frequency, with clear advantages for stabilization of octave-spanning femtosecond lasers and other supercontinuum light sources.     

Nano- and femtosecond UV laser pulses to immobilize biomolecules onto surfaces with preferential orientation

By relying on the photonic immobilization technique of antibodies onto surfaces, we realized portable biosensors for light molecules based on the use of quartz crystal microbalances, given the linear dependence of the method on the laser pulse intensity. Here, we compare the quality of the anchoring method when using nanosecond (260 nm, 25 mJ/pulse, 5 ns, 10 Hz rep. rate) and femtosecond (258 nm, 25 μJ/pulse, 150 fs, 10 kHz rep. rate) laser source, delivering the same energy to the sample with the same average power. As a reference, we also tethered untreated antibodies by means of the passive adsorption. The results are striking: When the antibodies are irradiated with the femtosecond pulses, the deposition on the gold plate is much more ordered than in the other two cases. The effects of UV pulses irradiation onto the antibodies are also analyzed by measuring absorption and fluorescence and suggest the occurrence of remarkable degradation when nanosecond pulses are used likely induced by a larger thermal coupling. In view of the high average power required to activate the antibodies for the achievement of the photonic immobilization technique, we conclude that femtosecond rather than nanosecond laser pulses have to be used.     

Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.     

Near-infrared continuum generation of femtosecond and picosecond pulses in doped optical fibers

We report broad continuum generation in GeO₂-doped silica fibers induced by femtosecond and picosecond pulses generated by novel high-power laser sources. Over 0.5 W of average power in a broad-band picosecond continuum is generated in the near-infrared part of the spectrum from 800 to 1500 nm. PACS 42.60.By; 42.55.Xi; 42.65.Wi; 42.81.Qb     

Asynchronous cross-correlation for weak ultrafast deep ultraviolet laser pulses

We have developed a technique for the temporal characterisation of weak deep-ultraviolet (DUV) ultrashort laser pulses by combining asynchronous optical sampling with difference-frequency mixing. The intensity profile of picosecond DUV pulses, with peak powers as low as 2.5 W, have been measured accurately with a resolution of 50 fs. The method can be extended to complete amplitude and phase characterisation of few femtosecond laser pulses at DUV wavelengths.     

High-repetition-rate femtosecond optical parametric chirped-pulse amplifier in the mid-infrared

We discuss a dual-stage optical parametric chirped-pulse amplifier generating sub-100-fs pulses in the mid-infrared at a repetition rate of 100 kHz. The system is based on a 1064 nm pump laser and a 3–4 μm difference frequency generation seed source derived from the output of a femtosecond fiber laser amplifier. Both lasers are commercially available, are diode-pumped, compact, and allow for turn-key operation. Here, we focus our discussion on the design and dimensioning of the optical parametric chirped-pulse amplifier. In particular, we review the available gain materials for mid-infrared generation and analyze the impact of different stretching scenarios. Timing jitter plays an important role in short-pulse parametric amplifier systems and is therefore studied in detail. The geometry of the amplifier stages is optimized through a full 3-dimensional simulation with the aim of maximizing gain bandwidth and output power. The optimized system yields output pulse energies exceeding 1 μJ and an overall gain larger than 50 dB. The high repetition rate of the pump laser results in an unprecedented average power from a femtosecond parametric system at mid-infrared wavelengths. First experimental results confirm the design and the predictions of our theoretical model.     

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.     

Generation of THz transients by photoexcited single-crystal GaAs meso-structures

We report a sub-picosecond photoresponse and THz transient generation of GaAs single-crystal mesoscopic platelets excited by femtosecond optical pulses. Our structures were fabricated by a top-down technique, by patterning an epitaxial, 500-nm-thick GaAs film grown on top of an AlAs sacrificial layer and then transferring the resulting etched away 10 × 20-μm² platelets onto an MgO substrate using a micropipette. The freestanding GaAs devices, incorporated into an Au coplanar strip line, exhibited extremely low dark currents and ~0.4 % detection efficiency at 10 V bias. The all-optical, pump–probe carrier dynamics analysis showed that, for 800-nm-wavelength excitation, the intrinsic relaxation of photocarriers featured a 310-fs-wide transient with a 290 fs fall time. We have also carried out a femtosecond, time-resolved electro-optic characterization of our devices and recorded along the transmission line the electrical transients as short as ~600 fs, when the platelet was excited by a train of 100-fs-wide, 800-nm-wavelength optical laser pulses. The platelets have been also demonstrated to be very efficient generators of free-space propagating THz transients with the spectral bandwidth exceeding 2 THz. The presented performance of the epitaxial, freestanding GaAs meso-structured photodevices makes them uniquely suitable for THz-frequency optoelectronic applications, ranging from ultrafast photodetectors to THz-bandwidth optical-to-electrical transducers and photomixers.     

The effect of different drying methods on band gap energy and femtosecond LIDT of nano-porous anti-reflective silica thin films

In this work, nano-porous anti-reflective silica thin films are deposited on the quartz samples by dip-coating in silica sol–gel. After dip-coating, the samples are divided into three groups and each group is heated in an oven with a different drying method. The effect of the heating methods on the morphology and optical properties of the coated layers are studied by SEM imaging and measuring optical transmission of the samples. Then based on the transmission data and calculation of absorption coefficient of the layers, the band gap energy of the silica thin films is calculated. In addition, laser induced damage threshold (LIDT) of the samples are measured by using 35 fs, 100 mJ femtosecond laser pulses. It can be seen that there is a distinct correlation between band gap energy and LIDT of the samples which both of them have been affected by heating method of the coated samples.     

Laser ablation of gallium arsenide in different solutions

The optical, structural, and nonlinear optical characteristics of GaAs nanoparticles obtained by laser ablation in different liquids were investigated. Thermally induced self-defocusing in GaAs solutions was observed using both a high pulse repetition rate and nanosecond pulses. In studying the nonlinear optical characteristics of GaAs solutions using picosecond and femtosecond pulses, two-photon absorption was observed. The nonlinear absorption coefficient of an aqueous GaAs solution measured by the Z-scan technique and the nonlinear susceptibility of GaAs nanoparticles were, respectively, 0.7 × 10^?9 cm W^?1 and 2 × 10^?9 esu at a wavelength of 795 nm.     

Generation of superintense femtosecond pulses by the Cr:forsterite laser system

A repetitively pulsed chromium-forsterite laser system is created. High-power femtosecond light pulses are generated at the fundamental (1.24 μm) and second-harmonic (0.62 μm) wavelengths. Theoretical analysis is performed to optimize the output pulse energy. Laser pulses with a duration of 110 fs, an energy of 1 mJ, and a repetition rate of 1–50 Hz are generated. The intensity of the focused beam is greater than 10¹⁶ W/cm². High-efficiency radiation conversion into the second harmonic is used to increase the energy contrast of the generated pulses. Original Text ? Astro, Ltd., 2006.     

Multi-millijoule, nonlinear preamplifier for high intensity femtosecond Yb:YAG chirped-pulse amplification lasers at 1030 nm

We report the highest energy broadband laser pulses at a center wavelength of 1030 nm based on optical parametric chirped-pulse amplification (OPCPA). We have demonstrated amplification of 1030 nm femtosecond laser pulses from a broadband Yb oscillator to over 6.5 mJ with a total gain of greater than 10⁷ achieved in a single pass through only 56 mm of gain material at a 10 Hz repetition rate. The amplified spectral bandwidth of 10.8 nm affords recompression to a 230 fs pulse duration following amplification. As an alternative to the regenerative amplifier (RA) this system is one of the more promising candidates for realizing compact, high intensity, direct diode-pumped, high repetition rate femtosecond Yb:YAG chirped-pulse amplification (CPA) in laser systems.     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

Optimization of a distributed feedback dye laser system to generate single tunable picosecond pulses from UV to IR

We report a new configuration of a picosecond distributed feedback dye laser system, which can be pumped by various wavelengths. The optical characteristics of the system are presented. By using a triangle mirror functioned beam splitter and a right-angle prism attached to the front face of the cell in the pumping geometry, continuously and widely tunable laser wavelengths have been obtained for a range of more than 100 nm. A quenching mirror adhered to a face on the side of the cell, functioning as a high-Q cavity shared common gain medium, has been employed to observe single short laser pulses with transform-limited bandwidth.     

Diode-pumped passively mode-locked Yb:LSO/SESAM laser

Diode-end-pumped passively mode-locked Yb³⁺:Lu₂SiO₅ (Yb:LSO) lasers both in the picosecond and the femtosecond regimes were demonstrated in this paper. The mode-locked lasers were initiated by using of semiconductor saturable absorber mirrors (SESAM). In the absence of intra-prisms for pulse compression, the laser emitted 1.6 W of output power with pulses width of about 4.3 ps. A pair of SF10 prisms was inserted into the laser cavity, pulses as short as 699 fs was generated around a center wavelength of 1043.6 nm with an average output power of 800 mW and a peak power of 11.0 kW.     

Development of the 10 TW ‘ATTILA’ Nd laser system

A high power laser system called ‘ATTILA’ (Almost-Table Top Terawatt Intense Laser) (10 TW) is under development at the University of Milano, Bicocca. Once fully functional ATTILA will produce 10 J, 1 picosecond pulses, reaching the power of 10 TW. The system is based on the chirped pulsed amplification technique and uses Nd:glass single-pass amplification.

The amplifier chain can be injected with two different oscillators: a Nd:YLF oscillator by Quantronix, stretched by an optical fiber, and a Nd:glass femtosecond oscillator by TimeBandwidth (Zurich), stretched by diffraction gratings, both delivering nanojoules pulses. A regenerative amplifier increasing the energy up to 1 mJ follows these.

Recently, we have implemented an automatic control system for the stabilization of mode-locking in the Quantronix oscillator. This is based on the active control of cavity length and on accurate stabilization of mode-locker temperature. A PC using LabView software drives the system. We are now beginning the installation of an adaptive optics system with a deformable mirror in order to optimize the optical quality and focusability of the beam.

Our final goal is to obtain an intensity of 10¹⁸ W/cm² on target. This will allow the production of relativistic electrons and energetic protons, and the study of relativistic plasma physics and of matter in extreme conditions.