Mechanisms of femtosecond laser ablation of dielectrics revealed by double pump–probe experiment

We study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectrics. A femtosecond pump–probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. To get more information in the complex process of excitation and relaxation mechanisms involved during and after the interaction, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a controlled density of carriers, and a second one at 800 nm with variable pulse duration, from 50 fs to 10 ps, reaches an excited solid. In Al₂O₃, we show that the total density of carriers never exceeds the sum of the densities excited by the two pulses sent independently. This means that the second pulse deposits further energy in the material by heating the previously excited carriers, and that no electronic multiplication occurs. On the other hand, in SiO₂, it is possible, under specific conditions, to observe an increase of carrier density due to impact ionization. All these results demonstrate that the avalanche process, which is often invoked in the laser breakdown literature, does not play a dominant role in optical breakdown induced by short pulses.     

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.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

Ultrashort pulse electron gun with a MHz repetition rate

We report the construction of an electron gun emitting ultrashort pulses with a repetition rate of 2.7 MHz. The gun works at an acceleration voltage of 20 kV and is operated with a laser oscillator having an ultralong cavity. A low number of electrons per pulse eliminates space charge broadening. Electron yield and beam profiles are measured for operation with laser wavelengths of 800, 400, and 266 nm. The initial energy spread of the electrons is determined for these three wavelengths, and pulse durations of 600, 390, and 270 fs are inferred from the data.     

The influence of ultra-fast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation

The effect of ultra-short laser-induced morphological changes upon irradiation of silicon with double pulse sequences is investigated under conditions that lead to mass removal. The temporal delay between 12 double and equal-energy pulses (E _p=0.24 J/cm² each, with pulse duration t _p=430 fs, 800 nm laser wavelength) was varied between 0 and 14 ps and a decrease of the damaged area, crater depth size and periodicity of the induced subwavelength ripples (by 3–4 %) was observed with increasing pulse delay. The proposed underlying mechanism is based on the combination of carrier excitation and energy thermalization and capillary wave solidification and aims to provide an alternative explanation of the control of ripple periodicity by temporal pulse tailoring. This work demonstrates the potential of pulse shaping technology to improve ultra-fast laser-assisted micro/nanoprocessing.     

Extending the High-Order Harmonics Plateau and Isolated Attosecond Pulse Generation in an Intense Three-Color Laser Field

We propose a method to extend the high-order harmonics plateau and generate an isolated sub-10-as pulse by adding a weak control pulse (10 fs, 1600 nm) to modify a two-color laser field (5 fs, 800 nm; 10 fs, 400 nm). The numerical results show that the plateau is extended obviously in the three-color laser field regime. Additionally, the ionization rate and classical returning kinetic-energy maps are calculated to better understand the physical origin of the high-order harmonics generation (HHG). By means of the ionization probability and the time-frequency distributions, more features of the HHG are revealed. Furthermore, our simulations show that the width of the plateau and the relative conversion efficiency of the harmonic spectra are highly dependent on the relative phase. Finally, by adjusting the relative phase and superposing a properly selected range of the HHG spectrum, an isolated attosecond pulse with a duration of 7 as is obtained.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

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.     

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.     

飞秒激光参数对非绝热耦合分子态布居的影响

利用含时波包法研究强飞秒泵浦-探测激光场中激光脉宽、波长和场强对非绝热耦合NaI分子各态布居的影响.波包在势能面上做周期性运动,周期约为1 000 fs.延时为200 fs时,波包第一次到达交叉区域分裂成两部分.波包在交叉区域的分裂情况影响各态布居.脉宽增长,NaI分子的激发概率增大,而解离概率减小.泵浦波长为共振波长318 nm时,激发概率最大.泵浦波长增长,NaI分子的解离概率减小.泵浦场强增大,激发概率增大,但解离概率不变.探测激光波长和场强不影响NaI分子各态布居分布.调节激光场参数可实现对波包运动的控制从而控制态布居的选择性分布.研究结果为实验上实现分子的光控制过程提供参考.     

Computational study of plasma-assisted photoacoustic response from gold nanoparticles irradiated by off-resonance ultrafast laser

The gold nanoparticles (AuNPs) are capable of enhancing the incident laser field in the form of scattered near field for even an off-resonance irradiation where the incident laser wavelength is far away from the localized surface plasmon resonance (LSPR). If the intensity of the pulse laser is large enough, this capability can be employed to generate a highly localized free electron (plasma) in the vicinity of the particles. The generated plasma can absorb more energy during the pulse, and this energy deposition can be considered as an energy source for structural mechanics calculations in the surrounding media to generate a photoacoustic (PA) signal. To show this, in this paper, we model plasma-mediated PA pressure wave propagation from a 100-nm AuNPs and the surrounding media irradiated by an ultrashort pulse laser. In this model, the AuNP is immersed in water and the laser pulse width is ranging from 70 fs to 2 ps at the wavelength of 800 nm (off-resonance). Our results qualitatively show the substantial impact of the energy deposition in plasma on the PA signal through boosting the pressure amplitudes up to ～1000 times compared to the conventional approach.     

Two-color two-photon excitation of indole using a femtosecond laser regenerative amplifier

The fluorescence emission from indole resulting from two-color two-photon (2C2P) excitation with 400 and 800 nm wavelengths is observed, using the second harmonic and fundamental wavelength of a 800 nm 40 fs pulsed Ti:Sapphire femtosecond (fs) regenerative amplifier operating at a repetition rate of 1 kHz. By delaying one fs laser pulse relative to the other, the cross correlation of fluorescence is observed, which indicates the generation of 2C2P fluorescence signal in the experiment. The strongest 2C2P fluorescence emission characterized by the peak of cross correlation curve suggests optimal temporal overlap of the two fs laser pulses. The 2C2P fluorescence signal is linearly dependent on the total excitation intensity. The fluorescence signals with 400 nm and 800 nm irradiation alone are also demonstrated and discussed in this paper.     

Femtosecond pulse compression based on second harmonic generation from a frequency chirped pulse

A 49 fs pulse at the wavelength of 800 nm was converted to a 26 fs pulse at the wavelength of 400 nm by temporal stretching (frequency chirping) and second harmonic generation from a chirped pulse with subsequent compression. The energy conversion efficiency of 35% was achieved.     

Chirped pulse amplification in single mode Tm:fiber using a chirped Bragg grating

We report femtosecond pulse generation and chirped pulse amplification in Tm:fiber. A mode-locked oscillator operating in the soliton regime produced 800 fs pulses with 5 nm spectral bandwidth, at 40 pJ pulse energy. This oscillator seeded a pre-amplifier that utilizes a Raman soliton self-frequency shift to produce wavelength tunable pulses with 3 nJ energy, reduced pulse duration of 150 fs, and increased bandwidth of 30 nm. For further amplification, the pulses were stretched up to 160 ps using a chirped Bragg grating (CBG). Stretched pulses were amplified to 85 nJ after compression in single-mode Tm:fiber and recompressed with the CBG as short as 400 fs. Compressed pulses were coupled into a highly nonlinear tellurite fiber to investigate the potential of this ultrashort pulse 2-μm fiber source as a pump for mid-IR supercontinuum generation.     

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.     

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.     

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.     

Ultrafast dynamics process of platinum nanoparticles under femtosecond laser

The energy relaxation dynamics process of Pt nanoparticles with a diameter of 25 nm after a 400-nm femtosecond laser excitation was investigated using pump-probe technology with 130 fs laser pulse. Experimental results show that the process includes a fast decay with time constant of 850 fs and a slow decay with time constant of 9 ps. Theoretical discussion indicates that the former results from electron-phonon coupling process, while the latter is from phonon-phonon coupling process.     

Pulse-duration dependency of femtosecond laser refractive index modification in poly(methyl methacrylate)

Refractive index modification of pure poly(methyl methacrylate) (PMMA) is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. It is observed that at 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. Results suggest that three- and two-photon absorption is responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation with short pulses of low laser fluence allows control of the photomodification via incubation, thus reducing bulk damage.