Femtosecond dynamics of electron-vibrational heating and desorption

The dynamics of vibrational heating and desorption induced by multiple inelastic hot electron scatterings is investigated by a microscopic quantum mechanical approach. Numerical simulations are found to be in good agreement with experimental results of O₂ desorbed from Pt(111) by femtosecond laser pulses. The subpicosecond response time of the desorption probability is shown to arise directly from the dynamics of vibrational heating of the molecule-surface bond.     

Infrared-laser-pulse control of bond- and state-selective excitation, dissociation and space quantization: application to a three-dimensional model of HONO2 in the ground electronic state

Selective control over the vibrational excitation and space quantization of the dissociation fragments by optimally designed linearly polarized and shaped infrared (IR) laser pulses of the picosecond (ps) and subpicosecond duration is demonstrated by means of quantum-dynamical simulations within the Schr?dinger wave-function formalism for a three-dimensional (3-D) model of HONO₂ in the ground electronic state, wherein the OH and the ON single-bond stretches are explicitly treated, together with the bending angle between them, on the basis of the ab initio defined 3-D potential-energy surface and dipole function. The high-lying zeroth-order vibrational states of the OH bond are prepared selectively both below and above the dissociation threshold of the ON single bond, and demonstrate a quasi-periodic oscillatory behaviour, manifesting intramolecular vibrational energy redistribution (IVR) on the picosecond timescale. Selective breakage of the ON single bond in HONO₂ with more than 97% probability is demonstrated, along with control of the space quantization of the dissociation fragments: the OH fragments rotating clockwise, OH(c), and anticlockwise, OH(a), are prepared selectively, with the OH(a)/OH(c) branching ratio being as high as 10.975. The results obtained show that optimally designed strong and short IR-laser pulses can compete against IVR and manipulate vibrational excitation and dissociation of polyatomic molecules. Received: 3 November 1999 / Published online: 13 July 2000     

Transient stimulated raman scattering in crystals during motion of populations of vibrational states

Transient stimulated Raman scattering (SRS) in crystals is analyzed taking into account the motion of populations of vibrational states under the action of subpicosecond (shorter than the dephasing time) pump pulses. Analytic expressions describing the dynamics of excitation of vibrations in SRS are derived. It is found that for a small wavelength of SRS interaction and high intensities of pump radiation observed for femtosecond SRS in crystals, avalanche excitation of vibrations can be responsible for SRS suppression. It is shown that when phase matching of Stokes-anti-Stokes parametric coupling in transient SRS is ensured, it is possible to elevate the efficiency of frequency conversion under conditions of motion of populations of vibrational states; this explains recent successful results in the experimental implementation of femtosecond SRS in crystals pumped by a Bessel beam.     

High power subpicosecond pulse generation from a synchronously pumped dye laser combined with an external compressor

A new scheme of subpicosecond pulse generation based on a dye laser is described. Output pulses of 15 ps from a rhodamine-6G laser pumped synchronously by a frequency-double mode-locked Nd: YAG laser are compressed into 0.8 ps pulses of 2 MW by two passes of saturable absorber (DODCI) and saturable amplifier (rhodamine-6G). The technique is useful for generating widely tunable, high power repetitive subpicosecond pulses.     

Determination of the Subpicosecond Laser Pulse Chirp in the Middle IR Range Based on the Fourth Harmonic Noncollinear Generation

A new method is proposed to determine the subpicosecond laser pulse chirp in the middle IR range at the central wavelength of 10 μm, based on the generation of the second-harmonic pulses both by the bandwidth-limited and frequency-modulated subpicosecond pulses and the subsequent noncollinear generation of the fourth-harmonic radiation by the corresponding second-harmonic pulses. The time dependences are given of the instantaneous frequency of the frequency-modulated second-harmonic pulse at the central wavelength of 5 μm, generated in the field of the frequency-modulated subpicosecond IR pulse, propagating in the negative uniaxial AgGaS₂ crystal along the direction of 61°36′ relative to the optical axis. These results can be used in designing a nonlinear optical phase correlator to determine the phase and time profile of the subpicosecond laser pulse in the middle IR range.     

Femtosecond multipulse polarization spectroscopy of molecular vibrations and rotations in liquids

Simultaneous control over the vibrational and rotational molecular responses in ultrafast optical Kerr-effect signals in acetonitrile, toluene, and chloroform at room temperature is achieved by means of non-resonance excitation with two 30-fs orthogonal linearly polarized laser pulses. It is shown that the orientational response of the molecules can be suppressed and their vibrational response isolated by enhanced the delay between the exciting pulses and their relative intensity.     

Superexcited state dynamics probed with an extreme-ultraviolet free electron laser

We present the first experimental results obtained with a high-gain harmonic generation extreme ultraviolet free electron laser. The experiment probes decay dynamics of superexcited states of methyl fluoride via ion pair imaging spectroscopy. Velocity mapped ion images of the fluoride ion, obtained with excitation via intense, coherent, subpicosecond pulses of 86-89 nm radiation, reveal low translational energy, implying very high internal excitation in the methyl cation cofragment. Angular distributions show changing anisotropy as the excitation energy is tuned through this region. The dynamics underlying the dissociation are discussed with the aid of theoretical calculations.     

Efficient excitation of Raman coherence by a gradient force

The vibrational molecular excitation by strong laser fields have been studied. Starting with the molecular Hamiltonian, we derive the vibrational excitation rate due to a gradient of laser field and show that the rate can be two to three orders of magnitude larger than the regular Raman rates. The developed theory contributes to understanding of interaction of powerful laser pulses with molecular systems, and it might be used in developing quantitative coherent Raman spectroscopy. Copyright © 2013 John Wiley & Sons, Ltd.     

PTCDA分子基态的飞秒激光控制研究

将优化控制理论和多组态含时Hartree（MCTDH）方法相结合,建立了适合于MCTDH方法的计算具有平面结构的PTCDA分子的多自由度振动量子模型,研究了在PTCDA分子激发后从分子激发态回落至分子基态的动力学过程.在理论上分析了约化目标态产生率与激发脉冲、分子的演变时间及优化场的有效能量之间的关系,对分子在各个振动坐标下波函数的振动分布做了分析与比较.研究发现,增加分子的回落演变时间在提高目标态产生率的同时可以使优化激光控制场的强度降低,这为实验上用低能量激光最大程度地实现目标态提供了有效手段. 关键词： PTCDA 多组态含时Hartree方法 飞秒激光控制     

Optical control of the anisotropy of the orientation of molecules in a liquid

The control of the anisotropy of the orientation of molecules in pure 1,2-dichlorobenzene, C₆H₄Cl₂ at room temperature is demonstrated experimentally. To accomplish the optical control, the medium is exposed to non-resonant excitation with two successive linearly polarized laser pulses with a duration of 60 fs. The state of transient anisotropy is probed with the third pulse by detecting the ultrafast optical Kerr effect via optical heterodyne detection and synchronous demodulation. It is shown that variations in the two parameters, the delay time between two pump pulses, and the angle between the polarization directions of the pump pulses ensure the control of the anisotropy of the orientation of molecules in the subpicosecond region.     

Simplified setup for high-resolution spectroscopy that uses ultrashort pulses

Using a broadband femtosecond laser and a simple optical setup, we demonstrate narrow-bandwidth-tunable excitation of vibrational modes in CCl4 and CHBr3 liquids. The resolution obtained is 80 times higher than the laser bandwidth. A pair of time-shifted, linearly chirped pulses is created by use of a high-order dispersion-compensated prism-interferometer setup. We use this pulse pair to selectively excite Raman-active transitions. Our setup represents a significant simplification with improved resolution, of previous approaches to the use of ultrashort pulses for chemically selective spectroscopy.     

Characterisation of a train of subpicosecond laser pulses by fringe resolved autocorrelation measurements

Fringe resolved autocorrelation functions generated by a train of subpicosecond laser pulses from a synchronously pumped ring dye laser were recorded. By comparison with calculated correlation functions one can not only derive the pulse duration of the center pulse, but also draw conclusions about the quasicontinuous background or satellite pulses of lower intensity.     

The transition from direct to delayed ionisation of C 60

The timescale for the coupling of electronic and vibrational excitation in isolated fullerenes is determined by recording positive ion time-of-flight mass spectra on excitation with ultrashort laser pulses at 790 nm of the same fluence but different pulse durations. The coupling leads to the onset of a delayed ionisation “tail” on the parent fullerene ion peak. This occurs for a pulse duration of 500-1000 fs, depending on laser fluence. Received 20 October 2000     

Femtosecond laser control of intramolecular vibrations in a liquid

Optical control of coherent intramolecular oscillations in chloroform CHCl₃ and dimethyl sulfoxide (CH₃)₂SO is attained experimentally under normal conditions by means of femtosecond polarization spectroscopy. Nonresonant excitation of the medium is accomplished by a sequence of two linearly polarized laser pulses. The state of the medium is probed by the third pulse via the optical Kerr effect. We show that control over the vibrational dynamics of molecules on a sub-picosecond scale can be achieved by varying the delay between the excitation pulses and their relative intensity.     

Nature of subpicosecond terahertz pulse propagation in practical dielectric-filled parallel-plate waveguides

It is analytically shown that the presence of submicrometer-sized air gaps between the dielectric and metal contact surfaces in a dielectric-filled metallic parallel-plate waveguide can have a dramatic effect on the guided-wave propagation of subpicosecond terahertz pulses. Through the use of metal-evaporated dielectric surfaces to overcome the imperfect contact problem, and a special air-dielectric-air cascaded waveguide geometry to avoid multimode excitation, undistorted subpicosecond terahertz pulse propagation via the single-TEM mode is demonstrated, for what is believed to be the first time, in a silicon-filled PPWG.     

Selective ablation of thin films with short and ultrashort laser pulses

Micromachining of CuInSe₂ (CIS)-based photovoltaic devices with short and ultrashort laser pulses has been investigated. Therefore, ablation thresholds and ablation rates of ZnO, Mo and CuInSe₂ thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti:sapphire laser. The experimental results were compared to the theoretical evaluation of the samples heat regime. In addition, the cells photo-electrical properties were measured before and after laser machining. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were employed to characterise the laser-induced ablation channels. Using nanosecond laser pulses, two phenomena were found to limit the laser-machining process. Residues of Mo that were projected onto the walls of the ablation channel and the metallization of the CuInSe₂ semiconductor close to the channel lead to a shunt. The latter causes the decrease of the photovoltaic efficiency. As a consequence of these limiting effects, only subpicosecond laser pulses allowed the selective or complete ablation of the thin layers without a relevant change of the photo-electrical properties.     

Near-infrared subpicosecond pulse generation in a synchronously mode-locked cw dye laser

Tunable subpicosecond pulses have been obtained from a synchronously mode-locked Oxazine-1 dye laser by tandem pumping with output pulses of a mode-locked Rhodamine 6G dye laser. The effects of cavity detuning on the pulse-width and the second harmonic power (the peak intensity of the autocorrelation trace) have been investigated. The experimental results are found to be in good agreement with those predicted by a recent model analysis.     

Amplification of ultrashort laser pulses by a resonant Raman scheme in a gas-jet plasma

Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.     

Vibrational quantum interference in Raman-induced Kerr effect: momentum conservation

The pump–probe Raman-induced optical Kerr effect (RIKE) of simple molecular liquids, studied with femtosecond laser pulses, exhibit long lasting beats ascribable to vibrational quantum interference (QI). While energy conservation entails vibrational resonances in RIKE, momentum conservation boils down to wave vector-matching in the pump and probe processes, which calls for the participation of a vibrational excitation wave. The refractive index dispersion around vibrational resonances is intimately related to the focusing angle of the pump (also probe) beam. The larger the focusing angle, the greater the excitation wave number, i.e. the more energetic the vibration in resonance; if the focusing angle is too small, energetic vibrations cannot be observed in vibrational QI, even if energy conservation is fulfilled. If the pump and probe beams are perfectly collimated, then all beams must be collinear in order to conserve momentum.     

Experimental study of relativistic self-focusing andself-channeling of an intense laser pulse in an underdense plasma

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW⩽P⩽15 TW) in an underdense plasma. We present results obtained with picosecond (τ=1 ps) and subpicosecond (τ=0.4 ps) pulses. In the “long pulse” regime, modifications in the laser propagation are observed for Pc, the critical power for self-focusing. By contrast, self-guiding of subpicosecond pulses is observed for P≈P_c. Using a paraxial envelope model describing the laser propagation and taking into account the plasma response to the ponderomotive force, it is shown that a maximum laser intensity of 5-15 times that reached in vacuum may be achieved when P is in the (1.25-4)×P_c range. It is also demonstrated that ion motion may significantly reduce the effective P_c