Few-cycle pulse characterization with an acousto-optic pulse shaper

An acousto-optic pulse shaper has been used to characterize few-cycle pulses generated in a hollow-core fiber. A grism pair precompensates for the dispersion of the acousto-optic crystal, allowing the full pulse-shaping window to be used for replica generation rather than self-compensation. A 9.4 fs pulse was measured, the shortest ever measured with an acousto-optic pulse shaper, to our knowledge.     

Three-dimensional spatiotemporal pulse characterization with an acousto-optic pulse shaper and a Hartmann-Shack wavefront sensor

We demonstrate a simplified arrangement for spatiotemporal ultrashort pulse characterization called Hartmann-Shack assisted, multidimensional, shaper-based technique for electric-field reconstruction. It employs an acousto-optic pulse shaper in combination with a second-order nonlinear crystal and a Hartmann-Shack wavefront sensor. The shaper is used as a tunable bandpass filter, and the wavefronts and intensities of quasimonochromatic spectral slices of the pulse are obtained using the Hartmann-Shack wavefront sensor. The wavefronts and intensities of the spectral slices are related to one another using shaper-assisted frequency-resolved optical gating measurements, performed at particular points in the beam. This enables a three-dimensional reconstruction of the amplitude and phase of the pulse. We present some example pulse measurements and discuss the operating parameters of the device.     

Quartic-phase-limited grism-based ultrashort pulse shaper

By replacing the dispersive element in a zero-dispersion pulse shaper with a grism, we have constructed a quartic-phase-limited pulse shaper. We demonstrate compensation of 4500 fs2 without the use of a dynamic element in the pulse shaping line, which is approximately the amount of dispersion induced by a typical multiphoton microscope. We also demonstrate that detuning the pulse shaper to compensate for quadratic phase induces negligible spatial chirp, thereby maintaining a high-quality focal spot for a microscopy setup.     

A short pulse mode-locked Nd-glass laser

A mode-locked Nd-glass laser with a ring cavity configuration has been made. With this laser, using a resonator made with prisms instead of mirrors, a stable TEM₀₀ mode is obtained. Two photon measurements performed on the whole pulse train consistently yield a pulsewidth of 3.5 ps, with the proper contrast ratio 3:1.     

A new high-resolution femtosecond pulse shaper

A novel liquid crystal display (LCD) with 640 stripes was successfully implemented and investigated for femtosecond pulse shaping. As compared to previously used devices, the large active area allows for operation in high-power laser systems. The increased number of pixels greatly enlarges the manifold of accessible pulse modulations, making the device an ideal tool for coherent control experiments and optical information processing. Received: 1 February 2001 / Published online: 21 March 2001     

Complete characterization of a spatiotemporal pulse shaper with two-dimensional Fourier transform spectral interferometry

Spatiotemporal pulse shaping is characterized with two-dimensional Fourier transform spectral interferometry. A deformable-mirror-based bidimensional pulse shaper is used to create simple spatiotemporal structures on a femtosecond pulse, structures that are directly calculated from the measured spatiospectral phases and intensities.     

Onset of crater formation during short pulse laser ablation

We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out. PACS 52.38.Mf; 79.20.Ds; 65.40.De     

Chirp control in the direct space-to-time pulse shaper

The dispersion properties of the direct space-to-time pulse shaper are investigated for the first time to our knowledge. We demonstrate that phase-front curvature of the input spatial profile leads to a chirp in the output temporal waveform, which one can compensate for by varying the separation between the pulse-shaping lens and slit. Furthermore, the output intensity profile remains invariant as the chirp is manipulated. These properties are fundamentally different than in the well-known Fourier-transform pulse shaper.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Electrostrictive counterforce on fluid microdroplet in short laser pulse

When a micrometer-sized fluid droplet is illuminated by a laser pulse, there is a fundamental distinction between two cases. If the pulse is short in comparison with the transit time for sound across the droplet, the disruptive optical Abraham-Minkowski radiation force is countered by electrostriction, and the net stress is compressive. In contrast, if the pulse is long on this scale, electrostriction is cancelled by elastic pressure and the surviving term of the electromagnetic force, the Abraham-Minkowski force, is disruptive and deforms the droplet. Ultrashort laser pulses are routinely used in modern experiments, and impressive progress has moreover been made on laser manipulation of liquid surfaces in recent times, making a theory for combining the two pertinent. We analyze the electrostrictive contribution analytically and numerically for a spherical droplet.     

Ultra short pulse laser annealing of FeB metallic glasses

We present the results of picosecond laser annealing of as-quenched Fe₈₅B₁₅ and Fe₈₂B₁₈ metallic glasses. The influence of the laser radiation on the surface and bulk properties are studied using CrK X-ray diffraction and transmission Mössbauer spectroscopy. The X-ray data show that the amorphous nature of the surface of the samples can be improved with laser treatment. The mat (cooling) surfaces of the ribbons appear to be more affected by the laser treatment, and show a higher stability. The Mössbauer data reveal that laser annealing of ribbon surfaces also affects the bulk properties of these materials due to induced stresses from the surface layer. The magnetic properties of these materials can be modified by laser annealing.     

On-chip optical pulse shaper for arbitrary waveform generation

We propose and demonstrate a silicon-on-insulator(SOI) on-chip optical pulse shaper based on four-tap finite impulse response. Due to different width designs in phase region of each tap, the phase differences for all taps are controlled by an external thermal source, resulting in an optical pulse shaper. We further demonstrate optical arbitrary waveform generation based on the optical pulse shaper assisted by an optical frequency comb injection. Four different optical waveforms are generated when setting the central wavelengths at 1533.78 nm and 1547.1 nm and setting the thermal source temperatures at 23℃ and 33℃, respectively. Our scheme has distinct advantages of compactness, capability for integrating with electronics since the integrated silicon waveguide is employed.     

The design and characterization of short pulse ultrasound transducers

The construction and testing of short pulse (bipolar and unipolar) ultrasound transducers are discussed and several examples are analysed. Equations predicting the radiation field of the short pulse transducer are derived using a new approach. Agreement was obtained between the theoretical predictions and the experimentally measured field for one short pulse transducer. Some discrepancy was noted between the predicted pulse shapes and those observed experimentally.     

High power short pulse generation in two-frequency flashpumped dye laser

We describe the simultaneous application of the self-injection and cavity dumping techniques to a flash pumped two-frequency dye laser. Short pulses with 1.5 ns fwhm were obtained with peak power in excess of 100 kW. A peak intensity gain by a factor 3 was observed with respect to normal cavity dumping operation. A simple model is presented to explain the power gain under self injected conditions and it is shown that for long cavities the power gain is limited by the upper laser level lifetime.     

Compensation of resonant atomic dispersion using a pulse shaper

An ultra-short pulse propagating in a resonant dense atomic medium experiences an important distortion due to a strong modification of its spectral phase. This distortion cannot be corrected using the usual simple dispersive devices (a pair of prisms, gratings, etc.). We present here an experimental demonstration of the compensation of this effect using a dual 640-pixel high-resolution pulse-shaper device. A cross-correlation intensity measurement combined with the XFROG (cross-correlated frequency-resolved optical gating) spectral phase measurement of the compensated pulse are performed; efficient correction of the resonant dispersive phase is shown. A spectacular temporal compression of the propagating pulse is then obtained. PACS 92.60.Ta; 42.50.Md; 42.65.Re; 42.79.-e     

Microscopic mechanisms of short pulse laser spallation of molecular solids

The mechanisms of photomechanical spallation are investigated in a large-scale MD simulation of laser interaction with a molecular target performed in an irradiation regime of inertial stress confinement. The relaxation of laser-induced thermoelastic stresses is found to be responsible for the nucleation, growth, and coalescence of voids in a broad sub-surface region of the irradiated target. The depth of the region subjected to void evolution is defined by the competition between the evolving tensile stresses and thermal softening of the material due to the laser heating. The initial void volume distribution obtained in the simulation of laser spallation can be well described by a power law. A similar volume distribution is obtained in a series of simulations of uniaxial expansion of the same molecular system performed at a strain rate and temperature realized in the irradiated target. Spatial and time evolution of the laser-induced pressure predicted in the MD simulation of laser spallation is related to the results of an integration of a thermoelastic wave equation. The scope of applicability of the continuum calculations is discussed. PACS 79.20.Ds; 61.80.Az; 02.70.Ns; 83.60.Uv     

Modeling and experimental investigation of short pulse Raman microchip laser

The results of experimental and theoretical investigations of passive Q-switch Raman microchip lasers based on Nd³⁺:LSB active medium and Ba(NO₃)₂ Raman crystal are presented. It has been demonstrated that intracavity Raman conversion in the microchip lasers is a simple and efficient method, capable of delivering high power pulses with sub-100 ps duration. Intracavity generation of the 1st Stokes pulses with duration from 180 down to 48 ps and a peak power of 48 kW has been performed and studied. High peak power and short duration of the 1st Stokes pulses in microchip laser with Ba(NO₃)₂ Raman crystal allows to easily perform extracavity harmonic generation and frequency sum mixing in LBO, BBO, and KTP crystals with discrete-tunable wavelength from ∼1200 down to ∼240 nm. We have developed a generalized model of Q-switched Raman microchip lasers, that takes into account spatial inhomogeneity of pump, laser, and Stokes beams, thermalization within the upper and lower multiplets of activator ions in laser medium, and saturable absorber bleaching and recovery. For the microchip lasers with different saturable absorbers, the model achieves very good agreement with the presented experimental results in a wide range of pump powers.     

Chromium oxide dissolution in steels via short pulse laser processing

Changes of microstructure, chemical and phase compositions in thin surface layers of low carbon steel saturated by chromium oxide have been studied by TEM, XPS and XRD methods. Ultrafine chromium oxide powder was spread on a steel surface and subjected to laser processing with nanosecond pulses. It was found that such conditions of processing as overheating of small volume of metal, high temperature gradient, rapid solidification and laser-induced plasma formation lead to dissolution of chromium oxide in the metal matrix. As a result of laser processing the surface layers contain chromium oxide, chrome-spinel FeO $\cdot $ Cr $_{2}$ O $_{3}$ and chromium in metal state dispersed in alpha and gamma iron. The processing technique allows to obtain surface layers whose chemical composition might be equivalent to the composition of stainless steels.     

Coherent matter waves for ultrafast laser pulse characterization

A technique for the characterization of ultrashort laser pulses using coherent matter waves is demonstrated. We emphasize the analogy between matter wave packets and electromagnetic wave packets propagating in dispersive media. Due to quadratic dispersion the wave packets are stretched and their temporal structure eventually converges to their spectrum, thus providing a possibility for energy measurements in conjugate space. This is demonstrated theoretically and experimentally and is the basis for our laser pulse characterization technique. We use energy resolved interferometrically recorded photoelectron spectra generated by above-threshold ionization in an autocorrelation setup to characterize ultrashort laser pulses at 800 nm wavelength. This approach is potentially applicable to the XUV wavelength region.     

Femtosecond electron pulse characterization using laser ponderomotive scattering

We demonstrate a method for the measurement of the instantaneous duration of femtosecond electron pulses using the ponderomotive force of an intense ultrashort laser pulse. An analysis procedure for the extraction of the electron pulse duration from the transient change of the transverse electron beam profile is proposed. The durations of the electron pulses generated in our setup were determined to be 410+/-30 fs.