In order to investigate the ultrafast dynamics of free carriers generated in bulk dielectrics by intense femtosecond laser pulses we have designed a setup for ultrafast time-resolved imaging Mach-Zehnder interferometry. The application of the 2D-Fourier-transform technique allows us to accurately reconstruct the actual laser-induced phase shifts and transmission changes for the probe pulses, which provide the properties of free carriers. Interferometric measurements in high-purity fused silica clearly demonstrate that the dominant ionization mechanism for intensities below 10 TW/cm<sup>2</sup> is multiphoton ionization.
Ultrashort laser pulse interaction with material involves a number of specialities as compared to longer irradiations. Applying femtosecond laser pulses, the fundamental physical processes such as excitation, melting and ablation are temporally separated, allowing a separate investigation of each of them. The irradiated material passes through highly non-equilibrium states of different kinds on different timescales after irradiation. Thus, the theoretical description of the investigated processes may differ strongly from the classical descriptions valid for equilibrium or steady-state conditions. On a femtosecond timescale we investigate the non-equilibrium of the laser-excited electron gas. With the help of a detailed microscopic approach we study the applicability of simplified macroscopic descriptions of laser absorption and free-electron excitation. We study different melting processes occurring on different timescales in the picosecond regime. The nature of the melting process depends on the laser and material parameters, respectively. Material removal, i.e. ablation, occurs on a pico- to nanosecond time scale, depending on excitation strength. We show theoretical and experimental investigations of the expansion dynamics of the excited material.
The mode structure of supercontinuum emission generated by 40-fs pulses of 800-nm Ti:sapphire laser radiation in fused silica microstructure fibers is experimentally studied. The long-wavelength and visible parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber models. With an appropriate spectral filtering, bell-shaped modes of the long-wavelength section (~720-900 nm) of the supercontinuum generated in a microstructure fiber with a small core diameter can be separated from either doughnut-like or bipartite modes of the visible part (~400-600 nm) of this supercontinuum. This effect can be employed to spectrally slice single modes of supercontinuum emission from microstructure fibers.
The formation of well-defined craters is a general feature of laser ablation with ultrashort laser pulses, indicative of a sharp ablation threshold. Results of a microscopic characterization of ablation craters on semiconductors after irradiation with single intense ultrashort laser pulses are presented.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).
The results of experimental and theoretical studies of the properties of holey fibers are presented. The fabrication of holey fibers with a pitch of the two-dimensional periodic structure of the cladding less than 500 nm allowed us to experimentally observe a photonic band gap in transmission spectra of holey fibers tunable within the range of 930 - 1030 nm. It is demonstrated that holey fibers provide an opportunity to considerably increase the efficiency of spectral broadening and phase control of short laser pulses as compared with conventional fibers.
The influence of the structure of the holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800 nm Ti:sapphire laser radiation transmitted through 2- and 3- micrometers -pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of holey fibers are considered. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.
Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.
Removal of material from the surface of metals and semiconductors following irradiation with pico- or femtosecond laser pulses is a thermal process involving states of matter having unusually thermodynamic, hydrodynamic and optical properties.
We have investigated femtosecond laser-induced ablation of gallium arsenide and silicon using time-of-flight mass spectroscopy. Below the ablation threshold we observe free flight desorption of atoms from the laster heated surface. The absence of collisions between particles leaving the solid allows to obtain the maximum surface temperature during laser irradiation of Gallium Arsenide. We estimated maximum surface temperatures of the order of 3500 K at the ablation threshold, where we observed a step-like increase in the number of detected particles. In the case of Silicon the existence of molecules of up to 6 atoms does not allow to measure the surface temperature. With increasing fluence free flight desorption transforms into a collisional expansion process. The behavior of Gallium particles can be quantitatively described through Knudsen-layer theory, indicating that Gallium particles expand as a non-ideal gas close to the ablation threshold ((gamma) equals Cv/Cp less than 5/3). Above fluences of approximately 2.5 Fth (gamma) approaches 5/3 indicating an ideal gas behavior for the expanding material. Dilution into the two phase regime of a superheated liquid characterizes ablation close to threshold.
Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 10<SUP>13</SUP> W/cm<SUP>2</SUP> with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.
We describe the generation of optical harmonics of high order during the interaction of intense femtosecond laser pulses with the surface of insulating and metallic solid targets. With a p- polarized fundamental beam from a titanium sapphire, chirped pulse amplification laser system ((lambda) equals 800 nm) we have observed even and odd harmonics up to the eighteenth order (lambda approximately equals 45 nm) for fundamental laser intensities of about 10<SUP>17</SUP> to 10<SUP>18</SUP> W/cm<SUP>2</SUP>.
Generation and amplification of femtosecond laser pulses is discussed. We discribe
different amplification schemes and present detailed results on the spatial and
temporal properties of the amplified pulses. Pulses of approximately 100 fs and
peak intensities of several times 1O are obtained.