W. E. Bron

FEMTOSECOND LASER HEATING OF MULTI-LAYER METALS. II: EXPERIMENTS

Abstract Femtosecond thermoreflectivity experiments are performed to investigate energy deposition and transport during the very early period of short-pulsed laser heating of gold and chromium multi-layer metal films. The chromium layer underneath the top gold layer is found to produce significant effects on the laser-energy deposition process. Experimental results show that radiation absorption by free electrons and the subsequent heating of the lattice occur not only at different times but also at different locations in a multi-layer metal film. The conventional radiation heating model fails to predict these results, and a more rigorous two-step model agrees well with the measured data.

Spectroscopy of high-frequency phonons

It has recently become possible to observe experimentally the spectral, spatial and temporal evolution of non-equilibrium phonon distributions in solids. The author reviews those techniques which are applicable to high-frequency (>or approximately=200 GHz) phonon propagation. Each technique is analysed for the physical principles involved, its characteristic parameters, and its range of applicability. The physical phenomena, to which the techniques have been applied, are discussed and the experimentally obtained results are compared to theoretical predictions whenever possible. It is shown that, although no universally applicable phonon spectrometer has so far been devised, those spectrometers presently in use are applicable to a broad range of phenomena inherent in phonon transport.

Ultrashort processes in condensed matter

Proceedings of the NATO Advanced Study Institute held near Lucca, Italy, Aug.-Sept. 1992, to consider theoretical and experimental aspects of ultrashort processes in condensed matter. One major subgroup of topics involves the ultrashort dynamics of excitations of various particles produced through

Elastic constants of EuF2 and SrCl2

Abstract The sound velocity in single-crystal EuF 2 between 77 and 300°K and the sound velocity in SrCl 2 between 195 and 300°K have been determined using an ultrasonic pulse-echo technique. From the sound velocity the elastic constants, anisotropy factor, Debye temperature and frequency have been calculated.

Raman frequencies of fluorite crystals

Abstract The frequency of th Raman active mode in CaF2, EuF2, PbF2, SrCl2, BaF2 and BaCl2 has been measured at 300 and 77°K.

Shock wave and cavitation bubble dynamics during photodisruption in ocular media and their dependence on the pulse duration

Shock waves and cavitation bubbles generated by optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond and picosecond laser pulses generated during photodisruption in corneal tissue and water. Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into both corneal tissue and water to create optical breakdown. Pulses with durations of 20 ps have been applied for comparative studies. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles. Femtosecond pulse engender rapidly decaying shock waves in both materials. The spatial range of shock waves induced by femtosecond laser pulses is considerably smaller than that of shock waves induced by picosecond optical breakdown. Cavitation bubbles excited by femtosecond pulses are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses. The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the application of femtosecond laser technology to intraocular microsurgery.

Dual synchronously pumped and synchronously amplified dye lasers

A dual synchronously pumped and synchronously amplified dye-laser system has been constructed and tested. A regenerative amplifier is used as the amplification source. An amplification factor of 6 × 104 is reached in each of two synchronously pumped dye-laser output beams. Thus, the total incident beam intensity available for two-photon nonlinear excitations reaches an amplification of the order of 4 × 109 times that available from unamplified laser beams.

Generation and kilohertz rate amplification of synchronized femtosecond and picosecond laser pulses

Synchronized femtosecond and tunable picosecond laser pulses have been generated using a linear cavity configuration in both lasers. The long-term stability of the synchronization, as well as the frequency of the synchronously pumped femtosecond dye laser, is assured by active stabilization of the cavity length. Additionally, the picosecond and femtosecond pulses have been amplified in dual amplifier chains to pulse energies in the microjoule range at a 1 kHz repetition rate. The total peak intensity available for two-photon, nonlinear excitations is on the order of 10/sup 9/ times greater than that available from unamplified beams. >

Transient electric field generated by nonequilibrium states in superconducting Pb films

Nonequilibrium states can be excited in superconducting Pb films with laser pulses of picosecond duration. We used the technique of electro‐optic sampling to observe the transient electric field generated by these states in the presence of a supercurrent. It follows from our experiment that the switching between the superconducting and normal states occurs within 1 ps after the laser excitation.

The U1 center in KBr

Abstract The infrared absorption from U1 centers in KBr has been measured in a variety of crystals. The results suggest that anharmonic coupling of the U1 center to lattice vibrations is not greater than that of the U center, that the U1 center is in an interstitial position of lower than cubic symmetry, and that it may be tunneling between equivalent sites.