Zone folded coherent acoustic phonons were generated in a multilayered GaSb/InAs epitaxial heterostructure via rapid heating by femtosecond laser pulses. These phonons were probed by means of ultrafast x-ray diffraction. Phonons both from the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the absorption index due to photogenerated free carriers.
The X-ray power absorption by the Beryllium compound refractive lenses (CRL) installed in the ESRF ID10 front-end reaches 139 W. This non-negligible power leads to an excessive temperature in the lens such that the induced thermal stress is much larger than the yield stress of Beryllium. The thermal fatigue damage of the lens occurred after certain number of operation cycles. Sudden loss of focusing ability was observed recently after 6 ~ 7 years frequent operation. SEM and phase contrast images confirmed the damage of the CRL. Following these observations, optimization of some design parameters (width, and thickness of the thin part between two holes) of the CRL has been carried out as well as some operational parameters (cooling of the lens, vertical aperture of the X-ray beam on the lens). An optimized Beryllium CRL for the ID10 front-end should have a width of 10 mm instead of 2 mm and the thickness of the thin part between two holes should be increased to 0.2 mm. The temperature of the CRL can be reduced by a better cooling of the lens, for instance by improving the thermal contact between the Beryllium and the copper cooling block, or by reducing the vertical aperture of the X-ray beam from 4 mm to 2 mm (eventually to 1 mm).
Although the realisation of femtosecond X-ray free electron laser (FEL) X-ray pulses is still some time away, X-ray diffraction experiments within the sub-picosecond domain are already being performed using both synchrotron and laser- plasma based X-ray sources. Within this paper we summarise the current status of some of these experiments which, to date, have mainly concentrated on observing non-thermal melt and coherent phonons in laser-irradiated semiconductors. Furthermore, with the advent of FEL sources, X-ray pulse lengths may soon be sufficiently short that the finite response time of monochromators may themselves place fundamental limits on achievable temporal resolution. A brief review of time-dependent X-ray diffraction relevant to such effects is presented.
The advent of CPA femtosecond lasers has opened the way to a new regime of interaction with atoms and molecules. In some experiments like time-resolved x-ray diffraction of laser- excited samples, the signal to be measured can contain very few photons and repetition rates up to 1 kHz are required. The laser-triggered x-ray streak camera system is therefore a promising tool for the study ultrashort x-ray events. We present the results of the characterization tests performed on our subpicosecond x-ray streak camera at the University of Michigan and at the European Synchrotron Radiation Facility. This new ultrafast diagnostic is triggered by a short laser pulse and can acquire images at rates up to 1 kHz and features a subpicosecond time resolution along with a 40 micrometer spatial resolution. We discuss the different issues related to the interaction between the laser pulse and photo-conductive switches, the synchronization of the detector.
The absorption band of titanium sapphire is excellently matched to the 510.6 nm and 578.2 nm emission lines of the copper vapor laser (CVL). The availability of high power and high repetition rate output from CVLs allows the generation of high average power tunable radiation from these crystals at the repetition rate of the pump. Such lasers fulfill the rapid data collection requirement of many spectroscopic applications such as resonance ionization mass spectrometry. A computer model of the absorption of CVL radiation in the crystal which incorporates the polarization and beam quality of the pump laser is used to map the gain distribution in the crystal so enabling parameters such as crystal doping level and length to be optimized. Gain switched analysis of the titanium sapphire laser predicts threshold, slope efficiency, pulse width and pulse build up time which are in good agreement with observed values. Thermal effects have also been computed. The experimental pump energy for threshold is typically 25 (mu) J and the slope efficiency for broadband lasing is greater than 20% with output powers up to 750 mW. The shortest pulse width and build-up time observed are 24 ns and 60 ns, respectively, for a resonator with a 0.85 m round-trip length. Tuning methods and resonator designs are also reviewed.
New Results on the trapping efficiency of cryogenic gas purifiers indicate that efficiencies of over 95 can be obtained. High power industrial excimer lasers operate at high buffer gas pressures and use large volume discharge vessels. A prototype gas purifier offering a circulation flow rate and operating pressure twice that of a conventional gas purifier has been developed. The performance and applicability of this device to industrial excimer lasers will be discussed.