Femtosecond pulse-shape modulation at nanosecond rates

We report high-rate, computer-controlled femtosecond pulse shaping by use of an electro-optical gallium arsenide optical phased-array modulator with 2304 controlled waveguides. It provides fast modulation speed of both spectral phases and amplitudes. Limited by the driving electronics of our current setup, we were able to update a pulse shape in approximately 30 ns. This technique paves the way toward individual shaping of every single pulse of typical femtosecond mode-locked oscillators.     

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers

A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of pi-conjugated oligomers and semiconductor nanocrystals.     

Femtosecond hard X-ray plasma sources with a kilohertz repetition rate

Laser-driven plasma sources of femtosecond hard X-ray pulses have found widespread application in ultrafast X-ray diffraction. The recent development of plasma sources working at kilohertz repetition rates has allowed for diffraction experiments with strongly improved sensitivity, now revealing subtle fully reversible changes of the geometry of crystal lattices. We provide a brief review of this development and present a novel plasma source with an optimized mechanical and optical design, providing a high flux of several 10¹⁰ photons/s at the Cu-Kα energy of 8.04 keV and a pulse duration of ≤300 fs. First experiments, including the generation of Debye–Scherrer diffraction patterns from Si powder, demonstrate the high performance of this source.     

High-harmonic generation from plasma mirrors at kilohertz repetition rate

We report the first demonstration of high-harmonic generation from plasma mirrors at a 1 kHz repetition rate. Harmonics up to nineteenth order are generated at peak intensities close to 101? W/cm2 by focusing 1 mJ, 25 fs laser pulses down to 1.7 μm FWHM spot size without any prior wavefront correction onto a moving target. We minimize target surface motion with respect to the laser focus using online interferometry to ensure reproducible interaction conditions for every shot and record data at 1 kHz with unprecedented statistics. This allows us to unambiguously identify coherent wake emission as the main generation mechanism.     

Generation of relativistic intensity pulses at a kilohertz repetition rate

By using adaptive optics to correct the wave-front distortion of a 21-fs, 0.7-mJ, 1-kHz laser, we are able to focus the pulses to a 1-mum spot with an f/1 off-axis parabolic mirror. The peak intensity at the focal position is 1.5x10(18) W/cm(2) , which is to the authors' knowledge the first demonstration of generating relativistic intensity at a kilohertz repetition rate.     

Femtosecond pulse shaping directly in the mid-IR using acousto-optic modulation

Pulse shaping directly in the mid-IR is accomplished by using a germanium acousto-optic modulator (Ge AOM) capable of programmable phase and amplitude modulation for IR light between 2 and 18 microm. Shaped waveforms centered at 4.9 microm are demonstrated in both the frequency and the time domains. With a 50% throughput efficiency, the Ge AOM can generate much more intense pulses with higher resolution than can indirect shaping methods. Furthermore, the phase stability of the shaped pulse proved sufficient for cross correlation with unshaped mid-IR pulses. Thus, phase- and amplitude-tailored pulses can now be readily incorporated into phase-sensitive experiments, such as heterodyned 2D IR spectroscopy.     

Femtosecond terahertz radiation from femtoslicing at BESSY

Femtosecond far-infrared radiation pulses in the THz spectral range were observed as a consequence of the energy modulation of 1.7 GeV electrons by femtosecond laser pulses in the BESSY storage ring in order to generate femtosecond x-ray pulses ("femtoslicing"). In addition to being crucial for diagnostics of the laser-electron interaction, the THz radiation itself is useful for experiments where intense ultrashort THz pulses of well-defined temporal and spectral characteristics are required.     

Femtosecond dynamics of chemical reactions at surfaces

One of the major goals in physical chemistry is to obtain a microscopic understanding of chemical reactions. Recent developments in femtosecond laser techniques provide the opportunity to resolve the timescale of elementary steps of chemical reactions at surfaces. This is exemplified for the femtosecond laser-induced oxidation of CO on Ru(001). Among other adsorbate-specific probes vibrational sum-frequency generation spectroscopy offers the possibility to monitor adsorbates or reaction intermediates directly at the surface. Recently, we have employed this technique to investigate the dynamics of the CO-stretch vibration of CO adsorbed on Ru(001) after optical excitation leading to CO desorption. Received: 4 August 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Dark matter experiment using Argon pulse-shape discrimination: DEAP-3600

DEAP-3600 is a single-phase liquid-argon dark matter detector that at time of writing is cooling down in preparation for filling at the SNOLAB facility near Sudbury, Ontario, Canada. DEAP-3600 is designed and constructed to achieve a sensitivity of 10^?46cm² for a WIMP-nucleon cross section for a 100 GeV WIMP. The steps taken in design and construction to achieve the ultra-low backgrounds required for such a sensitive WIMP search are reviewed.     

Ultrafast structural dynamics studied by kilohertz time-resolved x-ray diffraction

Ultrashort multi-ke V x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the femtosecond time domain. However, pulse-to-pulse instability and long data acquisition time restrict the application of ultrashort x-ray systems operating at low repetition rates. Here we report on the performance of a femtosecond laser plasma-induced hard x-ray source that operates at 1-k Hz repetition rate, and provides a flux of 2.0 × 1010 photons/s of Cu Kαradiation. Using this system for time-resolved x-ray diffraction experiments, we record in real time, the transient processes and structural changes induced by the interaction of 400-nm femtosecond pulse with the surface of a 200-nm thick Au(111) single crystal.     

Characterizing THz Coherent Synchrotron Radiation at Femtosecond Linear Accelerator

The generation and observation of coherent THz synchrotron radiation from femtosecond electron bunches in the Shanghai Institute of Applied Physics femtosecond accelerator device is reported. We describe the experiment setup and present the first result of THz radiation properties such as power and spectrum.     

Femtosecond quadrature-squeezed light generation in CdSe at 1.55 mum

We generated pulsed quadrature-squeezed light by cross-phase modulation in single-crystal hexagonal CdSe at wavelengths of 1.4 to 1.55mum . We measured 0.4-dB squeezing (0.7 dB is inferred at the crystal) with 100-fs laser pulses. The wavelength and the intensity dependence, as well as variations in the local oscillator configuration, are examined. At higher intensities squeezing is shown to deteriorate owing to competing nonlinear processes.     

Red-Shift Conical Emission by Femtosecond Pulses at Low Input Power

Red-shift conical emission （CE） is observed by femtosecond laser pulse propagating in BK7 at a low input power （compared to those input powers for generation of blue-shift CE）. With the increasing input power the blue-shift CE begins to appear whereas the red-shift CE ring （902 nm in our experiment） disappears accompanied by the augment of the central white spot size synchronously. The disappearing of red-shift CE in our experiment is explained such that the increase of axial intensity is much higher than that of ring emission and the augment of the central white spot size with the increasing input laser power.     

Femtosecond fiber laser at 780 nm for two-photon autofluorescence imaging

We present an Er-doped fiber(Er:fiber)-based femtosecond laser at 780 nm with 256 MHz repetition rate, 191 fs pulse duration, and over 1 W average power. Apart from the careful third-order dispersion management, we introduce moderate self-phase modulation to broaden the output spectrum of the Er:fiber amplifier and achieve 193 fs pulse duration and 2.43 W average power. Over 40% frequency doubling efficiency is obtained by a periodically poled lithium niobate crystal. Delivering through a hollow-core photonic bandgap fiber, this robust laser becomes an ideal and convenient light source for two-photon autofluorescence imaging.     

Femtosecond photon echo in dye-doped polymer films at room temperature

The signals of primary and stimulated femtosecond photon echo in polymer films doped with dye (phthalocyanine) molecules have been experimentally investigated at room temperature. A femtosecond echo spectrometer for these echo experiments is described. The decay curves of echo signals with increasing time intervals between excitation femtosecond pulses are obtained and blue shifts of the spectra of femtosecond echo signals with respect to the spectrum of excitation pulses are revealed. The possibilities of using the studied doped polymer films as recording media for high-temperature echo processors and coolants in optical refrigerators are analyzed.     

Femtosecond pump and probe measurement of all-optical modulation based on intersubband transition in n-doped quantum wells

An all-optical modulation of interband-resonant light (near-infrared signal light: 800 nm) by intersubband-resonant light (mid-infrared control light: 4–7 μm) in n-doped AlGaAs/GaAs multiple quantum wells is investigated by two-color femtosecond pump–probe experiments at room temperature. The modulation of the near-infrared signal light with an ultrafast recovery as short as 1 ps is successfully observed when the quantum wells are pumped by the mid-infrared control light pulse (4 fJ/μm²). The dependence of the modulation depth on the wavelength of the control light is also measured, which is shown to be consistent with the intersubband absorption spectrum of the quantum wells. The results indicate that the utilization of the intersubband transition is promising for the ultrafast all-optical modulation and switching.     

The New Macromolecular Femtosecond Crystallography (MFX) Instrument at LCLS

Macromolecular X-ray crystallography has developed, since its first use over 50 years ago to solve the structure of myoglobin, into a widely used method with broad impact in biological sciences and in society. It is today the primary technique used to obtain structural information on biomolecules that can shed light on their function and this information is often used in biomedical applications such as drug design. As this article is written, the Protein Data Bank [1 F.C. Bernstein, Journal of Molecular Biology 112(3), 535–542 (1977).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] has just reached the milestone of 100,000 deposited X-ray structures, with a continuing trend for an ever-increasing number of structures every year. The primary contribution to this success and the increasing number of X-ray structures is the broad availability of synchrotron radiation sources with many dedicated beamlines around the world providing rapid and efficient data collection along with standard data analysis tools capable of fast data interpretation.     

Femtosecond Er doped fiber laser using high modulation depth SESAM based on metal/dielectric hybrid mirror

We proposed and fabricated a metal/dielectric mirror based high modulation depth semiconductor saturable absorber mirror (SESAM), for ease of the growth restrictions. Using such a SESAM, we obtained self-starting mode-locking in a ring cavity fiber laser, which produced 102 fs pulses at a repetition rate of 36.2 MHz.