Ultrafast electron diffraction (UED) has the potential to capture changes in the structure of isolated molecules on the natural spatial and temporal scale of chemical reactions, that is, sub-Angstrom changes in the atomic positions that happen on femtosecond time scales. UED has the advantage that electron sources can easily reach sub-Angstrom spatial resolution, but so far femtosecond resolution had not been available for gas phase experiments due to the challenges in delivering short enough electron pulses on a gas target and the velocity mismatch between laser and electron pulses. Recently, we have used relativistic electron pulses at MeV energy to solve these challenges and reach femtosecond resolution. We have, for the first time, imaged coherent nuclear motion in a molecule with UED. In a proof-of-principle experiment, we captured the motion of a laser-excited vibrational wavepacket in iodine molecules. We are currently performing experiments in more complex molecules to capture laser-induced dissociation and conformational changes. We have also developed a table top 100 keV source that relies on a pulse compressor to deliver femtosecond electron pulses on a target and uses a tilted laser pulse to compensate for the velocity mismatch between the laser and the electrons. This source has a high repetition rate that will complement the high temporal resolution of the relativistic source.
Ultrafast time-dependent optical reflection and/or transmission spectroscopy can be used to measure time responses of materials and time arrivals between two unrelated ultrafast pulses. For example, a pump pulse, such as an x-ray pulse, excites a material, changing its refractive index. A spectrogram monitoring the change of intensity of a reflected or transmitted optical probe pulse can be used to indirectly monitor the refractive index change. Standard spectrogram deconvolution methods can be used to extract characteristics of both the material response and the probe pulse, but care must be taken to consider any experimental artifacts.
A recently demonstrated single-shot measurement of the relative delay between x-ray FEL pulses and optical laser pulses has now been improved to ~10 fs rms error and has successfully been demonstrated for both soft and hard x-ray pulses. It is based on x-ray induced step-like reduction in optical transmissivity of a semiconductor membrane (Si3N4). The transmissivity is probed by an optical continuum spanning 450 - 650 nm where spectral chirp provides a mapping of the step in spectrum to the arrival time of the x-ray pulse relative to the optical laser system.
Ultrashort pulse (USP) Ti:Sapphire oscillators are constantly improving in cost, performance, and reliability. These
improvements have been driven in part by improvements in the CW lasers used to pump the Ti:Sapphire gain medium.
Recent development of optically-pumped semiconductor (OPS) lasers heralds a USP pump source that reduces cost and
complexity while maintaining a high standard of performance and reliability. OPS lasers offer significant advantages
with respect to traditional diode-pumped solid state (DPSS) lasers in regards to wavelength flexibility, broad pump
tolerance, efficient spectral and spatial brightness conversion and high power scaling. In this paper, we report the
performance of different types of ultrashort pulse Ti:Sapphire oscillators pumped by OPS lasers: broad bandwidth
(approximately 100 nm) negative dispersion mirror based, broad bandwidth (approximately 100 nm) prism based, and
narrower bandwidth (approximately 10 nm) tunable prism based oscillator. We analyze the impact of multimode spatial
mode operation of the OPS pump laser on the mode quality, bandwidth and intensity noise of the USP oscillator output.
We compare the performance of USP oscillators pumped by multiple transverse mode OPS lasers with traditional single
transverse mode Nd:YVO4 DPSS lasers. We demonstrate excellent regenerative amplifier seeding with the OPS
pumped Ti:Sapphire oscillator.
Cryogenic cooling of Ti:Sapphire is a well known technique for improving its thermal performance. In particular the
improvement in thermal conductivity, temperature dependence of the index of refraction and thermal expansion around
77 K dramatically reduces the thermal lensing. This allows a significant increase in the possible pump power, while
keeping a very good beam quality over a wider range of operation. As an example we demonstrate a single-stage
regenerative amplifier that is capable of delivering compressed output powers of 7.5 W and 11.9 W at 1 and 5 kHz,
respectively, as well as a multi-pass amplifier delivering 13.2 W at 1 kHz.
A novel device designed for pulse shaping, characterization and phase compensation in ultrafast laser systems is
described. The pulse shaper exhibits low transmission loss and is widely applicable to lasers with spectral bandwidth
from 10 nm to over 400 nm. Pulse characterization and phase compensation is fully computer controlled in a closed loop
via MIIPS method. This system is designed to enhance performance of ultrafast oscillators and ultrafast amplifiers
including terawatt lasers and cryogenically cooled amplifier systems. Seed laser spectral amplitude shaping results in
increased bandwidth while preserving the output power in ultrafast regenerative amplifiers. Subsequent phase
compensation enables the robust delivery of output pulses within couple of percent of transform limit. Such system
could find numerous applications including MPE microscopy, CARS, and more general coherent control experiments.
Power scaling in laser systems is fundamentally constrained by detrimental effects of absorbed heat in the lasing medium. Cryogenic cooling is a well known technique for improving thermal performance in solid state laser materials. In particular the dramatic improvement in the thermal properties of Ti:sapphire at cryogenic temperatures has enabled a new class of commercial high-average-power femtosecond Ti:sapphire amplifiers. We review recent developments in this technology.