S. Backus

High power ultrafast lasers

In this article, we review progress in the development of high peak-power ultrafast lasers, and discuss in detail the design issues which determine the performance of these systems. Presently, lasers capable of generating terawatt peak powers with unprecedented short pulse duration can now be built on a single optical table in a small-scale laboratory, while large-scale lasers can generate peak power of over a petawatt. This progress is made possible by the use of the chirped-pulse amplification technique, combined with the use of broad-bandwidth laser materials such as Ti:sapphire, and the development of techniques for generating and propagating very short (10–30 fs) duration light pulses. We also briefly summarize some of the new scientific advances made possible by this technology, such as the generation of coherent femtosecond x-ray pulses, and the generation of MeV-energy electron beams and high-energy ions.

Intense 8-fs pulse generation in the deep ultraviolet

By use of the recently developed technique of guided-wave frequency conversion, the generation of sub-10-fs light pulses in the UV has been demonstrated for what is believed to be the first time. Cross-phase modulation of the light in a hollow waveguide produced a bandwidth of 16 nm, with a center frequency of 270 nm, at 1 kHz. A simple grating pair was used to compress the pulses to a duration of 8 fs, as measured by self-diffraction frequency-resolved optical gating. In the experiment the compressed energy was greater than 1 muJ , with a peak power of >100 MW ; the technique can be scaled to higher energy. Further improvements should make it possible to generate pulses as short as approximately 3 fs with this technique.

16-fs, 1-μJ ultraviolet pulses generated by third-harmonic conversion in air

We describe a simple method for generating sub-20-fs ultraviolet light pulses with useful average powers, using a kilohertz Ti:sapphire laser system. By focusing a 22-fs, 1-mJ laser pulse in air, we obtain ultraviolet pulses with an energy of 1 microJ and at a wavelength of 266 nm and with an average power of 1 mW. The pulse duration of the ultraviolet pulses was measured to be 16 fs with frequency-resolved optical gating.

Ultrabroadband phase-matched optical parametric generation in the ultraviolet by use of guided waves

We present what is believed to be the first experimental demonstration of guided-wave phase-matched frequency mixing and harmonic conversion in gases. Broad-bandwidth ultrafast pulses, tunable around 270 nm, were generated from an ultrafast Ti:sapphire amplifier system using 2? + 2? - ? parametric wave mixing in a capillary waveguide. We achieved nonresonant phase matching by coupling both the fundamental and the second-harmonic light into the lowest-order mode. The output 3? pulses have an energy of >4muJ at a 1-kHz repetition rate. Simple extensions of this method can generate higher-energy 10-20-fs pulses tunable throughout the vacuum ultraviolet.

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.

0.2-TW laser system at 1 kHz

We have developed a 1-kHz repetition-rate Ti:sapphire laser system that can simultaneously generate high peak and average powers of 0.2TW and 4W, respectively. The laser system generates 4-mJ energy pulses with a 20-fs pulse width. We eliminated thermal lensing in the system by cooling the Ti:sapphire crystal to 125K. The output 20-fs pulses were fully characterized by use of the new technique of transient-grating frequency-resolved optical gating. We demonstrate experimentally that the pulse duration at the output is limited only by fifth-order dispersion.

Coherent learning control of vibrational motion in room temperature molecular gases

An evolutionary learning algorithm in conjunction with an ultrafast optical pulse shaper was used to control vibrational motion in molecular gases at room temperature and high pressures. We demonstrate mode suppression and enhancement in sulfur hexafluoride and mode selective excitation in carbon dioxide. Analysis of optimized pulses discovered by the algorithm has allowed for an understanding of the control mechanism.

Attosecond time-scale feedback control of coherent X-ray generation

Abstract High-harmonic generation is an extreme, high-order, nonlinear process that converts intense, ultrafast, visible and infrared laser light pulses coherently into the soft X-ray region of the spectrum. We demonstrate that by optimizing the shape of an ultrafast laser pulse, we can selectively enhance this process by promoting strong constructive interference between X-ray bursts emitted from adjacent optical cycles. This work demonstrates that coherent control of highly nonlinear processes in the strong-field regime is possible by adjusting the relative timing of the crests of an electromagnetic wave on a sub-optical cycle, attosecond time scale.

Highly coherent light at 13 nm generated by use of quasi-phase-matched high-harmonic generation

By measuring the fringe visibility in a Youngs double pinhole experiment, we demonstrate that quasi-phase-matched high-harmonic generation produces beams with very high spatial coherence at wavelengths around 13 nm. To our knowledge these are the highest spatial coherence values ever measured at such short wavelengths from any source without spatial filtering. This results in a practical, small-scale, coherent, extreme-ultraviolet source that is useful for applications in metrology, imaging, and microscopy.

Design and implementation of a TW-class high-average power laser system

We describe the design, modeling and characterization of a titanium-doped sapphire multipass, kilohertz amplifier system with output pulses of energy 4.4 mJ and duration 17 fs, giving a peak power of 0.26 TW. The thermal lensing in the second amplifier stage is virtually eliminated by cryogenic cooling of the laser crystal. Gain-narrowing and shifting of the amplified spectrum are reduced by tailoring the output spectrum of the oscillator and by using a low-loss multipass amplifier chain. Fourth-order spectral dispersion was completely eliminated by using a prism pair in addition to adjusting the stretcher and compressor grating separation and angle. We also numerically modeled the evolution of the pulse energy and spectral phase and amplitude through the amplifier system. The results of the model are in excellent agreement with measurements made using the technique of transient-grating frequency-resolved optical gating.