Justin Peatross

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.

Ti:sapphire amplifier producing millijoule-level, 21-fs pulses at 1 kHz

We have developed a Ti:sapphire amplifier system capable of producing pulses of 1 mJ, with 20-22-fs pulse duration, at a 1-kHz repetition rate. The amplifier has a unique design consisting of a three-mirror multipass ring configuration with a highly doped Ti:sapphire crystal as the gain medium. Pulses of 15-fs duration from a Ti:sapphire oscillator are temporally stretched and injected into the amplifier, which is an eight-pass system with a total gain of 10(6). The amplif ier is more than 10% efficient, and the shot-to-shot energy fluctuation of the output is less than 2%. The output beam focuses to 1.8 times the diffraction limit.

Temporal decorrelation of short laser pulses

We describe a unique approach for extracting the temporal profile of ultrashort laser pulses from typical autocorrelation measurements. The use of the constraint that intensity is a nonnegative quantity enables an iterative numerical algorithm to reconstruct pulse shapes in a one-dimensional procedure. With the reconstruction of the intensity profile, the Gerchberg–Saxton algorithm can be used to retrieve the phase of the electric field from a spectral measurement. Because these procedures are carried out in one dimension, they are numerically much faster than two-dimensional techniques such as frequency-resolved optical gating. Their high computational efficiency can save substantial time by constructing good trial solutions for the more accurate but slower procedure of frequency-resolved optical gating.

Suppression of the pedestal in a chirped-pulse-amplification laser

The pedestal (prepulse and postpulse) associated with a chirped-pulse-amplification (CPA) laser is studied. Four components have been identified that contribute to the pedestal. Pulses are spectrally shaped by gain narrowing in a frequency-matched, regenerative amplifier, while self-phase modulation is avoided. The intensity contrast is further improved through the use of a saturable absorber, resulting in Gaussian pulses of ~0.9-ps duration with an intensity contrast exceeding 105:1. Both experimental and numerical descriptions of these processes are presented. This investigation makes possible the study of high-intensity ultrashort laser–plasma interactions with a fiber–grating CPA system.

High-order harmonic generation with an annular laser beam.

High-order harmonics have been generated by the use of an annular laser beam. The nonlinearity of harmonic production and the shorter wavelengths involved cause the harmonics to emerge strongly peaked on the laser axis. Thus the harmonics emerge from the focus inside the missing portion of the laser beam. This permits the laser to be blocked by an aperture that passes the harmonics.

Selective zoning of high harmonic emission using counter-propagating light

High harmonic production can be dramatically increased by utilizing an interaction region much longer than a coherence length. Counter-propagating light pulses can be used to disrupt the out-of-phase harmonic emission from selected zones in the focus so that the remaining emission builds constructively. Counter-propagating light creates a standing field modulation repeating over a half laser wavelength in which phase cancellations for harmonic emission occur. A simple power-law model is used to demonstrate how such pulses can be designed to counteract geometrical phase mismatches and improve emission for individual harmonics by more than two orders of magnitude.

Intensity-dependent phase-matching effects in harmonic generation

In high-order harmonic generation by an intense laser, intrinsic phases can develop at the atomic level between the laser field and the individual emitted harmonics. Because intrinsic phases can vary rapidly with the laser intensity, they can strongly influence phase matching to the extent that the laser intensity varies within the generating medium. Previously reported measurements of broad far-field harmonic emission patterns as well as measured asymmetries in the emission with respect to the axial positioning of the medium in the focus can be explained by intrinsic phases. An experimental method for further study of intrinsic phases is proposed that involves harmonic generation in two counterpropagating laser beams. The periodic intensity modulation created by the two beams coupled with the intensity-dependent intrinsic phases allows harmonic light to propagate in directions with otherwise extremely poor phase-matching conditions.

High harmonic generation in a semi-infinite gas cell

Ten-millijoule 35-femtosecond laser pulses interact with a cell of helium or neon that extends from a focusing lens to an exit foil near the laser focus. High harmonic orders in the range of 50 to 100 are investigated as a function of focal position relative to the exit foil. An aperture placed in front of the focusing lens increases the brightness of observed harmonics by more than an order of magnitude. Counter-propagating light is used to directly probe where the high harmonics are generated within the laser focus. In neon, the harmonics are generated in the last few millimeters before the exit foil, limited by absorption. In helium, the harmonics are produced over a much longer distance.

Photoemission of a Single-Electron Wave Packet in a Strong Laser Field

The radiation emitted by a single-electron wave packet in an intense laser field is considered. A relation between the exact quantum formulation and its classical counterpart is established via the electrons Wigner function. In particular, we show that the wave packet, even when it spreads to the scale of the wavelength of the driving laser field, cannot be treated as an extended classical charge distribution, but rather behaves as a pointlike emitter carrying information on its initial quantum state. We outline an experimental setup dedicated to put this conclusion to the test.

Direct observation of laser filamentation in high-order harmonic generation

We investigate the spatial evolution of a laser pulse used to generate high-order harmonics (orders ranging from 45 to 91) in a semi-infinite helium-filled gas cell. The 5 mJ, 30 fs laser pulses experience elongated focusing with two distinct waists when focused with f/125 optics in 80 Torr of helium. Extended phase matching for the generation of harmonics occurs in the region between the double foci of the laser, where the laser beam changes from diverging to converging.