A. Rundquist

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.

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of /spl sim/10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks.

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.

Demonstration of a Sub-Picosecond X-Ray Streak Camera

A novel design, magnetically focused, x‐ray streak camera was designed and tested using sub‐20 fs soft‐x‐ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.9 ps throughout the ultraviolet to soft‐x‐ray wavelength region. Our streak camera represents the fastest x‐ray detector developed to date.

ULTRASHORT OPTICAL WAVEFORM MEASUREMENTS USING FREQUENCY-RESOLVED OPTICAL GATING

We measure the intensity and phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser using the recently developed technique of frequency-resolved optical gating. These results represent to our knowledge the shortest complete optical waveform characterization measurements performed to date. We also verify recent theoretical calculations that predict that the main limitation on the pulse duration from these lasers is the presence of uncompensated higher-order dispersion.

GUIDED-WAVE PHASE-MATCHING OF ULTRASHORT-PULSE LIGHT

We review the use of hollow waveguides for frequency conversion of ultrafast laser pulses the ultraviolet and extreme ultraviolet. Phase-matching of these processes is reached through a balance of gas and waveguide dispersion. By mixing 400 nm with 800 nm light, ultrashort (8 fs) pulses are generated near 270 nm with high efficiency > 20%. Tuning of the longer-wavelength component in the mixing process allows tuning of the output from 215–308 nm. In the XUV, this guided-wave phase-matching has allowed an increase of conversion efficiency of high-order harmonic generation of 100–1000x over that obtained with a gas jet, in an experimentally-convenient geometry.

Demonstration of a 0.54-ps x-ray streak camera

A magnetically focused x-ray streak camera was designed and tested using sub-200 fs soft x-ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.54 ps for the ultraviolet and 0.88 ps in the soft-x-ray wavelength region. Our streak camera represents the fastest x-ray detector developed to date, and should allow sub-picosecond time resolution experiments to be performed using either synchrotron or laser-plasma-based x-ray sources.

Measurement of the Intensity and Phase of Ultrashort Pulses Using Frequency-Resolved Optical Gating

Recently, we developed a technique for measuring the full time-dependent intensity, I(t), and phase, ϕ(t), of the complex electric field, E(t), of an ultrashort laser pulse. This technique, Frequency-Resolved Optical Gating (FROG),1–5 has been demonstrated for pulses in the ultraviolet, visible, near-infrared, and mid-infrared. It can measure pulses from millijoules to a few picojoules (the latter using the second-harmonic-generation version). It is also routinely used to measure a single ultrashort laser pulse.

Femtosecond X-ray generation in the water window using 25-fs laser pulses

Summary form only given. Recent advances in laser technology have led to high-harmonic generation using ultrashort pulses with pulse durations in the range of 5-25 fs. We have shown that the cutoff harmonic photon energy, is inversely proportional to the square of the log of the laser pulse. Our experimental results for argon gas, are in excellent agreement with our predictions. The above formula also explicitly shows how the cutoff photon energy changes with the laser wavelength, the atomic species, and the electron quantum state. From the formula, under our experimental conditions (25 fs laser and helium gas), we expect to observe harmonics up to order 333 (corresponding to an energy of 518 eV). Experimentally, using a 25-fs Ti:sapphire laser centered at 800 nm, we observed discrete harmonic peaks in He up to the 221st order.

Controlling the intrinsic atomic phase in high-harmonic generation

Recently, the process of high-order harmonic generation has received considerable attention. The dependence of the harmonic spectra on laser intensity, wavelength, polarization, and pulse width have all been studied experimentally and numerically. However, more detailed comparisons of models with experiment has proven to be difficult, primarily because of the lack of detailed and reliable experimental data. In this work, we provide a theoretical explanation for our experimental observation that the chirp of the excitation laser used to generate the harmonics dramatically changes the observed spectra of the high harmonic output. For positively chirped pump pulses, the individual harmonic peaks are discrete and well defined, while for negatively chirped pump pulses, the harmonic output spectra merge into a continuum. By considering the intrinsic phase shift of the harmonic emission with respect to the driving laser pulse, we show that these effects can be successfully explained using both analytic semiclassical theory and quantum models.