Self-referenced spectral interferometry for single-shot measurement of sub-5-fs pulses.

We demonstrate a novel approach for the extension of self-referenced spectral interferometry to the temporal characterization of few-optical cycle pulses. The new experimental setup is characterized by low dispersion and a collinear geometry. 4-fs pulses have been characterized by performing single-shot measurements, with high dynamic range on a broad temporal region. An independent measurement of the pulse duration, obtained by using attosecond streaking, allowed us to cross-check the experimental technique.

Laser technology: source of coherent kiloelectronvolt X-rays.

Generating X-rays that have the properties of laser light has been a long-standing goal for experimental science. Here we describe the emission of highly collimated, spatially coherent X-rays, at a wavelength of about 1 nanometre and at photon energies extending to 1.3 kiloelectronvolts, from atoms that have been ionized by a 5-femtosecond laser pulse. This means that a laboratory source of laser-like, kiloelectronvolt X-rays, which will operate on timescales relevant to many chemical, biological and materials problems, is now within reach.

Pulse compression with time-domain optimized chirped mirrors.

Dispersive optical interference coatings (chirped mirrors - CMs) are designed by computer optimization of an analytically calculated initial multilayer. Traditionally, the relevant properties of the CM (reflectance and the frequency-dependence of the phase shift upon reflection) are optimized to match frequency-domain targets. We propose a novel target function that quantifies directly the capability of a multilayer to control the temporal shape of the reflected optical pulse. Employing this time-domain analysis/optimization one can design dispersive multilayers having air as medium of incidence and supporting the generation of pulses with durations in the sub-5-fs-range, as demonstrated in a proof-of-principle compression experiment.

Compact, low-cost Ti:Al2O3 laser for in vivo ultrahigh-resolution optical coherence tomography.

A compact, low-cost, prismless Ti:Al2O3 laser with 176-nm bandwidth (FWHM) and 20-mW output power was developed. Ultrahigh-resolution ophthalmic optical coherence tomography (OCT) ex vivo imaging in an animal model with approximately 1.2-microm axial resolution and in vivo imaging in patients with macular pathologies with approximately 3-microm axial resolution were demonstrated. Owing to the pump laser, this light source significantly reduces the cost of broadband OCT systems. Furthermore, the source has great potential for clinical application of spectroscopic and ultrahigh-resolution OCT because of its small footprint (500 mm x 180 mm including the pump laser), user friendliness, stability, and reproducibility.

X-ray pulses approaching the attosecond frontier.

Single soft-x-ray pulses of approximately 90-electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/-1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach.

Self-phase-matched high harmonic generation

Our theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period. This subcycle self-modulation is predicted to result in phase matching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the investigation of matter with x-ray techniques, such as time resolved x-ray diffraction and absorption.

Controlling the phase evolution of few-cycle light pulses

Using a coherent nonlinear optical technique, slipping of the carrier through the envelope of 6-fs light wave packets emitted from a mode-locked-oscillator/pulse-compressor system has been measured, permitting the generation of intense, few-cycle light with precisely reproducible electric and magnetic fields. These pulses open the way to controlling the evolution of strong-field interactions on the time scale of the light oscillation cycle and are indispensable to reproducible attosecond x-ray pulse generation.

Optical pulse compression: bulk media versus hollow waveguides.

Optical pulse-compression systems based on bulk materials and hollow waveguides are compared by use of coupled-mode theory. Our analysis reveals an intuitive picture of the temporal and spatial nonlinear processes involved in pulse compression. Further, simple formulas are derived that give an estimate of the spatial distortions and of the self-phase modulation induced by Kerr nonlinearity. Finally, a parameter regime is identified in which self-focusing in bulk media is suppressed, resulting in a substantial improvement in the spatial beam quality of the compressed pulses.

Generation of intense 8-fs pulses at 400 nm.

Frequency-doubled pulses from a sub-40-fs, 1-kHz Ti:sapphire amplifier system are spectrally broadened in an argon-filled hollow waveguide. Compression of the self-phase-modulated pulses is implemented with chirped mirrors and a prism pair, yielding 8-fs, 15-muJ pulses in the violet spectral range.

Mirror-dispersion-controlled sub-10-fs optical parametric amplifier in the visible.

Pulses from an optical parametric amplifier in the visible are compressed to sub-10-fs duration by a delay line made exclusively from chirped dielectric mirrors. We employ what we believe are the first ultrabroadband visible chirped mirrors, which provide negative group-delay dispersion up to the blue-green spectral region. The setup is compact and reliable, and we used it to observe, for what is to our knowledge the first time in organic molecules, coherent oscillations with periods as short as 16 fs.

Theory of self-focusing in a hollow waveguide.

We present a theoretical investigation of self-focusing in a hollow waveguide filled with noble gas. Our analysis was performed for a laser pulse that was predominantly in the fundamental mode and revealed the physical processes involved in self-focusing in a hollow waveguide. A critical power for self-focusing was obtained that was found to be substantially higher than the critical power for self-focusing in a bulk medium. Useful design criteria for pulse-compression systems are presented. We identify the parameter range for which the transverse variation of the pulse phase introduced by the Kerr nonlinearity is small.

Continuous-wave mode-locked Ti:sapphire laser focusable to 5 x 10(13) W/cm(2).

Generation of sub-10-fs pulses with an average power of 1 W and a peak of 1.5 MW from a Kerr-lens mode-locked mirror-dispersion-controlled Ti:sapphire laser is demonstrated. A specially designed lens triplet focuses the output of this compact all-solid-state source to a peak intensity in excess of 5x10(13) W/cm (2) . Nonperturbative nonlinear optics is now becoming feasible by use of the output of a cw mode-locked laser.

Nonlinear source for the generation of high-energy few-cycle optical pulses.

We investigate the evolution of optical pulses in a hollow waveguide filled with noble gas at pulse intensities for which tunnel ionization dominates the nonlinear response of the gas. A numerical analysis reveals that the spectral chirp generated by the plasma nonlinearity is to a good approximation linear over the whole pulse spectrum and can be compensated in a dispersive delay line. Our calculations predict the generation of 3-4-fs optical pulses with energies of a few milijoules. To our knowledge, these energies are an order of magnitude greater than the pulse energies that have been realized to date in hollow-fiber compressors based exclusively on the nonlinear Kerr effect.

Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate.

A compact all-solid-state femtosecond Ti:sapphire oscillator¿amplifier system using no grating-based pulse stretcher produces 20-fs, 1.5-mJ pulses at a 1-kHz repetition rate. The pulses are subsequently compressed in a hollow-fiber chirped-mirror compressor. The system delivers bandwidth-limited 5-fs, 0.5-mJ pulses at 780 nm in a diffraction-limited beam.