Survey of spatio-temporal couplings throughout high-power ultrashort lasers.

The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems.

Spatio-temporal characterization of attosecond pulses from plasma mirrors.

Reaching light intensities above 1025 W/cm2 and up to the Schwinger limit of the order of 1029 W/cm2 would enable testing fundamental predictions of quantum electrodynamics. A promising - yet challenging - approach to achieve such extreme fields consists in reflecting a high-power femtosecond laser pulse off a curved relativistic mirror. This enhances the intensity of the reflected beam by simultaneously compressing it in time down to the attosecond range, and focusing it to sub-micrometre focal spots. Here we show that such curved relativistic mirrors can be produced when an ultra-intense laser pulse ionizes a solid target and creates a dense plasma that specularly reflects the incident light. This is evidenced by measuring the temporal and spatial effects induced on the reflected beam by this so-called 'plasma mirror'. The all-optical measurement technique demonstrated here will be instrumental for the use of relativistic plasma mirrors with the upcoming generation of Petawatt lasers that recently reached intensities of 5 × 1022 W/cm2, and therefore constitutes a viable experimental path to the Schwinger limit.

Spectrally resolved point-spread-function engineering using a complex medium.

Propagation of an ultrashort pulse of light through strongly scattering media generates an intricate spatio-spectral speckle that can be described by means of the multi-spectral transmission matrix (MSTM). In conjunction with a spatial light modulator, the MSTM enables the manipulation of the pulse leaving the medium; in particular focusing it at any desired spatial position and/or time. Here, we demonstrate how to engineer the point-spread-function of the focused beam both spatially and spectrally, from the measured MSTM. It consists of numerically filtering the spatial content at each wavelength of the matrix prior to focusing. We experimentally report on the versatility of the technique through several examples, in particular as an alternative to simultaneous spatial and temporal focusing, with potential applications in multiphoton microscopy.

Controlling the velocity of a femtosecond laser pulse using refractive lenses.

The combination of temporal chirp with a simple chromatic aberration known as longitudinal chromatism leads to extensive control over the velocity of laser intensity in the focal region of an ultrashort laser beam. We present the first implementation of this effect on a femtosecond laser. We demonstrate that by using a specially designed and characterized lens doublet to induce longitudinal chromatism, this velocity control can be implemented independent of the parameters of the focusing optic, thus allowing for great flexibility in experimental applications. Finally, we explain and demonstrate how this spatiotemporal phenomenon evolves when imaging the ultrashort pulse focus with a magnification different from unity.

Spatio-spectral metrology at focus of ultrashort lasers: a phase-retrieval approach.

The complete characterization of an ultrashort laser beam ultimately requires the determination of its spatio-temporal electric field E(x, y, t), or its spatio-spectral counterpart E(x, y, ω). We describe a new measurement technique called INSIGHT, which determines E(x, y, ω), up to an unknown spatially-homogeneous spectral phase. Combining this information with a temporal measurement at a single point of the beam then enables the determination of the spatio-temporal field E(x, y, t). This technique is based on the combination of spatially-resolved Fourier-transform spectroscopy with an alternate-projection phase-retrieval algorithm. It can be applied to any reproducible laser source with a repetition rate higher than about 0.1 Hz, relies on a very simple device, does not require any reference beam, and circumvents the difficulty associated with the manipulation of large beam diameters by working in the vicinity of the beam focus. We demonstrate INSIGHT on a 100 TW-25 fs laser, and use the measurement results to introduce new representations for the analysis of spatio-temporal/spectral couplings of ultrashort lasers.

High-harmonic generation from plasma mirrors at kilohertz repetition rate.

We report the first demonstration of high-harmonic generation from plasma mirrors at a 1 kHz repetition rate. Harmonics up to nineteenth order are generated at peak intensities close to 10¹8 W/cm² by focusing 1 mJ, 25 fs laser pulses down to 1.7 µm FWHM spot size without any prior wavefront correction onto a moving target. We minimize target surface motion with respect to the laser focus using online interferometry to ensure reproducible interaction conditions for every shot and record data at 1 kHz with unprecedented statistics. This allows us to unambiguously identify coherent wake emission as the main generation mechanism.

Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses.

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.