Diagnostic of Laser-Plasmas: Single-shot Supercontinuum Spectral Interferometry

Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interactions, including those observed in the high intensity ultrashort laser pulse regime. We developed a new femtosecond optical diagnostic, single-shot supercontinuum spectral interferometry (SSSI), which measures ultra-rapid transients in the complex index of refraction induced by an intense laser pulse. This measurement provides a direct view on how the laserproduced perturbations evolve in time and space. To date, SSSI has been successfully used to diagnose femtosecond dynamics in the interaction of intense laser pulses with gases, nanometersized atomic or molecular clusters, and plasmas, including plasma waveguides. † Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. ‡ Present address: Naval Research Laboratory, Washington, D. C. 20375, USA. INTRODUCTION Frequency domain or spectral interferometry (SI) has been employed in many experiments to measure refractive index transients. These include measurements of self-phase modulation in optical fibers, induced phase modulation in solids, time evolution of femtosecond laser-plasmas [1], and laser-driven plasma wakefields [2]. In this technique, a reference pulse and a time delayed probe pulse, upon which a pumppulse-induced phase has been imposed, interfere in the frequency domain when combined in a spectrometer [1,2]. Recently, single-shot spectral interferometry (SSI) for the measurement of transient refractive index variations was realized by linearly chirping the reference and probe beams so that each temporal slice of the refractive index variation was projected onto a different frequency component [3,4]. The temporal phase variation was then obtained by applying a direct map from frequency to time. However, aside from resolution limits imposed by the spectrometer, this method has a fundamental limitation determined by the spectral bandwidth and the degree of chirp. Here, we demonstrate a SSI diagnostic that uses chirped supercontinuum (SC) pulses generated in air. We call this technique single-shot supercontinuum spectral interferometry (SSSI) diagnostic [5]. Using SSSI, we can achieve single-shot temporal resolution (~10 fs) up to an order of magnitude better than in previous work [3,4,6].