M. Petrarca

Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator

Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin–momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.

Femtosecond timing-jitter between photo-cathode laser and ultra-short electron bunches by means of hybrid compression

The generation of ultra-short electron bunches with ultra-low timing-jitter relative to the photo-cathode (PC) laser has been experimentally proved for the first time at the SPARC_LAB test-facility (INFN-LNF, Frascati) exploiting a two-stage hybrid compression scheme. The first stage employs RF-based compression (velocity-bunching), which shortens the bunch and imprints an energy chirp on it. The second stage is performed in a non-isochronous dogleg line, where the compression is completed resulting in a final bunch duration below 90 fs (rms). At the same time, the beam arrival timing-jitter with respect to the PC laser has been measured to be lower than 20 fs (rms). The reported results have been validated with numerical simulations.

High-power third-harmonic flat pulse laser generation

The generation of a high-power laser pulse at 266 nm that is longitudinally shaped according to a prefixed intensity profile is reported. The main features of the pulse shape modifications due to second- and third-harmonic conversions are measured, and the results are in good agreement with the theory. The UV temporal shape depends on the chirp of the fundamental pulse and on the crystal phase-matching angle. Exploiting the large stretching imposed on the third-harmonic signal, we show that the pulse intensity profile can be obtained by spectral single-shot measurements.

Femtosecond dynamics of energetic electrons in high intensity laser-matter interactions.

Highly energetic electrons are generated at the early phases of the interaction of short-pulse high-intensity lasers with solid targets. These escaping particles are identified as the essential core of picosecond-scale phenomena such as laser-based acceleration, surface manipulation, generation of intense magnetic fields and electromagnetic pulses. Increasing the number of the escaping electrons facilitate the late time processes in all cases. Up to now only indirect evidences of these important forerunners have been recorded, thus no detailed study of the governing mechanisms was possible. Here we report, for the first time, direct time-dependent measurements of energetic electrons ejected from solid targets by the interaction with a short-pulse high-intensity laser. We measured electron bunches up to 7 nanocoulombs charge, picosecond duration and 12 megaelectronvolts energy. Our ’snapshots’ capture their evolution with an unprecedented temporal resolution, demonstrat- ing a significant boost in charge and energy of escaping electrons when increasing the geometrical target curvature. These results pave the way toward significant improvement in laser acceleration of ions using shaped targets allowing the future development of small scale laser-ion accelerators.

The Project Plasmonx for Plasma Acceleration Experiments and A Thomson X-Ray Source at SPARC

We present the status of the project PLASMONX, recently approved by INFN. This project, based on a collaboration between INFN and CNR-IPCF, aims at a long term upgrade of the SPARC system with the goal to develop at LNF an integrated facility for advanced beam-laser-plasma research in the field of advanced acceleration techniques and ultra-bright X-ray radiation sources and related applications. The project, in its first phase, foresees the development at LNF of a High Intensity Laser Laboratory (HILL) whose main component is a 100 TW-class Ti: Sa laser system synchronized to the SPARC photo-injector. Experiments of self-injection and acceleration of electrons into laser driven plasma waves will be conducted at HILL-LNF, early in this first project phase. Eventually an additional beam line will be built in the SPARC bunker in order to transport the SPARC electron beam at an interaction point, where a final focus system will allow to conduct experiments either of laser-beam co-propagation in plasma waves for high gradient acceleration, or experiments of laser-beam head-on collisions to develop a Thomson source of bright ultra-short X-ray radiation pulses, with X-ray energies tunable in the range 20 to 1000 keV and pulse duration from 30 fs to 20 ps. Preliminary simulations of plasma acceleration with self-injection are illustrated, as well as external injection of the SPARC electron beam.

Measurements of fluence profiles in femtosecond laser filaments in air

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our approach is based on the dependence of single-shot laser ablation threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser filaments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the external beam focusing. The limiting value of the peak intensity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f-number above ∼500), is about 55  TW/cm2.

Single-shot non-intercepting profile monitor of plasma-accelerated electron beams with nanometric resolution

An innovative, single-shot, non-intercepting monitor of the transverse profile of plasma-accelerated electron beams is presented, based on the simultaneous measurement of the electron energy and the betatron radiation spectra. The spatial resolution is shown to be down to few tens of nanometers, important for high-precision applications requiring fine shaping of beams and detailed characterizations of the electron transverse phase space at the exit of plasma accelerating structures.

Sub-picosecond snapshots of fast electrons from high intensity laser-matter interactions

The interaction of a high-intensity short-pulse laser with thin solid targets produces electron jets that escape the target and positively charge it, leading to the formation of the electrostatic potential that in turn governs the ion acceleration. The typical timescale of such phenomena is on the sub-picosecond level. Here we show, for the first time, temporally-resolved measurements of the first released electrons that escaped from the target, so-called fast electrons. Their total charge, energy and temporal profile are provided by means of a diagnostics based on Electro-Optical Sampling with temporal resolution below 100 fs.

Universal threshold for femtosecond laser ablation with oblique illumination

We quantify the dependence of the single-shot ablation threshold on the angle of incidence and polarization of a femtosecond laser beam, for three dissimilar solid-state materials: a metal, a dielectric and a semiconductor. Using the constant, linear value of the index of refraction, we calculate the laser fluence transmitted through the air-material interface at the point of ablation threshold. We show that, in spite of the highly nonlinear ionization dynamics involved in the ablation process, the so defined transmitted threshold fluence is universally independent of the angle of incidence and polarization of the laser beam for all three material types. We suggest that angular dependence of ablation threshold can be utilized for profiling fluence distributions in ultra-intense femtosecond laser beams.

Experimental results with the SPARC emittance-meter

The SPARC project foresees the realization of a high brightness photo-injector to produce a 150-200 MeV electron beam to drive a SASE-FEL in the visible light. As a first stage of the commissioning a complete characterization of the photoinjector has been accomplished with a detailed study of the emittance compensation process downstream the gun-solenoid system with a novel beam diagnostic device, called emittance meter. In this paper we report the results obtained so far including the first experimental observation of the double emittance minimum effect on which is based the optimised matching with the SPARC linac.