M. Pascolini

Cluster effects in high-order harmonics generated by ultrashort light pulses

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

Optimization of high-order harmonic generation by adaptive control of a sub-10-fs pulse wave front.

We present a method for the optimization of high-order harmonic generation based on wave-front correction of the driving laser beam. The technique exploits wave-front adaptive control by means of a deformable mirror, governed by an optimization procedure.

Instrumentation for analysis and utilization of extreme-ultraviolet and soft x-ray high-order harmonics

The design and the performances of an extreme ultraviolet (EUV)spectrometer/monochromator for generation and diagnostics of high-order harmonics of an ultrashort (<30 fs) pulsed laser focused onto a gas jet are presented. The harmonic generation is optimized using an adaptive mirror before the laser focusing stage. A toroidal mirror is used to focus the XUV radiation in an intermediate stage for pump/probe experiments. A grazing-incidence flat-field spectrometer for the 5–75 nm spectral region has been designed: it adopts a stigmatic toroidal mirror and a varied line-space flat grating mounted in converging light. The almost flat stigmatic spectrum is acquired by a 40-mm-diameter microchannel plate intensifier; the whole detector can be moved to scan various portions of the spectrum. Different detectors allow one to acquire both integrated and single shot spectra at 1 kHz repetition rate. The absolute calibration of the spectrometer is provided. The same optical scheme can be applied to the design of a co...

Efficiency of gratings in the conical diffraction mounting for an EUV time-compensated monochromator

Recent measurements of the absolute diffraction efficiency of plane gratings in the conical diffraction mounting (in which the light approaches the grating in the plane parallel to the direction of the grooves) are presented. Three gratings have been tested at the beamline BEAR (Elettra Synchrotron, Trieste) in the 10-130 nm region, showing a peak efficiency as high as 70%. The aim of these measurements is the use of two gratings in the conical diffraction mounting for the realization of a high-throughput time-compensated monochromator for the spectral selection of high-order harmonic radiation produced by the interaction between an ultrashort laser pulse (less than 100 fs) and a gas jet. The monochromatic and ultrashort pulse at the monochromator exit can be used for the injection of a Free Electron Laser. The theory of the time-compensation with gratings will be briefly resumed, the design of the monochromator will be presented, and the results of the measurements at BEAR will be discussed.

Space applications of Si/B4C multilayer coatings at extreme ultra-violet region; comparison with standard Mo/Si coatings

In the extreme ultra-violet region, multilayer coatings are the only technique to obtain high reflectivity in normal incidence optical configurations. The interference process which regulates periodic multilayers behavior offers narrow-band spectral filtering without the use of additional filters, fact that makes these coatings particularly suitable for lines emission observations. Despite the large amount of possible materials combinations, Mo/Si multilayers are the standard choice for space research on plasma physics in the 13 - 30 nm spectral region. In this work Si/B4C is presented as an alternative material couple for the 30.4 nm selection. Attractive features are the better spectral purity and the second order reflectivity reduction. A possible application to the Sounding CORonagraph Experiment is described as an example. B4C thin films have been used to characterize this material in terms of optical constants in the 40 nm - 150 nm spectral region where, currently, only few data are available.

Imaging of recombination events in high-order harmonic generation by phase-stabilized few-optical-cycle pulses

A high-order harmonic generation process using carrier-envelope phase stabilized few-optical-cycle light pulses is investigated. A time–frequency analysis of generated spectra is used to retrieve direct information about the temporal structure of the generated extreme ultraviolet radiation.

Adaptive wavefront control based on genetic algorithm for the enhancement of high-order harmonic generation driven by two-cycle laser pulses

In this work we report on the optimization of the conversion efficiency of the harmonic generation process, by adaptive control of the wavefront of sub-10-fs light pulses, obtained by using a deformable mirror and a genetic algorithm. Sub-10-fs, 0.2-mJ energy light pulses, generated by the hollow-fiber compression technique, were focused in the gas target (argon or neon) by a 250-mm focal-length mirror. Pulse wavefront correction has been achieved by using a deformable mirror (DM) controlled by 37 actuators distributed on a honeycomb pattern of 15 mm diameter. The harmonic radiation was observed by a soft-X-ray spectrometer, with double output: time-integrated high-resolution bidimensional focal-plane image and real-time (1 kHz) intensity of a suitable spectral region. This latter signal was used as fitness parameter for the genetic algorithm; an initial population of DM configurations was initialized with random values of the actuator signals. A new generation of DM configurations is derived from ordering, selection and transformation of previous generation, up to the convergence to the fittest individual. Strong enhancement of the harmonic conversion efficiency of about one order of magnitude, as well as a significant extension of the harmonic spectrum is evident. The initial and optimal wavefronts of the fundamental beam were measured both in real time with an Hartmann sensor and off-line using a ZYGO interferometer. Using the measured beam wavefront were calculated the spatial characteristics of the fundamental beam.

Carrier envelope phase effects on electron quantum paths in high harmonics generation by few-cycle pulses

High-order harmonic spectra generated by few-optical-cycle pulses have been acquired on a single-shot basis. We show that the differences in the generated spectra are related to variations in the electron-quantum-paths contributing to the emission process.

High-order harmonic generation in alkanes: signature of the ground state wavefunction

In this work, XUV spectra from some alkanes (methane, I/sub p/ = 12.97 eV; propane, I/sub p/ = 11.07 eV; butane, I/sub p/ = 10.63 eV) are measured and compared with those acquired from xenon (I/sub p/ = 12.13 eV). The harmonic emission is obtained focusing femtosecond light pulses on a pulsed gas jet. Two different pulse durations are considered, namely 20 fs and 6 fs; in both cases the pulse energy could be continuously tuned without modification in temporal and spatial structure of the driving pulses. The experimental results have been compared to numerical simulation based on an atom-like Lewenstein model. Preliminary results reproduce the cutoff wavelength behavior of the investigated molecular species.

As carrier-envelope phase contributes to high-order harmonic phase: effects on the electric field of attosecond pulses

In this work we report on a self-referencing technique for the measurement of the effect of the carrier-envelope phase (CEP) of the driving pulses on the phase difference between consecutive harmonics. By using a numerical model based on the nonadiabatic saddle-point method we demonstrate that, in particular experimental conditions, it is possible to directly control the electric field of the attosecond pulses by controlling the electric field of the driving pulses. In conclusion, using a self-referencing technique, we have experimentally demonstrated that the harmonic phase is directly affected by the CEP of the driving pulses