[en] The development from a field spectrometric experiment of the chlorophyll vegetation index (CVI), a broad-band 6. sensitive to leaf chlorophyll concentration at the canopy scale, and of its optimized version (OCVI) are described. A single correction factor is incorporated in the OCVI algorithm to take into account the different spectral behaviours due to crop and soil types, sensor spectral resolution and scene sun zenith angle. The sensitivity of different broad-band 6., including CVI, to leaf cholophyll concentration is compared for a wide range of soils and crops conditions and for different sun zenith angles by the analysis of a PROSPECT+SAILH syntetic dataset

[en] Complete text of publication follows. Since the first experimental demonstration of the generation of isolated attosecond pulses, the attosecond technology has become an important branch of ultrafast science. So far, the reported applications of isolated attosecond pulses have been limited by the low photon flux of the available sources. We demonstrate a technique for the generation of isolated attosecond pulses with energy up to 2.1 nJ. The key elements are: the use of few-optical-cycle driving pulses with stable carrier-envelope phase (CEP), linear polarization and peak intensity beyond the saturation intensity of the gas use for HHG; and the optimization of the interaction geometry in terms of gas pressure, position and thickness of the gas cell. We used 5-fs driving pulses with stable CEP to generate XUV radiation by HHG in a 2.5-mm-long cell filled with xenon at static pressure (2.5-3 torr) at a peak intensity I = (2.3±0.3) x 10¹⁵ W/cm². The XUV spectra display an evolution from a continuous behavior to a modulated one by changing the CEP value. The energy of the XUV pulses in the case of continuous spectra was 2.1 nJ, after a 100-nm-thick aluminium filter used to block the fundamental radiation and the low order harmonics. We have also used argon, krypton and neon as generating media: also in such cases clear transition between modulated and continuous XUV spectra were observed upon changing the CEP of the driving pulses. We have measured the temporal characteristics of the attosecond pulses by using the FROGCRAB method. Figure 1 a shows a portion of the FROGCRAB trace; in the reconstructed temporal intensity profile of the XUV pulses, the pulse duration was 155±5 as (the transform limit was ∼ 120 as). The physical mechanism at the basis of this method is related to the ionization dynamics in the generating medium. We used a nonadiabatic three-dimensional numerical model. In agreement with experimental results, the calculated XUV spectra display an evolution from a continuous behavior to a modulated one by changing the CEP value. We have then calculated the propagation of the XUV beam from the gas cell to the target position by taking into account the effects of the toroidal mirror and of a 200-μm-diameter pinhole used for spatial filtering of the XUV radiation. With a proper choice of the CEP, isolated attosecond pulses are generated, with a constant pulse duration across the transverse profile of the beam, thus demonstrating the excellent spatial characteristics of the generated attosecond pulses.

[en] Complete test of publication follows. Control of the carrier-envelope phase (CEP) of light pulses is one of the most advanced frontiers of ultrafast optics. The generation of optical waveforms with reproducible electric field profile is important in the case of few-optical-cycle pulses for which a CEP variation produces a strong change in the waveform. CEP-dependent phenomena are observable in high-intensity interactions, such as above-threshold ionization and high-order harmonic generation (HHG), moreover CEP stabilization is a fundamental prerequisite for the production of isolated attosecond pulses through HHG. In this work we propose a novel scheme for the generation of high energy ultrabroadband near-IR pulses with passive CEP stabilization, starting from an amplified Ti:sapphire laser system. A phase stable seed is generated by difference frequency generation (DFG) between the frequency components of a hollow-fiber-broadened supercontinuum. This configuration intrinsically provides a high CEP stability due to the absence of mechanical delays between the frequency components undergoing the DFG process. The seed is then boosted in energy through a multistage near-IR optical parametric amplifier (OPA), exploiting the broad gain bandwidths available around degeneracy. We generated ∼ 300 μJ phase-stable pulses at 1.6 μm and, after compensation of the negative dispersion by propagation through bulk media, we have obtained nearly transform-limited sub-25 fs pulse duration, which corresponds to less than 4 optical cycles at the considered carrier frequency. To verify that CEP-stabilization is preserved after the OPA stages and that amplified parametric superfluorescence is negligible, we used an f-to-2f interferometer. The appearance of a stable, high-contrast fringe pattern upon averaging is a clear proof of the shot-to-shot stability of the CEP due to the self-phase-stabilization mechanism. Once compressed, these pulses could be ideal drivers for HHG experiments; indeed taking into account that the ponderomotive energy of the recolliding electron scales as the square of the laser carrier wavelength, the use of near-IR pulses will enable the generation of higher order harmonics. Even more interesting is the possibility of using this system as a front-end for high energy OPA pumped by a 100 mJ, 10 hZ Ti:sapphire laser system, enabling multi-mJ-level self-phase-stabilized pulses.

[en] Complete test of publication follows. In the last few years the field of attosecond science has shown impressive and rapid progress, mainly due to the introduction of novel experimental methods for the characterization of extreme ultraviolet (XUV) pulses and attosecond electron wave packets. This development has been also triggered by significant improvements in the control of the electric field of the driving infrared pulses. Particularly interesting for the applications is the generation of isolated attosecond XUV pulses using few-cycle driving pulses. In this case significant progresses have been achieved thanks to the stabilization of the carrier-envelope phase (CEP) of amplified light pulses. In this work we demonstrate that the polarization gating (PG) method with few-cycle phase-stabilized driving pulses allows one to generate few-cycle isolated attosecond pulses tunable on a very broad spectral region. The PG method is based on temporal modulation of the ellipticity of a light pulse, which confines the XUV emission in the temporal gate where the polarization is close to linear. The time-dependent polarization of phase-stabilized sub-6-fs pulses, generated by the hollow fiber technique, has been obtained using two birefringent plates. It is possible to create a linear polarization gate, whose position is imposed by the intensity profile of the pulse whilst the emission time is linked to the CEP of the electric field. The pulses have been analyzed by using a flat-field spectrometer. Continuous XUV spectra, corresponding to the production of isolated attosecond pulses, have been generated for particular CEP values. Upon changing the rotation of the first plate it was possible to tune the XUV emission in a broad spectra range. We have then achieved a complete temporal characterization of the generated isolated attosecond pulses using frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG CRAB). The measured parabolic phase indicates the presence of a predominant second order dispersion (positive chirp), which is intrinsic to the XUV generation process. As recently demonstrated in the case of trains of attosecond pulses, the positive chirp of the radiation produced by high-order harmonic generation can be compensated for by the negative group delay dispersion of thin aluminum foils. Upon increasing the thickness of an aluminum plate we have obtained XUV pulses with duration shorter than 300 as (at 37 eV), thus corresponding to few cycles of the electric field.

[en] We demonstrate the generation of broadband extreme ultraviolet (XUV) continua by high order harmonic generation in neon using carrier-envelope phase (CEP) stabilized, few-cycle pulses with a time-dependent polarization. The XUV spectra can support isolated attosecond pulses with a duration of about 50 as, opening the possibility of achieving time resolution close to the atomic unit of time. A theoretical model that calculates the spectral and temporal characteristics of the XUV radiation emitted by a single atom is used to interpret the CEP dependence of the harmonic structures.

[en] The development of novel tools for the generation of atto second light pulses is continuously triggering the introduction of new spectroscopic and measurement methods, which will offer the opportunity to investigate unexplored research areas with unprecedented time resolution. In this work we will report on recent advances in atto second science, with particular emphasis on the generation of isolated atto second pulses produced by laser-driven high-order harmonic generation in gases. We will consider several approaches to atto second metrology and we will present some applications of atto second pulses. Future perspectives in atto second technology will then be discussed.

[en] We report on our experimental and theoretical investigations on the generation of high-order harmonics driven by 1500 nm few-cycle laser pulses in xenon. In contrast to the common belief, we found experimental evidence suggesting that harmonic generation driven by mid-infrared laser pulses can be realized with high efficiency; in particular, an enhancement of very high harmonic orders can be achieved under suitable conditions of the laser-medium interaction. The experimental results were simulated by a 3D non-adiabatic model. The theoretical outcomes confirm the experimental findings and provide a physical explanation for the counter-intuitive results. In particular, a time-dependent phase-matching analysis threw light on the generation mechanisms at a timescale of half optical cycle of the fundamental pulse.

[en] In this review we report on recent advances in laser technology, which have contributed to the fast development of attosecond science. In particular we will concentrate on two experimental methods for the generation of high-peak-power, few-optical-cycle laser pulses with controlled electric field, which are crucial for the generation of isolated attosecond pulses. The first method is the hollow-fiber compression technique, introduced in 1996 and now routinely used in several laboratories. So far, isolated attosecond pulses have been generated by using few-cycle pulses produced by such compression technique, in combination with active stabilization of the carrier-envelope phase. More recently, few-cycle pulses tunable in the infrared region have been generated by optical parametric amplification with passive stabilization of the carrier-envelope phase. Such parametric sources represent excellent drivers for the generation of harmonic radiation with an extended cutoff, and offer the possibility to extend attosecond science towards the soft-X rays region. Finally, we will briefly discuss the basic elements of attosecond metrology

[en] The intensification and the duration shortening of the femtosecond laser pulses are responsible for the following: when the focused laser radiation interacts with the substance the generation of fast electrons, ions and various electromagnetic noises increases abruptly. It affects the recording of the X-ray spectra by the below-listed equipment: charge-coupled device (CCD) base detectors and photomultipliers. One studied a new mean-median algorithm enabling to increase essentially the signal/noise ratio in the X-ray spectra of the femtosecond plasma when using a CCD-detector to record them

[en] High-order harmonics of ultrashort laser pulses appear in the time domain as a sequence of attosecond bursts of coherent extreme ultraviolet radiation separated by half the optical cycle of the laser field. In order to confine this emission to an isolated attosecond pulse, suitable gating techniques have been proposed and investigated. In this work, we review a few gating techniques used in our laboratories, along with the basic tools for the generation and application of attosecond pulses. (invited review)