Search

We demonstrate the spatial separation of a considerable portion of the XUV from the fundamental IR driving beam in high-order harmonic generation by an integrated system of microchannels realized through Femtosecond Laser Micromachining. © 2020 The Author(s)

The fine-tuning of the growth conditions and post-deposition treatments is of fundamental importance to improve the efficiency of photovoltaic devices based on all-inorganic metal halide perovskites like CsPbBr3. In this work, we used time-resolved terahertz spectroscopy (TRTS) in combination with optical characterization techniques, x-ray diffraction (XRD), and scanning electron microscopy, to probe the different properties induced by a low-temperature (180 °C) annealing treatment on evaporated CsPbBr3 thin-films. We observed a faster build-up and relaxation dynamics in the annealed sample, accompanied by a remarkable decrease of the photoluminescence intensity and minor changes in the photoconductivity and XRD measurements as compared to the as-deposited sample. We estimated for both the samples a mobility of μ = 1.7 ± 0.5 × 10 2 cm2 V−1 s−1. Our results suggest that the lattice reorganization induced by low-temperature annealing of evaporated CsPbBr3 could lead to a different charge carrier-phonon coupling and to an increased contribution of non-radiative recombination channels. We found that TRTS can be effectively used to follow the changes induced by post-deposition thermal annealing of CsPbBr3.

We describe the implementation of nonlinear time-domain spectroscopy of rotovibrational IR-active modes in methane through broadband Four-Wave Mixing driven by resonant high-energy mid infrared laser pulses. At high driving pulse intensities we observe an efficient vibrational ladder climbing triggered in the molecules. This study opens the possibility to impulsively and selectively excite molecules of biological interest to high-lying vibrational states and to characterize their dynamics.

We report on the application of femtosecond laser micromachining to the fabrication of complex glass microdevices, for high-order harmonic generation in gas. The three-dimensional capabilities and extreme flexibility of femtosecond laser micromachining allow us to achieve accurate control of gas density inside the micrometer interaction channel. This device gives a considerable increase in harmonics&rsquo, generation efficiency if compared with traditional harmonic generation in gas jets. We propose different chip geometries that allow the control of the gas density and driving field intensity inside the interaction channel to achieve quasi phase-matching conditions in the harmonic generation process. We believe that these glass micro-devices will pave the way to future downscaling of high-order harmonic generation beamlines.

We report efficient high harmonic generation and control in glass devices fabricated by femtosecond laser micromachining. This work paves the way to microfluidic HHG beamlines for X-ray spectroscopy with attosecond resolution.

We exploited ultrafast THz spectroscopy to explore the carrier dynamics in 2D perovskite and estimate the transient dielectric properties. The conductivity spectra show the presence of polaronic carriers diluted in an excitonic bath.

Synopsis We present a high-energy mid-infrared optical parametric amplifier source tunable between 2.8 and 3.8 μm for strong-field applications. We exploited this source in combination with a 800 nm source for generating high-order harmonics in atoms and molecules in a two-color pulse scheme.

We report on the efficient excitation of high-lying rovibrational states in methane by a high-energy mid-IR source and on the time-domain study of the induced dynamics.

We investigate high-order harmonic generation (HHG) in ZnTe crystals driven by a mid-IR optical parametric amplifier as a function of the angle between the polarization direction of the driving laser field and the crystal axis.

We measured the chiral relaxation of photoexcited Fenchone at the Carbon K-edge. Our results demonstrate that ultrafast chiral dynamics can be probed using core level spectroscopy with circularly polarized free-electron laser pulses.

We generated high-order harmonics in ZnTe crystal driven by a mid-IR optical parametric amplifier. The dependence of the harmonic signal as a function of the crystal orientation is investigated and interpreted within the re-collision framework.

We demonstrate efficient high-order harmonic generation in fused-silica chips fabricated by femtosecond laser micromachining. This work provides a route toward the miniaturization of HHG beamlines and the implementation of X-ray spectroscopy with attosecond temporal resolution.

We report efficient high harmonic generation and control in glass chips fabricated by femtosecond laser micromachining. This work paves the way for the implementation of microfluidic HHG beamlines for X-ray spectroscopy with attosecond resolution.

We demonstrate efficient high-order harmonic generation in fused-silica chips fabricated by femtosecond laser micromachining. This work provides a route toward the miniaturization of HHG beamlines and the implementation of X-ray spectroscopy with attosecond temporal resolution.

Although Gallium Arsenide (GaAs) is one of the most studied semiconductors, its hot carrier and hot phonon dynamics are still debated. Here, we report two-phonon out-of-equilibrium absorption processes in the transient THz response of GaAs.

We present a robust, three-stage optical parametric amplifier driven by a Ti:Sapphire ultrafast laser system that implements passive carrier-envelope phase stabilization and directly generates five-cycle mid-IR pulses by dispersion compensation. The source is based on potassium titanyl arsenate crystals and exploits intra-pulse difference-frequency seed generation in the mid-IR. This source will be particularly suited for applications in strong-field physics, such as high order harmonic generation and photoelectron spectroscopy.

We report on the implementation of time-domain spectroscopy through broadband Four-Wave Mixing in the study of roto-vibrational excitation of hydrocarbons by resonant high-energy mid-IR pulses.

One way to increase the well known low efficiency of high order harmonics generation is to place the gas medium in a hollow core waveguide. The numerical model used to obtain driving and harmonics field configuration along and across the waveguide was developed for an arbitrary gas density profile and arbitrary fiber diameter modulation. The model was tested against experimental measurements and excellent agreement was obtained for the fluorescence emission along the waveguide. We analyse the influence of the diameter modulation which result from the fabrication process on both pulse propagation and on harmonic generation.

We study the nature of photo-excited charge carriers in CsPbBr3 nanocrystal thin films by ultrafast optical pump - THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

Since the first observation of High-order Harmonic Generation (HHG) in gas twenty years ago, the combination of intense research together with technological developments, has led to impressive progress in the field of extreme ultraviolet spectroscopy and attosecond science. Beam lines based on HHG extend over several meters and are based on instrumentation that requires careful alignment and even active stabilization systems. Therefore, a miniaturization of HHG beams will reduce the cost of these light sources and pave the way to their application in numerous new fields. Femtosecond laser micromachining followed by chemical etching (FLICE) has already demonstrated its high potential in the fabrication of fused silica lab-on-a-chip devices; it can directly produce microfluidic networks in a 3D geometry directly buried in the glass substrate. Until now, they have been extensively used for the manipulation of fluids but they are perfectly suitable for the manipulation of gas as well. In this work, we will demonstrate HHG in a gas filled microchannel network fabricated by the FLICE technique. The device structure will be based on hollow waveguides: several inlets will deliver the gas into a central hollow waveguide where the ultrafast laser will be coupled and HHG will take place. The high versatility of the FLICE technique will allow us to fabricate devices with modulated gas concentration and waveguide profile to achieve quasi-phase-matching conditions. Moreover, we will also demonstrate an integrated filtering chip that will allow to geometrically separate the main laser radiation from the XUV generated beam.

We unveil the large polaron fingerprints in the transient THz dielectric response of lead-halide perovskites. We clarify the mechanism underlying the physics of charge transport of full-inorganic lead-halide perovskites by combining ultrafast Thz spectroscopy with DFT calculations.

Generation of few-femtosecond (fs) pulses in the ultraviolet (UV) is important in ultrafast spectroscopy since several bio-relevant molecules, including DNA bases, possess electronic transitions in this spectral region [1]. Exploiting nonlinear processes driven by intense laser pulses has led to several advances to meet this challenge. We developed a source delivering 1.9 fs (2.2 cycles at 255 nm) deep-UV pulses with 150 nJ of energy on target [2].

We exploit high-order harmonic generation spectroscopy at the caustics of the recombining electron wave-packet as a method for directly comparing experimental spectra with ab-initio theories. Experimental results in xenon and comparison with ab-initio time-dependent configuration-interaction singles calculations allowed to assess the role of the wave-packet enhancement during the giant resonance. Results in argon show that this technique can also be applied to other targets.

The XUV field emitted by impulsively aligned ethylene molecules during high-order harmonic generation is retrieved as a function of molecular orientation. The results can be ascribed to multielectron contributions to the harmonic emission.

We present a tunable, Ti:Sa-pumped mid-IR OPA with CEP-stable, few-cycle output. The pulses are compressed in bulk, resulting in 56.5-fs, 300-µJ pulses at 3.4 micron with 308 mrad CEP noise.

Since the first observation of High-order Harmonic Generation (HHG) in gas twenty years ago, the combination of intense research together with technological developments, has led to impressive progress in the field of extreme ultraviolet spectroscopy and attosecond science. Beam lines based on HHG extend over several meters and are based on instrumentation that requires careful alignment and even active stabilization systems. Therefore, a miniaturization of HHG beams will reduce the cost of these light sources and pave the way to their application in numerous new fields. Femtosecond laser micromachining followed by chemical etching (FLICE) has already demonstrated its high potential in the fabrication of fused silica lab-on-a-chip devices; it can directly produce microfluidic networks in a 3D geometry directly buried in the glass substrate. Until now, they have been extensively used for the manipulation of fluids but they are perfectly suitable for the manipulation of gas as well. In this work, we will demonstrate HHG in a gas filled microchannel network fabricated by the FLICE technique. The device structure will be based on hollow waveguides: several inlets will deliver the gas into a central hollow waveguide where the ultrafast laser will be coupled and HHG will take place. The high versatility of the FLICE technique will allow us to fabricate devices with modulated gas concentration and waveguide profile to achieve quasi-phase-matching conditions. Moreover, we will also demonstrate an integrated filtering chip that will allow to geometrically separate the main laser radiation from the XUV generated beam.

We demonstrate the generation of high-order harmonics in a fused-silica device fabricated through femtosecond laser micromachining. This achievement paves the way for the miniaturization of HHG applications from large-scale laboratories to microstructures.

We generate attosecond pulse train (APT) in argon driven by the high repetition rate (HR) laser of the extreme light infrastructure-attosecond light pulse source (ELI-ALPS), providing 100 kHz, 80 W, 1030 nm, 40 fs pulses from a fiber chirped-pulse amplification (fiber-CPA) laser system. Under the current operating conditions of the high harmonic generation beamline (HR-GHHG), we observed the average pulse duration to be 395 as measured using the technique of reconstruction of attosecond beating by interference of two-photon transitions. The beamline uses an annular-shape laser beam so that the main part of the driving laser co-propagating with the APT can be eliminated by reflection on a holey mirror. An additional 100 nm aluminum foil is used to filter out the remaining laser and the low order harmonics, allowing 2 pJ APT with a bandwidth from 25 eV to 50 eV to be transported to the target position where the APT interacts with matter. The implementation of the HR-GHHG beamline in ELI-ALPS delivering attosecond pulse trains at 100 kHz paves the way for time-resolved experiments in the infrastructure, especially those that involve rare events and coincidence analysis, both of which need high statistics.

The use of fast closed-loop adaptive optics has improved the performance of optical systems since its first application. Here we demonstrate the amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier devoted to Attosecond Science exploiting two high speed adaptive optical systems for the correction of static and dynamic instabilities. The exploitation of multi actuator adaptive lenses allowed for a minimal impact on the optical setup.

High-order harmonic generation is a nonlinear effect appearing when atoms or molecules are exposed to intense laser pulses. It consists in the emission of ultrashort bursts of extreme ultraviolet and soft X radiation, giving rise to harmonics in the spectral domain. This emission encodes a wealth of information about the intimate structure of the targeted species, allowing the reconstruction of the outermost distribution of electrons. We will review here the most important aspects of high-order harmonic generation and we will show how this process can be applied to the reconstruction of molecular orbitals. A critical analysis will illustrate benefits, weaknesses and perspectives of this technique.

We demonstrate the generation of high-order harmonics in a fused-silica chip produced by femtosecond laser micromachining. This achievement paves the way to the miniaturization of HHG applications from large-scale laboratories to microdevices.

We report on the first application of adaptive deformable lenses to the stabilization of a high-energy mid-IR parametric source. Fluctuations of output pulse intensity and CEP were significantly suppressed by the adaptive optics system.

Adaptive Optics (AO) is a key technology for ground-based astronomical telescopes, allowing to overcome the limits imposed by atmospheric turbulence and obtain high resolution images. This technique however, has not been developed for small size telescopes, because of its high cost and complexity. We realized an AO system based on a Multi-actuator Adaptive Lens and a Shack-Hartmann wavefront sensor (WFS), allowing for a great compactness and simplification of the optical design. The system was integrated on a 11” telescope and controlled by a consumer-grade laptop allowing to perform Closed-Loop AO correction up to 400 Hz.

Spectral focusing of the recolliding electron in high-order harmonic generation driven by two-color fields is shown to be a powerful tool for isolating and enhancing hidden spectral features of the target under study. In previous works we used this technique for probing multi-electron effects in xenon and we compared our experimental results with time-dependent configuration-interaction singles calculations. We demonstrate here that this technique can be exploited for reconstructing the enhancement factor of the xenon giant dipole resonance and we discuss the sensitivity of this method to macroscopic effects. We then extend the technique to argon in order to test the applicability of this procedure to other targets.

We demonstrate amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier designed for Attosecond Science exploiting two high speed adaptive optical systems based on multi actuator adaptive lenses.

The availability of tunable mid-infrared laser systems capable to deliver high intensity few cycle pulses offers the perspective of generating ultrashort radiation in the extreme UV range through the high-order harmonic generation (HHG) of the fundamental. In particular, such systems make possible the superposition of multiple colors of incommensurable frequencies that gives rise to time gating of HHG, thereby realizing the necessary condition leading to isolated attosecond pulse (IAP) generation. In this paper, the HHG resulting from the superposition of two incommensurable frequencies is analyzed with the aim to find out the conditions that the relative intensity and frequency of the two fields have to fulfill in order to generate isolated attosecond pulses. It is observed that the two fields can be superimposed in parallel and perpendicular polarization, both giving rise to IAP under appropriate conditions.

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of π and π* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. Here we elucidate the role of the metallic support in determining the physical properties of the Si/Ag interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out contributions arising from different localization. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.

In this paper, we numerically investigate the possibility to obtain a lighthouse emission for the attosecond pulses produced by high-order harmonics of a strong mid-infrared fundamental pulse without any optical element inserted in the path of the generating beam. The parameters of the driving pulse, focusing geometry, gas medium and detection configuration are currently experimentally feasible. Here, we study in detail the specific propagation conditions of the laser beam, and describe the exact mechanism of the sensitive space-time variation of the medium's refractive index that lead to the dynamic wavefront rotation. This basic requirement for the lighthouse phenomenon is transmitted to the harmonic bursts, which are emitted with different divergence in successive optical half-cycles, thus can be detected in the far field at increasing distances from the optical axis. In this configuration, spectral filtering of the harmonics is not necessary, therefore the total harmonic pulse power might be used in further pump-probe experiments.

Intense ultrafast laser sources in the mid-IR have attracted in the recent years lot of attention in strong-field physics. A possible implementation for such sources relies in: a) the generation of a short IR seed by intra-pulse difference frequency generation (DFG) of a broadband continuum generated in an hollow-core fiber (HCF); b) optical parametric amplification (OPA) of the seed to the mJ level [1]. This approach yields a major advantage, namely the passive stabilization of the carrier-envelope phase (CEP) owing to the DFG process, enabling applications like the generation of isolated attosecond pulses in the soft X-rays. Nevertheless the CEP stability performances are limited with respect to those achievable with active stabilization techniques. Moreover, OPA setups require a very good beam quality as well as a robust stability of the pumping source and this can hinder the scalability towards higher pulse energies.

Over the last few decades, the investigation of ultrafast phenomena occurring in atoms, molecules and solid-state systems under a strong-field regime of light-matter interaction has attracted great attention. The increasing request for a suitable optical technology is significantly boosting the development of powerful ultrafast laser sources. In this framework, Optical Parametric Amplification (OPA) is currently becoming a leading solution for applications in high-power ultra-broadband light burst generation. The main advantage provided by the OPA scheme consists of the possibility of exploring spectral ranges that are inaccessible by other laser technologies, as the InfraRed (IR) window. In this paper, we will give an overview on recent progress in the development of high-power few-optical-cycle parametric amplifiers in the near-IR and in the mid-IR spectral domain. In particular, the design of the most advanced OPA implementations is provided, containing a discussion on the key technical aspects. In addition, a review on their application to the study of strong-field ultrafast physical processes is reported.