Time-frequency mapping of two-colour photoemission driven by harmonic radiation

The use of few-femtosecond, extreme ultraviolet (XUV) pulses, produced by high-order harmonic generation, in combination with few-femtosecond infrared (IR) pulses in pump-probe experiments has great potential to disclose ultrafast dynamics in molecules, nanostructures and solids. A crucial prerequisite is a reliable characterization of the temporal properties of the XUV and IR pulses. Several techniques have been developed. The majority of them applies phase reconstruction algorithms to a photoelectron spectrogram obtained by ionizing an atomic target in a pump-probe fashion. If the ionizing radiation is a single harmonic, all the information is encoded in a two-color two-photon signal called sideband (SB). In this work, we present a simplified model to interpret the time-frequency mapping of the SB signal and we show that the temporal dispersion of the pulses directly maps onto the shape of its spectrogram. Finally, we derive an analytical solution, which allows us to propose a novel procedure to estimate the second-order dispersion of the XUV and IR pulses in real time and with no need for iterative algorithms

Ultrafast Excited-State Decay Mechanisms of 6-Thioguanine Followed by Sub-20 fs UV Transient Absorption Spectroscopy

: Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment

Controlled Multibeam Supercontinuum Generation With a Spatial Light Modulator

We report on deterministic femtosecond multifilamentation in fused silica by encoding a diffractive microlens array into a spatial light modulator. The efficiency and focal length of each microlens are modified through the addressing voltage. This allows for a precise control on the energy coupled to the filaments thus obtaining a homogenized supercontinuum pattern from an inhomogeneous irradiance input distribution. Slight changes in the focal length of the microlenses allow for independent tailoring of the supercontinuum spectra

Frequency resolved wavefront retrieval and dynamics of diffractive focused ultrashort pulses

In this work we demonstrate the ability of the spatiotemporal characterization technique STARFISH to retrieve the wavelength dependent wavefront of focused ultrashort laser pulses. The high resolution achievable with this technique allows measuring the wavefront at the focal spot. In particular, the method is applied to study the effects of focusing with a kinoform diffractive lens. The evolution from converging to diverging wavefronts as the pulse propagates along the focal region is analyzed for each wavelength. The spatiotemporal intensity and spatially resolved spectrum structure of the pulses, as well as their profiles on axis, are also presented. Numerical simulations of the propagation of such pulses confirm the experimental results

Dynamic Control of Interference Effects Between Optical Filaments through Programmable Optical Phase Modulation

Light beams shaped by programmable megapixel spatial light modulators (SLMs) are key to broadening the applications of photonics. In this paper, we consider the application of a SLM for the generation of two mutually coherent white-light continuum optical sources by filamentation of infrared femtosecond pulses in bulk. We demonstrate that the inhomogeneity of the input beam and the longitudinal separation of the generated filaments are crucial parameters that break down the mutual coherence across neighboring filaments. We show that local control over the optical phase enables us to gain fine control over filament interference effects.This work was supported by Generalitat Valenciana through the programme (PROMETEO\2012\021), Spanish Ministry of Science under Grant FIS2013-40666-P) and University Jaume I through the project P1 1B2013-53

Diffractive optics for spectral control of the supercontinuum generated in sapphire with femtosecond pulses

We propose the use of kinoform diffractive lenses to focus near infrared femtosecond pulses in sapphire crystals for supercontinuum generation. It is shown that a strongly peaked structure appears in the blue region of the supercontinuum spectra. The central wavelength of this peak can be easily controlled by simply changing the lens-crystal distance. Moreover, when compared with the supercontinuum generated with a refractive lens in analogous conditions, the spectral extension of the so-generated continuum is larger. Our results were corroborated for sapphire plates with different thicknesses as well as in other transparent dielectrics such as fused silica.Support from Spanish Ministerio de Ciencia e Innovación (MICINN) through the Consolider Program SAUUL CSD2007-00013, research projects FIS2009-09522 and FIS2010-15746, and from Junta de Castilla y León through the Program for Groups of Excellence (GR27). CR and RBV acknowledge MICINN for support through grants BES-2007-17415 and AP2007-00202, respectively. GMV and OMY gratefully acknowledge partial financial support from Convenio UJI-Bancaixa under the project P1-1B2010-26.We also acknowledge support from the Centro de Láseres Pulsados (CLPU) (Salamanca, Spain

Frequency resolved wavefront retrieval and dynamics of diffractive focused ultrashort pulses

In this work we demonstrate the ability of the spatiotemporal characterization technique STARFISH to retrieve the wavelength dependent wavefront of focused ultrashort laser pulses. The high resolution achievable with this technique allows measuring the wavefront at the focal spot. In particular, the method is applied to study the effects of focusing with a kinoform diffractive lens. The evolution from converging to diverging wavefronts as the pulse propagates along the focal region is analyzed for each wavelength. The spatiotemporal intensity and spatially resolved spectrum structure of the pulses, as well as their profiles on axis, are also presented. Numerical simulations of the propagation of such pulses confirm the experimental results.We acknowledge support from Spanish Ministerio de Ciencia e Innovación (MICINN) through the Consolider Program SAUUL (CSD2007-00013), Research projects FIS2009-09522,FIS2010-15746 and P11B2010-26, and grant programs Formación de Profesorado Universitario (for B. Alonso and R. Borrego-Varillas) and Ramón y Cajal (for I. J. Sola); and from the Junta de Castilla y León through the Program for Groups of Excellence (GR27)

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, sub-20 fs broadband transient absorption spectroscopy in the near-UV are combined with CASPT2/MM decay path calculations to unravel the excited-state decay channels of water solvated 2-thio and 2,4-dithiouracil. These molecules feature linear absorption spectra with overlapping ππ* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ππ* absorbing state and via the intermediate nπ* dark state. Moreover, the 2,4-dithiouracil decay pathways is shown to be strongly correlated either to those of 2- or 4-thiouracil, depending on the sulfur atom on which the electronic transition localizes

Enhancement of filamentation postcompression by astigmatic focusing

The energy scaling up of pulse postcompression is still an open issue. In this work we analyze the use of astigmatic focusing to improve the output pulses in a filamentation based postcompression setup. Unlike spherical conditions, astigmatic focusing enhances the output energy and the spectral broadening of the filament. This is due to the increase of critical power, allowing a considerable improvement of the postcompression energy and stability in a simple way. We demonstrated compression from FWHM 100 fs, 10 nm, 3 mJ input pulses to 13 fs, 142 nm, near 1 mJ pulses.We acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the Consolider Program SAUUL (CSD2007-00013), Research project FIS2009- 09522, and grant programs Formación de Profesorado Universitario (for B. Alonso and R. Borrego-Varillas) and Ramón y Cajal (for I. J. Sola), and from the Junta de Castilla y León through the Program for Groups of Excellence (GR27). We also acknowledge support from the Centro de Láseres Pulsados, Salamanca, Spain

Spatial aberrations in high-order harmonic generation

We investigate the spatial characteristics of high-order harmonic radiation generated in argon, and observe cross-like patterns in the far field. An analytical model describing harmonics from an astigmatic driving beam reveals that these patterns result from the order and generation position dependent divergence of harmonics. Even small amounts of driving field astigmatism may result in cross-like patterns, coming from the superposition of individual harmonics with spatial profiles elongated in different directions. By correcting the aberrations using a deformable mirror, we show that fine-tuning the driving wavefront is essential for optimal spatial quality of the harmonics