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Ultrafast spectroscopy techniques use sequences of ultrashort light pulses (with femto- to attosecond durations) to study photoinduced dynamical processes in atoms, molecules, nanostructures, and solids. This field of research has experienced an impetuous growth in recent years, due to the technological progress in the generation of ultrashort light pulses and to the development of sophisticated spectroscopic techniques, which greatly increase the amount of information on the process under study. This paper aims at providing a non-exhaustive overview of the state of the art of the field and at pointing out future challenges. We first review the progress in ultrafast optics, which has enabled the generation of broadly tunable light pulses with duration down to a few optical cycles; we then discuss the pump-probe technique, showing examples of its capability to combine very high time resolution, down to the attosecond regime, with broad spectral coverage; we introduce two-dimensional spectroscopy and present results that demonstrate the additional information content provided by the combination of temporal and spectral resolution. Next, we review the achievements of ultrafast X-ray and electron diffraction, which provide time-dependent structural information on photochemical processes, and we conclude with a critical analysis of the future open challenges in the field.

Opsins are a broad class of photoactive proteins, found in all classes of living beings from bacteria to higher animals, which work either as light-driven ion pumps or as visual pigments. The photoactive function in opsins is triggered by the ultrafast isomerization of the retinal chromophore around a specific carbon double bond, leading to a highly distorted, spectrally red-shifted photoproduct. Understanding, by either experimental or computational methods, the time course of this photoisomerization process is of utmost importance, both for its biological significance and because opsin proteins are the blueprint for molecular photoswitches. This paper focuses on the ultrafast 11-cis to all-trans isomerization in visual rhodopsins, and has a twofold goal: (i) to review the most recent experimental and computational efforts aimed at exposing the very early phases of photoconversion; and (ii) discuss future advanced experiments and calculations that will allow an even deeper understanding of the process. We present high time resolution pump-probe data, enabling us to follow the wavepacket motion through the conical intersection connecting excited and ground states, as well as femtosecond stimulated Raman scattering data allowing us to track the subsequent structural evolution until the first stable all-trans photoproduct is reached. We conclude by introducing computational results for two-dimensional electronic spectroscopy, which has the potential to provide even greater detail on the evolution of the electronic structure of retinal during the photoisomerization process.

Time-resolved two-dimensional (2D) electronic spectra (ES) tracking the evolution of the excited state manifolds of the retinal chromophore have been simulated along the photoisomerization pathway in bovine rhodopsin, using a state-of-the-art hybrid QM/MM approach based on multiconfigurational methods. Simulations of broadband 2D spectra provide a useful picture of the overall detectable 2D signals from the near-infrared (NIR) to the near-ultraviolet (UV). Evolution of the stimulated emission (SE) and excited state absorption (ESA) 2D signals indicates that the S1 → SN (with N ≥ 2) ESAs feature a substantial blue-shift only after bond inversion and partial rotation along the cis → trans isomerization angle, while the SE rapidly red-shifts during the photoinduced skeletal relaxation of the polyene chain. Different combinations of pulse frequencies are proposed in order to follow the evolution of specific ESA signals. These include a two-color 2DVis/NIR setup especially suited for tracking the evolution of the S1 → S2 transitions that can be used to discriminate between different photochemical mechanisms of retinal photoisomerization as a function of the environment. The reported results are consistent with the available time-resolved pump-probe experimental data, and may be used for the design of more elaborate transient 2D electronic spectroscopy techniques.

This article is devoted to the exploration of the benefits of a new ultrafast confocal pump-probe technique, able to study the photophysics of different structured materials with nanoscale resolution. This tool offers many advantages over standard stationary microscopy techniques because it directly interrogates excited state dynamics in molecules, providing access to both radiative and non-radiative deactivation processes at a local scale. In this paper we present a few different examples of its application to organic semiconductor systems. The first two are focussed on the study of the photophysics of phase-separated polymer blends: (i) a blue-emitting polyfluorene (PFO) in an inert matrix of PMMA and (ii) an electron donor polythiophene (P3HT) mixed with an electron acceptor fullerene derivative (PCBM). The experimental results on these samples demonstrate the capability of the technique to unveil peculiar interfacial dynamics at the border region between phase-segregated domains, which would be otherwise averaged out using conventional pump-probe spectroscopy. The third example is the study of the photophysics of isolated mesoscopic crystals of the PCBM molecule. Our ultrafast microscope could evidence the presence of two distinctive regions within the crystals. In particular, we could pinpoint for the first time areas within the crystals showing photobleaching/stimulated emission signals from a charge-transfer state., (This journal is © The Royal Society of Chemistry 2012)

We introduce a novel configuration for broadband, time-resolved CARS spectroscopy/microscopy in which pump, Stokes and probe pulses are all derived from a single femtosecond Yb:KYW laser. The 1-MHz repetition rate of the system allows very intense CARS signals to be obtained over short acquisition times, while a delayed probe pulse ensures an efficient non-resonant background suppression.

We present an ultrafast transient absorption spectroscopy system in the visible combining high-sensitivity broadband detection with extreme temporal resolution. The instrument is based on an ultrabroadband sub-10 fs optical parametric amplifier coupled to an optical multichannel analyzer with fast electronics, enabling single-shot detection at 1 kHz repetition rate. For a given pump-probe delay tau, we achieve a differential transmission (DeltaTT) sensitivity of the order of 10(-4) over the lambda(pr)=490-720 nm probe wavelength range by averaging over 1000 shots, allowing the acquisition of complete two-dimensional DeltaTT (lambda(pr),Tau) maps within a few minute measurement time. We present application examples highlighting the capability of this instrument to observe ultrafast dynamical processes, follow impulsively excited vibrational motions with frequency as high as 3000 cm(-1) (11 fs period), and determine the probe wavelength dependence of amplitude and phase of the oscillations.

We demonstrate a novel near-field absorption spectrometer with 100 nm spatial resolution based on an ultrabroadband Ti:sapphire oscillator coupled to an aperture NSOM, enabling the measurement of nanoscale absorption spectra. The instrument is particularly suited for structural phase-selective imaging of organic materials at the nanoscale. We demonstrate that variations in the local absorption spectrum allow us to distinguish between the crystalline and the amorphous phases in polycrystalline phtalocyanine films, thus providing previously unavailable information on their mesoscopic texture.

Broadband Coherent Anti-Stokes Raman Scattering (B-CARS) is a powerful label-free nonlinear spectroscopy technique allowing one to measure the full vibrational spectrum of molecules and solids. B-CARS spectra, however, suffer from the presence of a spurious signal, called non-resonant background (NRB), which interferes with the resonant vibrational one, distorting the line shapes and degrading the chemical information. While several numerical techniques are available to remove this unwanted contribution and extract the resonant vibrational signal of interest, they all require the user's intervention and sensitively depend on the spectral shape of the NRB, which needs to be measured independently. In this work, we present a novel approach to remove NRB from B-CARS spectra based on deep learning. Thanks to the high generalization capability offered by the deep architecture of the designed neural network, trained through realistic simulated spectra, our fully automated model is able to process B-CARS spectra in real time and independently of the detailed shape of the NRB spectrum. This results in fast extraction of vibrational spectra without requiring user intervention or the measurement of reference spectra. We expect that this model will significantly simplify and speed-up the analysis of B-CARS spectra in spectroscopy and microscopy. [ABSTRACT FROM AUTHOR]

A laser-based system for time-resolved photoemission spectroscopy using up to 6.2 eV photons is presented. The versatility of the laser source permits several combinations of pump and probe photon energies with pulse durations of 50–100 fs. The ultrahigh vacuum system, equipped with evaporators, a low energy electron diffraction system and an Auger spectrometer, grants the possibility to grow and characterize thin films in situ. The electron energy analyzer is a time-of-flight spectrometer with a multianode detector allowing high count rates. The performance of the whole experimental setup is investigated on Cu(100), Cu(111), and Ag(111) single crystals. [ABSTRACT FROM AUTHOR]

Abstract: Ultrafast exciton dynamics is measured in films of functionalized single wall carbon nanotubes embedded in PMMA with time resolution down to sub-10-fs. Ultrafast bleaching decay and photoinduced absorption formation (depending on probe wavelength) are observed and assigned to decay of the second (or higher) exciton into the first exciton. The time constant of the relaxation process is ≈40fs. The origin of the phenomenon is discussed within the one-dimensional semiconductor picture. [Copyright &y& Elsevier]

We study the enhanced charge generation from higher excited states in a conjugated polymer via a modification of transient differential transmission spectroscopy. The first excited state S1 is reexcited via a second laser pulse towards a higher state Sn. The relaxation dynamics of Sn as well as the kinetics of the states created from Sn is revealed by a broad-band probe pulse.The push and probe durations below 20 fs allow the time-resolution of the Sn relaxation and enables us to identify the two main contributions to enhanced charge generation from Sn, energy migration and vibrational relaxation. [Copyright &y& Elsevier]

Abstract. Many fundamental processes of radiation-matter interaction, which take place on the ultra-short time scale, can now be directly investigated by using few-optical-cycles light pulses with duration < 10 fs. We discuss two techniques for generating such pulses: broad-band parametric amplification, which allows the generation of pulses in the visible range suitable for spectroscopy, and compression of high-energy light pulses in a hollow fiber. As an example of application in time-resolved spectroscopy we report results of pump-probe experiments in a prototypical conjugated molecule, namely sexithiophene. The new laser sources, due to their characteristics of peak power and coherence, also allow the exploration of new fields of experimental physics, such as extreme nonlinear optics. We focus on high-order harmonics, showing that a high-energy bunch of photons, up to the X-ray-energy region, with coherence typical of laser radiation and time duration comparable to or shorter than the exciting pulses, can be generated. [ABSTRACT FROM AUTHOR]

We characterize and compress sub-nJ visible white-light continuum (WLC) pulses generated by self-phase modulation in yttrium aluminium garnet. The spectral phase is retrieved by spectrally resolving the transient reflectivity from an optically excited transition metal oxide. This measured phase is compensated by applying the appropriate distortion to a deformable mirror. By comparing the response of two different materials, we show that the white-light pulses can be compressed to approximately 10 fs duration. [ABSTRACT FROM AUTHOR]

Presents the transverse effects in a Nd:YVO[sub 4] microchip solid-state laser due to gain-guiding. Role of transverse modulation of the gain profile and detuning in controlling spatial and spectral properties; Function of the pump beam dimension; Use of the Maxwell-Bloch equations to interpret laser behavior.

Abstract. We report on a sub-10-fs optical parametric amplifier in the visible, using noncollinear phase matching in beta-barium borate, with a pulse compressor made exclusively of chirped dielectric mirrors. We employ mirrors of novel design, providing negative group delay dispersion for bandwidths of over 200 THz, up to the blue-green spectral region. The resulting setup, compact, easy to operate and reliable, is an ideal tool for ultrafast spectroscopy with extremely high time resolution. We use these pulses to observe in the time domain coherent oscillations in organic molecules with period as short as 16 fs. [ABSTRACT FROM AUTHOR]

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400-1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in Г (1550 nm) and a second sizable peak due to photogeneration in the L valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at Г and L, with shorter spin relaxation times whether the X point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range. [ABSTRACT FROM AUTHOR]

Abstract: We use pump-probe spectroscopy with broadband detection to study electronic energy relaxation and coherent vibrational dynamics in carotenoids. A fast optical multichannel analyzer combined with a non-collinear optical parametric amplifier allows simultaneous acquisition of the differential transmission dynamics on the 500–700nm wavelength range with sub-10-fs temporal resolution. The broad spectral coverage enables on the one hand a detailed study of the ultrafast bright-to-dark state internal conversion process; on the other hand, the tracking of the motion of the vibrational wavepacket launched on the ground state multidimensional potential energy surface. We present results on all-trans β-carotene and on a long-chain polyene in solution. The developed experimental setup enables the straightforward acquisition and analysis of coherent vibrational dynamics, highlighting time–frequency domain features with extreme resolution. [Copyright &y& Elsevier]