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The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process., The formation dynamics of excitons in 2D transition metal dichalcogenides are challenging to probe directly because of their inherently fast timescales. Here, the authors use extremely short optical pulses to excite an electron-hole plasma, and show the formation of 2D excitons in MoS2 on the timescale of 30 fs.

We present a novel method, named STRIPE, for the complete temporal characterization of ultrashort light pulses. By applying STRIPE to both simulated and experimental data, we demonstrate its superior performances against other established reconstruction algorithms.

By combining UV transient absorption spectroscopy with sub-30-fs temporal resolution and CASPT2/MM calculations, we present a complete description of the primary photo-induced processes in solvated tryptophan. Our results shed new light on the role of the solvent in the relaxation dynamics of tryptophan. We unveil two consecutive coherent population transfer events involving the lowest two singlet excited states: a sub-50-fs non-adiabatic La-->Lb transfer through a conical intersection and a subsequent 220 fs reverse Lb-->La transfer due to solvent assisted adiabatic stabilization of the La state. Vibrational fingerprints in the transient absorption spectra provide compelling evidence of a vibronic coherence established between the two excited states from the earliest times after photoexcitation and lasting until the back-transfer to La is complete. The demonstration of response to the environment as a driver of coherent population dynamics among the excited states of tryptophan closes the long debate on its solvent-assisted relaxation mechanisms and extends its application as a local probe of protein dynamics to the ultrafast timescales.

Excitons are electron-hole quasi-particles, which strongly influence the electro-optical properties of solids and hold the potential for a variety of applications [1] . Acquiring a detailed understanding of their dynamic nature is thus essential to promote their exploitation in advanced technological areas. In particular, the ultrafast processes unfolding at the femto- and attosecond domain are of primary relevance in view of the desired extension towards the petahertz regime. Here we applied attosecond transient reflectivity spectroscopy to study the ultrafast response of a MgF2 single crystal and report the first observation of sub-fs core-exciton dynamics [2] . Combined with simulations, our results allowed us to disentangle the dual atomic-solid nature of this fundamental quasi-particle.

Miniaturized optical diagnostics might be greatly favored by the availability of effective, conformable UV light sources combining reduced size with mechanical flexibility. Here we report on our recent results on ZnO-incorporated nanofibers, exhibiting optical gain and polarized emission, used to obtain flexible UV lasers operating at room-temperature. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 682157, “xPRINT”), from the Italian Minister of University and Research (PRIN 2017PHRM8X) and from the University of Pisa (PRA “ANISE”).

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.

The electronic energy levels of cyclo(glycine–phenylalanine), cyclo(tryptophan–tyrosine) and cyclo(tryptophan–tryptophan) dipeptides are investigated with a joint experimental and theoretical approach. Experimentally, valence photoelectron spectra in the gas phase are measured using VUV radiation. Theoretically, we first obtain low-energy conformers through an automated conformer–rotamer ensemble sampling scheme based on tight-binding simulations. Then, different first principles computational schemes are considered to simulate the spectra: Hartree–Fock (HF), density functional theory (DFT) within the B3LYP approximation, the quasi-particle GW correction, and the quantumchemistry CCSD method. Theory allows assignment of the main features of the spectra. A discussion on the role of electronic correlation is provided, by comparing computationally cheaper DFT scheme (and GW) results with the accurate CCSD method.

Excitons characterize the ultrafast response of many materials of technological interest. While the development of attosecond science has unlocked the possibility of performing experiments with a suitable timeresolution, the access to the exciton properties remains a non-trivial step. We propose therefore a novel approach to disclose the physical properties behind the ultrafast exciton dynamics based on a phase-retrieval method.

Tryptophan is one of the three natural aromatic amino acids and can serve as local probe of photoreactions of proteins irradiated with UV light. However, the photophysics of tryptophan are complex, especially when the interaction with solvent is considered. The excited state evolution of tryptophan in condensed phase has been reported previously on picosecond and nanosecond time scales, using transient absorption (TA) spectroscopy [1] . Here we track the primary photoinduced processes in tryptophan using TA spectroscopy with unprecedented sub-30-fs temporal resolution [2] and compare them with state-of-the-art quantum mechanics/molecular mechanics computations [3] .

Recent development of spectroscopic techniques based on attosecond radiation has given the community the right tools to study the timing of the photoelectron process. In this work we investigate the effect of Fano resonances in attosecond streaking spectrograms and the application of standard phase-reconstruction algorithms. We show that while the existence of the infrared coupling (ac-Stark shift) hinders the applicability of FROG-like methods, under certain conditions it is still possible to use standard reconstruction algorithms to retrieve the photoemission delay of the bare resonance. Finally, we propose two strategies to study the strength of IR coupling using the attosecond streaking technique.

The electro-optical properties of most semiconductors and insulators of technological interest are dominated by the presence of electron-hole quasi-particles, called excitons. The manipulation of excitons in dielectrics has recently received great attention, with possible applications in different fields including optoelectronics and photonics. Here, we apply attosecond transient reflection spectroscopy in a sequential two-foci geometry and observe sub-femtosecond dynamics of a core-level exciton in bulk MgF2 single crystals. Furthermore, we access absolute phase delays, which allow for an unambiguous comparison with theoretical calculations. Our results show that excitons surprisingly exhibit a dual atomic- and solid-like character, which manifests itself on different time scales. While the former is responsible for a femtosecond optical Stark effect, the latter dominates the attosecond excitonic response. Further theoretical investigation reveals a link with the exciton sub-femtosecond nanometric motion and allows us to envision a new route to control exciton dynamics in the close-to-petahertz regime., The capability to follow electron motion in solids is necessary to explore the ultimate speed limits of optical charge manipulation and signal processing in optoelectronic devices. Here, the authors reveal the sub-femtosecond dynamics of core excitons in MgF2 and find the dual atomic-solid nature of the exciton quasi-particle to deeply affect its ultrafast dynamics.

DNA owes its remarkable photostability to the ability of its building blocks – the nucleosides – to efficiently dissipate the excess electronic energy acquired upon photoexcitation with ultraviolet light. The exact mechanism occurring on a sub-picosecond time scale has been a matter of intense debate. Here we combine sub-30-fs transient absorption spectroscopy with broad spectral coverage and state-of-the-art quantum dynamics simulations to resolve the early steps of the deactivation mechanisms of uridine (Urd) and 5-methyluridine (5mUrd) in aqueous solution. For both nucleosides we track the wave packet motion from the Franck-Condon region to the conical intersection (CI) with to the ground state and observe a direct spectral signature of an excited-state vibrational mode leading to the CI. We find that 5mUrd exhibits an order of magnitude longer excited state lifetime with respect to Urd and assign it to the larger inertia of the methyl group involved in the ring puckering mode leading to the CI. We argue that this longer lifetime enables the activation of potentially lesion-inducing coordinates such as ring opening, elucidating the origin of the susceptibility of thymine-containing DNA strands to photodamage.

4-Thiothymidine 20 fs broadband Transient Transmission Spectroscopy results and analysis provide evidence that along the photoexcited relaxation pathway, the intersystem crossing originates primarily from a dark intermediate state, which is vibrationally coherent accessible.

Broadband transient absorption with sub-20fs temporal resolution, supported by ab-initio calculations, quantitatively provides the strength of exciton-coherent phonon coupling in 1L-MoS2, showing a resonant profile around the C exciton.

The miniaturization of diagnostic devices that exploit optical detection schemes requires the design of light-sources combining small size, high performance for effective excitation of chromophores, and mechanical flexibility for easy coupling to components with complex and non-planar shapes. Here, ZnO nanowire-in-fiber hybrids with internal architectural order are introduced, exhibiting a combination of polarized stimulated emission, low propagation losses of light modes, and structural flexibility. Ultrafast transient absorption experiments on the electrospun material show optical gain which gives rise to amplified spontaneous emission, with threshold lower than the value found in films. These systems are highly flexible and can conveniently conform to curved surfaces, which makes them appealing active elements for various device platforms, such as bendable lasers, optical networks and sensors, as well as for application in bioimaging, photo-crosslinking, and optogenetics., 50 pages, 17 figures, 1 table, ACS Nano, 2020

We report sub-20-fs resolution UV transient absorption measurements, supported by ab-initio simulations, of the primary photoinduced processes of tryptophan in solution. We show internal conversion occurring in 300-fs followed by intersystem crossing in 1-ps.

Super-excited states of CO2 are studied by time-resolved photoelectron spectroscopy exploiting sub-15 fs vacuum-ultraviolet pulses. Comparison with quantum mechanical simulations allows us to identify the ultrafast mechanisms which dictate the super-excited state lifetime and coherence.

Sub-cycle MgF2 core-exciton dynamics was studied by attosecond reflection spectroscopy. We found that the interplay between the crystal and the core-exciton strongly alters the ultrafast material response, introducing a delay as big as 1 femtosecond.

We combine sub-20 fs transient absorption spectroscopy with state-of-the-art computations to study the ultrafast photoinduced dynamics of trans-azobenzene (AB). We are able to resolve the lifetime of the pi pi* state, whose decay within ca. 50 fs is correlated to the buildup of the n pi* population and to the emergence of coherences in the dynamics, to date unobserved. Nonlinear spectroscopy simulations call for the CNN in-plane bendings as the active modes in the subps photoinduced coherent dynamics out of the pi pi* state. Radiative to kinetic energy transfer into these modes drives the system to a high-energy planar n pi*/ground state conical intersection, inaccessible upon direct excitation of the lie state, that triggers an ultrafast (0.45 ps) nonproductive decay of the n pi* state and is thus responsible for the observed Kasha rule violation in UV excited trans-AB. On the other hand, cis-AB is built only after intramolecular vibrational energy redistribution and population of the NN torsional mode.

The deposition of perylene diimide-based aggregates (PDI) onto wide band gap n-type Sb-doped SnO2 (ATO) was investigated with the aim of finding efficient and versatile dye-sensitized platforms :for photoelectrochemical solar fuel generation. These ATO-PDI photoanodes displayed hydrolytic stability in a wide range of pH (from 1 to 13) and revealed superior perforthances (up to 1 mA/cm(2) net photocurrent at 1 V vs SCE) compared to both WO3-PDI and: undoped SnO2-PDI when used in a photoelectrochemical setup for HBr splitting. Although ATO, SnO2, and WO3 are endowed with similar conduction band edge energetics, in ATO the presence-of a:significant density of intrabandgap states, whose occupancy Varies with the applied potential, plays a substantial role in tuning the efficiency of photoinduced charge separation and collection. Furthermore, the investigation of the charge injection kinetics confirmed that, even in the absence of applied bias, ATO and WO3 are the best substrates for the-oxidative quenching of poorly reducing PDI excited states, with at least a fraction of them injecting within

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.

Thiobases are studied by transient absorption spectroscopy employing sub-20 fs UV-pulses combined with hybrid QM(CASPT2) / MM(AMBER) computations (static and dynamic) evidencing that intersystem crossing originates from a dark intermediate state along the photoexcited relaxation pathway.

The survival of photosynthetical organisms crucially depends on their ability to adapt to different light intensities. Under excess light conditions, the fastest photoprotection response consists in the turn-on of a dissipation mechanism of the energy absorbed by the chlorophylls into heat, called non-photochemical quenching (NPQ). NPQ requires the action of an intrathylakoidal protein, LHCSR3, composed of different chlorophylls and carotenoids bound to a tri-helicoidal carbon chain. During the light phase of photosynthesis, high light stress conditions generate a pH gradient across the thylakoid membrane which is sensed by the acid groups of LHCSR3 and triggers the protein quenching activity In this work, we reproduce the quenching of the LHCSR3 protein in vitro considering two different pH conditions and two protein aggregation states (HA: high aggregation, LA: low aggregation). NPQ was studied using a combination of picosecond time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) techniques in the visible and NIR spectral regions.

Two-dimensional electronic spectroscopy (2DES) is a powerful spectroscopic technique for the study of dynamics in coupled multi-chromophore systems, which has been successfully applied in the visible and infrared ranges. Its extension to the UV range (2DUV) is extremely promising for the study of biomolecules but poses several technical challenges: (i) the requirement of phase-locked pulse pairs; (ii) generation of extremely broadband UV pulses; (iii) dispersion management for the UV range. Here we present a setup able to generate ultrabroad phase-locked UV pump pulses with bandwidth from 260 to 310 nm. The UV pump pulse pairs are generated by Achromatic Second Harmonic Generation (ASHG) of a phase-locked, visible pulse pair (Fig. 1(a)). A non-collinear optical parametric amplifier (NOPA) produces 10-μJ

International audience; Using as a model the single adenine strand (dA)20, we study the ultrafast evolution of electronic excitations in DNA with a time resolution of 30 fs. Our transient absorption spectra in the UV and visible spectral domains show that internal conversion among photogenerated exciton states occurs within 100 fs. Subsequently, the ππ* states acquire progressively charge-transfer character before being completely trapped, within 3 ps, by fully developed charge-transfer states corresponding to transfer of an electron from one adenine moiety to another (A+A–).

Two-dimensional (2D) spectroscopy, originally developed for nuclear magnetic resonance, has been recently extended to the infrared and visible regimes. In this technique sequences of femtosecond light pulses are used to interrogate molecular systems and show, by a double Fourier transform, the correlation between excitation and detection frequencies. Extension to the ultraviolet (UV) regime is of great interest and promises to deliver rich structural and dynamical information on biomolecules such as DNA and proteins; however, it must overcome significant technical challenges. This review summarizes the current development status of 2DUV spectroscopy. After discussing the scientific case for the technique, we introduce its basic principles and review its experimental implementations, as well as the computational tools that have been developed to model the experiments. We conclude by giving a few application examples, which highlight the potential of 2DUV spectroscopy and motivate its further development.

[Image: see text] Photosynthetic organisms possess photoprotection mechanisms from excess light conditions. The fastest response consists in the pH-triggered activation of a dissipation channel of the energy absorbed by the chlorophylls into heat, called nonphotochemical quenching. In green algae, the pigment binding complex LHCSR3 acts both as a chlorophyll quencher and as a pH detector. In this work, we study the quenching of the LHCSR3 protein in vitro considering two different protein aggregation states and two pH conditions using a combination of picosecond time-resolved photoluminescence and femtosecond transient absorption in the visible and NIR spectral regions. We find that the mechanisms at the basis of LHCSR3 quenching activity are always active, even at pH 7.5 and low aggregation. However, quenching efficiency is strongly enhanced by pH and by aggregation conditions. In particular, we find that electron transfer from carotenoids to chlorophylls is enhanced at low pH, while quenching mediated by protein–protein interactions is increased by going to a high aggregation state. We also observe a weak pH-dependent energy transfer from the chlorophylls to the S(1) state of carotenoids.

4-thiouracil (4TU) is studied by transient absorption spectroscopy employing sub-20 fs UV-pulses and hybrid QM(CASPT2) / MM(AMBER) computations (static and dynamic), evidencing that, along the photoexcited relaxation pathway, intersystem crossing originates from a dark intermediate state.

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution ($\sim$20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS$_2$, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane $A'_{1}$ phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS$_2$ band structure induced by the $A'_{1}$ phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS$_2$ can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.

When ultraviolet (UV) radiation is absorbed by DNA, the electronic energy acquired by the molecule is efficiently converted into vibrational energy on an ultrafast timescale, preventing photochemical reactions which might induce mutations or strand breaks. In those ultrafast processes, conical intersections (CIs) play a crucial role [1]. Over the past years many studies have been performed to shed light in the fate of photoexcitations in DNA, although most of them with limited (>100 fs) temporal resolution. In this work we combine transient absorption (TA) spectroscopy using sub-20 fs UV pump pulses [2, 3] and broad UV spectral coverage with TA spectroscopy simulations from first principles based on mixed quantum mechanics /molecular mechanics (QM/MM) framework at the dynamically correlated multiconfiguration wavefunction level (CASPT2) [4, 5] to study the ultrafast relaxation dynamics of pyrimidine nucleosides uridine (Urd) and 5-methyluridine (MTU)

We combined sub-30 fs broadband transient absorption spectroscopy in the ultraviolet with state-of-the-art quantum mechanics/molecular mechanics simulations to study the ultrafast excited-state dynamics of the sulfur-substituted nucleobase 4-thiouracil. We observed a clear mismatch between the time scales for the decay of the stimulated emission from the bright pi pi* state (76 +/- 16 fs, experimentally elusive until now) and the buildup of the photoinduced absorption of the triplet state (225 +/- 30 fs). These data provide evidence that the intersystem crossing occurs via a dark state, which is intermediately populated on the sub-100 fs time scale. Nonlinear spectroscopy simulations, extrapolated from a detailed CASPT2/MM decay path topology of the solvated system together with an excited state mixed quantum-classical nonadiabatic dynamics, reproduce the experimental results and explain the experimentally observed vibrational coherences. The theoretical analysis rationalizes the observed different triplet buildup times of 4- and 2-thiouracil.

A precise temporal characterization of the pulses involved in pump-probe experiments is crucial for a proper investigation of the ultrafast dynamics in several physical systems. Indeed, it is required for the assessment of the dynamical properties under examination with sufficient temporal resolution. In the fewfs/attosecond domain, typical reconstruction procedures require time-consuming interative methods, which are also sensitive to the experimental noise and to the distortion of the measurement. We developed an approach, called Simplified Trace Reconstruction In the Perturbative regimE (STRIPE), which allows us for a precise characterization of the infrared (IR) and extreme ultraviolet (XUV) pulses, used in a pump-probe experiment. Our method is not based on a phase retrival algorithm, and for this it is typically much faster than the other ones currently known. Moreover, it allows for easily including in the reconstruction the experimental non-idealities that may affect the measurement, like possible distortion due to the measurement procedure itself.

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene dyes, more commonly known as BODIPY dyes, have been long since recognized for their excellent optical properties such as high absorption coefficients, high fluorescence quantum yields, and remarkable photostability1. Discovered for the first time in 1968 by Treibs and Kreuz2, BODIPY dyes are of wide importance and have a broad use as laser dyes in biological sensing, organic electronic, and other possible applications3. Recently BODIPY molecules dispersed in a polymeric matrix like polystyrene (PS) have been also used to achieve polariton condensation in strongly coupled microcavities4 . Here we present the synthesis, photophysical and gain properties of a series of BODIPY dyes functionalized with lateral substituents to improve their film forming ability and to reduce the effect of PL quenching. This will permit such dyes to be used at high concentration in a strongly coupled microcavity and will allow to reduce the threshold for polariton condensation. (1) Wood, T. E.; Thompson, A. Chem. Rev. 2007, 107, 1831-1861. (2) Treibs, A.; Kreuzer, F.-H. Justus Liebigs Ann. Chem. 1968, 718, 208-223. (3) Squeo, B. M.; Gregoriou, V. G.; Avgeropoulos, A.; Baysec, S.; Allard, S.; Scherf, U.; Chochos, C. L. Prog. Polym. Sci. 2017, 71, 26-52. (4) Grant, R. T.; Michetti, P.; Musser, A. J.; Gregoire, P.; Virgili, T.; Vella, E.; Cavazzini, M.; Georgiou, K.; Galeotti, F.; Clark, C.; Clark, J.; Silva, C.; Lidzey, D. G. Adv. Opt. Mater. 2016, 4, 1615-1623.

On the example of 2-, 4- and 2,4-thiouracil we demonstrate the performance of the RASPT2/RASSCF protocol in reproducing the spectral positions and line shapes of linear absorption spectra that have been recorded in water and documented in this work. Through a QM/MM scheme coupled to a room-temperature Wigner sampling we simulate condensed phase spectra, permitting to compare our results against experiments. We discuss the sensitivity of the simulations to: a) the active space size by pushing the limits beyond the full-valence active spaces; b) the consideration of “dark” nπ∗-states in the state averaging; c) the flavor of RASPT2 technique; d) the basis set. The benchmarking demonstrates that full-π valence active spaces tend to red-shift the absorption band. Increasing the active space rectifies the problem and we obtain near-quantitative agreement between our experiments and calculations. We, furthermore, demonstrate that the choice of RASPT2 flavor has to be made through rigorous benchmarking.

We describe a transient absorption (TA) spectroscopy system in the ultraviolet (UV) spectral range, for the study of the ultrafast optical response of biomolecules. After reviewing the techniques for the generation and characterization of ultrashort UV pulses, we describe the experimental setup of our ultrabroadband UV TA spectrometer. The setup combines sub-20-fs UV pump pulses tunable between 3.35 and 4.7 eV, with broadband white-light-continuum probe pulses in the 1.7&ndash, 4.6 eV range. Thanks to the broad tunability of the pump pulses in the UV spectral range, the extremely high temporal resolution and the broad spectral coverage of the probe, this TA system is a powerful and versatile tool for the study of many biomolecules. As an example of its potential, we apply the TA spectrometer to track ultrafast internal conversion processes in pyrene after excitation in the UV, and to resolve an impulsively excited molecular vibration with 85-fs period.

We introduce a birefringent interferometer for Fourier transform (FT) spectroscopy in the mid-infrared, covering the vibrational fingerprint region (5-10 µm, 1000-2000 cm-1), which is crucial for molecular identification. Our interferometer employs the crystal calomel (Hg2Cl2), which combines high birefringence (ne-no≈0.55) with a broad transparency range (0.38-20 µm). We adopt a design based on birefringent wedges, which is simple and compact and guarantees excellent delay accuracy and long-term stability. We demonstrate FTIR spectroscopy, with a frequency resolution of 3 cm-1, as well as two-dimensional IR (2DIR) spectroscopy. Our setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy.

Sub-20-fs transient absorption spectroscopy and simulations show that CNN-bendings dominate the sub-ps dynamics of ππ*-excited trans-azobenzene, thereby driving the system to the ground state through a non-productive decay channel in violation of the Kasha rule.

We measure exciton dynamics in single-layer MoS2 with unprecedented temporal resolution and we directly extract the characteristic time-scale for the Coulomb screening dynamics, which ranges between 15 and 35 fs.

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.

We introduce a scheme to generate collinear, interferometrically locked UV pulse pairs by combining birefringence and sum-frequency generation between a narrowband infrared light and broadband visible pulses. The scheme is applied to 2D electronic spectroscopy.

14th Workshop on Information Optics (WIO), Kyoto, 1-5 june 2015 We report on deterministic femtosecond multi-filamentation in fused silica by encoding a diffractive microlens array into a spatial light modulator. The efficiency and focal length of each microlens are electronically controlled. Even for an input inhomogeneous irradiance distribution, this allows for a precise control of the energy coupled to the filaments and the spectra of the generated supercontinuum beams. This method may be useful in different applications such as filamentation-based tumor treatment, laser processing, or spectroscopy. This research was funded by the Spanish Ministerio de Ciencia e Innovación through FIS2013-40666-P. Also financial support from the Generalitat Valenciana through project PROMETEO/2012/021 and Pla Propi d'Investigació de la Universitat Jaume I through project P1·1B2013-53 is acknowledged.

To overcome the challenges of ancillae preparation in two-dimensional spectrally sheared interferometry, we apply the approach based on spectral filtering in the Fourier plane of a 4-f zero-dispersion compressor, which allows the generation of two sheared and synchronized monochromatic fields. The scheme provides direct and precise measurement of their spectral shear, greatly improving the accuracy and reliability of spectral phase retrieval. By characterizing few-optical-cycle pulses spanning nearly two octaves of bandwidth, we prove the applicability of this method to sub-single-cycle pulses.

We present a complete experimental characterization and simulation of the spatiotemporal and spatio-spectral effects taking place when a femtosecond pulse is shaped by a diffractive optical element.

We present a scheme for the generation of tunable ultrashort UV pulses based on sum-frequency generation between a broadband visible pulse and a narrowband tunable pulse. It allows for broadband UV pulses tunable from 0.3 to 0.4 μm, with energy up to 1.5 μJ. Full characterization of the UV pulse is obtained by 2D spectral shearing interferometry. We demonstrate 8.4 fs UV pulses.

We introduce a scheme for the generation of tunable few-optical-cycle UV pulses based on sum-frequency generation between a broadband visible pulse and a narrowband pulse ranging from the visible to the near-IR. This configuration generates broadband UV pulses tunable from 0.3 to 0.4 mu m, with energy up to 1.5 mu J. By exploiting nonlinear phase transfer, transform-limited pulse durations are achieved. Full characterization of the UV pulse spectral phase is obtained by two-dimensional spectral shearing interferometry, which is here extended to the UV spectral range. We demonstrate clean 8.4 fs UV pulses. (C) 2014 Optical Society of America