Generation of high-energy sub-20 fs pulses tunable in the 250-310 nm region by frequency doubling of a high-power noncollinear optical parametric amplifier

Generation of high-energy, sub-20 fs deep-UV pulses in a twin-crystal third harmonic generation scheme

Ultrashort deep ultraviolet (DUV) pulses serve as indispensable tools for investigating molecular dynamics on the femtosecond scale. Nonlinear frequency upconversion of near-infrared (NIR) light sources in a sequence of nonlinear crystals is a common method for their generation. However, preserving the temporal duration of the starting source encounters challenges owing to phase-matching bandwidth limitations within the harmonic generation process. Here we propose an approach for circumventing this limitation and demonstrate it for the case of generation of the third harmonic of 800 nm pulses in a two-stage scheme (second harmonic generation succeeded by sum-frequency mixing of the fundamental and second harmonic pulses). Expanding the bandwidth of the DUV pulse involves the utilization for the last mixing process of two nonlinear crystals, detuned to convert opposite sides of the spectrum. The implementation of this approach yields 20 µJ, 263 nm DUV pulses as short as 19 fs after compression. The setup is very compact and extremely stable due to the common-path scheme, which makes it very interesting for a variety of advanced ultrafast spectroscopy applications.

Transient absorption spectroscopy based on uncompressed hollow core fiber white light proves pre-association between a radical ion photocatalyst and substrate

We present a hollow-core fiber (HCF) based transient absorption experiment, with capabilities beyond common titanium:sapphire based setups. By spectral filtering of the HCF spectrum, we provide pump pulses centered at 425 nm with several hundred nJ of pulse energy at the sample position. By employing the red edge of the HCF output for seeding CaF₂, we obtain smooth probing spectra in the range between 320 and 900 nm. We demonstrate the capabilities of our experiment by following the ultrafast relaxation dynamics of a radical cationic photocatalyst to prove its pre-association with an arene substrate, a phenomenon that was not detectable previously by steady-state spectroscopic techniques. The detected preassembly rationalizes the successful participation of radical ionic photocatalysts in single electron transfer reactions, a notion that has been subject to controversy in recent years.

Efficient and compact source of tuneable ultrafast deep ultraviolet laser pulses at 50 kHz repetition rate

Deep ultraviolet (DUV) laser pulses with tuneable wavelength and very short duration are a key enabling technology for next-generation technology and ultrafast science. Their generation has been the subject of extensive experimental effort, but no technique demonstrated thus far has been able to meet all requirements in one light source. Here we demonstrate a bright, efficient, and compact source of tuneable DUV ultrafast laser pulses based on resonant dispersive wave emission in hollow capillary fiber. In a total footprint of only 120cm×75cm, including the ytterbium-based drive laser, we generate pulses between 208nm and 363nm at 50kHz repetition rate with a total efficiency of up to 3.6%. Down-scaling of the DUV generation reduces the required energy sufficiently to enable the generation of two-color few-femtosecond DUV pulses.

Achromatic frequency doubling of supercontinuum pulses for transient absorption spectroscopy

We present achromatic frequency doubling of supercontinuum pulses from a hollow core fiber as a technique for obtaining tunable ultrashort pulses in the near UV and blue spectral range. Pulse energies are stable on a 1.1% level, averaged over 100 000 shots. By the use of conventional optics only, we compress a 0.2 µJ pulse at a center wavelength of 475 nm to a pulse duration of 12 fs, as measured by X-FROG. We test the capabilities of the approach by employing the ASHG-pulses as a pump in a transient absorption experiment on β-carotene in solution.

High-energy noncollinear optical parametric amplifier producing 4 fs pulses in the visible seeded by a gas-phase filament

We report on the design and characterization of a short-pulse-pumped, single-stage noncollinear optical parametric amplifier (NOPA) that achieves high pulse energies in the few-cycle pulse regime. Optimal pulse-front tilting and temporal compression of the short (35 fs) pump pulse are achieved using a 4f grating compressor, while spatial chirp at the NOPA crystal is eliminated with proper imaging using a pair of reflective telescopes. Gas-phase filamentation in an open-ended argon-filled cell provides a bright, stable seed source with little residual chirp that is suitable for temporal overlap with the short pump pulse without dispersion precompensation. Two seeding geometries are explored, and pulses as short as 3.5 fs are obtained by seeding with the entire filament bandwidth. Fourier-transform-limited 4 fs pulses are obtained by filtering the IR portion of the spectrum.

Microjoule-level, tunable sub-10 fs UV pulses by broadband sum-frequency generation

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 μm, with energy up to 1.5 μJ. 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.

Two MHz tunable non collinear optical parametric amplifiers with pulse durations down to 6 fs

We report on the development of a 2 MHz non collinear optical parametric amplifier (NOPA) for high repetition rate time resolved X-ray or optical spectroscopy. Our modular and very flexible device is pumped by the second and third harmonics of a commercial femtosecond Ytterbium-doped fiber laser. The amplified pulses are tunable from 520 nm to 1000 nm with pulse durations between 15 and 30 fs over the full tuning range. The same setup is also suitable for broadband amplification and we demonstrate the generation of 6 fs pulses at a central wavelength of 850 nm as well as the generation of a broadband spectrum supporting 4.2 fs transform limited pulse duration at a central wavelength of 570 nm. Very high stability and compactness is achieved thanks to an optimized mechanical design.

Disentangling Peptide Configurations via Two-Dimensional Electronic Spectroscopy: Ab Initio Simulations Beyond the Frenkel Exciton Hamiltonian

Two-dimensional (2D) optical spectroscopy techniques based on ultrashort laser pulses have been recently extended to the optical domain in the ultraviolet (UV) spectral region. UV-active aromatic side chains can thus be used as local highly specific markers for tracking dynamics and structural rearrangements of proteins. Here we demonstrate that 2D electronic spectra of a model proteic system, a tetrapeptide with two aromatic side chains, contain enough structural information to distinguish between two different configurations with distant and vicinal side chains. For accurate simulations of the 2DUV spectra in solution, we combine a quantum mechanics/molecular mechanics approach based on wave function methods, accounting for interchromophores coupling and environmental effects, with nonlinear response theory. The proposed methodology reveals effects, such as charge transfer between vicinal aromatic residues that remain concealed in conventional exciton Hamiltonian approaches. Possible experimental setups are discussed, including multicolor experiments and signal manipulation techniques for limiting undesired background contributions and enhancing 2DUV signatures of specific electronic couplings.

Generation of sub-20-fs deep-ultraviolet pulses by using chirped-pulse four-wave mixing in CaF2 plate

Sub-20-fs deep ultraviolet (DUV) pulses are generated by using nondegenerate, chirped-pulse four-wave mixing of the fundamental and second-harmonic pulses from a commercial Ti:sapphire amplifier in a CaF(2) plate. The energy of the DUV pulses is 3.8 μJ, with a conversion efficiency from total pump energy to DUV of ~3.8%. The DUV pulse is compressed using a pre-chirp, introduced via a fused silica window in the fundamental beam. The central wavelength of the DUV spectrum can be tuned from 257 to 277 nm by adjusting the cross angle between the two pump beams. The spectrum can reach a width of 16.8 nm, which can support a pulse duration of 8.7 fs.

Dissecting two-dimensional ultraviolet spectra of amyloid fibrils into beta-strand and turn contributions

We present an analysis of the contributions of various secondary structure elements of the amyloid β-protein to the two-dimensional far ultraviolet (2DFUV) signal of an amyloid fibril model. The contributions of the turns and the β-strands are affected by the geometry of the backbone peptide amide π → π* transition dipoles, the backbone interamide coupling in the excited state, and the exciton delocalization. These contributions are clearly distinguishable in the xyxy-xyyx pulse polarization configuration. The differences are attributed to the smaller splitting of the exciton energies and the larger fluctuations of the geometry of the peptide amide π → π* transition dipoles at the turns, while making the 2DFUV signal sensitive to the secondary structure. This signal may be used to determine the proportion of turns and β-strands.

Probing amyloid fibril growth by two-dimensional near-ultraviolet spectroscopy

Keeping track of the aggregation kinetics of amyloid fibrils is essential for understanding their formation mechanism and eventually developing treatments for misfolded protein-related diseases. A simulation study of a series of Aβ(9-40) amyloid fibrils with different size shows that novel two-dimensional near-ultraviolet (2DNUV) spectra contain characteristic signatures of interactions between peptides. Chiral 2DNUV signals show a larger degree of exciton delocalization compared to their nonchiral counterparts. Intensities of specific peaks provide a direct measure of the number of peptides in a fibril. These signals could be used to monitor the fibril growth kinetics, one peptide at a time.

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon

Generation of sub-20-fs UV pulses with more than 300 μJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.

Two-dimensional near-ultraviolet spectroscopy of aromatic residues in amyloid fibrils: a first principles study

We report a first principles study of two dimensional electronic spectroscopy of aromatic side chain transitions in the 32-residue β-amyloid (Aβ(9-40)) fibrils in the near ultraviolet (250-300 nm). An efficient exciton Hamiltonian with electrostatic fluctuations (EHEF) algorithm is used to compute the electronic excitations in the presence of environmental fluctuations. The through-space inter- and intra-molecular interactions are calculated with high level quantum mechanics (QM) approaches, and interfaced with molecular mechanics (MM) simulations. Distinct two dimensional near ultraviolet (2DNUV) spectroscopic signatures are identified for different aromatic transitions, and the couplings between them. 2DNUV signals associated with the transition couplings are shown to be very sensitive to the change of residue-residue interactions induced by residue mutations. Our simulations suggest that 2DNUV spectra could provide a useful local probe for the structure and kinetics of fibrils.

Gires-Tournois interferometer type negative dispersion mirrors for deep ultraviolet pulse compression

Typical femtosecond pulse compression of deep ultraviolet radiation consists of prism or diffraction grating pair chirp compensation but, both techniques introduce higher-order dispersion, spatial-spectral beam distortion and poor transmission. While negatively chirped dielectric mirrors have been used to compress near infrared and visible pulses to <10 fs, there has been no extension of this technique below 300 nm. We demonstrate the use of Gires-Tournois interferometer (GTI) negative dispersion multilayer dielectric mirrors designed for pulse compression in the deep ultraviolet region. GTI mirror designs are more robust than chirped mirrors and, can provide sufficient bandwidth for the compression of sub-30-fs pulses in the UV wavelength range. Compression of a 5 nm (FWHM) pulse centered between 266 and 271 nm to 30 fs has been achieved with less pulse broadening due to high-order dispersion and no noticeable spatial deformation, thereby improving the resolution of ultrafast techniques used to study problems such as fast photochemical reaction dynamics.

Generation of 30-fs ultraviolet pulses by four-wave optical parametric chirped pulse amplification

We report on the generation of approximately 30-fs ultraviolet pulses with approximately 10 microJ energy by means of four-wave optical parametric chirped pulse amplification in fused silica. The four-wave optical parametric amplifier is pumped by the second-harmonic of the Ti:sapphire laser and is seeded by visible broadband chirped signal pulses. The idler pulses are produced in the ultraviolet by four-wave mixing and are compressed in a medium with normal group velocity dispersion.

High gain broadband amplification of ultraviolet pulses in optical parametric chirped pulse amplifier

We report on a high gain amplification of broadband ultraviolet femtosecond pulses in an optical parametric chirped pulse amplifier. Broadband ultraviolet seed pulses were obtained by an achromatic frequency doubling of the output from a femtosecond Ti:Sapphire oscillator. Stretched seed pulses were amplified in a multipass parametric amplifier with a single BBO crystal pumped by a ns frequency quadrupled Nd:YAG laser. A noncollinear configuration was used for a broadband amplification. The total (after compression) amplification of 2.510(5) was achieved, with compressed pulse energy of 30 microJ and pulse duration of 24 fs. We found that the measured gain was limited by thermal effects induced by the absorption of the pump laser by color centers created in the BBO crystal.

Sub-15fs ultraviolet pulses generated by achromatic phase-matching sum-frequency mixing

A broadband ultraviolet pulse with a spectral width of 44 nm was generated by achromatic sum-frequency mixing of an 805-nm pulse and ultrabroadband visible pulse. Angular dispersion was introduced to achieve broadband phase matching by a prism pair. The UV pulse was compressed to 13.2 fs with another prism pair, with energy of 600 nJ.

Angular quasi-phase-matching experiments and determination of accurate Sellmeier equations for 5%MgO:PPLN

We validated the theory of angular quasi-phase-matching (AQPM) by performing measurements of second-harmonic generation and difference-frequency generation. A nonlinear least-squares fitting of these experimental data led to refine the Sellmeier equations of 5%MgO:PPLN that are now valid over the complete transparency range of the crystal. We also showed that AQPM exhibits complementary spectral ranges and acceptances compared with birefringence phase matching.

Generation of 200-microJ, sub-25-fs deep-UV pulses using a noble-gas-filled hollow fiber

High-energy 110-fs pulses of a KrF excimer laser system were spectrally broadened by self-phase modulation in a neon-filled hollow fiber and subsequently compressed by a grating pair. In this way, 25-fs pulses with energies as high as 200 microJ were generated at 248 nm. The pulses were characterized by an all-reflective single-shot transient grating frequency-resolved optical gating.