Ultrasensitive ultraviolet-visible 20 fs absorption spectroscopy of low vapor pressure molecules in the gas phase

Self-referencing for quasi shot-noise-limited widefield transient microscopy

Many applications of ultrafast and nonlinear optical microscopy require the measurement of small differential signals over large fields-of-view. Widefield configurations drastically reduce the acquisition time; however, they suffer from the low frame rates of two-dimensional detectors, which limit the modulation frequency, making the measurement sensitive to excess laser noise. Here we introduce a self-referenced detection configuration for widefield differential imaging. Employing regions of the field of view with no differential signal as references, we cancel probe fluctuations and increase the signal-to-noise ratio by an order of magnitude reaching noise levels only a few percent above the shot noise limit. We anticipate broad applicability of our method to transient absorption, stimulated Raman scattering and photothermal-infrared microscopies.

Two-Dimensional Infrared Spectroscopy of Isolated Molecular Ions

Two-dimensional infrared (2D IR) spectroscopy of mass-selected, cryogenically cooled molecular ions is presented. Nonlinear response pathways, encoded in the time-domain photodissociation action response of weakly bound N2 messenger tags, were isolated using pulse shaping techniques following excitation with four collinear ultrafast IR pulses. 2D IR spectra of Re(CO)3(CH3CN)3+ ions capture off-diagonal cross-peak bleach signals between the asymmetric and symmetric carbonyl stretching transitions. These cross peaks display intensity variations as a function of pump-probe delay time due to coherent coupling between the vibrational modes. Well-resolved 2D IR features in the congested fingerprint region of protonated caffeine (C8H10N4O2H+) are also reported. Importantly, intense cross-peak signals were observed at 3 ps waiting time, indicating that tag-loss dynamics are not competing with the measured nonlinear signals. These demonstrations pave the way for more precise studies of molecular interactions and dynamics that are not easily obtainable with current condensed-phase methodologies.

Multi-scale time-resolved electron diffraction: A case study in moiré materials

Ultrafast-optical-pump - structural-probe measurements, including ultrafast electron and x-ray scattering, provide direct experimental access to the fundamental timescales of atomic motion, and are thus foundational techniques for studying matter out of equilibrium. High-performance detectors are needed in scattering experiments to obtain maximum scientific value from every probe particle. We deploy a hybrid pixel array direct electron detector to perform ultrafast electron diffraction experiments on a WSe2/MoSe2 2D heterobilayer, resolving the weak features of diffuse scattering and moiré superlattice structure without saturating the zero order peak. Enabled by the detector's high frame rate, we show that a chopping technique provides diffraction difference images with signal-to-noise at the shot noise limit. Finally, we demonstrate that a fast detector frame rate coupled with a high repetition rate probe can provide continuous time resolution from femtoseconds to seconds, enabling us to perform a scanning ultrafast electron diffraction experiment that maps thermal transport in WSe2/MoSe2 and resolves distinct diffusion mechanisms in space and time.

Intense and Superflat White Laser with 700-nm 3-dB Bandwidth and 1-mJ Pulse Energy Enabling Single-Shot Subpicosecond Pulse Laser Spectroscopy

An optical spectrometer is a basic spectral instrument that probes microscopic physical and chemical properties of macroscopic objects but generally suffers from difficulty in broadband time-resolved measurement. In this work, we report the creation of ultrabroadband white-light laser with a 3-dB bandwidth covering 385 to 1,080 nm, pulse energy of 1.07 mJ, and pulse duration of several hundred femtoseconds by passing 3-mJ pulse energy, 50-fs pulse duration Ti:Sapphire pulse laser through a cascaded fused silica plate and chirped periodically poled lithium niobate crystal. We utilize this unprecedented superflat, ultrabroadband, and intense femtosecond laser light source to build a single-shot (i.e., single-pulse) subpicosecond pulse laser ultraviolet-visible-near-infrared spectrometer and successfully measure various atomic and molecular absorption spectra. The single-shot ultrafast spectrometer may open up a frontier to monitor simultaneously the ultrafast dynamics of multiple physical and chemical processes in various microscopic systems.

Ultrafast transient vibrational action spectroscopy of cryogenically cooled ions

Ultrafast transient vibrational action spectra of cryogenically cooled Re(CO)3(CH3CN)3+ ions are presented. Nonlinear spectra were collected in the time domain by monitoring the photodissociation of a weakly bound N2 messenger tag as a function of delay times and phases between a set of three infrared pulses. Frequency-resolved spectra in the carbonyl stretch region show relatively strong bleaching signals that oscillate at the difference frequency between the two observed vibrational features as a function of the pump-probe waiting time. This observation is consistent with the presence of nonlinear pathways resulting from underlying cross-peak signals between the coupled symmetric-asymmetric C≡O stretch pair. The successful demonstration of frequency-resolved ultrafast transient vibrational action spectroscopy of dilute molecular ion ensembles provides an exciting, new framework for the study of molecular dynamics in isolated, complex molecular ion systems.

Transient absorption microscopy setup with multi-ten-kilohertz shot-to-shot subtraction and discrete Fourier analysis

Recording of transient absorption microscopy images requires fast detection of minute optical density changes, which is typically achieved with high-repetition-rate laser sources and lock-in detection. Here, we present a highly flexible and cost-efficient detection scheme based on a conventional photodiode and an USB oscilloscope with MHz bandwidth, that deviates from the commonly used lock-in setup and achieves benchmark sensitivity. Our scheme combines shot-to-shot evaluation of pump-probe and probe-only measurements, a home-built photodetector circuit optimized for low pulse energies applying low-pass amplification, and a custom evaluation algorithm based on Fourier transformation. Advantages of this approach include abilities to simultaneously monitor multiple pulse modulation frequencies, implement the detection of additional pulse sequences (e.g., pump-only), and expand to multiple parallel detection channels for wavelength-dispersive probing. With a 40 kHz repetition-rate laser system powering two non-collinear optical parametric amplifiers for wide tuneability, we find that laser pulse fluctuations limit the sensitivity of the setup, while the detection scheme has negligible contribution. We demonstrate the 2-D imaging performance of our transient absorption microscope with studies on micro-crystalline molecular thin films.

Removal of correlated background in a high-order harmonic transient absorption spectra with principal component regression

We demonstrate a 40x mean noise power reduction (NPR) in core-to-valence extreme ultraviolet (XUV) femtosecond transient absorption spectroscopy with a high harmonic generation (HHG) light source. An adaptive iteratively reweighted principal component regression (airPCR) is used to analyze and suppress spectrally correlated HHG intensity fluctuations. The technique requires significantly less user input and leads to a higher mean NPR than a previously introduced edge-pixel PCR method that relies on the manual identification of signal-free spectral regions. Both techniques are applied in a time-resolved XUV absorption study of the 2snp¹P^o (n ≥ 2) autoionizing Rydberg states of helium, demonstrating sub-10^-3 optical density sensitivity.

Ultrafast-nonlinear ultraviolet pulse modulation in an AlInGaN polariton waveguide operating up to room temperature

Ultrafast nonlinear photonics enables a host of applications in advanced on-chip spectroscopy and information processing. These rely on a strong intensity dependent (nonlinear) refractive index capable of modulating optical pulses on sub-picosecond timescales and on length scales suitable for integrated photonics. Currently there is no platform that can provide this for the UV spectral range where broadband spectra generated by nonlinear modulation can pave the way to new on-chip ultrafast (bio-) chemical spectroscopy devices. We demonstrate the giant nonlinearity of UV hybrid light-matter states (exciton-polaritons) up to room temperature in an AlInGaN waveguide. We experimentally measure ultrafast nonlinear spectral broadening of UV pulses in a compact 100 μm long device and deduce a nonlinearity 1000 times that in common UV nonlinear materials and comparable to non-UV polariton devices. Our demonstration promises to underpin a new generation of integrated UV nonlinear light sources for advanced spectroscopy and measurement.

Nonlinearity enhancement in different materials is relevant for many scientific applications. Here the authors demonstrate pulse modulation in the UV regime due to polariton-based nonlinearity in an AlInGaN waveguide structure, including at room temperature.

Ultrahigh sensitive transient absorption spectrometer

Transient absorption (TA) spectroscopy is considered as a powerful technique that reflects the ultrafast dynamics of photogenerated carriers in photoelectric and photocatalysis materials. However, limited by its sensitivity, the photogenerated carrier density in TA measurements of solar energy materials is usually much higher than that in the real working condition. Here, we present a combination of kHz macro-pulse and MHz micro-pulse technique for an ultrahigh sensitive TA spectrometer, which improves the sensitivity to the 10^-7 level of ΔOD. It enables us to study ultrafast carrier dynamics pumped by very low power, which can avoid the influence of many-body interactions and the nonlinear effect associated with high carrier density. This work provides a novel TA method with ultrahigh sensitivity, which will play an important role in investigating the carrier dynamics of semiconductors in the working condition.

Broadband cavity-enhanced ultrafast spectroscopy

Broadband ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used to study molecular dynamics. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. In this article we discuss a cavity-enhanced ultrafast transient absorption spectrometer covering almost the entire visible range with a detection limit of ΔOD < 1 × 10-9, extending broadband all-optical ultrafast spectroscopy techniques to dilute beams of gas-phase molecules and clusters. We describe the technical innovations behind the spectrometer and present transient absorption data on two archetypical molecular systems for excited-state intramolecular proton transfer, 1'-hydroxy-2'-acetonapthone and salicylideneaniline, under jet-cooled and Ar cluster conditions.

Edge-pixel referencing suppresses correlated baseline noise in heterodyned spectroscopies

Referencing schemes are commonly used in heterodyned spectroscopies to mitigate correlated baseline noise arising from shot-to-shot fluctuations of the local oscillator. Although successful, these methods rely on careful pixel-to-pixel matching between the two spectrographs. A recent scheme introduced by Feng et al. [Opt. Express 27(15), 20323-20346 (2019)] employed a correlation matrix to allow free mapping between dissimilar spectrographs, leading to the first demonstration of floor noise limited detection on a multichannel array used in heterodyned spectroscopy. In addition to their primary results using a second reference spectrometer, Feng et al. briefly demonstrated the flexibility of their method by referencing to same-array pixels at the two spectral edges (i.e., edge-pixel referencing). We present a comprehensive study of this approach, which we term edge-pixel referencing, including optimization of the approach, assessment of the performance, and determination of the effects of background responses. We show that, within some limitations, the distortions due to background signals will not affect the 2D IR line shape or amplitude and can be mitigated by band narrowing of the pump beams. We also show that the performance of edge-pixel referencing is comparable to that of referencing to a second spectrometer in terms of noise suppression and that the line shapes and amplitudes of the spectral features are, within the measurement error, identical. Altogether, these results demonstrate that edge-pixel referencing is a powerful approach for noise suppression in heterodyned spectroscopies, which requires no new hardware and, so, can be implemented as a software solution for anyone performing heterodyned spectroscopy with multichannel array detectors already.

Optimized noise reduction scheme for heterodyne spectroscopy using array detectors

In this work, we optimize and further advance a noise reduction scheme for heterodyne spectroscopy. This scheme linearly combines data from reference detectors to predict the noise statistics in the signal detector through an optimized coefficient matrix. We validate this scheme for visible white-light-continuum and 800-nm light sources using un-matched CMOS arrays and show that the signal-to-noise ratio can approach the noise floor of the signal detector while using only ~5% of the energy for reference detection. We also optimize the strategy for estimating the coefficient matrix in practical applications. When combined with elaborate algorithms to perform pixel data compression and expansion, our scheme is applicable in difficult situations, including when the sample position is rapidly scanned, when detectors exhibit nonlinear response, and/or when laser fluctuations are large. The scheme is generalized to scenarios with complex chopping or phase cycling patterns, and a simple approach is provided for the chopping case. Finally, a robust and computationally efficient method is devised to remove multiplicative noise.

Octave-spanning single-cycle middle-infrared generation through optical parametric amplification in LiGaS2

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB₂O₄ (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS₂ (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

Reduction of laser-intensity-correlated noise in high-harmonic generation

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation. We show that the fluctuations of the extreme-ultraviolet (XUV) spectral power density can be predicted solely by monitoring the generating laser pulses; this method is in contrast with traditional balanced detection used in optical spectroscopy, where a replica of the signal is monitored. Such possibility emerges from a detailed investigation of high-harmonic generation (HHG) noise. We find that in a wide parameter range of the HHG process, the XUV fluctuations are dominated by a spectral blueshift, which is correlated to the near-infrared (NIR) driving laser intensity variation. Numerical simulations support our findings and suggest that non-adiabatic blueshift is the main source of XUV fluctuations. A straightforward post-processing of the XUV spectra allows for noise reduction and improved precision of attosecond transient absorption experiments. The technique is readily transferable to attosecond transient reflectivity and potentially to attosecond photoelectron spectroscopy.

Photometrics of ultrafast and fast broadband electronic transient absorption spectroscopy: State of the art

The physical limits of the photometric resolution in broadband electronic transient absorption spectroscopy are discussed together with solutions for how to reach these limits in practice. In the first part, quantitative expressions for the noise contributions to the transient absorption signal are derived and experimentally tested. Experimental approaches described in the literature are discussed and compared on this basis. Guide-lines for designing a setup are established. In the second part, a method for obtaining nearly shot-noise limited kinetics with photometric resolution of the order of 100 μOD in overall measurement times of a few minutes from femtosecond to microsecond time scale is presented. The results are discussed in view of other experiments of step-scan type which are subject to a background or to correlated noise. Finally, detailed information is provided on how to obtain transient absorption spectra where counting statistics are the sole source of noise. A method for how to suppress outliers without introducing bias is discussed. An application example is given to demonstrate the achievable signal-to-noise level and the fast acquisition time.

A compact design of a characterization station for far UV photodetectors

A newly fabricated characterization station is presented. It is a compact, cost-effective, and easily adjustable apparatus. Each part including 4-pin probe, manipulators, operating temperature, and applied bias can be independently controlled. The station can provide highly reliable, reproducible, and economical methods to quickly conduct and complete the characterizations of a large amount of sensing materials within a short period of time. It is particularly suitable for studies of various nanostructured materials and their related thermal effect, polarization effect, sensitivity, and electrical and electronic properties.

Coherent Two-Quantum Two-Dimensional Electronic Spectroscopy Using Incoherent Light

Two-quantum two-dimensional electronic spectroscopy (2Q 2D ES) may provide a measure of electron-correlation energies in molecules. Attempts to obtain this profound but elusive signal have relied on experimental implementations using femtosecond laser pulses, which induce an overwhelming background signal of nonresonant response. Here we explore theoretically the signatures of electron correlation in coherent 2Q 2D ES measurements that use spectrally incoherent light, I⁽⁴⁾ 2Q 2D ES. One can use such fields to suppress nonresonant response, and therefore this method may better isolate the desired signature of electron correlation. Using an appropriate treatment of the multilevel Bloch electronic system, we find that I⁽⁴⁾ 2Q 2D ES presents an opportunity to measure electron-correlation energies in molecules.

General noise suppression scheme with reference detection in heterodyne nonlinear spectroscopy

We devised a novel two-step reference scheme that can greatly suppress the additive and convolutional noises in heterodyne nonlinear spectroscopy. To optimally remove additive noise, we fully utilized the spectral correlation in multi-channel reference data through a linear combination and regression algorithm. Using our pump-probe 2D IR spectrometer, we demonstrated that our scheme can improve the signal-to-noise ratio by 10-30 times and reach the noise floor of the signal detector. The new algorithm is guaranteed to reduce noise, enables the use of unmatched reference detectors, and does not introduce baseline shift or signal distortion. This scheme is applicable to many heterodyne spectroscopic techniques.

Broadband UV-Vis vibrational coherence spectrometer based on a hollow fiber compressor

We describe a broadband transient absorption (TA) spectrometer devised to excite and probe, in the blue to UV range, vibrational coherence dynamics in organic molecules in condensed phase. A 800-nm Ti:Sa amplifier and a hollow fiber compressor are used to generate a 6-fs short pulse at 1 kHz. Broadband sum frequency generation with the fundamental pulse is implemented to produce a 400-nm, 8-fs Fourier limited short pulse. A UV-Vis white-light supercontinuum is implemented as a probe with intensity self-referencing to achieve a shot-noise-limited sensitivity. Rapid scanning of the pump-probe delay is shown very efficient in suppressing the noise resulting from low-frequency pump intensity fluctuations. Using either of the 800-nm or 400-nm broadband pulses as the pump for TA spectroscopy of organic molecules in solution, we resolve oscillatory signals down to the 320 nm probing wavelength with a 3200 cm^-1 FWHM bandwidth. Their Fourier transformation reveals the corresponding molecular vibrational spectra. Finally, we demonstrate the use of this setup as a vibrational coherence spectrometer for the investigation of the vibrational dynamics accompanying the sub-ps C=C photoisomerization of a retinal-like molecular switch through a conical intersection.

Stimulated Raman gas sensing by backward UV lasing from a femtosecond filament

We perform a proof-of-principle demonstration of chemically specific standoff gas sensing, in which a coherent stimulated Raman signal is detected in the direction anticollinear to a two-color laser excitation beam traversing the target volume. The proposed geometry is intrinsically free space as it does not involve back-scattering (reflection) of the signal or excitation beams at or behind the target. A beam carrying an intense mid-IR femtosecond (fs) pulse and a parametrically generated picosecond (ps) UV Stokes pulse is fired in the forward direction. A fs filament, produced by the intense mid-IR pulse, emits a backward-propagating narrowband ps laser pulse at the 337 and 357 nm transitions of excited molecular nitrogen, thus supplying a counter-propagating Raman pump pulse. The scheme is linearly sensitive to species concentration and provides both transverse and longitudinal spatial resolution.

Accurate convergence of transient-absorption spectra using pulsed lasers

Transient-absorption spectroscopy is a common and well-developed technique for measuring time-dependent optical phenomena. One important aspect, especially for measurements using pulsed lasers, is how to average multiple data acquisition events. Here, we use a mathematical analysis method based on covariance to evaluate various averaging schemes. The analysis reveals that the baseline and the signal converge to incorrect values without balanced detection of the probe, shot-by-shot detection, and a specific method of averaging. Experiments performed with sub-7 fs pulses confirm the analytic results and reveal insights into molecular excited-state vibrational dynamics.

100-kHz shot-to-shot broadband data acquisition for high-repetition-rate pump-probe spectroscopy

Shot-to-shot broadband detection is common in ultrafast pump-probe spectroscopy. Taking advantage of the intensity correlation of subsequent laser pulses improves the signal-to-noise ratio. Finite data readout times of CCD chips in the employed spectrometer and the maximum available speed of mechanical pump-beam choppers typically limit this approach to lasers with repetition rates of a few kHz. For high-repetition (≥ 100 kHz) systems, one typically averages over a larger number of laser shots leading to inferior signal-to-noise ratios or longer measurement times. Here we demonstrate broadband shot-to-shot detection in transient absorption spectroscopy with a 100-kHz femtosecond laser system. This is made possible using a home-built high-speed chopper with external laser synchronization and a fast CCD line camera. Shot-to-shot detection can reduce the data acquisition time by two orders of magnitude compared to few-kHz lasers while keeping the same signal-to-noise ratio.

A novel setup for femtosecond pump-repump-probe IR spectroscopy with few cycle CEP stable pulses

We present a three-color mid-IR setup for vibrational pump-repump-probe experiments with a temporal resolution well below 100 fs and a freely selectable spectral resolution of 20 to 360 cm(-1) for the pump and repump. The usable probe range without optical realignment is 900 cm(-1). The experimental design employed is greatly simplified compared to the widely used setups, highly robust and includes a novel means for generation of tunable few-cycle pulses with stable carrier-envelope phase. A Ti:sapphire pump system operating with 1 kHz and a modest 150 fs pulse duration supplies the total pump energy of just 0.6 mJ. The good signal-to-noise ratio of the setup allows the determination of spectrally resolved transient probe changes smaller than 6·10(-5) OD at 130 time delays in just 45 minutes. The performance of the spectrometer is demonstrated with transient IR spectra and decay curves of HDO molecules in lithium nitrate trihydrate and ice and a first all MIR pump-repump-probe measurement.

Polarization sensitive ultrafast mid-IR pump probe micro-spectrometer with diffraction limited spatial resolution

A setup of ultrafast transient infrared IR spectrometer is described in this paper that employed Schwarzschild objectives to focus the probe beam to a diffraction limited spot. Thus measurements were performed with very high spatial resolution in the mid-IR spectral region. Furthermore, modulating the polarization of the probe light enabled detecting transient dichroism of the sample. These capabilities of the setup were applied to study transient absorption of Photosystem II core complex and to image an organized film of methylene blue chloride dye. Moreover, a study of noise sources in a pump probe measurement is presented. The predicted noise level of the current setup was 8.25 μOD in 10(4) acquisitions and compared very well with the experimental observation of 9.6 μOD.

Capturing Ultrafast Quantum Dynamics with Femtosecond and Attosecond X-ray Core-Level Absorption Spectroscopy

Recent technical advances in ultrafast laser sources enable the generation of femtosecond and attosecond soft X-ray pulses in tabletop laser setups as well as accelerator-based synchrotron and free-electron laser sources. These new light sources can be harnessed via pump-probe spectroscopy to elucidate ultrafast quantum dynamics in atoms, molecules, and condensed matter with unprecedented time resolution and chemical sensitivity. Employing such ultrashort pulses in transient X-ray absorption spectroscopy combines the unique advantages of core-level absorption probing of chemical environments and oxidation states with the ability to obtain ultimately freeze-frame snapshots of electronic and nuclear dynamics. In this Perspective, we provide an overview of the progress in applying the recently developed technique of femtosecond to attosecond time-resolved soft X-ray transient absorption spectroscopy to the study of ultrafast phenomena, including some of our own efforts to elucidate the interaction of intense laser pulses with atoms and molecules in the strong-field, nonperturbative limit. Possible avenues for future work are outlined.

Seeding of picosecond and femtosecond optical parametric amplifiers by weak single mode continuous lasers

Optical parametric amplifiers are typically seeded with either parametric superfluorescence or broadband continuum pulses. We show both with picosecond and femtosecond pump pulses, that single longitudinal mode cw lasers with mW power can be well used to generate nearly Fourier-transform-limited output pulses. The 532 nm pumped picosecond system is seeded in the near infrared and fully tunable from 1260 to 1630 nm. The femtosecond system operates stable with just hundreds of seed photons. The output spectral width matches closely to the width of individual spectral features found in single shot spectra of parametric superfluorescence. Both systems provide interesting radiation sources for nonlinear optics experiments that need highly controlled and clean excitation.

Femtosecond pump/supercontinuum-probe spectroscopy: optimized setup and signal analysis for single-shot spectral referencing

A setup for pump/supercontinuum-probe spectroscopy is described which (i) is optimized to cancel fluctuations of the probe light by single-shot referencing, and (ii) extends the probe range into the near-uv (1000-270 nm). Reflective optics allow 50 μm spot size in the sample and upon entry into two separate spectrographs. The correlation γ(same) between sample and reference readings of probe light level at every pixel exceeds 0.99, compared to γ(consec)<0.92 reported for consecutive referencing. Statistical analysis provides the confidence interval of the induced optical density, ΔOD. For demonstration we first examine a dye (Hoechst 33258) bound in the minor groove of double-stranded DNA. A weak 1.1 ps spectral oscillation in the fluorescence region, assigned to DNA breathing, is shown to be significant. A second example concerns the weak vibrational structure around t=0 which reflects stimulated Raman processes. With 1% fluctuations of probe power, baseline noise for a transient absorption spectrum becomes 25 μOD rms in 1 s at 1 kHz, allowing to record resonance Raman spectra of flavine adenine dinucleotide in the S(0) and S(1) state.

Broadband tunable optical parametric amplification from a single 50 MHz ultrafast fiber laser

We have demonstrated a 0.7 microm - 1.9 microm wavelength-tunable light source based on a single-pass optical parametric amplification (OPA) in a multiperiod magnesium oxide-doped periodically poled lithium niobate crystal. The OPA pump was a frequency-doubled ultrafast ytterbium-doped fiber oscillator, and the residual 1040 nm laser power after frequency doubling was recycled to generate a supercontinuum seeding source. Compared with conventional OPAs, this system is free from timing jitter between the pump laser and the seeding source. Over 50% conversion efficiency was obtained with 10 nJ pump energy. Combined with a 50 MHz repetition rate, this versatile source is ideal for biomedical and spectroscopic applications.

Ultrafast internal conversion pathway and mechanism in 2-(2'-hydroxyphenyl)benzothiazole: a case study for excited-state intramolecular proton transfer systems

We study the ultrafast electronic relaxation of the proton transfer compound 2-(2'-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump-probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to approximately 100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S(0)/S(1) conical intersection at a 90 degrees twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S(1) surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S(1) state and after fast excited-state proton transfer (30-50 fs), HBT reaches the region of the S(1)/S(0) crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes approximately 100 ps for that step.