Wave packet dynamics and control in excited states of molecular nitrogen.

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.

Degradation of Proteoglycans and Collagen in Equine Meniscal Tissues.

Investigate meniscal extracellular matrix degradation. Equine menisci (n = 34 from 17 horses) were studied. Site-matched sections were cut and scored from three regions (ROIs; n = 102) and stained for histology, proteoglycan (safranin O and fast green), aggrecan, and collagen cleavage (NITEGE, DIPEN, and C1,2C antibodies, respectively). Picrosirius red and second harmonic generation microscopy were performed to investigate collagen ultrastructure. A total of 42 ROIs met the inclusion criteria and were included in the final analysis. The median (range) ROI histological score was 3 (0-9), providing a large spectrum of pathology. The median (range) proteoglycan score was 1 (0-3), representing superficial and central meniscal loss. The median (range) of DIPEN, NITEGE, and C1,2C scores was 1 (0-3), revealing immunostaining of the femoral and tibial surfaces. The proteoglycan scores exhibited significant positive associations with both histologic evaluation (p = 0.03) and DIPEN scores (p = 0.02). Additionally, a robust positive association (p = 0.007) was observed between the two aggrecanolysis indicators, NITEGE and DIPEN scores. A negative association (p = 0.008) was identified between NITEGE and histological scores. The C1,2C scores were not associated with any other scores. Picrosirius red and second harmonic generation microscopy (SHGM) illustrated the loss of the collagen matrix and structure centrally. Proteoglycan and collagen degradation commonly occur superficially in menisci and less frequently centrally. The identification of central meniscal proteoglycan and collagen degradation provides novel insight into central meniscal degeneration. However, further research is needed to elucidate the etiology and sequence of degradative events.

Tight-focusing parabolic reflector schemes for petawatt lasers.

A comparative study of three different tight-focusing schemes for high-power lasers is performed numerically. Using the Stratton-Chu formulation, the electromagnetic field in the vicinity of the focus is evaluated for a short-pulse laser beam incident upon an on-axis high numerical aperture parabola (HNAP), an off-axis parabola (OAP), and a transmission parabola (TP). Linearly- and radially-polarized incident beams are considered. It is demonstrated that while all the focusing configurations yield intensities above 1023 W/cm2 for a 1 PW incident beam, the nature of the focused field can be drastically modified. In particular, it is shown that the TP, with its focal point behind the parabola, actually converts an incoming linearly-polarized beam into an m = 2 vector beam. The strengths and weaknesses of each configuration are discussed in the context of future laser-matter interaction experiments. Finally, a generalization of NA calculations up to 4π-illumination is proposed through the solid angle formulation, providing a universal way to compare light cones from any kind of optics.

Complete characterization of a Yb-based OPA at a high repetition rate using frequency resolved optical switching.

We demonstrate experimentally that frequency resolved optical switching (FROSt) can be used to characterize ultra-broadband pulses at high repetition rates up to 500 kHz. Specifically, we present the complete temporal characterization of an optical parametric amplifier (OPA), from the supercontinuum (SC) to the second stage of amplification. Simultaneous characterization of co-propagating signal and idler pulses enables retrieval of their group delay, as well as their temporal phase and intensity. Our study focuses on an extensive frequency range spanning the infrared region (1.2 to 2.4 µm) and confirms the strength and convenience of FROSt as a single tool for characterizing a wide range of pulses at high repetition rates.

Mid-infrared frequency domain optical parametric amplifier.

We report on an optical architecture delivering sub-120 femtosecond laser pulses of 20 µJ tunable from 5.5 µm to 13 µm in the mid-infrared range (mid-IR). The system is based on a dual-band frequency domain optical parametric amplifier (FOPA) optically pumped by a Ti:Sapphire laser and amplifying 2 synchronized femtosecond pulses each with a widely tunable wavelength around 1.6 and 1.9 µm respectively. These amplified pulses are then combined in a GaSe crystal to produce the mid-IR few-cycle pulses by means of difference frequency generation (DFG). The architecture provides a passively stabilized carrier-envelope phase (CEP) whose fluctuations has been characterized to 370 mrad RMS.

Ultrafast Resonant State Formation by the Coupling of Rydberg and Dark Autoionizing States.

Studying the dynamics of dark states is challenging due to their inability to undergo single-photon emission or absorption. This challenge is made even more difficult for dark autoionizing states owing to their ultrashort lifetime of a few femtoseconds. High-order harmonic spectroscopy recently appeared as a novel method to probe the ultrafast dynamics of a single atomic or molecular state. Here, we demonstrate the emergence of a new type of ultrafast resonance state as a manifestation of coupling between Rydberg and a dark autoionizing state dressed by a laser photon. Through high-order harmonic generation, this resonance results in extreme ultraviolet light emission that is more than one order of magnitude stronger than for the off-resonance case. The induced resonance can be leveraged to study the dynamics of a single dark autoionizing state and the transient changes in the dynamics of real states due to their overlap with the virtual laser-dressed states. In addition, the present results allow the generation of coherent ultrafast extreme ultraviolet light for advanced ultrafast science applications.

Spectral phase sensitivity of frequency resolved optical switching for broadband IR pulse characterization.

In this work, we demonstrate the sensitivity of the frequency-resolved optical switching (FROSt) technique to detect a small amount of spectral phase shift for the precise characterization of ultrashort laser pulses. We characterized fs pulses centered at 1.75 µm that are spectrally broadened up to 700 nm of bandwidth in a hollow-core fiber and subsequently compressed down to 2.3 optical cycle duration by propagation in the air at atmospheric pressure. By inserting thin fused silica windows of different thicknesses in the beam path, we accurately retrieve group delay dispersion (GDD) variations as small as 10 fs2. Such GDD variations correspond to a change of the pulse duration of only 0.2 fs for a Fourier transform limited 2-cycle pulse at 1.75 µm (i.e., 11.8 fs). The capability to measure such tiny temporal variations thus demonstrates that the FROSt technique has sufficient sensitivity to precisely characterize single-cycle pulses.

Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence.

The effect of the incident UV pump wavelength on the subsequent excited state dynamics, electronic relaxation, and ultimate dissociation of formaldehyde is studied using first principles simulation and Coulomb explosion imaging (CEI) experiments. Transitions in a vibronic progression in the à ← X̃ absorption band are systematically prepared using a tunable UV source which generates pulses centered at 304, 314, 329, and 337 nm. We find, both via ab initio simulation and experimental results, that the rate of excited state decay and subsequent dissociation displays a prominent dependence on which vibronic transition in the absorption band is prepared by the pump. Our simulations predict that nonadiabatic transition rates and dissociation yields will increase by a factor of >100 as the pump wavelength is decreased from 337 to 304 nm. The experimental results and theoretical simulations are in broad agreement and both indicate that the dissociation rate plateaus rapidly after ≈2 ps following an ultrafast sub-ps rise.

Polarization-independent pulse retrieval based on frequency resolved optical switching.

We demonstrate experimentally that the frequency resolved optical switching (FROSt) method is independent of the polarization direction of the pulse to be characterized. In this perspective, it is employed to characterize two or three co-propagating pulses linearly polarized in orthogonal directions, enabling to retrieve simultaneously their temporal intensity and phase profiles together with their group delay. This technique is also applied to track a simple nonlinear process involving different polarization states: type-I second harmonic generation (SHG). We are able to characterize the depleted fundamental pulse along with the generated second-harmonic pulse, thus demonstrating that the FROSt technique is a practical and powerful tool to observe nonlinear processes both in the temporal and spectral domains even if it involves different polarization states.

Unsupervised hyperspectral stimulated Raman microscopy image enhancement: denoising and segmentation via one-shot deep learning.

Hyperspectral stimulated Raman scattering (SRS) microscopy is a label-free technique for biomedical and mineralogical imaging which can suffer from low signal-to-noise ratios. Here we demonstrate the use of an unsupervised deep learning neural network for rapid and automatic denoising of SRS images: UHRED (Unsupervised Hyperspectral Resolution Enhancement and Denoising). UHRED is capable of "one-shot" learning; only one hyperspectral image is needed, with no requirements for training on previously labelled datasets or images. Furthermore, by applying a k-means clustering algorithm to the processed data, we demonstrate automatic, unsupervised image segmentation, yielding, without prior knowledge of the sample, intuitive chemical species maps, as shown here for a lithium ore sample.

Compressed ultrafast tomographic imaging by passive spatiotemporal projections.

Existing streak-camera-based two-dimensional (2D) ultrafast imaging techniques are limited by long acquisition time, the trade-off between spatial and temporal resolutions, and a reduced field of view. They also require additional components, customization, or active illumination. Here we develop compressed ultrafast tomographic imaging (CUTI), which passively records 2D transient events with a standard streak camera. By grafting the concept of computed tomography to the spatiotemporal domain, the operations of temporal shearing and spatiotemporal integration in a streak camera's data acquisition can be equivalently expressed as the spatiotemporal projection of an (x,y,t) datacube from a certain angle. Aided by a new, to the best of our knowledge, compressed-sensing reconstruction algorithm, the 2D transient event can be accurately recovered in a few measurements. CUTI is exhibited as a new imaging mode universally adaptable to most streak cameras. Implemented in an image-converter streak camera, CUTI captures the sequential arrival of two spatially modulated ultrashort ultraviolet laser pulses at 0.5 trillion frames per second. Applied to a rotating-mirror streak camera, CUTI records an amination of fast-bouncing balls at 5,000 frames per second.

Time-resolved photoelectron imaging of complex resonances in molecular nitrogen.

We have used the FERMI free-electron laser to perform time-resolved photoelectron imaging experiments on a complex group of resonances near 15.38 eV in the absorption spectrum of molecular nitrogen, N2, under jet-cooled conditions. The new data complement and extend the earlier work of Fushitani et al. [Opt. Express 27, 19702-19711 (2019)], who recorded time-resolved photoelectron spectra for this same group of resonances. Time-dependent oscillations are observed in both the photoelectron yields and the photoelectron angular distributions, providing insight into the interactions among the resonant intermediate states. In addition, for most states, we observe an exponential decay of the photoelectron yield that depends on the ionic final state. This observation can be rationalized by the different lifetimes for the intermediate states contributing to a particular ionization channel. Although there are nine resonances within the group, we show that by detecting individual photoelectron final states and their angular dependence, we can identify and differentiate quantum pathways within this complex system.

All normal dispersion nonlinear fibre supercontinuum source characterization and application in hyperspectral stimulated Raman scattering microscopy.

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful label-free, chemical-specific technique for biomedical and mineralogical imaging. Usually, broad and rapid spectral scanning across Raman bands is required for species identification. In many implementations, however, the Raman spectral scan speed is limited by the need to tune source laser wavelengths. Alternatively, a broadband supercontinuum source can be considered. In SRS microscopy, however, source noise is critically important, precluding many spectral broadening schemes. Here we show that a supercontinuum light source based on all normal dispersion (ANDi) fibres provides a stable broadband output with very low incremental source noise. We characterized the noise power spectral density of the ANDi fibre output and demonstrated its use in hyperspectral SRS microscopy applications. This confirms the viability and ease of implementation of ANDi fibre sources for broadband SRS imaging.

Single-shot phase-matching free ultrashort pulse characterization based on transient absorption in solids.

The frequency-resolved optical switching (FROSt) method developed for ultrashort pulse characterization is implemented for single-shot measurements. In this basic demonstration, the delay axis of the spectrogram is spatially encoded by the pump beam having a small incident angle with the photoexcited material. We present the calibration procedure for spectrograms acquired in single-shot and the temporal characterization of 44 fs pulses with central wavelength at 800 nm both in scanning and single-shot FROSt configurations. The retrieved pulses are compared by means of the root-mean-square field error. Finally, the pulses are propagated through a known dispersive material to measure the added group-delay dispersion.

Raman effect in the spectral broadening of ultrashort laser pulses in saturated versus unsaturated hydrocarbon molecules.

A conventional hollow core fiber (HCF) scheme is implemented to investigate spectral broadening of Titanium:Sapphire (Ti-Sa) femtosecond laser pulses in saturated hydrocarbon molecules compared to unsaturated ones. While the saturated molecules exhibit a spectral broadening similar to noble gases, for the unsaturated ones with π bonds, broadening towards blue is restrained. Numerical simulations underpin that it is a combination of group velocity dispersion (GVD) and Raman scattering which limits the spectral broadening for the unsaturated molecules. Compression of low energy ∼40fs pulses to ∼8fs using saturated hydrocarbons is demonstrated, suggesting the feasibility of this media for high repetition rate laser pulse compression.

10.4 kW coherently combined ultrafast fiber laser.

An ultrafast laser delivering 10.4 kW average output power based on a coherent combination of 12 step-index fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at an 80 MHz repetition rate, and has a high beam quality (M2≤1.2) and a low relative intensity noise of 0.56% in the frequency range of 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared.

In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

Plasmonic Enhancement of Two-Photon Excitation Fluorescence by Colloidal Assemblies of Very Small AuNPs Templated on M13 Phage.

In this study, an engineered M13 bacteriophage was examined as a biological template to create a well-defined spacing between very small gold nanoparticles (AuNPs 3-13 nm). The effect of the AuNP particle size on the enhancement of the nonlinear process of two-photon excitation fluorescence (2PEF) was investigated. Compared to conventional (one-photon) microscopy techniques, such nonlinear processes are less susceptible to scattering given that the density of background-scattered photons is too low to generate a detectable signal. Besides this, the use of very small AuNPs in 2PEF microscopy becomes more advantageous because individual "isolated" AuNPs of this size do not sufficiently enhance 2PEF to produce a detectable signal, resulting in even less background signal. To investigate the 2PEF of the AuNP-M13 assemblies, a variety of sample preparation approaches are tested, and surface-enhanced Raman spectroscopy (SERS) is employed to study the strength of plasmon coupling within the gaps of AuNPs assembled on the M13 template. Results indicate that assemblies prepared with 9-13 nm AuNP were able to clearly label Escherichia coli cells and produce a 2PEF signal that was orders of magnitude higher than the isolated AuNP (below the threshold of detection). This study thus provides a better understanding of the opportunities and limitations relevant to the use of such small AuNPs within colloidal plasmonic assemblies, for applications in biodetection or as imaging contrast agents.

Probing Rydberg-Rydberg interactions in N2 by ultrafast EUV-NIR photoelectron spectroscopy.

The ultrafast dynamics of molecular nitrogen (N2) just below the ionization threshold has been investigated by time-resolved photoelectron spectroscopy using a single harmonic centered at hν = 15.38 eV. The evolution of the Rydberg wavepacket launched by the ultrashort EUV pulse is probed by a time-delayed femtosecond NIR laser pulse. The observed photoelectron spectra show two series of vibrational peaks to the ground X2Σg+ state and the first excited A2Πu state of N2+. Among these, two photoelectron peaks with the vibrational quantum numbers vX+ = 4 and vA+ = 1 exhibit clear anti-phase oscillation with a period of 300 fs, showing that two Rydberg states converging to the X2Σg+ and A2Πu ionic states interact with each other, thus causing periodic switching in the population of the ion core states.

Single-scan interferometric second harmonic generation microscopy using a kHz phase-scanner.

In conventional laser-scanning microscopy, images are formed by acquiring the signal from pixel to pixel. Here, we report more than one order of magnitude reduction in acquisition time of Interferometric Second Harmonic Generation (I-SHG) by scanning the phase within each pixel, to characterize the relative polarity of various samples. Using an electro-optic phase-scanner, we show that the phase-shift patterns required for interferometry can be applied at each pixel during the scanning of the sample, allowing single-scan I-SHG (1S-ISHG) measurements. Requiring exposure times comparable to standard SHG intensity images, the additional phase information of the signal can thus be retrieved in parallel to its amplitude at the time-scale of seconds. Moreover, slower modulations can be used to enhance the precision of the phase measurement, without any spatial or temporal shift between interferograms, in contrast to conventional frame phase-shifting I-SHG (standard I-SHG). This continues to extend I-SHG to dynamical processes, and opens it to large-scale studies, as well as to imaging samples where the signal-to-noise ratio is an issue.