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The vacuum ultraviolet (VUV) absorption spectra of cyclic ethers consist primarily of Rydberg ← n transitions. By studying three cyclic ethers of varying ring size (tetrahydropyran, tetrahydrofuran and trimethylene oxide, n = 6-4), we investigated the influence of ring size on the VUV excited-state dynamics of the 3d Rydberg manifold using time-resolved photoelectron spectroscopy (TRPES), time-resolved mass spectroscopy (TRMS) and ab initio electronic structure calculations. Whereas neither the electronic characters nor the term energies of the excited-states are substantially modified when the ring-size is reduced from n = 6 to 5 to 4, the excited-state lifetimes concomitantly decrease five-fold. TRPES and TRMS allow us to attribute the observed dynamics to a Rydberg cascade from the initially excited d-Rydberg manifold via the p-Rydberg manifold to the s-Rydberg state. Cuts through potential energy surfaces along the C-O bond reveal that a nσ* state crossing brings the s-Rydberg state along a path to the ring-opened ground state. The observed difference in excited-state lifetimes is attributed to an increasing slope along the repulsive C-O bond coordinate as ring size decreases.

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Photolabile protecting groups offer synthetic routes with facile deprotection by photolysis, providing higher yields with less workup. Protecting groups producing only gaseous byproducts upon removal are particularly desirable as sustainable reagents. Cheletropic reactions provide just this combination of photocleavable groups with gaseous byproducts. However, only few protecting groups for cheletropic reactions have been described and little is known about their photochemistry and associated stereochemistry. Here we show that the sulfolene protecting group, used for diene protection and functionalization, can be photochemically removed. First, using TD-DFT, we conclude that the photochemical ring-opening occurs conrotatorily in a concerted process, consistent with the Woodward-Hoffmann rules. Experimentally, we demonstrate that this process can be photoinitiated by radiation between 180 and 300 nm and confirm the proposed stereochemistry in solution. Using time-resolved photoelectron spectroscopy, we determine that the first steps in the photochemical ring-opening of sulfolene occur on ultrafast timescales (≤84 fs), consistent with a fully concerted process. Our findings expand applications of the sulfolene protecting group and promise an easy strategy for control of cis-trans isomerization.

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 tier broadband SRS imaging. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We demonstrate the use of a Convolutional Denoising Autoencoder Neural Network to denoise Hyperspectral Stimulated Raman Scattering microscopy images., 2020 Photonics North (PN), May 26-28, 2020, Niagara Falls, Ontario

Pump-probe measurements aim to capture the motion of electrons and nuclei on their natural timescales (femtoseconds to attoseconds) as chemical and physical transformations take place, effectively making “molecular movies” with short light pulses. However, the quantum dynamics of interest are filtered by the coordinate-dependent matrix elements of the chosen experimental observable. Thus, it is only through a combination of experimental measurements and theoretical calculations that one can gain insight into the internal dynamics. Here, we report on a combination of structural (relativistic ultrafast electron diffraction, or UED) and spectroscopic (time-resolved photoelectron spectroscopy, or TRPES) measurements to follow the coupled electronic and nuclear dynamics involved in the internal conversion and photodissociation of the polyatomic molecule, diiodomethane (CH_{2}I_{2}). While UED directly probes the 3D nuclear dynamics, TRPES only serves as an indirect probe of nuclear dynamics via Franck-Condon factors, but it is sensitive to electronic energies and configurations, via Koopmans’ correlations and photoelectron angular distributions. These two measurements are interpreted with trajectory surface hopping calculations, which are capable of simulating the observables for both measurements from the same dynamics calculations. The measurements highlight the nonlocal dynamics captured by different groups of trajectories in the calculations. For the first time, both UED and TRPES are combined with theory capable of calculating the observables in both cases, yielding a direct view of the structural and nonadiabatic dynamics involved.

We demonstrate the use a low noise all-normal-dispersion nonlinear fiber as a supercontinuum light source for Hyperspectral Stimulated Raman Scattering microscopy., 2019 Photonics North (PN), May 21-23, 2019, Quebec City, QC, Canada

Previous studies using nonlinear microscopy have demonstrated that osteoarthritis (OA) is characterized by the gradual replacement of Type II collagen with Type I collagen. The objective of this study was to develop a prototype nonlinear laser scanning microendoscope capable of resolving the structural differences of collagen in various orthopaedically relevant cartilaginous surfaces. The current prototype developed a miniaturized femtosecond laser scanning instrument, mounted on an articulated positioning system, capable of both conventional arthroscopy and second-harmonic laser-scanning microscopy. Its optical system includes a multi-resolution optical system using a gradient index objective lens and a customized multi-purpose fiber optic sheath to maximize the collection of backscattered photons or provide joint capsule illumination. The stability and suitability of the prototype arthroscope to approach and image cartilage were evaluated through preliminary testing on fresh, minimally processed, and partially intact porcine knee joints. Image quality was sufficient to distinguish between hyaline cartilage and fibrocartilage through unique Type I and Type II collagen-specific characteristics. Imaging the meniscus revealed that the system was able to visualize differences in the collagen arrangement between the superficial and lamellar layers. Such detailed in vivo imaging of the cartilage surfaces could obviate the need to perform biopsies for ex vivo histological analysis in the future, and provide an alternative to conventional external imaging to characterize and diagnose progressive and degenerative cartilage diseases such as OA. Moreover, this system is readily customizable and may provide a suitable and modular platform for developing additional tools utilizing femtosecond lasers for tissue cutting within the familiar confines of two or three portal arthroscopy techniques., A device capable of both wide field arthroscopic imaging and laser scanning imaging of second harmonic through a single lens has been implemented. Capable of working through a small incision, it has tested on essentially intact porcine knee joints.

Multi-modal nonlinear optical (NLO) microscopy, including stimulated Raman scattering (SRS) and second harmonic generation (SHG), was used to directly image mineralogical features of economic ore and rock samples. In SRS/SHG imaging, ore samples generally require minimal preparation and may be rapidly imaged, even in their wet state. 3D structural details, at submicron resolution, are revealed tens of microns deep within samples. Standard mineral imaging based on scanning electron microscopy (SEM), with elemental analysis via energy dispersive X-Ray spectroscopy, was used to independently validate the mineral composition of the samples. Spatially-resolved SRS from dominant Raman-resonant bands precisely maps the locations of specific minerals contained within the samples. SHG imaging reveals locally non-centrosymmetric structures, such as quartz grains. Competing absorption and nonlinear scattering processes, however, can reduce contrast in SRS imaging. Importantly, the correlation between standard electron microscopy and multi-modal NLO optical microscopy shows that the latter offers rapid image contrast based on the mineral content of the sample.

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The vacuum ultraviolet photoinduced dynamics of cyclopropane C3H6 were studied using time resolved photoelectron spectroscopy TRPES in conjunction with ab initio quantum dynamics simulations. Following excitation at 160.8 nm, and subsequent probing via photoionization at 266.45 nm, the initially prepared wave packet is found to exhibit a fast decay lt;100 fs that is attributed to the rapid dissociation of C3H6 to ethylene C2H4 and methylene CH2 . The photodissociation process proceeds via concerted ring opening and C C bond cleavage in the excited state. Ab initio multiple spawning simulations indicate that ring opening occurs prior to dissociation. The dynamics simulations were subsequently employed to simulate a TRPES spectrum, which was found to be in excellent agreement with the experimental result. On the basis of this agreement, the fitted time constants of 35 20 and 57 35 fs were assigned to prompt i dissociation on the lowest lying excited state, prepared directly by the pump pulse, and ii non adiabatic relaxation from higher lying excited states that lead to delayed dissociation, respectively

The Pixel-Imaging Mass Spectrometry (PImMS) camera allows for 3D charged particle imaging measurements, in which the particle time-of-flight is recorded along with $(x,y)$ position. Coupling the PImMS camera to an ultrafast pump-probe velocity-map imaging spectroscopy apparatus therefore provides a route to time-resolved multi-mass ion imaging, with both high count rates and large dynamic range, thus allowing for rapid measurements of complex photofragmentation dynamics. Furthermore, the use of vacuum ultraviolet wavelengths for the probe pulse allows for an enhanced observation window for the study of excited state molecular dynamics in small polyatomic molecules having relatively high ionization potentials. Herein, preliminary time-resolved multi-mass imaging results from C$_2$F$_3$I photolysis are presented. The experiments utilized femtosecond UV and VUV (160.8~nm and 267~nm) pump and probe laser pulses in order to demonstrate and explore this new time-resolved experimental ion imaging configuration. The data indicates the depth and power of this measurement modality, with a range of photofragments readily observed, and many indications of complex underlying wavepacket dynamics on the excited state(s) prepared.

We compare the excited state dynamics of diiodomethane (CH2I2) and bromoiodomethane (CH2BrI) using time resolved photoelectron spectroscopy. A 4.65 eV UV pump pulse launches a dissociative wave packet on excited states of both molecules and the ensuing dynamics are probed via photoionization using a 7.75 eV probe pulse. The resulting photoelectrons are measured with the velocity map imaging technique for each pump-probe delay. Our measurements highlight differences in the dynamics for the two molecules, which are interpreted with high-level ab initio molecular dynamics (trajectory surface hopping) calculations. Our analysis allows us to associate features in the photoelectron spectrum with different portions of the excited state wave packet represented by different trajectories. The excited state dynamics in bromoiodomethane are simple and can be described in terms of direct dissociation along the C–I coordinate, whereas the dynamics in diiodomethane involve internal conversion and motion along multiple dimensions.

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Stimulated Raman Scattering (SRS) microscopy is a nonlinear microscopy technique based on Raman vibrational resonances determined by the frequency difference between Pump and Stokes laser pulses. Modulation of one laser beam transfers the modulation to the other, as either a gain in Stokes (SRG) or a loss in Pump power (SRL). SRS microscopy does not exhibit the four-wave mixing nonresonant background characteristic of CARS microscopy. However, other background signals due to two-photon absorption, thermal lensing or cross-phase modulation (XPM) do reduce the detection sensitivity and can distort the hyperspectral scans. Phase sensitive lock-in detection can reduce contributions from two-photon absorption, which is out-of-phase for the SRG case. However, the background signal due to XPM, which can be in-phase with SRS, can reduce the detection sensitivity. We present a novel polarization modulation (PM) scheme in SRS microscopy which greatly reduces the nonresonant XPM background, demonstrated here for the SRL case. Since many Raman vibrational transitions are parallel polarized, the SRS signal is maximum (minimum) when the polarizations of the pump and the Stokes beams are parallel (perpendicular). However, in both parallel and perpendicular Pump-Stokes geometries, XPM is non-zero in many media. Therefore, PM can remove the XPM background without significantly reducing the SRS signal. Our results show that the PM-SRS successfully removes the nonresonant signal due to XPM. High imaging contrast is observed, concomitant with high sensitivity at very low analyte concentrations and undistorted Raman spectra.

Time-resolved photoelectron spectroscopy in combination with ab initio quantum chemistry calculations was used to study ultrafast excited state dynamics in formamide (FOR), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA) following 160 nm excitation. The particular focus was on internal conversion processes within the excited state Rydberg manifold and on how this behavior in amides compared with previous observations in small amines. All three amides exhibited extremely rapid (

We demonstrate the utility of multimodal coherent anti-Stokes Raman scattering (CARS) microscopy for the study of structured condensed carbohydrate systems. Simultaneous second-harmonic generation (SHG) and spectrally-scanned CARS microscopy was used to elucidate structure, alignment, and density in cellulose cotton fibers and in starch grains undergoing rapid heat-moisture swelling. Our results suggest that CARS response of the O-H stretch region (3000 cm(-1)-3400 cm(-1)), together with the commonly-measured C-H stretch (2750 cm(-1)-2970 cm(-1)) and SHG provide potentially important structural information and contrast in these materials.

We present a simple hyperspectral Stimulated Raman Scattering (SRS) microscopy method based on spectral focusing of chirped femtosecond pulses, combined with amplitude (AM) and polarization (PM) modulation. This approach permits the imaging of low concentration components with reduced background signals, combined with good hyperspectral resolution and rapid spectral scanning. We demonstrate, using PM-SRS in a Raman loss configuration, the spectrally resolved detection of deuterated dimethyl sulfoxide (DMSO-d6) at concentrations as low as 0.039 % (5.5 mM). In general, background signals due to cross-phase modulation (XPM), two-photon absorption (TPA) and thermal lensing (TL) can reduce the contrast in SRS microscopy. We show that the nonresonant background signal contributing to the SRS signal is, in our case, largely due to XPM. Polarization modulation of the Stokes beam eliminates the nonresonant XPM background, yielding high quality hyperspectral scans at low analyte concentration. The flexibility of our combined AM-PM approach, together with the use of variable modulation frequency and lock-in phase, should allow for optimization of SRS imaging in more complex samples.

Simplified, robust approaches to label-free Coherent Raman Microscopy allow for applications ranging from cellular imaging and disease progression, to biomaterials, to microimaging of mining ore distributions., Applied Industrial Optics: Spectroscopy, Imaging and Metrology, AIO 2014, July 13-17, 2014, Seattle, Washington, USA

Carotenoids are highly conjugated natural polyenic compounds with notoriously large optical nonlinearities. The commercial importance of carotenoids such as β-carotene and astaxanthin currently exceeds $500M, as these compounds are used as dyes, natural supplements, cosmetics, and food additives. Furthermore, biologically, carotenoids are used for many purposes, including as optically important components in photosynthis complexes. The large optical nonlinearities in carotenoids arises from the long conjugated backbone of the carotenoid molecule. Vibronic coupling between the highlydelocalized electron modes along the backbone and certain vibrational modes yields exceedingly large Raman and stimulated Raman responses, even at wavelengths far removed from traditional "resonance Raman" conditions. We report on the exploitation of such large natural nonlinear vibrational responses for enabling both coherent Anti-Stokes Raman scattering (CARS) and electronic four-wave-mixing (FWM) microscopy of dilute concentrations of carotenoids in vitro and in live microalgae. We find that CARS/FWM signals from astaxanthin in pump wavelength regimes between 800nm-920nm surpasses that from bulk diamond by orders of magnitude, allowing for nearly-nonresonant-background-free hyperspectral CARS imaging down to below 1mM. These results strongly support the potential for CARS/FWM to be used for live monitoring of carotenogenesis in plants and microorganisms, and thus may also apply to the study of natural and artificial photosynthesis.

We describe a stable kHz rate laser ablation/desorption supersonic molecular beam source for use in kHz rate laser experiments. With the development of modern lasers that typically operate at kHz rates, a need has arisen for stable molecular beam laser ablation/desorption sources for the study of involatile species. Many biomolecules of interest cannot be brought into the gas phase without thermal decomposition by simply heating the substrate and most (especially refractory) metals have melting and boiling points that are impossible to reach with conventional ovens. The source is based upon strong nonresonant interaction of a dithering laser focus with a rotating and translating solid rod, hydrodynamic transport of the ablated/desorbed material in helium or argon, and subsequent supersonic expansion. Further design details include flexible and easy adjustment of the source for rapid prototyping and optimization for kHz rate performance. Due to the high rate of sample removal, a major concern is clogging of the nozzle and laser input channel due to both material condensation and debris formation. In order to illustrate the range of applications, we demonstrate (1) the kHz laser ablation of a high temperature refractory metal (niobium) for use in studies of metal clusters; and (2) the kHz laser desorption and jet cooling of an involatile biomolecule (the DNA base guanine) for use in spectroscopic and dynamical studies. This kHz source design has been shown to be stable for over 12 continuous hours of operation (>4×107(>4×107 laser shots) and can be readily scaled to even higher repetition rates (>10 kHz).

Femtosecond time-resolved photoelectron spectroscopy is emerging as an important technique for investigating photodynamics in polyatomic molecules. Photoelectron spectroscopy is sensitive to both electronic configurations and vibrational dynamics and is therefore well suited to the study of ultrafast non-adiabatic processes. We emphasize the conceptual interpretation of wavepacket dynamics experiments, particularly stressing the critical role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to help to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from internal conversion in polyenes and polyaromatic hydrocarbons, electronic relaxation in substituted benzenes, excited state intramolecular proton transfer, azobenzene photoiosomerization dynamics and non-adiabatic photodissociation dynamics.

A simultaneous time and frequency-resolved photoluminescence setup based upon the concept of sum-frequency generation has been developed using the parallel collection capabilities of a liquid nitrogen cooled, high quantum efficiency charge coupled device (CCD) detector. This up conversion system can provide excellent time resolution (down to ~ 100 fs), when the detector is used in the single channel mode, with large dynamic range. When the CCD detector is used in its full capacity (1024 channels), wide spectral range as well as temporal information are obtained simultaneously from the up-converted signal, thus providing important information on the dynamics of various emission peaks which can occur simultaneously. Together with this, an efficient method for generation of high repetition rate, low energy infrared light between 1.44 and 1.62 �m is described. The technique uses difference frequency mixing of the fundamental wavelength of a mode-locked Nd:yttrium aluminum garnet laser and the output of a synchronously pumped Rhodamine 640 dye laser in a 10 mm long lithium triborate crystal. This source was then used to performed cw and time-resolved up-conversion photoluminescence measurements on a 1.54 �m emitting InGaAsP epitaxial layer.

Coherent anti-Stokes Raman scattering (CARS) and four-wave-mixing (FWM) microscopy are a related pair of powerful nonlinear optical characterization tools. These techniques often yield strong signals from concentrated samples, but because of their quadratic dependence on concentration, they are not typically employed for imaging or identifying dilute cellular constituents. We report here that, depending on the excitation wavelengths employed, both CARS and degenerate-FWM signals from carotenoid accumulations in alga cysts can be exceptionally large, allowing for low-power imaging of astaxanthin (AXN) deposits in Haematococcus pluvialis microalga. By use of a broadband laser pulse scheme for CARS and FWM, we are able to simultaneously collect strong intrinsic two-photon-excitation fluorescence signals from cellular chlorophyll in vivo. We show that CARS signals from astaxanthin (AXN) samples in vitro strictly follow the expected quadratic dependence on concentration, and we demonstrate the collection of wellresolved CARS spectra in the fingerprint region with sensitivity below 2mM. We suggest that multimodal nonlinear optical microscopy is sufficiently sensitive to AXN and chlorophyll concentrations that it will allow for non-invasive monitoring of carotenogenesis in live H. pluvialis microalgae., SPIE BiOS, February 1-2, 2014, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 8937

A bakeable (200 °C) ultrahigh vacuum 50 Omega coaxial coupling capacitor is described. The capacitor is compatible with the General Radio G874 coaxial standard, has a large capacitance (6 nF) allowing for efficient transmission of both fast pulses and long analog waveforms, can hold off dc voltages of up to 5 kV, and has a bandwidth greater than 2 GHz, allowing coupling of very fast rise-time signals from cathode ground microchannel-plate detectors, often used in photoelectron spectroscopy. The capacitor design also provides a convenient bias tee for applying high voltage to the detector anode.

Pathological understanding of arterial diseases is mainly attributable to histological observations based on conventional tissue staining protocols. The emerging development of nonlinear optical microscopy (NLOM), particularly in second-harmonic generation, two-photon excited fluorescence and coherent Raman scattering, provides a new venue to visualize pathological changes in the extracellular matrix caused by atherosclerosis progression. These techniques in general require minimal tissue preparation and offer rapid three-dimensional imaging. The capability of label-free microscopic imaging enables disease impact to be studied directly on the bulk artery tissue, thus minimally perturbing the sample. In this review, we look at recent progress in applications related to arterial disease imaging using various forms of NLOM.

We present a broadband vibrational sum frequency generation (BB-VSFG) scheme using a novel ps visible pulse shape. We generate the fs IR pulse via standard procedures and simultaneously generate an 'inverted' time-asymmetric narrowband ps visible pulse via second harmonic generation in the pump depletion regime using a very long nonlinear crystal which has high group velocity mismatch (LiNbO3). The 'inverted' ps pulse shape minimally samples the instantaneous nonresonant response but maximally samples the resonant response, maintaining high spectral resolution. We experimentally demonstrate this scheme, presenting SFG spectra of canonical organic monolayer systems in the C-H stretch region (2800-3000 cm(-1)).

Wavepacket Dynamics (Born–Oppenheimer wavepacket dynamics; Nonadiabatic wave-packet dynamics; Selection of experimental observables) Principles of Time-Resolved Photoelectron Spectroscopy (Photoelectron spectroscopy as a probe (Motivation; Description of photoionization); Description of electronic and nuclear degrees of freedom (Complementary Koopmans' correlations (Type I); Corresponding Koopmans' correlations (Type II); Franck–Condon Spectra)) Applications of Time-Resolved Photoelectron Spectroscopy (Experimental demonstration of TRPES; Interrogation of molecular dynamics employing TRPES (Excited state intramolecular proton transfer; Photostability of DNA bases)), Series: Advanced Series in Physical Chemistry

In this work, we found drastic changes in decay dynamics of photo-induced photoelectron signals depending on the excitation energy., Laser and Electro-Optics (CLEO Europe/EQEC), 2011 Conference on and the 12th European Quantum Electronics Conference, Munich, May 22-26, 2011, Munich, Germany

We demonstrate the simultaneous collection and separation of femtosecond-laser-based forward-collected coherent anti-Stokes Raman scattering (F-CARS) and two-photon-excitation-induced fluorescence lifetime images (FLIM) using time-correlated single photon counting (TCSPC). We achieve this in a nondescanned geometry using a single multimode fiber without significant loss of light, field of view, and most importantly, TCSPC timing fidelity. In addition to showing the ability to separate CARS images from FLIM images using time gating, we also demonstrate composite multimodal epicollected FLIM imaging with fiber-collected F-CARS imaging in live cells.

Short laser pulse technology has significantly contributed to biomedical research, especially via nonlinear optical microscopy. Coherent anti-Stokes Raman scattering (CARS) microscopy is a label-free, chemical-selective method that is growing in importance as improved methods and light sources develop. Here we discuss different approaches to laser source development for CARS microscopy and highlight the advantages of a multimodal CARS microscope, illustrated by selected applications in biomedical research.

Nonlinear optical (NLO) microscopy provides a minimally invasive optical method for fast molecular imaging at subcellular resolution with 3D sectioning capability in thick, highly scattering biological tissues. In the current study, we demonstrate the imaging of arterial tissue using a nonlinear optical microscope based on photonic crystal fiber and a single femto-second oscillator operating at 800nm. This NLO microscope system is capable of simultaneous imaging extracellular elastin/collagen structures and lipid distribution within aortic tissue obtained from coronary atherosclerosis-prone WHHLMI rabbits (Watanabe heritable hyperlipidemic rabbit-myocardial infarction) Clear pathological differences in arterial lumen surface were observed between healthy arterial tissue and atherosclerotic lesions through NLO imaging., Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues VII, 26 January 2009, San Jose, CA, USA

The influence of homoconjugation on ultrafast dynamics of cyclohexa-1,4-diene was investigated by time-resolved photoelectron spectroscopy in comparison with cyclohexene. While little influence is seen on excited states, both molecules exhibit clearly different ground state dynamics.

We utilize second-harmonic- and sum-frequency-generation at the objective focus to optimize spectral resolution in a single-fs-laser-source CARS microscope. Chirp-matching provides spectral focusing with a

Using an all-fiber laser system consisting of a femtosecond Er/Yb fiber oscillator as the pump and an ultra-highly nonlinear fiber for Stokes generation, we demonstrate multimodal (TPF+SHG+CARS) non-linear optical microscopy of both tissue samples and live cells. Multimodal imaging was successfully performed with pixel dwell times as short as 4 micros at low laser powers (40 mW total).

Time-resolved photoelectron spectroscopy of excited state intramolecular proton transfer is presented for the first time. In o-hydroxybenzaldehyde, it occurs within 100 fs on an almost barrierless potential. The excess energy dependence of the subsequent internal conversion is presented.

In this study we compare the single-photon autofluorescence and multi-photon emission spectra obtained from the luminal surface of healthy segments of artery with segments where there are early atherosclerotic lesions. Arterial tissue was harvested from atherosclerosis-prone WHHL-MI rabbits (Watanabe heritable hyperlipidemic rabbit-myocardial infarction), an animal model which mimics spontaneous myocardial infarction in humans. Single photon fluorescence emission spectra of samples were acquired using a simple spectrofluorometer set-up with 400 nm excitation. Samples were also investigated using a home built multi-photon microscope based on a Ti:sapphire femto-second oscillator. The excitation wavelength was set at 800 nm with a ~100 femto-second pulse width. Epi-multi-photon spectroscopic signals were collected through a fibre-optics coupled spectrometer. While the single-photon fluorescence spectra of atherosclerotic lesions show minimal spectroscopic difference from those of healthy arterial tissue, the multi-photon spectra collected from atherosclerotic lesions show marked changes in the relative intensity of two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) signals when compared with those from healthy arterial tissue. The observed sharp increase of the relative SHG signal intensity in a plaque is in agreement with the known pathology of early lesions which have increased collagen content., Photonics North 2009, 24 May 2009, Quebec, Canada

We demonstrate high performance coherent anti-Stokes Raman scattering (CARS) microscopy of live cells and tissues with user-variable spectral resolution and broad Raman tunability (2500 - 4100 cm(-1)), using a femtosecond Ti:Sapphire pump and photonic crystal fiber output for the broadband synchronized Stokes pulse. Spectral chirp of the fs laser pulses was a user-variable parameter for optimization in a spectral focusing implementation of multimodal CARS microscopy. High signal-to-noise, high contrast multimodal imaging of live cells and tissues was achieved with pixel dwell times of 2-8 micros and low laser powers (30 mW total).

Traditional CARS microscopy using picosecond (ps) lasers has been applied to a wide variety of applications; however, the lasers required are expensive and require an environmentally stable lab. In our work, we demonstrate CARS microscopy using a single femtosecond (fs) laser combined with a photonic crystal fiber (PCF) and optimal chirping to achieve similar performance to the ps case with important added advantages: fs-CARS utilizes versatile source that allows CARS to be combined with other multiphoton techniques (e.g. SHG, TPF) for multimodal imaging without changing laser sources. This provides an attractive entry point for many researchers to the field. Furthermore, optimal chirping in fs-CARS also opens the door to the combination and extension of other techniques used in ps CARS microscopy such as multiplex and FM imaging. The key advantage with chirped fs pulses is that time delay corresponds to spectral scanning and allows for rapid modulation of the resonant CARS signal. The combination of a fs oscillator with a PCF leads to a natural extension of the technology towards an all-fiber source for multimodal multiphoton microscopy. An all-fiber system should be more robust against environmental fluctuations while being more compact than free-space systems. We have constructed and demonstrated a proof of concept all-fiber based source that can be used for simultaneous CARS, TPF and SHG imaging. This system is capable of imaging tissue samples and live cell cultures with 4 μs/pixel dwell time at low average powers.

Coherent anti-Stokes Raman scattering (CARS) microscopy is a promising tool for live cell imaging and has been applied to the study of diseases such as multiple sclerosis, atherosclerosis and hepatitis. Combining PCFs with optimally chirped fs pulses, high performance multimodal (i.e. simultaneous two-photon fluorescence (TPF), second harmonic generation (SHG) and CARS) imaging of live cells and tissues is possible using only a single laser. Here the paper demonstrates a fully operational multimodal live cell and tissue imaging system based on a compact fs Er fiber oscillator system. A key component is a newly developed ultra-highly nonlinear silica fiber (UHNLF) which greatly reduces the peak power requirement for Stokes pulse generation. A commercial fully integrated frequency doubled fs Er fiber laser (IMRA F100) was the pump for TPF and SHG imaging. Here, in this paper a multimodal image of a fixed rabbit atherosclerotic arterial sample where TPF (elastin), SHG (collagen) and CARS (lipid) signals were collected simultaneously. This demonstrates for the first time the feasibility of a compact all-fiber source for multimodal CARS microscopy of live cells and tissue., Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference, June 14-19, 2009, Munich, Germany

Nonlinear optical (NLO) microscopy is used to follow key structural and biochemical changes associated with the progression of atherosclerosis. Arteries from WHHL-MI rabbits are examined using a 3 channel NLO microscope that can simultaneously monitor the coherent anti-stokes Raman scattered light (CARS), the two-photon excited fluorescence (TPEF) and the second harmonic generation (SHG) from a sample. Distinct differences in the nonlinear optical signals are observed that correlate with the age of the vessel and the presence of atherosclerotic plaque. These differences are attributed to the changing extracellular matrix and the increased lipid deposition associated with plaque development. The capability of NLO to perform 3D sectioning in thick highly scattering vessels in order to visualize structural details of the artery wall and highlight vessel pathology is demonstrated. These features make NLO a potentially valuable tool to help understand the progression of atherosclerosis.

Femtosecond time resolved photoelectron-photoion Coincidence Imaging Spectroscopy was used to study the non-adiabatic photodissociation dynamics of the NO dimer at 209 nm. Correlated photoelectron-photofragment energy and angular distributions reveal new details of the dissociation dynamics.

Femtosecond time resolved photoelectron-photoion Coincidence Imaging Spectroscopy was used to study the non-adiabatic photodissociation dynamics of the NO dimer at 209 nm. Correlated photoelectron-photofragment energy and angular distributions reveal new details of the dissociation dynamics.

Optically writable, thermally erasable surface relief gratings in thin Disperse Red 1 polymethyl methacrylate azopolymer films were used to demonstrate an arbitrarily reconfigurable fiber Bragg filter. Gratings were optically written on azopolymer-coated side-polished fiber blocks, and a write-erase-write cycle was demonstrated. Finite difference time domain simulations reveal that this optically reconfigurable device concept can be optimized in a silicon-on-insulator waveguide platform.

We report on the strong field ionization of small transition metal clusters (nickel, Ni(n) n=1-36) within the quasistatic regime at an infrared wavelength of 1.5 microm and at intensities up to 2 x 10(14) W/cm(2). From ion yields in a constant axial intensity beam, we obtained saturation intensities for the individual Ni(n) clusters. As compared to quasistatic, single active electron calculations, a dramatic suppression of ionization was observed. Dynamic polarization in the laser field likely leads to strong multielectron screening of the "active" electron. Representing the metal clusters as classical conducting spheres, we obtained, via a barrier suppression calculation, the classical ionization rates. Agreement was obtained for larger clusters with n>10 when the dynamic polarization was taken into account, emphasizing the multielectron nature of the ionization suppression.

Coherent anti-Stokes Raman scattering (CARS) microscopy [1] requires two input laser colours, called ‘Pump’ and ‘Stokes’ pulses; the frequency difference between them being tuned to the Raman resonance of interest. For live cell and tissue imaging, much of the focus has been on using picosecond lasers, either from two independent synchronized lasers or from an optical parametric oscillator (OPO) [2]. The use of femtosecond lasers has been explored, but more often in terms of multiplex CARS microscopy of cells [3, 4] and not tissue imaging. We have successfully employed a single femtosecond oscillator to perform tissue and live cell imaging with good spectral resolution, based upon conversion of a two-photon microscope.