Publications by authors named "François Légaré"

67 Publications

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

J Chem Phys 2021 Apr;154(14):144305

Elettra-Sincrotrone Trieste, SS 14, km 163.5, in Area Science Park, 34149 Basovizza, Trieste, Italy.

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, N, 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.
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http://dx.doi.org/10.1063/5.0046577DOI Listing
April 2021

Compressed ultrafast tomographic imaging by passive spatiotemporal projections.

Opt Lett 2021 Apr;46(7):1788-1791

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 (,,) 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.
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http://dx.doi.org/10.1364/OL.420737DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8050836PMC
April 2021

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

Opt Express 2020 Nov;28(24):35997-36008

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.
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http://dx.doi.org/10.1364/OE.404977DOI Listing
November 2020

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

Opt Express 2020 Nov;28(24):35807-35815

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.
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http://dx.doi.org/10.1364/OE.409342DOI Listing
November 2020

Capturing roaming molecular fragments in real time.

Science 2020 11;370(6520):1072-1077

Centre Énergie Matériaux Télécommunications, Institut National de la Recherche Scientifique, Varennes, Quebec J3X 1S2, Canada.

Since the discovery of roaming as an alternative molecular dissociation pathway in formaldehyde (HCO), it has been indirectly observed in numerous molecules. The phenomenon describes a frustrated dissociation with fragments roaming at relatively large interatomic distances rather than following conventional transition-state dissociation; incipient radicals from the parent molecule self-react to form molecular products. Roaming has been identified spectroscopically through static product channel-resolved measurements, but not in real-time observations of the roaming fragment itself. Using time-resolved Coulomb explosion imaging (CEI), we directly imaged individual "roamers" on ultrafast time scales in the prototypical formaldehyde dissociation reaction. Using high-level first-principles simulations of all critical experimental steps, distinctive roaming signatures were identified. These were rendered observable by extracting rare stochastic events out of an overwhelming background using the highly sensitive CEI method.
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http://dx.doi.org/10.1126/science.abc2960DOI Listing
November 2020

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

Opt Lett 2020 Sep;45(18):5295-5298

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.
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http://dx.doi.org/10.1364/OL.403362DOI Listing
September 2020

Information transfer via temporal convolution in nonlinear optics.

Sci Rep 2020 Sep 11;10(1):14969. Epub 2020 Sep 11.

INRS-EMT, 1650 Blvd Lionel-Boulet, Varennes, QC, J3X1S2, Canada.

Nonlinear parametric processes involving ultrashort pulses are typically carried out in time domain, which mathematically corresponds to a convolution of their frequency spectra. In contrast, this spectral convolution changes into a multiplication operation when performing the nonlinear interaction in frequency domain. Here, we extend the scope of frequency-domain nonlinear optics by demonstrating its ability to perform a temporal convolution. Through this approach, nonlinear optical operations that are inaccessible in time domain can be realised: specific optical information can be coherently advanced by picoseconds within a pulse sequence-a newly generated second harmonic pulse carries the amplitude and phase information of two input pulses. This central pulse is isolated when using an input field consisting of two cross-polarized input pulses in combination with type-II second harmonic generation. The effects of nonlinear temporal convolution can be viewed from the aspect of signal processing and pulse shaping, where the nonlinear interaction in the parametric crystal plays the role of a dynamic linear optical filter-in contrast to conventional static filters-with a shaping mask instantaneously adapting to the laser field.
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http://dx.doi.org/10.1038/s41598-020-72170-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7486391PMC
September 2020

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

Biomacromolecules 2020 07 1;21(7):2705-2713. Epub 2020 Jul 1.

EMT Research Center, Institut National de la Recherche Scientifique (INRS), Varennes, Quebec J3X 1S2, Canada.

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 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.
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http://dx.doi.org/10.1021/acs.biomac.0c00401DOI Listing
July 2020

10.4  kW coherently combined ultrafast fiber laser.

Opt Lett 2020 Jun;45(11):3083-3086

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 (≤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.
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http://dx.doi.org/10.1364/OL.392843DOI Listing
June 2020

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

Opt Express 2020 Jan;28(2):980-990

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.
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http://dx.doi.org/10.1364/OE.380941DOI Listing
January 2020

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

Opt Express 2019 Dec;27(26):38435-38450

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.
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http://dx.doi.org/10.1364/OE.27.038435DOI Listing
December 2019

Maturation of the Meniscal Collagen Structure Revealed by Polarization-Resolved and Directional Second Harmonic Generation Microscopy.

Sci Rep 2019 12 5;9(1):18448. Epub 2019 Dec 5.

Institut National de la Recherche Scientifique, Centre Énergie Matériaux Télécommunications (INRS-EMT); 1650 Boul. Lionel-Boulet, Varennes (QC), J3X 1S2, Canada.

We report Polarization-resolved Second Harmonic Generation (P-SHG) and directional SHG (forward and backward, F/B) measurements of equine foetal and adult collagen in meniscus, over large field-of-views using sample-scanning. Large differences of collagen structure and fibril orientation with maturation are revealed, validating the potential for this novel methodology to track such changes in meniscal structure. The foetal menisci had a non-organized and more random collagen fibrillar structure when compared with adult using P-SHG. For the latter, clusters of homogeneous fibril orientation (inter-fibrillar areas) were revealed, separated by thick fibers. F/B SHG showed numerous different features in adults notably, in thick fibers compared to interfibrillar areas, unlike foetal menisci that showed similar patterns for both directions. This work confirms previous studies and improves the understanding of meniscal collagen structure and its maturation, and makes F/B and P-SHG good candidates for future studies aiming at revealing structural modifications to meniscus due to pathologies.
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http://dx.doi.org/10.1038/s41598-019-54942-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6895152PMC
December 2019

Temporal characterization of femtosecond laser pulses using tunneling ionization in the UV, visible, and mid-IR ranges.

Sci Rep 2019 Nov 5;9(1):16067. Epub 2019 Nov 5.

Center for Relativistic Laser Science, Institute for Basic Science, Gwangju, 61005, Korea.

To generalize the applicability of the temporal characterization technique called "tunneling ionization with a perturbation for the time-domain observation of an electric field" (TIPTOE), the technique is examined in the multicycle regime over a broad wavelength range, from the UV to the IR range. The technique is rigorously analyzed first by solving the time-dependent Schrödinger equation. Then, experimental verification is demonstrated over an almost 5-octave wavelength range at 266, 1800, 4000 and 8000 nm by utilizing the same nonlinear medium - air. The experimentally obtained dispersion values of the materials used for the dispersion control show very good agreement with the ones calculated using the material dispersion data and the pulse duration results obtained for 1800 and 4000 nm agree well with the frequency-resolved optical gating measurements. The universality of TIPTOE arises from its phase-matching-free nature and its unprecedented broadband operation range.
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http://dx.doi.org/10.1038/s41598-019-52237-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6831698PMC
November 2019

Extremely broadband terahertz generation via pulse compression of an Ytterbium laser amplifier.

Opt Express 2019 Oct;27(22):32659-32665

We present a system for extremely broadband terahertz (THz) generation based on an Ytterbium (Yb) amplified laser emitting 170-fs-long pulses centered at 1030 nm. The pulses are first spectrally broadened in an Ar-filled hollow-core capillary fiber (HCF) and then recompressed down to ∼18 fs with a chirped-mirror pair. Extreme broadband THz pulses of bandwidths up to 60 THz and peak electric field as high as 55 kV/cm are obtained via two-color plasma generation. The combination of high-power Yb laser systems with gas-filled HCF opens the path towards the realization of the next generation high-repetition-rate, extremely broadband, and intense-field THz time-domain spectroscopy systems.
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http://dx.doi.org/10.1364/OE.27.032659DOI Listing
October 2019

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

Opt Express 2019 Jul;27(14):19702-19711

The ultrafast dynamics of molecular nitrogen (N) 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 XΣg+ state and the first excited AΠu state of N2+. Among these, two photoelectron peaks with the vibrational quantum numbers v = 4 and v = 1 exhibit clear anti-phase oscillation with a period of 300 fs, showing that two Rydberg states converging to the XΣg+ and AΠu ionic states interact with each other, thus causing periodic switching in the population of the ion core states.
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http://dx.doi.org/10.1364/OE.27.019702DOI Listing
July 2019

Elimination of imaging artifacts in second harmonic generation microscopy using interferometry.

Biomed Opt Express 2019 Aug 12;10(8):3938-3952. Epub 2019 Jul 12.

Institut National de la Recherche Scientifique, Centre Énergie Matériaux Télécommunications (INRS-EMT); 1650 Boul. Lionel-Boulet, Varennes (QC), J3X 1S2, Canada.

Conventional second harmonic generation (SHG) microscopy might not clearly reveal the structure of complex samples if the interference between all scatterers in the focal volume results in artefactual patterns. We report here the use of interferometric second harmonic generation (I-SHG) microscopy to efficiently remove these artifacts from SHG images. Interfaces between two regions of opposite polarity are considered because they are known to produce imaging artifacts in muscle for instance. As a model system, such interfaces are first studied in periodically-poled lithium niobate (PPLN), where an artefactual incoherent SH signal is obtained because of irregularities at the interfaces, that overshadow the sought-after coherent contribution. Using I-SHG allows to remove the incoherent part completely without any spatial filtering. Second, I-SHG is also proven to resolve the double-band pattern expected in muscle where standard SHG exhibits in some regions artefactual single-band patterns. In addition to removing the artifacts at the interfaces between antiparallel domains in both structures (PPLN and muscle), I-SHG also increases their visibility by up to a factor of 5. This demonstrates that I-SHG is a powerful technique to image biological samples at enhanced contrast while suppressing artifacts.
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http://dx.doi.org/10.1364/BOE.10.003938DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6701527PMC
August 2019

Generation of deep ultraviolet sub-2-fs pulses.

Opt Lett 2019 Mar;44(6):1308-1311

We demonstrate the generation of few-cycle deep ultraviolet pulses via frequency upconversion of 5-fs near-infrared pulses in argon using a laser-fabricated gas cell. The measured spectrum extends from 210 to 340 nm, corresponding to a transform-limited pulse duration of 1.45 fs. We extract from a dispersion-free second-order cross-correlation measurement a pulse duration of 1.9 fs, defining a new record in the deep ultraviolet spectral range.
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http://dx.doi.org/10.1364/OL.44.001308DOI Listing
March 2019

Molecular gases for pulse compression in hollow core fibers.

Opt Express 2018 Sep;26(19):25426-25436

We introduce hydrofluorocarbon molecules as an alternative medium to noble gases with low ionization potential like krypton or xenon to compress ultrashort pulses of relatively low energy in a conventional hollow core fiber with subsequent dispersion compensation. Spectral broadening of pulses from two different laser systems exceeded those achieved with argon and krypton. Initially 40 fs, 800 nm, 120 μJ pulses were compressed to few optical cycles duration. With the same approach a compression factor of more than 10 was demonstrated for an ytterbium-based laser (1030 nm, 170 fs, 200 μJ) leading to 15.6 fs.
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http://dx.doi.org/10.1364/OE.26.025426DOI Listing
September 2018

Coherent Tabletop EUV Ptychography of Nanopatterns.

Sci Rep 2018 Nov 12;8(1):16693. Epub 2018 Nov 12.

School of Chemistry, The University of Manchester, M13 9PL, Manchester, UK.

Coherent diffraction imaging (CDI) or lensless X-ray microscopy has become of great interest for high spatial resolution imaging of, e.g., nanostructures and biological specimens. There is no optics required in between an object and a detector, because the object can be fully recovered from its far-field diffraction pattern with an iterative phase retrieval algorithm. Hence, in principle, a sub-wavelength spatial resolution could be achieved in a high-numerical aperture configuration. With the advances of ultrafast laser technology, high photon flux tabletop Extreme Ultraviolet (EUV) sources based on the high-order harmonic generation (HHG) have become available to small-scale laboratories. In this study, we report on a newly established high photon flux and highly monochromatic 30 nm HHG beamline. Furthermore, we applied ptychography, a scanning CDI version, to probe a nearly periodic nanopattern with the tabletop EUV source. A wide-field view of about 15 × 15 μm was probed with a 2.5 μm-diameter illumination beam at 30 nm. From a set of hundreds of far-field diffraction patterns recorded for different adjacent positions of the object, both the object and the illumination beams were successfully reconstructed with the extended ptychographical iterative engine. By investigating the phase retrieval transfer function, a diffraction-limited resolution of reconstruction of about 32 nm is obtained.
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http://dx.doi.org/10.1038/s41598-018-34257-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6232105PMC
November 2018

Combining tensile testing and structural analysis at the single collagen fibril level.

Sci Data 2018 10 23;5:180229. Epub 2018 Oct 23.

Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada.

Tensile testing to failure followed by imaging is a simple way of studying the structure-function relationship of connective tissues such as skin, tendon, and ligament. However, interpretation of these datasets is complex due to the hierarchical structures of the tissues spanning six or more orders of magnitude in length scale. Here we present a dataset obtained through the same scheme at the single collagen fibril level, the fundamental tensile element of load-bearing tissues. Tensile testing was performed on fibrils extracted from two types of bovine tendons, adsorbed on a glass surface and glued at both ends. An atomic force microscope (AFM) was used to pull fibrils to failure in bowstring geometry. The broken fibrils were then imaged by AFM for morphological characterization, by second harmonic generation microscopy to assess changes to molecular packing, and by fluorescence microscopy after incubation with a peptide probe that binds specifically to denatured collagen molecules. This dataset linking stress-strain curves to post-failure molecular changes is useful for researchers modelling or designing functional protein materials.
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http://dx.doi.org/10.1038/sdata.2018.229DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6198748PMC
October 2018

Effect of refractive index mismatch on forward-to-backward ratios in SHG imaging.

Opt Lett 2018 Oct;43(20):5082-5085

Nonlinear optical imaging in the epi-direction is used to image subresolution features. We find that a refractive index mismatch between the object to be imaged and the background medium can change the far-field intensity image. As an example, we study second harmonic generation (SHG) microscopy where the forward-to-backward (F/B) ratio is used to quantify subresolution features. We show both theoretically and experimentally that the inhomogeneous refractive index in collagen tendon tissue creates near-field effects, which can change the F/B ratio by ∼20%-25%, even though the effect is negligible for most of the individual fibrils in the tissue. This is caused by the sensitivity of the backward signal on phase matching conditions.
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http://dx.doi.org/10.1364/OL.43.005082DOI Listing
October 2018

Direct compression of 170-fs 50-cycle pulses down to 1.5 cycles with 70% transmission.

Sci Rep 2018 Aug 7;8(1):11794. Epub 2018 Aug 7.

Centre Énergie Matériaux Télécommunications, Institut National de la Recherche Scientifique (INRS-EMT), 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.
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http://dx.doi.org/10.1038/s41598-018-30198-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6081375PMC
August 2018

Author Correction: Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers.

Sci Rep 2018 May 9;8(1):7536. Epub 2018 May 9.

Department of Physics, Advanced Research Complex, University of Ottawa, 25 Templeton Street, Ottawa, K1N6N5, Ontario, Canada.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.
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http://dx.doi.org/10.1038/s41598-018-25623-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5943455PMC
May 2018

Gouy phase shift measurement using interferometric second-harmonic generation.

Opt Lett 2018 May;43(9):1958-1961

We report on a simple way to directly measure the Gouy phase shift of a strongly focused laser beam. This is accomplished by using a recent technique, namely, interferometric second-harmonic generation. We expect that this method will be of interest in a wide range of research fields, from high-harmonic and attosecond pulse generation to femtochemistry and nonlinear microscopy.
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http://dx.doi.org/10.1364/OL.43.001958DOI Listing
May 2018

In tendons, differing physiological requirements lead to functionally distinct nanostructures.

Sci Rep 2018 03 13;8(1):4409. Epub 2018 Mar 13.

School of Biomedical Engineering, Dalhousie University, Halifax, Canada.

The collagen-based tissues of animals are hierarchical structures: even tendon, the simplest collagenous tissue, has seven to eight levels of hierarchy. Tailoring tissue structure to match physiological function can occur at many different levels. We wanted to know if the control of tissue architecture to achieve function extends down to the nanoscale level of the individual, cable-like collagen fibrils. Using tendons from young adult bovine forelimbs, we performed stress-strain experiments on single collagen fibrils extracted from tendons with positional function, and tendons with energy storing function. Collagen fibrils from the two tendon types, which have known differences in intermolecular crosslinking, showed numerous differences in their responses to elongation. Unlike those from positional tendons, fibrils from energy storing tendons showed high strain stiffening and resistance to disruption in both molecular packing and conformation, helping to explain how these high stress tissues withstand millions of loading cycles with little reparative remodeling. Functional differences in load-bearing tissues are accompanied by important differences in nanoscale collagen fibril structure.
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http://dx.doi.org/10.1038/s41598-018-22741-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5849720PMC
March 2018

Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers.

Sci Rep 2018 Feb 28;8(1):3789. Epub 2018 Feb 28.

Department of Physics, Advanced Research Complex, University of Ottawa, 25 Templeton Street, Ottawa, K1N6N5, Ontario, Canada.

Structural isomers, molecules having the same chemical formula but with atoms bonded in different order, are hard to identify using conventional spectroscopy and mass spectrometry. They exhibit virtually indistinguishable mass spectra when ionized by electrons. Laser mass spectrometry based on photoionization of the isomers has emerged as a promising alternative but requires shaped ultrafast laser pulses. Here we use transform limited femtosecond pulses to distinguish the isomers using two methods. First, we probe doubly charged parent ions with circularly polarized light. We show that the yield of doubly charged ortho-xylene decreases while para-xylene increases over a range of laser intensities when the laser polarization is changed from linear to circular. Second, we probe high harmonic generation from randomly oriented isomer molecules subjected to an intense laser field. We show that the yield of high-order harmonics varies with the positioning of the methyl group in xylene isomers (ortho-, para- and meta-) and is due to differences in the strength of tunnel ionization and the overlap between the angular peaks of ionization and photo-recombination.
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http://dx.doi.org/10.1038/s41598-018-22055-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5830629PMC
February 2018

Study of middle infrared difference frequency generation using a femtosecond laser source in LGT.

Opt Lett 2017 Sep;42(18):3698-3701

We demonstrate phase-matched difference frequency generation in the emerging nonlinear crystal LaGaTaO. Tunable wavelengths between 1.4 and 4.7 μm are generated by using femtosecond sources. We also report on the measurements of the optical damage threshold in the femtosecond regime and on the nonlinear refractive index n.
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http://dx.doi.org/10.1364/OL.42.003698DOI Listing
September 2017

Decoupling Frequencies, Amplitudes and Phases in Nonlinear Optics.

Sci Rep 2017 08 11;7(1):7861. Epub 2017 Aug 11.

INRS-EMT, 1650 Blvd. Lionel Boulet, Varennes, J3X1S2, Qc, Canada.

In linear optics, light fields do not mutually interact in a medium. However, they do mix when their field strength becomes comparable to electron binding energies in the so-called nonlinear optical regime. Such high fields are typically achieved with ultra-short laser pulses containing very broad frequency spectra where their amplitudes and phases are mutually coupled in a convolution process. Here, we describe a regime of nonlinear interactions without mixing of different frequencies. We demonstrate both in theory and experiment how frequency domain nonlinear optics overcomes the shortcomings arising from the convolution in conventional time domain interactions. We generate light fields with previously inaccessible properties by avoiding these uncontrolled couplings. Consequently, arbitrary phase functions are transferred linearly to other frequencies while preserving the general shape of the input spectrum. As a powerful application, we introduce deep UV phase control at 207 nm by using a conventional NIR pulse shaper.
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http://dx.doi.org/10.1038/s41598-017-07510-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5554166PMC
August 2017

Highly stable, 54mJ Yb-InnoSlab laser platform at 0.5kW average power.

Opt Express 2017 Jul;25(15):17549-17555

We report on the leap of Yb-InnoSlab laser technology towards high pulse energies of 54mJ combined with high average power exceeding half a kW. The system features pulse durations of 1.5 picoseconds (ps) at 10kHz repetition rate with excellent beam properties (M of 1.1) combined with superb power and pointing stability in the sub-% range. It provides different output ports to facilitate optical synchronization for pumping parametric amplifiers. Tunable, femtosecond seed pulses are derived directly from the ps Yb pump pulses. We investigate the long term stability of this ps driven white light continuum and demonstrate 100-fold pulse compression down to 10fs duration. Ultra-broadband IR spectra centred at 2µm wavelength are subsequently generated via difference frequency generation of selected white light components.
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http://dx.doi.org/10.1364/OE.25.017549DOI Listing
July 2017

Fiber optic humidity sensor using water vapor condensation.

Opt Express 2017 Jun;25(13):15313-15321

The rate of vapor condensation on a solid surface depends on the ambient relative humidity (RH). Also, surface plasmon resonance (SPR) on a metal layer is sensitive to the refractive index change of its adjacent dielectric. The SPR effect appears as soon as a small amount of moisture forms on the sensor, resulting in a decrease in the amount of light transmitted due to plasmonic loss. Using this concept, we developed a fiber optic humidity sensor based on SPR. It can measure the ambient RH over a dynamic range from 10% to 85% with an accuracy of 3%.
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http://dx.doi.org/10.1364/OE.25.015313DOI Listing
June 2017