Research

The Ultra-Fast Science & Technology group focuses on four categories of research: Laser Source Development, Ultra-Fast Molecular Imaging and Control, Ultra-Fast X-Ray Spectroscopy and Dynamics in Solids and Non-Linear Optical Spectroscopy and Microscopy.

Laser Source Development
Ultra-Fast Molecular Imaging
Ultra-Fast X-Ray Spectroscopy and Dynamics in Solids
Non-Linear Optical Spectroscopy and Microscopy

Laser Source Development

Sources

  1. High peak power IR system (0.33 TW)

    1. As the last stage of an optical parametric amplifier (OPA), we utilized a large aperture, 2 mm thick BBO crystal which was pumped with 50 mJ of 800 nm light. This produces 1.8 μm idler pulses with 10 mJ 30 fs duration at 100 Hz repetition rate. Together with the signal pulses, we have obtained energies of 23 mJ, which is the highest infrared energy femtosecond OPA ever demonstrated [1].

    2. Tunable IR source (3-10 μm) with energy of a tenth of a microjoule, based on difference frequency generation (DFG) between signal (1.3-1.5 μm) and idler (1.7-2.1 μm) beams in a AgGaS2 crystal.

  2. High harmonics in the water window

    1. Based on the high peak power IR system and the design of a new gas cell, we have been able to generate high harmonics in the water window (284-540 eV) with a flux superior to 105 photons per shot. This source is well suited for the study of ultrafast demagnetization or biological dynamics. [2]

  3. Few-cycle fiber (5mJ, 2-cycle pulses at 1.8 μm) [3]

  4. FOPA [4,5]

Pulse Characterization
  1. FrosT: Frequency resolved optical switching. A cool pulse characterization technique based on transient absorption in solids. It is phase matching-free and works from VIS to IR, no matter how short or long the pulses are. The phase retrieval is based on ptychography [7].

  1. N. Thiré et al., App. Phys. Lett., 106(9):091110
  2. V. Cardin et al., J. Phys. B: At. Mol. Opt. Phys. 51, 174004 (2018)
  3. few-cycle.com
  4. B. E. Schmidt et al, Nat. Comm., 5, 3643 (2014)
  5. V. Gruson et al., Optics Express 25,27714 (2017)
  6. H. Ibrahim and François Légaré in L. Young et al., ‘Roadmap of ultrafast x-ray atomic and molecular physics’, J. Phys. B: At. Mol. Opt. Phys. 51, 32003 (2018)
  7. Leblanc et al., submitted

Publications

Ultra-Fast Molecular Imaging and Control

How do atoms within a molecule re-arrange upon excitation by ultrashort laser pulses? To image such molecular dynamics, and shoot so-called ‘molecular movies’, we use Coulomb explosion imaging (CEI). This technique enables us to capture the subtle and irregular structural changes that can occur within a single small molecule undergoing a chemical reaction on a femtosecond (fs) timescale and with atomic resolution. When a high intensity laser pulse hits a molecule, its electrons are stripped–off almost immediately and the positively charged fragments explode and fly apart. They basically keep their relative angles among each other, and so by measuring their momenta one can reconstruct which fragments originated from the same molecule. Since momentum conservation is valid, one can find those fragments, whose momentum sum is zero.

Figure 1: Detection mechanism of CEI machine. A gas jet defined by two skimmers interacts with ultrashort laser pulses. The generated fragments are accelerated towards a position and time sensitive detector. From this information the 3d momentum vectors of the fragments are reconstructed.

With this technique we have observed such things as one proton migrating from one side of the linear organic acetylene molecule, HC-CH, to the other in a time resolved way. Proton migration is one of the fundamental processes in chemistry and biology. We have found a way to observe proton migration in the cation of acetylene and to shoot frames of this molecular movie. So far, this kind of system could only be studied with VUV light from free electron lasers [1].

Figure 2: Newton plots for time delays from 20 to 100 fs after subtraction of contribution at time zero. Proton migration evolves from blue (origin) to red. Positions for acetylene, transition state and vinylidene, obtained by classical simulations are indicated with “o”, “+” and “x”, together with structural information.

The CEI technique has also been used to visualize the coherent control of electric charges in small molecules that are induced by asymmetric two-colour laser fields, like in the hydrogen molecule, for example [2,3].

  1. H. Ibrahim et al., Nat. commun. 5:4422 (2014)
  2. V. Wanie, H. Ibrahim, S. Beaulieu, N. Thiré, B. E. Schmidt, Y. Deng, A. S. Alnaser, I. V. Litvinyuk, X.-M. Tong, F. Légaré, "Coherent control of D2/H2 dissociative ionization by a mid-infrared two-color laser field," J. Phys. B: At. Mol. Opt. Phys. 49 025601 (2016).
  3. H. Ibrahim, C. Lefebvre, A. D. Bandrauk, A. Staudte, F. Légaré, "H2: The benchmark molecule for ultrafast science and technologies," J. Phys. B: At. Mol. Opt. Phys., 51 042002 (2018)
Publications

Ultra-Fast X-Ray Spectroscopy and Dynamics in Solids

High harmonic generation (HHG) is a nonlinear process through which an electron freed via ionization is then accelerated by a driving optical field to re-collide with its parent ion. In the process, it can release its acquired kinetic energy by emission of a single high energy photon. Photons emitted through such a process can have energies of hundreds of electron-volts, placing them in the XUV/soft X-ray range of the electromagnetic spectrum.

HHG is a very powerful spectroscopic tool. As the probability of the re-collision is proportional to the photoionization cross-section, the amplitude of the recorded HHG spectra is a direct window to the inner workings of the generation medium. In ALLS, for example, the cooper minimum of krypton [1] and a giant resonance from multi-electron correlated effects in xenon [2] were observed while driving HHG with ultrashort, long wavelength laser pulses.

Figure 1: Scaling the HHG cutoff energy and giant resonance in Xenon. Taken from [2]
Measured HHG spectra in xenon for different driving wavelengths (Gray: 0.8 μm – Blueish: 1.4 μm – Reddish: 1.8 μm) and pulse duration. The laser intensities used were close to saturation. The figure highlights the effect of these parameters on the achievable cutoff energy. The increase in yield at 95 eV is due to the giant resonance from multi-electron correlation effect upon recombination to the parent ion.

The generation process is perfectly coherent, and the soft X-Rays are emitted in short, sub-femtosecond, bursts. The train of attosecond pulses will be shorter than the duration of the driving pulse. This make the process an ideal source of ultrafast X-Ray pulses, complementing very well femto-slicing synchrotron beamlines and SASE FELs that suffer from both a lack of temporal coherence and significant temporal jitter (> 100fs). Recent achievements using HHG as an X-Ray source in ALLS include:

  1. Generation of collimated water window harmonics up to the oxygen K-edge and introduction of the intensity clamping propagation regime in long gas-cell [3].

  2. Figure 2: Water window harmonics in ALLS.
    The upper image shows the recorded HHG spectra for different pressure of the generating medium. The lower left image presents successive single-shot absorption spectra of a mylar film. It shows the capability of the light source to find the carbon K-edge in a single shot within a 6 eV precision. On the lower right is a far-field image of the beam profile. It shows nicely Gaussian features and very low divergence.
  3. Coherent diffractive imaging of nanopatterns using a ptychography technique [4]. This was done by a user of the facility, Dr. Truong Nguyen and Prof. Melissa Denecke from the University of Manchester.

  4. Study of the ultrafast demagnetization response of a Co/Pt multilayer magnetic sample using X-Ray resonant magnetic scattering. [5]

  5. Figure 3: Ultrafast demagnetization signal of a Co/Pt multilayer measured by XRMS
    The upper frames show the quenching of the scattered photons when the magnetization of the sample drops following an 800 nm laser excitation. The curves on the lower part of the figure represent the integrated intensity of the diffraction peaks at different pump-probe delays. This intensity is proportional to the second power of the macroscopic magnetization of the sample.

  1. Shiner, A. D. et al. Observation of Cooper minimum in krypton using high harmonic spectroscopy. J. Phys. B At. Mol. Opt. Phys. 45, 74010 (2012).
  2. Schmidt, B. E. et al. High harmonic generation with long-wavelength few-cycle laser pulses. J. Phys. B At. Mol. Opt. Phys. 45, 074008 (2012).
  3. Cardin, V. et al. Self-channelled high harmonic generation of water window soft x-rays. J. Phys. B At. Mol. Opt. Phys. 51, (2018).
  4. Truong, N. X. et al. Coherent Tabletop EUV Ptychography of Nanopatterns. Sci. Rep. 8, 16693 (2018).
  5. Cardin et al. 2019 In Redaction
Publications

Non-Linear Optical Spectroscopy and Microscopy

Optical microscopy is widely recognized as an indispensable technique to image biological tissues. Confocal microscopy is widely used for this purpose, since it allows for the isolation of one cross-section of the sample in order to better resolve it.

Multiphoton microscopy (MPM) comes as a further improvement of these techniques, as it can selectively image one type of structure in the sample at a time. Second Harmonic Generation (SHG) only reveals the components lacking a center of inversion and can characterize their structure and 3D orientation, Coherent Anti-Stokes Raman Scattering (CARS) shows the different chemical bounds, and Two-Photons Excited Fluorescence (2PEF) serves to complete this view by revealing some components that exhibit auto-fluorescence. The numerous advantages of this imaging paradigm include its low invasiveness (no staining or marker injected), its high contrast (because nonlinear processes are involved), its high penetration and the fact that these are instantaneous processes, that do not decrease in large time scale [1].

Figure 1: Multiphoton imaging of biological tissues.
(a) Central part of a coronal cut of the body section of a meniscus, from a horse knee joint (by polarization SHG) (b) En-face cut of a human cornea imaged by intensity SHG. (c) Polarity of the microtubules revealed by Interferometric SHG (zebrafish cell). (d) Myosin sarcomeres inside a skeletal muscle (zebrafish). Scale-bars for all: 25 μm.

In particular, the SHG allows us to characterize, in 3D, the structures in a material, by changing the polarization of the light or by doing interferometry. The latter was developed to measure not only the intensity of the signal but also its phase, which enables imaging of the polarity [2].

Figure 2: SHG imaging of non-biological materials, such as ferroelectrics and crystals.
(a) Structured ferroelectric material (CBN) viewed by SHG (scale-bar: 500 μm). (b) Periodically-poled non-linear crystal (LiNbO3) by SHG (scale-bar: 20 μm). (c) SHG image and degradation effect observed in a ferroelectric epitaxial layer (ε-Fe2O3, scale-bar: 5 μm).

This technique was later improved in our lab to image more complex structures [3] and even probe biological processes in a time-scale of a few tens of seconds [4]. Later, some properties of the excitation process itself were studied [5]. The imaging artifacts and more complex structures in biology are currently being investigated.

Figure 1 shows some images obtained with the microscope of various biological samples while Figure 2 gathers a few examples of non-organic materials such as ferroelectrics and crystals.

  1. W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: Multiphoton microscopy in the biosciences,” Nat. Biotechnol., vol. 21, no. 11, pp. 1369–1377, 2003.
  2. M. Rivard et al., “Imaging the bipolarity of myosin filaments with Interferometric Second Harmonic Generation microscopy,” Biomed. Opt. Express, vol. 4, no. 10, p. 2078, 2013.
  3. C. A. Couture et al., “The Impact of Collagen Fibril Polarity on Second Harmonic Generation Microscopy,” Biophys. J., vol. 109, no. 12, pp. 2501–2510, 2015.
  4. S. Bancelin, C.-A. Couture, M. Pinsard, M. Rivard, P. Drapeau, and F. Légaré, “Probing microtubules polarity in mitotic spindles in situ using Interferometric Second Harmonic Generation Microscopy,” Sci. Rep., vol. 7, no. 1, p. 6758, 2017.
  5. S. Bancelin et al., “Gouy phase shift measurement using interferometric second-harmonic generation,” Opt. Lett., vol. 43, no. 9, p. 1958, 2018.
Publications