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1. Simultaneous measurements of plasma densities and electron collision times in plasma via time-resolved interferometry

   Nagar, Garima; Dempsey, Dennis; Shim, Bonggu (November 2020, APS Division of Plasma Physics Meeting 2020)

   We present time-resolved interferometry to simultaneously measure plasma densities and electron collision times for strong field laser-matter interactions. First, an intense femtosecond pump pulse generates plasma in a solid and second, a weak 800-nm femtosecond probe traverses the pump-induced plasma and is sent to an interferometer with controlled time delay between pump and probe. By analyzing the interferograms using Fourie methods, we can extract plasma densities and electron collision times in plasma simultaneously with micrometer spatial and femtosecond temporal resolutions. Using the technique, we study the plasma dynamics when a wavelength-varied (λ= 1.2-2.3 μm) pump pulse undergoes laser filamentation in solid materials. 

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2. Expanding the bandwidth of fluorescence-detected two-dimensional electronic spectroscopy using a broadband continuum probe pulse pair

   https://doi.org/10.1364/OE.516963  

   Sanders, Stephanie_E; Zhang, Muyi; Javed, Ariba; Ogilvie, Jennifer_P (February 2024, Optics Express)

   We demonstrate fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) with a broadband, continuum probe pulse pair in the pump-probe geometry. The approach combines a pump pulse pair generated by an acousto-optic pulse-shaper with precise control of the relative pump pulse phase and time delay with a broadband, continuum probe pulse pair created using the Translating Wedge-based Identical pulses eNcoding System (TWINS). The continuum probe expands the spectral range of the detection axis and lengthens the waiting times that can be accessed in comparison to implementations of F-2DES using a single pulse-shaper. We employ phase-cycling of the pump pulse pair and take advantage of the separation of signals in the frequency domain to isolate rephasing and non-rephasing signals and optimize the signal-to-noise ratio. As proof of principle, we demonstrate broadband F-2DES on a laser dye and bacteriochlorophylla. 

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3. Transient Absorption Microscopy Using Widefield Lock-in Camera Imaging

   https://doi.org/10.1021/acs.jpcc.4c02984  

   Wilson, Kelly S; Volek, Tanner S; Gross, Niklas; Link, Stephan; Baiz, Carlos R; Roberts, Sean T (July 2024, The Journal of Physical Chemistry C)

   Over the past decade, the proliferation of pulsed laser sources with high repetition rates has facilitated a merger of ultrafast time-resolved spectroscopy with imaging microscopy. In transient absorption microscopy (TAM), the excited-state dynamics of a system are tracked by measuring changes in the transmission of a focused probe pulse following photoexcitation of a sample. Typically, these experiments are done using a photodiode detector and lock-in amplifier synchronized with the laser and images highlighting spatial heterogeneity in the TAM signal are constructed by scanning the probe across a sample. Performing TAM by instead imaging a spatially defocused widefield probe with a multipixel camera could dramatically accelerate the acquisition of spatially resolved dynamics, yet approaches for such widefield imaging generally suffer from reduced signal-to-noise due to an incompatibility of multipixel cameras with high-frequency lock-in detection. Herein, we describe implementation of a camera capable of high-frequency lock-in detection, thereby enabling widefield TAM imaging at rates matching those of high repetition rate lasers. Transient images using a widefield probe and two separate pump pulse configurations are highlighted. In the first, a widefield probe was used to image changes in the spatial distribution of photoexcited molecules prepared by a tightly focused pump pulse, while in the second, a widefield probe detected spatial variations in photoexcited dynamics within a heterogeneous organic crystal excited by a defocused pump pulse. These results highlight the ability of high-sensitivity lock-in detection to enable widefield TAM imaging, which can be leveraged to further our understanding of excited-state dynamics and excitation transport within spatially heterogeneous systems. 

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4. Ultrafast time-resolved 2D imaging of laser-driven fast electron transport in solid density matter using an x-ray free electron laser

   https://doi.org/10.1063/5.0130953  

   Sawada, H.; Yabuuchi, T.; Higashi, N.; Iwasaki, T.; Kawasaki, K.; Maeda, Y.; Izumi, T.; Nakagawa, Y.; Shigemori, K.; Sakawa, Y.; et al (March 2023, Review of Scientific Instruments)

   High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10 fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam. 

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5. XFELs for structure and dynamics in biology

   https://doi.org/10.1107/S2052252517005760  

   Spence, J. C. (July 2017, IUCrJ)

   The development and application of the free-electron X-ray laser (XFEL) to structure and dynamics in biology since its inception in 2009 are reviewed. The research opportunities which result from the ability to outrun most radiation-damage effects are outlined, and some grand challenges are suggested. By avoiding the need to cool samples to minimize damage, the XFEL has permitted atomic resolution imaging of molecular processes on the 100 fs timescale under near-physiological conditions and in the correct thermal bath in which molecular machines operate. Radiation damage, comparisons of XFEL and synchrotron work, single-particle diffraction, fast solution scattering, pump–probe studies on photosensitive proteins, mix-and-inject experiments, caged molecules, pH jump and other reaction-initiation methods, and the study of molecular machines are all discussed. Sample-delivery methods and data-analysis algorithms for the various modes, from serial femtosecond crystallography to fast solution scattering, fluctuation X-ray scattering, mixing jet experiments and single-particle diffraction, are also reviewed. 

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