Improved spectral resolution of the femtosecond stimulated Raman spectroscopy achieved by the use of the 2nd-order diffraction method

Mastering Femtosecond Stimulated Raman Spectroscopy: A Practical Guide

Femtosecond stimulated Raman spectroscopy (FSRS) is a powerful nonlinear spectroscopic technique that probes changes in molecular and material structure with high temporal and spectral resolution. With proper spectral interpretation, this is equivalent to mapping out reactive pathways on highly anharmonic excited-state potential energy surfaces with femtosecond to picosecond time resolution. FSRS has been used to examine structural dynamics in a wide range of samples, including photoactive proteins, photovoltaic materials, plasmonic nanostructures, polymers, and a range of others, with experiments performed in multiple groups around the world. As the FSRS technique grows in popularity and is increasingly implemented in user facilities, there is a need for a widespread understanding of the methodology and best practices. In this review, we present a practical guide to FSRS, including discussions of instrumentation, as well as data acquisition and analysis. First, we describe common methods of generating the three pulses required for FSRS: the probe, Raman pump, and actinic pump, including a discussion of the parameters to consider when selecting a beam generation method. We then outline approaches for effective and efficient FSRS data acquisition. We discuss common data analysis techniques for FSRS, as well as more advanced analyses aimed at extracting small signals on a large background. We conclude with a discussion of some of the new directions for FSRS research, including spectromicroscopy. Overall, this review provides researchers with a practical handbook for FSRS as a technique with the aim of encouraging many scientists and engineers to use it in their research.

Electromagnetically actuated 3D-printed tunable optical slit device

This paper presents the design, manufacturing, and characterization of a three-dimensional (3D)-printed and electromagnetically actuated adjustable optical slit structure. The device comprises magnet-attached slits connected to the main frame via two springs controlled by external coils. To analyze the forces acting on the springs and simulate the mechanical behavior of the device, we developed both analytical and finite-element models. After fabricating the device using fused deposition, we conducted a series of tests to evaluate its performance. These tests included (1) analyzing the opacity of the slit blade as a function of its thickness, (2) measuring the temperature increase resulting from the power applied to the coils to determine the operable range of the structure, and (3) evaluating the hysteresis, repeatability, and resolution (minimum step) of the device. The experimental works were crucial to assessing the device's practicality and optimizing its performance for specific applications, which reveals a maximum slit width of ∼450µm, with ∼6.4µm step size within this study. Overall, our developed slit device has the potential to be useful in various optics-related laboratories due to its compatibility with conventional 1-inch (25.4 mm) diameter optomechanical mounts, compact form, low power consumption, and rapid prototyping capability with hybrid materials in a cost-friendly fashion, owing to the 3D-printing technology. We discuss an application where the adjustable slit is employed in a combined laser-scanning microscope and a spectrometer, highlighting its versatility and potential for the future.

Design of coherent wideband radiation process in a Nd3+-doped high entropy glass system

We discover that the spatially coherent radiation within a certain frequency range can be obtained without a common nonlinear optical process. Conventionally, the emission spectra were obtained by de-exciting excited centers from real excited energy levels to the ground state. Our findings are achieved by deploying a high-entropy glass system (HEGS) doped with neodymium ions. The HEGS exhibits a much broader infrared absorption than common glass systems, which can be attributed to be high-frequency optical branch phonons or allowable multi-phonon processes caused by phonon broadening in the system. A broadened phonon-assisted wideband radiation (BPAWR) is induced if the pump laser is absorbed by the system. The subsequent low-threshold self-absorption coherence modulation (SACM) can be controlled by changing excitation wavelengths, sample size, and doping concentrations. The SACM can be red-shifted through the emission of phonons of the excited species and be blue-shifted by absorbing phonons before they are de-excited. There is a time delay up to 1.66 ns between the pump pulse and the BPAWR when measured after traveling through a 35 mm long sample, which is much longer than the Raman process. The BPAWR-SACM can amplify the centered non-absorption band with a gain up to 26.02 dB. These results reveal that the shift of the novel radiation is determined by the frequency of the non-absorption band near the absorption region, and therefore the emission shifts can be modulated by changing the absorption spectrum. When used in fiber lasers, the BPAWR-SACM process may help to achieve tunability.

Raman Spectroscopy as a Neuromonitoring Tool in Traumatic Brain Injury: A Systematic Review and Clinical Perspectives

Traumatic brain injury (TBI) is a significant global health problem, for which no disease-modifying therapeutics are currently available to improve survival and outcomes. Current neuromonitoring modalities are unable to reflect the complex and changing pathophysiological processes of the acute changes that occur after TBI. Raman spectroscopy (RS) is a powerful, label-free, optical tool which can provide detailed biochemical data in vivo. A systematic review of the literature is presented of available evidence for the use of RS in TBI. Seven research studies met the inclusion/exclusion criteria with all studies being performed in pre-clinical models. None of the studies reported the in vivo application of RS, with spectral acquisition performed ex vivo and one performed in vitro. Four further studies were included that related to the use of RS in analogous brain injury models, and a further five utilised RS in ex vivo biofluid studies for diagnosis or monitoring of TBI. RS is identified as a potential means to identify injury severity and metabolic dysfunction which may hold translational value. In relation to the available evidence, the translational potentials and barriers are discussed. This systematic review supports the further translational development of RS in TBI to fully ascertain its potential for enhancing patient care.

Fluorescence Modulation by Ultrafast Chromophore Twisting Events: Developing a Powerful Toolset for Fluorescent-Protein-Based Imaging

The advancement of modern life sciences has benefited tremendously from the discovery and development of fluorescent proteins (FPs), widely expressed in live cells to track a myriad of cellular events. The chromophores of various FPs can undergo many ultrafast photophysical and/or photochemical processes in the electronic excited state and emit fluorescence with different colors. However, the chromophore becomes essentially nonfluorescent in solution environment due to its intrinsic twisting capability upon photoexcitation. To study "microscopic" torsional events and their effects on "macroscopic" fluorescence, we have developed an integrated ultrafast characterization platform involving femtosecond transient absorption (fs-TA) and wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS). A wide range of naturally occurring, circularly permuted, non-canonical amino-acid-decorated FPs and FP-based optical highlighters with photochromicity, photoconversion, and/or photoswitching capabilities have been recently investigated in great detail. Twisting conformational motions were elucidated to exist in all of these systems but to various extents. The associated different ultrafast pathways can be monitored via frequency changes of characteristic Raman bands during primary events and functional processes. The mapped electronic and structural dynamics information is crucial and has shown great potential and initial success for the rational design of proteins and other photoreceptors with novel functions and fluorescence properties.