Coherent Raman Scattering Research Articles

Temperature Dependence of Coherent versus Spontaneous Raman Scattering

Temperature Dependence of Coherent versus Spontaneous Raman Scattering

Ultra‐Broadband Visible‐DUV Supercontinuum Generation by Non‐Resonant Coherent Raman Scattering in Air

AbstractTo date, supercontinuum light in the visible and near‐infrared ranges is readily realizable by the optical Kerr effect through self‐phase modulation of ultrashort laser pulses in transparent media. However, it is still a challenge to extend the supercontinuum spectrum down to the deep‐ultraviolet (DUV) range, which is particularly needed for exploring ultrafast dynamics in chemistry, materials, and biology. Here, an approach of non‐resonant coherent Raman scattering is developed to generate ultra‐broadband visible‐DUV supercontinuum in ambient air with a spectral range spanning over 250 nm and a wavelength down to 220 nm. A rovibrational coherence is established in air molecules by filamentation of a near‐infrared femtosecond 800 nm pulse and two femtosecond Raman laser pulses at 267 and 400 nm are introduced into the coherent media to induce non‐resonant coherent Stokes and anti‐Stokes Raman scatterings, which serve as the spectral bridges to link the neighboring Raman pump laser spectra, resulting in ultra‐broadband supercontinuum light. The mechanism is further verified by examining the broadening of picosecond N2+ laser lines with narrow bandwidths (10–30 cm−1), which forms a supercontinuum spectrum spanning over 150 nm. The work provides a viable route for the establishment of coherent DUV supercontinuum in the gas media at designed wavelength ranges.

Pinpointing Fat Molecules: Advances in Coherent Raman Scattering Microscopy for Lipid Metabolism.

Pinpointing Fat Molecules: Advances in Coherent Raman Scattering Microscopy for Lipid Metabolism.

Coherent Raman Scattering Spectral Shapes in a Strong Excitation Regime (Model Calculations)

Coherent Raman Scattering Spectral Shapes in a Strong Excitation Regime (Model Calculations)

Quick Large‐Area Detection of Thin Silicone Films with Coherent Raman Scattering Imaging

Coherent Raman Scattering Imaging offers quick and surface‐sensitive large‐area scans of specimens including fingerprinting of the substances, combined with low detection limits. By using a multi‐photon process, it offers orders of magnitude stronger surface signal in comparison to spontaneous Raman (SR) and fluorescence signals of the substrate are avoided by the detection principle and the near‐infrared laser wavelength. By exciting at a specific wavelength instead of measuring whole spectra the measurement time is remarkably reduced. Here, detecting organic thin films on glass surfaces is investigated. The detection of those thin films is relevant for production processes where the interfacial layer is influenced by the low surface energy of contaminants, especially polysiloxanes. The samples are manufactured by pipetting diluted polydimethylsiloxane (PDMS) in heptane in various concentrations (1%, 0.1%, 0.01%, and 0.001%) on glass substrates. After evaporation of the solvent, thin films of the silicone remain. The resulting average coverage of the thin films equals 100–0.1 μg cm−2. Measurements determine the detectability threshold with SR to 10 μg cm−2, while the acquired spectrum is used to determine characteristic peaks for the pump laser. Afterwards the samples are imaged at those wavenumbers with Coherent Raman Scattering (CRS) Imaging techniques providing a chemically‐specific contrast.

Measurement of Carbon Dioxide Isotopes with Air-Lasing-Based Coherent Raman Spectroscopy.

Isotope detection is crucial for geological research, medical diagnostics, industrial production, and environmental monitoring. Various spectroscopic techniques are continually emerging for isotopic identification and accurate measurement. Herein, coherent Raman scattering (CRS) spectroscopy is developed for the quantitative detection of carbon dioxide isotopes, in which the N2+ air lasing coherently created in the interaction region is used as the probe. Benefiting from the narrow spectral width of air lasing, the Raman peaks of 12CO2 and 13CO2 can be well discerned, although their spectra partially overlap. The overlapped signals were proven to be the result of the coherent superposition of individual Raman signals. Based on that fact, a deconvolution algorithm was designed to retrieve the concentration ratio of the two isotopes. The relative error of the measurement is less than 6%. The CRS technique based on air lasing offers a potential approach for the quantitative characterization of molecular isotopes, especially in application scenarios of remote sensing or in situ detection.

Advancing cerumen analysis: exploring innovative vibrational spectroscopy techniques with respect to their potential as new point-of-care diagnostic tools.

Cerumen, commonly known as earwax, is a complex mixture composed of secretions from ceruminous glands. These secretions are heterogeneous mixtures mainly composed of lipids and proteins. Despite its prevalence, the potential diagnostic value of cerumen remains largely unexplored. Here, we present an in-depth analysis of cerumen utilizing well-known vibrational approaches such as conventional Raman spectroscopy or surface-enhanced Raman spectroscopy (SERS) together with advanced vibrational spectroscopy techniques such as coherent Raman scattering (CRS), i.e. broadband coherent anti-Stokes Raman scattering (CARS) or stimulated Raman scattering (SRS), as well as optical photothermal infrared (OPTIR) spectroscopy. Through the integration of these vibrational spectroscopic methods, lipids and proteins can be identified as the main components of cerumen; however, they contribute to the final spectral information to various extents depending on the vibrational detection scheme applied. The inherently weak Raman signal could be enhanced by linear (SERS) and non-linear (CRS) processes, resulting in efficient acquisition of fingerprint information and allowing for the detection of marker modes, which cannot be addressed by conventional Raman spectroscopy. OPTIR spectroscopy provides complementary information to Raman spectroscopy, however, without the contribution of a fluorescence background. Our findings underscore the utility of these cutting-edge techniques in unveiling the intricate molecular landscape of cerumen, paving the way for novel point-of-care diagnostic methodologies and therapeutic interventions.

Time-Domain Coherent Raman Scattering Techniques (Invited)

Time-Domain Coherent Raman Scattering Techniques (Invited)

Hyperspectral Coherent Raman Scattering (CRS) Microscopy Based on a Rapidly Tunable and Environmentally Stable Fiber Laser

AbstractCoherent Raman scattering (CRS) microscopy is an attractive label‐free imaging method providing high chemical sensitivity, sub‐micron spatial resolution, and video‐rate imaging speed. When extended to hyperspectral CRS (HS‐CRS), it is capable of distinguishing molecules even in complex heterogeneous systems by probing multiple Raman resonances. Solid‐state lasers, associated with their high cost, large footprint, and slow tuning speed, currently limit the widespread application of HS‐CRS. Fiber lasers with comparable properties usually suffer from poor noise performance or instabilities during wavelength tuning. A tunable two‐color fiber laser is demonstrated that can be electronically tuned at rates of up to 1 kHz. The use of polarization‐maintaining fibers and an all‐optical synchronization mechanism guarantees its mode‐locking stability. The superior noise performance of this system is evaluated by the side‐by‐side comparison with data obtained by a commercial optical parametric oscillator (OPO)‐based laser. Large‐area multimodal imaging of liver tissue sections surgically resected from a patient suffering from a non‐neoplastic liver parenchyma with micronodular cirrhosis in combination with a multilocular hepatocellular carcinoma is demonstrated. By exploiting the fast‐hyperspectral scanning capability of the laser and multivariate statistical analysis, virtual staining of tissue sections and a comparison of the diagnostic properties of HS‐CRS to the classical histopathological approach are achieved.

Numerical evaluation of spectral coverage and spectral resolution in coherent Raman scattering spectroscopy using a broadband fiber laser source

AbstractWe theoretically investigate the feasibility of using a fiber laser source for high resolution and broadband coherent Raman scattering (CRS) spectroscopy/imaging employing spectral focusing technique. To accomplish this task, we have developed a simulation tool where the laser pulse parameters can be imported directly from any pulse propagation software using split‐step Fourier method to evaluate the spectral resolution and spectral coverage of CRS spectroscopy. Thus, the simulation tool can accommodate optical pulses of arbitrary shape and central wavelength, being also a user‐friendly and versatile open‐source software. We validate the numerical simulation tool by comparing its results with published theoretical and experimental data. We then use the tool to study the feasibility of using an all‐fiber laser source for high resolution CRS spectroscopy in both the C‐H window and the fingerprint region. This tool has a broad scope of applications in multiple fields where CRS spectroscopy is useful such as biomedicine, materials characterization, and biological imaging.

Coherent Raman scattering microscopy of lipid droplets in cells and tissues.

Lipid droplets (LDs) play a key role as the hub for lipid metabolism to maintain cellular metabolic homeostasis. Understanding the functions and changes of LDs in different pathological conditions is crucial for identifying new markers for diagnosis and discovering new targets for treatment. In recent years, coherent Raman scattering (CRS) microscopy has been popularized for the imaging and quantification of LDs in live cells. Compared to spontaneous Raman scattering microscopy, CRS microscopy offers a much higher imaging speed while maintaining similar chemical information. Due to the high lipid density, LDs usually have strong CRS signals and therefore are the most widely studied organelle in the CRS field. In this review, we discuss recent achievements using CRS to study the quantity, distribution, composition, and dynamics of LDs in various systems.

Ultrafast Spectral Tuning of a Fiber Laser for Time-Encoded Multiplex Coherent Raman Scattering Microscopy

Coherent Raman scattering microscopy utilizing bioorthogonal tagging approaches like isotope or alkyne labeling allows for a targeted monitoring of spatial distribution and dynamics of small molecules of interest in cells, tissues, and other complex biological matrices. To fully exploit this approach in terms of real-time monitoring of several Raman tags, e.g., to study drug uptake dynamics, extremely fast tunable lasers are needed. Here, we present a laser concept without moving parts and fully electronically controlled for the quasi-simultaneous acquisition of coherent anti-Stokes Raman scattering images at multiple Raman resonances. The laser concept is based on the combination of a low noise and spectrally narrow Fourier domain mode-locked laser seeding a compact four wave mixing-based high-power fiber-based optical parametric amplifier.

Coherent Raman scattering imaging of lipid metabolism in cancer

Cancer cells dysregulate lipid metabolism to accelerate energy production and biomolecule synthesis for rapid growth. Lipid metabolism is highly dynamic and intrinsically heterogeneous at the single cell level. Although fluorescence microscopy has been commonly used for cancer research, bulky fluorescent probes can hardly label small lipid molecules without perturbing their biological activities. Such a challenge can be overcome by coherent Raman scattering (CRS) microscopy, which is capable of chemically selective, highly sensitive, submicron resolution and high-speed imaging of lipid molecules in single live cells without any labeling. Recently developed hyperspectral and multiplex CRS microscopy enables quantitative mapping of various lipid metabolites in situ. Further incorporation of CRS microscopy with Raman tags greatly increases molecular selectivity based on the distinct Raman peaks well separated from the endogenous cellular background. Owing to these unique advantages, CRS microscopy sheds new insights into the role of lipid metabolism in cancer development and progression. This review focuses on the latest applications of CRS microscopy in the study of lipid metabolism in cancer.

Coherent N2 + emission mediated by coherent Raman scattering for gas-phase thermometry.

We report on the generation of coherent emission from femtosecond (fs) laser-induced filaments mediated by ultrabroadband coherent Raman scattering (CRS), and we investigate its application for high-resolution gas-phase thermometry. Broadband 35-fs, 800-nm pump pulses generate the filament through photoionization of the N2 molecules, while narrowband picosecond (ps) pulses at 400 nm seed the fluorescent plasma medium via generation of an ultrabroadband CRS signal, resulting in a narrowband and highly spatiotemporally coherent emission at 428 nm. This emission satisfies the phase-matching for the crossed pump-probe beams geometry, and its polarization follows the CRS signal polarization. We perform spectroscopy on the coherent N2 + signal to investigate the rotational energy distribution of the N2 + ions in the excited B2Σu + electronic state and demonstrate that the ionization mechanism of the N2 molecules preserves the original Boltzmann distribution to within the experimental conditions tested.

Pulse-Picking Multimodal Nonlinear Optical Microscopy.

Nonlinear optical microscopy techniques can map chemical compositions in biological samples in a label-free manner. Commonly used nonlinear optical processes for imaging include multiphoton excitation fluorescence (MPEF), second harmonic generation (SHG), and coherent Raman scattering (CRS). Femtosecond lasers are typically used for MPEF and SHG due to the requirement of high peak power for excitation, while picosecond lasers are preferred for CRS due to the need for high spectral resolution. Therefore, it is challenging to integrate CRS with MPEF and SHG for chemical imaging. We develop a pulse-picking strategy based on an acousto-optic modulator that can program the duty cycle of the laser pulse train, significantly increasing the pulse peak power at low input average power. This approach offers strong enhancement of nonlinear optical signals and makes hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy compatible with MPEF and SHG for multimodal imaging at low laser average power. The pulse-picking method also enables the evaluation and comparison of phototoxicity of laser pulses at different average and peak power levels. The photo-perturbations to biological samples are evaluated using cellular dynamics and sample morphological changes, allowing the selection of optimal laser power for the best sensitivity and minimal phototoxicity.

Novel aspects of Raman spectroscopy in skin research.

The analytical technology of Raman spectroscopy has an almost 100‐year history. During this period, many modifications and developments happened in the method like discovery of laser, improvements in optical elements and sensitivity of spectrometer and also more advanced light detection systems. Many types of the innovative techniques appeared (e.g. Transmittance Raman spectroscopy, Coherent Raman Scattering microscopy, Surface‐Enhanced Raman scattering and Confocal Raman spectroscopy/microscopy). This review article gives a short description about these different Raman techniques and their possible applications. Then, a short statistical part is coming about the appearance of Raman spectroscopy in the scientific literature from the beginnings to these days. The third part of the paper shows the main application options of the technique (especially confocal Raman spectroscopy) in skin research, including skin composition analysis, drug penetration monitoring and analysis, diagnostic utilizations in dermatology and cosmeto‐scientific applications. At the end, the possible role of artificial intelligence in Raman data analysis and the regulatory aspect of these techniques in dermatology are briefly summarized. For the future of Raman Spectroscopy, increasing clinical relevance and in vivo applications can be predicted with spreading of non‐destructive methods and appearance with the most advanced instruments with rapid analysis time.

Direct Comparison of Hyperspectral Stimulated Raman Scattering and Coherent Anti-Stokes Raman Scattering Microscopy for Chemical Imaging.

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging technologies. Hyperspectral SRS and CARS imaging offer Raman spectral information at every pixel, which enables better separation of different chemical compositions. Although both techniques require two excitation lasers, their signal detection schemes and spectral properties are quite different. The goal of this protocol is to perform both hyperspectral SRS and CARS imaging on a single platform and compare the two microscopy techniques for imaging different biological samples. The spectral focusing method is employed to acquire spectral information using femtosecond lasers. By using standard chemical samples, the sensitivity, spatial resolution, and spectral resolution of SRS and CARS in the same excitation conditions (i.e., power at the sample, pixel dwell time, objective lens, pulse energy) are compared. The imaging contrasts of CARS and SRS for biological samples are juxtaposed and compared. The direct comparison of CARS and SRS performances would allow for optimal selection of the modality for chemical imaging.

A Review of Raman-Based Technologies for Bacterial Identification and Antimicrobial Susceptibility Testing

Antimicrobial resistance (AMR) is a global medical threat that seriously endangers human health. Rapid bacterial identification and antimicrobial susceptibility testing (AST) are key interventions to combat the spread and emergence of AMR. Although current clinical bacterial identification and AST provide comprehensive information, they are labor-intensive, complex, inaccurate, and slow (requiring several days, depending on the growth of pathogenic bacteria). Recently, Raman-based identification and AST technologies have played an increasingly important role in fighting AMR. This review summarizes major Raman-based techniques for bacterial identification and AST, including spontaneous Raman scattering, surface-enhanced Raman scattering (SERS), and coherent Raman scattering (CRS) imaging. Then, we discuss recent developments in rapid identification and AST methods based on Raman technology. Finally, we highlight the major challenges and potential future efforts to improve clinical outcomes through rapid bacterial identification and AST.

Hyperspectral Coherent Raman Scattering and Its Applications

Hyperspectral Coherent Raman Scattering and Its Applications

Visualizing and Quantifying Pharmaceutical Compounds within Skin using Coherent Raman Scattering Imaging.

Cutaneous pharmacokinetics (cPK) after topical formulation application has been a research area of particular interest for regulatory and drug development scientists to mechanistically understand topical bioavailability (BA). Semi-invasive techniques, such as tape-stripping, dermal microdialysis, or dermal open-flow microperfusion, all quantify macroscale cPK. While these techniques have provided vast cPK knowledge, the community lacks a mechanistic understanding of active pharmaceutical ingredient (API) penetration and permeation at the cellular level. One noninvasive approach to address microscale cPK is coherent Raman scattering imaging (CRI), which selectively targets intrinsic molecular vibrations without the need for extrinsic labels or chemical modification. CRI has two main methods-coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS)-that enable sensitive and selective quantification of APIs or inactive ingredients. CARS is typically utilized to derive structural skin information or visualize chemical contrast. In contrast, the SRS signal, which is linear with molecular concentration, is used to quantify APIs or inactive ingredients within skin stratifications. Although mouse tissue has commonly been utilized for cPK with CRI, topical BA and bioequivalence (BE) must ultimately be assessed in human tissue before regulatory approval. This paper presents a methodology to prepare and image ex vivo skin to be used in quantitative pharmacokinetic CRI studies in the evaluation of topical BA and BE. This methodology enables reliable and reproducible API quantification within human and mouse skin over time. The concentrations within lipid-rich and lipid-poor compartments, as well as total API concentration over time are quantified; these are utilized for estimates of micro- and macroscale BA and, potentially, BE.