Coherent Raman Spectroscopy Research Articles

Coherent Ultrafast Stimulated X-Ray Raman Spectroscopy of Dissipative Conical Intersections.

The quantum coherent dynamics of a vibronic wave packet in a molecule passing through a conical intersection can be revealed using attosecond transient coherent Raman spectroscopy. In particular, the time evolution of the electronic coherence can be monitored in the presence of vibrational dynamics. So far, the technique has been investigated without including environmental quantum noise. Here, we employ the numerically exact hierarchy equation of motion approach to show that the transient coherent Raman signals are robust and accessible on times of up to a few hundred femtoseconds with respect to electonic and vibrational dephasing.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies.

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

Low-Frequency Coherent Raman Imaging Robust to Optical Scattering.

We demonstrate low-frequency interferometric impulsive stimulated Raman scattering (ISRS) imaging with high robustness to distortions by optical scattering. ISRS is a pump-probe coherent Raman spectroscopy that can capture Raman vibrational spectra. Recording of ISRS spectra requires isolation of a probe pulse from the pump pulse. While this separation is simple in nonscattering specimens, such as liquids, scattering leads to significant pump pulse contamination and prevents the extraction of a Raman spectrum. We introduce a robust method for ISRS microscopy that works in complex scattering samples. High signal-to-noise ISRS spectra are obtained even when the pump and probe pulses pass through many scattering layers.

P-235 Lipid profile changes during neurodevelopment (ND) of mouse embryos cultured in vitro

Abstract Study question Does in vitro culture (IVC) induce changes in lipid profile of the embryo? Summary answer IVC induces changes in embryonic lipid homeostasis that subsequently affect placenta-brain axis (PBA) function and are then maintained in the brain of adult mice. What is known already During ART, embryos are exposed to oxidative stress (OS)-prone environment. Cellular lipids are particularly sensitive to OS. Lipids are a crucial component for proper brain development and any aberrancies in lipid profile of the embryo can affect neurodevelopment (ND) through the PBA, leading to functional and behavioural deficits during adulthood. In particular, deficiencies of ether-linked glicerophospolipids (e.g. plasmalogens), which are highly abundant in the brain, and polyunsaturated fatty acids (PUFAs), have been associated with psychiatric and ND disorders. Study design, size, duration C57BL6 mouse embryos were cultured in vitro for 72hs, and analysed or transferred to 25 recipient females for further development. From 6 pregnant mice, conceptuses (placentae and brain) were collected at near-term pregnancy (19dpc, n > 8/group). Remaining 8 pregnant mice were allowed to deliver and raise litters. Subsequently, brain was collected from male offspring at 4 month of age for lipid homeostasis evaluation. Control group consisted of embryos and offspring obtained from naturally mated C57BL6 mice. Participants/materials, setting, methods C57BL6 blastocysts (n > 15/group) were fixed and subjected to lipid analysis by Fourier transform infra-red (FTIR) and Coherent antistokes Raman spectroscopies (CARS). To reveal lipid changes indicative for ND disorders, selected PUFAs were analyzed by liquid chromatography-mass spectrometry (LC-MS) (n > 8/group) in placenta and brains from prenatal and adult mice. Additionally, placental expression of genes regulating biosynthesis of plasmalogens and proteome of adult males brain, was performed. Values were considered different when p < 0.05, Mean±SEM; Mann-Whitney test. Main results and the role of chance Our results show that IVC induced lipid composition changes in blastocyst, characterised by increased proportion of phospholipids, fatty amides and cholesterol (59±8.2 vs.44.7±8.8; 149±4.5vs. 104±12; 44±2.9vs. 27±4.6 sum of absorbance, p < 0.05); and lipid droplets (LDs) distribution changes, such as, increase size (2.3±0.11 vs 1.8±0.07 µm2, p = 0.0021), and number of large LDs (5.5±0.7 vs. 3.3±0.2, p = 0.006). LC-MS revealed increased levels of linolenic (LA), arachidonic (AA) and docosahexaenoic (DHA) acids in placenta (LA:15.49±1.7 vs. 6.92±2; AA:263.6±29.1 vs. 62.7±14.7 DHA:0±2.5 vs. 2.7±0.8 pmol/mg; p < 0.03) and foetal brain (LA:13.6±5.5 vs. 1.5±0.9; AA:240.5±100 vs. 39.3±9.6; 13.9±6.9 vs. 2.11±0.6 pmol/mg; p < 0.03) of IVC conceptuses, which may condition brain development and function. Conversely, levels of LA and AA were low in adult brain from IVC offspring (LA:4.4±1.5 vs 14.5±3.8; AA: 581±95 vs 1842±454 pmol/mg, p < 0.04). Additionally, the expression of genes regulating biosynthesis of plasmalogens was reduced in IVC placentae (Gnpat:0.2±0.04 vs 1.2±03; Tmem189:0.3±0.1 vs. 1.3±0.3 fold change; p ≤ 0.01). Proteome analysis demonstrated that pathways regulating lipid homeostasis, such as fatty acid oxidation and phospholipid binding and those involved in IL-17a signaling and neurogenesis were upregulated in brain of adult IVC offspring. Limitations, reasons for caution Our results may not entirely reflect what occurs in human embryo, as mouse oocytes have a higher lipid content than human oocyte. Differences in lipid content may cause an increased or decreased penetrance and exhibition of developmental changes of the embryos. Wider implications of the findings Being of non-genetic background, lipid changes induced by IVC may similarly threaten embryos regardless of species causing ND disorders. Our study sheds new light on non-genetic origins of ND disorders and will likely ignite further studies on embryonic lipid changes and their consequences, relevant for human pregnancy and neuropsychiatry. Trial registration number not applicable

Fluorescence-Encoded Time-Domain Coherent Raman Spectroscopy in the Visible Range.

Fluorescence-encoded vibrational spectroscopy has attracted increasing attention by virtue of its high sensitivity and high chemical specificity. We recently demonstrated fluorescence-encoded time-domain coherent Raman spectroscopy (FLETCHERS), which enables low-frequency vibrational spectroscopy of low-concentration fluorophores using near-infrared (800-900 nm) light excitation. However, the feasibility of this study was constrained by the scarcity of excitable molecules in the near-infrared range. Consequently, the broader applicability of FLETCHERS has not been investigated. Here we extend the capabilities of FLETCHERS into the visible range by employing a noncollinear optical parametric amplifier as a light source, significantly enhancing its versatility. Specifically, we use the method, which we refer to as visible FLETCHERS (vFLETCHERS), to individually acquire Raman spectra from five visible fluorophores that have absorption peaks in the 600-700 nm region. These results not only confirm the versatility of vFLETCHERS for a wide range of molecules but also allude to its widespread applicability in biological research through highly sensitive supermultiplexed imaging.

Coherent Raman spectroscopy: Quo vadis?

Although the potential of Coherent Raman Spectroscopy (CRS) in the area of biomedicine, life sciences and material sciences has been well demonstrated, its wide-spread practical application is still rather limited. The two main CRS techniques are Coherent Anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS) spectroscopy or microscopy. Here we present the current state of the art and challenges facing CRS. Although many technological challenges have been addressed to date, showing how to improve resolution, sensitivity and selectivity of CRS, significant efforts are still needed to increase the awareness of these techniques in the academic community, develop reliable protocols, and extend them to practical applications. For this purpose it is also necessary to initiate national and international research networks that can significantly contribute to the development of CRS approaches in areas that have so far made little use of CRS alongside other Raman spectroscopic methods. The purpose of this perspective paper is to present the current state-of-the-art of CRS with a historical background, assess the challenges and present some future development visions.

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.

Chemical States of Water in Anion Exchange Membrane for Fuel Cells Using Raman and Coherent Anti-Stokes Raman Spectroscopies

Purpose Low membrane durability remains a major challenge in anion exchange membrane fuel cells (AEMFCs). This is despite the AEMFCs’ proven potential to eliminate the dependency of Fuel cells on Pt catalysts and bring about the mass commercialization of the technology. This problem can be effectively addressed through combined optimization of the AEMFCs’ operating conditions during evaluation, followed by finetuning of the membrane chemistry to address any observed weaknesses. After synthesis, water distribution is the primary operation condition determining a given AEM’s performance and lifespan [1]. This is partly because water affects the ionic conductivity and, thus, maximum current density, which is known to play a significant role in the longevity of membranes. Therefore, establishing the water distribution during power generation for any given membrane is highly desirable. For this to happen, suitable and reliable techniques must exist to measure and compare the water distribution in situ and operando. In this paper, we used in-house assembled Raman spectroscopy and coherent anti-Stokes Raman scattering spectroscopy (CARS) systems to measure the water in an operational AEMFC for the first time. CARS comes with a high time resolution of 0.1 s, which is essential in the transient response study of FCs. Experimental Method QPAF-4 membrane film (Fig. 1) of the thickness of 30 µm and IEC of 2.0 meq g-1 was solution cast as reported [2]. The cell was then assembled, as reported [3]. An airtight transparent quartz window 200 μm thick was mounted at the cathode endplate for optical access to the QPAF-4 membrane through a 500 mm pinhole in the GDE. A thin Pt disk was placed on the opposite side of the membrane to reflect the signal to the spectrometer through the ×50 objective lens used. For Raman spectroscopy, a 632 nm, 1 mW laser with 5.5 min total exposure was used. For CARS, an 11 mW 785nm pump laser and 16 mW Stokes laser with 200 ms total exposure were used. The flow rate was 100 ml min-1 for all gases. The cell temperature was maintained at 60 °C. The relative humidity of the gas varied from 30% to 100% in steps of 10% RH. The system was allowed to stabilize for 3 hours at each humidity level before measurement. Results and discussion Figures 2 and 3 show the normalized in situ spectra recorded at the center of the membrane at different relative humidity values using CARS and Raman, respectively. The peak positions are comparable. However, the OH peak in CARS is taller relative to C=C than in Raman. CARS signal intensity is generally stronger than Raman [3]. Within the CARS peak, the purely OH portion of the signal is more prominent relative to the rest of the peak than in Raman. This is attributed to the fact that the Infrared laser associated with the stokes light (817-1144nm) of CARS is known to be more sensitive to OH vibration than the lower wavelength laser (632nm) used in Raman [4]. Conclusion We successfully developed a 785nm CARS system capable of operando water measurement in AEMFCs. The results from this technique are comparable to those from micro-Raman spectroscopy but with better signal intensity and time resolution. This increases the number of practical methods available to researchers interested in measuring water content in AEMFCs allowing for faster development of the technology. The water distribution results during power generation will be discussed at the conference. With CARS, higher temporal/time resolution is achieved, which is advantageous when cell response needs to be studied.

Air-laser-based coherent Raman spectroscopy of atmospheric molecules in a filamentary plasma grating.

Coherent Raman spectroscopy (CRS) with air-laser-based hybrid femtosecond/picosecond (fs/ps) pulses has shown promising potential for remote detection and surveillance of atmospheric species with high temporal and frequency resolution. Here, to enhance the sensitivity and extend the detection distance, we generate the CRS spectra of air molecules in situ in a filamentary plasma grating, and show that the grating can efficiently enhance the intensities of the coherent vibrational Raman lines of N2, O2, and N2 + by 2-3 orders of magnitude at an extended distance. By examining the intensities of the Raman lines, fs-pulsed supercontinuum, and ps-pulsed air laser produced under different grating conditions, we reveal that the optimization of the Raman lines is achieved by the dynamic balance between the supercontinuum-induced vibrational coherence and air-laser-induced polarization of the air species.

Coherent Raman Spectroscopy of Equilibrium Phonons with Subwavenumber Resolution

Time-domain coherent Raman spectroscopy technique with excellent time (<120 fs) and equivalent spectral resolutions (up to ∼0.1 cm–1) has been applied to selectively measure ultrafast decay rates of optical phonons in technologically important wide-bandgap materials. The decays of intrinsic vibrations have been traced in time within multiple orders and phonon decay times varied broadly within 0.45–1.7 ps range. Primary decay routes via third- and fourth-order parametric phonon interactions have been analyzed to provide important estimates for zero-temperature decay rates and line widths for the main Raman active vibrations.

Electronic‐Resonance‐Enhanced Coherent Raman Spectroscopy with a Single Femtosecond Laser Beam

AbstractCoherent Raman scattering (CRS) spectroscopy holds versatile applications in many fields ranging from measuring temperature and concentration in reacting flows to imaging biological molecules in living cells. Although the powerful spectroscopic technique has been studied for a few decades, significant attention is still paid to developing state‐of‐the‐art CRS spectroscopy with the simpler design and higher sensitivity for practical applications in harsh or complex environments. Herein, a novel electronic‐resonance‐enhanced coherent Raman scattering (ERE‐CRS) technique is developed with a single femtosecond laser beam. Such single‐beam ERE‐CRS scheme is accomplished with the narrowband air lasing signal naturally generated in the interaction region as a probe source. The developed technique not only avoids fine spatio‐temporal control in the conventional multibeam configuration, but also accomplishes 1–2 orders of magnitude enhancement of Raman signal due to the resonance of air lasing with electronic transition of the target gas. The dramatic enhancement effect is also manifested through the comparative measurements in CO2 isotopes. The single‐beam ERE‐CRS spectroscopy shows an unprecedented opportunity for the high‐sensitivity standoff detections of gas‐phase molecules and plasmas.

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy.

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

Standoff detection of bacterial spores by field deployable coherent Raman spectroscopy

Vibrational spectroscopies offer great potential for standoff detection of chemical and biological warfare agents, avoiding contamination to the operator and equipment. Among them, particularly promising is Coherent anti-Stokes Raman scattering (CARS) spectroscopy, using synchronized pump/Stokes laser pulses to set up a vibrational coherence of target molecules at a laser focus, which is read by further interaction with a probe pulse, resulting in the emission of a coherent beam detectable at a distance. CARS has previously demonstrated the capability to detect bacterial spores based on the Raman spectrum of the characteristic molecule calcium dipicolinate (CaDPA); however, a complex and bulky laser technology, which is only suitable for a laboratory environment, was employed. Here we develop a broadband CARS setup based on a compact, industrial grade ytterbium laser system. We demonstrate high signal-to-noise ratio detection of Bacillus atrophaeus spores at a concentration of 105 cfu/mm2, at a standoff distance of 1 m, and an acquisition time of 1 s. Our system, which combines chemical specificity and sensitivity along with improved ruggedness and portability, paves the way to a new generation of instruments for real-world standoff detection of chemical and biological threats.

Entangled photons enabled time-frequency-resolved coherent Raman spectroscopy and applications to electronic coherences at femtosecond scale

Quantum entanglement has emerged as a great resource for spectroscopy and its importance in two-photon spectrum and microscopy has been demonstrated. Current studies focus on the two-photon absorption, whereas the Raman spectroscopy with quantum entanglement still remains elusive, with outstanding issues of temporal and spectral resolutions. Here we study the new capabilities provided by entangled photons in coherent Raman spectroscopy. An ultrafast frequency-resolved Raman spectroscopy with entangled photons is developed for condensed-phase molecules, to probe the electronic and vibrational coherences. Using quantum correlation between the photons, the signal shows the capability of both temporal and spectral resolutions not accessible by either classical pulses or the fields without entanglement. We develop a microscopic theory for this Raman spectroscopy, revealing the electronic coherence dynamics even at timescale of 50fs. This suggests new paradigms of optical signals and spectroscopy, with potential to push detection below standard quantum limit.

Coherent Raman spectroscopy on hydrogen with in-situ generation, in-situ use, and in-situ referencing of the ultrabroadband excitation.

Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H2). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H2 and the non-resonant CRS spectra in a laminar H2/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H2 CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.

Background-penalty-free waveguide enhancement of CARS signal in air-filled anti-resonance hollow-core fiber.

We study coherent anti-Stokes Raman spectroscopy in air-filled anti-resonance hollow-core photonic crystal fiber, otherwise known as "revolver" fiber. We compare the vibrational coherent anti-Stokes Raman signal of N2, at ∼2331 cm-1, generated in ambient air (no fiber present), with the one generated in a 2.96 cm of a revolver fiber. We show a ∼170 times enhancement for the signal produced in the fiber, due to an increased interaction path. Remarkably, the N2 signal obtained in the revolver fiber shows near-zero non-resonant background, due to near-zero overlap between the laser field and the fiber cladding. Through our study, we find that the revolver fiber properties make it an ideal candidate for the coherent Raman spectroscopy signal enhancement.

Femtosecond coherent Raman system with >75 dB dynamic range for probing vibration modes across 250-2400 cm-1.

We report on the design and performance of a time-resolved Coherent Raman spectroscopy system with time resolution of better than 120 fs. The coherent transients can be traced with more than 75 dB dynamic range while accessing and probing Raman active modes across a 250-2400 cm-1 frequency. The system delivers an equivalent spectral resolution of better than 0.1 cm-1 regarding line bandwidth parameters for probed Raman resonances.

Enhanced Chemical Sensing with Multiorder Coherent Raman Scattering Spectroscopic Dephasing.

Molecular vibrational spectroscopy is widely used in various sensing and imaging applications, providing intrinsic information at the molecular level. Nonlinear optical interactions using ultrashort laser pulses facilitate the selective coherent excitation of molecular vibrational modes by focusing energy into specific molecular bonds, boosting the signal level for multiple orders of magnitude. The dephasing of such coherence, which is susceptible to the local molecular environment, however, is often neglected. The unique capability of vibrational dephasing dynamics to serve as a unique probe for complex molecular interactions and the effect of local nano- and microenvironments are beyond the reach of conventional, intensity-based spectroscopy. Here, we developed a novel multiorder coherent Raman spectroscopy platform with a special focus on the temporal evolution of molecular vibrational dephasing, termed as time-resolved coherent Raman scattering (T-CRS) spectroscopy. By utilizing a high dynamic range detection, molecular vibrational dynamics and the environmental effects are demonstrated with multidimensional spectroscopic sensing, which promises a new range of applications in biology, materials, and chemical sciences.

Label-Free Super-Resolution Imaging Techniques.

Biological and material samples contain nanoscale heterogeneities that are unresolvable with conventional microscopy techniques. Super-resolution fluorescence methods can break the optical diffraction limit to observe these features, but they require samples to be fluorescently labeled. Over the past decade, progress has been made toward developing super-resolution techniques that do not require the use of labels. These label-free techniques span a variety of different approaches, including structured illumination, transient absorption, infrared absorption, and coherent Raman spectroscopies. Many draw inspiration from widely successful fluorescence-based techniques such as stimulated emission depletion (STED) microscopy, photoactivated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM). In this review, we discuss the progress made in these fields along with the current challenges and prospects in reaching resolutions comparable to those achieved with fluorescence-based methods.

Background-free single-beam coherent Raman spectroscopy assisted by air lasing.

We develop a background-free single-beam coherent Raman scattering technique enabling the high-sensitivity detection of greenhouse gases. In this scheme, Raman coherence prepared by a femtosecond laser is interrogated by self-generated narrowband air lasing, thus allowing single-beam measurements without complex pulse shaping. The unique temporal and spectral characteristics of air lasing are beneficial for improving the signal-to-noise ratio and spectral resolution of Raman signals. With this method, SF6 gas present at a concentration of 0.38% was detected in an SF6-air mixture. This technique provides a simple and promising route for remote detection due to the low divergence of Raman signals and the availability of high-energy pump lasers, which may broaden the potential applications of air lasing.