Coherent Stokes Raman Scattering Research Articles

Frequency conversion to the telecom O-band using pressurized hydrogen.

Large-scale quantum networks rely on optical fiber networks and photons as so-called flying qubits for information transport. While dispersion and absorption of optical fibers are minimum at the infrared telecom wavelengths, most atomic and solid state platforms operate at visible or near-infrared wavelengths. Quantum frequency conversion is required to bridge these two wavelength regimes, and nonlinear crystals are currently employed for this process. Here, we report a novel approach of frequency conversion to the telecom band. This interaction is based on coherent Stokes Raman scattering (CSRS), a four-wave mixing process resonantly enhanced in a dense molecular hydrogen gas. We show the conversion of photons from 863 nm to the telecom O-band and demonstrate that the input polarization state is preserved. This process is intrinsically broadband and can be adapted to any other wavelength.

Coherent Stokes Raman scattering microscopy (CSRS)

We report the first implementation of laser scanning coherent Stokes Raman scattering (CSRS) microscopy. To overcome the major challenge in CSRS imaging, we show how to suppress the fluorescence background by narrow bandpass filter and a lock-in based demodulation. Near background free CSRS imaging of polymer beads, human skin, onion cells, avocado flesh and the wing disc of a drosphila larva are presented. Finally, we explain and demonstrate numerically that CSRS solves a major obstacle of other coherent Raman techniques by sending a significant part (up to 100%) of the CSRS photons into the backward direction under tight focusing conditions. We believe that this discovery will pave the way for numerous technological advances, e.g., in epi-detected coherent Raman multi-focus imaging, real-time laser scanning based spectroscopy or efficient endoscopy.

Background-free time-resolved coherent Raman spectroscopy (CSRS and CARS): Heterodyne detection of low-energy vibrations and identification of excited-state contributions

Coherent Raman scattering (CRS) spectroscopy techniques have been widely developed and optimized for different applications in biomedicine and fundamental science. The most utilized CRS technique has been coherent anti-Stokes Raman scattering (CARS), and more recently, stimulated Raman scattering. Coherent Stokes Raman scattering (CSRS) has been largely ignored mainly because it is often strongly affected by fluorescence, particularly for resonance enhanced measurements. However, in the cases of resonant excitation, the information contained in the CSRS signal can be different and complementary to that of CARS. Here, we combine the approaches of pulse shaping, interferometric heterodyne detection, 8-step phase cycling, and Fourier-transform of time-domain measurements, developed in CARS and 2D electronic spectroscopy communities, to measure resonant CSRS and CARS spectra using a titanium:sapphire oscillator. The signal is essentially background-free (both fluorescent and nonresonant background signals are suppressed) with high spectral resolution and high sensitivity and can access low-energy modes down to ∼30 cm−1. We demonstrate the ability to easily select between CSRS and CARS schemes and show an example in which acquisition of both CSRS and CARS spectra allows vibrational modes on the excited electronic state to be distinguished from those on the ground electronic state.

Fast vibrational imaging of single cells and tissues by stimulated Raman scattering microscopy.

ConspectusTraditionally, molecules are analyzed in a test tube. Taking biochemistry as an example, the majority of our knowledge about cellular content comes from analysis of fixed cells or tissue homogenates using tools such as immunoblotting and liquid chromatography–mass spectrometry. These tools can indicate the presence of molecules but do not provide information on their location or interaction with each other in real time, restricting our understanding of the functions of the molecule under study. For real-time imaging of labeled molecules in live cells, fluorescence microscopy is the tool of choice. Fluorescent labels, however, are too bulky for small molecules such as fatty acids, amino acids, and cholesterol. These challenges highlight a critical need for development of chemical imaging platforms that allow in situ or in vivo analysis of molecules. Vibrational spectroscopy based on spontaneous Raman scattering is widely used for label-free analysis of chemical content in cells and tissues. However, the Raman process is a weak effect, limiting its application for fast chemical imaging of a living system.With high imaging speed and 3D spatial resolution, coherent Raman scattering microscopy is enabling a new approach for real-time vibrational imaging of single cells in a living system. In most experiments, coherent Raman processes involve two excitation fields denoted as pump at ωp and Stokes at ωs. When the beating frequency between the pump and Stokes fields (ωp – ωs) is resonant with a Raman-active molecular vibration, four major coherent Raman scattering processes occur simultaneously, namely, coherent anti-Stokes Raman scattering (CARS) at (ωp – ωs) + ωp, coherent Stokes Raman scattering (CSRS) at ωs – (ωp – ωs), stimulated Raman gain (SRG) at ωs, and stimulated Raman loss (SRL) at ωp. In SRG, the Stokes beam experiences a gain in intensity, whereas in SRL, the pump beam experiences a loss. Both SRG and SRL belong to stimulated Raman scattering (SRS), in which the energy difference between the pump and Stokes fields is transferred to the molecule for vibrational excitation. The SRS signal appears at the same wavelengths as the excitation fields and is commonly extracted through a phase-sensitive detection scheme. The detected intensity change because of a Raman transition is proportional to Im[χ(3)]IpIs, where χ(3) represents the third-order nonlinear susceptibility, Ip and Is stand for the intensity of the pump and Stokes fields.In this Account, we discuss the most recent advances in the technical development and enabling applications of SRS microscopy. Compared to CARS, the SRS contrast is free of nonresonant background. Moreover, the SRS intensity is linearly proportional to the density of target molecules in focus. For single-frequency imaging, an SRS microscope offers a speed that is ∼1000 times faster than a line-scan Raman microscope and 10 000 times faster than a point-scan Raman microscope. It is important to emphasize that SRS and spontaneous Raman scattering are complementary to each other. Spontaneous Raman spectroscopy covers the entire window of molecular vibrations, which allows extraction of subtleties via multivariate analysis. SRS offers the speed advantage by focusing on either a single Raman band or a defined spectral window of target molecules. Integrating single-frequency SRS imaging and spontaneous Raman spectroscopy on a single platform allows quantitative compositional analysis of objects inside single live cells.

Three-pulse multiplex coherent anti-Stokes/Stokes Raman scattering (CARS/CSRS) microspectroscopy using a white-light laser source

We have developed a three-pulse non-degenerate multiplex coherent Raman microspectroscopic system using a white-light laser source. The fundamental output (1064nm) of a Nd:YAG laser is used for the pump radiation with the white-light laser output (1100–1700nm) for the Stokes radiation to achieve broadband multiplex excitations of vibrational coherences. The second harmonic (532nm) of the same Nd:YAG laser is used for the probe radiation. Thanks to the large wavelength difference between the pump and probe radiations, coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) can be detected simultaneously. Simultaneous detection of CARS and CSRS enables us to obtain information on the electronic resonance effect that affects differently the CARS and CSRS signals. Simultaneous analysis of the CARS and CSRS signals provides us the imaginary part of χ(3) without introducing any arbitrary parameter in the maximum entropy method (MEM).

Manipulation of coherent Raman scattering process in CS2 byusing chirpped circularly polarized femtosecond pulses

The coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) signals of vibration mode at 397cm-1 of CS2 liquid are manipulated by using two chirped circularly polarized femtosecond pulses in non-collinear excitation geometry. Along the phase matching direction intensity distributions, polarizations and wavelengths of CARS and CSRS signals vary regularly as the polarizations of the laser beams are changed. The intensity distributions of CARS and CSRS reflect the scattering cross sections of the stimulated raman scattering of the sample under different circularly polarizations. The polarizations of CARS and CSRS follow the conservation law of the angular momentum of the excitations. The frequency shifts of CARS and CSRS demonstrate the transitions between the rotational energy levels of CS2. The above information is helpful for understanding the mechanism and the behaviors of the CARS and CSRS of CS2 as well as other molecules.

Coherent anti-Stokes Raman scattering microscopy.

Coherent anti-Stokes Raman scattering microscopy.

Linewidth modelling of C2H2N2 mixtures tested by rotational CARS measurements

AbstractThe acetylene–nitrogen system is investigated in a combined theoretical and experimental effort in view of its thermometry applications. Accurate values of the rotational Raman linewidths of acetylene are of crucial importance for the computation of spectra from rotational coherent anti‐Stokes Raman scattering (CARS) used for temperature and concentration evaluations. Since the direct measurements of these linewidths are not available, they are rigorously calculated by a semi‐classical Robert–Bonamy formalism employing exact trajectories. The computed linewidths are used as input parameters in the theoretical spectra simulation, which is also improved by the inclusion of an interbranch interference with coherent Stokes Raman scattering (CSRS). These theoretical spectra are used to evaluate the temperature from experimental CARS spectra of acetylene‐nitrogen mixtures recorded in the pressure range 0.1–0.6 MPa. The new linewidths (NLWs) and the CSRS interference inclusion result in a clear improvement of spectral fitting as well as in reasonable values of evaluated temperatures. Copyright © 2006 John Wiley & Sons, Ltd.

A possibility of distinguishing the inhomogeneous broadening via coherent Raman spectra

An alternative possibility of determining the inhomogeneous broadening based on the excitation spectra of the coherent Raman scattering, coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) is proposed. By using the transform relations between the coherent Raman scattering and the absorption, it is shown that in the case of a Gaussian distribution of inhomogeneities, the inhomogeneous broadening can directly be determined from the difference in CARS and CSRS intensities.

Coherent Raman spectroscopy of CdTe/MnTe short-period superlattices

Short-period $(\mathrm{CdTe}{)}_{m}/{(\mathrm{MnTe})}_{n}$ superlattices were investigated by coherent Stokes Raman scattering (CSRS) in backscattering geometry. Temperatures ranged between 1.6 and about 120 K, and magnetic fields up to 7 T were applied in Voigt configuration. The method allows the observation of spin splittings of delocalized band carriers as well as of electrons localized to magnetic ions. The magnetic properties are probed by measuring the paramagnetic (PMR) and antiferromagnetic resonances (AFMR) of the ${\mathrm{Mn}}^{2+}$ d electrons and the spin splittings of photoexcited free electrons [conduction-electron spin flip (CSF)] in the CdTe wells, the wave functions of which are extended more or less into the MnTe barriers. Since resonant CSRS is particularly sensitive to Mn ions located in regions with high expectation probabilities for electrons, the investigations yield some spatial resolution in growth direction. A simultaneous observation of AFMR and PMR shows that, besides the antiferromagnetically coupled ${\mathrm{Mn}}^{2+}$ ions, uncoupled ones exist. The CSF data are analyzed in the framework of a mean-field model. The N\'eel temperatures are substantially decreased with respect to bulklike MnTe due to the reduced dimensionality, but on the other hand are increased by a magnetic coupling of the MnTe layers across the diamagnetic CdTe. We prove that the probability of finding electrons close to the CdTe/MnTe interface is enhanced as compared to results from superlattice band-structure calculations. Furthermore the experimental data show the existence of two-dimensional bound magnetic polarons.

CCD broadband detection technique for the spectral characterization of the inhomogeneous signal in femtosecond time‐resolved four‐wave mixing spectroscopy

The spectral analysis of the inhomogeneous signal obtained from femtosecond (fs) time-resolved degenerate four-wave mixing spectroscopy on iodine vapor with the help of CCD broadband detection is reported. It is shown with this spectral analysis that the broad band detection technique makes the simultaneous observation of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS), coherent Stokes Raman scattering (CSRS) and degenerate four-wave mixing (DFWM) processes possible. The fs lasers employed in our studies possess spectral widths which allow not only for a DFWM process but also other forms of four-wave mixing (FWM) processes such as CARS and CSRS to take place simultaneously in the molecular sample. These processes show themselves at different spectral positions within the FWM signal. The separation of the FWM signal with help of a monochromator and subsequent broadband detection with a CCD camera allow all the spectral components of the signal to be detected simultaneously under identical experimental conditions. As a result, the different FWM processes can be compared with one another directly and the continuous transition between these processes can be observed. Furthermore, the high quantum efficiency innate to a CCD camera allows for an evident improvement in the quality of the recorded transient FWM signal in comparison with detection with a fast photomultiplier. Copyright ” 1999 John Wiley & Sons, Ltd.

Femtosecond time-resolved four-wave mixing spectroscopy in iodine vapour

Femtosecond time-resolved four-wave mixing (FWM) spectroscopy is performed using degenerate laser frequencies in order to investigate molecular dynamics in iodine molecules in the gas phase. It is demonstrated that by varying the timing as well as the wavenumber position for the signal detection different dynamics can be accessed. This is due to a selection of contributions (e.g. described by Feynman diagrams) to the nonlinear polarization of the process. The wave packet dynamics of the electronic excited B as well as the X ground state of iodine can be monitored by degenerate FWM, coherent anti-Stokes Raman scattering or coherent Stokes Raman scattering (CSRS) processes all included in the same experiment.

Theory of time-resolved coherent Raman scattering with spectrally tailored noisy light

The theory for the application of spectrally tailored (‘two color’) noisy light in an interferometric coherent Raman scattering (CRS) experiment called I(2)CRS(3c) is presented. The theory is concerned with the purely resonant contribution to the intensity level I(2)CRS(3c) signal. The tailored beam is modeled as a sum of two Lorentzians where the center frequency of each Lorentzian is assumed to be under experimental control. It is found that not only are the coherent Stokes–Raman scattering (CSRS) and coherent anti-Stokes–Raman scattering (CARS) signals emitted, but a third ‘hybrid’ frequency is also contained in the signal. It offers the promise of a new way to up-convert low frequency Raman active modes, while the same dynamic information contained in the radiation difference oscillations (RDOs) of previous incoherent CRS experiments remains. More generally, it offers an additional degree of freedom that allows for arbitrary experimental control over the position in frequency space in which the hybrid signal will emerge. In addition it is shown that the hybrid signal frequency contains dynamically important information in its arbitrarily placed (in frequency space) signal. Plots of monochromatically detected I(2)CRS(3c) interferograms calculated for different parameter sets are shown together with the signal spectra. © 1997 John Wiley & Sons, Ltd.

Simple computer‐controlled scanning for a coherent Stokes Raman scattering spectrometer

Synchronous wave number‐shift scanning of a Nd:YAG pumped dye laser and the detection monochromator for a coherent Stokes Raman scattering (CSRS) system were realized using an inexpensive analog to digital board. With this improvement the CSRS spectrometer may be used to scan over a large wave number shift range and/or may be used at relatively high resolution using a photomultiplier tube as the detector. This is especially useful in the probing of luminous systems.

Polarization‐sensitive resonance CSRS of deoxy‐and oxyhaemoglobin

AbstractPolarization‐sensitive coherent Stokes Raman scattering (CSRS) measurements of oxy‐ and deoxyhaemoglobin in aqueous solutions are reported. The excitation wavelengths used were chosen in the region of the Q absorption bands to achieve twofold electronic resonance. The dispersion profiles of all independent susceptibility χ(3) components and purely anisotropic and anti‐symmetric scattering contributions were resolved within the frequency non‐degenerate CSRS scheme. Eight bands of oxyhaemoglobin and five bands of deoxyhaemoglobin were observed in the range 1500‐1680 cm−1. Simultaneously fitting sets of polarization spectra provided vibrational parameters (positions, bandwidths, amplitudes, phases and CSRS depolarization ratios) for each compound. Major bands were assigned to the non‐totally symmetric v10, v11 and v19 modes of the porphyrin macrocycle. The phases calculated exhibited a correlation with the symmetry of the vibrations. On the basis of the spectral fits, the three additional peaks arising in the oxyhaemoglobin spectra could be ascribed to the bands of intermediate deoxyhaemoglobin. The occurrence is due to the partial photolysis of oxyhaemoglobin. Vibrational parameters of these bands were found to be essentially similar to the parameters of the bands observed in the spectra of the stable deoxyhaemoglobin. Despite the asymmetric character predicted, the major bands were all contributed to by a considerable isotropic component. A decrease in the depolarization ratio P of the anomalously polarized v19 mode from 7.7 in oxyhaemoglobin to 4.3 in deoxyhaemoglobin was observed. Such a decrease in anti‐symmetric character of the vibration on release of the ligand supports the occurrence of deformation of the haem ring system.

Transient resonance coherent stokes Raman scattering spectra of photoinduced radical ions of diphenyloctatetraene in polar solvents

AbstractTransient resonance coherent Stokes Raman scattering (CSRS) spectra of the photoinduced radical ions of all‐trans‐1, 8‐diphenylocta‐1, 3, 5, 7‐tetraene (DPO) were observed in polar solvents such as acetonitrile, N, N‐dimethylformamide and dimethyl sulphoxide. The identification was based on the long lifetimes and the dependence of their formation on the electron‐donating and/or ‐accepting nature of the solvents. The fluorescence spectra of DPO were also observed in the presence of N, N‐dimethylaniline (DMA) and p‐dicyanobenzene (DCNB) in p‐dioxane and in polar solvents without DCNB and DMA. The absence of an indication of exciplex formation, in contrast to the cases of diphenylbutadiene and diphenylhexatriene, can be related to the extremely low efficiency of the radical ion formation by solvent‐assisted photoionization in DPO—DMA and DPO—DCNB in p‐dioxane systems. The strong dependence of the CSRS signal of the DPO radical ions on the content of polar solvent in the high‐concentration region of the binary solvent systems with p‐dioxane suggests a collective behaviour of the polar solvents surrounding an S1 solute for the CT interaction to generate the radical ions. The observed Raman wavenumbers were compared with those of DPO in the S0, S1 and T1 states and qualitative correlation among them is discussed.

Transient resonance CARS and CSRS of diphenylbutadiene‐d4 in the S0, S1T1 and photoinduced radical ion states

AbstractThe Raman, resonance CARS (coherent anti‐Stokes Raman scattering) and CSRS (coherent Stokes Raman scattering) spectra of α, α′, β, β′‐deuterated trans, trans‐1,4‐diphenylbuta‐1,3‐diene (DPB‐d4) in the So, S1, T1, radical cation and radical anion states were studied. The radical ions were produced transiently by ns‐pulsed UV irradiation in a solution containing electron donors ar acceptors. A comparative study of the observed Raman frequencies with those of DPB‐d0 permitted vibrational assignment of the transient species of DPB. Features relating to spectral and structural characteristics of the transient species of the shorter diphenylpolyenes are briefly discussed.

Resonant coherent anti-Stokes and Stokes Raman scattering spectra of molecular impurities at low temperatures

In the present paper, the results of the measurements of steady-state resonant coherent anti-Stokes Raman scattering (CARS) and coherent Stokes Raman scattering (CSRS) spectra of a tetracene impurity in the monocrystals of anthracene and a glassy matrix of polystyrene at low temperatures are presented. The inhomogeneous widths of the pure electronic lines of the samples differ over two orders of magnitude. In agreement with theory, it is shown that the widths of the spectral lines corresponding to the vibrations in both the ground and the excited electronic state of tetracene are determined by the relatively small inhomogeneous variation of the Raman frequencies and not by the much larger inhomogeneous variation of pure electronic or vibronic transition frequencies. The CARS excitation profiles reveal a shift with respect to the absorption band and hole–shape peculiarities that could be explained as the result of interference of different light waves in the sample. In the methodological part of the paper, some restrictions to the experimental possibilities determined by the resonant character of excitation have been outlined.

Radiation-matter oscillations and spectral line narrowing in field-correlated four-wave mixing. II. Experiment.

New radiation-matter phenomena in four-wave mixing spectroscopies, given a theoretical basis in the preceding paper [Dugan and Albrecht, Phys. Rev. A 43, 3877 (1991)], are examined experimentally here. A new class of Rabi detuning oscillations is recorded in interferometric coherent Stokes Raman scattering (CSRS). Two identical nanosecond incoherent light fields are mutually delayed, while simultaneously mixing in a nonlinear medium with a narrow-band field of a different color. Spectral filtering of the CSRS signal is essential. From such signals the frequencies and the dephasing rate constants can be a measure of the Raman-active vibrations in both the electronic ground state and the electronic excited state of the scattering chromophore. In principle, analogous probing of vibronic transitions is possible. Detuning oscillations for multimode systems are presented. It is shown how they are transformed into quantum beats, exactly those seen with femtosecond CSRS, when the narrow-band field is made broad and/or the detection is ``white.'' Finally, an example of the sub-Raman linewidth spectral feature, predicted in the preceding paper, is presented. It determines a Raman frequency, but does not carry its linewidth.

Radiation-matter oscillations and spectral line narrowing in field-correlated four-wave mixing. I. Theory.

New radiation-matter phenomena in four-wave mixing spectroscopies, given a theoretical basis in the preceding paper [Dugan and Albrecht, Phys. Rev. A 43, 3877 (1991)], are examined experimentally here. A new class of Rabi detuning oscillations is recorded in interferometric coherent Stokes Raman scattering (CSRS). Two identical nanosecond incoherent light fields are mutually delayed, while simultaneously mixing in a nonlinear medium with a narrow-band field of a different color. Spectral filtering of the CSRS signal is essential. From such signals the frequencies and the dephasing rate constants can be a measure of the Raman-active vibrations in both the electronic ground state and the electronic excited state of the scattering chromophore. In principle, analogous probing of vibronic transitions is possible. Detuning oscillations for multimode systems are presented. It is shown how they are transformed into quantum beats, exactly those seen with femtosecond CSRS, when the narrow-band field is made broad and/or the detection is ``white.'' Finally, an example of the sub-Raman linewidth spectral feature, predicted in the preceding paper, is presented. It determines a Raman frequency, but does not carry its linewidth.