Transient Stimulated Raman Scattering Research Articles

Transient stimulated Raman scattering spectroscopy and imaging

Stimulated Raman scattering (SRS) has been developed as an essential quantitative contrast for chemical imaging in recent years. However, while spectral lines near the natural linewidth limit can be routinely achieved by state-of-the-art spontaneous Raman microscopes, spectral broadening is inevitable for current mainstream SRS imaging methods. This is because those SRS signals are all measured in the frequency domain. There is a compromise between sensitivity and spectral resolution: as the nonlinear process benefits from pulsed excitations, the fundamental time-energy uncertainty limits the spectral resolution. Besides, the spectral range and acquisition speed are mutually restricted. Here we report transient stimulated Raman scattering (T-SRS), an alternative time-domain strategy that bypasses all these fundamental conjugations. T-SRS is achieved by quantum coherence manipulation: we encode the vibrational oscillations in the stimulated Raman loss (SRL) signal by femtosecond pulse-pair sequence excited vibrational wave packet interference. The Raman spectrum was then achieved by Fourier transform of the time-domain SRL signal. Since all Raman modes are impulsively and simultaneously excited, T-SRS features the natural-linewidth-limit spectral line shapes, laser-bandwidth-determined spectral range, and improved sensitivity. With ~150-fs laser pulses, we boost the sensitivity of typical Raman modes to the sub-mM level. With all-plane-mirror high-speed time-delay scanning, we further demonstrated hyperspectral SRS imaging of live-cell metabolism and high-density multiplexed imaging with the natural-linewidth-limit spectral resolution. T-SRS shall find valuable applications for advanced Raman imaging.

Stimulated Raman scattering of chirped Ti:sapphire laser pulse in BaWO4, SrMoO4 and Ca3(VO4)2 crystals

Transient stimulated Raman scattering (SRS) of chirped, temporally stretched up to 45 ps, Ti:sapphire laser pulses was studied in three different Raman-active crystals (BaWO4, SrMoO4 and Ca3(VO4)2) at the same experimental conditions. BaWO4 and SrMoO4 crystals had the same SRS “threshold” energy/intensity, which was associated with their similar integrated cross sections for spontaneous Raman scattering. The highest energy SRS efficiency was obtained in SrMoO4 crystal and reached 8%, which was two-times higher than one in BaWO4. Higher SRS efficiency in SrMoO4 was observed due to lower nonlinear absorption. The Ca3(VO4)2 crystal had significantly higher SRS “threshold” and lower efficiency despite the lowest nonlinear absorption losses. A simple expression of exponential gain for transient SRS of chirped laser pulses was proposed and verified by comparison with experimental results. This expression was used to estimate dephasing time T2 of Ca3(VO4)2 crystal which was 8.5 ± 1.0 ps.

Phase complementarity and magnification effect of optical pump rogue wave and Stokes rogue wave in a transient stimulated Raman scattering system

In this article, we study the phase complementarity and magnification effect of the optical pump wave and Stokes wave in a transient stimulated Raman scattering (TSRS) system. We construct new pump and Stokes rogue wave modulus solutions by applying Darboux transformation method. According to the number and layout of peak and trough, there are three phases for both the pump and Stokes rogue waves. Furthermore, it is novel that there does exist phase complementarity between the pump and Stokes rogue waves. Then, the magnification effect from the pump rogue wave to the Stokes rogue wave is also investigated.

Characterisation of optical phonons within epitaxial Ge2Sb2Te5/InAs(111) structures

Femto-second pump-probe and micro-Raman spectroscopy (RS) measurements have been made to identify optical phonons in Ge2Sb2Te5/InAs(111) and an InAs(111) substrate. A theory of transient stimulated Raman scattering (TSRS) incorporating the Raman tensor predicts which phonon modes may be observed in transient reflectance (R) and anisotropic reflectance (AR) pump-probe measurements, and how their amplitudes depend upon angles φ and θ that describe the orientation of the pump and probe beam electric fields within the sample plane. AR measurements of an InAs(111) substrate revealed the 6.5 THz T2 transverse optical phonon with amplitude proportional to sin(2(θ−φ)), as expected for both TSRS and the specular optical Kerr effect (SOKE), confirming that TSRS and SOKE are equivalent descriptions of the same phenomenon. The AR responses of Ge2Sb2Te5/InAs(111) revealed a single coherent optical phonon (COP) mode at about 3.4 THz with sin(2(θ−φ)) amplitude variation that confirms the T2-like character of the mode and hence the underlying cubic structure of the epilayer. This mode was also observed in the R measurement for one sample, with amplitude independent of φ and θ as predicted by TSRS theory. Both R and AR signals were heavily damped, which is attributed to dephasing of T2x, T2y and T2z modes that become non-degenerate due to structural distortions. RS measurements revealed three modes for Ge2Sb2Te5/InAs(111) and three modes for InAs(111). Taken together the TSRS and RS measurements provide rich information about optical phonons in the phase change material Ge2Sb2Te5 and the InAs(111) surface.

Steady-state and transient Raman gain coefficients of weakly-polar magnetoactive doped semiconductors

Assuming that the origin of stimulated Raman scattering (SRS) lies in third-order effective (Raman) susceptibility arising due to nonlinear induced polarization, we obtained expressions for steady-state and transient Raman gain coefficients of weakly-polar magnetoactive doped semiconductors under various geometrical configurations. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. The dependence of Raman gain coefficients on doping concentration, external magnetostatic field, magnetostatic field inclination, scattering angle and pump pulse duration are explored in detail with aim to determine suitable values of these controllable parameters to enhance Raman gain coefficients at lower threshold intensities, and to search the feasibility of efficient semiconductor nonlinear devices based on Raman nonlinearities. Most expectedly, the technological potentiality of weakly-polar magnetoactive doped semiconductors as hosts for compression of scattered pulses and fabrication of efficient nonlinear devices such as frequency converters, Raman amplifiers and oscillators etc. based on Raman nonlinearities are established.

Novel cone-like and four-leaf-like optical rogue waves in a transient stimulated Raman scattering system

A new transient stimulated Raman scattering (TSRS) system in nonlinear optical fibers is studied. The analytical optical rogue wave solutions are constructed via the Lax–Darboux technique. Furthermore, novel cone-like rogue wave for the off-diagonal density-matrix element, four-leaf-like rogue waves for the pump and Stokes waves are observed. The results provide a new way to understand the propagation mechanism of the optical pulse in the stimulated Raman scattering media.

Propagation patterns of pump and Stokes breathers of the transient stimulated Raman scattering system in gas-filled hollow-core crystal fibers

In this article, under investigation is the optical pulse propagation of the transient stimulated Raman scattering (TSRS) system in gas-filled hollow-core crystal fibers. The pump and Stokes breather solutions are constructed via Darboux transformation (DT) method. Novel spatio-temporal propagation patterns of breather waves are observed. The results show that the pump and Stokes breathers not only have rich phase patterns, but also demonstrate certain complementarity.

Explanation of the competition between O- and E-wave induced stimulated Raman and supercontinuum in calcite under ultrafast laser excitation.

Key optical properties of calcite were measured to unravel the difference between stimulated Raman scattering (SRS) and self-phase modulation (SPM) for the supercontinuum (SC) for ordinary (O) wave and extraordinary (E) wave. These properties are group velocity dispersion, walk-off, spontaneous Raman spectra and cross section, optical 1086cm-1 phonon linewidth, nonlinear susceptibility (χ3), steady-state and transient SRS, and SC caused from SPM. These are investigated for O-waves and E-waves from a 2.7 cm thick calcite crystal. Using 390 fs pulses (∼0.8µJ pulse energy) at 517 nm, the O-wave produced a stronger sharp SRS peak at 1086cm-1 and a weaker SC spectrum in the visible range than the E-wave. The salient difference found between the O- and E-waves for SRS and SPM in calcite is attributed to the larger Raman cross section and the size of nonlinear susceptibility (χ3) for O-waves as compared to E-waves.

Determination of the stimulated raman scattering threshold for a pump pulse of arbitrary width

A theoretical solution to the problem of determining the stimulated Raman scattering (SRS) threshold has been found within the undepleted pump approximation for a pump pulse of arbitrary width, which distinguishes it from the known solutions for the limiting cases of very short (highly transient SRS) and very long (quasi-steady-state SRS) pump pulses with respect to the oscillation dephasing time of the SRS medium. The general formula of the theoretical estimate of SRS threshold, in dependence of not only the pump radiation intensity and the SRS interaction length but also the pump-pulse width, is obtained based on the found solution. The theoretical estimate of the SRS threshold has been shown to be in good agreement with the experimental results on the excitation of picosecond SRS in crystals, which justifies the new express method for estimating the SRS gain in experimental measurements of the picosecond SRS threshold.

Efficient picosecond traveling-wave Raman conversion in a SrWO4 crystal pumped by multi-Watt MOPA lasers at 1064 nm

Raman conversion with a 50-mm-long SrWO4 crystal in a single-pass, traveling-wave setup has been investigated in both purely steady-state and transient stimulated Raman scattering (SRS) regimes. For steady-state SRS experiment, we employed as a pump source a Q-Switched master oscillator power amplifier (MOPA) laser system at 1064 nm, delivering 325 μJ, 550-ps-long pulses with diffraction limited beam quality and high spectral purity. At 2-kHz repetition rate, we obtained up to 90 μJ pulse energy and 250 ps pulse duration at 1180 nm, with a conversion slope efficiency close to quantum limit. To approach the transient SRS regime, we pumped the same crystal with 16-ps-long pulses from a hybrid MOPA laser system based on a mode-locked Yb-fiber oscillator followed by a diode-pumped bulk Nd:YVO4 power amplifier. At the maximum incident pump average power of 3.75 W, we obtained 1.4 W at the first Stokes Raman-shifted wavelength of 1180 nm (37 % optical-to-optical conversion efficiency), with 15 ps pulse duration and 70 % conversion slope efficiency.

Theoretical study of collinear optical frequency comb generation under multi-wave, transient stimulated Raman scattering in crystals

Using mathematical modelling we have studied the conditions of low-threshold collinear optical frequency comb generation under transient (picosecond) stimulated Raman scattering (SRS) and parametric four-wave coupling of SRS components in crystals. It is shown that Raman-parametric generation of an octave-spanning optical frequency comb occurs most effectively under intermediate, transient SRS at a pump pulse duration exceeding the dephasing time by five-to-twenty times. We have found the optimal values of not only the laser pump pulse duration, but also of the Raman crystal lengths corresponding to highly efficient generation of an optical frequency comb from the second anti-Stokes to the fourth Stokes Raman components. For the (high dispersion) and (low dispersion) crystals pumped at a wavelength of and a pulse duration five or more times greater than the dephasing time, the optimum length of the crystal was 0.3 and , respectively, which is consistent with the condition of the most effective Stokes – anti-Stokes coupling , where is the wave detuning from phase matching of Stokes – anti-Stokes coupling, determined by the refractive index dispersion of the SRS medium.

Transient stimulated raman scattering in crystals during motion of populations of vibrational states

Transient stimulated Raman scattering (SRS) in crystals is analyzed taking into account the motion of populations of vibrational states under the action of subpicosecond (shorter than the dephasing time) pump pulses. Analytic expressions describing the dynamics of excitation of vibrations in SRS are derived. It is found that for a small wavelength of SRS interaction and high intensities of pump radiation observed for femtosecond SRS in crystals, avalanche excitation of vibrations can be responsible for SRS suppression. It is shown that when phase matching of Stokes-anti-Stokes parametric coupling in transient SRS is ensured, it is possible to elevate the efficiency of frequency conversion under conditions of motion of populations of vibrational states; this explains recent successful results in the experimental implementation of femtosecond SRS in crystals pumped by a Bessel beam.

Direct Observation of Self-Similarity in Evolution of Transient Stimulated Raman Scattering in Gas-Filled Photonic Crystal Fibers

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.

Control of radiation spatial coherence under synchronous amplification in crystalline LiF:F 2 − amplifier

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F −2 color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F −2 crystals and the Stokes radiation amplification in the color center crystal.

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.

Transient stimulated Raman scattering in a two-component gas mixture

Generation of the first Stokes components of stimulated Raman scattering (SRS) in two-component gas mixtures, including the combination component (resulting from scattering of the first (second) Stokes component in the second (first) gas) is studied by numerical methods. The condition for efficient generation of the combination Stokes component is derived and interpreted. It is shown theoretically and by numerical simulation that the optimal ratio of the pressures for the transient SRS generation of the combination component is inversely proportional to the ratio of the Raman scattering cross sections of the two gases. The results of numerical simulation agree well with experimental data.

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.

Numerical simulations of pulsed and quasi-cw regimes of synchronously pumped Raman oscillators

Synchronously pumped Raman oscillators allow for an efficient frequency conversion of pump pulses whose power is well below the threshold for extracavity stimulated Raman scattering (SRS). Numerical simulations show that the cavity mismatch of pump laser and Raman oscillator is critical for maximum conversion efficiency and minimum pulse duration for transient and stationary SRS. At a certain mismatch the simulations predict a quasi-cw Raman oscillator output even at moderate cavity quality factors. Optimum operation conditions are discussed for pump pulse trains of finite lengths. The results of the numerical calculations are in qualitative agreement with a simple phenomenological model.

Spatial coherence of transient stimulated Raman scattering

The Stokes wave evolution at transient stimulated Raman Scattering (SRS) in compressed hydrogen was simulated taking into account diffraction effects. A corresponding numerical model was developed to study different amplitude-phase SRS characteristics, such as space–time and phase dependencies, spectrum and spatial coherence function of pump and Stokes waves. Simulation results showed significant differences in these characteristics between transient and quasi-stationary cases. The strong oscillatory process of energy exchange between pump and Stokes waves in the transient case results in complicated intensity and phase space–time dependencies. However, under quasi-stationary conditions the Stokes wave phase varies in wider limits, which leads to spatial coherency lowering. The module value of the spatial coherency function lowers to a threshold and then becomes stable as the conversion coefficient increases for both the transient and quasi-stationary cases. Stokes beam focusing is shown for SRS, which arises from competition between diffraction and non-uniform in a cross-section strong Raman amplification. Results of simulations are in good agreement with experimental data.

Multiwave quasi-phase-matched anti-Stokes stimulated Raman scattering

In our method of anti-Stokes generation at stimulated Raman scattering (SRS) with quasi-phase-matched conditions we took into account the generation of high-order Stokes and anti-Stokes components. The dependence of Raman gain dispersion on SRS generation was studied using numerical simulation. The possibility of an effective periodical structure for hydrogen was determined. For a multiwave media model we found that the efficiency of first anti-Stokes generation reached ~40%. We determined that for transient SRS the maximum efficiency of quasi-phase-matching anti-Stokes generation does not depend on pulse duration while the medium length greatly increases as transience increases. Our calculations showed that for hydrogen and barium nitrate if the duration of interacted pulses is 3 ns and more then we can neglect the influence of transitivity on quasi-phase-matching conditions. The results of our research can be used for the creation of new effective nonlinear optical frequency converters and Raman amplifiers.