Stimulated Raman Scattering Process Research Articles

Stimulated Raman photothermal microscopy toward ultrasensitive chemical imaging.

Stimulated Raman scattering (SRS) microscopy has shown enormous potential in revealing molecular structures, dynamics, and couplings in complex systems. However, the sensitivity of SRS is fundamentally limited to the millimolar level due to shot noise and the small modulation depth. To overcome this barrier, we revisit SRS from the perspective of energy deposition. The SRS process pumps molecules to their vibrationally excited states. The subsequent relaxation heats up the surroundings and induces refractive index changes. By probing the refractive index changes with a laser beam, we introduce stimulated Raman photothermal (SRP) microscopy, where a >500-fold boost of modulation depth is achieved. The versatile applications of SRP microscopy on viral particles, cells, and tissues are demonstrated. SRP microscopy opens a way to perform vibrational spectroscopic imaging with ultrahigh sensitivity.

Investigation of high beam quality methane Raman laser

Although lasers generated by stimulated Raman scattering (SRS) were reported to have better beam quality in comparison with pump laser; once pump laser intensity exceeded the threshold of laser-induced-breakdown (LIB), Raman laser beam quality was deteriorated dramatically. In this study, the backward SRS (BSRS) process of methane pumped by 532 nm laser was applied to produce high beam quality seed Raman laser beam, and stimulated Raman amplification with configuration of collimated seed laser and pump laser counter-propagation (BSRA) was used to increase the pulse energy of Raman laser. With a seed Raman laser of 51.4 mJ, ∼127.2 mJ Raman laser could be produced by BSRA process. Raman laser beam quality achieved Mx2 = 2.52 and My2 = 2.40, which were better than pump laser. The technique could be applied repeatedly; pulse energy and peak power of Raman laser could be further improved. This work provides of possible method to achieve high peak power (large pulse energy) Raman lasers with excellent beam quality.

Competition between nonlinear processes excited by picosecond laser pulses in a disodium ditungstate Raman crystal for two excitation polarizations

Two nonlinear processes, namely, two-photon absorption (TPA) and stimulated Raman scattering (SRS) in Na2W2O7 crystal at a wavelength of 523.5 nm, are studied theoretically and experimentally depending on the polarization of the incident picosecond laser radiation. It was found that, due to the anisotropy of the band gap structure, the TPA coefficient at 523.5 nm strongly depends on the pump radiation polarization. For radiation along the c axis with polarization parallel to the a axis, the TPA coefficient was measured to be 1.28 cm GW−1, and no SRS process was registered. However, for polarization parallel to the b axis, no measurable TPA was detected and SRS with a gain of 6.5 cm GW−1 was observed. To the best of our knowledge, this is the first demonstration of the competition of two nonlinear processes in one crystal depending on its orientation.

Highly efficient diode-pumped passively Q-switched Nd:YVO4/KGW Raman lasers at yellow and orange wavelengths.

Efficient diode-pumped passively Q-switched Nd:YVO4 yellow and orange lasers are developed with the pulse pumping scheme and the intracavity stimulated Raman scattering (SRS) and second harmonic generation (SHG). A Np-cut KGW is exploited in the SRS process to generate the yellow 579 nm laser or the orange 589 nm laser in a selectable way. The high efficiency is achieved by designing a compact resonator to include a coupled cavity for intracavity SRS and SHG and to provide a focused beam waist on the saturable absorber for reaching an excellent passive Q-switching. The output pulse energy and peak power can reach 0.08 mJ and 50 kW for the orange laser at 589 nm. On the other hand, the output pulse energy and peak power can be up to 0.10 mJ and 80 kW for the yellow laser at 579 nm.

Picosecond random Raman lasing in the spectral range of 360–630 nm using powders of potassium, sodium and strontium nitrates

Stimulated Raman scattering (SRS) of picosecond laser radiation at 532 and 355 nm in microcrystalline powders of potassium, sodium, and strontium nitrates has been studied. One anti-Stokes and three Stokes components were generated in all samples when pumping at 532 nm. For the first time to our knowledge, random Raman lasing in the used nitrates has been obtained upon their excitation at 355 nm. In this case, the multifrequency Stokes SRS is also observed in all nitrates. We have analyzed all possible loss channels when ultraviolet radiation is used for excitation of SRS. In general, picosecond random SRS generation has been obtained at 19 spectral lines in the spectral range of 360–630 nm. In all cases, the SRS process is due to the internal symmetric mode of the (NO3)− ion with the frequency shifts of 1050 cm−1 (KNO3), 1069 cm−1 (NaNO3), and 1057 cm−1 (Sr(NO3)2).

Vibrational mode-specific polarization effect in circularly polarized stimulated Raman scattering.

As one of the popular coherent Raman scattering techniques, stimulated Raman scattering (SRS) has made significant progress in recent years, especially in label-free biological imaging. Polarization provides an additional degree of freedom to manipulate the SRS process. In previous studies, only linearly polarized SRS was fully investigated, in which both pump and Stokes laser fields are linearly polarized. Here, we theoretically analyzed the SRS process excited by two circularly polarized laser fields and then experimentally demonstrated it by taking a spherical symmetric CH4molecule as a model system. The experimental results are in good agreement with the theoretical ones. It is shown that circularly polarized SRS (CP-SRS) has unique characteristics different from linear polarization. When the handedness of circular polarization states of two laser fields is the same, CP-SRS further suppresses the depolarized vibrational band while keeping the polarized band almost unaffected. On the other hand, when the handedness is opposite, CP-SRS enhances the depolarized band while suppressing the polarized band. Therefore, the CP-SRS not only allows us to resolve the symmetry of vibrational modes but also can enhance vibrational contrast based on symmetry selectivity by suppressing or enhancing the signal from a specific vibrational mode. These results will have potential applications in improving chemical selectivity and imaging contrast as well as spectral resolution SRS microscopy. In addition, the CP-SRS has the ability to determine the depolarization ratio ρ and identify the overlapping Raman bands.

Cascaded amplification via three-beam double stimulated Raman scattering in benzene

As stimulated Raman scattering (SRS) microscopy has emerged as a powerful tool for label-free imaging of biomolecules in biological systems, SRS has also received much attention in the field of nonlinear optics. Here, we enhanced the cascade SRS of benzene through an energy competition mechanism in the three-beam double SRS processes. A three-beam (pump, Stokes, and anti-Stokes beam) SRS scheme was formulated and demonstrated to simultaneously induce the parallel and cascaded SRS processes, which were related to the Raman active modes in the same molecule. Both two SRS processes generated high-order Stokes SRS through resonance enhancement of the double sample cell configuration, which effectively suppressed the CARS (Coherent anti-Stokes Raman Scattering) signal. It was shown in the experimental results that the cascaded SRS up to six orders from 562 to 780 nm were generated, and the SRS threshold of the multi-order Stokes components was lowered. The present work can potentially contribute to the development of super-resolution SRS microscopy.

(INVITED) Stimulated Raman lineshapes in the large light–matter interaction limit

Stimulated Raman scattering (SRS) represents a powerful tool for accessing the vibrational properties of molecular compounds or solid state systems. From a spectroscopic perspective, SRS is able to capture Raman spectra free from incoherent background processes and typically ensures a signal enhancement of several orders of magnitude with respect to its spontaneous counterpart. Since its discovery in 1962, SRS has been applied to develop technological applications, such as Raman-based lasers, frequency shifters for pulsed sources and Raman amplifiers. For the full exploitation of their potential, however, it is crucial to have an accurate description of the SRS processes under the large gain regime. Here, by taking as an example the stimulated Raman spectrum of a model solvent, namely liquid cyclohexane, we discuss how the spectral profiles and the lineshapes of Raman excitations critically depend on the pump excitation regime. In particular, we show that in the large light–matter interaction limit the Raman gain undergoes an exponential increase (decrease) in the red (blue) side of the spectrum, with the Raman linewidths that appear sharpened (broadened).

Investigated coherent anti-Stokes Raman scattering in the process of cascaded stimulated Raman scattering in liquid and ice-Ih D2O.

Stimulated Raman scattering (SRS) of liquid and ice-Ih D2O was investigated using a pulsed Nd:YAG laser with a wavelength of 532nm. The high-order Stokes peaks and corresponding anti-Stokes SRS [Coherent Anti-Stokes Raman Spectroscopy (CARS)] peaks were obtained. Two symmetric and antisymmetric Raman modes of stretching vibrations were observed in liquid D2O, while only a symmetric stretching vibration mode was observed in ice-Ih D2O. Pure Stokes SRS is always collinear with the pump beam along the axial direction. Some ring-like Stokes SRS and CARS shifts, which originate from four-wave mixing processes, can also be observed only in the forward direction along with different angles meeting the phase-matching criteria, respectively. Simultaneously, the temporal behavior of SRS in liquid and ice-Ih D2O was examined, and the temporal waveforms of the pump laser pulse, transmitted pump pulse, and the forward SRS pulse were measured. In both cases, SRS was the dominant contributor to stimulated scattering. However, the efficiency values drastically decreased due to the self-termination behavior of SRS in liquid D2O, which arose from the thermal self-defocusing of both the pump beam and the SRS beam, owing to the Stokes shift-related opto-heating effect. In contrast, for the SRS process in ice-Ih D2O, the thermal self-defocusing influence was negligible, benefitting from a much greater thermal conductivity and a higher conversion efficiency of SRS generation retained under both of the conditions.

Narrow-band random Raman lasing from Rhodamine 6G assisted by cascaded stimulated Raman scattering effect

This study reports the first experimental observation of cascaded stimulated Raman scattering (SRS) generation in a colloidal disordered medium. Generation of the cascaded effect requires both a high Raman gain and pump power in the disordered medium. Here, to extend effective path lengths of photons into the Raman gain medium for producing additional SRS processes, ZnO microspheres with abundant nano-protrusions as suitable scattering centers are proposed. It is explained that nano-protrusions on the surface of the spheres can act as nano reflectors and significantly provide potent feedback in the disordered system. This provided feedback via nano-protrusions boosts cascaded SRS generation to allow the appearance of higher Raman signals of Rhodamine 6G dye solution at a low scatterer concentration of 5 mg/ml. The threshold for the formation of the first Raman signal is measured at about 60 mJ/pulse. Also, the evolution of Raman signals under several fixed pump pulses is examined to investigate the stability from pulse to pulse. Our findings provide promising perspectives for achieving the single-frequency laser sources and generate desirable wavelengths for specific applications.

CO2-based hollow-core fiber Raman laser with high-pulse energy at 1.95 µm.

In this Letter, we present a high-pulse energy (>10µJ) Raman laser at 1946 nm wavelength directly pumped with a 1533 nm custom-made fiber laser. The Raman laser is based on stimulated Raman scattering (SRS) in an 8 m carbon dioxide (CO2)-filled nested anti-resonant hollow-core fiber. The low-energy phonon emission combined with the inherent SRS process along the low-loss fiber allows the generation of high-pulse energy up to 15.4 µJ at atmospheric CO2 pressure. The Raman laser exhibits good long-term stability and low relative intensity noise of less than 4%. We also investigate the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at the 2 µm spectral range. Our results constitute a novel, to the best of our knowledge, and promising technology towards high-energy 2 µm lasers.

Shock compression-induced enhancement of stimulated Raman scattering in heavy water

Stimulated Raman Scattering (SRS) of shock-compressed liquid heavy water was observed using a Nd:YAG laser in both the forward and backward directions of the laser beam. SRS spectra exhibited different characteristic features at weak and intense pump energies. As for the intense pump energy, the main SRS peak occurred at approximately 2400 cm −1 and subsequently shifted to a lower frequency. Interestingly, some new peaks were observed in both directions. Specifically, a lower frequency peak close to 2200 cm −1 and 2nd-order anti-Stokes peaks were observed in the backward direction, which further indicated that an ice-VII cubic structure was formed under shock compression conditions. In addition, the normalized intensity ratio of the two main peaks exhibited quite different energy-dependent behaviors. The results demonstrated that the shock-induced high pressure environment significantly enhanced SRS. The results could be attributed to the enhancement effect of Kerr self-focusing and the competitive Backward Stimulated Brillouin Scattering (BSBS) and Forward SRS (FSRS) processes.

Experimental investigation of the polarization modulation instability and stimulated Raman scattering in a chalcogenide optical fiber

In this work, a low-loss As2S5 optical fiber with 0.1 dB/m at 1545 nm was designed and fabricated, whose residual birefringence provided an opportunity for investigating the polarization modulation instability (PMI). Using a nanosecond laser operated at ∼1545 nm as the pump source, PMI and the stimulated Raman scattering (SRS) were experimentally investigated in a 3 m As2S5 optical fiber. During the SRS process, the amplified CW signal at 1632 nm and 1633 nm induced the PMI. To the best of our knowledge, this is the first report on the PMI effect induced by the amplified signal in the first-order SRS. The PMI gain was measured to be 24.4 dB at the signal of 1551 nm with the pump power of 20 mW, and the Raman gain was measured to be 31.2 dB at the signal of 1632 nm with the pump power of 41 mW. This investigation on the PMI gain and Raman gain in the As2S5 optical fiber may provide a reference for the development of fiber amplifiers, fiber lasers, and optical communication.

High-power diode-pumped Nd:GdVO4/KGW Raman laser at 578 nm.

A diode-pumped neodymium-doped gadolinium vanadate (Nd:GdVO4) laser is developed as a compact efficient yellow light at 578 nm by means of intracavity stimulated Raman scattering (SRS) in a potassium gadolinium tungstate (KGW) crystal and the second-harmonic generation in a lithium triborate crystal. The SRS process with a shift of 768cm-1 is achieved by setting the polarization of the fundamental wave along the Ng axis of the KGW crystal. The self-Raman effect arising from the Nd:GdVO4 crystal is systematically explored by employing two kinds of coating specification for the output coupler. With a specific coating on the output coupler to suppress the self-Raman effect, the maximum output power at 578 nm can reach 3.1 W at a pump power of 32 W. Moreover, two different lengths for the Nd:GdVO4 crystal are individually used to verify the influence of the self-Raman effect on the lasing efficiency.

3 W tunable 1.65 µm fiber gas Raman laser in D2-filled hollow-core photonic crystal fibers

We demonstrate here a multi-watt tunable 1.65 µm fiber laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs) by stimulated Raman scattering (SRS). Owing to the fiber transmission properties of the HC-PCFs, pure rotational Stokes wave at 1.65 µm can be generated when pumped with a tunable 1.5 µm erbium-doped fiber amplifier (EDFA). When pumped at 1540 nm, with a 20-meter-long HC-PCF filled with 16 bar deuterium, a record average Stokes power of ∼2.91 W at 1645 nm is achieved, which is about 3.64 times higher than the similar experiment previously reported, and the corresponding power conversion efficiency is about 58%. Over the whole Stokes tunable range of 1645 to 1656 nm, the average output power is >2.5 W and the conversion efficiencies are >54%. A numerical model is also established to analyze the rotational SRS process in deuterium-filled HC-PCFs, the simulation results are in good agreement with experiments. This work opens a new opportunity for high-power tunable fiber lasers operating in the 1.7 μm band.

An Efficient Switching-Off of Coherent Anti-Stokes Raman Scattering via Double Stimulated Raman Scattering Processes of Heteromolecular Vibrational Modes.

Recently, we introduced a new switching-off technique applicable to subdiffraction-limited coherent Raman imaging, where a coherent anti-Stokes Raman scattering (CARS) signal can be selectively suppressed via competitive stimulated Raman scattering (SRS) processes between vibrational modes of a single molecular species. Here, we show that such a three-beam CARS suppression can be made via double SRS processes between vibrational modes of heteromolecular species, a mixture of paraffin oil and benzene. We achieve more than 80% suppression of the pump-Stokes-beam CARS signal for the ring-breathing mode (target mode) of benzene when the C-H stretching mode (acceptor mode) of paraffin oil is used to deplete the pump photons via the pump-depletion-beam SRS process. The freedom in the choice of acceptor mode for depletion, which could be a critical advantage of the present switching-off scheme, can be of use for overcoming current challenges of depletion-based super-resolution coherent Raman imaging of biomolecules.

Tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in RbTiOPO4 crystal.

Stimulated polariton scattering (SPS) and stimulated Raman scattering (SRS) in ${{\rm RbTiOPO}_4}$RbTiOPO4 (RTP) crystal are combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7-1076.0 nm, 1077.7-1080.4 nm, 1081.8-1082.2 nm, and 1084.8-1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7-1108.1 nm, 1109.0-1112.7 nm, 1114.3-1115.1 nm, and 1117.8-1121.1 nm, respectively. A maximal average output power of 970 mW is achieved for the SRS-Stokes wave at the peak wavelength of 1118.8 nm. It is obtained when the diode power is 7.9 W, and the pulse repetition frequency (PRF) is 10 kHz.

High-peak-power 1.7 µm pulse generation by cascaded second- and third-order nonlinearity

We report an efficient and novel method for generating high-peak-power 1.7 µm laser pulses by cascaded optical parametric oscillation (OPO) and stimulated Raman scattering (SRS). The 1064 nm fundamental wave was first converted to 1535 nm by the KTA OPO, and further extended to 1.7 µm by a SRS process. The configuration of OPO + SRS can provide high-intensity pumping light for subsequent Raman conversion, and allows for better wavelength expansibility benefitting from the non-phase-matching requirement of SRS. Two types of Raman conversion using the low-frequency Raman shift in KY(WO4)2 and high-frequency Raman shift in YVO4 were further studied. Up to the 8th-order cascaded KY(WO4)2 Raman laser (KRL) using the high gain 87 cm−1 Raman mode and a YVO4 Raman laser (YRL) using the 890 cm−1 Raman mode emitting at 1.7 µm were realized, respectively. The output wavelengths at 1556, 1577, 1599, 1622, 1646, 1670, 1695, 1720 nm and the output wavelength at 1778 nm were observed in the KRL and YRL, respectively. The maximum total average output powers of 1.26 W and 1.05 W, minimum pulse widths of 8.4 and 24 ns and maximum pulse peak powers of 33.3 kW and 9.4 kW were obtained respectively from the KRL and YRL, enabling the 1.7 µm laser source to have practical applicability in medical imaging, industrial processing, and mid-infrared laser generation.

Study on picosecond pulse-train multi-wavelength Raman laser operating at two Raman modes without tuning

The Stokes line of multi-mode Raman laser was limited by low conversion efficiency of higher-order Stokes and the competition between pure cascaded stimulated Raman scattering (SRS) and Four-wave-mixing (FWM) process which will deplete the pump of SRS process. Through developed pulse-train synchronously pumping technique and two-Raman-mode operating, the orders of output Stokes was improved. With average power of 220mW, the developed a Raman laser providing up to 5 orders Stokes output, containing up to 16 laser lines without tuning operating and four-wave-mixing process.

Vibrational spectroscopy and imaging with non-resonant coherent anti-Stokes Raman scattering: double stimulated Raman scattering scheme.

We introduce a new coherent anti-Stokes Raman scattering (CARS) suppression scheme based on measuring a non-resonant CARS loss signal by three-beam (pump-Stokes-depletion) double stimulated Raman scattering (SRS) processes, which can be potentially of use for super-resolution Raman microscopy. In the converging configuration with employing both pump-depletion and Stokes-depletion SRS processes, we obtained approximately 94% suppression of non-resonant CARS signal, which is about 1.5 times more efficient than that with the parallel configuration with pump-Stokes and pump-depletion SRS processes. Such an enhanced suppression efficiency in the converging configuration results from a simultaneous loss of photons both in the pump and Stokes beams by double SRS processes, leading to an efficient suppression of the pump-Stokes-pump CARS signal. Based on the present method, we further propose two potential applications: (1) non-resonant background-free CARS imaging and (2) label-free super-resolution Raman imaging, and carry out simple numerical simulations to show their feasibility.