A femtosecond stimulated raman spectrograph for the near ultraviolet

The design of a femtosecond stimulated Raman spectrograph (FSRS) optimized for measurements in the near UV is presented. It features a 387.5 nm Raman pump pulse causing a (pre-)resonance enhancement for molecular absorbing in the UV. As many photoreactive molecules absorb there, the set-up is particularly suited to study photochemical reactions. The 387.5 nm pulses are generated by frequency adding two femtosecond laser pulses with linear chirps, equal in magnitude but opposite in sign. This results in intense and spectrally narrow (∼10 cm^-1) Raman pump pulses which allow recording of Raman spectra with a good spectral resolution. The power of the spectrograph to trace ultrafast photoreactions is illustrated by measurements on the photochemistry of o-nitrobenzaldehyde. PACS 33.20.Fb; 39.30.+w     

光学击穿对宽带受激布里渊散射及受激拉曼散射特性的影响

实验上分析了在多模激光抽运的条件下,水中宽带受激布里渊散射(WSBS)及前向受激拉曼散射(FSRS)产生的物理机制及二者之间能量变化的关系。实验结果表明,在多模激光抽运过程中,WSBS能量与FSRS能量处于不断变化的状态。聚焦长度较短或抽运光能量相对较低时,WSBS在散射过程中占据主导地位;随着聚焦长度和抽运光能量的增加以及光学击穿的产生,FSRS在散射过程中逐渐占据优势。     

Experimental investigation on the competition between wideband stimulated Brillouin scattering and forward stimulated Raman scattering in water

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.     

飞秒受激拉曼光谱线形:基于泵浦及探测脉冲光的共振条件研究

Resonance enhancement has been increasingly employed in the emergent femtosecond stimulated Raman spectroscopy (FSRS) to selectively monitor molecular structure and dynamics with improved spectral and temporal resolutions and signal-to-noise ratios. Such joint efforts by the technique-and application-oriented scientists and engineers have laid the foundation for exploiting the tunable FSRS methodology to investigate a great variety of photosensitive systems and elucidate the underlying functional mechanisms on molecular time scales. During spectral analysis, peak line shapes remain a major concern with an intricate dependence on resonance conditions. Here, we present a comprehensive study of line shapes by tuning the Raman pump wavelength from red to blue side of the ground-state absorption band of the fluorescent dye rhodamine 6G in solution. Distinct line shape patterns in Stokes and anti-Stokes FSRS as well as from the low to high-frequency modes highlight the competition between multiple third-order and higher-order nonlinear pathways, governed by different resonance conditions achieved by Raman pump and probe pulses. In particular, the resonance condition of probe wavelength is revealed to play an important role in generating circular line shape changes through oppositely phased dispersion via hot luminescence (HL) pathways. Meanwhile, on-resonance conditions of the Raman pump could promote excited-state vibrational modes which are broadened and red-shifted from the coincident ground-state vibrational modes, posing challenges for spectral analysis. Certain strategies in tuning the Raman pump and probe to characteristic regions across an electronic transition band are discussed to improve the FSRS usability and versatility as a powerful structural dynamics toolset to advance chemical, physical, materials, and biological sciences.     

Pump power dependence in resonance femtosecond stimulated Raman spectroscopy

Femtosecond stimulated Raman spectroscopy (FSRS) has emerged as a powerful new technique that is capable of obtaining resonance Raman spectra of fluorescent species and transient photochemical intermediates. Unlike related transient infrared absorption techniques, the FSRS signal is quite sensitive to the laser power utilized in the vibrational probing event. In particular, FSRS spectra are highly sensitive to the intensity of the picosecond Raman‐pump pulse. We have measured the power dependence of the FSRS signal using pulse energies from ~10⁻⁹ to ~10⁻⁵ J and molecules with a range of molar absorptivities at the Raman‐pump wavelength of 400 nm, including β‐carotene (ε₄₀₀ = 58 300 M⁻¹ cm⁻¹), para‐nitroaniline (17 800 M⁻¹ cm⁻¹), nitronaphthalene (247 M⁻¹ cm⁻¹) and ferrocene (57 M⁻¹ cm⁻¹). We show that for strongly absorbing molecular systems, such as β‐carotene and para‐nitroaniline, the ground‐state (GS) FSRS signal actually decreases with increasing pump power at pump fluences above ~10⁻² J cm⁻², due to depletion of the GS population. However, for weakly absorbing species like nitronaphthalene and ferrocene, the signal increases linearly with increasing pump fluence until ~0.5 J cm⁻², at which point two‐photon absorption by the solute induces nonlinear absorption of the pump pulse and attenuation of the FSRS signal. The data are quantitatively simulated with a photophysical kinetic model, and the results are analyzed to provide simple guidelines for acceptable Raman‐pump powers in resonance FSRS experiments. The acceptable Raman‐pump power is proportional to the focused beam area and depends inversely on the sample's molar absorptivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature measurements in condensed phases using non‐resonant femtosecond stimulated Raman scattering

We have previously demonstrated the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the temperature (T) of condensed matter at the molecular vibrational level. [Phys. Rev. Lett. 2011, 107, 43001] In this paper, we expand the theory for the FSRS temperature dependence by considering the effects of an isolated change of T as well as a coupled change of T and chemical concentration. We point out that the origin of the temperature sensitivity of the Stokes to anti‐Stokes ratio of FSRS lies in the exponential nonlinearity of the gain and loss. We establish that FSRS of two Raman modes can be used to simultaneously determine the vibrational temperature and the change in concentration of the species contributing to those two modes. Single‐shot experimental results using FSRS are presented to demonstrate over four orders of magnitude higher efficiency than spontaneous Raman in small volume samples with picosecond resolution. Copyright © 2012 John Wiley & Sons, Ltd.     

Time—dependent Theory of Raman Scattering with Pulses—Application to Continuum Raman Spectroscopy

A theory of real-time dependence of Raman scattering for a pulse-mode laser is developed within second-order perturbation theory and using the wavepacket terminology.We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency,For an initial ground virational state,it is shown that the rate of Raman emission as a funcition of time and pulse carrier frequency is structureless for all pulses,and for pulses that are longer than the dissociation time the rate also decays with the pulses.This is contrary to recently reported resonance fluorescence type structures at long times (M.Shapiro,J.Chem.Phys.99,2453(1993),We explain why such structures are unphysical for continuum Raman scattering.     

Effect of a fluorescing impurity on the characteristics of stimulated Raman scattering from acetone

Asymmetry in the intensities between backward and forward stimulated Raman scattered radiations (BSRS and FSRS respectively) in acetone is investigated in the presence of a fluorescing impurity (rhodamine 6G). In the case of pure acetone above a threshold pump power, BSRS becomes more intense than FSRS. On the other hand intensity of BSRS decreases with concentration of the fluorophore in solution, while that of FSRS is found to increase. It is observed that absorption saturation also affects the asymmetry between FSRS and BSRS.     

Femtosecond stimulated Raman scattering picosecond molecular thermometry in condensed phases

We demonstrate the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the temperature of condensed matter at the molecular vibrational level. We report the temperature dependence of Raman loss to Raman gain ratios for low frequency modes (<300 cm(-1)) in a CaCO3 single crystal from cryogenic to room temperature, which is shown to be in agreement with theoretical predictions. We also report the measurements of nonequilibrium time evolution of mode specific vibrational temperatures in the CaCO3 single crystal to demonstrate that FSRS can measure temperature on picosecond time scales. Finally, we point out the unique origin of this temperature dependent anti-Stokes to Stokes ratio in stimulated Raman, which is not present in other coherent Raman spectroscopies. These measurements require no material dependent parameters or prior calibration.     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

Picosecond pulse generation in a benzene raman generator amplifier system

Picosecond ruby laser pulses generate simultaneously Stokes pulses of the 992 cm^–1 ring breathing mode and of the 3063 cm^–1 CH-stretching mode of benzene in a Raman generator cell by stimulated Raman scattering under self-focusing conditions. The Raman generator light is spectrally filtered, collimated, and amplified in three Raman amplifier cells pumped by ruby laser pulses. Both first Stokes lines have been selectively amplified to short, low divergence, intense light pulses.     

Efficient excitation of Raman coherence by a gradient force

The vibrational molecular excitation by strong laser fields have been studied. Starting with the molecular Hamiltonian, we derive the vibrational excitation rate due to a gradient of laser field and show that the rate can be two to three orders of magnitude larger than the regular Raman rates. The developed theory contributes to understanding of interaction of powerful laser pulses with molecular systems, and it might be used in developing quantitative coherent Raman spectroscopy. Copyright © 2013 John Wiley & Sons, Ltd.     

1064nm纳秒激光脉冲激发的外腔式PbWO4拉曼激光器

研究了外腔式PbWO4拉曼激光器在纳秒脉冲抽运下的输出特性。利用主动调Q的Nd:YAG激光器产生的脉冲宽度为31.4ns,最大输出能量为200mJ的1064nm激光作为抽运源。拉曼激光谐振腔采用的是平凹腔设计。测量了输出的拉曼光脉宽与抽运能量的关系,分析了输出的拉曼光脉冲波形图和光谱图,测量了输出的拉曼光脉冲能量与抽运能量的关系,计算了转换效率与抽运能量的关系。当注入抽运光能量达到42mJ时,得到了一阶斯托克斯光脉冲的最大能量和转换效率分别为10mJ和24%,获得外腔式PbWO4拉曼激光器的一阶斯托克斯光脉冲波长为1177.6nm,典型的一阶斯托克斯光脉冲脉宽为20ns。     

Femtosecond stimulated Raman spectroscopy

Advances in the field of Femtosecond Stimulated Raman Spectroscopy (FSRS), a new time‐resolved structural technique that provides complete vibrational spectra on the ultrafast timescale, are reviewed. When coupled with a femtosecond optical trigger, the time evolution of a reacting species can be monitored with unprecedented <25 femtosecond temporal and 5 cm^‐1 spectral resolution. New technological and theoretical advances including the development of tunable FSRS and a background‐free FSRS format are discussed. The most recent experimental studies focus on ultrafast reaction dynamics in electronically excited states: isomerization in cyanobacterial phytochrome, ultrafast spin flipping in a solar cell sensitizer, and excited state proton transfer in green fluorescent protein. The use of FSRS to directly map multidimensional reactive potential energy surfaces and to probe the mechanism of reactive internal conversion is prospectively discussed.     

Compression of ultraintense laser pulses in inhomogeneous plasma upon backward stimulated raman scattering

The potential for time compression of ultraintense laser pulses in plasma based on nonlinear interactions between waves during backward stimulated Raman scattering is analyzed analytically and numerically. The optimal conditions for obtaining a maximal energy conversion coefficient of 10-ps laser pulses to the energy of 100-fs Stokes pulses are found.     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser

Generation of efficient subnanosecond self-stimulated Raman pulses by a diode-pumped passively Q-switched Nd:YVO4/Cr4+:YAG laser is demonstrated. Experimental results reveal that self-frequency Raman conversion can be achieved with a c-cut Nd:YVO4 crystal in a nearly hemispherical cavity. At 2.0 W of incident pump power, the self-stimulated Raman laser produces stable 710-ps pulses at a Stokes wavelength of 1178.6 nm with 7.2 microJ of pulse energy at a 17-kHz repetition rate.     

Multifrequency parametric infrared Raman generation in KGd(WO(4))(2) crystal with biharmonic ultrashort-pulse pumping

Mutlifrequency parametric Raman generation was carried out in a KGd(WO(4))(2) crystal by use of a dual-wavelength Ti:sapphire laser system. It was found that with femtosecond pump pulses the efficiency of Raman generation is low because of the onset of self-phase modulation. The mechanism for suppression of stimulated Raman scattering by self-phase modulation is discussed. Employing 2-ps-long chirped pulses generated four Stokes and one anti-Stokes component.     

Short-pulse, all-fiber, Raman laser with dispersion compensation in a holey fiber

The use of Raman gain in conventional fiber followed by dispersion compensation in a holey fiber in a synchronously pumped laser configuration allowed compression by a factor of 8.5 of output pulses at a selected wavelength with respect to the pump pulses. We obtained 2-ps output pulses at 1.14 microm from a totally fiber-integrated laser pumped with 17-ps pulses at 1 microm. Higher pulse compression should be possible with nonlinear chirp compensation. Ultrashort-pulse, all-fiber Raman lasers at wavelengths shorter than 1.3 microm are feasible.