Efficient compression of femtosecond pulses by stimulated Raman scattering

Femtosecond pulses at 496 nm were Raman-shifted in methane with 20% efficiency. The pulse duration could be reduced up to 6.5 times from 560 fs at the fundamental to 85 fs at the Stokes frequency (580nm), which is the shortest pulse duration generated in this way. It was shown experimentally that chirped-pulse Raman scattering avoids the limitations arising from self-phase modulation.     

Direct observation of anharmonic coupling in the time domain with femtosecond stimulated Raman scattering

Off-resonant impulsive Raman excitation is used to initiate nonstationary nuclear motion in low-frequency vibrational modes of deuterio-chloroform. Femtosecond time-resolved stimulated Raman probing of the high-frequency C-D stretch reveals additional vibrational sidebands that arise as a result of anharmonic coupling between the impulsively excited low-frequency and the probed high-frequency vibrations. These experiments illustrate the detailed molecular information on anharmonic and reactive surfaces available from multidimensional femtosecond stimulated Raman techniques.     

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.     

Applications of stimulated Raman scattering

The applications of stimulated Raman scattering reviewed in this paper include: determination of spontaneous Raman parameters and nonlinear susceptibilities by gain measurements and coherent anti-Stokes Raman spectroscopy, excitation of molecular vibrations and lattice waves and measurement of their relaxation times, generation of intense tunable light in the infrared region of the spectrum. The main results of the theory of stimulated Raman scattering relevant to the understanding of these applications are discussed.     

Chaos in stimulated Raman scattering

Coherent state technique and maximal Lyapunov exponent method are used for the study of deterministic chaos in a system governing stimulated Raman scattering. The influence of the external pumping on dynamics of the system is investigated. Phase portraits and FFT spectra are presented.The author would like to express his sincere thanks to prof. J. Peina for his comments and stimulating discussions.     

Single-pulse stimulated Raman scattering spectroscopy

We demonstrate the acquisition of stimulated Raman scattering spectra with the use of a single femtosecond pulse. High-resolution vibrational spectra are obtained by shifting the phase of a narrow band of frequencies within the input pulse spectrum, using spectral shaping. The vibrational lines are resolved via amplitude features formed in the spectrum after interaction with the sample. Using this technique, low-frequency Raman lines (<100 cm?1) are observed on both the Stokes and anti-Stokes sides.     

Studies of stimulated Raman scattering

The dependence of the excitation threshold and the intensity of the first Stokes component on the properties of the scattering material, the concentration of its molecules, and the exciting light intensity has been studied. It has been found that the intensity of stimulated Raman scattering lines depends exponentially on the intensity of the exciting light and the concentration of the scattering molecules. This dependence is in agreement with the theory developed.The author are gratful to P. A. Bazhuln, N. G. Basov, and A. M. Prokhorov for the present study.     

The buildup of stimulated Raman scattering from spontaneous Raman scattering

We present a quantum electrodynamic theory which shows for the first time how stimulated Raman scattering builds up, both spatially and temporally, from spontaneous Raman scattering. We analytically solve operator Maxwell-Bloch equations to find the Stokes field operator, and thus the Stokes intensity, for both monochromatic and broad-band (phase diffusion) pump lasers.     

Disturbing interference patterns in femtosecond stimulated Raman microscopy

Femtosecond stimulated Raman microscopy (FSRM) is an upcoming technique in nonlinear microscopy which facilitates rapid chemical mapping. It employs femtosecond white‐light pulses as probe pulses and intense picosecond pulses as pump pulses. Stimulated Raman scattering (SRS) occurs at the focus of a scanning microscope. Chemical constituents in the sample are identified via their Raman signatures. In this article, disturbing interference patterns in FSRM are reported. They are caused by a broadening of the pump pulse due to nonlinear interactions in the focal region of the microscope and reduce the signal‐to‐noise ratio. The properties of these modulations are explored, and the methods to suppress them are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Stokes-anti-Stokes entanglement in stimulated Raman scattering

A cavity model of Raman scattering from phonons by an undepleted laser field is considered. The fields in the interaction are coupled to the boson reservoir that produces the damping mechanism in the model. An analysis for the origination of quantum entanglement between the Stokes and anti-Stokes fields, depending on their coupling constants with the reservoir bosons and on the state of the reservoir, is performed under the assumption of the initial coherent state of phonons.     

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 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.     

Soliton experiments in stimulated Raman scattering

Experimental observations of solitons in stimulated Raman scattering are reported. Soliton formation resulted from the introduction of a phase shift in the incident Stokes beam as predicted by theory. Pulse sharpening and retardation on propagation in the Raman medium have been observed along with amplitude diminution. The first two features were predicted and the third was not. Spontaneous soliton formation has been observed in the absense of any amplitude modulation or apparent phase shift in the optical fields, indicating that additional sets of initial conditions may result in soliton formation.Work performed under the auspices of the U.S. Department of Energy.     

Efficient stimulated Raman scattering in silicon

Stimulated Raman scattering in silicon was investigated at liquid helium temperatures. A Q-switched single mode Nd: YAG laser was used to generate Stokes radiation at 1.127 μm. Time resolved measurements give a maximum power conversion efficiency of 20%. The effects of intensity induced losses are discussed.     

Frequency shifts in stimulated Raman scattering

The nonresonant contributions to the nonlinear susceptibility χ⁽³⁾ produce a frequency chirp during stimulated Raman scattering. In the case of transient stimulated Raman scattering, the spectrum of the generated Stokes pulse is found at higher frequencies than expected from spontaneous Raman data. The frequency difference can be calculated from the theory of stimulated Raman scattering.     

Origin of spectral interferences in femtosecond stimulated Raman microscopy

In femtosecond stimulated Raman microscopy (FSRM), a spectrally narrow (Raman pump) pulse and a broad (Raman probe) laser pulse are employed to generate the Raman spectra of microscopic objects. The resulting spectra exhibit, in addition to the Raman bands, spectral modulations of comparable amplitude. Here a model is devised that attributes these modulations to a four‐wave mixing (FWM) process. Two light fields of the probe pulse and one field of the pump pulse serve as input fields. The resulting FWM field experiences a heterodyne amplification by the probe field. Simulations based on this model reproduce the appearance of the spectral modulations. Furthermore, the amplitude of the modulations exhibits dependences on the energies of pump and probe pulses as well as on the nonlinear refractive index n₂, which are in line with the model. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Multimode stimulated Raman scattering with squeezed light

Stimulated Raman scattering of a multimode pump light on an one-mode phonon system is examined. The pump light is expected to be strong, so that it can be treated classically. The phonon mode and scattered Stokes and anti-Stokes modes are described quantally. The phonon is prepared in a coherent state and the scattered modes are prepared in displaced and squeezed states at the input.The author would like to thank Dr. J. Peina for fruitful discussions, and Dr. A. Luk for comments on the results.