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 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.     

Raman scattering in impurity-induced anharmonic crystals

The theory of first-, second-, and third-order Raman scattering is investigated for isotopically disordered anharmonic crystals. The theory of time-dependent thermodynamic Zubarev Green's functions is adopted to obtain the Raman tensor, intensity of Raman lines, and differential cross sections of various orders of scatterings. It is observed that each class of scattering can be separated into diagonal and nondiagonal parts. The first-order and nondiagonal parts are absent in the case of chemically pure crystals. The diagonal parts are separated into anharmonic and interference terms. The interference terms arise due to the interactions of anharmonic phonons with the local phonons. The temperature and defect dependencies are discussed in detail along with the nature of continuous and line spectra. It is proposed that very high-power laser sources will reveal the third-order spectra, and that the resulting structure can be explained with the help of temperature-dependent one-, two-, and three-phonon density of states.     

受激喇曼散射的高斯-厄米模展开方法

运用高斯-厄米模展开受激喇曼散射波动方程,在忽略泵浦倒空情况下,求得了解析解,并用于阈值条件和波前再现讨论。     

Transient coherent Raman scattering in the time and frequency domain

A new type of Raman spectroscopy is presented: After transient excitation of molecular modes coherently scattered Raman spectra are investigated in a depayed probing experiment. The spectral position of the Raman mode is observed after long delay times. The dephasing time is obtained from the time dependence of the scattered amplitudes. Frequency disturbing non-resonant susceptibilities are eliminated. We report on first experimental results of transient coherent Raman spectroscopy of liquid CH₃CCl₃.     

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.     

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.     

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.     

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.     

Monitoring stimulated Raman scattering with photoacoustic detection

A capability of high-frequency ultrasound detection to monitor the process of energy deposition into a molecular system via Raman excitation is experimentally demonstrated. It is shown that the generated ultrasound signal is directly proportional to the optical signal generated in stimulated Raman scattering. Ultrasound detection provides a simple way to discriminate against laser-induced breakdown and allows for the quantification of the stimulated Raman scattering process where direct optical detection is not available. Additionally, it can be used for stimulated Raman imaging in deep tissue, provided that the generated photoacoustic signal is sufficiently strong.     

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.     

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.     

Generation of flat-top waveform in the time domain based on stimulated Brillouin scattering

A method of generating a flat-top waveform in the time domain based on stimulated Brillouin scattering (SBS) is proposed. The transmitted pulses are simulated with pump wavelength at 1064 nm and 532 nm, respectively, and validated in the experiment performed with the Nd:YAG seed-injected laser. The experimental results agree well with the theoretical simulation. With 532 nm pump wavelength, the top of the transmitted pulse is almost a platform, while there is a peak in the front and a platform in the back with 1064 nm pump wavelength. The mechanism behind the generation of flat-top waveform with 532 nm pump wavelength is analyzed in details. PACS 42.65-k; 42.65.Es; 42.65.Hw