High-order stimulated Raman scattering in a highly transient regime driven by a pair of ultrashort pulses

Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes

Raman scattering has found renewed interest owing to the development of gas-filled hollow-core fibers, which constitute a unique platform for exploration of novel ultrafast nonlinear phenomena beyond conventional solid-core-fiber and free-space systems. Much progress has been made through models for particular interaction regimes, which are delineated by the relation of the excitation pulse duration to the time scales of the Raman response. However, current experimental settings are not limited to one regime, prompting the need for tools spanning multiple regimes. Here, we present a theoretical framework that accomplishes this goal. The theory allows us to review recent progress with a fresh perspective, makes new connections between distinct temporal regimes of Raman scattering, and reveals new degrees of freedom for controlling Raman physics. Specific topics that are addressed include transient Raman gain, the interplay of electronic and Raman nonlinearities in short-pulse propagation, and interactions of short pulses mediated by phonon waves. The theoretical model also accommodates vector effects, which have been largely neglected in prior works on Raman scattering in gases. The polarization dependence of transient Raman gain and vector effects on pulse interactions via phonon waves is investigated with the model. Throughout this Perspective, theoretical results are compared to the results of realistic numerical simulations. The numerical code that implements the new theory is freely available. We hope that the unified theoretical framework and numerical tool described here will accelerate the exploration of new Raman-scattering phenomena and enable new applications.

Intermolecular energy transfer-enhanced super-broadband stimulated Raman scattering in cyclohexane-benzene mixtures

Supercontinuum radiation has found numerous applications in diverse fields encompassing spectroscopy, pulse compression, and tunable laser sources. Anomalous enhanced stimulated Raman scattering (SRS) of cyclohexane-benzene mixtures was obtained in this study. SRS of the pure solvent, the multi-order Stokes of the strongest fundamental vibration modes, and energy transfer in intra-molecular modes were observed. SRS of the mixture revealed that the cross-pumping effect was generated between the C-H stretching (v₂) mode of cyclohexane and the C=C ring skeleton (v₁) mode of benzene, thereby producing the intermolecular secondary stimulated Raman emission and the appearance of two super-broadband radiations at 664.36-673.9 nm and 704.62-729.22 nm. The results suggest that the energy transfer of intermolecular vibrational modes, where the strongest vibrational mode excites other vibrational modes, is a simple approach for generating supercontinuum coherent radiation.

Efficient frequency conversion and the crossing-pump effect of stimulated Raman scattering in an aqueous sodium sulfate solution

The cascaded stimulated Raman scattering (SRS) of an aqueous sodium sulfate solution was investigated as well as the generation of the crossing-pump effect. With the introduction of dual sample cells, the first-order Stokes of the O-H stretching vibrational mode was able to act as the pump light to excite the Stokes of the S-O stretching vibrational mode, and a new Raman peak was obtained at 4423 cm^-1. The dual sample cell device not only lowered the SRS threshold, but also enhanced the four-wave mixing (FWM) process. Compared to the input laser of 7 ns/pulse, the first-order Stokes of O-H was compressed to a pulse width of 413 ps after passing through the dual sample cells. The SRS of aqueous sodium sulfate solution covered an ultrabroad wavelength ranging from 441 nm to 720 nm (a Raman shift ranging from -3859 cm^-1 to 4923 cm^-1). The cone-shaped launch ring of the FWM process was also recorded. This work provides a reference for the establishment of laser frequency conversion devices using an aqueous sodium sulfate solution as the Raman medium.

Deep- to near-ultraviolet Raman frequency conversion pumped by femtosecond pulses in a hollow-core waveguide

Broadband vibrational/rotational Raman generation ranging from deep ultraviolet (DUV) to blue wavelengths is demonstrated by using molecular hydrogen in a hollow-core waveguide as a Raman-active medium pumped by a femtosecond DUV laser. We find the high-order transient stimulated Raman scattering is drastically enhanced for input beams including a circularly polarized component; a circularly polarized input beam achieves the highest conversion efficiency. Coherent vibrational anti-Stokes Raman emission is observed only for a circularly polarized pump beam, indicating that the waveguide effect also contributes to the upconversion of a DUV pulse via transient stimulated Raman scattering.

Intramolecular Vibration Energy-Transfer-Driven Cascaded Stimulated Raman Scattering and Four-Wave Mixing in Benzene Series

Cascaded stimulated Raman scattering (SRS) of benzene, bromobenzene, chlorobenzene, ethylbenzene, and toluene was investigated by a pulsed Nd:YAG laser with 532 nm wavelength. The results showed that the third-order Stokes SRS of the ring skeleton vibration (CC at 3006 cm^-1) accompanied by another higher-frequency Stokes SRS of the CH stretching vibration (at 3066 cm^-1), which arose only when the third-order Stokes SRS of the ring skeleton was produced, can be attributed to the vibration energy transfer between vibration energy levels of CC and CH. The Stokes and anti-Stokes SRS rings, which originated from the intramolecular energy-transfer-enhanced four-wave mixing (FWM) processes, can be observed only in the forward direction along different angles apart from the pump beam direction. The phenomenon also existed in other derivatives of benzene. We propose the intramolecular energy-transfer-enhanced SRS for the first time, which can be used for a broadband Raman laser.

Broadband visible-near infrared and deep ultraviolet generation by four-wave mixing and high-order stimulated Raman scattering from the hybrid metasurfaces of plasmonic nanoantennae and Raman-active nanoparticles

Broadband visible-near infrared and deep ultraviolet generation from Raman-active hybrid metasurfaces by two-color pumping.

Generation of intense subcycle optical pulses in a gas

The generation of intense subcycle laser pulses during the propagation of two-color femtosecond pulses in a gas medium is investigated theoretically and experimentally. Four-wave mixing induced by the laser pulses in a gas medium generates multi-octave laser radiation from the ultraviolet to the infrared, which forms stable subcycle laser pulses after a certain propagation distance in a gas medium with group-velocity dispersion. The intense subcycle laser pulses would allow the coherent control of the waveforms of soft-x-rays generated via high-harmonic generation.

High-order Raman sidebands generated from the near-infrared to ultraviolet region by four-wave Raman mixing of hydrogen using an ultrashort two-color pump beam

A two-color pump beam consisting of a fundamental beam of a Ti:sapphire laser (35 fs, 802 nm) and a signal beam generated by optical parametric amplification (55 fs, 1203 nm) was utilized to generate multiple Raman sidebands by vibrational four-wave Raman mixing. The second harmonic emission (401 nm) was further employed as a seed beam for enhancing efficiency. Numerous sidebands emitting at 602, 481, 344, 301, 267, 241, 219, 200, and 185 nm were observed by irradiating the beam onto a screen coated with sodium salicylate. The spectral band width of these emission lines was capable of generating 0.9-fs optical pulses by Fourier synthesis.

Molecular modulation in a hollow fiber

We report the extension of the technique of molecular modulation to a deuterium-filled optical fiber. Using driving lasers at 807 and 1064 nm, each with a pulse energy of several millijoules and a 200 microm diameter fiber with a length of 22.5 cm, we generate 12 sidebands with wavelengths spanning 1.56 microm to 254 nm.

Analysis of adiabatic passage by light-induced potentials with chirped laser pulses in three- and four-level diatomic systems

This paper describes an investigation into the process of adiabatic passage by light-induced potentials (APLIP), which was previously suggested as a method for employing two strong picosecond laser pulses to transfer the population between two electronic states. We have extended earlier numerical studies in order to assess the feasibility of an experimental implementation of the APLIP concept. APLIP has been modeled in a three-level model system based on Na2 with chirped pulses, using laser parameters available from a typical chirped pulse amplified Ti:sapphire laser. The model showed that the APLIP process remains essentially unchanged for chirped pulses of equal magnitude and the opposite, or equal and positive sign of chirp as compared to the transform-limited case. We also examined the case of additional electronic states by introduction of a fourth state that lies close to the "target," i.e., final, state. The investigation showed that there are circumstances in which a significant fraction of the population gets transferred to this state which will disrupt the APLIP process. However, by switching to this fourth state as the target state in an experiment, good transfer efficiency is recovered. The results of the extension of the original APLIP modeling to chirped pulses and additional electronic states indicate that an APLIP experimental realization should be feasible in Na2.

Femtosecond lasers in gas phase chemistry

This critical review is intended to provide an overview of the state-of-the-art in femtosecond laser technology and recent applications in ultrafast gas phase chemical dynamics. Although "femtochemistry" is not a new subject, there have been some tremendous advances in experimental techniques during the last few years. Time-resolved photoelectron spectroscopy and ultrafast electron diffraction have enabled us to observe molecular dynamics through a wider window. Attosecond laser sources, which have so far only been exploited in atomic physics, have the potential to probe chemical dynamics on an even faster timescale and observe the motions of electrons. Huge progress in pulse shaping and pulse characterisation methodology is paving the way for exciting new advances in the field of coherent control.