External seeding of stimulated Raman scattering in microdroplets

Short-pulse broadband stimulated Raman scattering in carbon disulfide via resonance cascading

Cascaded stimulated Raman scattering (SRS) of carbon disulfide (CS₂) was investigated by a pulsed Nd:YAG laser with a wavelength of 532 nm. The fourth-order Stokes and second-order anti-Stokes lines were generated when the pump laser energy was about 1.909 mJ in one sample cell (C₁) only. However, the same result was obtained in the second sample cell (C₂) with a pump energy of 0.883 mJ. At the same time, the fifth-order Stokes line was produced in C₂ when the pump energy increased to 1.208 mJ, and the coherent radiation wavelength ranged from 498 to 644 nm. The result was attributed to the resonance enhancement effect, where the frequency difference between the pump laser and the Stokes light emitted from the working medium (CS₂) self-matched with the vibrational energy level of C=S, which resulted in the generation of the cascaded broadband SRS. The anti-Stokes SRS was attributed to four-wave mixing. Simultaneously, the pulse durations of the Stokes and anti-Stokes were compressed to about 380 ps by SRS and laser-induced breakdown. The resonance effect not only reduced the threshold, but it also generated broadband and short-pulse SRS.

Detection of pesticide model compounds in ethanolic and aqueous microdroplets by nonlinear Raman spectroscopy

Pesticide model compounds are detected in microdroplets by means of in situ nonlinear Raman spectroscopy. The chloro- and nitro-substituted aromatics are dissolved in ethanol as well as in water. Their vibrational spectroscopic fingerprint, that is, Raman bands characteristic of the functional groups attached to the aromatic system, is determined in bulk medium by linear Raman spectroscopy. As a result of the concept of group vibrations, the chosen compounds can be regarded as representatives for a whole class of pesticides containing these residues. External seeding of stimulated Raman scattering (SRS) is applied for the detection of these group vibrations of the pesticide model compounds. This technique can be utilized for in situ diagnostics and a noninvasive vibrational spectroscopic analysis of the chemical composition of microdroplets with natural or anthropogenic origin.

Minority species detection in aerosols by stimulated anti-Stokes-Raman scattering and external seeding

A novel technique applicable to remote sensing has been developed for determination of the chemical composition of microdroplets. Enhancement of stimulated anti-Stokes-Raman scattering (SARS) by external seeding of stimulated Raman scattering (SRS) at the Stokes shift lowers the detection limit of the minority species in multicomponent microdroplets. The technique is most useful in the investigation of microdroplets that contain fluorophores that can obscure the SRS signal. The SARS signal is to the blue of the pump laser and out of the fluorescence region of the fluorophore. Information about majority and minority species in multicomponent microdroplets can be determined from the enhanced SARS signals.