Higher order processes in random Raman lasing

Narrow-band random Raman lasing from Rhodamine 6G assisted by cascaded stimulated Raman scattering effect

This study reports the first experimental observation of cascaded stimulated Raman scattering (SRS) generation in a colloidal disordered medium. Generation of the cascaded effect requires both a high Raman gain and pump power in the disordered medium. Here, to extend effective path lengths of photons into the Raman gain medium for producing additional SRS processes, ZnO microspheres with abundant nano-protrusions as suitable scattering centers are proposed. It is explained that nano-protrusions on the surface of the spheres can act as nano reflectors and significantly provide potent feedback in the disordered system. This provided feedback via nano-protrusions boosts cascaded SRS generation to allow the appearance of higher Raman signals of Rhodamine 6G dye solution at a low scatterer concentration of 5 mg/ml. The threshold for the formation of the first Raman signal is measured at about 60 mJ/pulse. Also, the evolution of Raman signals under several fixed pump pulses is examined to investigate the stability from pulse to pulse. Our findings provide promising perspectives for achieving the single-frequency laser sources and generate desirable wavelengths for specific applications.

Stimulated multifrequency Raman scattering of light in a polycrystalline sodium bromate powder

The observation of multifrequency stimulated Raman scattering of light in a thin powder layer of a gyrotropic crystal of sodium bromate when excited by intense ultrashort pulses of the second optical harmonic (λ = 532 nm) of an yttrium aluminum garnet laser is reported. Five Stokes and anti-Stokes satellites corresponding to the excitation of the intramolecular optical mode of NaBrO₃ crystal were present in the recorded spectra. In addition, Raman satellites corresponding to the excitation of lattice modes, as well as their combinations with intramolecular modes, were found in the spectra of stimulated Raman scattering. The prospects of using stimulated Raman scattering spectroscopy for express analysis of gyrotropic media: gyrotropic polycrystals, crystalline amino acids, proteins, DNA, etc. have been established.

Enabling time resolved microscopy with random Raman lasing

Optical imaging of fast events and processes is essential for understanding dynamics of complex systems. A bright flash of illuminating light is required to acquire sufficient number of photons for superior image quality. Laser pulses can provide extreme brightness and are typically employed to achieve high temporal resolution; however, the high degree of coherence associated with the lasing process degrades the image quality with speckle formation. Random lasers are low-coherence sources of stimulated emission and do not suffer from speckle, but are rather broadband and have a relatively low output power limiting the scope of their potential applications. In this report, we demonstrate the use of random Raman lasing as a novel imaging light source with unprecedented brightness for a speckle-free and narrowband light source. We showcase the advantages of a random Raman laser to image the nanosecond scale dynamics of cavitation formation in water and quantitatively compare these images to those taken with incoherent fluorescent emission and coherent laser light as illumination source.

Modeling focusing Gaussian beams in a turbid medium with Monte Carlo simulations

Monte Carlo techniques are the gold standard for studying light propagation in turbid media. Traditional Monte Carlo techniques are unable to include wave effects, such as diffraction; thus, these methods are unsuitable for exploring focusing geometries where a significant ballistic component remains at the focal plane. Here, a method is presented for accurately simulating photon propagation at the focal plane, in the context of a traditional Monte Carlo simulation. This is accomplished by propagating ballistic photons along trajectories predicted by Gaussian optics until they undergo an initial scattering event, after which, they are propagated through the medium by a traditional Monte Carlo technique. Solving a known problem by building upon an existing Monte Carlo implementation allows this method to be easily implemented in a wide variety of existing Monte Carlo simulations, greatly improving the accuracy of those models for studying dynamics in a focusing geometry.