Enhanced raman scattering spectra for gaussian light beams

Abstract

We show that effect of light-induced focusing of the low-frequency Raman scattering components and defocusing of the high-frequency components in the field of a gaussian exciting beam lead to a slight shift of the enhanced Raman line relative to the center of the spontaneous Raman line on the low-frequency side. We propose and provide a rationale for an efficient algorithm for calculating the average power for enhanced Raman scattering, based on numerical solution of the wave equation for the field of the Stokes beam developing from spontaneous Raman scattering.Introduction. Raman spectroscopy methods are widely used today for obtaining information about the composition, structure, and other properties of molecules and supramolecular formations [1]. In this case, very often relatively high-power focused laser beams are used for excitation of Raman scattering. Under such conditions, significant enhancement of Raman scattering may occur due to development of the process of stimulated Raman scattering (SRS), and the parameters of the scattered radiation and their relationship to the parameters of the studied medium and the exciting beam may be considerably different from the case of purely spontaneous Raman scattering.Spontaneous Raman scattering power, as we know, is proportional to the power of the exciting beam and the thickness of the scattering medium; when excited by a spectrally narrow beam, its spectral composition is completely determined by the parameters of the medium. Development of the enhancement process especially leads to breakdown of the linear relationship between the scattered beam power and the exciting beam power. Narrowing of the spectral composition of the scattered radiation also occurs, which can be easily explained by the greater enhancement of the central spectral components of the spontaneous Raman scattering compared with the components of the wings of the scattering line [2,3]. Additionally, in the cases when Raman scattering is excited by a focused light beam, light-induced focusing or defocusing of the scattered radiation in the nonuniform field of the excitation beam may occur. This especially involves "gain focusing" [4]: compression of the scattered beam due to its higher gain near the axis of the focused exciting beam compared with the gain in the peripheral region, and also light-induced focusing of low-frequency scattering components and defocusing of high-frequency components of the beam of scattered radiation, due to the contribution of the real part of the SRS susceptibility of the medium to the refractive index.The above-indicated focusing and defocusing effects depend on frequency. Consequently, they have an effect on both the power characteristics and the spectral characteristics of the scattered radiation. This paper is devoted to study of this effect for excitation of Raman scattering by a gaussian beam under conditions when its attenuation does not occur. We have obtained relatively simple relations allowing us with sufficient accuracy to calculate the power and the spe...

Stationary generation of diode-pumped self-Raman Nd:YVO4/YVO4 composite crystal laser

“…Of course, for multi-modal generation the Stokes and laser modes overlap in a complicated manner and the difference of their diffraction losses should result in the transverse cross section of the Stokes beam always being less than that of the laser beam. Narrowing as a result of Raman-gain should also cause a decrease of the Stokes transverse cross section [16,17]. The limiting cross section of the laser beam should be the pumping transverse cross section.…”

Analysis of energy characteristics of a ring-cavity Raman laser

“…This problem can be solved analytically just as correctly only in the simplest cases and only by neglecting the effect of the SRS process on the pump radiation. It has been shown [8][9][10] that the transverse cross section of the Stokes beam is always less than the pump transverse cross section and decreases with increasing pump power when the Stokes radiation develops with spontaneous scattering. Considering this, Eq.…”

Light-induced frequency shift of stimulated Raman scattering in Bessel light beams

“…Therefore, it should lead with nonuniform (focused) pumping to focusing of the scattered beam near the more intense pumping and, eventually, to more efficient gain. For ω s < ω p -ω v , the SRS fraction to the index of refraction is negative and; therefore, should cause defocusing and a decreased gain of the corresponding frequency components of the Stokes beam.The real part of SRS susceptibility in the Stokes emission spectrum with excitation by a Gaussian beam has been studied [3][4][5]. It was demonstrated theoretically and experimentally [4] that the real part of SRS susceptibility (or, which is the same, the imaginary part of the SRS gain coefficient) causes a shift of the SRS gain maximum towards higher frequencies than that of the exact Raman resonance.…”

“…The real part of SRS susceptibility in the Stokes emission spectrum with excitation by a Gaussian beam has been studied [3][4][5]. It was demonstrated theoretically and experimentally [4] that the real part of SRS susceptibility (or, which is the same, the imaginary part of the SRS gain coefficient) causes a shift of the SRS gain maximum towards higher frequencies than that of the exact Raman resonance.…”

“…(1). One possible method for overcoming this has been published [5]. The frequency dependence of σ δ is defined by the relationship ⏐σ δ ⏐ 2 ~ σ ⁄ (1 + δ 2 ) regardless of the method for calculating the spontaneous scattering.…”