Raman scattering is a two-photon process [1]. As illustrated in Fig. la, an incident photon is annihilated and a new photon of a different frequency is created. This implies that the scattering is inelastic, and the medium is left in a different energy state. When the scattered photon is lower in energy, or frequency, it is said to be Stokes shifted. The shift in frequency of the Stokes photon is related to a characteristic frequency of the medium. This is illustrated in Fig. 2a. Calling this characteristic material excitation frequency ω)v, the Stokes frequency is then ωs = ΩL — ωv, where ωL is the incident laser frequency. Thus, Raman (Stokes) scattering results in an excitation from the ground state of the medium to an excited state mediated by a two-photon inelastic scattering. The internal material excitation can involve the creation of an excited electronic state, an excited vibrational-rotational state, a lattice vibration, a spin flip in semiconductors, or electron waves in plasmas [2]. The phenomenon occurs in gases, liquids, solids, and plasmas.
Raman scattering is a two-photon process [1]. As illustrated in Fig. la, an incident photon is annihilated and a new photon of a different frequency is created. This implies that the scattering is inelastic, and the medium is left in a different energy state. When the scattered photon is lower in energy, or frequency, it is said to be Stokes shifted. The shift in frequency of the Stokes photon is related to a characteristic frequency of the medium. This is illustrated in Fig. 2a. Calling this characteristic material excitation frequency ω)v, the Stokes frequency is then ωs = ΩL — ωv, where ωL is the incident laser frequency. Thus, Raman (Stokes) scattering results in an excitation from the ground state of the medium to an excited state mediated by a two-photon inelastic scattering. The internal material excitation can involve the creation of an excited electronic state, an excited vibrational-rotational state, a lattice vibration, a spin flip in semiconductors, or electron waves in plasmas [2]. The phenomenon occurs in gases, liquids, solids, and plasmas.
The traditional Raman process alluded to in Chap. 13 is a three-state sequence of transitions in which radiative excitation (induced by a pump field) is followed by spontaneous emission that produces a final state differing from the initial state [Her50b] [Sho90, § 17.5]. When the final state of the sequence is more energetic than the initial state the resulting emission line (to the red of the pump wavelength) is known as a Stokes spectral line. The difference between the pump frequency and the Stokes frequency, the Raman frequency , defines the excitation energy of the final state relative to the initial state. When, instead, the final state has lower energy than the initial state, as can occur when the initial quantum state is already excited, then the emission is an anti-Stokes line, at a bluer wavelength than the pump field. The overall Raman scattering is a two-photon process. Typically Raman spectroscopy deals with molecules; the two-photon transitions are then between vibrational-rotational states, through electronically excited intermediate states, that are characterized in part by vibrational quantum number v and rotational angular momentum quantum numbers J, M . From any given excited electronic state there are many fluorescing transitions, corresponding to various vibrational and rotational quantum numbers of the final state. The wavelengths of the various Stokes and anti-Stokes lines (i.e. the Raman frequencies) characterize the particular molecular species, and so they have provided a valuable diagnostic tool for spectroscopists.
The Raman effect is the inelastic scattering of photons by matter. When a monochromatic light beam (pump beam) propagates in an optical fibre, spon- taneous Raman scattering transfers some of the photons to new frequencies. The probability of a photon scattering to a particular frequency shift, depends on that frequency shift, forming a characteristic spectrum. The scattered photons may lose energy (Stokes) or gain energy (anti-Stokes). If the pump beam is linearly polarized, then the polarization of scattered photons may be the same (parallel scattering) or orthogonal (perpendicular scattering). If photons are already present at other frequencies then the probability of scattering to those frequencies is enhanced (stimulated scattering). In this thesis, the equations governing the Raman effect, both in bulk glass and single mode fibre, are restated. The probability of spontaneous scattering (parallel and perpendicular) in an optical fibre has been measured for frequency shifts between 4THz and 58THz. Particular attention has been given to those frequency shifts where the scattering is intrinsically weak. This has identified those frequency shifts where it might be possible to eliminate Raman noise from experiments designed to generate quantum states of light in optical fibres using four wave mixing. From the measured scattering probabilities, the Raman equations can be used to predict the scattering cross-section in bulk glass. The prediction is found to be in reasonable agreement with measurements of the bulk glass cross-section published by other authors. By increasing the pump power, the transition to stimulated scattering has been observed. In the case of Stokes scattering there is reasonable agreement between the Raman equations and experimental observations. In the case of anti-Stokes scattering, and in the limit of very high pump power, behaviour is observed which is not predicted by the Raman equations, indicating that other processes must be included to successfully describe such experiments.
Based on the modern idea that a photon is a stable, indivisible particle, it is necessary to abandon the generally accepted idea that in the elementary act of Raman scattering of light, a photon splits into two particles. Such a rejection and the transition to a four-photon scheme of the elementary act with the participation of an axion make it possible to preserve the integrity of the photons involved in the elementary act of Raman scattering. The laser beam interacts with vibrations of atoms in molecules, with electrons in atoms, and with phonons or other excitations in the medium under study, as a result of which frequency components appear in the radiation spectrum at the output of the medium under study, shifted relative to the laser frequency toward high or low values. Raman scattering of light is the inelastic scattering of optical radiation in the medium under study, accompanied by a noticeable change in the frequency of radiation. Unlike Rayleigh scattering, which occurs when the refractive indices of the medium and the pumping radiation coincide in phase, in Raman scattering, spectral lines appear in the spectrum of scattered radiation that are not present in the spectrum of primary (exciting) light. Raman scattering of light can be observed in various media: liquids, semiconductors, and gases. It is noteworthy that with the advent of lasers, spontaneous Raman scattering has been supplemented by forced scattering, which allows for a clear result. Using the example of laser radiation scattering in a cell with atomic potassium vapor, it is shown that axions participate in the process of Raman scattering of light. Received: 26 November 2024 | Revised: 29 July 2025 | Accepted: 28 August 2025 Conflicts of Interest The author declares that he has no conflicts of interest to this work. Data Availability Statement The data that support the findings of this study are openly available at https://journalofscience.org/index.php/GJSFR/article/view/102768, reference number [5]. Author Contribution Statement Valeriy Evgenievich Ogluzdin: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration.
A simple description of the stimulated Raman emission of Stokes radiation, and of the induced absorption found by Stoicheff at the anti-Stokes frequency, is based on the semiclassical theory of radiation. This theory gives the well-known formulas for absorption and stimulated emission when carried to the first order in the time-dependent perturbation due to the interaction of the light beam with the molecules. If there are two light beams of frequencies ωL and ω, the second-order perturbation may be associated with stimulated emission at the Stokes frequency ω=ω−1=ωL—ωR and with induced absorption at the anti-Stokes frequency ω=ω1=ωL+ωR, where ωL is the laser frequency and ωR a Raman-active vibrational frequency; in both cases, there is a molecular transition to the excited vibrational state. The highly directional and very sharp anti-Stokes emission at ω1 arises from the conversion by molecules in intense beams at ωL and ω−1 of two ωL photons into an ω−1, ω1 pair. Like spontaneous emission from excited states, the production of this anti-Stokes radiation requires the full quantum theory of radiation for an adequate description. The emission of ω1 and the observed directional absorption of ω−1 due to the reverse conversion ω−1+ω1→2ωL are explained. The anti-Stokes emission is proportional to the Stokes intensity and to the square of that of the laser, and to the square of the molecular number density; its sharpness is due to the absence of a molecular transition. The generation of photon pairs ω−n+ωn, where ωn=ωL+nωR, can occur for all values of n, but it is most efficient for n=1, due to resonance in the appropriate molecular hyperpolarizability and to the intensity of the Stokes beam at ω−1.
The process of stimulated Raman scattering is described by the model of 2D oscillator. The photons of Stokes mode are described by one mode of the oscillator and phonons of the medium are described by the other mode of oscillator. The interaction of photons and phonons is taken to be quadratic in the creation and annihilation operators of the photons and phonons. New time-dependent integrals of motion are found for stimulated Raman scattering within the framework of this model. The photon-phonon probability-distribution function is found in terms of Hermite polynomials of four variables. It is shown that the squeezing phenomenon can be transferred from one mode to the other due to interaction. The statistical dependence appears due to interaction as well. An explicit formula of the photon-number probability distribution as a function of the laser-field intensity and the medium parameters is obtained both in terms of Hermite polynomials of two variables and in terms of Legendre polynomials after the averaging of the photon-phonon probability-distribution function over the phonon mode. The mean photon number and its dispersion in the Stokes mode are expressed as functions of the medium parameters and a parameter of the interaction. It is shown that if the medium is initially prepared, then after the interaction with laser field the photons are created in squeezed state. The classical propagator for stimulated Raman scattering is found within the framework of the symplectic tomography scheme.
Light gets scattered when it encounters an obstacle or inhomogeneity even on a microscopic scale. A well-known example is the blue color of the sky, resulting from Rayleigh scattering of light by molecules in the air. Such redirection of energy can be used to amplify signals by taking power from a “pump” wave co-propagating with the signal in an appropriate optical medium. An example of this is provided by Raman scattering. Having said that, it is important to realize that scattering does not always occur when light interacts with a material [1]. In some cases, photons get absorbed in the medium, and their energy is eventually dissipated as heat. In other cases, the absorbed light may be re-emitted after a relatively short time delay in the form of a less energetic photon [2], a process known as fluorescence. If fluorescence takes place after a considerable delay, the same process is called phosphorescence [3]. For a photon to get absorbed by a material, its energy must correspond to the energy required by the atoms or molecules of that material to make a transition from one energy level to a higher energy level. In contrast, the scattering of photons from a material can take place without such a requirement. However, if the energy of the incident photon is close to an allowed energy transition, significant enhancement of scattering can occur.