Induced Light Scattering

scattering of light in a medium that is due to variation in the motion of its component elementary particles (electrons, atoms, and molecules) that takes place in response to both an incident light wave and the scattered radiation itself. A distinction is made among induced combination scattering (Raman scattering), which takes place with the participation of the intramolecular vibrations of atoms, the rotations of molecules, or the motions of electrons within atoms; induced Mandel’shtam Brilloum scattering, which involves the elastic displacements of molecules (that is, sonic or hypersonic waves); induced scattering of light on polaritons (coupled vibrations of molecules and an electromagnetic field); and other types. Induced light scattering is observed in solids, liquids and gases.

If the intensity of incident light is low, spontaneous light scattering occurs, in which the variation in the motion of the microscopic particles takes place only under the influence of the incident wave field. In this case the intensity of the scattered light is low (in 1 cu cm it is 10⁻⁸ to 10⁻⁶ of the incident light intensity), and its frequency ω′ differs from that of the incident light by the value Δω, which is equal to the frequency of vibration of the microscopic particles.

If the intensity of the incident light is very high, nonlinear effects appear in the medium. The microscopic particles of the medium are affected not only by forces with frequencies of the incident ω and scattered ω′ radiations but also by a force acting at the difference frequency Δω—that is, at the frequency of the microscopic particles’ natural vibrations—which leads to resonance excitation of vibrations. We will consider this by using as an example induced combination scattering with the participation of intramolecular atomic vibrations. In response to the total electric field of the incident and scattered light, a molecule is polarized and an electric dipole moment proportional to the total electric field intensity appears on it. The potential energy of the atomic nuclei is changed by a quantity proportional to the product of the dipole moment and the square of the total electric field intensity. As a result, the external force that acts on the nucleus contains a component with difference frequency Δω, which produces resonance excitation of the atoms’ vibrations. This in turn leads to an increase in the intensity of the scattered radiation, which once more reinforces the vibrations of the elementary particles, and so on. In this way the scattering of light itself induces (stimulates) the subsequent scattering process; that is why it has been called induced (stimulated) scattering. The intensity of the scattered light during induced light scattering may be on the order of the incident light intensity.

The excitation of intramolecular vibrations during induced combination scattering (the excitation of hypersound during induced Mandel’shtam-Brillouin scattering, and so on) occurs when induced light scattering takes place in a substance that is in a state of near-equilibrium. Here the frequency of the scattered light ω′ is found to be lower than the frequency ω of the incident light: ω′ = ω − Δω (Stokes process). However, it is possible during induced light scattering not only to excite the motion of microscopic particles but also to suppress it if the initial state of the substance is not in equilibrium. Here ω′ = ω + Δω (anti-Stokes process).

If the scattered radiation during induced light scattering leaves the scattering space without reflections from its boundaries, the scattered light is incoherent, just as in the case of spontaneous light scattering, and the angular distribution of the scattered light depends on the shape of the scattering body. For example, for an elongated shape the scattered radiation is concentrated mainly along its axis. However, if the scattering body is located in an open cavity, then as a result of the multiple reflections of the scattered light from mirrors in the cavity, coherent radiation forms at the scattering frequency ω′. (This is achieved only when the incident light intensity exceeds a certain threshold value.) In this case the directivity of the scattered radiation is controlled by the configuration of the cavity.

Since the intensities of the incident and scattered radiations are high (10⁶-10⁹ watts per sq cm) in cases of induced light scattering, other nonlinear effects—for example, parametric processes, which cause the production of radiation with a complete set of new frequencies ω_n = ω + nΔω, where n = ±1, ±2, ±3, … (Figure 1)—also appear frequently in the substance simultaneously with the induced light scattering. The components for which n >̳ are called anti-Stokes components, and those for which n –2 are called higher Stokes components. The radiation of these components after they have emerged from the scatterer occurs mainly along conical surfaces with various small (1°-10°) apex angles (for the various components). In an isotropic medium the axes of all the cones coincide with the direction of the scattered beam. In crystals these cones may have various orientations and each component may radiate in two cones. When a photographic film is positioned behind a test specimen perpendicularly to the transmitted beam of frequency ω, rings are produced on it that correspond to the various components of the induced light scattering.

Figure 1. Spectrum of scattered light for induced combination scattering: ω is the frequency of the incident light

Since the intensity of the scattered light during induced light scattering can be of the same order as that of the incident light, the scattered radiation can in turn become a source of induced light scattering. The development of this process can also lead to the production of an entire series of components whose frequencies will coincide with the parametric frequencies ω_n. However, their other properties are essentially different from parametric radiation. Sometimes two or more kinds of induced light scattering that affect one another develop simultaneously in a substance.

Induced light scattering is used to efficiently convert intensive radiation in a laser to radiation with greater brightness and other characteristics; to produce intense hypersound and other types of motion of microscopic particles; and to study the microscopic structure of substances.