scattering by the particles
three phonon Bragg diffraction
V.M. Kotov, G.N. Shkerdin
A method for the determination of the particles’ sizes by means of the analysis of the speckle structure of the particles’ scattered light with the using of the filtering properties of the three phonon acousto-optic (AO) Bragg scattering is described. The method is based on the using of the collimating system by means of which the optical field is spatially divided in to two areas – the area where the beam interference occurs and area without beam interference. For the simplicity of the analysis it is assumed that the collimating system is a lens. The analysis is made by the example of the light scattering by two particles. Let’s the particles are located in the lens focal plane, the distance between particles is . The lens forms the each of the scattered radiations into the parallel beam, so in the lens output two crossed beams propagate. In the crossed area the beams interfere, there the speckle pattern is created. On the distance more than some value A counted off from the lens the beams don’t cross, don’t interfere, and don’t create the speckle pattern. Beyond of the area A every beam keeps the information about a single particle only. The distance A is defined with a good approximation as
where F is the focus distance of the lens, Di is the particle size, λ is the wavelength of light. The beam diameter D1 in the outside of the inter-ference area is bonded with Di by the expression
It should be noted that the diameter D1 does not change along the propagation direction of the beam. By means of the experimental measuring f the D1 the size of the particle Di can be defined.
When the particles are located not in the focus but at some distance m from the lens the beams on the output of the lens will be divergent. The angle φ2 of the divergence of each beam is
Of course at the same time the distance A is changing. The spot diameter D1 is not constant; it changes with the moving away from the lens. From the expression (3) it can be seen that the smaller the particle size Di the wider the divergence angle φ2. On this fact can be the based the angular filtration of the optical radiation directed for the separation of the beams kept the information of different scattering particles.
Two filtering elements were used in the experiments: a hole and followed AO element. They are passive and active angular filters correspon-dently. It is shown that the filtering properties of the AO diffraction are depend on the number of reasons which are defined by the scattering nature of the divergent radiation on the «thick» anisotropic acoustic grating.
It is revealed that the diffraction orders can be considered as the «angular sub bands». They have different angular selectivity that allows the obtaining the information about the particles having different sizes.