I doing an experiment now.

My structure has 700nm periodicity.

The surface of the unit structure is sloped at 45 degrees from the normal incidence angle.

My laser is 500 nm.

I can expect there will be diffraction.

I wonder if the geometrical optics can be applicable for my case?

Or just I need to consider the diffraction?

I am asking this question because I have something that I can not explain.


1 Answer 1


It depends on what you mean by whether "geometrical optics can be applicable".

Geometric optics will work fine with a system with a grating like the one you mention, although you need to know the rules for calculating the directions and strengths of the transmitted / reflected rays. There are three main points to heed here:

  1. In general, several separate rays will emerge from the incidence of one ray;

  2. Their directions are governed by the Bragg Resonance Condition.

  3. Their relative strengths are governed by the actual shape of the periodic structure: these are related to the amplitudes of the components in the Fourier series describing the periodic index variation.

The reason that geometric optics works well here is that a ray stands for essentially a plane wave i.e. a wavefront that has minimal aberration over length scales that are typically much longer than the length scales in the grating you mention. Plane waves incident on a grating give rise to a set of plane waves, one for each integer solution in the Bragg resonance law.


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