Photoelasticity
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In materials science, photoelasticity describes changes in the optical properties of a material under mechanical deformation. It is a property of all dielectric media and is often used to experimentally determine the stress distribution in a material.
Plastic utensils in a photoelasticity experiment
Photoelastic model to validate the stiffener model. Isochromatic fringe patterns around a steel platelet in a photo-elastic two-part epoxy resin.
Tension lines in a plastic protractor seen under cross-polarized light
Photoelastic experiment showing the internal stress distribution inside the cover of a Jewel case
Acousto-optic modulator
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An acousto-optic modulator (AOM), also called a Bragg cell or an acousto-optic deflector (AOD), uses the acousto-optic effect to diffract and shift the frequency of light using sound waves. They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric transducer is attached to a material such as glass. An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the material. These can be thought of as moving periodic planes of expansion and compression that change the index of refraction. Incoming light scatters off the resulting periodic index modulation and interference occurs similar to Bragg diffraction. The interaction can be thought of as a three-wave mixing process resulting in sum-frequency generation or difference-frequency generation between phonons and photons.
A sketch to explain the Bragg condition for an AOD. Λ is the wavelength of the sound wave, λ is that of the light wave, and n is the refractive index of the crystal in the AOD (which should be omitted. This is a mistake). The +1 order has a positive frequency shift compared to the incident light; The 0th order has the same frequency as the incident light. The minor transverse displacement of 0th order from the incident light represents the refraction inside the crystal.