Development of a Novel Thermal Interferometer

There have been considerable achievements based on optical interferometry in the past, suggesting that the development of a thermal interferometer might be advantageous for testing the thermal properties of different materials. The concept of thermal wave interference originates from the measurement of thermal diffusivity in thin films using the photo-acoustic effect.

In a photo-acoustic cell, a sample absorbs a modulated light beam and converts it into thermal energy. This leads to periodic temperature variations on the sample surface that cause the immediate air layer in contact with the surface to expand and contract periodically. This results in the formation of acoustic waves in the cell, which is detected by a pressure sensitive microphone detector.

By comparing the phase lag between the modulated light beam and acoustic signals, the thermal diffusivity of the sample can be worked out provided the thickness of the sample is known or vice-versa. The phase lag between the incident radiation and the resulting acoustic waves is known as the ‘thermal phase lag’ in photo acoustic spectroscopy. In samples that are not thermally thick, an additional thermal phase lag is observed and can be explained using two different approaches:

1. By solving the thermal diffusion differential equations with appropriate boundary conditions. [1, 2]
2. Bennett and Patty [3] reformulated the thermal response of the Rosencwaig-Gersho theory [1] as a superposition of thermal waves on the sample surface using interference theory. The thermal energy propagates in the sample as complex thermal waves that decay rapidly due to the highly damping nature of the medium. However, in samples that are thinner than the diffusion length, the thermal waves travel forward and backward, striking the rear and front surfaces of the sample.

The first approach is applicable to all cases, where boundary conditions can be defined mathematically; however, the mathematical treatment is simpler in the second approach, but appears to be applicable only to special cases. Since then, several researchers have discussed thermal-wave interference and diffraction . Thus the concept of thermal wave interference can be used to explain the thermal response of suitable samples in the photo acoustic cell.

The aim of this project is to develop a laboratory prototype of a novel thermal interferometer, which can be applied to the non-destructive testing of metals and coatings. There is further scope for the development of a commercial model in future for use in different applications in the health industry e.g. the detection of protein, biomass, etc.

[1]. Rosencwaig A and Gersho A 1976 J. Appl. Phys. 47 64
[2]. Charpentier P, Lepoutre F and Bertrand L 1982 J. Appl. Phys. 53 608
[3]. Bennett C A Jr and Patty R R 1982 Appl. Opt. 21 49