Single and multi-phonon interaction studies for low-emittance applications

Stefan Andersson

The infrared optical properties of materials are of interest for a number of applications, such as solar absorbers, radiative coolers, frost prevention coatings, heat mirrors and of course infrared components such as windows and lenses. This thesis emphasises on materials of interest for low emittance applications in both the 3-5 mm and 8-13 mm atmospheric windows. Electromagnetic radiation with wavelength within these bands transmits through the atmosphere and can therefore be detected with an infrared sensor. Due to advanced detector materials and microelectronics it is possible to detect distant objects which are only moderately overheated. Primarily low emittance dielectric materials for radomes have been studied, but also materials interesting for infrared windows and frost prevention have been investigated.

The mechanism that contributes to the infrared optical properties of dielectric compounds are the lattice vibrations, optical phonons, with their transverse optical (TO) and longitudinal optical (LO) resonance frequencies. Transverse electromagnetic waves interact with transverses optical phonons and the quantum of the coupled field is called a polariton. Between the TO and LO phonon mode, the real part of the dielectric function is negative, which leads to high bulk reflectance. In addition the material is strongly absorbing, and the imaginary part of the dielectric function exhibits a resonance at the transverse optical phonon frequency. The high reflectance frequency interval is called reststrahlen (after a German word meaning residual rays) band. Beryllium oxide has a strong reststrahlen band in the interval 9.5 to 15 mm, Loh, 1968 #118 i.e., closely matching the 8 to 13 mm atmospheric window, and exhibits therefore selective low emittance in the region. In addition to the favourable optical properties BeO has excellent thermal properties, low thermal expansion and good thermal conductivity, which makes beryllium oxide an interesting candidate material for radomes. This studies the infrared properties of beryllia and demonstrates some physical phenomena that occur for BeO, but are general for all reststrahlen band materials, e.g. how the structure of the reststrahlen band is affected by the surface roughness and how the reststrahlen band "expands" with increasing angle of incidence. Angle dependent spectral reflectance and emittance measurements have been made in addition to integrated radiometer measurements. However, because of the strong infrared dispersion of beryllia, the radiometric measurements must be corrected. A model for radiometric measurements for "non gray" materials has therefore been developed. Other candidate radome materials, investigated in the thesis, with strong reststrahlen band in the 8-13 mm region are SIALON and b-SiC.

To determine the infrared optical properties of a material fully it is important to also consider higher harmonics of the fundamental lattice vibrations (multiphonon processes), in which more than one phonon is interacting with a single photon. These higher harmonics occur because of the anharmonicity of the lattice potential and give and important contribution to the absorption in the atmospheric 3-5 mm window. Multiphonon absorption is temperature dependent and increases with temperature. Also extrinsic effects, e.g. material defects, impurities, and scattering, can become important contributors to infrared properties in the 3-5 mm window. Multiphonon absorption for polycrystalline b-SiC, polycrystalline MgF2 and for 10 different Schott glasses is reported in the thesis. The thesis will also demonstrate, both experimentally and theoretically, that the photon-multiphonon interaction also effects the index of refraction as well as the reststrahlen band. Temperature dependent measurements on single crystalline BaF2 and Al2O3 are represented in the study.

 


Uppsala University | The Ångström Laboratory | Updated 97-03-07 | Bengt Götesson