A thorough understanding of laser-materials interactions is crucial when designing and building optical systems. An ideal test method would probe both the thermal and optical properties simultaneously for materials under large optical loads where detrimental thermal effects emerge. An interesting class of materials are those used for infrared wavelengths due to their wide spectral transmission windows and large optical nonlinearities. Since coherent sources spanning the mid-wave and long-wave infrared wavelength regions have only become widely available in the past decade, data regarding their thermal and optical responses is lacking in literature. Photothermal Lensing (PTL) technique is an attractive method for characterizing the optical and thermal properties of mid-infrared materials as it is nondestructive and can be implemented using both continuous wave and pulsed irradiation. Analogous to the well-known Z-scan, the PTL technique involves creating a thermal lens within a material and subsequently measuring this distortion with a probe beam. By translating the sample through the focus of the pump laser, information can be obtained regarding the nonlinear absorption, thermal diffusivity and thermo-optic coefficient. This thesis evaluates the effectiveness and scope of the PTL method using numerical simulations of low loss infrared materials. Specifically, the response of silicon, germanium, and As2Se3 glass is explored. The 2 µm pump and 4.55 µm probe beam geometries are optimized in order to minimize experimental error. Methodologies for estimating the thermal diffusivity, nonlinear absorption coefficient and thermo-optic coefficient directly from the experimentally measured PTL signal are presented. Finally, the ability to measure the nonlinear absorption coefficient without the need for high-energy or ultrashort optical pulses is demonstrated.
Master of Science (M.S.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Masters Thesis (Campus-only Access)
Cook, Justin, "Photothermal Lensing in Mid-Infrared Materials" (2017). Electronic Theses and Dissertations. 5638.