We are currently building a photothermal deflection spectrometer. This is a unique opportunity for students to learn Labview coding, instrument design, optomechanical alignment, and metrology techniques.
Photothermal deflection spectroscopy uses a broadband lamp modulated by a chopper as a pump source and a HeNe laser as the probe. The sample is immersed in an inert deflection fluid (C6F14) with a strong change in refractive index as a function of temperature. When the pump is absorbed by the sample, the photoexcitation energy is eventually converted to heat to create a transient lens on the sample surface as heat transfers from the sample surface into the deflection fluid. The HeNe probe samples this lens and the temperature-dependent deflection is recorded. This deflection is proportional to the photoexcitation-dependent absorption and allows for measurements of absorption values >1000x lower than radiometric UV-Vis spectroscopies.
Further Reading:
T.C.M. Müller and T. Kirchartz. “Absorption and Photocurrent Spectroscopy with High Dynamic Range.” Advanced Characterization Techniques for Thin Film Solar Cells 1 (2016): 189-214. https://onlinelibrary.wiley.com/doi/pdf/10.1002/9783527699025.ch8
W. B. Jackson, N. M. Amer, A. C. Boccara, and D. Fournier “Photothermal deflection spectroscopy and detection.” Applied Optics, 20, 8, 1333-1344 (1981). https://www.osapublishing.org/ao/fulltext.cfm?uri=ao-20-8-1333&id=24810