Visible photothermal deflection spectroscopy using microcantilevers

Photothermal deflection spectroscopy based on bi-material cantilevers combines the sensitivity of miniature sensors and the selectivity of optical spectroscopy. In this paper, we report on the photothermal response of the microcantilevers functionalized with nanometer thin organic films in the visible region. Unlike responses in the infrared regime, in the optical region, light absorption by all the cantilever constituents must be considered for extraction of the physical parameters of the organic layer. A model of photothermal deflection for the optical region has been developed for two absorbing layers consisting of a thick metal (>200 nm) and a thin organic film. In order to develop a realistic model, all the optical constants of the cantilever components were experimentally measured in the visible region using spectroscopic ellipsometry, which enabled the determination of the optical transmission and, therefore, the heat transfer properties of the stratified cantilever structure. The results of the model were compared with the experimental spectra for two thin films of dye layers: rhodamine and methylene blue (MB). We present a simple method to calibrate the sensitivity of the visible photothermal deflection spectroscopy for quantification in addition to chemical identification. The accuracy and reproducibility of the method are discussed, as are the experimental parameters (incident light power, cantilever design) required for the selective and sensitive detection of organic molecules deposited as thin layers on the cantilevers.