Magnification in Excess of 100-Times of the Microscopic Photothermal Lensing Signal from Solute Molecules by Two-Color Excitation with Continuous-Wave Lasers.

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A microscopic photothermal lensing measurement under two-color continuous-wave laser excitation was performed to investigate a signal enhancement owing to the transient absorption by photoexcited solute molecules in liquid solutions. An intensity-modulated 409 nm laser beam and an un-modulated 532 nm laser beam were used for excitation, and a 670 nm probe beam was used for detecting the modulation amplitude of thermal lensing signals generated with a microscopic objective lens focusing laser beams into a capillary flow cell of 0.1 mm optical path length. The amplitude of the modulated signal increased as the power of the un-modulated laser beam increased, and a 143-times magnification was observed for an iso-propanol solution of naphthacene having 4.6 × 10-4 absorbance at 409 nm and a negligible absorbance at 532 nm. A four-level model explaining the signal enhancement is proposed, and an important role of the transient absorption by photoexcited molecules is discussed.
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