We present photothermal images of melanin using modulation with two laser beams. Strong melanin absorption followed by efficient nonradiative relaxation caused heating and an increase in temperature. This temperature effect was used as an imaging contrast to detect melanin. Melanin from several samples including Sepia officinalis, black human
hair, and live zebra fish, were imaged with a high signal-to-noise ratio. For the imaging, we focused two near infrared
laser beams (pump and probe) collinearly with different wavelengths and the pump was modulated in amplitude. The
thermally induced variations in the refractive index, at the modulation frequency, were detected by the scattering of the probe beam. The Photothermal method brings several imaging benefits including the lack of background interference and the possibility of imaging for an extended period of time without photodamage to the melanin. The dependence of the photothermal signal on the laser power, modulation frequency, and spatial offset of the probe is discussed. The new photothermal imaging method is promising and provides background-free and label-free imaging of melanin and can be
implemented with low-cost CW lasers.
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Josef Kerimo and Charles A. DiMarzio
Photothermal imaging of melanin
", Proc. SPIE 8589, Three-Dimensional and Multidimensional Microscopy: Image Acquisition and Processing XX, 858910 (February 22, 2013); doi:10.1117/12.2003201; http://dx.doi.org/10.1117/12.2003201