Abstract

Photothermal deformation techniques (PTD) measure optical energy absorption of surfaces as a surface displacement due to thermal expansion.¹ Typical sensitivities have usually been in the ≤5 μW range when detecting the surface buckling with HeNe-laser interferometric techniques. ² Significant enhancements are currently described based on mechanically resonating thin bars in a flexural mode of vibration.³ The exciting light source, a CO₂ laser, is modulated at the resonant frequency, typically 0.5 to 2 kHz (Fig, 1). While optically driven resonant vibrations have often been reported in opaque materials, this appears to be the first time resonant photothermal signals have been used in conjunction with a tunable laser to make surface-sensitive absorption scans. Because resonant Qs of up to 10⁵ are readily achieved, the surface displacement and hence PTD response are similarly increased, e.g., to sensitivities in the nanowatt and even picowatt ranges of absorbed power. Equally important are the following advantages: excitation and sense beams are displaced from each other, hence avoiding "crosstalk" due to direct reflectivity modulation, and optical alignment requirements are relaxed from microns to mms. Also, the present measurements do not require ultra high vacuum.

© 1991 Optical Society of America