Optically enhancing and controlling photoacoustic signals using ultra-thin semiconductor coatings on metal surfaces.

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We present a simple approach to enhance the signal strength and detection sensitivity of weak photoacoustic signals, by using ultrathin, high refractive index, absorbing semiconductor layers on metal surfaces. They form etalon resonances with reflection minima already for layer thicknesses of only a few tens of nm. Strain waves induce changes in the physical/optical thickness of the layers and/or changes in the phase and amplitude of light upon reflection from the metal-semiconductor interface. When the reflection is optically probed near the steep slope of the etalon resonance as a function of semiconductor layer thickness, the optical response is enhanced. With a 12 nm thick Ge layer on Au, we observed a 15-fold increase in the absolute reflection changes induced by strain waves compared to the signal without the Ge layer. Moreover, thin semiconductor layers can be used as transducers, enabling the generation of higher-frequency strain waves in the metal where the layer thickness determines the spectrum of these waves. Additionally, by properly choosing the semiconductor thickness and the pump-probe wavelengths, increased control over where light is absorbed is obtained. This offers some flexibility in tailoring the light absorption in both the metal and the semiconductor. We demonstrate this with a Si layer on Pt, where at 800 nm pump wavelength, most light is absorbed by Pt, while at 400 nm, both Pt and Si absorb light. Our results show the potential of this approach to address the challenge of detecting weak photoacoustic signals and to provide some control over strain wave generation.