Photoacoustic detection of low duty cycle gratings through optically opaque layers

We report on the use of ultra-high frequency photoacoustics to detect gratings with linewidths as narrow as 75 nm, buried underneathoptically opaque metal layers. Our results show that buried gratings can be detected by observing diffraction from the spatially periodicacoustic replica of the buried grating at the glass/metal interface and from replicas of the acoustic wave inside the glass substrate. Themeasured diffraction signals show a linear dependence on grating duty cycle rather than the expected quadratic one. We find that this is dueto the presence of a coherent background optical field, which interferes with and coherently amplifies the weaker fields diffracted off thegrating-shaped acoustic waves. Our measurements show that ultra-high frequency photoacoustics is a promising technique for detection ofsub-wavelength periodic nanostructures.