Broadband optical detection of strain waves in a thin ruthenium film

Optimizing the optical detection of ultrafast strain waves is essential for potential applications in semiconductor metrology. A crucial element in this is the wavelength-dependent optical response to these waves, which is, unfortunately, not known for many materials. Here, using an ultrafast spectrometer, we investigate the strain-wave-induced changes in reflectance of a 30 nm ruthenium layer at wavelengths ranging from 475 nm and 1000 nm. We find that the maximum detectable strain-wave frequency is inversely proportional to the optical penetration depth of the probe wavelength. Consequently, the spectrum peak of the reflectance change induced by the strain-wave shifts from 80 GHz to 300 GHz, as the probe wavelength decreases from 1000 nm to 560 nm, respectively. Surprisingly, certain strain-wave frequencies are not observed below the maximum detectable frequency for specific probe wavelengths. This emphasizes that there is no simple relation between detection wavelength and the strength of the optical response of the material to strain waves.