Wavelength-dependent optical detection of strain waves near intrinsic and artificial optical resonances

Laser-induced, ultrafast strain waves are potentially interesting for subsurface metrology in the semiconductor industry. These waves are commonly detected by measuring their effect on the reflectance of a material. Changes in reflectance are typically small, making detection difficult. In this Letter, we compare strain-wave-induced changes in reflectance at and around an interband transition (IBT) and a surface plasmon polariton resonance (SPR). Both are present on an Au-covered segmented grating, at different wavelengths. Using a white light continuum (WLC) probe pulse, we measure ultrafast reflectance changes over a broad wavelength range. We find that the strain-wave-induced changes at the IBT are only about 37% smaller than those measured with probe wavelengths close to an SPR. This contrasts with the different appearance of the SPR and the IBT in the static reflectance spectrum of the sample. Our results show that the static reflectance spectrum is not a good predictor for the strain-wave-induced reflectance changes and emphasize the importance of experiments to find the optimum wavelength to detect strain waves.