We present a home-built Brillouin spectrometer based on a Virtually Imaged Phased Array (VIPA), designed for the investigation of acoustic vibrational modes of  nanoparticles. This system enables rapid Brillouin spectral mapping, with acquisition times on the order of one second per point.

Brillouin light scattering (BLS) is a powerful technique for probing acoustic vibrational modes in phononic crystals composed of submicron polystyrene (PS) particles, typically above 100 nm in diameter. In this work, we demonstrate the capabilities of our VIPA-based spectrometer by focusing on PS nanoparticles opals in the 20–100 nm size range. Particular attention is given to a low-frequency mode that arises from interparticle mechanical coupling that emerges from a mechanical interaction-induced feature absent in isolated nanoparticles. This mode follows an inverse dependence on particle diameter (ω ∝ 1/d) and becomes indistinguishable in larger particles (d > 200 nm), where intrinsic Lamb modes dominate the spectrum.

Using the VIPA spectrometer, we perform fast hyperspectral mapping to uncover spatial heterogeneities linked to variations in particle packing, strain, and size dispersion. The presence of interparticle coupling leads to red- or blue-shifts of Lamb modes depending on the local mechanical environment, making the vibrational spectrum sensitive to both intrinsic nanoparticle properties and collective interactions.

These results establish VIPA-based Brillouin spectroscopy as a promising technique for the comprehensive mechanical characterization of nanostructured materials. Potential implementations and further improvements will be discussed.