Flash
Microcavity enhancement of low-frequency Raman scattering from CsPbI₃ at room temperature

Raman spectroscopy is a powerful technique for identifying chemicals and characterizing materials. Modern laser filters, based on volume holographic gratings amongst other approaches, now make it relatively straightforward to obtain Raman spectra much closer spectrally to the incident laser, in particular in the spectral range of 100 cm^-1 down to 5 cm^-1 away from the laser`s wavelength. In this spectral range, Low-Frequency Raman (LFR) scattering is sensitive to the phonon dispersion relation and to the vibrational modes associated with the nanostructural dimensions of the material, with broad applications to chiral purity of organics, biomolecular assemblies, hybrid organo-metal halide perovskites, and semiconductor super-lattices. However, the signal strength from LFR is very weak. Here we show that introducing a film of material into a photonic crystal structure, in this case a 1D optical microcavity consisting of two distributed Bragg reflector (DBR) mirrors greatly enhances the LFR signal. This is the first demonstration of Cavity Enhanced LFR (CE-LFR). In particular, we situated thin films of CsPbI₃, which in some forms are Halide Perovskites, into a microcavity prepared from stacks of ZnS and CaF₂ alternating layers. The resultant microcavities had a Quality Factor Q = 23. A TiO₂ layer with a gradient in thickness was also located between the DBRs to enable tuning of the cavity resonance from a wavelength of 490 nm to 545 nm. We investigated the effect of cavity tuning on LFR scattering intensity. We observed that the width of the cavity peak is sufficiently broad to resonate both the incident laser and the scattered LFR peak from the CsPbI₃ film. This double resonance greatly increased the light-matter interaction and hence the LFR signal. We have achieved a 47-fold increase in the LFR intensity. Our finding suggest CE-LFR is a promising route for sensitive characterization of nanoscale structured materials.