Modifying the Optical Phonon Response of Nanocrystals inside Terahertz Plasmonic Nanocavities

Phonons are quantized lattice vibrations that represent a major energy dissipation channel in solid-state systems [1], both at the macro- and at the nano-scale. Although the phonon response of a specific nanomaterial is usually considered as its intrinsic fingerprint, here we show how it can be altered by exploiting the unique properties of terahertz (THz) plasmonic nanocavities [2]. Specifically, we obtained such nanocavities from the end-to-end coupling (30-nm gap size) of few-μm-long plasmonic gold nanoantennas. We fabricated a series of plasmonic arrays featuring different nanoantenna lengths, spanning from 4.75 μm to 6.75 μm, thus tuning their resonances between approximately 7 and 9 THz. We tested our approach on cadmium sulphide (CdS) nanocrystals (NCs), spin-coated over the array surfaces (Fig. 1a), since these NCs feature a dipole-active (Fröhlich) phonon mode at 7.85 THz. We performed THz transmission measurements using a Fourier-transform THz microscope coupled to synchrotron light (ELETTRA, Trieste), showing the splitting of the nanoantenna resonance into two new vibro-polariton bands, as shown in Fig. 1b. This anti-crossing behaviour represents a distinctive signature of the strong coupling between the plasmon and phonon modes, the splitting (Rabi) at the crossing point being directly related to the coupling strength. More intriguingly, we also observed the phonon resonance modification without any THz illumination, just exploiting the vacuum electric field of the nanocavities [3] (estimated to be as high as 4.6× 105 V/m). To this end, we performed a series of micro-Raman measurements on individual nanocavity areas, finding evidence of the two new hybrid states (P− and P+ in Fig. 1c) even in THz "dark" conditions. The evidence of phonon mode splitting both in THz and Raman characterizations confirms the possibility of altering the intrinsic phonon response of a nanomaterial using properly tailored plasmonic resonators, which could open new avenues for the manipulation of energy dissipation in nanodevices. Novel cavity geometries that promise to further boost the strong vibrational coupling in these systems will be presented on site.