Joint Seminar on Quantum Information and Technologies

Among a number of spectroscopic techniques developed to characterize the electronic and vibrational structure of atoms and molecules, one - Substrate Enhanced Raman Scattering (SERS) - has taken hold in a rather unexpected territory of plasmonics. Plasmonics resonances occur in metallic nanostructures as the free electron gas sloshes within the metal, in response to the electric field of incident light, creating the so-called "hot-spots" - tiny volumes near the metal surface in which the intensity of incident illumination is amplified by orders of magnitude. While this effect can be directly used to enhance the light-matter interaction, its applicability is suppressed by the extremely lossy nature of plasmonic excitations, resulting in extremely low populations of plasmons.Nevertheless, very recent implementations of SERS experiments [1] appear to provide results which escape the standard semi-classical description of the Raman process based on the classical treatment of the electromagnetic enhancement of fields in these hot-spots [2]. To address these issues, we have proposed a novel theoretical framework, inherited from the community of quantum cavity optomechanics [3-5] and applied it to predict a plethora of effects which are not encompassed by the semi-classical framework. This approach allows us to discuss the coherences of the non-linear scattering processes, role of vacuum fluctuations of the plasmon population on the vibrational pumping of molecules, etc. [5-7] By reciprocity, we show that SERS constitutes a very atypical implementation of the optomechanical dynamics, placing it in the largely unexplored regime of THz mechanical frequencies, low-Q "bad" cavities and large single-photon couplings.[1] R. Zhang, et al., Nature 498, 82 (2013).[2] H. Xu, et al., Phys. Rev. Lett. 93, 243002 (2004).[3] P. Roelli, et al., Nat. Nano. 11, 164 (2016).[4] M.K. Schmidt and J. Aizpurua, Nature Nanotech. 11, 114 (2016).[5] M.K. Schmidt, et al., ACS Nano 10, 6291 (2016).[6] M.K. Schmidt, et al., in preparation.[7] F. Benz, M.K. Schmidt, et al., Science, in press (2016).