Nanoparticle on mirror plasmonic nanostructure for molecular cavity optomechanics

The plasmonic response of gold can be utilized to enhance the incident electromagnetic (EM) field by several orders of magnitude. When a gold nanoparticle is placed in close proximity to a gold film, the incident field can excite a deeply confined plasmon, strongly enhancing the Raman interaction with molecules placed within the gap. Recent theoretical predictions suggest the suitability of such arrangements to optically amplify the internal vibrations of molecules. This paper presents the fabrication and characterization of such plasmonic nanocavities.

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

Light matter interaction can be boosted by several orders of magnitude by tailoring the photonic environment, thus enabling a wide range of applications. One particular example are plasmonic nanostructures that support localized surface plasmon polariton (LSPP) leading to enhancement and confinement of incident electromagnetic fields. These novel properties provide access to a large number of optically driven effects within matter placed in the vicinity of such nanostructures. In this thesis, we investigate the effects of optical excitation on gold nanostructures in the presence of Raman active molecules. This is accomplished by fabricating nanoparticle on mirror (NPoM) plasmonic nanostructures where a gold nanoparticle is placed at a fixed distance from a smooth gold film using molecular monolayers. As the nanoparticle and mirror are placed only a few nanometers apart, optical excitation leads to formation of hybridized LSPP modes due to coupling between the plasmons within the nanoparticle and the mirror. This plasmonic coupling also results in confinement and subsequent enhancement of the incident light within the gap and is extremely sensitive to structural parameters of the NPoM. The creation of a homogeneous gap between the nanoparticle and mirror is quite challenging due the roughness of the mirror. Moreover, due to the large number of NPoM nanostructures present on the sample it is quite difficult to locate a specific NPoM. To bypass these difficulties atomically smooth and patterned substrates were developed using template stripping combined with conventional photolithography techniques. Next, in order to characterize the optical response of the NPoMs, a custom optical setup was built. The setup was primarily designed to investigate the plasmonic properties of the NPoM using elastic scattering spectroscopy along with surface enhanced Raman scattering (SERS) from the molecules within the gap. The combination of these two techniques was later used to investigate the effect of laser power on the NPoMs. It was discovered that the scattering spectra of the NPoMs changed irreversibly after laser exposure even with a few ÎŒW/ÎŒm2 incident intensities. Moreover, the plasmonic response of the NPoMs fabricated with well organized molecular monolayer changed much faster than the NPoMs prepared with disorganized ones. This phenomenon was also investigated in NPoMs prepared using a dielectric gap layer, by changing the gap conductivity, and by changing the nanoparticle shape. The experimental results combined with simulations suggest a decrease in gap height due to the reorientation of the molecular monolayer under optical excitation.

EPFL2021

Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering

Christophe Marcel Georges Galland, Tobias Kippenberg, Nicolas Piro Mastracchio

The exceptional enhancement of Raman scattering by localized plasmonic resonances in the near field of metallic nanoparticles, surfaces or tips (SERS, TERS) has enabled spectroscopic fingerprinting down to the single molecule level. The conventional explanation attributes the enhancement to the subwavelength confinement of the electromagnetic field near nanoantennas. Here, we introduce a new model that also accounts for the dynamical nature of the plasmon-molecule interaction. We thereby reveal an enhancement mechanism not considered before: dynamical backaction amplification of molecular vibrations. We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled. We express the vacuum optomechanical coupling rate for individual molecules in plasmonic 'hot -spots' in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems. Remarkably, the frequency of commonly studied molecular vibrations can be comparable to or larger than the plasmon's decay rate. Together, these considerations predict that an excitation laser blue-detuned from the plasmon resonance can parametrically amplify the molecular vibration, leading to a nonlinear enhancement of Raman emission that is not predicted by the conventional theory. Our optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross -sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode -selective enhancements. It also provides a quantum mechanical framework to analyse plasmon-vibrational interactions in terms of molecular quantum optomechanics.

Nature Publishing Group2015

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

EPFL2021