Light-Matter Interactions In Plasmonic Cavities:Theories And Spectroscopic Studies

Plasmon resonance in metallic nanostructures enables nanoscale confinement and manipulation of light.The deep sub-wavelength confined light in plasmonic nanocavities thus offer huge coupling strength with matter,which enables mediation of the optoelectronic properties,understanding of light-matter interactions and paves new way for exploring new quantum phenomena and applications.For example,plasmons can amplify molecular spectral signals by several orders of magnitude through the enhanced local electric field.It triggers the development of a series of plasmon-enhanced spectroscopy such as surface-enhanced Raman spectroscopy(SERS),surface-enhanced fluorescence(SEF)and surface-enhanced infrared absorption(SEIRA),which have been widely applied in surface analysis and nanoscience.In previous studies,most efforts are focused on designing nanostructures to improve the enhancement factors,while the plasmon-molecules interactions induced unique spectral behaviors are rarely discussed.Here we combine theoretical simulations as well as experiments to investigate the light-matter interactions in plasmonic nanocavities and probe its unique cavity quantum electrodynamic(CQED)properties.The main research results of this thesis are outlined as follows:1.We established a generic theoretical framework fur light-matter coupling system.Based on this,we studied the spectral response under weak and strong light-matter coupling regime.The interactions lead to the perturbation and dressing of the excited state.The change in imaginary part of the eigenvalues give rise to modified spontaneous emission rate of the excited states,which results in the spectral reshaping effect observed in SERS and SEF process.Based on the finding that SERS and SEF share similar enhancement mechanism,we propose a universal method to correct the reshaped SERS spectra.This method is useful for studying complex molecular-interface interactioris such as surface selection rule and surface adsorption configuration.Furthermore,the interactions will also contribute to modifying the real part of the eigenvalues,which leads to a shift of transition energy that known as "Lamb shift" effect.We observed a single-molecule Lamb shift of 7 meV in the plasmonic cavity and found correlations between Lamb shifi and molecular position in the cavity.The results provide new methods for controlling light and matter interaction at the nanoscale and also for detecting the spatial distribution of excited states of single molecules.In addition,we also investigate the Fano interference between plasmons and phonons in surface-enhanced infrared absorption(SEIRA)process.We systematically studied the role of energy detuning,cavity damping rate,interaction distance and oscillation intensity on the Fano interference.This work systematically shows the effect of coupling strength on the absorption spectrum and provides a new way to control quantum interference.2.We unveiled the excited states of Exciton-Polaritons(EPs)via nonlinear spectroscopy and probed the phonon scattering process of EPs via resonant Raman spectroscopy.In the strong coupling regime,the light and matter hybrid together and forms new quasiparticle EPs.Due to the intrinsic hybrid nature,EPs can expand the range of parameters for undergoing quantum phenomena by orders of magnitude,such as realize room-temperature superfluid and Bose-Einstein condensation(BEC).These phenomena are critically dependent on properties of EPs excited states the phonon scattering from excited states to ground states.However,a clear understanding of EP excited states is still lacking,due to limited access to these states and the decay processes.Here we introduce the unique valley degree of freedom as a quantum mark to the EPs studies.We,for the first time,directly probe the EP excited states including the optical forbidden 2p states and dynamic instable LP states via two-photon luminescence(TPL)and second harmonic generation(SHG).Based on the spectroscopic analysis and polarization dependence in these states,the transition process and valley dynamics in 2D EP are also resolved.Besides,we probed the phonon scattering of EPs via resonant Raman spectroscopy and found electron-phonon interactions also inherit the hybrid nature of EPs,such as valley correlated phonon scattering and stimulated phonon scattering.Our findings can help to understand the ultrafast relaxation processes in 2D EPs and represent the first step towards a clear understanding of EP excited states.It also provides a generic method for exploring complex physical mechanisms in CQED systems.3.The mechanical and thermal effects of plasmons in plasmonic cavities are explored.The ultra-confined plasmonic field also generates optical trapping potential weils that even sufficient to overcome thermal fluctuations.Based on this,we developed single?molecule plasmonic optical tweezer technique and demonstrated room-temperature trapping and releasing of single molecules in solutions.We found that the surface atomic roughness insider the cavities is one of the keys to constructing the single molecule optical traps.This work extends the scale of manipulation of objects down to single molecule level and can be widely applied to other related applications in nanoscience.In addition,we also demonstrated that plasmonic cavities can be used as nanoscale heat sources.We proposed the plasmon-mediated local thermal process and thermal reactions.It provides new possibilities for plasmonic chemistry and solar energy conversion.