Investigation Of Controllable Construction And Nonlinear Surface Enhanced Raman Scattering Of Nanocavities

When light hits a metallic nanoparticle surface it induces resonant charge oscillations of the free electrons at the surface.This effect is called‘surface plasmons’and promising example to gain control over light-matter interactions way below the diffraction limit.With the development of nanofabrication and nano-synthesis technology,various artificial nanostructures based on plasmonic materials can be used to manipulate electromagnetic waves.In particular,ultrathin dielectric gaps between metals can shrink plasmonic optical modes with surprisingly low loss and with volumes below 100 nm³.Based on this extremely strong confinement,the nanocavities have important applications in enhanced light-matter interaction,ultrasensitive sensors,plasmon chemistry,and quantum light sources.At present,the most extensive and effective method to construct the nanocavity is to use the nanoparticles on film structure.The core of this method is to find suitable gap layer materials or methods to achieve large area,stable and controllable nanocavity.The emerging two-dimensional transition metal chalcogenides（TMDs）have atomic thickness and excellent optical properties,which fit well with the characteristics of the local optical field of the nanocavity.Surface-enhanced Raman spectroscopy（SERS）is a technique capable of detecting and chemically identifying single molecules in the vicinity of nanostructured metal surfaces.Over the past few years,the quality of SERS substrates has improved rapidly and has been accompanied by notable advances in measurement techniques.This has led to a number of experimental results that escape interpretation within conventional electromagnetic theory formalisms.A framework derived from cavity optomechanics was suggested to shed light on these latest results and predict novel phenomena.However,at present,the study of the SERS system from the optomechanical perspective still needs to be improved,the theory is still limited to the molecular system,and the relevant experimental evidence is still few.Based on this,in this article,a kind of high quality and high stability nanocavities was constructed by two-dimensional materials and characterized.Moreover,based on such nanocavity,we have observed strong optomechanical effects,which are quantitatively in good agreement with the extended phonon-nanocavities optomechanical theory.The main research results are as follows:1.The cavities system with two-dimensional material as the spacer layer completely was constructed,and each component of the cavity has good single-crystal properties in the cavity range.The thermal stability of this kind of nanocavity has been investigated,where the optical response is almost unaffected by 10 min of irradiation with an average pulsed laser light intensity of 1μW/μm².In addition,uniform plasmonic nanocavities were constructed over a large range and were confirmed by remote photoluminescence and Raman spectroscopy of TMDs in the range of 40×40μm.2.A phononic cavity optomechanical system consisting of a monolayer MoS₂ placed inside a plasmonic nanojunction is studied.The power dependence of SERS intensity strongly related to the excitation detuning was observed for the first time,and the threshold value of nonlinear SERS was observed to be one order of magnitude lower under blue detuning.The experimental results were in good agreement with the extended phononic cavity optomechanical theory.3.In the phonon-cavity optomechanical system,the exceptionally strong nonlinear effect of second-order Raman was observed for the first time,which has not been observed in the molecular system.These findings indicate that the second-order Raman may benefit from stronger optomechanical effects,and the phononic cavity optomechanical system may have richer physical and application than the molecular system.