Light-Matter Interactions In Confined Space At Nanoscale

Light-matter interactions in confined space at nanoscale has been playing a great role in surface science and optical physics.In this dissertation,we studied two types of confined light-matter interactions.One is that the light is focused onto confined surface on nanostructure through surface plasmon resonance and thus generates stronger interaction with surface molecules and metal atoms,which results in surface-enhanced Raman spectroscopy and plasmon-enhanced photoluminescence of metal.The other one is that the semiconductor material is confined in two dimension and thus generates two-dimensional excitons with giant oscillation strength,which leads to stronger light-matter interaction.The main research results of this dissertation are outlined as follows:1.For the first time,we observed one-photon photoluminescence from single silver nanorods.The photoluminescence of plasmonic nanostructure has been mostly believed to originate from interband transition inside metal.Since the interband transition energy of silver is too large(-3.9 eV)to be excited by a visible laser,the visible photoluminescence of silver nanorods excited at both 532 nm and 633 nm visible laser cannot be explained by the long-standing interband transition mechanism.Instead,we propose that the one-photon photoluminescence of silver nanorods comes from the intraband hot-electron luminescence.The photoluminescence quantum yield(10-6)of silver nanorods is found to be similar to that of gold nanorods,indicating an efficient intraband excitation of hot electrons.The intraband hot electron photoluminescence mechanism points to the high-energy nature of hot electrons excited through intraband transition,which sets the basis for high efficient hot electron harvesting for photovoltaics and photocatalysis.2.The local field offered by localized surface plasmon resonances(LSPR)not only enhances the Raman scattering of molecules at the surface,but also distorts the spectral relative intensity providing chemical informations,termed plasmonic spectral shaping effect(PSSE).We reveal the PSSE on SERS using single nanoparticle spectroscopy.We further propose a robust method to correct the PSSE and retrieve the fingerprint of intrinsic chemical information.The method is established based on the finding that the mysterious SERS continuum background is mostly contributed by LSPR-modulated photoluminescence of metal,which contains the local field dispersion in wavelength shared by SERS.We validate this method in single gold nanorods of varying aspect ratios.We further demonstrate its generality in more complicated systems such as tip-enhanced Raman spectroscopy and SERS on silver nanoaggregates.3.SERS has attracted tremendous interest as a label-free sensitive analytical method.For optimization of SERS activity,we systematically investigated the size effect of nanoparticles on not only the SERS enhancement but also the SERS signal to background ratio,constituting the fundamental limit of the SERS sensitivity.By using single gold nanorods with different size but the same LSPR,we show that smaller nanorods(>25 nm)have much larger SERS enhancement factor due to the stonger lightning-rod effect.However,the SERS signal to background ratio does not vary with the size of nanrods,which indicates the SERS sensitivity is not straightforwardly correlated with SERS enhancement factor.Surprisingly,we found that the gold-silver complex nanoparticle,i.e.silver nanobar with a gold nanorod core,show more than one magnitude higher SERS signal to background ratio than that of gold nanorods.This enhancement comes from strongly suppressed photoluminescence quantum yield of gold-silver complex nanoparticle,whose mechanism needs further investigation.4.Electromagnetically induced transparency(EIT)in atomic gases gives rise to lasing without inversion and slow light generation,and can be used for optical quantum-information processing.If EIT were to be implemented in the solid state,the range of applications would largely widen since optoelectronics and photonics are based on semiconductor heterostructures.To date,quantum wells have been the most promising candidates for optoelectronic integration,but are limited to the IR spectral region.We demonstrated for the first time EIT in a single-layer WSe2 crystal,a two-dimensional semiconductor supporting giant excitonic oscillator strength and long coherence times of optical transitions in the visible.We exploit second-harmonic generation(SHG)to probe the optically dressed states.EIT manifests itself as a sharp dip in the SHG spectrum.The most striking characteristic of EIT in SHG is a polariton-like anti-crossing as a function of pump wavelength,predicted by a ladder-type three-level model.Single-layer WSe2 is well suited for integration in optoelectronic devices,opening up a new solid-state platform for EIT.