All-optical modulation of linear and nonlinear emissions in plasmonic nanogap systems

Abstract

The past decades have witnessed the rapid growth of studies regarding light-matter interactions in plasmonic systems. It is well-known that light can be confined at subwavelength scale in plasmonic nanostructures, which could generate electromagnetic (EM) field enhancement and boost many optical phenomena such as Raman, fluorescence, harmonic generation and two-photon luminescence (TPL). In this respect, considerable attention has been paid to further improve the light concentration in plasmonic systems. Meanwhile, most plasmonic nanostructures are passive systems, which largely limit the scope and flexibility of their applications. Therefore, it is of great importance to realize all-optical modulation in active plasmonic systems. Based on above-mentioned points, plasmonic nanogap systems (PNSs) have been recently selected as one of the best candidates for both passive and active control of optical emissions. In the PNS, EM field is highly localized in the gap region, thus leading to ultrasmall mode volume compared with conventional dielectric microcavities. In addition, all-optical modulation of the optical responses in PNS can also be achieved by filling the gap with active materials. This thesis covers my research on investigating all-optical modulation of both linear and nonlinear responses in two specific PNSs, i.e. plasmonic metal particle-on-film nanocavities (MPoFNs) and gold sphere plasmonic nanomatryoshkas (GSPNs). On the one hand, I collected and analyzed the light scattering and Raman enhancement in graphene- and molecule-sandwiched MPoFNs to probe two common quantum size effects, namely spatial nonlocality and quantum charge transport, respectively. I also achieved reversible plasmon resonance tuning in photoswitchable molecule-sandwiched MPoFNs, which demonstrates my study on all-optical modulation of linear optical emissions in the PNS. On the other hand, I continued to explore the influence of electron transport on TPL response of GSPNs embedded with molecular nanojunctions, which lays the foundation for active control of nonlinear optical phenomena in PNSs. The main contents and conclusions for each work are listed as follows: First and foremost, surface-enhanced Raman spectroscopy (SERS) was taken as a tool to probe the horizontal near-field enhancement limit in graphene-coupled MPoFNs, where one to four layers of graphene were sandwiched between a gold nanosphere (Au NS) and the underlying gold thin film (Au TF). In combination with the high-resolution transmission electron microscopy (TEM) cross-sectional imaging and calculations based on nonlocal hydrodynamic model (NLHD), the gap distance correlated SERS and dark-field scattering spectroscopies were performed on graphene-sandwiched MPoFNs, which unravel that the intrinsic nonlocal effect of gold sets a limit to the near-field enhancement factors (EFs) and mitigates the red-shift of plasmon resonance when the gap distance was reduced to sub-nanometer level. The results not only prove former theoretical predictions under both near- and far-field regime but also show the feasibility of tuning the optical response in the versatile graphene-sandwiched MPoFNs, which has a great potential in designing the graphene-based optical devices ranging from visible to near-infrared frequencies. Next, I carried out far- and near-field optical characterizations on MPoFNs embedded with two types of organic molecules. Specifically, a clear blue-shift of major plasmon resonance modes was observed in conductive molecule (BPDT)-sandwiched MPoFNs in comparison to that in insulating counterparts (B4T). The physical origins of the hybridized plasmon modes were also disclosed via polarization-dependent dark-field spectroscopy, and the reduced SERS intensity of the vibrational modes verifies the quenching of near-field plasmonic enhancement which originates from electron transport in the molecular tunnel junctions. After discussing quantum size effects in the passive plasmonic system, I further investigated the active modulation of plasmon resonance in MPoFNs embedded with photoactive molecules under ultraviolet (UV)-visible light irradiation, and reversible tuning of major plasmon resonance was preliminarily realized in individual MPoFN under ambient conditions. Based on the findings in this part, molecule-sandwiched MPoFNs can be employed as a versatile platform to achieve active control of optical signals under quantum regime, thus opening up a new avenue in the study of molecular electronics. In the final part of this thesis, TPL spectroscopy was utilized to explore the influence of electron transport on the plasmonic characteristics of GSPNs with varied junction width. Together with the measured linear and TPL responses of different GSPNs, theoretical and numerical analyses unraveled that the TPL emission of the nanojunctions is closely related to the near-field enhancement within the metal regions, and is largely influenced by the electron transport across the molecular nanojunctions. Besides, the excitation-wavelength dependent TPL intensities of three typical nanojunctions (0.7 nm, 0.9 nm and 1.5 nm junction widths) and an Au NS of the similar size were measured under femtosecond laser illumination, and no perceivable contribution from the low-energy plasmon modes (LEM) of those nanojunctions was discovered. This experimental observation is consistent with the numerical results based on quantum-corrected model (QCM), assuming the value of conductance for molecular layers and the efficient electron transport across the nanojunctions. These results provide possibilities for investigating charge transport in molecular nanojunctions by plasmon-mediated nonlinear spectroscopies and can be further applied in active PNSs.