SPP/LSP coupled hybrid mode directed multi-functional surface-enhanced Raman system
Date of Issue2012
School of Electrical and Electronic Engineering
Photonics Research Centre
Surface plasmons (SPs) have generated a tremendous amount of interest in recent decades due to their intriguing features such as lateral spatial confinement, surface sensitivity and field enhancement. The consistent investigation of SPs leads to a wide scope of applications including ultrahigh-sensitivity biosensing, super-resolution imaging, sub-wavelength nanolithography, miniaturized photonic circuits, surface- enhanced Raman spectroscopy (SERS), etc. In this thesis, a multi-functional surface-enhanced Raman system based on the SP-hybrid-mode is proposed and investigated. The system is grounded on a novel idea, which utilizes the giant electromagnetic enhancement arising from the strong coupling between surface plasmon polaritons (SPPs) and localized surface plasmon (LSP), the two different types of SP modes, and deals with the problems facing with SERS and other SP-based techniques. First of all, as a fundamental investigation, two types of excitation configurations are proposed to achieve the plasmon-hybrid-mode: through a periodic metallic structure and via an attenuated total reflection configuration, respectively. In both approaches, the numerical investigation with three-dimensional finite-difference time-domain method shows that the coupling between the LSP and SPPs could result in a great improvement of electromagnetic enhancement compared to that from separate SP mode. Raman enhancement of more than 109 was predicted from the numerical simulation. SERS experiment was carried out afterwards to verify the Raman enhancement. In the experiment, SPPs are excited by a tightly-focused radially polarized beam and interact with the nanospheres to generate the plasmon-hybrid-mode, exactly according to the second configuration aforementioned. The experimental result demonstrates that silver nanospheres within the propagation region of SPPs are effectively activated and detected by a CCD camera. Raman enhancement presents 20 times improvement compared to the conventional nanoparticle-induced SPPs/LSP co-enhanced Raman spectroscopy. Surface-enhanced Raman scattering (also termed as SERS) from single nanosphere-film junction is realized, which is of significance for SERS as a quantitative analytical tool. It is also the basis for the following experimental work. Subsequently, surface plasmon coupled-emission (SPCE) of SERS was studied. It is found that Raman signal originated from molecules sitting at the nanosphere-film junction can couple back to SPPs, and eventually radiates into the substrate side with high refractive index at SPP resonant angles. Collection efficiency of SERS can be improved with the help of SPCE. Meanwhile, due to the extremely narrow linewidth of Raman peaks, the SPCE curve of SERS can also be employed for measuring the propagation length of SPPs and quantitatively characterizing the point spread function of an SPCE microscopy. Last but not least, as a promising application, the two-dimensional mapping of the strongly-dominant longitudinal field component of SPPs was realized based on the SERS imaging. Our method takes advantage of SERS from a single nanosphere-film junction where the enhancement factor is determined by the longitudinal component of coupled SPPs between the nanosphere and metal film. By scanning the nanosphere immobilized on the film over the propagation region of SPPs, We can map out the near-field SPP longitudinal field with super-resolution. To sum up, a plasmon-hybrid-mode directed surface-enhanced Raman system is proposed in this thesis. By utilizing the coupling-effect between SPPs and LSP, Raman enhancement of more than 109 was predicted numerically and more than 108 was achieved experimentally. Moreover, collection efficiency of SERS originating from the nanosphere-film junctions can also be improved because of the plasmon hybridization. Finally, our system can also serve as a powerful SPP-characterization tool, for measuring the propagation length and mapping the near-field profiles and dynamics of SPPs.
DRNTU::Science::Physics::Optics and light