Study Of Gain-assisted Localized Surface Plasmon Resonance And Its Optical Applications

Localized surface plasmon resonance(LSPR) of metallic nanoparticles can be used in various areas including medicine, biology, and chemistry. However, due to the great loss of metal absorption in the visible light band, its application is limited. It is quite difficult to completely eliminate metallic absorption losses by optimizing the structure of the device alone. To overcome this problem, a novel strategy has recently been proposed by integrating plasmonic nanostructures with gain materials.The electric field enhancement effect of LSPR can be applied in many fields, such as medicine, bio molecular sensor, chemistry and so on. Limited to the losses, the near-field enhancement of plasmonics and metamaterials is insufficient for many practical applications. In the thesis, we have proposed a simple method to significantly enhance the local-field of a gap plasmonic system by placing a metallic nanoparticle in close proximity to a substrate covered with a thin gain film. Numerical results show that a thin gain film with 100 nm thickness can contribute to several, or dozens, of times more intense local electric fields in the gap between the nanoparticle(NP) and the substrate. For a given NP radius, there is an optimal refractive index of the gain film that enables to achieve a maximal field enhancement. Moreover, the optimal refractive index of the gain substrate will increase as the NP radius decreases. It means that the optimal refractive index of the gain film can be accommodated to any available materials by using metal nanoparticles with an appropriate radius.Raman scattering provides a means for identifying molecules through their “fingerprint” vibrational spectra. Extensive studies have been conducted on the design of various plasmonic devices that are able to enhance and spatially confine electromagnetic fields for surface enhanced Raman scattering(SERS). However, the near-field enhancement from individual metallic NPs is insufficient for ultrasensitive detections because of the losses inherent to the interaction of light with metals. In the present thesis, a novel gap plasmonic system consisting of a silver nanoshell with a gain core above a silver substrate has been presented to achieve ultrastrong plasmonic hot spots for single-molecule SERS(SM-SERS) applications. The proposed system can contribute to a super high G factor with a low gain coefficient. The G factor is five orders of magnitude greater than the highest values reported in theory for SM-SERS applications.Third harmonic generation(THG) has a very important role in solar cells, photodynamic therapy, etc. Regarded as one of the most effective methods to substantially enhance nonlinear upconversions by using LSPR, there is currently significant interest in the construction of various gap systems. We have presented a novel design to enhance THG using individual Au nanorods coated with gain materials. The present design avoids the use of conventional gap systems, where complicated semiconductor techniques must be used in order to realize the corresponding high-resolution planar structures. As such, this design can contribute to a significant THG enhancement from nanoparticles; this THG is especially favorable for many applications(e.g., photovoltaic devices, bioimaging, and therapy). Results revealed the outer gain layer has an optimal gain coefficient, where an ~239.7-fold third harmonics can be obtained compared to those using the same structure without the gain coefficient. The results also show that the THG enhancement of the gain-assisted Au nanorod with a critical gain coefficient is one to two orders of magnitude greater than the highest values reported for gap THG systems.