Fano Resonance And Spaser Of Metallic Nanostructures

With the development of science and technology, the metallic structures can be reduced to the scale of nanometers. The surface plasmon of the metallic nanostructures supports a very high electric field and extraordinary sensitive to surface conditions and the the nanostructure. This properties lead the surface plasmon to be used in chemical and biological sensors, surface enhanced Raman scattering, second harmonic generation (SHG), etc. In the project, we focused on the studies of the characteristics of surface plasmon resonances. We propose plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure, and study the quadrupolar Fano resonance of the RCSRD structure. We also study the spaser based on Fano resonance of the RCSRD. In the last, we introduce the periodic ripples induced by the femtosecond laser and the role of surface plasmon in formation of the ripples. The innovative results are as follow:(1) We propose a plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure, which can support a Fano resonance. Fano interference is an important way to decrease the resonance line-width and enhance the spectral detection resolution, but realizing a Fano line-shape with both a narrow line-width and high spectral contrast-ratio is still a challenge. Fano line-width and spectral contrast-ratio of the RCSRD structure can be actively manipulated by adjusting the gap between the nanorod and CSRD, and by adjusting the gap between the ring and disk in CSRD. When the gap size in CSRD is reduced to5nm, the quadrupolar Fano line-width is of0.025eV, with a contrast ratio of80%, and the figure of merit reaches15. So the RCSRD structure has a very extensive application in the biological chemical sensing.(2) We reports a spaser based on Fano resonance of a plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure coated with a layer of gain media. The amplification of the dark quadrupolar mode at the Fano resonance wavelength causes the spaser with a high Purcell factor of3.24×107, a high signal to noise ratio (SNR) of4.4×106and a lower threshold of0.02086. These significant optical properties are attributed to the greatly enhanced spontaneous emission and depressed radiation loss supported by the strong localized dark mode at the Fano resonance wavelength. Reducing the loss of the metal is an important focus in the study of the Spaser. The amplification of the dark mode provide a new way to reduce the loss of the Spaser.(3) Surface plasmon play an important role in the formation of the ripples induced by the femtosecond laser. Three kind of periodic ripples formed in the ablation spots in sequence:LSFRs (A>0.45λ), MSFRs (0.2A<A<0.45λ) and HSFRs(A<0.2A). The formation of high spatial frequency ripples (HSFRs) in stainless steel irradiated by50fs,800nm,1kHz femtosecond laser pulses in water. We further conduct theoretical calculation to study the ultrafast dynamics, and find that double split of LSFRs plays the decisive role in the formation of HSFRs. Theoretical calculations show that as the groove depths reach to a certain value, the incident laser localizes on the protuberance, which induces the split of ripples. Similar experiments are conducted in copper, however, no split of LSFRs is observed. The different experimental results on stainless steel and copper conducted in water and in air are also discussed. The split process is affected by the laser pulse numbers, fluences, surrounding medium and thermal parameters of samples. These nanostructures are with features of1/6-1/8laser wavelength, which has potential applications in nanophotonics, metamaterials, data storage, and nano-optoelectronics.