| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 99 === Recently the behavior of extraordinary transmission (EOT) has attracted lots of attentions. This property can be applied on biosensors due to its high environmental sensitivity especially for gold nano-grating. In such sample, surface plasmon ploaritons (SPPs) on the dielectric/metal interface and TM light resonance in nanoslits (cavity mode, CM) play important roles for EOT. SPPs can be excited both on air/metal and substrate/metal interface. SPPs on air/metal interface can be observed easily by near-field scanning optical microscopy (NSOM). However SPPs on the metal/substrate interface are hard be observed. Here we generate coherent acoustic phonons (CAPs) by typical pump-probe spectroscopy from bottom of the substrate. CAPs can modulate the refractive index of substrate. Then we observe the transmission difference induced by CAPs by NSOM. Finally we can investigate the near-field interaction between SPPs and CAPs.
In our operating wavelength we can generate SPPs both on the gold/air interface and gold/GaN interface. By obtaining the NSOM image induced by CAPs we can know that the transmission start to change when CAPs enter the SPPs below the gold. Once CAPs enter this area, the refractive index is modulated and the energy of this area starts to transfer to the CM in the nanoslit. Therefore the transmission change is significant in the nanoslit. The energy transfer reaches its maximum when CAPs on the interface between metal and substrate. Since the acoustic impendence of gold and air are totally different, CAPs will be reflected when it arrive at the gold/air interface. The reflected CAPs have the opposite strain relative to the former one. Therefore the transmission change is opposite.
From our experiment we can observe the near-field interaction between SPPs and CAPs. The CAPs modulated signal in the far-field is due to the coupling between SPPs under the metal and CM in the nanoslit. Since SPPs is very sensitive to the refractive index change of the environment, it has a great potential in acoustic imaging. By understanding such phenomena we can design other plamonic structure such as nanohole array and extend our current structure from 1D space to 2D space.
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