The Near-Field Interaction between Surface Plasmon Polaritons and Coherent Acoustic Phonons

近來金屬格柵的異常穿透行為受到了很高的重視，由於異常穿透光的行為對周遭環境的改變非常敏感，因此最近廣泛的應用到生物、化學感測器，且重要性與日俱增。此行為通常由表面電漿極化子以及金屬狹縫中的光場共振所貢獻。表面電漿極化子會在金屬/空氣介面，以及金屬/基板介面被激發，利用近場顯微鏡可以觀察到金屬/空氣介面的表面電漿極化子，但是卻無法觀察到金屬以下的表面電漿極化子。因此本實驗利用了超快頻譜技術由基板底部來產生同調聲子，利用同調聲子來影響金屬格柵基板的折射率，並使用近場顯微鏡來觀察穿透光的變化，進而了解同調聲子如何與表面電漿極化子產生交互作用。 在我們的實驗條件下可在金與空氣及金與砷化鎵基板間介面產生表面電漿極化子。藉由從基板底部激發同調生子，並取得聲子在不同位置時之近場穿透光影像，來觀察此影像變化。可得知當同調聲子進入金下方表面電漿的區域時，聲子開始影響此區域的折射率，使得能量開始由表面電漿區域轉換到金屬狹縫的光場共振膜態。因此穿透光的變化主要集中在狹縫區。此能量轉換可在聲子到達金與砷化鎵基板介面時達到最大值。當聲子到達金及空氣介面時，由於空氣與金屬的音波阻抗差異過大，聲子無法傳遞到空氣中，故在此介面音波會被反彈，反彈的音波造成的應變跟原本的音波相反。因此在穿透光變化的行為上，會造成與原本的音波相反的結果。 由我們的實驗可以看到聲子與表面電漿極化子之近場交互作用之過程，並且得知此交互作用在遠場下的訊號變化與光場在金屬狹縫間的共振互相耦合有關。由於表面電漿對於折射率的改變非常敏感，未來在奈米聲波的影像應用上有很大的潛力，藉由了解此現象未來我們可以設計其他的表面電漿產生結構(例如金屬孔洞排列結構)，將現在的一維金屬格柵延伸至二維空間中，成為音波偵測的陣列。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