垂直腔納米表面等離激元（SPP）中的共振模式形成物理機制

垂直腔面發射激光器(VCSEL)在光通信和傳感器等領域有着重要的應用，由于衍射極限的限制，光的VCSEL通常無法做到納米尺度，為了獲得納米尺度的垂直腔激光器，就需要利用表面等離激元(SPP)的垂直腔。

最近，俞大鵬教授團隊和張家森教授團隊繼續合作，由博士生朱新利等提出了SPP垂直納米腔，該腔為中空的長方體結構，五面為金屬銀，一面為空氣窗口作為輸出鏡。他們利用前期提出的基于電子束曝光的PMMA模闆剝離法[Xinli Zhu, Yang Zhang, Jiasen Zhang, Jun Xu, Yue Ma, Zhiyuan Li and Dapeng Yu, “Ultrafine and Smooth Full Metal Nanostructures for Plasmonics”, Advanced Materials 22(39), 4345-4349 (2010)]，成功地制備出三種SPP垂直納米腔，分别包含一個、兩個和三個空氣窗口。SPP在垂直腔中以金屬-介質-金屬（MIM）結構的對稱模式存在，該模式不會随着腔的寬度的減小而截止，因此可以獲得遠小于波長的寬度。利用寬度的改變還可以調控SPP的色散，獲得極小的模體積。他們利用掃描電子顯微鏡中安裝的陰極熒光光譜儀對SPP垂直納米腔進行了系統的研究，結合理論模拟，給出了腔中模式的形成機理，得到了SPP的色散關系。實驗中在70nm寬的腔中獲得的最小模體積達0.0014 m3。在垂直納米腔中利用調控SPP的色散可以獲得高的Q因子與模體積的比值，将在研究熒光粒子與局域電磁場的相互作用及納米激光器等方面有着重要的應用。該結果于2月1日發表在《納米快報》（Nano Letters, Xinli Zhu et al., http://pubs.acs.org/doi/abs/10.1021/nl104024j）的網絡版上。

該研究工作得到了國家自然科學基金委、科技部973計劃以及介觀物理國家重點實驗室自主科研項目等的大力資助。

Figure 1. Plasmonic vertical nanocavity. (a) Schematic of a single nanocavity. (b) 30°-tilted SEM image of a representative cavity with length, width, and height of 360, 120, and 500 nm, respectively.

Figure 2. Resonances and mode patterns of plasmonic nanocavities with 70 nm widths, 500 nm heights, and increasing lengths. (a) CL spectra of 16 nanocavities with cavity lengths increasing from 120 (bottom) to 770 nm (top). (b) Calculated spectra using the FDTD method. Spectra are offset vertically for clarity. The black arrow at 655 nm indicates that the spectra were collected twice at the same position due to the limited measuring range of the spectrophotometer. (c), (d) Calculated mode field intensity patterns and electric field amplitude of a 260 nm-long cavity at 635 nm in the y = -35 nm and x = 0 planes, respectively. (e), (f) Calculated mode patterns in the y = -35 nm plane of 420 and 610 nm-long cavities at 539 and 523 nm, respectively.

Figure 3. Experimental and calculated dispersions of plasmonic modes (1, 1), (1, 2), and (1, 3) in vertical nanocavities. The grey and black lines respectively represent the dispersions of light in a vacuum and SPPs on an infinite Ag film.

Plasmonic Vertical Resonant Nanocavities, Xinli Zhu, Jiasen Zhang,* Jun Xu, and Dapeng Yu,* Nano Letters, in press (2011).