奈米非晶矽之光熱效應致光學雙穩態及其超解析技術應用

Abstract

近年來，有許多研究投入光學的雙穩態系統，以應用於積體光學元件的開發。挾帶奈米結構的製造優勢以及高折射率，以矽為材料的波導搭配矽本身光學非線性特質的光學雙穩態系統具有製造技術成熟的優勢，成為目前市面上積體光學常見的元件。由波導組成的膜態與足夠大的非線性效應是組成光學雙穩態的必要條件，然而矽材料本身克爾效應的非線性特質並不明顯，約 10-9 µm2/mW。因此其光學雙穩態系統傳統上利用高品質因子的波導，品質因子甚至可高至數百萬，用以增幅元件的非線性特質，進而形成雙穩態。但是高品質因子波導之邊長大小至少有微米層級，並不容易縮減到奈米層級，使得元件大小成為矽光學雙穩態系統的一大劣勢。不過在我們先前的研究中，發現了矽的奈米方塊邊長約 100 nm且品質因子低於十，卻能透過光熱效應的升溫回饋改變自身的光學性質，累積出巨大的等效非線性達 10-1 µm2/mW。這篇研究利用此巨大非線性擺脫了傳統對高品質因子波導的仰賴，同時運用在超線性提升解析度的影像技術。更進一步透過穩態間急劇的轉換，產生的散射強度相依性相當於 10 次方函數，此非線性應用至奈米結構的超解析影像技術，將雷射掃描顯微系統的解析度提升了 3 倍。

Optical bistability plays an indispensable role in the field of photonic integrated circuits. With the advantage of mature manufacturing skills in nanostructure and high refractive index, silicon waveguides with optical nonlinearity are promising optical bistable devices. The prerequisite condition for optical bistability is a resonant cavity plus enough nonlinear feedback. Conventionally, because the intrinsic nonlinearity of silicon is as low as 10-9 µm2/mW via the Kerr effect, silicon-based bistable devices require a high-Q cavity, such as ring resonator or photonic crystal cavity, whose Q-factor is as high as million, but the size becomes difficult to decrease to the nanometer scale. Recently, we found that silicon nanoblock Mie resonators show a giant nonlinearity (10-1 µm2/mW) enhanced by photo-thermo-optical reactions. In this work, we take advantage of the giant nonlinearity of nano-silicon to produce optical bistability in a single silicon Mie resonator, whose size is 100 nm and whose Q-factor is less than 10. Both size and Q-factor are record-low in the field of optical bistability. Moreover, we propose a novel application of optical bistability toward enhancing microscopic resolution. It is known that super-linear power dependency offers point spread function reduction of laser scanning microscopy. The higher the super-linearity order, the better the resolution enhancement. Here we demonstrate that the nano-silicon bistability state transition provides a super-linear jump whose slope is equivalent to the 10th order of nonlinear response. The unprecedentedly large nonlinearity leads to a 3-fold resolution enhancement in a densely packed periodic silicon nanocuboid array. This research suggests the potential of optical signal modulation in a nanoscale device and sets a benchmark for label-free silicon nanostructure inspection.