大調幅全光調制之奈米材料

Abstract

在通訊系統中，調變深度是一個重要的參數，因為較高的調變深度能降低信號傳輸過程中發生位元錯誤的可能性。在數十微米等級大小的傳統光波導可以產生超過100%的調變深度。然而，當元件尺寸縮小至次微米尺度時(例如米氏震盪器)，可實現的調變深度通常會顯著下降，僅能達到50%左右。為了克服這一限制，本論文研究了兩種提升全光開關調變深度的可行策略：(1) 在奈米結構超材料中調控米氏震盪；(2) 整合奈米級厚度的相變材料。並透過時間解析激發-探測技術量測調變深度的研究之外，同時也系統性地調查弛豫時間。 在第一種策略中，非晶矽奈米方塊經週期性排列形成二維陣列，以產生電偶極”晶格”震盪。藉由精準調整方塊之間的週期，電偶極晶格震盪的特徵波長發生改變，還與磁偶極震盪產生耦合，導致反射受到抑制。再透過非晶矽的光能損耗特性，使得此奈米結構在特定波長下會吸收大部分入射光，而產生具有高品質因子的狹窄頻譜。當狹窄頻譜受到另一束光照射而產生熱調變時，會導致於頻譜位移而產生極大的調變深度。其理論峰值振幅與非耦合的情況相比有兩個數量級的增益，在實驗結果上也測得一個數量級增益。本論文也在實驗上觀察到大於100%的調頻深度以及數奈秒的弛豫時間。 第二種方法則是採用二氧化釩(具有金屬-絕緣體相變特質)作為介質。當加熱超過相變溫度，二氧化釩會發生可逆的結構與電子轉變，引發強烈的非線性光學反應，這個變化可以產生極大的調變深度。文獻指出，超快激發光也能在數皮秒內觸發相變，使二氧化釩有潛力成為全光開關。在大於室溫的條件下，該文獻也提及二氧化釩的三個特徵相位：絕緣、緩增長以及完全相變。在本研究中，透過溫度控制或脈衝光激發方式探討二氧化釩在不同相位的調頻深度與弛豫時間。在溫控操作下，於 68°C 時觀察到近 2% 的調變深度，較室溫提升了 5.5 倍。此外，在 75°C 至 85°C 之間發現了新的「混合」相，其特徵同時具有緩增長與完全相變的光學反應。另一方面，藉由極大能量的光進行激發(在完全相變相位)，可以產生最大調變深度(12.5%)。而對應到的弛豫時間可以從數十皮秒變化至幾奈秒。我們也發現絕對大的調變深度與快的響應速度無法同時追求。然而，我們引入了自定義的品質因子並進行分析後發現，在光激發驅動下，最佳操作區間位於慢增長相位，能提供快速響應與相對較大調變深度的平衡組合。 最後，我們利用 COMSOL軟體模擬二氧化釩奈米結構絕緣態與金屬態下的光學光譜並計算其調頻深度，以找出能支持極大調變深度全光開關的幾何設計。

In communication systems, modulation depth is a critical parameter because higher modulation depth reduces the likelihood of bit errors during signal transmission. Conventional optical waveguides with dimensions on the order of tens of micrometers can achieve modulation depths exceeding 100%. However, when device dimensions are reduced to the sub-micrometer scale, as in Mie resonators, the achievable modulation depths typically decrease significantly, often reaching only around 50%. To overcome this limitation, this study explores two practical strategies for all-optical switches to enhance their modulation depth: (1) engineering Mie resonances in nanostructured metamaterials and (2) incorporating a phase-transition material with nanometer-scale thickness. Both approaches were experimentally investigated using time-resolved pump–probe measurements. The relaxation times, which are crucial for determining the switching speed, were also systematically examined. In the first strategy, amorphous silicon nano-blocks were periodically arranged to form a two-dimensional array that supports additional lattice resonances determined by the periodicity. By precisely tuning the periodicity, the electric-dipole lattice resonance not only shifts spectrally but also couples with the magnetic-dipole resonance, resulting in strong suppression of reflection. Due to the moderate optical loss of amorphous silicon, the nanostructure absorbs most of the incident light at a specific wavelength, producing a narrower spectrum with a high-quality factor. When thermal modulation is activated under this matched-resonance condition by an additional light source, the dynamic spectral shift generates a colossal modulation depth. Comparing the matched and mismatched cases, the enhancement reaches two orders of magnitude in theoretical peak-amplitude calculations and one order of magnitude in experimental measurements. Notably, under the matched condition, a modulation depth greater than 100% with a relaxation process of only a few nanoseconds was observed experimentally. The second approach explored in this work employs vanadium dioxide (VO2) as the active medium, leveraging its well-known metal–insulator transition (MIT). When heated above the transition temperature, VO2 undergoes a reversible structural and electronic transformation that induces strong nonlinear optical responses and enables large modulation depths. Prior studies have shown that ultrafast photoexcitation can also trigger the MIT within a few picoseconds, making VO2 a promising platform for all-optical switching. At temperatures above room temperature, these works further identify three characteristic regimes of VO2: the insulating phase, the slow-growth phase, and the full transition phase. In this study, a 30-nm-thick VO₂ thin film was driven across different phases either by temperature control with a weak perturbative pulse or by direct ultrafast excitation at room temperature (25 oC). Under temperature-controlled operation, a modulation depth of nearly 2% was achieved at 68 oC, representing a 5.5-fold enhancement relative to room temperature. We additionally identified an intermediate “hybrid” phase between 75 and 85 oC, exhibiting optical signatures characteristic of both the slow-growth and full-transition regimes. Under ultrafast laser excitation, a maximum modulation depth of 12.5% was obtained in the full-MIT regime. The corresponding relaxation dynamics, ranging from tens of picoseconds to several nanoseconds across different operating conditions, reveal a clear trade-off between modulation depth and response time. To evaluate and optimize VO2 for high-performance optical modulation, we introduce a self-defined figure of merit that accounts for both modulation magnitude and speed. Our analysis indicates that the optimal operational regime lies within the slow-growth phase when driven by ultrafast photoexcitation, offering a balanced combination of fast response and relatively large modulation depth. Furthermore, we employed COMSOL simulations to investigate the modulation depths of VO2 nanostructures. The optical spectra of VO2 in its insulating and metallic states were calculated, and the resulting modulation depths were analyzed to identify geometric designs that support all optical switching with exceptionally large modulation depths.