電漿子的奈米結構和超材料系統，很有機會被應用在操控和增強次波長尺度的線性和非線性光學系統。在可見光到近紅外的範圍，銀金屬被視為擁有最好的表面電漿子光學特性。因為表面電漿子在銀裡面，比在所有其他的金屬中，擁有更低的能量損耗。然而，常規技術(真空濺鍍和真空蒸鍍)所製備的銀膜或銀結構，其金屬銀是由多種晶格所組成的。這種金屬的表面晶界以及表面粗糙度會對表面電漿子造成額外的散射損耗，以至限制它真正的效能。因此，發展超平坦、大面積的單晶銀是減少散射損耗的關鍵。這種單晶銀在製作奈米結構上，擁有極高的再現性，這種特性有機會實現表面電漿子集成奈米元件。在本研究中，我們利用化學合成的方法，在室溫中成長出釐米級的單晶銀板，這是在過去文獻中利用膠體溶液所合成出最大的銀板，也比過去最大的銀板要超過一至兩個數量級。這個銀板應用在低損耗的線性和非線性的表面電漿子系統中，擁有比多晶銀更好的光學特性。我們發現，這個銀板可以讓表面電漿子傳播的距離超過過去所有文獻的實驗紀錄，在紅光，傳播的距離超過100微米。這個傳播的距離，也超過用Johnson-Christy量測銀的介電常數所推算的傳播距離。過去，Johnson-Christy的銀參數被視為是損耗最低的介電常數。另外，聚焦離子蝕刻術在這個單晶銀板上製作納米結構時，奈米結構擁有很高的再現姓與可調性。我們利用這個特性，設計並製作出一個特殊的雙共振頻率的奈米溝槽陣列結構，可應用在二次諧波訊號的產生(SHG)。相對於使用粗糙表面的金屬膜或是top-down製程方式所製作的奈米天線陣列(多晶金屬)，使用單晶銀所產生的二次諧波訊號，擁有很高的空間均勻性(標準差為10%)。另外，使用單晶銀，可精準微調不同深度的奈米溝槽(100 nm到175 nm)，藉以大幅調控奈米溝槽的表面電漿子的共振能量(444 nm ~ 624 nm)，這使我們可以把結構輕易設計在感興趣的波長上，產生全可見光的二次諧波訊號。

Plasmonic nanostructures and metamaterials offer unique possibilities for manipulating and amplifying linear and nonlinear optical processes at subwavelength scales. At optical frequencies, silver (Ag) is the best plasmonic material in optical property owing to its lowest intrinsic loss among all metals. However, additional scattering losses originated from grain boundaries and surface roughness limit the performance of polycrystalline Ag plasmonic structures prepared by conventional techniques. Therefore, the development of ultrasmooth, macroscopic-sized Ag crystals exhibiting reduced scattering losses is critical to fully understand the ultimate performance of Ag as a plasmonic material and can also lead to cascaded and integrated plasmonic devices with reproducible characteristics. Here, we demonstrate the growth of single-crystalline Ag plates with millimeter lateral size—the largest colloidal Ag crystals ever reported—for low-loss linear and nonlinear plasmonic applications. Using these Ag crystals, we have achieved record-breaking surface plasmon polariton propagation lengths beyond 100 m in the red wavelength region. These lengths even exceed the predicted propagation lengths using the Johnson-Christy optical constants. Furthermore, these crystals allow the fabrication of highly tunable and reproducible plasmonic nanostructures by focused-ion-beam milling. We have designed and fabricated novel double resonant nanogroove arrays using these crystals for spatially uniform and spectrally tunable second-harmonic generation (SHG). In contrast to “hot”-spot-based nonlinear optical processes such as surface-enhanced Raman scattering (SERS) and SHG using either randomly roughened films or top-down fabricated plasmonic nanoantenna arrays, our approach can achieve nonlinear signal generation over a larger sample area with dramatically improved uniformity and controllability.

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