雙曲超穎材料之研究進展

Abstract

雙曲線超穎材料（hyperbolic metamaterial，HMM）已經成為具有令人興奮的功能的新型材料，特別是對於光電設備。但是，它們獨特的功能在激光作用下的實現受到很大限制。另外，為了滿足下一代可穿戴設備對物聯網的需求，非常需要開發可附接到任意基板上的柔性HMM。鑑於以往報導的不足，本論文旨在解決現有的難題，嘗試提高HMM的功能性，並擴展HMM的潛在應用。本論文的主要突破如下： 雙曲超穎材料可以增強隨機雷射。 我們提供了將HMM與發光奈米結構集成在一起的首次嘗試，這可以在降低雷射閾值的情況下極大地增強隨機雷射作用。有趣的是，差分量子效率可以提高四倍以上。基於HMM激發的高階波向量模態可以極大地增加形成閉環的可能性，從而降低了矩陣中散射光子的傳播所消耗的能量，這一事實可以很好地解釋。另外，由於與發光奈米顆粒的隨機分佈的耦合，高階波向量模態的外耦合傳播到達遠場而不會被困在HMM內。執行從有限差分時域（finite-difference time-domain，FDTD）方法派生的電磁仿真以支持我們的解釋。在HMM的幫助下實現對雷射作用的強烈增強，為開發高性能光電元件（包括光電晶體管和許多其他固態照明系統）提供了一種有吸引力，非常簡單且有效的方案。此外，由於借助HMM結構增加了光吸收，因此我們展示的方法對於高效太陽能電池的應用也很有用。 雙曲超穎材料可以用作瞬態和柔性超穎材料。 瞬態技術被認為是其特定功能的最重要突破，它可以在特定時間實施然後完全溶解。具有用於負折射的高階波向量模態或具有高光子態密度（photonic density of states，PDOS）的HMM，可以有效地增強量子轉換效率，這些HMM代表了用於生成尚未實現的光電元件的新興關鍵要素之一。但是，尚未探索將HMM應用於瞬態技術。在這裡，我們展示了將瞬態技術與HMM（即瞬態HMM）集成在一起的首次嘗試，該過程由水溶性和生物相容性聚合物和金屬的多層組成。我們證明了我們新設計的瞬態HMM也可以具有高階波向量模態和高PDOS，這可以顯著增強覆蓋在HMM頂部的發光器。我們表明，這些瞬態HMM設備在5分鐘內浸入去離子水中後會失去功能。此外，當瞬態HMM與柔性基板集成在一起時，該元件在超過3000個彎曲循環中表現出出色的機械穩定性。我們預計，這裡開發的瞬態HMM可以用作通用平台，以提高瞬態技術的廣泛應用範圍，包括固態照明、光通信和可穿戴光電設備等。 奈米級核－殼雙曲結構可以增強隨機雷射作用。 等離子材料以其出色的定制發光，重塑能態密度（density of states，DOS）和聚焦次波長光而出現了多種功能。但是，受其傳播損耗和窄帶共振的限制，等離子體材料提供寬帶DOS來推進其應用是一個挑戰。在這裡，我們開發了一種新穎的奈米級核－殼雙曲結構，由於與等離激元基的純金屬奈米粒子相比，DOS更高且電子的集體振盪時間更長，因此在多殼奈米級複合材料內部具有顯著的耦合效應。隨後，在表面形成表面等離子體共振之巨大局部電磁波，引起明顯的外耦合效應。具體來說，奈米級核－殼雙曲線結構很好地限制了能量而不會衰減，從而減少了傳播損失，然後以前所未有的超低閾值（∼30 μJ/cm2）實現了前所未有的受激發射（染料分子的隨機激射作用）。此外，由於奈米級核－殼雙曲線結構的徑向對稱性，高波矢量模態的激發和誘導的附加DOS很容易獲得。我們認為，奈米級核－殼雙曲線結構為擴大基於等離子體的應用的發展鋪平了道路，例如：太陽能電池的高光電轉換效率、大功率提取發光二極管、寬光譜光電探測器、將發射器內部攜帶於球殼部分的定量螢光顯微鏡，以及生物發光成像系統用於人體體內和體外研究。 柔性和可捲曲雙曲超穎材料可以增強隨機雷射作用。 在保持其原始功能的同時，能夠適應尺寸減小的自由曲面的可滾動光子裝置是非常需要的。在光子元件中，動量空間中具有雙曲線色散的HMM擁有大型PDOS，事實證明，該PDOS可以促進光物質相互作用。但是，這些元件主要在剛性基板上開發，從而限制了它們的功能。在這裡，我們提出了在紙質基材上整合由聚合物和金屬多層組成的柔性和可捲曲HMM的首次嘗試。有趣的是，這種獨特的設計能夠展現出高PDOS和散射效率，從而增強了受激輻射並引起了明顯的雷射作用。柔性且可捲曲的HMM結構在曲率半徑為1 釐米的自由曲面上仍能很好地保持其功能，並且可以承受反覆彎曲而不會降低性能。與平坦表面相比，雷射作用的強度提高了3.5倍。我們預計，這種靈活且可捲曲的HMM結構可以用作靈活的光子技術的多樣化平台，例如發光元件、可穿戴光電和光通信。

Hyperbolic metamaterials (HMMs) have emerged as novel materials with exciting functionalities, especially for optoelectronic devices. However, the implementation of their unique features to laser action is rather limited. Additional, to meet requirement of the next generation of wearable devices for internet of things, the development of flexible HMMs attachable on arbitrary substrates is greatly desirable. Based on the shortcoming of the previous reports, in this Dissertation, we aim to resolve the existing difficulties, try to advance the functionality of HMMs, and extend the potential application of HMMs. The major achievements of this Dissertation are as follows. Enhanced random laser action by HMMs We provide the first attempt to integrate HMMs with light emitting nanostructures, which enables to strongly enhance random laser action with reduced lasing threshold. Interestingly, the differential quantum efficiency can be enhanced by more than four times. The underlying mechanism can be interpreted well based on the fact that the high-k modes excited by HMMs can greatly increase the possibility of forming close loops decreasing the energy consumption for the propagation of scattered photons in the matrix. In addition, out-coupled propagation of the high-k modes reaches to the far-field without being trapped inside the HMMs due to the coupling with the random distribution of light emitting nanoparticles also plays an important role. Electromagnetic simulations derived from the finite-difference time-domain (FDTD) method are executed to support our interpretation. Realizing strong enhancement of laser action assisted by HMMs provides an attractive, very simple and efficient scheme for the development of high performance optoelectronic devices, including phototransistors, and many other solid state lighting systems. Besides, because of increasing light absorption assisted by HMMs structure, our approach shown is also useful for the application of highly efficient solar cells. Transient and flexible HMMs Transient technology is deemed as a paramount breakthrough for its particular functionality that can be implemented at a specific time and then totally dissolved. HMMs with high wave-vector modes for negative refraction or with high photonic density of states (PDOS) to robustly enhance the quantum transformation efficiency represent one of the emerging key elements for generating not-yet realized optoelectronics devices. However, HMMs has not been explored for implementing in transient technology. Here we show the first attempt to integrate transient technology with HMMs (i.e., transient HMMs), composed of multilayers of water-soluble and bio-compatible polymer and metal. We demonstrate that our newly designed transient HMMs can also possess high-k modes and high PDOS, which enables to dramatically enhance the light emitter covered on top of HMMs. We show that these transient HMMs devices loss their functionalities after immersing into deionized water within 5 min. Moreover, when the transient HMMs are integrated with a flexible substrate, the device exhibits an excellent mechanical stability for more than 3000 bending cycles. We anticipate that the transient HMMs developed here can serve as a versatile platform to advance transient technology for a wide range of application, including solid state lighting, optical communication, and wearable optoelectronic devices, etc. Core-shell hyperbolic structure for enhancement of random laser action Plasmonic material has emerged with multi-functionalities for its remarkable tailoring light emission, reshaping density of states (DOS), and focusing subwavelength light. However, restricted by its propagation loss and narrowband resonance in nature, it is a challenge for plasmonic material to provide a broadband DOS to advance its application. Here, we develop a novel nanoscale core-shell hyperbolic structure that possesses a remarkable coupling effect inside the multi-shell nanoscale composite owing to a higher DOS and a longer time of collective oscillations of the electrons than the plasmonic-based pure-metal nanoparticles. Subsequently, a giant localized electromagnetic wave of surface plasmon resonance is formed at the surface, causing pronounced out-coupling effect. Specifically, the nanoscale core-shell hyperbolic structure confines the energy well without being decayed, reducing the propagation loss and then achieving an unprecedented stimulated emission (random lasing action by dye molecule) with a record ultralow threshold (∼30 μJ/cm2). Besides, owing to the radial symmetry of the nanoscale core-shell hyperbolic structure, the excitation of high wave-vector modes and induced additional DOS are easily accessible. We believe that the nanoscale core-shell hyperbolic structure paves a way to enlarge the development of plasmonic-based applications, such as high optoelectronic conversion efficiency of solar cells, great power extraction of light-emitting diodes, wide spectra photodetectors, carrying the emitter inside the core part as quantitative fluorescence microscopy and bioluminescence imaging system for in vivo and in vitro research on human body. Flexible and rollable HMMs for the enhancement of random laser action Rollable photonic devices that can adapt to freeform surfaces with reduced dimensions while maintaining their original functionalities are highly desirable. Among photonic devices, metamaterials with hyperbolic dispersion in momentum space, defined as HMM, possess a large PDOS that has been proven to boost light-matter interaction. However, these devices are mainly developed on rigid substrates, restricting their functionalities. Here we present the first attempt to integrate flexible and rollable HMMs consisting of polymer and metal multilayers on paper substrate. Quite interestingly, this unique design enables to exhibit high PDOS and scattering efficiency to enhance stimulated emission and induce pronounced laser action. The flexible and rollable HMM structure remains well its functionalities on freeform surfaces with curvature radius of 1 mm, and can withstand repeated bending without performance degradation. The intensity of laser action is enhanced by 3.5 times as compared to the flat surface. We anticipate that this flexible and rollable HMM structure can serve as a diverse platform for flexible photonic technologies, such as light-emitting devices, wearable optoelectronics, and optical communication.