發展利用非金屬材料之THz波前與振幅調製技術

Abstract

太赫茲輻射具有眾多獨特的特性，包括光子能量低、對金屬的高反射性、對水表現出強烈的吸收，以及對大多數介電材料表現出極高的穿透性。這些特性賦予太赫茲輻射在多個研究領域中的優越性。太赫茲輻射可提供有關分子之間低頻震動模式、氣體的旋轉模式和晶格內聲子模式等分子訊息，進而使太赫茲的吸收頻譜能夠清晰地分析同分異構物的組成模式。因此，太赫茲輻射在各種研究和實際應用中得到廣泛應用。然而，由於太赫茲波段相關設備仍相對缺乏，因此低耗損且高效的元件變得尤為重要。為了解決這一問題，本研究針對波前和振幅的調製分別使用了不同的方法。在波前的調製方面，採用了三種主要方法，包括透鏡、螺旋相位板以及超穎介面。透鏡利用造鏡者公式並搭配3D列印技術，透過常見3D列印材料的改良，最後所選用的材料為光固化樹脂混合30%的Al2O3，成功地製造耗損較小的太赫茲元件並有效的降低製造成本，可應用於多種系統架設及應用。隨著未來6G通訊波段提供更高速度、更大容量和極低延遲的可能性，相關研究正積極進行。在這方面，具有軌道角動量的螺旋光束展現出相當大的應用潛力。螺旋光束的拓樸數可以是任意整數，且不同拓樸數的螺旋光束呈現正交性，適用於增加通訊通道。因此要能產生這種具有軌道角動量的螺旋光束就非常重要，本研究利用了兩種方式來實現，分別是螺旋相位板以及超穎介面。螺旋相位板利用3D列印方式製造，所選用的材料與製作透鏡時相同，形成可以隨著空間旋轉的螺旋階梯狀結構，進而產生相位隨著空間旋轉的環形光斑。而超穎介面則利用表面電漿共振及Pancharatnam-Berry (PB) phase的原理，設計出漏斗結構的超穎表面，達到空間上不同相位分佈的狀態，進而實現螺旋狀的波前。在振幅的調製方面，有兩種主要方法。首先，利用磁流體其優異的磁致特性進行調製。這種方式在施加弱磁場時，磁流體中的奈米粒子會排列成鏈狀分布。透過調整磁場大小，使得鏈狀結構的緊密程度發生變化，當鏈狀之間的間距與入射光達到共振時，即實現了振幅的調製效果。其次，受到磁流體和3D列印技術的啟發，提出了第二種方式。這種方法將鏈狀結構類比為一維光子晶體，類似光柵結構。由於太赫茲對於許多介電材料有著高穿透的特性，通過適當設計結構尺寸並搭配合適材料，使其能與入射光產生高品質因子的共振。因此這種方法可以成為一種設計簡單、製程單純的元件製作方式，並在振幅的調製方面發揮作用。總而言之，這項研究不僅探討了太赫茲調製器在波前和振幅方面的特性與設計，更成功地實際製作出符合成品來匹配模擬結果。這一系列有效的調製方法為太赫茲技術的發展和應用開啟了嶄新的前景。Terahertz (THz) radiation possesses numerous distinctive characteristics, including low photon energy, high reflectivity to metals, strong absorption in water, and exceptionally high penetration through most dielectric materials. These attributes confer superior advantages to THz radiation in various research domains. THz radiation offers valuable insights into low-frequency vibrational modes between molecules, rotational modes of gases, and phonon modes within crystal lattices, enabling clear analysis of composition patterns among isomeric substances through absorption spectra. Consequently, THz radiation finds widespread applications in diverse research and practical scenarios. However, due to the relative scarcity of THz band-related devices, components with low lossand high efficiency become particularly crucial. Addressing this challenge, our study employs distinct methods for modulation in both wavefront and amplitude.In the realm of wavefront modulation, three primary approaches are employed: lenses, spiral phase plates, and superluminal interfaces. Lenses, utilizing the Fresnel formula coupled with 3D printing technology, are crafted from a light-curing resin blended with 30% Al2O3. This material choice minimizes THz component losses and effectively reduces manufacturing costs, rendering it applicable to various system setups and applications. As the future unfolds with the promise of higher speeds, larger capacities, and extremely low latency in 6G communication bands, research in this area is actively progressing. In this context, vortex beams of light carrying orbital angular momentum exhibit significant application potential. The spectrum numbers of vortex beams can be arbitrary integers, and those with different spectrum numbers exhibit orthogonality, suitable for enhancing communication channels. Thus, the generation of these vortex beams with orbital angular momentum is crucial, and our study employs two methods to achieve this: spiral phase plates and metasurface. Spiral phase plates, manufactured through 3D printing using the same material as the lenses, form a helical step-like structure that rotates in space, generating a circular light spot with a phase that varies spatially. Metasurface, utilizing the principles of surface plasmon resonance and Pancharatnam-Berry (PB) phase, design funnel-shaped superluminal surfaces, achieving different spatial phase distributions and realizing spiral wavefronts.In amplitude modulation, two main methods are employed. Firstly, modulation using magnetic fluids takes advantage of their excellent magneto-optical characteristics. When subjected to a weak magnetic field, nanoparticles in the magnetic fluid align into chain-like distributions. Adjusting the magnetic field size changes the tightness of the chain-like structure. When the spacing between the chains resonates with incident light, amplitude modulation is achieved. Secondly, inspired by magnetic fluids and 3D printing technology, we propose another method. This approach analogizes the chain-like structure to a one-dimensional photonic crystal, similar to a grating structure. Because THz radiation has high penetration through many dielectric materials, resonance can be achieved by appropriately designing structure dimensions and using suitable materials. Therefore, this method can serve as a straightforward and efficient way to produce components, playing a role in amplitude modulation.In summary, our study not only delves into the characteristics of THz modulators in terms of wavefront and amplitude but also proposes a series of effective modulation methods, opening up new prospects for the development and application of THz technology.