次波長兆赫波光纖

Abstract

在電磁波頻譜中，兆赫波所位在的頻段是一個非常重要的區域，這個頻段所包含的頻率範圍大約從100GHz - 10THz，是介於微波與光波之間的過渡地帶。由於許多重要的物理現象都發生在這個頻段，兆赫波的相關研究近年來日益受到重視，各種產生及偵測兆赫波的方法也相繼被提出。然而由於大部份的物質對於兆赫波都有很強的吸收，在過去的兆赫波研究中大多都使用自由空間中的金屬反射鏡來改變兆赫波的傳播方向。利用這種方法所建構出來的系統很容易受到外界的干擾且在架設上十分沒有彈性，因此若是缺少一種可靠且低損耗的傳導方法，兆赫波系統在未來的實際應用上勢必會遭遇到很大的困難。有鑒於此，我們提出了一種次波長兆赫波光纖，可以有效地傳導兆赫波並且具有極低的衰減係數(~0.01cm-1)。 我們所設計的兆赫波光纖採用線徑比波長還小的聚乙烯細線或圓管作為核心(core)，並以空氣作為外殼(cladding)。由於線徑小於波長，這種光纖所能傳播的模態只有HE11，且大部份光場都會在空氣中傳播，可以大幅減低兆赫波在傳播過程中被核心介質吸收的程度。此外由於聚乙烯在兆赫波頻段的吸收常數較其他材質為小，因此被限制在核心的光場在傳播過程中也會損失較少的能量。採用上述的架構，我們成功地減少兆赫波在傳輸中的損耗。相信這樣子的兆赫波光纖能夠有效地縮減兆赫波系統的大小並降低系統複雜度，對未來的兆赫波生物影像及分子偵測等等應用的實現有很大的幫助。

In the electromagnetic spectrum, terahertz frequencies, which usually defined as those in the range of 100 GHz – 10 THz, form a significant region that connects the microwave and optical-wave bands. Since various important physical phenomena happened in this region, more and more attention had been paid to the investigations of terahertz science in recent years. Many different techniques regarding the generation and detection of terahertz wave were also proposed one after another. However, for the reason that most of the materials have a great absorption constant in the terahertz frequency, the propagation of terahertz wave is usually controlled by metallic reflectors in almost all the applications, which makes the terahertz systems not only vulnerable to environmental disturbance but also inflexible in the construction of application setups. Hence, there will be a lot of difficulties in the future applications of terahertz system for lack of a reliable and low-loss guiding method. In view of this, we proposed and demonstrated a terahertz subwavelength fiber with a very low attenuation constant (~0.01 cm-1) for guiding terahertz wave. In our design, the sub-wavelength fiber core is made of polyethylene wires or tubes that surrounded by the air serving as the cladding. Due to the sub-wavelength characteristic of the fiber core, the only sustained mode is HE11 which delivers most of the fields in the air cladding, which enormously reduces the degree of core absorption in the propagation. In addition, as polyethylene has a lower absorption constant in the terahertz region, the power delivered in the core will be attenuated less than the cores made of other materials. By adopting above structure, we successfully reduce the loss in terahertz wave guiding which can make the terahertz systems compact and less complex. We believe that it will be very helpful to the realization of terahertz applications such as fiber sensing, biomedical-imaging, and so on.