Highly Birefringent, Low-Loss, and Near-Zero Flat Dispersion ENZ Based THz Photonic Crystal Fibers

Md Shahjalal Hossain,Md Selim Habib,Md Samiul Habib,Nguyen Hoang Hai,S M Abdur Razzak,Christos Markos

doi:10.1109/jphot.2020.2994108

Abstract

In this paper, we present and compare two experimentally feasible photonic crystal fiber (PCF) designs ( $\textit{Type-I}$ and $\textit{Type-II}$ ) which ensure near-zero flattened dispersion with ultra-high phase and group birefringence at THz frequencies. Both structures are based on a subset of a triangular array of circular air-holes, which define the cladding of the PCF and a central elliptical air-hole which breaks the symmetry of the structure, thus introducing high levels of birefringence. Additionally, we investigate the possibility of further enhancing the birefringence properties of $\textit{Type-II}$ structure by selectively filling the air-holes with Potassium Chloride (KCl) as strong Epsilon-Near-Zero (ENZ) material. Our investigation reveals that significant enhancement of birefringence can be achieved than its original counterpart with birefringence to be as high as 0.0627 at 6.2 THz and near-zero flat dispersion of $-$ 0.54 $\pm$ 0.04 ps/THz/cm over the frequency range of 6.2 $-$ 6.3 THz.

Highlights

Terahertz (THz) radiation is one of the significant parts of the electromagnetic spectrum, which bridges the two main technologically developed frequency bands, i.e., microwave and infrared frequencies and is roughly defined to lie between 0.1−10 THz
In Type-I structure, the photonic crystal fiber (PCF) constitutes of only air-holes and represents the conventional type of PCF, in which high birefringence is obtained by introducing geometrical asymmetry with single elliptical air-hole at the core region
The main guiding parameters, which define the performance of a PCF are phase and group birefringence, waveguide dispersion and effective material loss (EML) and are defined mathematically as [9], [11]

Summary

Introduction

Terahertz (THz) radiation is one of the significant parts of the electromagnetic spectrum, which bridges the two main technologically developed frequency bands, i.e., microwave and infrared frequencies and is roughly defined to lie between 0.1−10 THz. The major drawbacks of free-space propagation systems are that they severely affected by the unwanted absorption losses, and are difficult to integrate with other components towards monolithic devices. PCFs offer unique guiding properties, such as, endlessly single mode propagation, tailorable dispersion, high birefringence and good confinement ability [3]. Birefringence is one of the key factors in THz PCFs, and is highly desirable in many applications, e.g., in sensing, filtering, splitting, and polarization-based THz wave guiding [4]. Another important application of THz PCFs is in the data transmission purpose, where zero-flat dispersion is the primary requirements. The zero-flat dispersion prevents an optical signal from pulse spreading at the receiving end, reducing the high bit error rates [5]

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Conclusion