Contents lists available at ScienceDirect Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte Microstructured optical fibers for terahertz waveguiding regime by using an analytical field model Dinesh Kumar Sharma a,c, ⁎ , Anurag Sharma a , Saurabh Mani Tripathi b a Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b Department of Physics and Center for Lasers & Photonics, Indian Institute of Technology Kanpur, Kanpur 208016, India c Ajay Kumar Garg Engineering College, Adhyatmik Nagar, Ghaziabad 201009, India ARTICLE INFO Keywords: Terahertz radiations Microstructured optical fibers Second-order mode Effective index Variational method Analytical field model ABSTRACT Microstructured optical fibres (MOFs) are seen as novel optical waveguide for the potential applications in the terahertz (THz) band as they provide a flexible route towards THz waveguiding. Using the analytical field model (Sharma et al., 2014) developed for index-guiding MOFs with hexagonal lattice of circular air-holes in the photonic crystal cladding; we aim to study the propagation characteristics such as effective index, near and the far-field radiation patterns and its evolution from near-to-far-field domain, spot size, effective mode area, and the numerical aperture at the THz regime. Further, we present an analytical field expression for the next higher- order mode of the MOF for studying the modal properties at terahertz frequencies. Also, we investigate the mode cut-off conditions for identifying the single-mode operation range at THz frequencies. Emphasis is put on studying the coupling characteristics of MOF geometries for efficient mode coupling. Comparisons with available experimental and numerical simulation results, e.g., those based on the full-vector finite element method (FEM) and the finite-difference frequency-domain (FDFD) method have been included. 1. Introduction Terahertz (1 THz = 10 12 Hz) radiations or THz waves, frequently referred to as T-rays, fall in the electromagnetic spectrum located in between two domains of operation and bridges the gap between the microwave and the optical frequencies. In general, this radiation band ranges from 0.1 to 10 THz (or 0.4–40 meV), corresponding to the sub millimetre wavelength range [2–7]. Recently, this domain has been extended to 40–50 THz, so-called THz gap in the frequency domain [8]. Terahertz radiation band has strong potential for applications such as biomedical sensing, noninvasive imaging and spectroscopy, astronomy, label-free detection of proteins, and the pharmaceutical drugs [9–14]. Terahertz radiation, in particular THz time-domain spectroscopy, stands on the cusp of becoming a routine research tool used by scien- tists in the disparate fields [10,15]. Moreover, there has been increased interest in outstanding potential of terahertz detection for imaging of concealed weapons, explosive, chemical and the biological agents [11–16]. Apart from that, T-rays are extremely useful for non-invasive medical diagnostics, tissue imaging and also in the study and better understanding of the dynamics of complex natural biological systems [17–21]. T-rays can create images and transmit information in the same way that visible light create a photograph, radio waves transmit sound and X-rays view within the human body [11–14]. Terahertz radiations are not suitable for the long distance free space communication or the ground based atmospheric monitoring but they are extremely useful for tomography and the short distance communication [22–24]. Several THz radiation emitters such as the photoconductive switch [3], free electron lasers [7] and the semiconductor surfaces [4] have been reported. The non-resonant optical rectification of ultrashot laser pulses based on nonlinear dielectric crystals facilitated by the induced polarization in the electro-optic crystals such as LiNbO 3 , ZnTe, ZnSe and GaAs have been used for generating the broad-band THz radiation [8,25]. The frequency of T-rays is too high for electronics based op- eration while it is too low for dielectric-based wave guiding structures. With the need for a compact, reliable and flexible terahertz system for various applications, a low-loss THz waveguide is essential. Several innovative ideas have been made for low-loss guiding of terahertz ra- diations via dielectric waveguides and the metal dielectric hybrid wa- veguides [24,26]; moreover, a number of different fiber-based and the metal-based waveguides with various geometries have been reported such as metal tubes, parallel plate metal waveguide, sapphire fiber [3–7], polymer waveguide and the plastic ribbon waveguide [26–28]. In 2004, an interesting waveguide based on bare metal wire to guide THz pulses was presented by Wang and Mittleman [24]. http://dx.doi.org/10.1016/j.yofte.2017.09.025 Received 1 June 2017; Received in revised form 16 September 2017; Accepted 30 September 2017 ⁎ Corresponding author at: Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India. E-mail addresses: dk81.dineshkumar@gmail.com (D.K. Sharma), asharma@physics.iitd.ac.in (A. Sharma), smt@iitk.ac.in (S.M. Tripathi). Optical Fiber Technology 39 (2017) 55–69 1068-5200/ © 2017 Elsevier Inc. 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