1 3 DOI 10.1007/s00340-014-5996-6 Appl. Phys. B THz mode‑coupling in photonic‑crystal–surface‑plasmon‑coupled waveguides Triranjita Srivastava · Ritwick Das · Punnag Padhy · Rajan Jha Received: 19 May 2014 / Accepted: 19 December 2014 © Springer-Verlag Berlin Heidelberg 2014 1 Introduction At optical frequencies, the plasmonic geometries compris- ing metals like gold and silver offer an attractive route to generate surface waves such as surface-plasmon polariton (SPP)-based waveguides [1]. This is primarily because the lower limit for plasmonic activity of metals depends on the plasma frequency which usually lies in ultraviolet (UV) or visible region of the electromagnetic (em) spectrum [2]. However, realizing plasmonic waveguides much below the plasma frequency of metals turns out to be a major bottle- neck, as extremely large imaginary component of the com- plex dielectric constant strongly prohibits penetration of em field in the metal thereby exciting only loosely bound surface modes [3]. Alternately, a few semiconductors such as GaAs [4] and InAs [5], having lower plasma frequency than metals have been used as a substitute for metals to excite the THz SPPs. Also, a few researches have shown that sub-wavelength aperture arrays based on semiconduc- tor–metal (Al) composites exhibit enhanced transmission at THz frequencies [6]. Such characteristics are expected to originate from localized surface-plasmon resonances. Recently, a ferroelectric polymer, namely polyvinylidene fluoride (PVDF), has been synthesized which gives rise to the possibility of realizing plasmon-like behavior at THz frequencies essentially due to their low plasma frequencies lying between 0.1 and 1.0 THz [7, 8]. Moreover, the usage of PVDF offers an opportunity to realize a fully polymeric device for THz applications. The excitation of SPPs at THz frequencies offers another critical challenge in terms of high-index lossless prisms. Although, silicon and complex metal–prism geometries [9] and semiconductor gratings [10] have been reported, there are other avenues which are yet to be investigated in detail. One alternate route would be to employ polymer-based Abstract We present a detailed design principle and propagation characteristics of a channel plasmon-photonic– crystal-coupled-waveguide (PPCCW) for realizing tera- hertz (THz) waveguides and waveguide-based devices. The avoided-crossing behavior exhibited by dispersion curves of the supermodes due to modal interference leads to a sharp change in group velocity close to the phase-matching wavelength. As a consequence, the group-velocity disper- sion (GVD) peaks to a maxima (~1.40 × 10 5 ps/km·μm) and dips to a minima (~−1.60 × 10 5 ps/km·μm), thus exhibiting extremely large GVD. The propagation length is obtained in the order of a few centimeters. The analysis is substantiated by the frequency dependence of mode field distribution and propagation loss suffered by the super- modes of PPCCW geometry. Also, the obtained result is in good agreement with FEM-based study. The proposed waveguide is aimed at facilitating enhanced performance THz-waveguide sensors and GVD compensation modules. T. Srivastava Department of Physics, Kalindi College, University of Delhi, New Delhi 110008, India R. Das School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar 751005, India P. Padhy School of Electrical Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar 751013, India R. Jha (*) Nanophotonics and Plasmonics Laboratory, School of Basic Sciences, Indian Institute Technology Bhubaneswar, Bhubaneswar 751007, India e-mail: rajaniitd@gmail.com; rjha@iitbbs.ac.in