Tuning SPP propagation length of hybrid plasmonic waveguide by manipulating evanescent field

Highlights

• Modal characteristics of plasmonic waveguide tuned by evanescent field disturbance.

• Propagation length could be tuned more than 100%.

• Mode area is independent to the propagation length tuning.

Abstract

Exploring hybrid plasmonic waveguide is a major breakthrough for achieving low-loss lightwave propagation at subwavelength size. Various types of passive hybrid plasmonic waveguides have been reported, but research on active hybrid plasmonic waveguides is very limited. In this study, we propose a tunable hybrid plasmonic waveguide in which its modal characteristics could be dynamically tuned by manipulating the evanescent field of a dielectric waveguide using a closely suspended tuning waveguide. The device can obtain a long-distance lightwave propagation at centimeter scale while maintaining the propagation mode at deep-subwavelength size (2.44${\lambda }^2$/10⁵). Using the proposed tuning method, the propagation length can be tuned more than 100% at the same propagation mode area. The proposed device with a propagation length that can be tuned in a wide range is a promising candidate for building active photonic components.

Introduction

Hybrid plasmonic waveguides (HPWs) have recently attracted substantial research interest for developing the next-generation and high-density integrated photonic circuits [1], [2], [3], [4], [5] owing to the long-distance plasmonic wave propagation at the sub-wavelength propagation mode size. Various types of HPW have been proposed for improving light confinement without large propagation loss, such as planar metal-gap–dielectric waveguides [6], dielectric wires set on a metallic surface [7], dielectric-loaded wedge waveguide structures [8], [9] or wedge-to-wedge waveguide structures [10]. The propagation length of a HPW at the optical communication wavelength, 1.$55\mathrm{\mu }\mathrm{m}$, has achieved hundreds of micrometers while the propagation mode is confined at ${\lambda }^2$/10⁴ [10]. Recent emerging research trends focus on developing the tunable plasmonic waveguides for the reconfigurable photonic circuit technology [11], [12], [13], [14]. In general, the modal characteristics of a HPW can be tuned by changing the gap (g) between the metal wedge and the dielectric waveguide using electromechanical actuation, by changing the refractive index (RI) of the gap medium, or by changing the RI of the dielectric waveguide. In the latter mechanism, the RI can be modulated by the thermo-optic (TO) or electro-optic (EO) effects. However, these mechanisms are usually not effective due to the weak strength of the EO and the TO effects in the available waveguide materials, especially in the high-RI material such as silicon. This issue leads to limit the tuning range of the devices. To increase the TO strength, hence expand the tuning range of modal characteristics, the dielectric waveguides can be made of low-RI materials such as active polymers [12]. However, these materials lead to a high power consumption, in the order of mW, and a slow response time. The tuning can also be done by microelectromechanical (MEMS) actuation. In this method, the power consumption is usually negligible due to the non-current actuation operation, however, in the HPW applications, the propagation mode area is expanded faster than the propagation length when compared to that of the two former mechanisms. The trade-off between the propagation length tuning and the variation of the effective mode area limits the application of MEMS tuning mechanism in the tunable HPWs. So far, the HPW based on the high-RI dielectric waveguide made of silicon is still a better choice because it offers the possibility to scale down the waveguide dimensions for the high-density integrated photonic circuits, and the well-established fabrication of the silicon nanotechnology.

In this work, we propose a HPW with tunable propagation length that resolves the limitations of the above mentioned methods. To dynamically control the propagation characteristics of this HPW, the effective refractive index of the high-refractive-index dielectric waveguide is modified by manipulating the evanescent field using an index-tuning waveguide suspended near the dielectric waveguide. The dependence of the modal characteristics on the key geometrical parameters and the displacements of the tuning waveguide are analyzed to show the effectiveness of the proposed tuning method.