Slow waves on long helices

Lauren E Barr,Euan Hendry,J Roy Sambles,Alastair P Hibbins,Gareth P Ward

doi:10.1038/s41598-022-05345-1

Abstract

Slowing light in a non-dispersive and controllable fashion opens the door to many new phenomena in photonics. As such, many schemes have been put forward to decrease the velocity of light, most of which are limited in bandwidth or incur high losses. In this paper we show that a long metallic helix supports a low-loss, broadband slow wave with a mode index that can be controlled via geometrical design. For one particular geometry, we characterise the dispersion of the mode, finding a relatively constant mode index of sim 45 between 10 and 30 GHz. We compare our experimental results to both a geometrical model and full numerical simulation to quantify and understand the limitations in bandwidth. We find that the bandwidth of the region of linear dispersion is associated with the degree of hybridisation between the fields of a helical mode that travels around the helical wire and an axial mode that disperses along the light line. Finally, we discuss approaches to broaden the frequency range of near-constant mode index: we find that placing a straight wire along the axis of the helix suppresses the interaction between the axial and high index modes supported by the helix, leading to both an increase in bandwidth and a more linear dispersion.

Highlights

Slowing light in a non-dispersive and controllable fashion opens the door to many new phenomena in photonics
In the 1960s and 1970s it was found that waves supported on structures with glide- and screw-symmetry demonstrated a dispersion that crossed at some Brillouin zone boundaries in reciprocal space without forming a band-gap, and dispersed almost linearly over a wide frequency r ange[20]
We find that the bandwidth of the near-constant mode index behaviour is constrained by the hybridisation of a high-index helical mode and an axial mode that disperses along the light line

Summary

Introduction

Slowing light in a non-dispersive and controllable fashion opens the door to many new phenomena in photonics. Solutions based on solid state systems are more practical, such as waveguides encased in negative-index m etamaterials[8,9], or high quality-factor resonances in photonic crystals[10] Such structured media can be used to control the direction and speed of propagation of electromagnetic waves, and there are many examples of photonic crystal w aveguides11–13, metamaterials[14], and metasurfaces[15,16] that slow the speed of a propagating wave. In the 1960s and 1970s it was found that waves supported on structures with glide- and screw-symmetry demonstrated a dispersion that crossed at some Brillouin zone boundaries in reciprocal space without forming a band-gap, and dispersed almost linearly over a wide frequency r ange[20] This analysis, based only on symmetry arguments, was supported by experimental results in the microwave range of the electromagnetic (EM) s pectrum[21] and more complete theoretical s tudies[22–24]. We find that placing a straight wire at the centre of the helix suppresses the interaction between the axial and high index modes supported by the helix, leading to both an increase in bandwidth and reduced frequency dispersion

Methods

Conclusion