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Broadband wave plates made by plasmonic metamaterials
SCieNtifiC RePoRTS |
Broadband wave plates made by plasmonic metamaterials
Lin Chen
Xianmin Ke
Huijie Guo
Junhao Li
Xun Li
Lei Zhou
OPEN Published: xx xx xxxx Although metamaterials wave-plates have been demonstrated previously, many of them suffer from the issue of narrow bandwidth since they typically rely on resonance principles and thus exhibit inevitable frequency dispersions. Here, we show that the dispersion of spoof surface plasmon (SSP) mode supported by a fishbone structure can be freely modulated by varying the structural parameters. This motivates us to establish a general strategy of building broadband wave-plates by cascading two fishbone structures with different propagation constants of SSP modes. We derive a criterion under which the cross-polarization phase-difference across the whole device can maintain at a nearly constant value over a wide frequency band, with frequency dispersions in the two fishbone structures cancelled out. As an illustration, we design and fabricate an efficient microwave quarter-wave plate and experimentally characterize its excellent polarization-control performances over a broad frequency band (7-9.2 GHz). Our findings can stimulate making dispersion-controlled high-performance optical functional devices in different frequency domains.
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The ability to manipulate the polarization states of electromagnetic (EM) waves is of central importance in both
fundamental optics physics and photonics applications1. Compared with linear polarization, circular
polarization is particularly important in sensing biological structures with chiral geometry2 and chiral imaging3.
Conventionally, conversions between different types of polarizations are realized with wave plates made by
birefringent crystals with cross-polarization phase-difference determined by the thickness and birefringence of the
crystals. Unfortunately, such devices inherently exhibit narrow working bandwidth since the birefringent index
is highly frequency-dependent.
Metamaterials (MTMs), artificial materials constructed by subwavelength-sized building blocks, have
attracted intensive interests recently due to their strong abilities to control EM waves4?7. In particular, many
MTM-based wave plates were proposed and/or demonstrated, which exhibit many advantages over conventional
devices in terms of compactness, flexibility, and easy integration8?24. However, since the building blocks of MTMs
are typically some resonant structures, such devices again exhibit limitted working bandwidths, since the
inevitable frequency dispersions of MTMs make the cross-polarization phase-difference deviate quickly from the
desired values at frequencies out of the designed working frequency. While plenty of works have been proposed
to enlarge the working bandwidth of such MTM-based devices by cascading multiple resonant modes, many of
them are for reflection geometry where the reflection amplitude is typically near 100% and one only need to
control the dispersion of reflection phase10?15. For transmission geometry which is more useful in applications, one
can not directly use this scheme since the transmission phase is locked with the amplitude and their frequency
dispersions should be considered simultaneously22?29.
In this paper, we establish a general strategy to build high-performance broadband wave-plates based on
spoof surface plasmon (SSP) modes with phase dispersions well controlled. Previous study has demonstrated
that a single fishbone structure array supporting SSP modes fails to construct a broadband wave plate since the
cross-polarization phase difference could not be kept constant within a wide spectral band30,31. We first show
that the dispersions of SSP modes can be strongly modulated by the fishbone structural parameters30?35. Jointing
two fishbone structures exhibiting opposite signs of frequency dispersions to form a single device, we show that
the physical responses of such a device (e.g., cross-polarization phase difference and transmittance ratio) can be
nearly dispersionless over a broad frequency range if the structural parameters of two fishbone structures satisfy
certain conditions, thanks to the dispersion cancellation effect. As a proof-of-concept demonstration, we design
and fabricate a microwave quarter-wave plate and experimentally show that it exhibits excellent linear-to-circular
polarization conversion abilities over a wide frequency range (7?9.2 GHz). Our results, based on a general
dispersion-control strategy, can inspire making broadband transmission-mode optical devices with other
functionalities, such as half-wave plate, and in different frequency domains.
Working Principle
For light incident upon birefringent metamaterials, it will encounter different effective refraction index
coefficients for two orthogonal polarizations. Here we take arrays of fishbone structures supporting SSP modes as an
example to illustrate how to manipulate the phase dispersion to function as a broadband quart (...truncated)