Magnetoplasmonic enhancement of Faraday rotation in patterned graphene metasurfaces

Abstract

Faraday rotation is a fundamental property present in all non-reciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here we show experimentally that in specifically design metasurfaces, magneto-plasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide new physical insights and predictions on these phenomena. Finally, we demonstrate strong tuneability in these metasurfaces using electric and magnetic field biasing. PACS numbers:Graphene is considered a very promising material for non-reciprocal magneto-optical applications at microwave, terahertz and infrared frequencies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Two of the most common non-reciprocal devices are isolators and circulators, and both are realizable starting from a Faraday rotator [9,11,17,20]. Faraday rotation (FR) observed in uniform graphene typically exhibits a maximum at low frequency (< 1 THz), and is barely present at higher frequencies, apparently precluding applications above 3 THz. [3,4,12]. It was experimentally found that the magneto-optical response in transmission is enhanced at the plasmonic resonance frequency, in structures such as graphene dots [8], antidots [21], and ribbons [22]. The effect of magneto-plasmonic resonance on the FR are currently experimentally unexplored even if it was numerically demonstrated that such plasmonic structures should also induce a blue-shifting of the Faraday rotation maximum [17,23]. In continuous graphene, the impedance of the continuous monolayer is given for the two opposite circular polarisations by [5] :where ω is the photon frequency, τ the carriers' scattering time, σ DC = e 2 τ |E F |(π 2 ) −1 is the low-temperature lowfrequency limit of graphene's conductivity for no mag- * Electronic address: mtamagnone@seas.harvard.edu † Electronic address: Jean-Marie.Poumirol@unige.ch netic bias, and ω c = ev 2 f B|E F | −1 is the semi-classical cyclotron frequency. In such a system the maximum FR always appear at energies bellow the cyclotron resonance. To go even further, when considering the highly doped (and/or low magnetic field regime) where τ −1 is dominant over ω c , the Faraday rotation will peak at zero frequency and will not extend above a cutoff frequency given by ω = τ −1 . Explaining the experimentally observed strong reduction of FR above 3 THz [3,4,12].Trying to circumvent these limitations, in this paper we have studied both experimentally and numerically the behaviour of the FR in three different patterned structures: a periodic array of graphene square dots (GSD), a graphene square anti-dot lattice (GSA), and a hybrid metal-graphene patterned structure (HMG). We have confirmed for all structures the presence of nonreciprocal magneto-plasmons blue shifting the Faraday rotation above the cyclotron resonance and compared the relative merits of the different geometries.All the samples measured ...

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