Graphene-Based Mid-Infrared Emitters

Cost-effective and sustainable mid-infrared light sources are required for applications such as gas sensing and infrared beacons. Semiconductor LEDs are a natural replacement for the conventional incandescent sources still widely used in many applications, but to achieve emission at long wavelengths requires the realization of devices with narrow effective bandgaps, inherently leading to relatively poor internal and external quantum efficiencies. Recently, the technological potential of graphene-based incandescent emitters has been recognized, in part due to the ability of graphene to sustain extremely large current densities. Mid-infrared sources based on such emitters potentially offer an alternative, less complicated, approach to mid-infrared semiconductor LEDs [1], where the low thermal mass of graphene offers the potential for high frequency modulation [2]. Encapsulation of the emitting layer with hexagonal boron nitride allowed devices to run continuously in air for over 1000 hours [3], with the emission spectrum covering the absorption bands of many important gases. The h-BN encapsulation also allows the incorporation of a frequency selective surface metamaterial layer and we have previously shown that a metasurface consisting of ring resonators acts to tailor the broadband thermal emission into a dual band radiation, with measurements of both reflection and emission spectra agreeing well with simulations [4]. Here we describe a simple architecture, consisting of a back-reflector behind a multilayer graphene filament, which we use to produce emitters with wall-plug-efficiencies comparable to state-of-the art semiconductor cascade LEDs. Such high relative efficiencies demonstrate the feasibility of developing a graphene based mid-infrared light emitting device, which could be more cost effective and sustainable to manufacture than either silicon MEMs or compound semiconductor based alternatives. We show that the spectral characteristics of the emission could also be tailored for specific applications, such as gas sensing, through the incorporation of a metasurface.<br/><br/>[1] L. M. Lawton, N. H. Mahlmeister, I. J. Luxmoore, and G.R. Nash, “Prospective for graphene based thermal mid-infrared light emitting devices” AIP Adv. <b>4</b>, 087139 (2014).<br/>[2] N. H. Mahlmeister, L. M. Lawton, I. J. Luxmoore, and G. R. Nash, “Modulation characteristics of graphene-based thermal emitters”, Appl. Phys. Express <b>9</b>, 012105 (2016).<br/>[3] H. R. Barnard, E. Zossimova, N. H. Mahlmeister, L. M Lawton, I. J. Luxmoore, and G. R. Nash, “Boron Nitride Encapsulated Graphene Infrared Emitters”, Appl. Phys. Lett. <b>108</b>, 131110 (2016).<br/>[4] C. Shi, N. H. Mahlmeister, I. J. Luxmoore, and G. R. Nash, “Metamaterial Graphene Thermal Emitter”, Nano Res. <b>11</b>, 3567 (2018).