Dynamically-tunable nonreciprocal epsilon-near-zero photonic platform enabled by current-biased Dirac semimetals

Engineering the nonreciprocal reflection of light enables the existence of reflectors, absorbers, and emitters whose performance is not limited by constraints such as Kirchhoff’s law of radiation. However, the deployment of on-chip integrated nonreciprocal photonics is hindered by a lack of materials with desirable electromagnetic properties or, when suitable materials are available, a lack of designs capable of multi-spectral, broadband, or wavelength-tunable operation. To address these challenges, we show that DC current bias can induce dynamically-tunable nonreciprocity in three-dimensional (3D) Dirac semimetals via a mechanism known as plasmon Fizeau drag. We illustrate the unique performance advantage of this dynamic tuning mechanism by designing a flat, subwavelength-thin infrared absorber using the Dirac semimetal Cd3As2 as an active material, whose dielectric tensor can be modulated in situ via direct current (DC) bias. We present a closed-form expression for the nonreciprocal and nonlocal Cd3As2 permittivity tensor under plasmon Fizeau drag. Then, we use this material model to create a sub-wavelength thermal absorber operational at an infrared wavelength of about 20 µm with 100% peak absorptance, a high nonreciprocal absorptance contrast of ∼87%, and an in situ dynamic tunability range of about 2 𝜇m in wavelength. The performance can be attributed in part to coupling to Berreman modes, which become nonreciprocal due to the shifting of the epsilon-near-zero (ENZ) point as a function of angle of incidence (that is, lateral momentum of light). The presented model can be used to simulate other 3D Dirac and Weyl materials under DC current bias and paves the way for the development of dynamically tunable nonreciprocal ENZ materials and devices for broadband photonic applications.