Magic-Angle Graphene for Highly Efficient Tunable 2D Photonic Surfaces and Waveguide-Coupled Modulators

Tuning the refractive index of optical materials via external stimuli (e.g. electric field, temperature, etc.) has broad applications from optical communications to display technologies. While conventional semiconductor quantum well modulators can achieve a fast and reversible optical tunability, the refractive index change is small (Δn, Δk &lt;&lt;0.1). Phase change materials can achieve large Δn&gt;1, yet the response is much slower due to the phase transition rate limitation. Tunable 2D photonic surfaces provide a unique opportunity in their atomic thickness and a large refractive index change (Δn, Δk &gt;1) comparable to phase change materials, while simultaneously offering a response time even faster than conventional semiconductors, enabling unique opportunities in tunable photonic devices. Magic angle graphene (twisted bilayer graphene with the twist angle around 1.1°) is famous for its topological insulator and unconventional superconductivity behavior. However, until now, few studies have been conducted on its applications in photonics to benefit from its unique band structures. <br/>In this research, we extract the optical conductivity and dielectric function of the magic angle graphene by tight-binding model and Kramers-Kronig relations. We then calculated the refractive index of the magic angle graphene at different Fermi levels upon electrical gating. Interestingly, while regular monolayer graphene changes from a semimetal (n~k) to a dielectric material (n&gt;&gt;k) upon gating at near infrared (NIR) spectral regime, magic angle graphene behaves in the opposite fashion in that its changes from a semimetal to a good metal (n&lt;&lt;k). This behavior gives magic angle graphene a unique application prospective in photonics.<br/>As a demonstration, we use finite element model to design magic-angle graphene based optical devices. We report 2D surface photonic modulators based on coupling between magic angle graphene and regular single-layer graphene to best utilize their opposite behavior upon electrical gating. This modulator can work both in reflective mode and transmission mode, and the former can be applied to efficient modulating retroreflectors for free-space optical communication and light detection and ranging (LiDAR) applications. With optimized parameters and dielectric materials in an optical slot-antenna coupled cavity (SAC), the broadband reflection difference in 1500-2000 nm wavelength range between the on and off states can reach 70%, with a low insertion loss of 0.7 dB and a high extinction ratio of 7 dB almost independent of incident angles. This performance far exceeds our previous design optimization of surface-incident modulator based on regular single layer graphene in a similar optical SAC structure, which only achieved an extinction ratio of 1.62 dB ^[1]. This device structure also enables a low driving voltage for free-space optical modulation, as opposed to their commercial bulk LiNbO₃ counterparts that require more than 100 V to drive [2].<br/>Our second design is the magic-angle graphene on silicon waveguide electroabsroption modulator. This waveguide could work at the wavelength around 1550 nm with an on-off ratio larger than 14 (&gt;11 dB extinction ratio) and transmission loss less than 0.013 dB. To increase the absorption difference, we compared slot waveguide vs. channel waveguide structures. According to the previous work on graphene coupled silicon waveguide electroabsroption modulator, our device can work in a similar infrared region, but our simulation results showed a larger modulation 0.22 dB than their results 0.1 dB.<br/> Finally, the device concept for such large refractive change 2D photonic system may also extend to other 2D materials and other electromagnetic wave range. Currently, tunability of flat metalenses and other optical components is of great interest for these 2D surface photonic systems. <br/> <br/>References<br/>[1] S Fu et al. ACS Photonics 6 (1), 50-58<br/>[2] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2729