Strain-concentration for fast, compact photonic modulation and non-volatile memory

Abstract

A critical figure of merit (FoM) for electro-optic (EO) modulators is the transmission change per voltage, dT/dV. Conventional approaches in wave-guided modulators maximize dT/dV via a high EO coefficient or longer light-material interaction lengths but are ultimately limited by material losses and nonlinearities. Optical and RF resonances improve dT/dV at the cost of spectral non-uniformity, especially for high-Q optical cavity resonances. Here, we introduce an EO modulator based on piezo-strain-concentration of a photonic crystal cavity to address both trade-offs: (i) it eliminates the trade-off between dT/dV and waveguide loss—i.e., enhancement of the resonance tuning efficiency dv c /dV for the fixed EO coefficient, waveguide length, and cavity Q—and (ii) at high DC strains it exhibits a non-volatile (NV) cavity tuning Δvc,NV for passive memory and programming of multiple devices into resonance despite fabrication variations. The device is fabricated on a scalable silicon nitride-on-aluminum nitride platform. We measure dv c /dV=177±1MHz/V, corresponding to Δv c =40±0.32GHz for a voltage spanning ±120V with an energy consumption of δU/Δv c =0.17nW/GHz. The modulation bandwidth is flat up to ωBW,3dB/2π=3.2±0.07MHz for broadband DC-AC and 142±17MHz for resonant operation near a 2.8 GHz mechanical resonance. Optical extinction up to 25 dB is obtained via Fano-type interference. Strain-induced beam-buckling modes are programmable under a “read-write” protocol with a continuous, repeatable tuning range of 5±0.25GHz, allowing for storage and retrieval, which we quantify with mutual information of 2.4 bits and a maximum non-volatile excursion of 8 GHz. Using a full piezo-optical finite-element-model (FEM) we identify key design principles for optimizing strain-based modulators and chart a path towards achieving performance comparable to lithium niobate-based modulators and the study of high strain physics on-chip.