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

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 dv0/dV for the fixed EO coefficient, waveguide length, and cavity Q -and (ii) at high DC strains it exhibits a nonvolatile (NV) cavity tuning Av 0 , 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 dv0/dV = 177 f 1 MHz/V, corresponding to Av0 = 40 f 0.32 GHz for a voltage spanning f 120 V with an energy consumption of delta U /Av 0 = 0.17 nW/GHz. The modulation bandwidth is flat up to omega BW , 3 dB / 2 pi = 3.2 f 0.07 MHz for broadband DC-AC and 142 f 17 MHz 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 f 0.25 GHz, 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. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement