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 dv_c/dV for the fixed EO coefficient, waveguide length, and cavity Q—and (ii) at high DC strains it exhibits a nonvolatile (NV) cavity tuning 1v_cNV 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 ± 1 MHz/V, corresponding to 1v_c = 40 ± 0.32 GHz for a voltage spanning ± 120 V with an energy consumption of δU/1v_c = 0.17 nW/GHz. The modulation bandwidth is flat up to ω_{BW3 dB}/2π = 3.2 ± 0.07 MHz for broadband DC-AC and 142 ± 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 ± 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.