Ultra-Low Index-Contrast Polymeric Photonic Crystal Nanobeam Electro-Optic Modulator

A novel nanocavity-based polymer-on-insulator (POI) electro-optic modulator (EOM) is proposed. It consists of a polymeric photonic-crystal nanobeam cavity (PCNC) with ultra-low index-contrast ( n$_{\text{cavity}}$/n$_{bg}$ = 1.17). Based on three-dimensional (3D) finite-difference time-domain (FDTD) method, the PCNC design and optimization are investigated theoretically. A high quality-factor ( Q ) of 3.4 $\,\times\, 10^4$ and small mode-volume of 22.8 ${({\lambda }/n_{SEO}})^{3}$ can be achieved. In order to judge the efficiency of the cavity design for electro-optic (EO) modulation, the optical modes and the electric field distribution are computed using an electromagnetic finite-element solver. Benefiting from the fast and strong EO effect in polymers, the modulator shows an EO modulation efficiency of up to 16 pm/V, which is over an order of magnitude higher than that in lithium niobate (LN) PCNCs. Moreover, the device is only 80  $\mu$ m in length, leading to a voltage-length product $V_{\pi }$ × L = 0.05 V $\cdot$ cm, which is much smaller than those of Mach-Zehnder modulators. To the best of our knowledge, this is the first on-chip POI-based EOM that features both ultra-compact size and high modulation efficiency. Hence, it is potentially an ideal platform for applications in optical communications, electric-field sensing, and tunable photonic circuits.