High-Speed Low Voltage Electro-Optic (EO) Modulators Enabled by Transparent Conducting Materials

High speed electro-optic(EO) modulators with ultra-low driving voltages and compact sizes are of continuous great interest because they are the enabling technology for many important and novel application areas like defense, aerospace, telecommunications, RF photonics and on-chip optical interconnect. Current commercial EO modulators based on LiNbO3 have typical electrical switching power of 500mW (P = Vpi 2/50Ω, Vpi = 5V). Noting that the typical output power of semiconducter laser is 1mW∼10mW, this means a nearly 100 to 1 electrical to optical power conversion efficiency which is quite excessive and inefficient. A reduction of the electrical driving voltage by 10X to 0.5V region, or the driving voltage by 100X to 5mW region, means a 1 to 1 electrical to optical power conversion efficiency. To achieve this, previous works have focused on (1)Engineering organic EO materials at the molecular level, to enhance their EO responses by 10X--20X. (2) Minimizing the size of the optical waveguide to a only few hundred nanometers in thickness, so that the distance between the metal electrodes is 10X--15X smaller than conventional waveguide. However, the conventional device structure of the EO modulators requires two thick cladding layers separating the metal electrodes far away from the EO waveguide core, in order to avoid metal induced optical loss. Therefore a large portion of the driving voltage is applied on the thick cladding layers rather than on the EO waveguide core. If the modulator structure can be modified so that the electrical driving voltage is conducted from the metal electrodes directly to the EO waveguide core by transparent conducting (TC) materials based bridge electrodes, one can achieve additional 25X--225X reduction of electrical driving power (5X--15X in driving voltage). We have successfully demonstrated voltage-size figure of merit VpiL as low as 0.6V-cm using organic EO materials. A comprehensive modeling work show that the TC based EO modulator can achieve modulation bandwidth from 40GHz--80GHz for organic modulator and 15GHz--50GHz for GaAs based modulator while the Vpi can still be as low as 0.5V.