Theoretical investigation of surface plasmonic polariton-based electro-optical modulator with low polarization dependence

Overview: Photonic integratedcircuits (PICs) have made remarkable progress in the past few decades with thedevelopment of applications in the fields of optical communication, sensing,and imaging. Si photonics is deemed to be a promising solution for futurehigh-speed on/off-chip optical interconnections. Typical Si waveguide modulatorsleverage electrically altering either the refractive or the absorptiveproperties of a material to modulate the transmission of light through adevice. Due to the weak plasma dispersion effect of Si and the diffractionlimit of the Si waveguides, the Si MZI modulators suffer from large footprintsof ~103 μm²~104 μm². Plasmonics provides an approach tominiaturize optical devices beyond the diffraction limit. Alternatively, fullyCMOS-compatible plasmonic modulators using Si as an active material aredemonstrated. Relying on the manipulation of the transverse magnetic (TM) modeexcited on the metal-dielectric interface, most of the previous demonstrationsare designed to response only for specific polarization state. In this case, itwill lead to a high polarization dependent loss, when the polarizationsensitive modulator integrates to a fiber with random polarization state.Herein, we propose a plasmonic modulator utilizing a metal-oxide- indium tin oxide(ITO) wrap-around the silicon waveguide and investigate its optical modulationability for both the vertical and horizontal polarized guiding light by tuningelectro-absorption of ITO with the field-induced carrier injection. ITO wasreported to have much larger variation of its dielectric constant. Similar toSi based field-effect, MOS device where carrier accumulation is formed under anapplied voltage bias, carrier density (NITO) can be tuned at the ITO/dielectricinterface with an applied bias. When the real part of the permittivity of theITO material is tuned to near zero, at a certain NITO, which is referred as the“epsilon-near-zero” (ENZ) state, while it has the maximal absorption loss dueto the strong confinement of the guided mode. By simultaneously optimizing themode field confinement and the ITO carrier dispersion effect in these stacks,similar optical modulation performance can be achieved for both polarizations,which has not been reported in the previous work. In an optimized structure, aΔER (a difference between the extinction ratios of two polarizationmodes) about 0.005 dB/μm is demonstrated at ITO “ENZ”-state by simulation. Theenergy flux clearly shows the polarization-selective coupling between the polarizedguided modes in the feeding silicon waveguide and those in the combinedwaveguide. Coupling efficiency above 74% is obtained for both polarizations.The proposed plasmonic combined modulator has a potential application inguiding and processing of light from a fiber with a random polarization state.