Multiple p-n junction subwavelength gratings for transmission-mode electro-optic modulators

Abstract

We propose a free-space electro-optic transmission modulator based on multiple p-n-junction semiconductor subwavelength gratings. The proposed device operates with a high-Q guided-mode resonance undergoing electro-optic resonance shift due to direct electrical control. Using rigorous electrical and optical modeling methods, we theoretically demonstrate a modulation depth of 84%, on-state efficiency 85%, and on-off extinction ratio of 19 dB at 1,550 nm wavelength under electrical control signals within a favorably low bias voltage range from −4 V to +1 V. This functionality operates in the transmission mode and sustainable in the high-speed operation regime up to a 10-GHz-scale modulation bandwidth in principle. The theoretical performance prediction is remarkably advantageous over plasmonic tunable metasurfaces in the power-efficiency and absolute modulation-depth aspects. Therefore, further experimental study is of great interest for creating practical-level metasurface components in various application areas.

A Dynamically Modulated All‐Dielectric Metasurface Doublet for Directional Harmonic Generation and Manipulation in Transmission

“…A periodic array of subwavelength high‐index dielectric resonators can also support Fano resonances originating from the interference of dark and bright modes which are known for their sharp spectral features, narrow linewidth, and high Q factors yielding highly dispersive and steep phase spectra . The Fano resonances are generated as a result of excitation of guided mode resonances (GMRs) or quasi‐bound states in continuum (quasi‐BICs).…”

“…The Fano resonances are generated as a result of excitation of guided mode resonances (GMRs) or quasi‐bound states in continuum (quasi‐BICs). While the BICs are symmetry‐protected states and require oblique incidence or a broken in‐plane inversion symmetry in order to couple into radiation modes via transformation into quasi‐BICs, the GMRs can be excited under normal incidence in a symmetric lattice with large filling fractions and moderate subwavelength periodicities . From a design perspective, the use of quasi‐BICs or GMRs in high‐contrast metasurfaces holds a great promise to induce large phase shifts with limited spectral shift of the resonance and refractive index change.…”

“…One of the fundamental limitations associated with conventional all‐dielectric metasurfaces is their fixed optical response after fabrication which restricts their application scope. This has motivated investigation of several tuning approaches for all‐dielectric metasurfaces, including nonlinear effects, mechanical reconfiguration, phase‐change materials, thermo‐optical effects, and carrier‐induced electro‐optical effects . Among these tuning mechanisms, electro‐optical effects based on modulation of free carriers are of particular interest as they allow for robust and continuous tuning of the scattering from individual elements with short response time and low power consumption without suffering from crosstalk .…”

“…In this approach, operating at the Huygens' regime yields a limited dynamic phase span while operating at the critical coupling regime does not allow for a reflectionless operation and the wide phase tunability comes at the cost of a very low transmission amplitude . Recent studies have shown that modulation of free carriers at p‐n and p‐i‐n junctions can be used to directly induce refractive index changes in the constituent dielectric resonators of a metasurface . Although this tuning mechanism allows for a larger volume of interaction between free carrier effects with the fields distributed within the resonators, the limited carrier‐induced changes of refractive index at infrared wavelengths limit the spectral shift of resonances.…”

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

“…It should be noted that despite the high conversion efficiency and directionality afforded in the Huygens regime, it is strongly dependent on the incident angle and wavelength [73,74]. Higher conversion efficiencies may be maintained over the spectral and angular regime of operation using elements with more exotic geometries exhibiting multimodal directionality [75] or using multigate active elements [25,27,76] with cascaded time-modulated layers which can also potentially serve as isolators by engineering the bandgaps to realize an asymmetric dispersion [43,44].…”