Electro-optical modulation of light polarization in a nonlocal lithium niobate metasurface

TL;DR

This work addresses fast, low-dissipation polarization control in integrated photonics by implementing a LiNbO3 metasurface that leverages a TE-polarized quasi-bound state in the continuum near the telecom wavelength. The authors design a 1D grating of asymmetric LiNbO3 nanowires on SiO2 to host a high-Q quasi-BIC excited by TE polarization and use in-plane electrodes to actuate the EO coefficient r33, shifting the resonance and modulating polarization. They quantify static and dynamic polarization changes via Stokes parameters, obtaining about ΔS ≈ 0.05 and polarization-ellipse changes of approximately ±3° at Vpp = 10 V, with a retardance modulation Δδ ≈ 0.06 rad and a modulation bandwidth near 800 MHz. This demonstrates that high-Q resonant LiNbO3 metasurfaces can achieve fast, subwavelength polarization and phase modulation, enabling compact on-chip EO devices for advanced photonic modulation tasks.

Abstract

We report the experimental realization of a LiNbO3 metasurface for electro-optic modulation of light polarization in the telecommunication band. High-Q quasi-bound states in the continuum are emploied to enhance the modulation of amplitude and phase of an impinging beam by a driving electric field, leading to efficient polarization rotation and conversion. We quantified modulation effects under a CMOS-compatible bias at 1 MHz frequency, achieving a variation of 5% in the Stokes parameters and a variation of the polarization ellipse angles of about 3° for the transmitted light. These results demonstrate that dynamic polarization and phase modulation can be attained in a compact platform, highlighting the potential of high-Q resonant LiNbO3 metasurfaces for enhanced light-matter interaction in subwavelength electro-optic devices.

Figures (5)

• Figure 1: a) Illustration of a polarization modulator made by an $x$-cut lithium niobate (LiNbO3) on insulator grating. Any mixed polarization state (e.g., an imput beam polarized at 45 with respect to both the $x$ and $y$ axes) is dynamically tuned by an external bias $V(t)$, which modifies the relative phase $\delta(t)=\delta_\mathrm{TE}(t)-\delta_\mathrm{TM}$ between the transverse electric (TE) and magnetic (TM) components (blue and orange, respectively) of the electric field via the electro-optic effect. b) SEM micrograph of the investigated LiNbO3 grating, with out-of-plane crystallographic $x$-axis, periodicity ${p = 800\nm}$, and asymmetry ${(D_1-D_2)/(D_1+D_2) = 0.2}$. The blue and orange arrows identify the TE and TM electric field components of the excitations. c,d) COMSOL finite-element simulation of (c) the transmittance $|S_{21}| ^2$, where ${S}_{21}$ is the complex transmission coefficient, and (d) of the phase of $S_{21}$ for the two orthogonal excitations. e) Measured transmitted intensity through the metasurface for an impinging beam at normal incidence with the electric field polarized either TE or TM polarization, in blue and orange, respectively.
• Figure 2: Across-resonance static and dynamic characterization of the transmitted polarization for an impinging beam linearly polarized at 135 with respect to the LiNbO3z axis (see sketch on top). a)--c) Plot of the orthogonal polarization states in blue (orange), in the linear z (y), linear diagonal 45(135) and circular left (right) basis, respectively. The intensities in each panel are normalized to the total intensity $S_0=I_a+I_b$, where $I_{a,b}$ are the two measured projections in the corresponding basis. In grey, the corresponding normalized Stokes parameters $\overline{S}_1$, $\overline{S}_2$ and $\overline{S}_3$. d) -- f) Variation of the normalized Stokes parameters upon application of a sinusoidal driving voltage with peak-to-peak amplitude $V_\mathrm{pp} = 10V$ and frequency $f_\mathrm{mod} = 1MHz$.
• Figure 3: Dispersion (in grey) and modulation (in blue) of the a) ellipse rotation angle $\psi$ and b) of the ellipticity angle $\chi$ of a 135-polarized beam, depending on the laser wavelength. The sinusoidal modulating voltage is characterized by $V_\mathrm{pp}= 10V$ and $f_\mathrm{mod}=1MHz$.
• Figure 4: Spectra of a the retardance $\delta$, b the diattenuation magnitude $D$ and c the depolarization magnitude $\Delta$. The solid lines correspond to the average values obtained by calculating the Mueller matrices from four different polarization combinations, while the shaded areas are the corresponding standard deviations.
• Figure 5: Wavelength dependence of the retardance $\delta =\delta_\textsc{te}-\delta_\textsc{tm}$ (in gray) and of its variation $\Delta \delta=\delta(V_\mathrm{pp}/2)-\delta(0)$ (in blue) upon application of a sinusoidal driving voltage with peak-to-peak amplitude $V_{\mathrm{pp}}=10V$ and frequency $f_\mathrm{mod} = 1MHz$. The difference $\Delta\delta$ has been calculated by evaluating $\delta$ from the metasurface Mueller matrices at 0V and $V_\mathrm{pp}/2$. The solid lines are obtained by averaging the results of the decomposition algorithm applied to four different polarization combinations. The shaded areas are the corresponding standard deviations.