All-Dielectric Resonant Cavity Electro-Optic Transduction Between Microwave and Telecom

TL;DR

All-dielectric resonant cavity electro-optic transduction addresses the need for efficient microwave–optical conversion for quantum information and sensing. The authors implement a bulk LiNbO3 platform that uses dielectric confinement to avoid metal electrodes and achieve triply resonant operation within a Fabry-Perot cavity. They report a single-photon EO coupling rate g0/2pi = 1.5 ± 0.3 Hz and cooperativity C = (1.7 ± 0.8) × 10^-2, with optical normal-mode splitting validating the coupling and agreement with simulations, indicating potential to reach the strong coupling and single-photon regime at room temperature. The all-dielectric approach offers higher optical power handling and lower noise, enabling precise sensing of microwave fields and a viable path to quantum transduction between microwave and telecom photons.

Abstract

We present a resonant electro-optic transducer for efficient conversion between microwave and telecom wavelength photons. Our platform employs a bulk lithium niobate crystal whose large dielectric constant creates wavelength-scale confinement of microwave photons. By incorporating this crystal within a high-finesse Fabry - Perot optical cavity, microwave photons couple to optical photons through the electro-optic effect. We demonstrate the ability to tune our system into triply resonant operation, where microwave photons, optical pump photons, and upconverted optical photons are simultaneously resonant with high quality factor electromagnetic modes of the system. The device achieves photon number conversion efficiency at the percent level, comparable to state-of-the-art devices at room temperature -- sufficient to resolve the thermal occupation of the microwave mode -- while avoiding the noise and loss associated with metal electrodes. These results establish our all-dielectric devices as a promising platform for high-precision sensing of optically detected microwave fields and as a viable route toward single-photon-level microwave - optical quantum transduction.

Figures (3)

• Figure 1: All-dielectric electro-optic resonator design. (a) Finite element simulation (COMSOL) of the electric field energy distribution of the $TM_{131}$ microwave dielectric resonator mode of a $4\times12\times8\ \mathrm{mm}$ LN slab. The black arrows indicate the direction of the electric field. The red line depicts the optical beam path, which passes through the region of maximum microwave field. Here, both the optical and microwave electric fields are along the crystal $z$ axis, where the electro-optic coefficient is largest. (b) Overlap of microwave and optical modes in a Fabry--Perot cavity half-filled with LN. The $z$ component of the microwave field along the laser path is shown in dark blue. The optical cavity consists of a curved mirror and a mirror coating on the back of the LN slab. A zoomed-in segment of the optical field near the air--dielectric interface is shown in red.
• Figure 2: Simplified schematic of the experimental set-up. The LN slab (green) is affixed to an aluminum ground plane (light gray), which is mounted on a 5-axis translation stage. An opposing ground plane reduces radiative losses of the microwave modes. Microwave signals from a VNA, and from a signal generator are introduced via independent loop antennas. The optical cavity, completed by a curved mirror (light blue), is pumped by a $1550\ \mathrm{nm}$ laser, and the reflected light is collected on a high speed photodetector (HS PD). The curved mirror is attached to a ring piezoelectric actuator (brown) to scan and lock the optical cavity. The transmitted light is collected on a photodetector (PD) as well to characterize the optical response. See Fig. S1 for a more detailed diagram of the experimental set-up.
• Figure 3: Tuning into triply resonant electro-optic transduction. (a) A stacked series of $S_{21}$ traces taken as the air gap of the optical cavity is coarsely lengthened. The vertical lines correspond to the microwave modes which remain at a constant frequency. The diagonal lines correspond to the difference frequency between the pump and an output mode ($\Delta_{op}$), which vary with cavity length. The hot-spots where a diagonal line intersects a vertical line correspond to triply resonant transduction. (b) Electric field energy distributions of three lowest order $TM$ microwave modes. The $TM_{111}$ and $TM_{131}$ modes have good spatial overlap with the optical mode, yielding strong transduction peaks. (c) $S_{21}$ traces for varying laser wavelengths while maintaining constant cavity length, to within the piezo actuator's scan range. The bold magenta trace corresponds to triply resonant conditions. The black dots represent $\Delta_{op}$ which tunes with laser wavelength. The $S_{21}$ transduction peak is maximized as $\Delta{op}$ nears $\omega_m/2\pi=9.44\ \mathrm{GHz}$ for the $TM_{131}$ mode.