Reconfigurable on-chip vortex beam generation via acoustically stimulated Brillouin nonlinear optical radiation

Abstract

The integrated devices that generate structured optical fields with non-trivial orbital angular momentum (OAM) hold great potential for advanced optical applications, but are restricted to complex nanostructures and static functionalities. Here, we demonstrate a reconfigurable OAM beam generator from a simple microring resonator without requiring grating-like nanostructures. Our approach harnesses Brillouin interaction between confined phonon and optical modes, where the acoustic field is excited through microwave input. The phonon stimulate the conversion from a guided optical mode into a free-space vortex beam. Under the selection rule of the radiation, the OAM order of the emitted light is determined by the acousto-optic phase matching and is rapidly reconfigurable by simply tuning the microwave frequency. Furthermore, this all-microwave control scheme allows for the synthesis of arbitrary high-dimensional OAM superpositions by programming the amplitudes and phases of the driving fields. Analytical and numerical models predict a radiation efficiency over 25\% for experimentally feasible on-chip microcavities. This work introduces a novel paradigm for chip-to-free-space interfaces, replacing fixed nanophotonic structures with programmable acousto-optic interactions for versatile structured light generation.

Figures (4)

• Figure 1: Principle of Brillouin NOR and OAM beam radiation. (a) Radiation of OAM beams from a microring cavity supporting both photonic and phononic resonant modes. The phonons are excited by microwave using IDT through piezoelectric effect. $k_{1}(\omega)$ and $k_{2}(\Omega)$ are the wave vectors of the photonic and phononic modes in the waveguide. IDT: interdigital transducer. (b) Illustration of the wave vectors of the guided photons, guided phonons and free-space photons along the waveguide. (c) Selection rules of the radiation. Four types of radiation processes between one photonic mode and one phononic mode. SFR: sum-frequency radiation. DFR: difference-frequency radiation.
• Figure 2: Field distribution of OAM beam radiated from a microring cavity. (a) Real part of the radiation field at $z = 100µ m$ in the XY plane with a radius of $10µ m$ for OAM orders $-5,-3,0,3,5$, respectively. (b) The power density distribution in the $r$-space. $r$ is the distance from the ring center. (c) The power density distribution in the $k$-space. $\theta$ is the angle of the wave vector $k$ to z-axis. (d) Relationship between the angle $\theta_{\mathrm{max}}$ with maximal intensity and OAM order $\widetilde{M}$. (e) Relationship between the OAM order $\widetilde{M}$ and the phonon frequency for $R =$ 30, 50, and 70$\,µ m$. The dashed lines show the frequency range of the phonon mode restricted by Eq. (\ref{['eq:wavevector']}), which bounds the maximum OAM order for different ring radius $R$.
• Figure 3: Radiation efficiency of OAM beam. (a) Relationship between the radiation efficiency and the quality factors of resonant modes in a $50\,\mathrm{\mu m}$-radius microring. The ring is fixed at critical coupling for both photon and phonon modes. The input powers are $P_{\mathrm{phonon}} = 100\,\mathrm{\mu W}$ and $P_{\mathrm{photon}} = 1\,\mathrm{mW}$, respectively. Here $\widetilde{M}=0$ is assumed. (b) The intensity-normalized nonlinear radiation rate to the air of different OAM orders for microcavities of various radii. There is a trade-off between the maximal OAM order and radiation efficiency.
• Figure 4: Universal synthesis of OAM superpositions. A single photonic WGM is excited with multi-tone optical field, while a group of phononic WGMs are excited via corresponding multi-tone microwave drive. A narrow-band spectral filter isolates the desired frequency-degenerate OAM superposition.