GHz acousto-optic angular momentum with tunable topological charge

Abstract

Controlling the symmetry of optical and mechanical waves is pivotal to their full exploitation in technological applications and topology-linked fundamental physics experiments. Leveraging on the control of orbital angular momentum, we introduce here a device forming acoustic vortices which can impart an orbital angular momentum modulation at super-high-frequency on reflected light beams. Originated by shape-engineering of a single-contact bulk acoustic wave resonator, acoustic vortices are generated in a wide band of frequencies around 4 GHz with topological charge ranging from 1 to beyond 13 tunable by the device geometry and/or excitation frequency. With all electrical control and on-chip integration our device offers compact solutions for angular-momentum-based light communication, three-dimensional particle manipulation, as well as alternative interaction schemes for optomechanical devices.

Fig. 1. Acousto-optical generation of chiral optical beams.

a Device concept. A properly engineered bulk acoustic wave resonator (BAWR) launches an acoustic vortex into the substrate and modulates in time the OAM of a reflected optical beam. The BAWR is engineered starting from a circular contact (with the expected acoustic field sketched in (b)) that is perturbed into a spiral (expected field sketched in (c)). d, e Experimental amplitude and phase map of a vortex with ℓ = 3 at 1.139 GHz. f, g Simulated maps for the same drive and device geometry extracted from full, 3D simulations.

Fig. 2. Electrical response.

a Simulated mechanical displacement (Δz, in log-scale) originating from a circular BAWR in a 2D, azimuthally symmetric cell. b Experimental electrical reflection (corresponding to the S₁₁ rf-scattering parameters) of the BAWR in a limited frequency range, highlighting the 13 MHz separated dips corresponding to substrate resonances. c Simulated and d experimental phase profile around the path γ_c of Fig. 1g, evaluated precisely at one of the substrate resonances in panel (b) (on-peak) and far from them (off-peak). The dots are experimental points, with the solid line representing an average. The phase excursion decreases at the resonance frequency in both simulation and experiment.

Fig. 3. Mechanical and optical angular momentum.

a, b Sketch of the moving boundary coupling mechanism, relying upon the phase shift of reflected waves. c Amplitude and phase polar maps of the acoustic vortex with ℓ = 1. d Amplitude and phase polar maps of a perfectly reflected planar optical wavefront, considering a maximum acoustic displacement of λ_o/4. e, f Gram matrices of the mechanical and optical bases considering the range of ℓ from 1 to 3.

Fig. 4. Electrically tunable vortex topological charge.

a Phase profile around a vortex with topological charge ℓ = 1 generated by driving the BAWR at 750 MHz. Experimental data points are the blue dots, with the red lines showing their moving mean value. The upper and lower left insets show the measured and simulated phase profiles. The dashed circle in the upper left inset shows the circulation path for the determination of the phase profile. b, c Corresponding data for vortecies with ℓ = 2 and ℓ = 3 excited at 927 MHz and 1150 MHz, respectively.

Fig. 5. High topological charge vortices.

Comparison of phase profiles for spiral BAWRs of different Ms, operated at the same frequency of 1158 MHz. a M = 1 spiral, b M = 2 spiral, c M = 3 spiral. All spiral shapes have been sketched in the upper-right corner of their respective plots. Experimental data points are the blue dots, with the red lines showing their moving mean value.