Fig. 4. Optimization of acousto-optic impedance match.

a An illustration of transducers with thickened bus bars. The layer stack is otherwise identical to Fig. 1a. b An optical microscope image of the new transducer design with relevant geometric parameters denoted. Contrast and color has been modified to make the waveguides below the transducer more visible. c The ∣S₁₁∣² of these transducers. This subfigure can be directly compared to Fig. 2c to see a greater fraction of incident power being dissipated into the acoustic wave. d The modulation index for all acoustic modes present, showing a substantial performance increase when compared to Fig. 3b. Incident RF power, P_RF, and optical wavelength, λ, are fixed while measuring this spiral with C = 38 modulated segments. e Sweep of RF frequency, f_RF, over a node of the highest frequency acoustic mode while incident RF power, P_RF, is gradually increased after each frequency sweep. The coloring expresses the modulation efficiency. The contours indicate the modulation index, β, achieved. f. The carrier,  ± 1, and  ± 2 sidebands resulting in a β = 0.58π rad modulation index are displayed. The power in the  ± 1 sidebands are maximized near this modulation index. The measurement setup (Supplementary Note 5) manifests the sidebands 100 MHz from harmonics of the RF drive frequency, f_RF. The y-axis is normalized such that 0 dB corresponds to the total optical power present in all sidebands. Note that this is the same device as (d), but the RF power has been increased from 30 dBm to 34 dBm. g A measurement of modulation efficiency as a function of wavelength for the four acoustic modes. There are 1000 equally spaced data points for each mode. The RF drive frequency, f_RF, is fixed at the f_RF value denoted on the plot for the collection of these 1000 data points.