  Acousto-optic interactions involving propagating phonons can break the
time-reversal and frequency-modulation symmetry of light. However, conventional
acousto-optic modulators based on bulk materials have frequency bandwidth
limited to hundreds of megahertz due to their large structural sizes. Here, we
experimentally demonstrate gigahertz single-sideband acousto-optic modulation
on an etchless lithium niobate integrated platform by using photonic bound
states in the continuum. The upper- or lower-sideband modulation of light can
be obtained conveniently by choosing specific combinations of input and output
channels. Under this scheme, we have realized a 3-GHz frequency shifter, which
operates in the C-band with a 3-dB bandwidth of ~35 nm. The extinction ratios
of the upper(lower)-sideband modulated light to the lower(upper)-sideband
modulated and unmodulated light are >44 (47) and 25 (23) dB in the 3-dB
operating bandwidth. The frequency-shifted light can be further processed with
amplitude and frequency modulation. Therefore, the demonstrated gigahertz
single-sideband acousto-optic modulation can enable many photonic applications
such as optical signal processing, sensing, and ion trapping.
