  Intensity modulators are an essential component in optics for controlling
free-space beams. Many applications require the intensity of a free-space beam
to be modulated at a single frequency, including wide-field lock-in detection
for sensitive measurements, mode-locking in lasers, and phase-shift
time-of-flight imaging (LiDAR). Here, we report a new type of single frequency
intensity modulator that we refer to as a longitudinal piezoelectric resonant
photoelastic modulator. The modulator consists of a thin lithium niobate wafer
coated with transparent surface electrodes. One of the fundamental acoustic
modes of the modulator is excited through the surface electrodes, confining an
acoustic standing wave to the electrode region. The modulator is placed between
optical polarizers; light propagating through the modulator and polarizers is
intensity modulated with a wide acceptance angle and record breaking modulation
efficiency in the megahertz frequency regime. As an illustration of the
potential of our approach, we show that the proposed modulator can be
integrated with a standard image sensor to effectively convert it into a
time-of-flight imaging system.
