  Recent observations of gravitational waves motivate investigations for the
existence of Primordial Black Holes (PBHs). We propose the observation of
gravitational microlensing of distant quasars for the range of infrared to the
submillimeter wavelengths by sub-lunar PBHs as lenses. The advantage of
observations in the longer wavelengths, comparable to the Schwarzschild radius
of the lens (i.e. $R_{\rm sch}\simeq \lambda$) is the detection of the wave
optics features of the gravitational microlensing. The observation of
diffraction pattern in the microlensing light curve of a quasar can break the
degeneracy between the lens parameters and determine directly the lens mass as
well as the distance of the lens from the observer. We estimate the wave optics
optical-depth, also calculate the rate of $\sim 0.1$ to $\sim 0.3$ event per
year per a quasar, assuming that hundred percent of dark matter is made of
sub-lunar PBHs. Also, we propose a long-term survey of quasars with the cadence
of almost one hour to few days to resolve the wave optics features of the light
curves to discover PBHs and determine the fraction of dark matter made of
sub-lunar PBHs as well as their mass function.
