  Gravitational microlensing has become a mature technique for discovering
small gravitational lenses in the Universe which are otherwise beyond our
detection limits. Similarly, plasma microlensing can help us explore cosmic
plasma lenses. Both pulsar scintillation and extreme scattering events of
compact radio sources suggest the existence of ~ AU scale plasma lenses in the
ionized intersteller medium (IISM), whose astrophysical correspondence remains
a mystery. We propose that plasma microlensing events by these plasma lenses
recorded in the form of wide-band dynamic spectra are a powerful probe of their
nature. Using the recently developed Picard-Lefschetz integrator for the
Kirchhoff-Fresnel integral, we simulate such dynamic spectra for a
well-motivated family of single-variable plasma lenses. We demonstrate that the
size, strength, and shape of the plasma lens can be measured from the location
of the cusp point and the shape of spectral caustics respectively, with a
combination of distances and the effective velocity known a priori or measured
from the widths of the interference pattern. Future wide-band observations of
pulsars, whose plasma microlensing events may be predictable from parabolic arc
monitoring, are the most promising ground to apply our results for a deeper
insight into the fine structures in the IISM.
