  Light fields with spatially varying backgrounds can modulate cosmic
preheating, and imprint the nonlinear effects of preheating dynamics at tiny
scales on large scale fluctuations. This provides us a unique probe into the
preheating era which we dub the "cosmic microscope.'' We identify a distinctive
effect of preheating on scalar perturbations that turns the Gaussian primordial
fluctuations of a light scalar field into square waves, like a diode. The
effect manifests itself as local non-Gaussianity. We present a model,
"modulated partial preheating," where this nonlinear effect is consistent with
current observations and can be reached by near future cosmic probes.
