  The possibility of making an object invisible for detectors has become a
topic of considerable interest over the past decades. Most of the studies so
far focused on reducing the visibility by reshaping the electromagnetic
scattering in the spatial domain. In fact, by manipulating the electromagnetic
scattering in the time domain, the visibility of an object can also be reduced.
Importantly, unlike previous studies on phase-switched screens and time-varying
metasurfaces, where the effect is narrow band due to the dispersive resonance,
for microwave frequency range, we introduce a broadband switchable metasurface
integrated with p-i-n diodes. The reflection phase of the metasurface can be
changed by approximately {\pi} over a fractional bandwidth of 76%. By
modulating the metasurface quasirandomly in the time domain, the incident
narrow-band signal is spread into a white-noiselike spectrum upon reflection,
creating a spectral camouflage. The broadband feature of the proposed
time-varying metasurface can provide practical insight for various
applications, including radar stealth and ultrawide-band wireless
communication.
