  Active control of heat transfer at the nanoscale has great potentials in
thermal logic and energy conversion devices. In the present work, we
theoretically propose a radiative thermal switch (RTS) composed of a pair of
asymmetric black phosphorus (BP) gratings, with BP nanoribbons periodically
patterned in different directions. The simply mechanical rotation between the
gratings enables substantial modulation of near-field radiative heat transfer,
especially when combined with the use of non-identical parameters, i.e.,
filling factors and electron densities of BP. Among all the cases including
asymmetric BP gratings, symmetric BP gratings, and BP films, we find that the
asymmetric BP gratings possess the most excellent switching performance. The
optimized switching factors can be as high as 90% with the vacuum separation
d=50 nm and higher than 70% even in the far-field regime. The high-performance
switching is basically attributed to the rotatable-tunable matching degree
between the surface characteristics of the two asymmetric gratings. Moreover,
due to the twisting principle, the RTS can work at any temperature, which has
great advantage over the phase change materials-based RTS. The proposed
switching scheme has great significance for the applications in thermal
management and thermal circuits.
