  Spintronic terahertz (THz) emitter provides the advantages such as apparently
broader spectrum, significantly lower cost, and more flexibility in compared
with the commercial THz emitters, and thus attracts great interests recently.
In past few years, efforts have been made in optimizing the material
composition and structure geometry, and the conversion efficiency has been
improved close to that of ZnTe crystal. One of the drawbacks of the current
designs is the rather limited laser absorption - more than 50% energy is wasted
and the conversion efficiency is thus limited. Here, we theoretically propose
and experimentally demonstrate a novel device that fully utilizes the laser
intensity and significantly improves the conversion efficiency. The device,
which consists of a metal-dielectric photonic crystal structure, utilizes the
interference between the multiple scattering waves to simultaneously suppress
the reflection and transmission of the laser, and to reshape the laser field
distributions. The experimentally detected laser absorption and THz generations
show one-to-one correspondence with the theoretical calculations. We achieve
the strongest THz pulse emission that presents a 1.7 times improvement compared
to the currently designed spintronic emitter. This work opens a new pathway to
improve the performance of spintronic THz emitter from the perspective of
optics.
