  Subwavelength gratings play a fundamental and pivotal role in numerous
science and applications for wave manipulation, exhibiting distinctive features
such as filtering, phase manipulation, and anti-reflection. However,
conventional fabrication methods for ultrasmall periodic structures are
constrained by the fundamental optical diffraction limit, making it challenging
to produce subwavelength gratings for optics. Here, we demonstrate a novel
technique to build a reconfigurable subwavelength photorefractive grating (SPG)
in a thin-film lithium niobate on the platform of an optical microcavity. Such
SPGs are optically induced through the photorefractive effect and the
subwavelength features originate from the spatial phase modulations of the
pump's standing wave. The resulting SPGs lead to the mode splitting of two
counter-propagating modes inside the microcavity, exhibiting an
Electromagnetically Induced Transparency (EIT)-like transmission spectrum.
Moreover, the unique subwavelength characteristic of SPGs enables first-order
quasi-phase-matching for backward second-harmonic generation, a long-standing
problem in nonlinear optics. Also, free-space-to-chip vertical nonlinear
frequency conversion can be achieved in a similar manner. These results provide
a flexible approach towards fabricating subwavelength gratings, which holds
significant potential in various applications such as nonlinear frequency
conversion, optical communication, sensing, and quantum technologies.
