Polarization-insensitive unidirectional meta-retroreflector.

• Metasurface-based retroreflectors have been demonstrated with great attractions and application values. • The concept of high-order EP in 2D scattering channel is firstly presented. • The unidirectional retroreflector based on non-Hermitian physics further provide a new perspective for angular-asymmetric photonic devices for wave manipulation. • Polarization-insensitive retroreflector based on isotropic structure can be realized. Extraordinary angularly asymmetric electromagnetic manipulation is desirable in angle-encoded steganography, directional display, one-side sensing and other unconventional applications, which however is challenging. Here, we build the non-Hermitian surface scattering systems by silicon nano-posts with spatially modulated loss, showing the extraordinary angular-asymmetry in the near infrared regime under different polarized light incidence. Especially a polarization-insensitive unidirectional retroreflector with non-Hermitian metasurface is first demonstrated at a fourth-order exceptional point (EP), related to the collapsed EPs of a generalized 4 × 4 non-Hermitian scattering matrix under TM and TE polarization states. Our investigations provide a new perspective for designing polarization-insensitive asymmetric photonic devices for wave manipulation and applying more generalized non-Hermitian physics to surface scattering systems. [ABSTRACT FROM AUTHOR]