Unidirectional reflectionlessness of passive PT-symmetric organic thin-film waveguides.
(A) Waveguide composition calculated to support the fundamental leaky TE mode together with its corresponding position-dependent complex effective index neff+ikeff displayed in the top panel. (B) Cross-sectional SEM images of a typical CuPc-coated photoresist grating before (top) and after (bottom) planarization with the top photoresist layer. Faint contrast between the upper and lower photoresist layers remains visible in the bottom panel, confirming that the internal grating structure is maintained. (C) Diagram showing the optical measurement setup, where a laser beam is Kretschmann coupled into the leaky TE0 mode supported by the composite waveguide layer. The intensity of the 0 order reflected and −1 order diffracted beam is measured as a function of incidence angle for forward and backward illumination corresponding to light incident on the left and right sides of the prism, respectively. (D) Dispersion diagram illustrating the process of unidirectional scattering within the complex index modulated waveguide that occurs at its EP. (E) Reflectivity and −1 order diffraction efficiencies measured as a function of incidence angle (in the glass substrate) for a bare waveguide with no CuPc. The diffracted intensities for forward and backward incidence peaks near the Littrow condition (θinc~56°), which is designed to coincide with resonant in-coupling to the waveguide mode marked by the reflectivity dip. (F) The same measurement carried out for a waveguide with CuPc (dPC=6 nm, θdep=55°) targeted to achieve δ≈1 .