Unidirectional coupling of surface plasmons with ultra-broadband and wide-angle efficiency: potential applications in sensing

摘要

A cascaded-grating coupling scheme for unidirectionally coupling and launching surface plasmons from free-space light with ultra-broadband and wide-angle efficiency is proposed and theoretically investigated, and the potential applications in sensing are discussed. The cascaded sub-gratings of different periods couple the incident light over a relatively wide wavelength and angular range into surface plasmon polaritons (SPPs). After traveling a short distance, the SPPs interact with the sensed medium, and then are coupled back into freely propagating light through the nanoslit in the metal film. By observing the changes of SPP-mediated light coupled out through the slit, broadband sensing is achieved. A quantitative theoretical model is developed for performance prediction and validated with aperiodic Fourier modal method. Proof-of-principle demonstrations show that the spectral full-width-half-maximum reaches 400 nm and meanwhile the angular width is up to 25°, both of which are almost four times compared to the reference single-periodic gratings. A wavelength sensitivity as high as 852 nm RIU-1 (RIU, refractive index units) and a relative intensity change up to ∼5 × 104% RIU-1 are obtained.