Mid-Infrared Nanofocusing Using Fragmented High-Order Transformation Optics

An approach to design a hyperbolic lenslet for mid-infrared nanofocusing based on fragmented high-order transformation optics (TO) (FHTO) method is proposed, which can be realized with binary realistic metal and dielectric nanolayers. The conceptual idea is achieved by discretizing the lenslet into N(N > 1) concentric fragments and perform high-order TO in each fragment with different values of azimuthal s parameter - an unconstrained parameter that defines the operation point of the nanofocusing device. To highlight the superior performance of using the FHTO in the nanofocusing versus the unitary (N=1) high-order TO (UHTO), both methods are adopted to design the hyperbolic lenslet. The numerical simulation results show that the FHTO (e.g., N\to \infty and N = 5 ) exhibits superior focusing performance compared with the UHTO. For example, the performance of the FHTO is better than that of the UHTO by similar to 2.5 (5.3) times in spectral resolution (power throughput). Remarkably, even the manufacturable lenslet designed using FHTO-approach with realizable number of metal-dielectric binary layers (five pairs) demonstrates superior performance than the UHTO. It is noteworthy to mention that the design of FHTO-based devices enabled by circular effective-medium theory (CEMT) provides a means to bridge the gap between theoretical design and practical realization of nanophotonics systems.