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 ~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.