Dielectric slot-coupled half-Maxwell fisheye lens as octave-bandwidth beam expander for terahertz-range applications

We present a paradigm for integrated photonic devices based upon broadband slab-confined collimated beams that are launched with half-Maxwell fisheye lenses. Although it is challenging to match to the low-index focus of the lens whilst maintaining adequate field confinement for a well-defined point source, integrated dielectric slot waveguides prove highly suitable, yielding collimators of 90% efficiency and over one octave bandwidth. Terahertz technology will benefit from such broadband slab-confined beams to replace free-space optics, toward compact, mass-producible systems that do not require manual optical alignment. We present two prototype systems to demonstrate the versatility of this concept, namely a diagonally-set distributed Bragg reflector as frequency-division diplexer for terahertz communications, and an attenuated total reflection-based liquid sensor. Both are enabled by oblique in-slab reflections that are collected at a location other than the originating lens, which is not attainable using ordinary single-mode channel waveguides.