Abstract
We present a paradigm for integrated photonic devices based on 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 while maintaining adequate field confinement for a close approximation of a point source, integrated dielectric slot waveguides prove highly suitable, yielding collimators of 90% efficiency and bandwidth greater than one octave. 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 a 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.
Highlights
A dielectric slab waveguide supports infinite in-plane propagating modes, and this presents complexities in comparison to single-mode channel waveguides, there are several advantages
It is well understood that periodic alternation of medium’s dielectric properties can produce frequency-dependent transmission and reflection.31,32. This concept may be implemented in a slab waveguide by means of an array of stripes of the effective medium, thereby realizing a distributed Bragg reflector (DBR) or dichroic beam splitter, which serves as an optical filter in the present work
We wish to remark that effective medium techniques are not the sole viable means to realize GRIN terahertz optics, as the dispersion of a parallel-plate metallic waveguide may serve to realize engineerable refractive index that is mediated by plate separation, and this has been demonstrated for Luneburg and Maxwell fisheye lenses
Summary
A dielectric slab waveguide supports infinite in-plane propagating modes, and this presents complexities in comparison to single-mode channel waveguides, there are several advantages. With in-slab beamforming techniques and integrated optics, this mode can propagate as a well-defined collimated beam.2 The fact that these beams are free to propagate in in-plane directions allows for diagonally set features to generate oblique reflections and thereby direct the scattered radiation away from its originating direction. We devise two practical use-cases: a distributed Bragg reflector (DBR)-based broadband frequency-division diplexer for terahertz communications applications and an attenuated total internal reflection (ATR)-based terahertz liquid sensor. Both of these prototypes leverage oblique reflections that are generated within the free-propagation region of the silicon slab, as examples of the great versatility that is offered by this integrated-optical paradigm
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