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Epsilon-Near-Zero Photonics Wires for Mid-Infrared Optical Lumped Circuitry

There has been recent interest in the development of optical analogues of lumped element circuitry, where optical elements act as effective optical inductors, capacitors, and resistors. Such optical circuitry requires the photonic equivalent of electrical wires, structures able carry optical frequency signals to and from the lumped circuit elements while simultaneously maintaining signal carrier wavelengths much larger than the size of the lumped elements. Here we demonstrate the design, fabrication, and characterization of hybrid metal/doped-semiconductor 'photonic wires' operating at optical frequencies with effective indices of propagation near-zero. Our samples are characterized by polarization and angle-dependent FTIR spectroscopy and modeled by finite element methods and rigorous coupled wave analysis. We demonstrate coupling to such photonic wires from free space, and show the effective wavelength of the excited mode to be approximately an order of magnitude larger than the free-space wavelength of our light. The operational length of the photonic wires approaches twice the free space wavelength, significantly longer than what is achievable with bulk epsilon near zero materials. The novel architecture utilized in our hybrid waveguides allows for significant design flexibility by control of the semiconductor material's optical properties and the sample geometry. In addition, by utilizing a semiconductor-based architecture, our photonic wires can be designed to monolithically integrate the optical equivalents of capacitive, inductive, and resistive lumped circuit elements, as well optoelectronic sources and detectors. As such, the demonstrated photonic wires have the potential to provide a key component, and a realistic framework, for the development of optical circuitry.