Microwave Waveguide‐Type Hyperbolic Metamaterials

Hyperbolic metamaterials (HMs) supporting hyperbolic isofrequency curves (IFCs) provide unprecedented control on wave propagation and light‐matter interaction. However, in the microwave regime, ultrahigh wave vectors and customizable permittivity tensors cannot be supported simultaneously by current common HMs. Based on the waveguide principle, low‐loss waveguide‐type HMs (WHMs) are suggested as a new geometry format of microwave HMs, flexibly mixing positive and negative components in their effective permittivity tensors. Then, a deeply subwavelength WHM cavity, confirming the existence of giant wave vectors, is demonstrated, whose transverse size has a potential to be reduced to three orders smaller than the working free‐space wavelength. Such a WHM cavity is also demonstrated to follow the fascinating anomalous scaling law originating from the unique shape of hyperbolic IFCs. As a low‐loss platform, WHMs are predicted to produce distinctive physical phenomena based on HMs and provide novel functional devices at low frequencies.