The strong optical anisotropy of
hyperbolic metamaterials has enabled
remarkable optical behavior such as negative refraction, enhancement
of the photonic density of states, anomalous scaling of resonators,
and super-resolution imaging. Resonators fashioned from these optical
metamaterials support the confinement of light to dimensions much
smaller than the diffraction limit. These ultrasmall resonators can
be used to increase light–matter interactions for new applications
in photonics. Here, we present subdiffraction mid-infrared resonators
based on all-semiconductor hyperbolic metamaterials. Importantly,
these resonators are fully compatible with epitaxial growth techniques
and can be engineered to incorporate quantum well intersubband transitions
that are degenerate with the mode of the resonators, enabling an entirely
new generation of quantum optoelectronic devices. The strongest optical
confinement achieved is λ/33 for a free-space wavelength of
10 μm, and the measured Q-factors are in the
range of 14–17. The dispersion of the resonance mode is presented
through both experimental data and numerical solutions, and greater
than 10% tuning of the resonance frequency (106 cm–1) is demonstrated. Radiation patterns and radiative Q-factors are also mapped out using experimental results. Finally,
the resonator structures are investigated with finite element simulations
and the field profile indicates the presence of a strong vertical
polarization, which is essential for coupling to intersubband transitions
in quantum well structures. These extreme subdiffraction resonators
could be useful for engineering novel light-matter interactions and
devices in the mid-infrared.