  Semiconductor lasers with ultra-low thresholds and minimal footprints are a
topic of active research. Such devices require a combination of high quality
factor laser cavities with small active region volumes, which drives the quest
for novel cavity geometries exploiting nano-optic concepts. For
high-reflectivity coated ridge lasers, where light is tightly confined in the
waveguide, a low threshold can only be achieved by strongly reducing the
diffraction losses arising at the laser facet. We show here that, somewhat
counter-intuitively, opening a carefully designed aperture in a metallic facet
coating can simultaneously enhance both its transmission and modal reflectivity
by correcting the phase front at the subwavelength scale. Numerical simulations
and experimental results demonstrate a reduction of optical mirror loss by up
to 40% while the transmission is increased by four orders of magnitude.
Applying this approach to both facets of a short cavity quantum cascade laser,
we achieve laser operation at room temperature with an electrical dissipation
of only 143 mW. Such light sources are especially suitable for portable and
battery-operated chemical agent sensing applications operating in the
mid-infrared wavelength range, where multiple greenhouse and pollutant gases
have their fundamental absorption lines. Our work suggests possibilities for
further applications including frequency comb dispersion engineering, and can
be implemented in a broad range of optoelectronic systems.
