  Coherent nanoscale photon sources are of paramount importance to achieving
all-optical communication. Several nanolasers smaller than the diffraction
limit have been theoretically proposed and experimentally demonstrated using
plasmonic cavities to confine optical fields. Such compact cavities exhibit
large Purcell factors, thereby enhancing spontaneous emission, which feeds into
the lasing mode. However, most plasmonic nanolasers reported so far have
employed resonant nanostructures and therefore had the lasing restricted to the
proximity of the resonance wavelength. Here, we report on an approach based on
gold nanorod hyperbolic metamaterials for lasing. Hyperbolic metamaterials
provide broadband Purcell enhancement due to large photonic density of optical
states, while also supporting surface plasmon modes to deliver optical feedback
for lasing due to nonlocal effects in nanorod media. We experimentally
demonstrate the advantage of hyperbolic metamaterials in achieving lasing
action by its comparison with that obtained in a metamaterial with elliptic
dispersion. The conclusions from the experimental results are supported with
numerical simulations comparing the Purcell factors and surface plasmon modes
for the metamaterials with different dispersions. We show that although the
metamaterials of both types support lasing, emission with hyperbolic samples is
about twice as strong with 35% lower threshold vs. the elliptic ones. Hence,
hyperbolic metamaterials can serve as a convenient platform of choice for
nanoscale coherent photon sources in a broad wavelength range.
