  Nanoscale light sources are being intensively investigated for their
potential to enable low-energy, high-density optical communication and sensing
systems. Both nano-light-emitting diodes (nanoLEDs) and nanolasers have been
considered, based on advanced nanophotonic concepts such as photonic crystals
and plasmonic structures, with dimensions well into the sub-micrometer domain.
With decreasing dimensions, light-matter interaction becomes stronger,
potentially leading to efficient and ultrafast radiative emission, both in the
spontaneous and stimulated regime. These features have created wide
expectations for the practical prospects of such nanoscale light sources, in
particular for optical interconnects. In this article we examine the limits to
the downscaling of LEDs and lasers, and ask ourselves which type of source is
most suited to ultralow-power optical communications. Based on simple physical
considerations on the scaling of spontaneous and stimulated emission rates for
semiconductor active regions at room temperature, we analyze the speed and
energy limits for nanoLEDs and nanolasers as a function of their size. The role
of spontaneous emission enhancement (Purcell effect) in practical nanophotonic
sources is also revisited. The main conclusion is that nanoLEDs reach a
fundamental energy/speed limit for data rates exceeding a few Gb/s, whereas
nanolasers with active dimensions in the range of few 100s nm may enable direct
modulation rates larger than 40 Gb/s at power levels adequate for
short-distance and low-energy optical interconnects.

Applied Physics;Optics