Lasers are often considered to be highly directional light sources:
their beams are able to propagate over long distances without
substantial spreading. This, however, is not always the case.
Semiconductor lasers, the most commonly used among all lasers, suffer
from a large beam divergence. Such divergence is governed by the
principle of diffraction, which predicts bending and spreading of light
around small obstacles or apertures. Light beams endure strong
diffraction when emerging from the small light-emitting regions of
semiconductor lasers (the dimensions of which are comparable to the
laser wavelength). This leads to a beam divergence angle of tens of
degrees for most semiconductor lasers.
Laser beams with small divergence angles are important for many
applications such as free-space communication, remote sensing, and
pointing. High directionality is desirable for efficiently coupling
laser power into waveguides and optical fibers without the need for
lenses. Beam collimation is usually achieved using lenses or other bulky
optical devices that typically require meticulous alignment.
To create semiconductor lasers with highly directional output, the
researchers incorporated a properly tailored metallic structure, named a
plasmonic collimator, directly onto the laser facet. The plasmonic
collimator consists of an aperture centered on the laser active region
and a periodic array of grooves nearby. (Note: High resolution figure
available on request.) The aperture couples part of the emitted light
into surface electromagnetic waves (so-called surface plasmons) on the
laser facet. As the surface waves propagate on the facet, they are
progressively scattered by the grooves and are reemitted into the far field.
These beams are in phase when they arrive at the same position in the
far field, so that the optical energy is concentrated into a small solid
angle. Stated slightly differently, grooves in the plasmonic collimator
act essentially as an array of coherent light sources that interfere
constructively so that optical energy is projected into the far field in
a single direction perpendicular to the laser facet with small
divergence. The collimation effect in the innovative laser resembles
that of the phased antenna array (an array of antennas emitting in
phase), which has already been widely used in applications such as
directional broadcasting and space communication.
In the present work low beam divergence has been achieved in the
vertical direction, parallel to the direction of the polarization of the
laser. By replacing the metallic structure with a series of concentric
grooves of circular shape one can achieve also a very small divergence
in the horizontal direction. This will result in full beam collimation.
Preliminary results have shown that this scheme works very well: a
divergence of a few degrees in the horizontal and vertical planes has
been achieved in a quantum cascade laser, in accordance with simulations.