In optoelectronic devices, spontaneous emission has to be directed toward a
single mode channel, for the sake of efficiency, noise reduction or speed
enhancement. However in general, this is not achieved and a large portion g
of the radiation escapes out of this mode, so that only a small fraction b
is effectively coupled. To overcome this difficulty, one may use 3D
photonic-crystal materials, with theoretically vanishing ã’s in the
bandgap, but they are difficult to manufacture. Alternatively, one may use
microresonators to dynamically boost the photons (Purcell effect) into the
cavity mode, but the restrictive matching between the quantum-dot source
and cavity energies severely reduces the device yield.
A new Bloch-mode formalism has been used to predict that single-row-defect
waveguides in photonic-crystal membranes offer large b-factors in excess
of 95%. In this 2D system, it is the photon emission into the spurious
modes that is naturally lowered, like in 3D bandgap materials, while the
emission into the desired channel is comparable to that in the bulk
material. Since no Purcell effect is involved, the efficient funnelling is
achieved over a remarkably large spectral interval of 40-nm, a value
comparable to the inhomogeneous dot linewidths at l=950nm. This prediction
may impact the design of future single mode LEDs or low threshold lasers.
***
Nanoscale Machines Controlled by Light Get a Leg Up
We present experiments revealing the first detailed images of individual
molecules reversibly changing their shape in response to light. We were
successful because we added tiny ¿legs¿ to molecules that allowed them
to stand up above the fray of their clinging nanoscale environment.
These molecules (azobenzenes) are very special because they perform
mechanical work when they change shape. They are exciting for
nanotechnology applications as they could perform as the smallest
imaginable remote-controlled mechanical pistons or electrical switches
(i.e. nanomachines). While previously these molecules have been studied
tumbling in solution or organic polymers, nanotechnology applications
require them to be mounted very closely on a surface. But mounting
molecules on a surface generally destroys their mechanical properties:
nanomachines interact with their nano bench tops. The legs on our
molecules surmounted this challenge by causing the molecular engine to
stand up away from the surface just enough (fractions of a nanometer) to
escape surface interference, while still being firmly mounted to the
nano bench top. Our images make clear the previously murky but essential
dependence of nanomachines on their detailed nanoscale environments, and
pave the way towards future nanotechnology applications.
