Biology Reference
In-Depth Information
The resulting structures are randomized owing to the chemical
interactions between the QDs and the substrate, as well as between
each other. Packing of QDs in the two-layer method is denser with
QD separation of 5
10 nm, whereas QD separation for the DNA
process may be up to an order of magnitude larger. This is explained
by the latter method involving more intermediate stages [5]. Both
methods are suitable for mass production, in that advanced optical
lithography with sufficient resolution can be utilized for the definition
of the waveguide patterns. The DNA-mediated process provides a
programming capability while the two-layer method is more suitable
for the fast assembly of devices with selective deposition of QDs of
different size and thereby different emission wavelength.
Waveguide behavior of the QD cascades was studied using a signal
laser at the input and a photodetector with nanowatt detection limit
at the output. Collimated pump light for device illumination resulted
in excitation of electron
hole pairs and created significant gain [6].
In addition, modeling and simulations for QDWs fabricated by DNA-
directed self-assembly and the two-layer method were carried out
to theoretically determine the light propagation mechanism and its
characteristics such as absorption, emission, and gain [5,7]. Dot-to-
dot interactions were evaluated using finite-different-time domain
simulations (FDTD). Light transmission was found to occur owing
to a sequence of stimulated emission through the QD array. Small
separation between the QDs results in near-field energy transfer
due to interdot coupling [5]. For single line (one-dimensional) QDW
array formation, subwavelength dimensions allow for optical energy
transfer below the diffraction limit [7]. Monte Carlo modeling of a 2D
device structure with randomized QD placement revealed the cross-
coupling to improve the signal throughput and, therefore, to result in
the same gain as the one-dimensional system. Auger recombination
was found to be an important nonradiative competing process to
stimulated photon emission [5].
The linear waveguide increases transmitted signal levels given
sufficient pump power. Since the net signal was shown to increase
proportionally to the pump power, and it was consistently higher on
the waveguide than on the substrate, a gain mechanism was proposed.
For highly efficient coupling between stimulated photons from the
QDs, transfer of electromagnetic energy can be achieved with low
device gain. However, lower coupling efficiency demands higher
gain to achieve comparable output intensity. Similar results were
Search WWH ::




Custom Search