Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 46

military communication systems. Secure communications will take place through
hardware and software designed for the purpose. Compressing the essential data
in order to transmit with minimum bandwidth requires knowledge of the data
being transmitted and is largely a software development.
Optical materials and devices are used in many communications operations.
These include photonic switches and waveguides that may be fabricated with
greater precision and smaller size using materials and processes emerging from
research in nanotechnology, such as vertical cavity surface-emitting lasers
(VCSELs), quantum dot lasers, and photonic bandgap materials.
New materials for both optical and RF applications are emerging from con-
tinuing investigations of semiconductors. Two examples are self-assembled quan-
tum dots for laser systems and gallium nitride (GaN) and related materials for
both optical (and ultraviolet) emission and high-power RF devices. These mate-
rials are discussed in Chapter 4.
Both optical and RF MEMS devices are of growing importance for commu-
nications and are discussed below.
Optical Devices
Micro- and nanotechnologies have already had significant impact on III-V,
fiber, and nonlinear optoelectronic devices. For III-V devices, control of growth
thickness using modern epitaxial growth techniques such as molecular beam
epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD, also
known as organometallic vapor-phase epitaxy, OMVPE) has progressed to the
point that individual atomic layers (~0.5 nanometer thick) are routinely grown
with differing material compositions. Figure 3-4 shows the evolution of the low-
est threshold current for cw diode lasers. 67 It gives some perspective on the
impact of improved growth capabilities and the advantages of nanostructures for
optical devices. The first diode lasers, demonstrated in 1962, 68 were composed of
homojunction materials (p-n junctions in a single material, GaAs). They were
pulsed devices that were incapable of cw operation because of the power dissipa-
tion associated with inefficiencies inherent in this simple structure. The first cw
lasers employed double heterostructures, where an index step was built in by
varying material composition to confine the optical field to the junction region
and dramatically lower the optical losses. The first data point on the double
heterostructure curve is due to Alferov and was part of the work for which he was
awarded the Nobel Prize in Physics in 2000. The scale, in the growth direction, of
the double heterostructure waveguide is on the order of several hundred nanom-
eters. The next major breakthrough was the addition of quantum wells to confine
the carriers in the center of the optical mode and modify the electronic wave
function and density of states as a consequence of the confinement. These quan-
tum wells are on the scale of the de Broglie wavelength of the electrons and are
typically tens of nanometers thick in the growth direction. The most recent curve