nanotechnology Communications

Nanoscale structuring of optical devices, such as vertical-cavity surface-emitting lasers (VCSELs), quantum dot lasers, and photonic crystal materials, is leading to additional advances in communications technology.

VCSELs have nanoscale layers of compound semiconductors epitaxially grown into their structure—alternating dielectric layers as mirrors and quantum wells. Quantum wells allow the charge carriers to be confined in well-defined regions and provide the energy conversion into light at desired wavelengths. They are placed in the laser’s cavity to confine carriers at the nodes of a standing wave and to tailor the band structure for more efficient radiative recombination. One-dimensional nanotechnology techniques involving precise growth of very thin epitaxial semiconductor layers were developed during the 1990s. Such nanostructuring has enhanced the efficiency of VCSELs and reduced the current required for lasing to start (called the threshold current). Because of improving performance and their compatibility with planar manufacturing technology, VCSELs are fast becoming a preferred laser source in a variety of communications applications.

More recently, the introduction of quantum dots (regions so small that they can be given a single electric charge) into semiconductor lasers has been investigated and found to give additional benefits—both further reductions in threshold current and narrower line widths. Quantum dots further confine the optical emission modes within a very narrow spectrum and give the lowest threshold current densities for lasing achieved to date in VCSELs. The quantum dots are introduced into the laser during the growth of strained layers, by a process called Stransky-Krastanov growth. They arise because of the lattice mismatch stress and surface tension of the growing film. Improvements in ways to control precisely the resulting quantum dots to a more uniform single size are still being sought.

Photonic crystals provide a new means to control the steering and manipulation of photons based on periodic dielectric lattices with repeat dimensions on the order of the wavelength of light. These materials can have very exotic properties, such as not allowing light within certain wavelengths to be propagated in a material based on the particular periodic structure. Photonic lattices can act as perfect wavelength-selective mirrors to reflect back incident light from all orientations. They provide the basis for optical switching, steering, and wavelength separation on unprecedented small scales. The periodic structures required for these artificial crystals can be configured as both two- and three-dimensional lattices. Optical sources, switches, and routers are being considered, with two-dimensional planar geometries receiving the most attention, because of their greater ease of fabrication.

Another potentially important communications application for nanotechnology is microelectromechanical systems (MEMS), devices sized at the micrometre level (millionths of a metre). MEMS are currently poised to have a major impact on communications via optical switching. In the future, electromechanical devices may shrink to nanodimensions to take advantage of the higher frequencies of mechanical vibration at smaller masses. The natural (resonant) frequency of vibration for small mechanical beams increases as their size decreases, so that little power is needed to drive them as oscillators. Their efficiency is rated by a quality factor, known as Q, which is a ratio of the energy stored per cycle versus the energy dissipated per cycle. The higher the Q, the more precise the absolute frequency of an oscillator. The Q is very high for micro- and nanoscale mechanical oscillators, and these devices can reach very high frequencies (up to microwave frequencies), making them potential low-power replacements for electronic-based oscillators and filters.

Mechanical oscillators have been made from silicon at dimensions of 10 × 100 nanometres, where more than 10 percent of the atoms are less than one atomic distance from the surface. While highly homogeneous materials can be made at these dimensions—for example, single-crystal silicon bars—surfaces play an increasing role at nanoscales, and energy losses increase, presumably because of surface defects and molecular species absorbed on surfaces.

It is possible to envision even higher frequencies, in what might be viewed as the ultimate in nanomechanical systems, by moving from nanomachined structures to molecular systems. As an example, multiwalled carbon nanotubes are being explored for their mechanical properties. When the ends of the outer nanotube are removed, the inner tube may be pulled partway out from the outer tube where van der Waals forces between the two tubes will supply a restoring force. The inner tube can thus oscillate, sliding back and forth inside the outer tube. The resonant frequency of oscillation for such structures is predicted to be above one gigahertz (one billion cycles per second). It is unknown whether connecting such systems to the macro world and protecting them from surface effects will ever be practical.