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NC State physics advance leads to a better understanding of optics at the atomic scale

May lead to faster, more efficient computers

North Carolina State University

An advance by North Carolina State University physicists improves our understanding of how light interacts with matter, and could make possible the development of new integrated-circuit technologies that result in faster computers that use less energy.

Distinguished University Professor Dr. David Aspnes and post-doctoral Research Associate Dr. Eric Adles published a paper in the April 15 issue of Physical Review B on second-harmonic generation - or how wavelengths of light are shortened upon interaction with materials. The editors highlighted the work as an exceptional paper in the issue.

Aspnes explains that the research could be used to further our understanding of how materials bond to each other - such as silicon and next-generation insulating materials for integrated-circuit technologies. Application of this advance could aid researchers in selecting and processing materials that bond to silicon more uniformly, resulting in faster computers that utilize energy more efficiently.

Adles says the research allows scientists and engineers to use nonlinear-optical spectroscopy - which examines light reflected, absorbed or produced by a substance to determine its physical properties - to obtain more accurate information on a substance at the atomic scale. For example, the research could be used to get better data on the physical properties of the "interface" - the one-atom-thick layer where two materials bond to each other. Essentially, Adles says, the results provide a "key" that can be used by researchers to analyze spectroscopy data. Previously, scientists could collect such data on the interface, but had no means of interpreting it correctly on the atomic scale.

Aspnes says the goal of the research was to "improve our understanding of how things work," but notes that it also gives others the tools to better analyze data and therefore gives manufacturers and industry scientists the opportunity to make better decisions about how best to move forward.


Aspnes is a professor of physics at NC State and a member of the National Academy of Sciences.

Note to editors: The paper's abstract follows.

"Application of the anisotropic bond model to second-harmonic generation from amorphous media"
Authors: Drs. Eric J. Adles and David E. Aspnes, North Carolina State University
Published: Physical Review B, April 15, 2008

Abstract: As a step toward analyzing second-harmonic generation SHG from crystalline Si nanospheres in glass, we develop an anisotropic bond model ABM that expresses SHG in terms of physically meaningful parameters and provide a detailed understanding of the basic physics of SHG on the atomic scale. Nonlinear-optical NLO responses are calculated classically via the four fundamental steps of optics: evaluate the local field at a given bond site, solve the force equation for the acceleration of the charge, calculate the resulting radiation, then superpose the radiation from all charges. Because the emerging NLO signals are orders of magnitude weaker and occur at wavelengths different from that of the pump beam, these steps are independent. Paradoxically, the treatment of NLO is therefore simpler than that of linear optics LO, where these calculations must be done self-consistently. The ABM goes beyond previous bond models by including the complete set of underlying contributions: retardation RD, spatial-dispersion SD, and magnetic MG effects, in addition to the anharmonic restoring force acting on the bond charge. Transverse as well as longitudinal motion is also considered. We apply the ABM to obtain analytic expressions for SHG from amorphous materials under Gaussian-beam excitation. These materials represent an interesting test case not only because they are ubiquitous but also because the anharmonic-force contribution that dominates the SHG response of crystalline materials and ordered interfaces vanishes by symmetry. The remaining contributions, and hence the SHG signals, are entirely functions of the LO response and beam geometry, so the only new information available is the anisotropy of the LO response at the bond level. The RD, SD, and MG contributions are all of the same order of magnitude, so none can be ignored. Diffraction is important in determining not only the pattern of the emerging beam but also the phases and amplitudes of the different terms. The plane-wave expansion that gives rise to electric quadrupole magnetic dipole effects in LO appears here as retardation. Using the paraxial-ray approximation, we reduce the results to the isotropic case in two limits, that where the linear restoring force dominates glasses and that where it is absent metals. Both forward- and backscattering geometries are discussed. Estimated signal strengths and conversion efficiencies for fused silica appear to be in general agreement with data where available. Predictions that allow additional critical tests of these results are made.

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