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Topic Name: New Physical Phenomenon Seeing High Frequency Waves by Combining Molecular Dynamics Simulations of Shock Waves

Category: Nanocharacterization

Research persons: Evan Reed, Michael Armstrong

Location: Lawrence Livermore National Laboratory, United States

Details

Acoustic waves play many everyday roles - from communication between people to ultrasound imaging. Now the highest frequency acoustic waves in materials, with nearly atomic-scale wavelengths, promise to be useful probes of nanostructures such as LED lights.

However, detecting them isn't so easy.

Enter Lawrence Livermore National Laboratory scientists, who discovered a new physical phenomenon that enables them to see high frequency waves by combining molecular dynamics simulations of shock waves with an experimental diagnostic, terahertz (THz) radiation. (The hertz is the base unit of frequency. One hertz simply means one cycle per second. A terahertz is 10^12 hertz.).

The Livermore scientists performed computer simulations of the highest frequency acoustic waves forming spontaneously at the front of shock waves or generated by sub-picosecond pulse-length lasers.

They discovered that, under some circumstances, when such a wave crosses an interface between two materials, tiny electric currents are generated at the interface. These currents produce electromagnetic radiation of THz frequencies that can be detected a few millimeters away from the interface. Part of the wave is effectively converted to electromagnetic radiation, which propagates out of the material where it can be measured.

Most molecular dynamics simulations of shock waves connect to experiments through electronic properties, such as optical reflectivity.

"But this new approach connects to the much lower frequency THz radiation produced by the individual atoms moving around in the shock wave," said Evan Reed, lead author of a paper that appears in the July 7 edition of the journal, Physical Review Letters. "This kind of diagnostic promises to provide new information about shocked materials like the dynamics of crystals pushed to ultra-high strain rates."

Using molecular dynamics simulations, the team, made up of Livermore's Reed and Michael Armstrong in collaboration with Los Alamos National Laboratory colleagues shows that the time-history of the wave can be determined with potentially sub-picosecond, nearly atomic time and space resolution by measuring the electromagnetic field.

Reed and colleagues studied the effect for an interface between two thin films, which are used in LED (light-emitting diode) nanostructures, and are piezoelectric (electric currents that are generated when they are squeezed). Piezoelectric materials have been used for decades as arrival time gauges for shock-wave experiments but have been limited by electrical equipment that can only detect acoustic frequencies less than 10 gigathertz (GHz), precluding observation of the highest frequency acoustic waves. The new THz radiation technique can help improve the time resolution of such approaches.

The technique has other applications as well. It can be applied to determine the structure of many kinds of electronic devices that are constructed using thin film layered structures, such as field-effect transistors.

"The detection of high frequency acoustic waves also has been proposed for use in imaging of quantum dot nanostructures used in myriad optical devices, possibly including solar cells in the future," Reed said. "The technology is not there yet for that application, but our work represents a step closer."

Note for Molecular Dynamics
Molecular dynamics (MD) is a form of computer simulation in which atoms and molecules are allowed to interact for a period of time under known laws of physics, giving a view of the motion of the atoms. Because molecular systems generally consist of a vast number of particles, it is impossible to find the properties of such complex systems analytically; MD simulation circumvents this problem by using numerical methods. It represents an interface between laboratory experiments and theory, and can be understood as a "virtual experiment". MD probes the relationship between molecular structure, movement and function. Molecular dynamics is a multidisciplinary method. Its laws and theories stem from mathematics, physics, and chemistry, and it employs algorithms from computer science and information theory. It was originally conceived within theoretical physics in the late 1950's, but is applied today mostly in materials science and biomolecules.

Before it became possible to simulate molecular dynamics with computers, some undertook the hard work of trying it with physical models such as macroscopic spheres. The idea was to arrange them to replicate the properties of a liquid. J.D. Bernal said, in 1962: "... I took a number of rubber balls and stuck them together with rods of a selection of different lengths ranging from 2.75 to 4 inches. I tried to do this in the first place as casually as possible, working in my own office, being interrupted every five minutes or so and not remembering what I had done before the interruption." Fortunately, now computers keep track of bonds during a simulation.

Molecular dynamics is a specialized discipline of molecular modeling and computer simulation based on statistical mechanics; the main justification of the MD method is that statistical ensemble averages are equal to time averages of the system, known as the ergodic hypothesis. MD has also been termed "statistical mechanics by numbers" and "Laplace's vision of Newtonian mechanics" of predicting the future by animating nature's forces and allowing insight into molecular motion on an atomic scale. However, long MD simulations are mathematically ill-conditioned, generating cumulative errors in numerical integration that can be minimized with proper selection of algorithms and parameters, but not eliminated entirely. Furthermore, current potential functions are, in many cases, not sufficiently accurate to reproduce the dynamics of molecular systems, so the much more demanding Ab Initio Molecular Dynamics method must be used. Nevertheless, molecular dynamics techniques allow detailed time and space resolution into representative behavior in phase space.

Note for Piezoelectricity
Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. The word is derived from the Greek piezein, which means to squeeze or press.

The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.

The effect finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultra fine focusing of optical assemblies. It is also the basis of a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies such as STM, AFM, MTA, SNOM etc.

Note for Acoustic Wave
An acoustic wave is a weak wave (meaning a small pressure change) that moves at the speed of sound. As opposed to a shock wave, the pressure change is continuous, and the wave is isentropic, meaning energy is conserved as it propagates. A further difference is that shock waves can move faster than the speed of sound. Alternating compression and expansion acoustic waves moving in unison cause sound.

In figure 1, Top: Comparison of strain rate in a molecular dynamics simulation at a AlN/GaN interface to the calculated electric field measured 1 mm from the inter- face for a shock wave with properties given in the text. The time-dependence of the electric field is closely related to the strain rate at the interface enabling direct resolution of THz frequency strain waves. Bottom: Frequency as a function of time for the electromagnetic radiation. An 8 THz harmonic of a 4 THz fundamental corresponds to atomic scale spatial structure around 5 angstroms.

In figure 2, Electromagnetic radiation is produced when an acoustic wave (purple) generates electric currents (red) as it propagates past an interface between two piezoelectric materials. The radiation propagates outside of the materials and can be detected to determine the shape of the acoustic wave with nearly atomic scale resolution.