Since its invention in 1958, the laser has found use in every segment of our society. In contrast to common light sources, which emit incoherent photons at many frequencies, laser beams are emitted from “coherent” or single-wavelength light sources. However, exceedingly few sources can generate the narrow-bandwidth, intense laser light.
We have found a new source of coherent optical radiation that is fundamentally distinct from lasers and other existing sources of coherent radiation. Using analytical theory and computational experiments, we have simulated a mechanical shock wave in simple table salt (NaCl) and observed a new light source. Where only incoherent photons and sparks would be expected to emerge from the NaCl crystal, we found much more. Remarkably, weak but measurable coherent light was observed, typically in the frequency range 1–100 THz.
|Figure 1. Simulations of a shock propagating through NaCl (black) show narrow-bandwidth coherent peaks that do not exist in the simulations without shocks (red). The emission frequencies show a coherence length of 5 mm from the 16-THz peak, comparable to that of some commonly used lasers.|
Relevance to CMELS Research Themes
CMELS has several strategic themes that are directed toward understanding and clarifying the ultrafast dynamics of matter under shock compression. Using the coherent property of this light, we can develop new and novel experimental diagnostics to better understand the picosecond-timescale dynamics of shocked matter. For example, because the coherent emission frequencies are determined by the shock speed and the lattice constants of the crystal, they can potentially be used to determine atomic-scale properties of the shocked material.
Major Accomplishments in 2005
In collaboration with the Massachusetts Institute of Technology, our preliminary theoretical work suggested that coherent light might be emitted from shock waves under some circumstances. However, experimentally relevant predictions would require simulations of tens of millions of atoms.
In 2005, we performed molecular dynamics simulations of shock waves propagating through crystalline NaCl. The simulations of such large numbers of atoms were made possible by using the LLNL 23-TFLOP/s Thunder computer.
These simulations solved the classical equations of motion for each atom in the shocked NaCl crystal, addressing interaction, thermal eects, and deformation of the crystal lattice. Additionally, each atom in the crystal has a static charge. Our goal was to calculate the total electric current in the computational cell and relate it to the emitted electromagnetic radiation to make a prediction about the emission of coherent light.
|Figure 2. (a) The coherent polarization current occurs for the duration of the shock propagation with roughly constant amplitude. (b) The frequency as a function of position (in the shock propagation direction) shows that the coherent current originates at the shock front (between the white dotted lines) rather than behind it.|
Molecular dynamics simulations of NaCl in Figure 1 show coherent (narrow) peaks in the electric current for shocks propagating in two different crystallographic directions [111, 100]. The peak frequencies are in excellent agreement with our analytical results.
We discovered that, when a shock wave propagates through a crystal, the synchronized motion of large numbers of atoms can produce narrow-bandwidth radiation. The molecular dynamics simulations demonstrate coherence lengths on the order of millimeters (around 20 THz) and potentially greater, comparable to some lasers. Figure 2 shows the time- and space-dependent origin of the coherent currents in a simulation.
The emission frequencies are determined by both the shock speed and the lattice constants of the crystal and, therefore, can potentially be used to determine atomic-scale properties of the shocked material. We expect that this effect will be observable in a wide variety of material systems under realizable shock-wave conditions. The predicted spatial distribution of emitted radiation shows that the radiation can be concentrated in a particular direction (Figure 3).
The generation of terahertz radiation caused by shocking crystals will have significant basic science implications because other mechanisms are few and fundamentally different. In the near term, we expect to use this form of coherent light as a subpicosecond probe to understand properties of shock waves. The frequency, magnitude, and spatial distribution of the emitted radiation contain information about the shock speed, roughness or shape of the shock front, and the degree of crystallinity of the lattice.
|Figure 3. The calculated spatial distribution of the coherent electric field magnitude from a 1-mm2 shock front shows that the emitted radiation can be concentrated in a particular direction under some circumstances.|
Potential longer-term applications include a practical source of long-time coherence radiation if the emission power level can be made sufficiently large. This new source of radiation may lead to new methods of medical imaging and new detectors for national-security applications.
E. J. Reed et al., “Coherent Optical Photons from Shock Waves in Crystals,” Phys. Rev. Lett. 96, 013904 (2006).
E. J. Reed et al., “Reversed Doppler Effect in Photonic Crystals,” Phys. Rev. Lett. 91, 133901 (2003).
E. J. Reed et al., “The Color of Shock Waves in Photonic Crystals,” Phys. Rev. Lett. 90, 203904 (2003).
Contact: Evan Reed, reed firstname.lastname@example.org.
We continue to explore the coherent emission effect through molecular dynamics simulations and are developing a theoretical picture of coherent effects in shocked crystals. In addition, we will verify our simulation results with physical experiments to observe the effect.
We anticipate that the biggest challenge will be to detect the weak signal using existing relatively inefficient detection techniques. However, our experimental approach incorporates a number of elements designed to boost the coherent signal strength. One element includes using a technique capable of generating repeated shock waves at rates of around 1 kHz.
In these experiments, we will employ ultrafast subpicosecond laser-driven shock waves in conjunction with an ultrafast terahertz detection scheme. At potentially subnanometer spatial resolution, our experiments will bring us close to the scale of our simulations.