Nonlinear Electromagnetic Interactions in Energetic Materials

M. A. Wood, D. A. R. Dalvit, and D. S. Moore

Phys. Rev. Applied 5, 014004 – Published 12 January 2016

Abstract

We study the scattering of electromagnetic waves in anisotropic energetic materials. Nonlinear light-matter interactions in molecular crystals result in frequency-conversion and polarization changes. Applied electromagnetic fields of moderate intensity can induce these nonlinear effects without triggering chemical decomposition, offering a mechanism for the nonionizing identification of explosives. We use molecular-dynamics simulations to compute such two-dimensional THz spectra for planar slabs made of pentaerythritol tetranitrate and ammonium nitrate. We discuss third-harmonic generation and polarization-conversion processes in such materials. These observed far-field spectral features of the reflected or transmitted light may serve as an alternative tool for standoff explosive detection.

Received 5 October 2015

DOI:https://doi.org/10.1103/PhysRevApplied.5.014004

Physics Subject Headings (PhySH)

High-order harmonic generation Nonlinear optics

2-dimensional systems Functional materials

Molecular dynamics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. A. Wood^1,2, D. A. R. Dalvit², and D. S. Moore³

¹School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
²Theoretical Division, MS B213, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
³Explosive Science and Shock Physics Division, MS P952, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

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Issue

Vol. 5, Iss. 1 — January 2016

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Images

Figure 1
(a),(c) Representative orthoscopic views of the [100] crystallographic direction for PETN and ammonium nitrate. Panels (b),(d) show the [010] direction in either material which exemplifies the unique molecule arrangement in either direction. (e) Perspective view of the simulation domain showing how the free surface is created in MD; perpendicular to this free surface is the propagation direction of both applied and emitted fields. This simulation setup enables realistic absorption and emission polarizations within MD. Atom colors correspond to carbon (gray), hydrogen (white), oxygen (red), and nitrogen (blue).
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Figure 2
(a) Two example Gaussian electric field pulses with different carrier frequencies applied in PETN. Absorption is measured as the difference between the total energy after and before the pulse is applied. (b) Absorbance (normalized by the peak value for each polarization), as a function of the input carrier frequency and input polarization, for PETN (red) and AN IV (blue). The unique molecule geometry in each direction leads to polarization-dependent absorbance.
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Figure 3
ReaxFF-calculated power spectrum for (a) PETN and (b) AN at 50 K and 1 atm of pressure. These spectra aid in analyzing the radiated electric field signals shown in Sec. 3. The sum of all element contributions normalized by total kinetic energy yields the vibrational density of states.
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Figure 4
Emission signals from PETN along the [100] ( $x$ direction) for electric field pulses applied in the (a) [100] and (b) [010] directions. Complementary emission signals with pulses along the [010] ( $y$ direction) for the (c) [100] and (d) [010] applied field directions in PETN. Where the emission signals are aligned with the pulse polarization, Rayleigh scattering dominates the observed emission. Perpendicular directions do show emission at unique frequencies different from the applied pulse, confirming the frequency conversion due to internal scattering.
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Figure 5
Emission signals from AN along the [100] ( $x$ direction) for electric field pulses applied in the (a) [100] and (b) [010] directions. Complementary emission signals with pulses along the [010] ( $y$ direction) for the (c) [100] and (d) [010] applied field directions in AN. Third-harmonic emission is clearly seen for parallel emission directions to the applied field and is much weaker in orthogonal directions.
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Figure 6
Average emission signals for all output polarizations for impulses applied in the [100] direction for (a) PETN and (d) AN along with field pulses in the orthogonal, [010] direction in panels (b) and (e). Total emission signals for all absorption and exit polarizations for (c) PETN and (f) AN.
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Physical Review Applied