Photonic crystal changes color when exposed to an explosion

Blast-induced traumatic brain injury has been the signature wound of soldiers in Afghanistan and Iraq who have been exposed to explosion shockwaves on the battlefield. However, little is known about exposure thresholds that induce brain injury. While the soldiers may appear normal with no visible wounds, they could suffer brain damage from the supersonic blast. If returned to the field, they risk repetitive brain injury that could have long-term effects.

There have been many efforts to develop new sensor devices that can measure the overpressure generated by explosions. However, most require power to operate and can be expensive and cumbersome. It would be ideal if we could develop a power-free, wearable patch that can change color immediately to indicate the blast level. By identifying the specific color change on badges worn by soldiers, a field medic could instantly diagnose the damage level and decide when a soldier can return to the field and what treatment he or she should receive.

Three-dimensional photonic crystals are microstructured crystalline materials with periodically modulated refractive indices on lengths comparable to the light wavelength of interest.¹ The light waves scattered from the dielectric lattice interfere with each other (which is known as Bragg scattering), leading to omnidirectional stop bands or photonic bandgaps (PBGs) where the light is totally reflected over a particular wavelength range. Photonic crystals are of interest for numerous applications, including ultrahigh-bandwidth integrated optical circuits, lasers, sensing, spectroscopy, and pulse shaping.

The 3D photonic crystals are fabricated using multibeam interference lithography (MBIL)—see Figure 1(a)—of a commercially available, negative-tone photoresist (SU-8) using a visible (wavelength: 532nm) laser.² The prepared crystalline films have a colorful reflection from the diffraction grating of the underlying pattern, which is a characteristic of the periodicity (∼1μm), structural symmetry, porosity, refractive-index contrast between high (SU-8, n = 1.6) and low (air, n = 1) dielectric materials, and viewing angle: see Figure 1(b)–(d). The displayed colors caused by the interference, diffraction, or light scattering by arrays of microstructured materials are often referred to as structural colors. They often appear considerably brighter than those of pigments whose colors are caused by selective absorption by chemical substances.³ Therefore, the color change from a thin film consisting of only a few layers of ordered structures may be highly visible to the naked eye. In addition, SU-8 photonic crystals are thermally and mechanically stable up to 300°C and chemically inert because of aromatic functionality and high crosslink density. When exposed to a high-energy blast wave, however, the microstructures are broken apart preferentially in the weakly connected cleavage planes, resulting in color degradation (see Figure 2) or complete loss.⁴

Figure 1. Fabrication of 3D photonic crystals by multibeam interference lithography. (a) Fabrication process. (b) Scanning-electron-microscope (SEM) image of the diamond-like 3D SU-8 negative-tone photoresist photonic crystal. (c) Higher-magnification SEM image of (b). (d) Optical image of 3D SU-8 photonic crystals from different angles.

Figure 2. Optical microscopy images of 3D photonic crystal (a) before and (b) after exposure to a single-pulse overpressure at 320kW/m². (c) SEM image of the damaged photonic structure corresponding to (b).

So far, we have shown proof-of-concept results that exploit the material failure of 3D photonic crystals to detect blast exposure. It will be important to determine what the blast threshold should be for a specific color change and the corresponding type of brain injury, and how the shock wave triggers structural damage within the crystal or brain over time. The photonic-crystal patch can be made with a size of a few mm². Our next step will be to scale up the new materials over a larger area (a few cm²) to be useful in real field conditions.

MBIL is mask-free and versatile. We can change the beam geometry to create various periodic 3D optical structures at different lengths (a few hundred nanometers to a few micrometers) with various patterns.^5,6 Much work needs to be done to establish physical and theoretical models of the colorimetric change as a function of material properties (pattern size and symmetry, porosity, and mechanical properties). In addition, we have made 3D photonic crystals in organic/inorganic hybrid materials⁷ as well as inorganic materials, such as silicon,⁸ titania dioxide,² and silicon carbide,⁹ thus providing a wider range of thermal and mechanical characteristics to measure the magnitude of explosions to which a soldier may be exposed over time. Because of the small, lightweight design of photonic-crystal films, we expect them to be worn on soldiers' helmets and uniforms across multiple locations, allowing field medics to better assess the explosion levels.

In the future, we hope to build a quantitative physical model that correlates the blast time and intensity to the 3D photonic crystals' structural change and the resulting color change. To achieve this, we will create 3D photonic crystals with different symmetries and from materials with various elasticity or mechanical properties.