Why trap light?

In 1865 James Clerk Maxwell showed how light propagates as electromagnetic waves. In 1984 it was realized that light can also be trapped1. My journey into this subject began as a PhD student at Harvard University, focusing on the possibility to trap or localize any classical wave in a suitable material2. This was not one of the ‘big questions of science’ at the time, but my supervisor Michael J. Stephen encouraged me to pursue it. Electrons can easily be trapped in atoms, but classical waves such as light have the disadvantage of being restricted to positive energy states. Maxwell’s equations require that in any non-absorbing dielectric material, the energy of photons surpasses all potential barriers. This made trapping of light seemingly impossible. Signatures of light localization occur in strong randomly scattering media1,3. However, it would require a fundamentally new class of materials to achieve the goal of systematic light trapping. Photonic crystals are artificial periodic dielectrics that trap light with manifold consequences. In basic science, they provide a way to engineer light–matter interactions at will. Light trapping in photonic crystals allows tailoring of the fundamental electromagnetic force. For practical applications, they enable processing of laser light on microchips for information technology, trapping of sunlight in thin films for energy conversion, and ways of guiding laser light for medical diagnostics and therapy. After completing my thesis on wave localization, I investigated a seemingly unrelated question of electronic band tails in disordered semiconductors4. Band tails arise from localized electronic states below the band edge within the bandgap of the semiconductor. It was my hunch that creating a photonic band edge was the key to strong localization of light. Previously, in optics, only ‘stop-gaps’ in one-dimensional (1D) periodic dielectrics that could trap light in one dimension had been considered. Light was still free to escape in the other two dimensions. To completely trap light in three dimensions it would be necessary to create a spectral gap for light that persisted in all directions in 3D space.