After 25 years of their discovery, photonic crystals continue to provide new fundamental physics as well as novel potential applications. In this talk, I will give examples of both.
Scientifically, photonic crystals will be an important playground for Bosonic “topological insulators/semimetals“. Quite some examples have already been demonstrated. I will discuss the 2D Quantum Hall state of light, and how 3D Weyl points and line nodes are realized in gyroid photonic crystals along with their phase diagrams and topologically nontrivial interfacial states.
Technologically, 2D photonic crystal nanocavities are important for on-chip lasers and single-photon sources. Furthermore, 2D broad-area semiconductor lasers provide higher power and richer functionalities than current chip-scale single-mode sources of DFB lasers and VCSELs, which are both essentially 1D photonic crystal lasers.

"120 µW peak output power from edge-emitting photonic crystal double-heterostructure nanocavity lasers" Ling Lu, Adam Mock, Tian Yang, Min Hsiung Shih, Eui Hyun Hwang, Mahmood Bagheri, Andrew Stapleton, Stephen Farrell, John O'Brien and P. Daniel Dapkus, Applied Physics Letters Vol. 94, Issue 11, p. 111101, (2009)

“Larger-area single-mode photonic crystal surface-emitting lasers enabled by the accidental Dirac-point '' Song-Liang Chua, Ling Lu and Marin Soljačić, in preparation (2013)
"Waveguiding at the edge of a three-dimensional photonic crystal" Ling Lu, John D. Joannopoulos and Marin Soljačić, Physical Review Letters, Vol. 108, Issue 24, pp. 243901, (2012)

"Weyl points and line nodes in gyroid photonic crystals" Ling Lu, Liang Fu, John D. Joannopoulos and Marin Soljačić, Nature Photonics, Vol. 7, No. 4, P. 294-299 (2013)