  The control over light propagation and localization in photonic crystals
offers wide applications from sensing and on-chip routing to lasing and quantum
light-matter interfaces. While in electronic crystals magnetic fields can be
used to induce a multitude of unique phenomena, the uncharged nature of photons
necessitates alternative approaches to bring about similar control over photons
at the nanoscale. Here, we experimentally realize pseudomagnetic fields in
two-dimensional photonic crystals through engineered strain of the lattice.
Analogous to strained graphene, this induces flat-band Landau levels at
discrete energies. We study the spatial and spectral properties of these states
in silicon photonic crystals at telecom wavelengths with far-field
spectroscopy. Moreover, taking advantage of the photonic crystal's design
freedom, we realize domains of opposite pseudomagnetic field and observe
topological edge states at their interface. We reveal that the strain-induced
states can achieve remarkably high quality factors despite being phase-matched
to the radiation continuum. Together with the high density of states and high
degeneracy associated with flat bands, this provides powerful prospects for
enhancing light-matter interactions, and demonstrates a design principle to
govern both on-chip and radiating light fields.
