Bridging the gap between ultrafast optics and resonant photonics via omni-resonance

High-finesse planar Fabry-P{é}rot (FP) cavities spectrally filter the incident field at discrete resonances, and thus cannot be utilized to resonantly enhance the field of ultrashort pulses. Introducing judicious angular dispersion into a pulse can give rise to `omni-resonance', whereby the entire bandwidth of a spatiotemporally structured ultrafast pulse couples to a single longitudinal cavity resonance, even when the pulse bandwidth far exceeds the resonant linewidth. Here we show that omni-resonance increases the intra-cavity peak intensity above that of a pulse having equal energy and bandwidth when tightly focused in free space -- maintained across its entire bandwidth and along a cavity longer than the Rayleigh length of the focused pulse. This paves the way towards broadband resonant enhancement of nonlinear optical effects, thereby bridging the gap between ultrafast optics and resonant photonics.