  The ability to highly localize light with strong electric field enhancement
is critical for enabling higher efficiency solar cells, light sources, and
modulators. While deep subwavelength modes can be realized with plasmonic
resonators, large losses in these metal structures preclude most practical
applications. We developed an alternative approach to achieving subwavelength
confinement that is not accompanied by inhibitive losses. We experimentally
demonstrate a dielectric bowtie photonic crystal structure that supports mode
volumes commensurate with plasmonic elements and quality factors that reveal
ultra-low losses. Our approach opens the door to the extremely strong
light-matter interaction regime with simultaneously both ultra-low mode volume
and ultra-high quality factor that has remained elusive in optical resonators.
