Dielectric cavities support record low mode volumes by incorporating subwavelength features into photonic crystal unit cells.

The fields of photonic crystals and plasmonics have taken two different approaches to increasing light–matter interaction. Photonic crystal cavities increase temporal confinement of light in a material, as represented by their high quality factor, while plasmonic structures increase spatial confinement, as represented by their low mode volume. However, the inability to simultaneously attain extreme temporal and spatial confinement of light remains a barrier to realizing ultimate control of light in a material and maximum performance in photonic devices. Here, by engineering the photonic crystal unit cell to incorporate deep subwavelength dielectric inclusions, we show that it is possible in a single structure to achieve a mode volume commensurate with plasmonic elements while maintaining a quality factor that is characteristic of traditional photonic crystal cavities. Manipulating the geometric design of the unit cell leads to precise control of the band structure and mode distribution in the photonic crystal. With a dielectric bow-tie unit cell, photonic crystals can achieve a mode volume as small as 0.0005 (λ/n)3 with a quality factor as large as 1.75 × 106. Our results demonstrate that there exists a promising alternative to lossy metals for extreme light concentration and manipulation.

We investigate the wavelength- and polarization-dependence of photocurrent signals generated at few-layer MoS2-metal junctions through spatially resolved photocurrent measurements. When incident photon energy is above the direct bandgap of few-layer MoS2, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, which can be attributed to photovoltaic effects. In contrast, if incident photon energy is below the direct bandgap of MoS2, the photocurrent response is maximized when the incident light is polarized in the direction perpendicular to the electrode edge, indicating different photocurrent generation mechanisms. Further studies show that this polarized photocurrent response can be interpreted in terms of the polarized absorption of light by the plasmonic metal electrode, its conversion into hot electron-hole pairs, and subsequent injection into MoS2. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in metal-semiconductor junctions, opening the door for engineering future two-dimensional materials based optoelectronics through surface plasmon resonances.

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A ring resonator is patterned on a porous silicon slab waveguide to produce a compact, high quality factor biosensor with a large internal surface area available for enhanced recognition of biological and chemical molecules. The porous nature of the ring resonator allows molecules to directly interact with the guided mode. Quality factors near 10,000 were measured for porous silicon ring resonators with a radius of 25 μm. A bulk detection sensitivity of 380 nm/RIU was measured upon exposure to salt water solutions. Specific detection of nucleic acid molecules was demonstrated with a surface detection sensitivity of 4 pm/nM.