400%/W second harmonic conversion efficiency in 14 μm-diameter gallium phosphide-on-oxide resonators.

Second harmonic conversion from 1550 nm to 775 nm with an efficiency of 400% W is demonstrated in a gallium phosphide (GaP) on oxide integrated photonic platform. The platform consists of doubly-resonant, phase-matched ring resonators with quality factors Q ∼ 10, low mode volumes V ∼ 30(λ/n), and high nonlinear mode overlaps. Measurements and simulations indicate that conversion efficiencies can be increased by a factor of 20 by improving the waveguide-cavity coupling to achieve critical coupling in current devices.

Frequency Control of Single Quantum Emitters in Integrated Photonic Circuits.

Generating entangled graph states of qubits requires high entanglement rates with efficient detection of multiple indistinguishable photons from separate qubits. Integrating defect-based qubits into photonic devices results in an enhanced photon collection efficiency, however, typically at the cost of a reduced defect emission energy homogeneity. Here, we demonstrate that the reduction in defect homogeneity in an integrated device can be partially offset by electric field tuning.

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit.

Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage.

Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics.

The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI and a monolayer of WSe. We observe unprecedented control of the spin and valley pseudospin in WSe, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe valley splitting and polarization via flipping of the CrI magnetization.

Graphene-graphite oxide field-effect transistors.

Graphene's high mobility and two-dimensional nature make it an attractive material for field-effect transistors. Previous efforts in this area have used bulk gate dielectric materials such as SiO(2) or HfO(2). In contrast, we have studied the use of an ultrathin layered material, graphene's insulating analogue, graphite oxide.