Research

Nanoscale magnetism and magnetic materials using time resolved, super-resolution magnetic imaging

We use time-resolved thermal gradients as the basis for spatiotemporal magnetic microscopy.  Taking advantage of the magneto-thermoelectric interactions, such as the time-resolved anomalous Nernst effect (TRANE), we use heat and light to transduce the local magnetic moment into an electrical signal.  This enables new, quantitative studies that image magnetization dynamics on picosecond time-scales.  We are currently using this method to examine magnetic, electronic, orbital, and thermal interactions in a variety of emerging magnetic systems, including spin Hall effect systems, chiral magnetic materials, and topological insulators.

An exciting aspect of using heat rather than light for magnetic imaging is that there is no fundamental limit to the spatial resolution because it is determined by the generation and spatiotemporal evolution of a thermal gradient rather than on principles of optical diffraction.  Using this fact, we are developing a super-resolution magnetic microscope for imaging nanoscale magnetism and the dynamical properties of magnetic materials on 50-100 nm length scales.  This capability will enable studies of magnetic systems and devices on their fundamental scales: spatial scales comparable to domain wall widths and time scales smaller than the period of ferromagnetic resonance.

Relavent publications:
• "Imaging Magnetization Structure and Dynamics in Ultrathin Y3Fe5O12/Pt Bilayers with High Sensitivity Using the Time-Resolved Longitudinal Spin Seebeck Effect." Phys. Rev. Appl. 7, 044004 (2017).
• "Ferromagnetic resonance phase imaging in spin Hall multilayers." Phys. Rev. B 93, 144415 (2016).
• "Phase-sensitive imaging of ferromagnetic resonance using ultrafast heat pulses." Phys. Rev. Applied 4, 044004 (2015).
• "Towards a table-top microscope for nanoscale magnetic imaging using picosecond thermal gradients." Nature Communications 6, 8460 (2015).

 

Diamond NV centers for precision measurement and quantum control

Individual nitrogen vacancy (NV) defect center spins in diamond form a unique platform to study the quantum properties of matter because they are simple, they can be studied one at a time, and they have quantum coherence even under ambient conditions.  Because NV centers are point defects, they can be used to sense magnetic fields with high spatial resolution provided that they are close to the surface of the diamond host.  We are currently focused on developing effective methods of robust quantum control, making use of gigahertz frequency magnetic fields and gigahertz frequency lattice strain.  These studies shed light on fundamental interactions between spins, electromagnetic fields, and phonons, but also may find application in magnetometry, inertial sensing, and quantum information processing

Relavent publications:
• Optically Detected Ferromagnetic Resonance in Metallic Ferromagnets via Nitrogen Vacancy Centers in Diamond." arXiv:1607.07485 (2016).
• "Cooling a Mechanical Resonator with a Nitrogen-Vacancy Center Ensemble Using a Room Temperature Excited State Spin-Strain Interaction." Nature Communications 8, 14358 (2017).
• "Continuous dynamical decoupling of a single diamond nitrogen-vacancy center spin with a mechanical resonator." Phys. Rev. B 92, 224419 (2015).
• "Coherent Control of a Nitrogen-Vacancy Center Spin Ensemble with a Diamond Mechanical Resonator." Optica, 2, 233 (2015).
• "Mechanical spin control of nitrogen-vacancy centers in diamond," Phys. Rev. Lett. 111, 227602 (2013).

Watch a video of Greg's lecture III at ITAMP/B2 Institute Winter Graduate School on AMO Physics where he talks about our diamond spin-mechanics work

 

Spin-based defects in wide-bandgap oxide semiconductor materials

Wide bandgap semiconductors are host to a zoo of optically active paramagnetic point defects.  By identifying point defects with (1) sufficiently strong optical dipoles, (2) an optically differentiable spin signal, and (3) a nearly nuclear-spin free environment, we can cultivate new defect-based spin systems for quantum information processing and precision measurement technology.  Alternatives to diamond may provide new avenues of growth, processing, and control over quantum states.  One interesting candidate is ZnO, which has a mainly spinless nuclear lattice combined with a band-gap that is capable of supporting optical transitions between defect levels without interaction with the bands.  In addition, ZnO is piezoelectric, offering new functional control over defect states that is not available in diamond.    These properties, combined with the rich literature of optically-detected magnetic resonance of mid-gap defect states, provide a roadmap to the discovery of new defects for quantum information processing, quantum communication, and quantum optics.   

Relavent publications:
• "Temperature Dependence of Wavelength Selectable Zero-Phonon Emission from Single Defects in Hexagonal Boron Nitride." Nano Letters 16, 6052-6057 (2016).

• "Polarization spectroscopy of defect-based single photon sources in ZnO." ACS Nano 10, 1210 (2016).
• "A single-molecule approach to ZnO defect studies: single photons and single defects." J. Appl. Phys. 116, 043509 (2014).