Current Research

Diffusive Transport at Dirac Points is an intriguing wave transport phenomenon in photonic crystals with Dirac cones, where the energy flux transmits omnidirectionally while a uniform phase front is automatically guaranteed. By integrating photonic graphene with photonic topological insulator (PTI) protected edges, this phenomenon can be used to design horns with high aperture efficiency or to harvest energy from free space to PTI waveguide.

Chiral Surface Waves Spin-momentum locking is a phenomenon recently explored in electromagnetic waves that possess an evanescent tail which is accompanied by a transverse spin due to out-of-plane field rotation. We introduce novel metasurface design that supports chiral surface waves. The chiral surface waves are circularly polarized waves that possess two transverse spins, one due to out-of-plane field rotation which is intrinsic to any surface wave, and the other is due to in-plane field rotation which is imposed by the design, and both follow spin-momentum locking.

Steering Circularly Polarized Waves Over the last decade, much research has been directed towards making conventional optical devices such as lenses and waveguides using flat metasurfaces. Using planar optics provides compact on-chip platforms that are easily integrated. By carefully engineering metasurfaces’ equifrequency contours, we can allow propagation of highly collimated, circularly polarized surface waves and route them along defined paths.

Phototonic Topological Insulators (PTIs) are a new type of electromagnetic material that can create extremely robust waveguides, cavities, and filters. Their defining feature is the existence of modes only on the boundary of the material that are intrinsically immune from backscatter. They come in multiple types, both reciprocal and nonreciprocal, and can be analyzed via band structure simulations and custom numerical codes. Shown below is a numerical example of the Berry curvature for a valley-type PTI photonic crystal.

Electromechanical resonances in biological systems are investigated using custom biophysical simulations to investigate non-thermal effects of EM waves on biological structures. While electric fields between charged and polar biomolecules (such as DNA and microtubules) are typically screened, high frequency fields can overcome this screening and may result in novel interactions.

Photoemission based devices take advantage of the extreme field enhancement that is possible with nano-scale resonant plasmonic structures. With these devices, high electron emission current can be generated with low power lasers and low static voltage bias. Applications include vacuum electronics devices, all-metal photovoltaics, and new approaches for photochemistry.

Anisotropic patterning techniques for generating a wide range of impedance surfaces enable varying size, direction, and shape of individual cells in the pattern regardless of the local environment. These surfaces allow control over surface wave propagation and scattering behavior. This example shows a surface designed to guide surface waves smoothly around a curve.

Nonlinear metasurface absorbers for high power surface waves consist of periodic structures with nonlinear circuit components that change states depending on input power levels. The large scale metasurface and associated measurement data demonstrate increasing absorption with higher input power. Other examples can vary their absorption based on the incoming waveform, or adapt their absorption band to match the incoming frequency.

Broadband active metasurfaces allow us to exceed the bandwidth limits of conventinal artificial impedance surfaces using non-Foster circuit elements. This will enable control of scattering properties at frequencies that would otherwise be impractical. This example provides a broadband tunable impedance boundary with nearly an octave of bandwidth in the UHF range.

Superluminal waveguides based on non-Foster circuits demonstrate measured phase and group velocity greater than the speed of light. Of course, these waveguides obey the causality limit, and thus cannot transmit information faster than the speed of light. Nonetheless they provide an important medium for understanding the limits of superluminal propagation.

Nonlinear self-focusing of microwaves is demonstrated using a nonlinear impedance surface. The front surface has tuning varacotors, controlled by detectors and signal conditioning circuits on the back. The measured data shows self-focusing of a surface wave beam, counteracting diffraction.

Scalable high power microwave sources using periodic structures act as a coherent array of switched ring-down oscillators to produce a high power microwave beam in the far field. Optical triggering can enable beam steering and focusing.

Non-Foster matching of a small loop antenna (ka=0.28) shows a broadband gain improvement over a passive match. However, the noise added by the active non-Foster circuit can exceed the gain improvement. We are currently exploring these performance trades to identify fundamental limits.

Physics of micro plasmas are studied with a simple oscillator circuit. We have shown that both the Townsend and Fowler-Nordheim theorems cannot be used in the form of traditional or modified Paschen curves to completely predict the behavior of point-to-point microplasmas. With this technique we can measure the separate breakdown and quenching voltages, which are related to the presence of metastable Argon states.

W-band quasi-optic power combiner consisting of a 4X4 tapered slot antenna array couples the field from a transmitting horn antenna to 16 MMICs. After amplification, the power is fed into another 4X4 array and combined. A near-uniform power division from the waveguide to the 16 MMICs is shown.

Dense magnetic coil arrays for high resolution transcranial magnetic stimulation enable us to pattern the magnetic field inside the body to produce arbitrary spatial patterns and temporal waveforms for next generation neurological therapies. This example shows a measurent of a high magnetic field region tuned to produce an L-shaped pattern.

Receptor coated quantum dots are provide cellular-level resolution of neural activities inside the brain. The functionalized quantum dots are selectively bound to different neurotransmitters in the extra-synaptic regions of neurons. Shown here are the emission spectra of a commercially available CdSe/ZnS core/shell quantum dots before and after binding of neurotransmitters leading to a 6nm redshift of the peak wavelength.

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