Research

Molecular beam epitaxyGENxplor_photo

The Law group uses molecular beam epitaxy (MBE) to grow thin films of interesting materials and heterostructures.  MBE is a technique whereby a film is deposited one atomic layer at a time in an ultra high vacuum.  This results in crystalline films with few defects and smooth surfaces and also enables abrupt transitions between materials with flat interfaces.  We collaborate with the Zide group on the growth of III/V materials and use our own Veeco GenXplor MBE to grow chalcogenide-based topological insulators.

Unique optical excitations in topological insulators (DOE Early Career award)

The overall objective of this research is to understand how light interacts with topological insulator (TI) films and layered structures.  Unlike normal materials, the electrons in TI films are trapped at the top and bottom surfaces of the film.  These electrons have unusual properties, including low mass and high velocity.  Light shining on these trapped electrons will excite electron density waves, called plasmons, which inherit the unusual properties of the electrons.  This project aims to understand how these plasmons interact with each other and how the plasmon properties change as the film dimensions change.  By controlling the physical properties of the films, the optical response of the film can also be controlled.  In addition to single TI films, the project will also investigate the properties of stacks of TI films layered with normal insulating films.  Stacking these materials results in multiple layers of trapped electrons whose plasmons can interact in ever more complex ways.  After these interactions are understood, we can begin to engineer complex TI structures to obtain designer optical phenomena in the far-infrared and THz, wavelength ranges of interest for environmental monitoring and chemical sensing.  This research directly addresses DOE Grand Challenges, including understanding how properties of matter emerge from complex electronic correlations and learning how to control these properties as well as the mission of the Basic Energy Sciences program to understand and control matter at the electronic/atomic level.

OP: Investigating High-K Modes in Metamaterial Structures (funded by the NSF)

The goal of this project is to understand and control light propagation through infrared hyperbolic metamaterials (HMMs).  HMMs are materials comprised of alternating layers of metal and dielectric materials, where the thickness of each layer is much less than the wavelength of light.  In our case, the HMMs are built from alternating layers of doped and undoped semiconductors grown by molecular beam epitaxy.  This method enables the growth of crystalline HMMs with low losses, sharp interfaces, high lateral uniformity, tunable optical properties, and atomic control over layer thickness.  This project has two major components.  The first is investigating light transmission through subwavelength gratings fabricated on top of the HMMs.  If a subwavelength aperture were fabricated on a normal material, light would not be able to propagate through the tiny opening.  However, if a subwavelength aperture is fabricated on an HMM, the unusual optical properties of the HMM allow the light to transmit.  By fabricating an array of apertures, we will focus the infrared light into subwavelength volumes, with applications in light harvesting and infrared concentrators.  The second topic aims to incorporate quantum wells into the HMM structure.  The quantum wells will be engineered such that the ground to first excited state transition wavelength aligns with the HMM working wavelengths.  This will allow strong coupling between the quantum well intersubband transition and the high-wavevector optical modes of the HMM.  This would be the first ever demonstration of strong coupling in a non-resonant HMM and lay the foundation for the integration of semiconductor optoelectronic components with infrared HMMs.  This research develops the fundamental understanding required for the creation of monolithically integrated infrared optical systems based on designer semiconductor HMMs.

Spintronics based on topological insulators (collaboration with J. Xiao, funded by DOE, description from DOE award)

Conventional electronic devices have components that serve two separate functions. Logic operations are implemented with circuits that leverage the “charge” of electrons. One of the most fundamental circuit elements is the transistor, which conditions the flow of electrical current depending on the presence of absence of electrons at a specific gate. Memory operations are implemented by leveraging magnetic properties that arise due to the “spin” of the electron. Hard drives and other nonvolatile memory elements store information by changing the orientation of the spins in small domains.

In recent years it has become clear that there are interactions between the charge and spin properties of electrons and that these properties can be leveraged to create new paradigms for device operation that combine logic and memory functions. The potential advantages of devices based on “spintronics” are significant improvements in processing speeds and dramatic reductions in energy consumption. This proposal is focused on understanding the interactions between charge and spins in heterostructures that contain ferromagnetic materials, normal metals, and topological insulators. Independently, these three materials components have magnetic, normal electrical, and coupled spin-charge electrical transport properties.  When the materials are combined, interesting new physical effects can arise at the interfaces between the materials, and these effects can probably be exploited to create new electronic device paradigms that realize the promise of spintronics.

The proposed work has three parts. First, we will grow heterostructures that contain the different material components and measure how the properties of the heterostructure differ from the properties of the components by themselves. Second, we will perform time-resolved measurements of the reorientation of spins in response to applied electric currents. This measurement is important for developing an understanding of the physical interactions that underlie the observed properties and also for developing materials that can be engineered into devices with high processing speeds. Finally, we will develop theoretical models that allow us to understand the emergence of these unique properties in heterostructures and guide the search for new heterostructures with improved performance. The proposal addresses the two grand challenges for condensed-matter and materials physics for the next decade: (1) how to meet the energy demands of future generations; and (2) how to extend the information technology revolution.

MRI: Development of a system for low temperature optical measurement of 3D magnon, plasmon and spin torque transfer dynamics (collaboration with M. Doty, funded by NSF)

As part of this project, we are building a new instrument to measure a variety of optical effects, including plasmon propagation in topological insulators.  Topological insulators are a recently-discovered class of materials such as Bi2Se3 and Bi2Te3 which have an insulating bulk, but conducting surfaces on which the electron spin and momentum are locked: an electron with momentum +kx will have spin up (for example), while an electron with momentum -kx will have spin down. These unusual properties are due to the strong spin orbit coupling in these materials, which lead to a band inversion in which the valence band is higher in energy than the conduction band.  There is a great deal of interest in the fundamental properties of topological insulators, including the possibility of topological quantum computation.  However, we are interested in them for their unique optical properties, specifically, their plasmons.

A plasmon is simply a collective excitation of electrons.  Topological insulators are predicted to have unique two-dimensional, spin-polarized plasmons.  These are interesting for fundamental reasons, as spin-polarized plasmons do not exist in any other system, as well as for applications in spintronics.  The instrument being developed in this project will allow us to demonstrate that the plasmons in TIs are spin polarized as well as investigate their unique dynamics.