News & Highlights

The light emitted by a monolayer semiconductor MoSe2 is modulated as we reversibly change the distance between the semiconductor and the mirror.

Jan 2020: Mechanical tuning of light-matter interactions

Non-technical description: An object (emitter) in its high energy state can release the stored energy by emitting light. This process, called spontaneous emission, happens almost ubiquitously, such as in light bulbs. Though one may think how fast an emitter radiates light is the intrinsic properties of the emitter, scientists realized it in fact also depends on the environment of the emitter. Controlling the radiative properties of an emitter by embedding it into complex optical structures (called cavities and photonic crystals) has since then become a cornerstone of modern optics.

In this work, we control the radiation of excitons (bound electron-hole pairs) in an atomically thin semiconductor using a simple mirror. We change the environment of the excitons by moving the semiconductor towards/away from the mirror and demonstrate a substantial modification of the light emission from excitons (left figure). We show that such control can even be dynamic, which can create new applications in both classical and quantum optical information processing.

The paper was published in Physical Review Letter and was done in collaboration with the Kim and Lukin groups at Harvard.

Unlike normal materials, thermal radiation from samarium nickelate does not change significantly as its temperature rises.

Dec 2019: Unusual thermal radiation

Non-technical description: All matter emits thermal radiation above absolute zero temperature (aka 'blackbody radiation'). Normally the hotter an object gets, the more thermal power it radiates, which forms the principle for infrared thermal imaging.

In this work, we show that an oxide, SmNiO3, violates this fundamental relationship by exhibiting temperature-independent thermal radiation (left image). This is realized by engineering the metal-insulator transition in SmNiO3: intuitively, with increasing temperature, the material becomes more metallic and appears cooler than its actual temperature. This result could enable new technologies in thermal camouflage and thermal management.

The paper appeared in PNAS, and was a collaborative work with groups of Kats (UW Madison), Ramanathan (Purdue), and Comin (MIT).

A monolayer of MoSe2 containing merely three layers of atoms can act as a highly reflecting mirror thanks to the excellent coherence properties of its excitons.

Jan 2018: An atomically thin mirror

Non-technical description: Metals reflect light but become transparent if thinned down to less than tens of atoms thick. So ultimately, how thin can a mirror be? Can we make mirrors made of just a few layers of atoms?

One way to do so is to use 'resonance' effect: incident light at a particular frequency can resonantly create optical excitations in a material. If all of the excited states lose their energy by emitting photons, all of the incident light will be reflected back because of the interference between the re-emitted and incident light. In reality, however, energy stored in excited states can get lost by heat, leading to light being absorbed rather than reflected.

In this work, we show that optical excitations called excitons in high-quality MoSe2 monolayers decay almost entirely by emitting light at low temperatures. Therefore these materials, despite being merely three atoms thick, can act almost as a perfect mirror. Furthermore, their reflection can be turned on and off by a voltage. This result represents the ultimate miniaturization of a mirror and enables unique applications, ranging from nonlinear quantum optics to metasurfaces.

This paper was in Physical Review Letters and was done in collaboration with the Kim and Lukin groups at Harvard. News coverage by Physics, Nature, Chemistry World.

By bringing a two-dimensional semiconductor, WSe2, close to an ultra-flat metal surface, we enhance and directly probe the emission of the nominally spin-forbidden dark excitons.

Jan 2017: Seeing the dark states

Non-technical description: Excitons — bound electron and hole pairs — are an elementary optical excitation of semiconductors. Inside an exciton, the spins of the electron and hole can be either parallel or antiparallel. Excitons with antiparallel spins (bright excitons) can quickly decay by emitting light, while those with parallel spins do not usually emit light. These so-called 'dark excitons' are attractive for information processing because they live longer than bright excitons, but are also harder to study experimentally.

In this work, we introduce a method for probing dark excitons in a two-dimensional semiconductor, WSe2, by placing the material near a metallic surface. The dark excitons in WSe2 interact strongly with the coupled motions of the electrons and electromagnetic waves at the metal surface (a.k.a. surface plasmon polaritons), leading to enhanced light emission that can then be optically measured. This result significantly improves experimental capabilities for probing and manipulating excitons, thus opening up new avenues for active metasurfaces and optoelectronics.

This work appeared in Nature Nanotechnology, and done in collaboration with the Kim and Lukin groups at Harvard. Additional news coverage by Nature Nanotechnology.

In the newly developed high-performance electrolyte, ionic conduction is sustained by protons, while electronic conduction is prevented by a fuel-induced metal-insulator transition. This enables high performance low-temperature solid oxide fuel cells.

May 2016: New design rules for fuel cells

Non-technical description: Fuel cells can generate electricity by oxidizing fuels in an environmentally friendly fashion. Electrolyte — a component of fuel cells which separates the fuel from the oxidizer— needs high ionic conductivity but low electronic conductivity. Typically ionic conduction in the solid electrolyte is created by vacancies (missing atoms) in the lattice. These missing atoms, however, also often creates electronic conduction, leading to efficiency loss or even catastrophic failure of fuel cells.

In this work, we report a fundamentally new electrolyte design strategy to circumvent this issue by using a quantum material, SmNiO3. Unlike many other materials, SmNiO3 becomes much more electronically insulating upon exposure to fuels due to a metal-insulator transition caused by electron correlation effects. At the same time, it also sustains high ionic conductivity, which is comparable to the best performing electrolytes at similar temperatures. This result enables improved low-temperature fuel cell performance and new ways of designing materials for electrochemical energy conversion.

The paper appeared in Nature, with news coverage by Nature Energy, Harvard, Argonne National Lab, IEEE Spectrum, and National Science Review. This work was done in collaboration with the groups of H. Zhou and D. Fong at Argonne.

Strongly correlated materials show promises for future memory and neuromorphic devices.

Aug 2015: Brain-inspired computing devices

Non-technical description: Recently, brain-inspired computation employing artificial neural networks has led to tremendous progress in artificial intelligence, including image and speech recognition. Such computation, however, consumes much more power than human brains when implemented on digital computers, which becomes especially problematic for energy-constrained mobile platforms. Implementing new computing architectures that mimic the brain at the hardware level promises for more energy-efficient brain-like computing.

Here we review recent progress towards building these hardware — neuromorphic devices and circuits — using correlated materials and their phase transitions. We made particular emphasis on materials synthesis and device physics, and critically evaluate their prospects. The paper was published in Proceedings of IEEE, and you can read the full text here.

Electron doping turns SmNiO3 from a reflective metal to a transparent insulator at room temperature, opening new directions for electronics and optics.

Sep 2014: New metal-insulator phase transitions

Non-technical description: The ability to dramatically change a material's properties by controlling its 'electronic phase transitions' (see the previous post for more details) can lay the foundation for a plethora of electronic and optical applications.

Here we report the discovery of a new type of metal-to-insulator transition in an oxide material, SmNiO3, driven by electron doping. The oxide turns from a shiny metal to a transparent insulator when doped with lithium-ions or protons. Its electronic conductivity changes by more than eight orders of magnitude, among the highest ratio achieved in phase change materials at room temperature. We attribute such a phase transition to a Mott transition driven by a quantum mechanical effect called electron correlation. Such a drastic phase transition forms an emerging platform for studying many-body physics and for realizing oxide-based reconfigurable electronic/optical devices.

The paper was in Nature Communications with news coverage from Harvard,

Using a electrical pulse, we can switch an insulating material into a metal within nanoseconds.

Feb 2013: Ultrafast metal-insulator transitions

Non-technical description: Electrons can move freely in a metal but not so in an insulator. Intriguingly, certain materials can switch between being a metal and an insulator depending on external conditions, just as water can change its phase between solid, liquid, and gas. Perhaps the famous material hosting such so-called metal-insulator transitions is vanadium dioxide, whose electrical and optical properties can dramatically change when heated above 68 ºC. This phase change, if triggered electrically, may enable a plethora of new electronic and optical devices.

Here, by optimizing materials growth and device structures, we are able to trigger the phase change in VO2 by a voltage within a few millionths of a second—the fasted ever achieved electrically. Such a study on the dynamics of the phase transition not only forms the foundation for new electronics and optics, but can also help to resolve the everlasting debate over VO2's phase transition mechanism initiated by J.B. Goodenough and N.F. Mott.

The paper appeared in IEEE Electron Device Letters and was done in collaboration with Micron Technology, Inc.. See related discussion in Nature.