Making the world’s best atomic clock

The relentless pursuit of spectroscopy resolution has been a key drive for many scientific and technological breakthroughs over the past century, including the invention of laser and the creation of ultracold matter. State-of-the-art lasers now maintain optical phase coherence over many seconds and provide this piercing resolution across the entire visible spectrum. The new capability in control of light has enabled us to create and probe novel quantum matter via manipulation of dilute atomic and molecular gases at ultralow temperatures.

Application of STEM/EELS to Plasmon-Related Effects in Optical Spectroscopy

The last decade has seen an explosion in the study of plasmonic materials, with current applications including surface-enhanced spectroscopy, imaging beyond the diffraction limit, solar energy harvesting, and ultrasensitive detection.  Our group has been working to characterize the near-field enhancements encountered upon excitation of the localized surface plasmon resonance.

Seeding a New Kind of Garden: Synthesis of Symmetrically Stellated Bimetallic Nanocrystals as a New Class of Plasmonic Colloids

Branched gold-based nanoparticles are ideal platforms for plasmon-enhanced surface spectroscopies to which the introduction of a second metal can impart multi-functionality in catalysis and chemical sensing. However, most examples of stellated nanostructures are asymmetric, with branches randomly distributed from the cores of the nanoparticles. As revealed from single particle light scattering measurements, small deviations in architecture can give rise to variable properties from one particle to the next.

Exciton Fission and Solar Energy Conversion Beyond the Limit

The maximum solar-to-electric power conversion efficiency of a conventional solar cell is determined by the Shockley-Queisser limit of ~31%. One viable approach to exceed this limit is to create two or more electron-hole pairs from the absorption of one photon in a process called singlet fission or multiple exciton generation.  Recent measurements in our group by time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state.

Characterization of the Hydration Structure of Aqueous Carbonic Acid by X-ray Absorption Spectroscopy

Aqueous carbonic acid(H2CO3) is the centerpiece of both the global carbon cycle and physiological buffer systems, yet it remains poorly characterized despite enormous effort.  This reflects the fact that carbonic acid is intrinsically unstable  upon contact with even a single water molecule, reacting in via a proton chain mechanism to ultimately form aqueous bicarbonate and carbonate anions and hydrated protons, which comprises the reversible mechanism of dissolution of CO2 gas.  While solid and gaseous carbonic acid have been studied in some

Hyperbolic Polaritons

In 1987, the search for a medium that expels vacuum fluctuations in a prescribed bandwidth and rigorously forbids spontaneous emission led to the concept of the photonic crystal. Here, we argue that the search for the opposite effect: enhancing vacuum and thermal fluctuations inside a medium within a prescribed bandwidth can be accomplished by an artificial medium known as a hyperbolic metamaterial.

From fundamental reaction mechanisms to improved heterogeneous catalysts

First-principles calculations for heterogeneously catalysed reactions have emerged as a major source of information for understanding the underlying reaction mechanism. Combining computational chemistry derived insights with experimentally measured reaction rates and reaction orders, along with input from spectroscopies and materials characterization techniques can lead the quest for the nature of the active site.

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