Two dimensional (2D) electronic systems serve as a fundamental platform for condensed matter physics. The recent discoveries of new classes of 2D materials, such as graphene, transition metal dichalcogenides, and topological insulators, have provided opportunities to investigate new physics and device applications. In the first part of my talk, I will cover our resent optical investigation of atomically thin transition metal dichalcogenides with electrical control. We show that this mono or bilayer semiconductor not only behave as remarkable excitonic systems in the truly 2D limit, but also provide an ideal system for optical generation and electrical control of valley degrees of freedom, which is a manifestation of control of Berry phase effects in Bloch bands. In the second part of may talk, I will show the experimental demonstration of chiral edge photocurrent in nano beams with strong spin-orbit coupling, which resembles the spin Hall effect in GaAs system. We attribute the observation to the chiral nature of the surface spin states generated by the Rashba spin-orbit coupling effect.

Scaling theory has become a key approach to understanding complex systems, unifying diverse phenomena including nature, society and technology. Here we first present a scaling analysis applied to real-world networks, uncovering the self-similar nature of their structures. By applying renormalization group (RG) theory, we classify networks into universality classes in the space of configurations, characterized by a small/large-world phase transition. These findings help to understand the emergence of the scale-free property in complex networks, and further suggest a unified scaling framework that plays an essential role in our understanding of real-world networks.

We next focus on scaling theory on human mobility. In this case, we introduce two principles that govern human trajectories, allowing us to build a statistically self-consistent microscopic model for individual human mobility. The model account for the empirically observed scaling laws, but also allows us to analytically predict most of the pertinent scaling exponent.

My research group is interested in infrared photovoltaics, i.e. the capture of IR radiation and its transformation into usable energy [1].  In studying the interaction between infrared radiation and matter we have discovered many interesting phenomena involving radiative polaritons in thin oxide films [2], and solitons and chaos in thermoelectric devices.  I will discuss these phenomena and illustrate a possible related experiment with the infrared free electron laser.

[1] A.J. Vincent-Johnson, K.A. Vasquez, J.E. Bridstrup, A.E. Masters, X. Hu, and G. Scarel, “Heat recovery mechanism in the excitation of radiative polaritons by broadband infrared radiation in thin oxide films”.  Appl. Phys. Lett. 99, 131901 (2011).
[2] A.J. Vincent-Johnson, Y. Schwab, H.S. Mann, M. Francoeur, J.S. Hammonds, and G. Scarel, “Origin of the radiation emitted by radiative polaritons”.  Submitted.

Tailoring of functional nanomaterials for nanotechnology and assessing the potential impacts of these materials on human health and ecosystems relies on developing a comprehensive understanding of their reactivity and stability (i.e., transformation behavior), which are inextricably related to their atomic structures and physico-chemical properties. In this seminar, I will give an overview of these topics and include examples from my work aimed at advancing our understanding of the atomic structure and nature of structural and compositional disorder for several technologically relevant and environmentally important nanomaterials. I will also give an overview of high-energy x-ray total scattering and pair distribution function (PDF) analysis, a synchrotron-based scattering method that is a powerful tool for evaluating poorly crystalline phases. Developing a deeper understanding of the fundamental structural aspects of nanosized materials is important for evaluating and predicting their functional potential, behavior in natural and artificial systems, and their possible impact on the environment.

The National High Magnetic Field Laboratory (NHFML) in Tallahassee, FL recently commissioned the Split-Florida Helix magnet, which is a 25 Tesla Bitter magnet that provides wide, free-space optical access to high magnetic fields.  This magnet will enable the implementation of high-resolution and modern optical spectroscopies, which have been limited in the past at high magnetic fields due to the requirement of using optical fibers for routing light in and out from the magnets.  I will discuss this exciting current development as well as highlight recent experimental results coming from both my own research group and the NHMFL’s user program in visible continuous-wave and ultrafast optics.

New phosphors have been discovered in recent years that glow (after excitation stops) longer and brighter than those previously known. Among other applications, these materials allow for some useful teaching demos, a few of which will be shown. Aspects of the history of fluorescence will be discussed, touching on contributions by Stokes, Einstein, Hopfield, Chalfie, and J. Zallen. The new materials and proposed mechanisms for their long-persistent (> 10 h) luminescence will be described, as will a connection (?) to Sherlock Holmes and "The Hound".

The genomes of Thermotogales species show evidence of significant horizontal gene transfer (HGT) from the Archaea. The Thermotogales have acquired genes from archaeal lineages that grow at higher temperatures than they do. Previous in silico analyses of Thermotogales whole genome protein sequences predicted that the ancestor of the Thermotogales lived at a higher OGT than extant species. We tested this hypothesis by measuring the activity and thermostability of extant and reconstructed ancestral myo-inositol-3-phosphate synthase (MIPS) that was inherited from archaea by HGT. An in silico analysis of the reconstructed ancestral Thermotoga proteins predicted that they would be more thermostable than the extant proteins. The measured temperature optima (T_opt) of recombinant extant MIPSs were found to be near the OGTs of their source organisms. Four possible variants of an ancestral Thermotoga MIPS were constructed and found to have higher T_opt than the MIPSs from Thermotoga sp. str. RQ2 and Tt. maritima MSB8. The melting temperature (T_m) values of the reconstructed proteins were determined by differential scanning fluorimetry and are at least 3.4°C higher than that of Thermotoga sp. str. RQ2, the most thermostable extant Thermotoga MIPS tested. The average T_m of the reconstructed proteins values is 5.7°C higher than that of the average of the extant Thermotoga species' proteins, which is consistent with our hypothesis. Two reconstructed ancestral Thermococcus MIPSs are active and highly thermostable with higher T_m values than those of the extant and reconstructed Thermotoga MIPSs. Two reconstructed ancestral MIPSs from deeper in the archaeal lineage had higher T_m than the extant and reconstructed thermococcal MIPSs and the Pyrococcus furiosus MIPS, the most thermostable extant MIPS tested. These results suggest that the ancestral Thermotogales and Thermococcales may have grown at higher OGTs than their descendants.

Gold and silver nanoparticles (NPs) are attractive nanomaterials because of their distinguished optical property-localized surface plasmon resonances (LSPRs). The resonance happens when the frequency of the incident light matches the natural frequency of the surface electrons oscillating determined by the restoring force of the nuclei. The natural frequency is highly dependent on the size and shape of the NP, the refractive index of the surrounding medium, and the nearby metal material. Combined with different polymers, silver and gold NPs can be used in many sensors for chemical and biological molecule detection in the use of LSPRs. In this talk, silver nanocubes, as an example, will be discussed from synthesis to poly(allylamine hydrochloride)-dithiocarbamate (PAH-DTC) coating. The peak wavelength of LSPRs can be tuned by different concentration of the PAH-DTC during coating. And a hybrid gold nanosphere-film structure using a polyelectrolyte multilayer film (poly(allylamine hydrochloride) (PAH) /Poly(sodium 4-styrenesulfonate) (PSS)) as a spacer and its pH-triggered tunable LSPR will be demonstrated.

We will describe two advanced imaging methods developed in our group. First, we will discuss how to probe the photonic density of states (PDOS) in the vicinity of plasmonic nanostructures. Specifically, we find that by measuring the changes in fluorescence lifetime, we can accurately map the strength of PDOS as well as quantify nanoscale morphological changes induced by polymer swelling / deswelling. Second, we will describe two strategies of "looking" through turbid media such as biological tissues and bioengineered scaffolds used in tissue engineering. In particular, we will discuss our progress towards imaging endothelial cells on blood vessel lumen.

Recent years have seen remarkable progress in our understanding of physical aging in nondisordered systems with slow, i.e. glassy-like dynamics. In many systems a single dynamical length L(t), that grows as a power-law of time t or, in much more complicated cases, as a logarithmic function of t, governs the dynamics out of equilibrium. In the aging or dynamical scaling regime these systems are best characterized by two-times quantities, like dynamical correlation and response functions, that transform in a specific way under a dynamical scale transformation. The resulting dynamical scaling functions and the associated non-equilibrium exponents are often found to be universal and to depend only on some global features of the system under investigation. In this talk I discuss two different types of systems with complex aging properties, namely driven diffusive systems with a logarithmic growth law and a non-equilibrium polymer network that is supposed to capture important properties of the cytoskeleton of living cells. For the driven diffusive systems, we focus on the ABC model and a related domain model and measure two-times quantities in systems undergoing logarithmic growth. For the polymer network model, we explain in some detail its relationship with the cytoskeleton, an organelle that is responsible for the shape and locomotion of cells. Our study of this system sheds new light on the non-equilibrium relaxation properties of the cytoskeleton.