The quantization of electron motion in magnetic fields generates a plethora of fascinating phenomena observed in condensed materials. One of the well-known examples is the Shubnikov-de Haas (SdH) resistance oscillations. In two dimensional electron systems, SdH oscillations can be very pronounced leading to the Quantum Hall Effect (QHE) at low temperatures. 

Landau quantization produces a remarkable effect on Joule heating of two dimensional (2D) electrons. The heating forces 2D electrons into exotic electronic states in which voltage (current) does not depend on current (voltage). In contrast to the linear response at low temperatures (SdH, QHE), the quantization affects Joule heating in a significantly broader temperature range. At temperatures significantly exceeding the cyclotron energy the dc heating produces a multi-tiered electron distribution containing as many tiers as the number of Landau levels inside the energy interval kT. This quantal heating preserves the overall broadening of the electron distribution. Surprisingly the distribution resulting from quantal heating is, in some respect, similar to the one created by the quantum microwave pumping between Landau levels. Indicated phenomena produce a broad variety of nonlinear effects in quantizing magnetic fields and present an exciting area of the contemporary research. In this talk a recent experimental investigations of the dynamics of quantal heating are presented indicating an important role of the electron-electron interaction in the relaxation of the electron distribution.  

Noble metal nanoparticles can support localized surface plasmons, which lead to enhanced electromagnetic fields at the nanoparticle surface and allow for a host of surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS).  While extensive theoretical calculations have been performed that predict how these enhanced electromagnetic fields are distributed on the nanoparticle surface, confirming these results using optical techniques is extremely challenging due to the diffraction limit of light.  Because the metal nanoparticles are smaller than the wavelength of light, they appear as diffraction limited spots in optical images, obscuring the local electromagnetic field enhancements.  This talk will describe recent efforts to use high resolution single molecule imaging techniques to measure how electromagnetic fields are locally enhanced on the surface of noble metal nanoparticles for applications in SERS.  Single molecule spectroscopy allows us to beat the diffraction limit by over an order of magnitude, providing the necessary resolution to optically image electromagnetic field enhancements on noble metal nanoparticle surfaces.

Rapidly evolving acute respiratory infectious diseases (for example, Influenza, Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), and West Nile Flavivirus (WNF)) now have significantly deleterious impacts on human health and economic productivity worldwide. Due to their highly contagious nature, and rapid negative impact on human health and economies, these diseases require developing a simple, high throughput, and immediate (within 30 minutes) screening methodology that can affordably and accurately determine virus diagnosis, so that treatments can be administered in a timely fashion. Furthermore, the expense of anti-virals now prohibits broad distribution even in developed countries. The diagnostic approaches that we are developing in the Pennathur lab enables rapid regionally based deployment of medications to stymie the spread of viruses. These approaches include (1) the development of a nanofluidic conductivity sensor for general nucleic acid detection, (2) fluorescent silver nanocluster DNA probes (AgNC-DNA) combined with microfluidic capillary electrophoresis (mCE), to detect and identify DNA sequences from HepA, HepB and HepC viruses, and (3) microfluidic tangential flow filtration (μTFF) of blood and serum for efficient on-chip sample preparation. 

Specifically, we have developed a novel nanofluidic-based platform for the efficient detection of nucleic acids. The transduction method is label-free, inducing the formation DNA complexes that result in changes in flow velocity and current in a nanofluidic channel. This innovation takes into account the changes in surface and bulk conductivity in a nanochannel due to the concentration of ions in the bulk. Furthermore, we have developed a method for modifying a low cost, molecular beacon-like AgNC-DNA probe so that multiple DNA sequences can be detected and identified simultaneously and rapidly using microfluidic capillary electrophoresis. As a demonstration, we used this technique to design probes for nucleic acid targets of Hepatitis A, B and C virus. Finally, to truly make this work translational, we have developed a microfluidic based method for biological sample filtration. Such a method allows for facile integration with the above diagnostic sensors, and uses tangential flow filtration methods to effectively isolate targets of interest. 

Because of the possibility of electromagnetic interference between neighboring electronic systems, there is an urgent need to quantify the entry and distribution of electromagnetic (EM) energy within complicated metallic enclosures and to understand the manner in which this energy couples to sensitive electronic devices within such enclosures.  When the wavelength of the impinging radiation is much smaller than the typical length scale of the enclosure, the distribution of energy within such cavities is highly sensitive to small changes in the frequency, the structure of the cavity, as well as the nature of the channels which couple EM energy into the cavity.  Thus, a statistical approach to understanding this problem is called for.  
There is great interest in the wave and quantum properties of systems that show chaos in the classical (short wavelength) limit.  These ‘wave chaotic’ systems appear in many contexts: nuclear physics, acoustics, two-dimensional quantum dots, and electromagnetic enclosures, for example. Random Matrix Theory (RMT) predicts the universal fluctuating properties of quantum/wave systems that show chaos in the classical/ray limit.  
In this context we developed a stochastic model, the “Random Coupling Model” (RCM) [1,2], which can accurately predict the probability density functions (PDFs) of voltages and electromagnetic field quantities on objects within such cavities, given a minimum of information about the cavity and the nature of its internal details.   The RCM is formulated in terms of electrical impedance, essentially equivalent to Wigner’s reaction matrix in quantum mechanics, rather than the more commonly studied scattering matrix.  The RCM predictions have been tested in a series of experiments using normal metal and superconducting quasi-two-dimensional and three-dimensional electromagnetic billiards [3].  We have extended the RCM in a number of directions, for example by examining the effects of ‘short orbit’ ray trajectories that enter the cavity, bounce a small number of times, and then leave the cavity.  We are able to account for the effects of these orbits using a semi-classical theory, and find excellent agreement between theory and experiment [4].  Our current efforts are focused on testing predictions for the statistical properties of multiple inter-connected enclosures [5], enclosures irradiated through apertures [6], and enclosures characterized by a mixed chaotic and regular phase space [7], using scaled model structures.
For more information see: http://anlage.umd.edu/AnlageQChaos.htm. 

[1] S. Hemmady, et al., Phys. Rev. Lett. 94, 014102 (2005).
[2] X. Zheng, T. M. Antonsen and E. Ott, Electromagnetics 26, 3 (2006); Electromagnetics 26,     37 (2006).
[3]  S. Hemmady, et al., IEEE Trans. Electromag. Compat. 54, 758-771 (2012); Z. B. Drikas, et     al., IEEE Trans. Electromag. Compat. 56, 1480-1487 (2014).
[4]  J.-H. Yeh, et al.,  Phys. Rev. E 81, 025201(R) (2010); Phys. Rev. E 82, 041114 (2010). 
[5]  G. Gradoni, et al., Phys. Rev. E 86, 046204 (2012).
[6]  G. Gradoni, et al., IEEE Trans. Electromag. Compat., 57, 1049-1061 (2015).
[7]  Ming-Jer Lee, et al., Phys. Rev. E 87, 062906 (2013).

​The resonant metallic nanoparticles are proven to be efficient systems for the electromagnetic field control at nanoscale, owing to the ability to localize and enhance the optical field via excitation of strong plasmon resonances. In turn, high refractive index dielectric nanoparticles with low dissipative losses in the visible range, possessing magnetic and electric Mie-type resonances, offer great opportunity for light control via designing of scattering properties. Such resonant nanoparticles made of high refractive index dielectrics (Si, Ge etc.) revolutionized the field of nanophotonics, opening a new branch – All-dielectric Nanophotonics. In this talk, we will discuss recent advances in the all-dielectric and hybrid (metal/dielectric) nanophotonics, including such effects as nonlinear reconfiguration of nanoparticle scattering properties and enhanced optical frequency conversion. Additionally, I will present our novel methods for fabrication of resonant all-dielectric and hybrid nanoparticles.

100-plus years of theoretical and experimental advances have reduced kinematical scattering formalisms for powder diffraction to routine, vendor-supplied, black-box analysis programs accessible to users at all training levels. Understanding what really goes on in the analysis, however, is a non-trivial task. We used computer modeling to analyze the powder diffraction process from nanoparticle ensembles.
Our results showed, surprisingly, that the classical formulations described in diffraction textbooks were inadequate; venerable concepts like reflection multiplicity, the "Lorentz factor", sampling statistics, etc. actually depended on the size of the crystalline particles contributing to the diffraction profile. We expect modeling of scattering experiments to yield more surprises as the phase space hidden behind canonical assumptions becomes accessible for exploration.

Classical novae and supernovae were long thought to be completely separate astrophysical phenomena. This is no longer true; at least some supernovae may have symbiotic nova precursors. I’ll review the current state of  knowledge of the temporal evolution of the white dwarfs in novae, and the Tree of Death of Supernovae. These will help illuminate the still-controversial but ultimately testable, hypothesized connection between novae and supernovae.

In recent years, graphene has emerged as a potential material in optoelectronic devices ranging from optical modulators, photodetectors to saturable absorbers for mode-locking lasers. Significant effort has also been made to develop graphene-based materials and devices for biotechnological applications such as biosensors, drug delivery, cell imaging and detection.
The aim of this talk is to introduce the optoelectronic properties of graphene and possible applications to various devices. The devices covered will range from nonlinear optical devices such as optical limiters and saturable absorbers for mode-locking lasers, to graphene field effect transistors (FETs) for chemical sensing. 

Manipulating small spin textures that can serve as bits of information by electric currents is one of the main challenges in the field of spintronics. In this work we study the stability and current driven dynamics of Skyrmions and their lattices in two-dimensional ferromagnets [1,2] and antiferromagnets [3] with Dzyaloshinskii-Moriya interaction.
 
Ferromagnetic Skyrmions recently attracted a lot of attention because they are small in size and are better than domain walls at avoiding pinning sites while moved by electric current. Nevertheless, ferromagnetic Skyrmions also have certain disadvantages, such as the presence of stray fields and transverse dynamics, making them harder to employ in spintronic devices. To avoid these unwanted effects, we propose a novel topological object: the antiferromagnetic (AFM) Skyrmion [3] and explore its properties using analytical theory based on generalized Thiele equation and micromagnetic simulations. This topological texture has no stray fields and we show that its dynamics are faster compared to its ferromagnetic analogue. We obtain the range of stability and the dependence of AFM Skyrmion radius on the strength of Dzyaloshinskii-Moriya interaction coming from relativistic spin-orbit effects.
 
Moreover, we study the temperature effects on the stability and mobility of AFM Skyrmions. We find that the thermal properties, e.g. such as the antiferromagnetic Skyrmion radius and diffusion constant, are rather different from those for ferromagnetic Skyrmions. More importantly, we show that due to unusual topology the AFM Skyrmions do not have a velocity component transverse to the current (no topological Hall effect), and thus may be interesting candidates for spintronic memory and logic applications.
 
[1] I. A. Ado, O. A. Tretiakov, M. Titov, arXiv:1603.07994 (2016).
[2] U. Gungordu, R. Nepal, O. A. Tretiakov, K. Belashchenko, and A. A. Kovalev, Phys. Rev. B 93, 064428 (2016).
[3] J. Barker and O. A. Tretiakov, Phys. Rev. Lett., 116, 147203 (2016).

A spatially varying gap leads to the creation of edge states. These very robust states are associated with quantized currents, the foundation of the quantum Hall effect in electronic systems. Here we discuss a randomly distributed gap in photonic systems. Despite the presence of strong disorder, the behavior of photons is not characterized by conventional Anderson localization: Rather than confining the photons to an area of the size of the localization length, the random gap creates geometric states. This type of confinement can be understood as angular localization, where the photons of a local light source can propagate only along waveguides in certain directions. The directions are determined by the boundary of the spectrum. Thus, the system's properties on the shortest scales determine the behavior of the photon propagation on the largest scales.

Light scattering from non-spherical particles and aggregates exhibit complex structure that is revealed only when observed in two angular dimensions. However, due to variations in shape, packing, and orientation of such aerosols, the structure of two-dimensional angular optical scattering (TAOS) patterns varies among particles. The spectral dependence of scattering contributes further to the observed complexity, but offers another facet to consider. By leveraging multispectral TAOS data from flowing aerosols, we have identified novel morphological descriptors that may be employed in multivariate statistical algorithms for “unknown" particle classification. While these descriptors provide a means for grouping particles as a class, they provide little information about particle orientation. For this, we implement digital holography, which can be recorded simultaneously with TAOS data on a single camera to enhance particle characterization. This talk will discuss the underlying principles behind the two strategies and their synergy for particle characterization.

Disorder effects on the transport of classical and matter waves (EM & electrons, for instance) have been widely investigated from fundamental and practical points of view. It is widely believed that the presence of disorder in 1D random media leads to an exponential spatial localization of waves, i.e., Anderson localization. We have recently proposed, however, a model of disorder that induces anomalous localization or delocalization of electrons in disordered quantum wires. Following that model, we provide experimental evidence demonstrating that anomalous localization of electromagnetic waves can be induced in microwave waveguides with dielectric slabs randomly placed: if the random spacing between the slabs follows a distribution with a power-law tail (Lévy-type distribution), unconventional properties in the microwave-transmission fluctuations take place revealing the presence of anomalous localization. We obtain both theoretical and experimental distributions of the transmission through random waveguides and show that only two parameters, both of them experimentally accessible, determined the complete transmission distribution.

Concerning matter waves (electrons), numerical simulations of disordered armchair graphene nanoribbons reveal the presence of anomalous electron localization, while the statistical properties of the conductance are also described by our model.

In this talk we will give some general and basic ideas of our theoretical framework (random-matrix theory) for describing wave transport phenomena in the presence of standard-Anderson and anomalous localizations.

Despite the fact that there are still abundant natural petroleum reserves (supplies will last for more than a century), significant carbon mitigation cannot be achieved without the development of environmentally sustainable and renewable fuels. Owing to their high productivity-to-biomass ratio, ease of cultivation, and ability to grow in saline water, algae have been considered as a leading biodiesel feedstock. To displace fossil fuels, however, algae must be grown at a scale that yields approximately 10 million barrels of oil per day – which would supply approximately 50% of the total U.S. consumption. For the last few decades, researchers have searched for the “sweet spot” between algae’s triacylglycerols (TAG) production and biomass accumulation to obtain a strain with increased lipid production that can be developed as a commercially viable algal feedstock for biofuel production. Diatoms, a unique algal taxon, naturally accumulate TAGs as storage components, which can be readily converted to biodiesel. In fact, lipids derived from fossil diatoms are a major component of the highest quality petroleum.  Therefore fast growing, lipid accumulating, diatoms can be an excellent platform for biodiesel production. For many years, studies have been performed to environmentally optimize diatoms’ lipid production and biomass accumulation, yet no economically sustainable strain has been reported. In my talk, I will present our unique, genetically modifies, strains generated from the model diatom Phaeodactylum tricornutum, that could be used as a test-case for economical sustainable biofuel production. These strains are characterized by high lipid yield, yet keep relatively fast growth rates and are more efficient in using solar energy for lipid production.