Longitudinal electric fields exist in the presence of electric charges, either real or induced. They often hide in the near-field of an object, give rise to local-field factors, stock energy, but alone do not induce a Poynting vector.
I will discuss two cases where their role is far from innocent.  In  a classical transport theory for electromagnatic waves inside media with electric dipoles, longitudinal waves mix with  transverse waves and induce a novel transport channel. This imposes a "minimum electromagnetic conductivity" and rules out Anderson localization. In QED,  longitudinal electric fields are strongly connected to the vector potential.  In the presence of an external magnetic field,  the presence of longitudinal fields inside a medium with   give rise to a diamagnetic Einstein - De Haas effect induced by the quantum vacuum.

Cells compute with chemistry and semiconductors compute with transistors – but both operate by controlling the flow of electrons. Biochemistry typically allows electron flow in proteins only over a few nanometers, whereas semiconductors use wires that can conduct quickly over long distances. What if cells have designed biomolecules that behave like wires? To breathe, living cells typically use soluble, membrane-ingestible molecules, like oxygen, to dispose of electrons generated during metabolism. But, we have found that to “breathe” in deep ocean and underground anoxic environments, soil bacteria, Geobacter have evolved nanowires to export electrons to extracellular acceptors that could be hundreds of cell lengths away.
I will present our recent discoveries that solve a longstanding mystery of how nanowires move electrons to soil minerals or help generate electricity. By correlating cryo-electron microscopy with multimodal functional imaging and a suite of electrical, biochemical and physiological studies, we find that nanowires are made up of polymerized cytochrome proteins that transport electrons via seamless stacking of metal-containing heme molecules over micrometer distances (Cell 2019, Nature Chem.Bio. 2020, Nature 2021). As metalloproteins were not known to polymerize, the discovery of these cytochrome nanowires opens an entirely new field for the development of next-generation living bioelectronics. My recent experimental studies on individual nanowires show inherent spin polarization and the highest electronic conductivity reported on proteins (> 100 S/cm with ultrafast electron transfer rate of ~200 fs). Computational studies suggest that quantum coherent transport accounts for the high conductivity of these nanowires. Identifying the role of quantum effects in these processes will help understand, predict, and ultimately control extracellular electron transfer by protein nanowires used by diverse environmentally important microbes to capture, convert and store energy. Moreover, cytochrome nanowires acting as Biocompatible Quantum Probes at room temperature will enable a route to engineer quantum technologies based on biology.

In vertebrate embryos the sequential segmentation of the pre-somitic mesoderm (PSM) creates a tissue structure associated with the vertebral column. Before a given segment is created, the tissue is pre-patterned by the expression of different genes at different locations. Do cells create these patterns by expressing genes independently within cells, or do the patterns emerge by collective behaviour? In this talk I'll show that for zebrafish embryos, a significant part of the patterning process is encoded within single cells. More specifically, I'll show that a model of a cell containing a stochastic genetic timer and a stochastic genetic clock reproduce the observed gene expression dynamics observed in vivo and in vitro. Finally, this model is extended by introducing the entry rate of the cells to the PSM, and reproduces non-trivial features observed in the patterning of the embryo. This work extends our understanding on how cells pattern the PSM before creating a segment, and also introduces a new strategy for tissue patterning within embryos.

Employing tools from statistical physics and complex systems, my research focuses on understanding collective behavior in biological systems. Using a mix of experimental studies and physics driven modeling, I have been able to elucidate rules and equations that can explain the complex, collective behavior in a variety of systems, from bacterial populations to neural networks. In this talk, I will discuss my previous research examining the dynamics in a mixed population of antibiotic resistant and sensitive bacterial cells as well as population decoding studies of neural signals in the small nematode C. elegans. Through these examples, I will demonstrate how my collective systems approaches can generate insights into how groups of simple actors- such as bacterial cells or neurons- can lead to complex emergent outcomes. As I look towards my future work, I will apply these collective systems approach towards another complex system, bacteria in flow. I will discuss my preliminary results and future plans for studying bacterial adherence and dynamics in complex flow environments, inspired by clinical endocarditis infections.
 

Analogous to the role of highways in our macroscopic world, the cytoskeleton organizes the cellular cytoplasm. Micron-sized cytoskeletal polymers, such as microtubules, link distant cellular sites. Nanometer-sized motor proteins walk on complex multi-microtubule highway systems to drive intracellular transport and also remodel the "highway". I will present two different aspects of microtubule organization in my seminar. First, I present an unexpected discovery that nanometer-sized proteins separated by several microns on microtubules can sense and respond to each other. This challenges the long-held view of microtubules as a passive platform and reveals how the microtubule is like a wooden bridge rather than a concrete highway. Second, I present the development of an Atomic Force Microscopy assay that enables us to directly visualize the dynamic features of individual microtubules within complex microtubule arrays. This imaging modality bridges the resolution gap between light and electron microscopy to reveal new insights by which complex microtubule arrays can be remodeled by associated proteins.

The health of our body is manifested at the cellular level by interactions between biomolecular machines. A mechanistic understanding of cellular processes at the molecular level is crucial for designing effective strategies to combat various diseases. In this seminar, I will present my work on developing a “computational microscope” that combines physics, molecular simulations, and machine learning to investigate exemplary molecular motor systems that play key roles in genome maintenance and cardiac muscle contraction. Firstly, I have developed rare-event sampling techniques based on statistical physics to search for the most probable transition pathways for biomolecular processes. The methodology enables me to capture the millisecond dynamics of helicase motors translocating nucleic acid substrates. Secondly, by integrating informatics approaches with molecular simulations, I was able to reveal how the myosin-actin complex generates force in the human heart muscle, providing insights into the allosteric network encoded in the machine. The computational platform directly links the protein sequence space to their functions. Finally, I will discuss my plans to study the emergent behaviors of the actin-myosin systems, engineer molecular motors with novel functions, and develop strategies for treating cardiomyopathy.

Despite being an old material in optical and microwave technologies in its bulk form, thin-film lithium niobate (TFLN) has recently emerged as one of the most promising integrated photonic platforms owing to its strong electro-optic (EO) coefficient, quadratic optical nonlinearity, and broadband optical transparency ranging from 250 nm to 5 µm. In this talk, I will first overview the basic optical properties of LN, and how LN nanophotonics can grant us new regimes of nonlinear light-matter interactions. Then I will present some of our recent experimental results on the realization and utilization of dispersion-engineered and quasi-phase-matched ultrafast photonic devices in both classical and quantum domains. I will discuss the realization of 100 dB/cm optical parametric amplification [1], 1.5-3 µm widely tunable optical parametric oscillator (OPO) [2], ultra-wide bandwidth quantum squeezing [3], femtosecond and femtojoule on chip all-optical switching [4], and the integrated mode-locked lasers based on TFLN [5]. 

[1] L. Ledezma*, R. Sekine*, Q. Guo*, R. Nehra, S. Jahani, and A. Marandi, "Intense optical parametric amplification in dispersion-engineered nanophotonic lithium niobate waveguides," Optica, vol. 9, pp. 303-308, 2022.
[2] L. Ledezma, A. Roy, L. Costa, R. Sekine, R. Gray, Q. Guo, et al., "Widely-tunable optical parametric oscillator in lithium niobate nanophotonics," arXiv preprint arXiv:2203.11482, 2022.
[3] R. Nehra*, R. Sekine*, L. Ledezma, Q. Guo, R. M. Gray, A. Roy, et al., "Few-cycle vacuum squeezing in nanophotonics," Science, 2022.
[4] Q. Guo*, R. Sekine*, L. Ledezma*, R. Nehra, D. J. Dean, A. Roy, et al., "Femtojoule femtosecond all-optical switching in lithium niobate nanophotonics," Nature Photonics, vol. 16, pp. 625-631, 2022.
[5] Q. Guo et. al. Actively mode-locked laser in nanophotonic lithium niobate with Watt-level peak power (To be submitted).
 

In this talk, I'll discuss the confining and scattering phenomena of electrons in a Lieb lattice subjected to the influence of a rectangular electrostatic barrier. In this setup, hopping amplitudes between nearest neighbors in orthogonal directions are considered different, and the next-nearest neighbor interaction describes spin-orbit coupling. This makes it possible to confine electrons and generate bound states, the exact number of which is exactly determined for null parallel momentum to the barrier. In such a case, it is proved that one even and one odd bound state is always generated, and the number of bound states increases for non-null and increasing values of the parallel momentum. That is, these bound states carry current. In the scattering regime, the exact values of energy are determined where the resonant tunneling occurs. The existence of perfect tunneling energy in the form of super-Klein tunneling is proved to exist regardless of the bang gap opening. Finally, it is shown that perfect reflection appears when solutions are coupled to the intermediate flat-band solution.

The most powerful means of understanding nature at the smallest length scales is through the use of particle colliders. Colliders smash particles together at high energies, briefly producing new particles through quantum fluctuations, which then decay into complicated sprays of energy in surrounding detectors. Much in analogy with how the details of our cosmic history are imprinted in the cosmic microwave background, the detailed features of the interactions of elementary particles are imprinted into macroscopic correlations in the energy flow of the collision products. Understanding the underlying microscopic physics in collider experiments therefore relies on our ability to decode these complicated correlations in energy flow. In turn, the desire to understand how to compute collider observables from an underlying quantum field theory (QFT) description has been a driver of theoretical developments and insights into the structure of QFT.

Accreting compact objects such as neutron stars and black holes in X-ray Binaries (XRBs) and the supermassive black holes at the center of galaxies, called Active Galactic Nuclei (AGN), exhibit complex variability in their emission. The variability is ubiquitous and operates over a wide range of timescales from seconds up to years. Traditional studies of XRBs and AGN in large time domain surveys aim to connect stochastic characterizations of the variability to the underlying physical processes responsible for accretion onto the compact object, but frequently fail to capture nonlinear or higher-order modes of variability. Methods from topology and nonlinear dynamics distinguish between deterministic, chaotic, and stochastic behavior, and can be used to identify modes of variability that relate to dominant accretion processes. I will review several applications of nonlinear dynamics and chaos theory to the study of both XRBs and AGN that show a connection between nonlinear behavior and the underlying accretion processes. I will outline an approach from nonlinear time series analysis – called recurrence analysis – applied to the systematic and ensemble study of variable sources that can inform current and future time domain missions.

By deliberately introducing disorder into the lattices, we manipulate propagation to produc e conductive or localized transport. The role of dimensionality, in particular, is important in such phenomena. In this talk, I will present the statistical properties of photons transport in 1D quasi-periodic amplifying structures with inherent non-Hermiticity. Specifically, I will talk about the lasing in Anderson localized states near a critical degree of disorder. Because of the presence of non-Hermiticity and amplification, we can investigate gain/loss phenomena as well as coupling between Anderson localized modes.

Cargo inside cells is transported by molecular motors that move ballistically on cytoskeleton, interspersed with diffusive episodes when motor-cargo complexes detach from the cytoskeleton.  A common type of cytoskeletal morphology involves multiple converging microbutubules with their minus ends collected at the microtubule organizing center (MTOC) in the interior of the cell.  This arrangement enables MTOC to serve as a trap, enabling agglomeration of cargo.  The general principles governing dynamics, efficiency, and tunability of transport and trapping ability of MTOC is not fully understood.  To address this, we develop a one-dimensional model that includes advective transport towards an attractor (such as the MTOC), and diffusive transport that allows particles to reach absorbing boundaries (such as cellular membranes).  We show that the mean first passage time (MFPT) to reach the boundaries experiences a dramatic growth in magnitude, transitioning from a low to high MFPT regime over a window of cargo attachment-detachment rates that is close to in vivo values.  Furthermore, we find that increasing either the attachment or detachment rate can result in optimal dispersal away from MTOC when the attractor is placed asymmetrically.  Finally, we also describe a regime of rare events where the MFPT scales exponentially with advective velocity towards the attractor and the escape location becomes exponentially sensitive to the attractor positioning.  Taken together, our results suggest that structures such as the MTOC allow for the sensitive control of the spatial and temporal features of transport and corresponding function under physiological conditions. 

Subwavelength confinement, chiral sensing, and frequency conversion of light at the nanoscale are highly desirable for future photonic and optoelectronic applications of quantum materials. By breaking the time-reversal symmetry, magnetic field enables novel light-matter interactions with important real-space features such as chiral magnetopolaritons, unidirectional edge photocurrent, and nonreciprocal light propagation at magnetic interfaces. However, due to many technical difficulties, these important magneto-optical phenomena and their applications at the nanoscale have not been investigated in real space at infrared (IR) or terahertz (THz) frequencies. In this talk, I report a direct visualization of the infrared magnetoexciton polaritons due to quantized Landau transitions in near-charge neutral graphene, using a novel magneto scanning near-field optical microscope (m-SNOM) working in a magnetic field up to 7 Tesla. We map the magnetic field-dependent polariton excitation and propagating at the edge of graphene and hBN and explore its associated enhanced chiral edge photocurrent down to the quantum Hall region. Our approach establishes m-SNOM as a versatile platform for exploring magneto-optical effects at the nanoscale. This preliminary research sets the stage for future spectroscopic investigations of the topological and chiral photonic phenomena in complex quantum materials using low-energy photons.

Mengkun Liu (Ph.D. 2012 Boston University) is an associate professor at the Department of Physics and Astronomy of Stony Brook University (since Jan. 2015). His post doc research was at UC San Diego from 2012-2014. His research interests include physics of correlated electron systems, low-dimensional quantum materials, infrared and terahertz nano-optics and ultrafast time-domain spectroscopy.  Prizes include NSF career award (2021) and Seaborg Institute Research Fellowships at Los Alamos National Lab (2009, 2010).