Propagation of waves can be blocked and wave modes localized by disorder. First discovered for electrons at low temperatures, this “Anderson localization” takes place for other waves as well. Theoretical studies and experimental observations have been reported for sound and matter waves in various space dimensionalities, going from one-dimensional (1D) chains to three-dimensional (3D) bulk disordered materials. Interestingly, electromagnetic waves in general and light in particular seem to stand out from this general picture. First, the common belief is that Anderson localization is promoted by strong disorder. Localization of light, however, can be observed at weak disorder by confining light propagation to (quasi-)1D tubes or 2D planes but difficulties exist in 3D where even the strongest reachable disorder didn’t allow to demonstrate Anderson localization of light convincingly. Second, recent theoretical studies indicate that strong disorder seems to disadvantage localization by opening a new channel of wave transport involving non-propagating longitudinal fields. These fields are specific for electromagnetism and do not exist for scalar waves (sound or matter waves) or other vector waves (elastic waves in solids). Moreover, the point-scatterer model that allows for an exact solution, predicts diffuse scattering of light and exhibits no sign of localization at any disorder strength, even when the widely accepted Ioffe-Regel criterion of localization is obeyed by far. It seems therefore that in contrast to other waves, light can be localized by disorder in low-dimensional systems only, whereas a structured medium – a photonic crystal or a hyperuniform structure – is required to observe localization in 3D. In these latter materials, localization is expected to take place at weak disorder, similarly to low-dimensional systems and in agreement with the idea that strong disorder impedes the localization of light.
 

Nanoscale ZnO particles are known to inhibit the growth of bacteria.  The fundamental mechanisms driving this process, however, are not completely understood.  While there are many contributing factors to consider, we hypothesize that the antimicrobial action is most fundamentally derived from the ZnO surface and its interaction with growth media and the bacteria’s extracellular material.  In this work, we implement minimum inhibition concentration and novel comparative assays to evaluate the antibacterial activity of ZnO microcrystals produced by us using a hydrothermal chemical growth method.  The samples were synthesized in the range of sizes from 1µm to 5µm with varying abundances of surfaces with different polarities.  This approach prevents the ZnO particles to be internalized by the bacterial cells with diameters ca. 500 nm, thus allowing one to study correlations between overall surface polarity and antibacterial action.  These experiments were performed in conjunction with optoelectronic studies of ZnO crystals (photoluminescence, surface photovoltage) to characterize electronic structure and dominant charge transport mechanisms as fundamental phenomena, which could potentially govern the processes leading to an antibacterial behavior in our samples.  We report on the results of these comparative studies relating antibacterial properties with surface morphology and electronic behavior.

Computational problems may be solved by realizing physics systems that can simulate them. Here we present a new system of up to >1000 coupled lasers that is used to solve difficult computational tasks. The well-controlled dissipative coupling anneals the lasers into a stable phase-locked state with minimal loss, that can be mapped on different computational minimization problems. We demonstrate this ability for simulating XY spin systems and finding their ground state, for phase retrieval, for imaging through scattering medium and more.

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common.  Driving the field is the study of exoplanet atmospheres, with the goal of detecting a gas that might be indicative of life. A suitable “biosignature gas” is not just one that might be produced by life, but one that: can accumulate in an atmosphere against atmospheric radicals and other sinks; has strong atmospheric spectral features; and has limited abiological false positives. Which gases might be potential biosignature gases in an as yet unknown range of exoplanetary environments? New computer simulations and next-generation telescopes coming online means the ambitious goal of searching for “biosignature gases” in a rocky exoplanet atmosphere is within reach.  

Answering the titular question has become a central motivation in the field of quantum biology, ever since the idea was raised following a series of experiments demonstrating wave-like behavior in photosynthetic complexes. Here, we report a direct evaluation of the effect of quantum coherence on the efficiency of three natural complexes. An open quantum systems approach allows us to simultaneously identify their level of “quantumness” and efficiency, under natural physiological conditions. We show that these systems reside in a mixed quantum-classical regime, characterized by dephasing-assisted transport. Yet, we find that the change in efficiency at this regime is minute at best, implying that the presence of quantum coherence does not play a substantial role in enhancing efficiency. However, in this regime, efficiency is independent of any structural parameters, suggesting that evolution may have driven natural complexes to their parameter regime to “design” their structure for other uses.

Hyperuniformity is a new type of long-range order that encompasses all perfect crystals, perfect quasicrystals, and some exotic disordered states of matter. Disordered hyperuniform many-particle systems [1,2] can be regarded to be new states of disordered matter in that they behave more like crystals or quasicrystals in the manner in which they suppress large-scale density fluctuations, and yet are also like liquids and glasses because they are statistically isotropic structures with no Bragg peaks. Thus, these special correlated disordered materials possess a "hidden order" that is not apparent on large length scales. A variety of groups have found that disordered hyperuniform materials possess desirable photonic and electronic bandgap properties. More recently, we have shown that they possess nearly optimal transport and elastic properties. I will review the salient ideas behind the hyperuniformity concept and  procedures to design a variety of different disordered hyperuniform materials as well as their corresponding physical properties, including novel transport, mechanical, electromagnetic and elastodynamic characteristics [3,4,5]. It has been a numerical and experimental challenge to create very large samples that are hyperuniform with high fidelity. I will discuss recent progress that we have made in this direction [6] and its implications for novel physical properties.
 

1. S. Torquato and F. H. Stillinger, "Local Density Fluctuations, Hyperuniform Systems, and Order Metrics," Phys. Rev. E, 68, 041113 (2003).
2. S. Torquato, "Hyperuniform States of Matter," Phys. Reports, 745, 1 (2018).
3. G. Zhang, F. H. Stillinger, and S. Torquato, "Transport, Geometrical, and Topological Properties of Stealthy Disordered Hyperuniform Two-phase Systems," J. Chem. Phys., 145, 244109 (2016).
4. S. Torquato and D. Chen, "Multifunctional Hyperuniform Cellular Networks: Optimality, Anisotropy and Disorder," Multifunctional Materials, 1, 015001 (2018).
5. J. Kim and S. Torquato, Multifunctional Composites for Elastic and Electromagnetic Wave Propagation, Proc. Nat. Acad. Sci., 117, 8764 (2020).
6. J. Kim and S. Torquato, "New Tessellation-Based Procedure to Design Perfectly Hyperuniform Disordered Dispersions for Materials Discovery, Acta Materialia, 68, 143 (2019).

Gamma rays comprise the highest energy band of the electromagnetic spectrum. Much of the celestial and solar radiation in this band could only be studied once instruments were carried above most of the Earth’s atmosphere by large balloons or by satellites around 1960.  The instruments had their heritage in the nuclear and particle physics communities and encountered challenges due to their weight and the severe background radiation that they encounter.  Tremendous strides have been made since then.  With current space gamma-ray instruments spanning the energy range from a fraction of an MeV (million electron volts) to hundreds GeV, the sky is both static and highly variable.  Most clearly visible at energies above 100 MeV, there is a faint isotropic glow from objects in the early universe and a striking band of radiation from our milky way galaxy produced by discrete stellar-like objects, such as pulsars, and from radiation produced when cosmic rays interact with the interstellar medium.   In the MeV nuclear energy range, diffuse galactic emission is observed in lines from the annihilation of positrons and decay of radioactive nuclei in the relics of stellar explosions. Gamma-ray bursts sometimes reaching energies of 100 MeV range and typically lasting seconds to minutes occur a few times a day announcing a stellar explosion in the early universe. Flares from the thousands of detected active galactic nuclei powered by massive black holes occur a few times a year.  The Sun is faintly observed above 100 MeV as it moves along the ecliptic plane with radiation produced by cosmic-ray interactions.  The Sun is mostly dark as viewed at MeV energies except during periods of high solar activity when flares accelerate electrons and ions to high energies which then interact in the solar atmosphere producing electron bremsstrahlung and nuclear gamma-ray lines lasting for minutes to hours.  These lines provide both information on the solar elemental abundances and on the composition and directionality of the accelerated ions.  I will highlight some of the studies that I am most acquainted with since I graduated from Queens College in 1961.