View abstract at https://physics.qc.cuny.edu/uploads/colloquia/Jianbo_Liu_F2025.pdf

Using an established simulation technique [1] and motivated by recent work [2] showing the rapid onset of system-spanning rigid structures (identified by a pebble game algorithm) with increase of volume fraction in a highly-concentrated (or dense) suspension, we show that the number of contacts in the network is an increasing function of imposed shear stress, but it also fluctuates during flow.  The microscopic interactions between particles and the general behavior of this shear-thickening suspension will be outlined briefly to provide insight to the network development leading to the changes in rheological properties. 
 
We will then explore, or two-dimensional suspensions (monolayers) the development of minimally rigid structures in the shear-thickened suspension as it approaches jamming at high stress, and show that the onset of large rigid clusters exhibits critical behavior.  The extension to 3D and the possible relationship of this behavior to other work showing critical scaling at the onset of shear thickening and jamming will be discussed.
 

1. R. Mari, R. Seto, J. F. Morris & M. M. Denn 2014 Shear thickening, frictionless and frictional rheologies. J. Rheol. 58, 1693.
2. M. van der Naald, A. Singh, T.T. Eid, K. Tang, J. J. de Pablo & H. M. Jaeger, H.M. 2024. Minimally rigid clusters in dense suspension flow. Nature Physics 20, 653–659.

At Home with QC presents:

QC astronomer Keaton Bell uses video recordings from space telescopes to measure vibrations of dead stars called white dwarfs. White dwarf stars are the glowing hot embers left over when most stars run out of nuclear fuel. Some white dwarfs vibrate spontaneously, revealing resonant frequencies of the stars that can be used to map their interior structures. This presentation will describe the physics of stellar vibrations by analogy with musical instruments. We will review how the QC White Dwarf Research Group interprets video recordings of vibrating stars to study their structures and discuss the importance of studying white dwarf stars. This talk will premiere an exciting new discovery that has never been seen by a public audience.

For more information about the presentation and Keaton Bell, click here.

RSVP: bit.ly/AHWQC-KeatonBell

Cavity optomechanics typically relies on mechanical modulation of optical frequencies to couple optical and mechanical degrees of freedom. Here we study a different mechanism, based on mechanically induced symmetry breaking that couples otherwise independent optical modes. Systems with this type of interaction exhibit non-trivial Hamiltonian dynamics even in the absence of external drive and dissipation, in contrast to standard models where the absence of pumping leads only to a trivial equilibrium shift. This dynamics is marked by a bifurcation: above a critical photon number the trivial equilibrium becomes unstable. In the stable regime, optical and mechanical degrees of freedom share a common spectrum and undergo amplitude-modulated oscillations. Beyond the bifurcation, their behavior diverges: the mechanical oscillator settles into periodic motion at its natural frequency, while the optical modes oscillate at much higher frequencies determined by the mechanical amplitude, with adiabatic modulation. Such adiabatic behavior is interrupted by sudden jumps reminiscent of Landau–Zener transitions in a quantum two-level system. This symmetry-breaking-mediated interaction provides an alternative route for controlling energy exchange between optical and mechanical subsystems, and establishes symmetry breaking as a principle for engineering optomechanical interactions in degenerate cavities.

Light is an electromagnetic wave defined by several degrees of freedom (DoF), including frequency, momentum, amplitude, phase, and polarization. Controlling these properties is crucial across scientific disciplines, and is a key goal of a wide array of technologies. Nanophotonic devices called “metasurfaces” fill this need by structuring common materials (such as silicon, glass, and metals) at subwavelength scales (micrometer and smaller). The result is tailored light-matter interactions determined by the details of the structure, no longer limited by the materials we are given by nature. These interactions can be customized at will—a sandbox for invention in a platform that is readily manufacturable.
My research aims to both (1) invent, design and develop new devices using novel and emerging physical phenomena and (2) apply these new tools to exciting applications. In particular, starting from an array of uniform subwavelength structures, we have found that introducing small geometric perturbations that break specific symmetries can impart remarkable control to light point-by-point across the device—in some cases, “complete” control over the physically relevant DoF. Applications include new methods for optical combiners in augmented reality systems, custom couplers in integrated photonics for communications systems, and controlling the directionality and polarization of thermal emission in surprising ways.

Adam Overvig is an Assistant Professor in the Department of Physics at Stevens Institute of Technology, since Fall 2023, where he was awarded the AFOSR YIP. He received his PhD in applied physics from Columbia University in 2020, where he was an NSF IGERT fellow. He was then a postdoctoral researcher at the Advanced Science Research Center at the City University of New York until 2023, where he was recognized as a Finalist for the Blavatnik Regional Awards for Young Scientists. His research interests include metasurfaces, symmetries in open systems, holography, thermal photonics, and applications of photonics to emerging quantum technologies.

Optical metasurfaces and planar optics provide wavefront manipulation within a sub-wavelength distance, which hold great promise for miniaturizing optical systems with a reduced footprint and improved functionality. My lab aims to create ultra-compact meta-optics for imaging/sensing with diagnostic applications, and light delivery with therapeutic applications. The mismatch of dimensions and mechanical properties between bulky, rigid substrates and soft biological tissues is a general challenge for in vivo applications. Using computational inverse design strategies, we create ultrathin resonant metasurfaces to facilitate the transfer and heterogeneous integration with various electronic and photonic devices. For example, the integration of metasurfaces on optical fiber tips will improve the compactness and precision of endoscopic optical probes. Furthermore, we leverage resonant metasurfaces that are sensitive to their dielectric environments. We create dynamic tunable metasurfaces, where the resonances of meta-atoms are tuned by the surrounding medium, in order to modulate the far-field optical functions with spatiotemporal control for dynamic light delivery. As a proof of concept, we demonstrate medium-switchable meta-holograms, which also provide direct visual reporting for refractometric sensing. Eliminating plasmonic ohmic loss and heating issues, dielectric metasurfaces enable broad biosensing and diagnostic applications with higher Q factors, better repeatability, reliability, and biocompatibility.