We present a microscopic theory for nonlinear optical spectroscopy of N molecules in an optical cavity. Using the Heisenberg-Langevin equation, an analytical expression is derived for the time- and frequency-resolved signals accounting for arbitrary numbers of vibrational excitations. We identify clear signatures of the polariton-polaron interaction from multidimensional projections of the signal, e.g., pathways and timescales. Cooperative dynamics of cavity polaritons against intramolecular vibrations is revealed, along with a crosstalk between long-range coherence and vibronic coupling that may lead to localization effects. Our results further characterize the polaritonic coherence and the population transfer that is slower.

Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.103001

Metals become softer when they are heated up, as their stiffness decreases due to thermally induced expansion. More than a decade ago, Swiss physicist Charles Edourad Guillaume, who received the Nobel Prize in Physics in 1920, discovered a nickel based alloy with temperature insensitive elastic modules, which he named Elinvar. Such Elinvar alloys are very rare and the underlying mechnisms are still under debate. Recently, a research team led by Prof. Yang Yong at CityU together with international collaborators has discoveried a super-elastic high-entropy alloy, which exhibits an extraordinary Elinvar effect, retaining near-constant elastic modulus from room temperature to more than 600 degree Celsius. Furthermore, normal metals unusally have the elastic strain limit less than one percent, except those having pseuado-elasticity associated with phase transformation and large energy dissipation. This newly discovered alloy can attain a high elastic strain limit of two percent. We performed in situ high energy x-ray diffraction studies which demonstrated that the high elastic strain limit is not related to stress-induced phase transformation, but rather arises from the high lattice friction. Such a novel alloy with unprecendented mechanical properties has a great potential for aerospace engineering.

Read more at Nature:
https://www.nature.com/articles/s41586-021-04309-1

The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2022/02/10/super-elastic-high-entropy-elinvar-alloy-discovered-potential-aerospace-engineering