YANG GROUP @ HKU PHYSICS
Photonics at the extreme nanoscale
Many optical phenomena and devices rely on the optical response of materials at the extreme nanoscale (e.g. tens of nanometers and even smaller). Equipped with a new nonclassical framework, we aim at quantifying nonclassical material responses and are interested in applications on nanoresonators, nanolasers, color displays, and 2D-material functionalities within this regime.
Some selected representative works are as follows.
Broadband measurement of Feibelman's quantum surface response functions
we develop an ellipsometric
approach based on the quantum Fresnel equations to measure d-parameters in the far-field and
demonstrate this at a gold-air interface.
Gold shows spill-in and spill-out at different
frequencies, and exhibits the surface-assisted Bennett mode around 2.5 eV.
Our measurements are
validated by passivity and causality constraints. The finding could enhance the electron field emission.
A general framework for nanoscale electromagnetism
The d parameters, first introduced by Feibelman, are a convenient mathematical parametrization for surface-related, quantum corrections. They can be derived from a careful analysis of the reflection of an external potential off a planar interface by going beyond the conventional assumptions of local and stepwise material response: the d parameters then introduce the leading-order corrections to the classical reflection coe
fficients.
The d parameters drive an effective nonclassical surface polarization with contributing an out-of-plane surface dipole density π(r) and contributing an in-plane surface current density K(r). These surface terms can be equivalently incorporated as a set of mesoscopic boundary conditions (here without external interface currents or charges) for the conventional macroscopic Maxwell equations.
We translate the mesoscopic d parameter directly into observables—spectral shifting and broadening—and measure them in specially designed plasmonic systems that exhibit pronounced nonclassical corrections. Our experimental testbed enables a direct procedure to extract d parameters from standard dark-field measurements, in a manner analogous to ellipsometric measurements of the local bulk permittivity.