Tunable circular conversion dichroism of single-layer twisted graphene-patterned metasurface.

Two-dimensional (2D) chirality-induced asymmetric transmission/reflection has great potential for polarization applications. Usually, asymmetric effects resulting from circular conversion dichroism (CCD) occur in chiral metasurfaces. Here, we propose a single-layer twisted graphene-patterned (with tilted elliptical hole arrays) metasurface and theoretically reveal its tunable CCD in the terahertz (THz) region. The unit cell of the metasurface is achiral. Merely by altering the in-plane orientation of holes for structural 2D chirality, a tunable CCD can be achieved at normal incidence. Interestingly, the reflection phase can be considered an intuitive method to show this metasurface's anisotropy, which complements the conventional CCD measurement in characterizing chiral materials. Furthermore, we can achieve active CCD based on the tunability of graphene. Due to the Fabry-Pérot resonance, a multiband enhancement of CCD spectrum will happen by changing the dielectric layer thickness. The proposed metasurface provides more flexible opportunities for designing active THz devices for polarization manipulation.

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus.

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

Non-Hermitian physics for optical manipulation uncovers inherent instability of large clusters.

Intense light traps and binds small particles, offering unique control to the microscopic world. With incoming illumination and radiative losses, optical forces are inherently nonconservative, thus non-Hermitian. Contrary to conventional systems, the operator governing time evolution is real and asymmetric (i.e., non-Hermitian), which inevitably yield complex eigenvalues when driven beyond the exceptional points, where light pumps in energy that eventually "melts" the light-bound structures. Surprisingly, unstable complex eigenvalues are prevalent for clusters with ~10 or more particles, and in the many-particle limit, their presence is inevitable. As such, optical forces alone fail to bind a large cluster. Our conclusion does not contradict with the observation of large optically-bound cluster in a fluid, where the ambient damping can take away the excess energy and restore the stability. The non-Hermitian theory overturns the understanding of optical trapping and binding, and unveils the critical role played by non-Hermiticity and exceptional points, paving the way for large-scale manipulation.

Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications.

Surface plasmonic sensors have been widely used in biology, chemistry, and environment monitoring. These sensors exhibit extraordinary sensitivity based on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) effects, and they have found commercial applications. In this review, we present recent progress in the field of surface plasmonic sensors, mainly in the configurations of planar metastructures and optical-fiber waveguides. In the metastructure platform, the optical sensors based on LSPR, hyperbolic dispersion, Fano resonance, and two-dimensional (2D) materials integration are introduced. The optical-fiber sensors integrated with LSPR/SPR structures and 2D materials are summarized. We also introduce the recent advances in quantum plasmonic sensing beyond the classical shot noise limit. The challenges and opportunities in this field are discussed.

Giant circular dichroism enhancement and chiroptical illusion in hybrid molecule-plasmonic nanostructures.

Recently, there are great interest in studying the interaction between chiral molecules and plasmonic particles, because a weak circular dichroism (CD) signal in the ultraviolet (UV) region from chiral molecules can be both enhanced and transferred to the visible wavelength range by using plasmonic particles. Thus, ultrasensitive probe of tiny amounts of chiral substance by CD are worth waiting for. Here we present another way to strongly enhance CD of chiral molecules by using plasmonic particle cluster, which need not transfer to the visible wavelength. The method to calculate CD of chiral molecules in nanosphere clusters has been developed by means of multiple scattering of electromagnetic multipole fields. Our calculated results show that 2 orders of magnitude CD enhancement in the UV region for chiral molecules can be realized. Such a CD enhancement is very sensitive to the cluster structure. The cluster structure can cause chiroptical illusion in which a mirror symmetry in the CD spectra of opposite enantiomeric molecules is broken. The correction of quantum size effect on the phenomenon has also been considered. Our findings open up an alternative avenue for the ultrasensitive detection and illusion of chiral information.