Chirality refers to the structural property of an object that cannot be superposed onto its mirror image. The existence of chirality in nature is universal, ranging from molecules at the nanoscale to gastropod shells at the macroscale. Light can be chiral as well. Circularly polarized light with opposite helicity has its electric field vector rotating clockwise or counterclockwise during propagation. Chiral light-matter interactions are widely used in molecule detection, optical communication and quantum information processing. In this talk, I will discuss how to engineer the chiral light-matter interaction based on metamaterials and nano photonic structures towards novel imaging and sensing applications. I will first present a new concept of chiral metamirrors, which can achieve near-perfect reflection of designated circularly polarized light without reversing its handedness, yet complete absorption of the other polarization state . Such a metamaterial can be used for polarimetric imaging to extract the polarization information of light . Recently, we have applied the deep learning approach to accelerate the design of chiral metamaterials with prescribed chiroptical responses . Finally, I will discuss the generation of chiral hotspots in silicon nanocube dimers that can amplify circular dichroism signals by one order of magnitude . Our findings would lead to integrated devices for circular dichroism spectroscopy, enantioselective sensing, sorting and synthesis.
References:  Z. J. Wang et al., "Circular Dichroism Metamirrors with Near-Perfect Extinction", ACS Photonics 3, 2096 (2016);  L. Kang et al., "Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror", Nano Letters 17, 7102 (2017);  W. Ma et al., "Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials", ACS Nano 12, 6326 (2018). Research Highlight in Nature Photonics 12, 443 (2018);  K. Yao and Y. M. Liu, "Enhancing circular dichroism by chiral hotspots in silicon nanocube dimers", Nanocale 10, 8779 (2018).
A superlens that can create sub-diffraction-limited imaging has attracted extensive interest over the past decade. In this paper, we discuss our recent work of infrared superlenses based on perovskites and doped semiconductors. Perovskite oxides show pronounced phonon resonances in the range of 10 to 30 μm, giving rise to negative permittivities around the resonant frequencies. Consequently, we can match a pair of perovskite materials with permittivities in opposite signs to fulfill the superlensing condition. Using a scattering-type scanning near-field optical microscope (s-SNOM) coupled with a tunable free-electron laser, we investigate the evanescent waves in the image plane of perovskite superlenses to address precisely the surface polariton modes, which are important to enhance imaging resolution. Sub-diffractionlimited images with resolution of λ/14 have been achieved at the superlensing wavelength. We also demonstrate a nearfield superlens based on doped semiconductors in the mid-infrared region. Highly doped n-GaAs induces a resonant enhancement of evanescent waves, leading to a significantly improved spatial resolution at the wavelength around 20 μm that is adjustable by changing the doping level. Experimentally, gold stripes below the GaAs superlens are imaged with a λ/6 subwavelength resolution by s-SNOM. Full-wave simulation results are in very good agreement with the observed superlensing effect. These results promise a wide range of applications for infrared imaging, spectroscopy and biochemical sensing at the nanoscale.
Surface plasmon polaritons, sometimes referred to as Surface Plasmons (SPs) have brought us great opportunities to
work in nanoscale at optical frequencies. The SPs at the two surfaces of a thin metal film interact with each other, hence
generate new modes which are either symmetric or anti-symmetric. For anti-symmetric modes, the dispersion curve turns
to be of negative slope at large wave vectors, so two different anti-symmetric modes can be excited at the same
frequency. These two modes can form beats with novel features. The envelope (profile) of the beating SP waves could be
stationary, which means its shape will not change in time. Our simulation results clearly showed such phenomena, which
is a strong evidence of the SPs dispersion relations at the thin metal film. It is a proof of the existence of negative group
velocity of SPs. Beats can help us determine the difference in k and the amplitudes ratio of the two beating waves. We
also studied beating between anti-symmetric mode and symmetric mode SPs with the same frequency. The study of the
energy density distribution showed that the output from such system can be well controlled through beats formation.
Example by using NSOM (Near-field Scanning Optical Microscopy) has been simulated. The beating phenomena have a
potential application in the integrated optical circuits.
By tailoring the dispersion curve of surface plasmons (SPs) of a thin metallic film surrounded by dielectric half-spaces, it is shown that the group velocity of the symmetric mode is always positive, while the group velocity of the anti-symmetric mode can be negative. Consequently, the forward and backward propagation of SPs, in which the energy flow is respectively parallel or antiparallel to the wave vector, can be realized. The physical origin of the intriguing backward SPs is given. Furthermore, schemes for the negative refraction and imaging of SPs are proposed by incorporating two plasmon modes with group velocities of opposite signs.