Yuzhang Liang

Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays

Conventional metasurface reflector-arrays based on metallic resonant nanoantenna to control the wavefront of light for focusing always suffer from strong ohmic loss at optical frequencies. Here, we overcome this challenge by constructing a non-resonant, hybrid dielectric-metal configuration consisting of TiO2 nanofins associated with an Ag reflector substrate that provides a broadband response and high polarization conversion efficiency in the visible range. A reflective flat lens based on this configuration shows an excellent focusing performance with the spot size close to the diffraction limit. Furthermore, by employing the superimposed phase distribution design to manipulate the wavefront of the reflected light, various functionalities, such as multifocal and achromatic focusing, are demonstrated for the flat lenses. Such a reflective flat lens will find various applications in visible light imaging and sensing systems.

Ultra-thin plasmonic color filters incorporating free-standing resonant membrane waveguides with high transmission efficiency

We propose an ultra-thin plasmonic color filtering device based on subwavelength metal grating engraved on a dielectric membrane waveguide without substrate. As experiments demonstrate, the fabricated free-standing plasmonic color filters have more than 70% transmission efficiency at different resonant wavelengths in the visible spectral region and are capable of generating arbitrary colors. Experimental results are in good agreement with the theoretical calculations. These artificial nanostructured color filtering devices may find potential applications in high resolution color imaging and sensing systems.

Subradiant dipolar interactions in plasmonic nanoring resonator array for integrated label-free biosensing

With the development of advanced nanofabrication technologies over the past decade, plasmonic nanostructures have attracted wide attention for their potential in label-free biosensing applications. However, the sensing performance of nanostructured plasmonic sensors is primarily limited by the broad-line-width features with low peak-to-dip signal ratio in the extinction spectra that result from strong radiative damping. Here, we propose and systematically investigate the in-plane and out-of-plane dipolar interactions in an array of plasmonic nanoring resonators that are from the spatial combination of classic nanohole and nanodisk structures. Originating from the strong coupling of the dipolar modes from parent nanohole and nanodisk structures, the subradiant lattice plasmon resonance in the nanoring resonator array exhibits narrow-line width spectral features with high peak-to-dip signal ratio and strong near-field electromagnetic enhancement, making it an ideal platform for high-sensitivity chemical and biomedical sensing. We experimentally demonstrate that the plasmonic nanoring resonator array can be used for high-sensitivity refractive index sensing and real-time monitoring of biomolecular specific binding interactions at nanomolar concentration. Moreover, due to its simple normal incident illumination scheme and polarization independent optical response, we further transfer the plasmonic nanoring resonator array onto the optical fiber tip to demonstrate an integrated and miniaturized platform for label-free remote biosensing, which implies that the plasmonic nanoring resonator array may be a potential candidate for developing high performance and highly integrated photonic biosensing systems.

Dual-channel narrowband polarization absorber with high field enhancement and refractive index sensitivity based on a nanorod array

In this paper, we present a dual-channel narrowband polarization absorber based on a metal-dielectric-metal structure, which consists of a top metallic nanorod array, a metal substrate, and an ultrathin middle dielectric spacer. The proposed structure can achieve high absorptance above 96% in a wide angular range of incidence around ±20° at two remarkable absorption peaks for transverse magnetic polarization under normal incidence. Most significantly, the extremely highly confined enhancement of electromagnetic fields between Au film and nanorods has been observed by employing numerical simulation based on a finite element method, which is up to 110 times compared with the incident electric field. The underlying physics mechanism of a strong gap plasmon resonance is analyzed, and it is primarily attributed to simultaneous excitation of multiple localized electric dipole and magnetic dipole resonance modes in this film-coupled nanorods system. Additionally, we also investigate the dependence of dual resonance peaks on structural parameters as well as their sensitivities to the refractive index of media surrounding nanorods. The wavelength modulation and intensity modulation are also shown simultaneously. This structure is near-perfect absorbing, plasmonic refractive index sensing, and surface-enhanced Raman spectroscopy, all rolled into one. It will have great significance and potential in developing new miniaturized multifunctional photonics devices and their high integrations.

Free-standing plasmonic metal-dielectric-metal bandpass filter with high transmission efficiency

Plasmonic spectrum filtering devices based on metallic nanostructures have attracted wide attention due to their good reliability, ease of fabrication, and wideband tunability. However, the presence of thick substrate significantly limits the structure’s longitudinal size for further optoelectronic integration and reduces the devices’ performance. Here we propose and demonstrate an ultra-thin plasmonic bandpass filter based on free-standing periodic metal-dielectric-metal stack geometry working in the near-infrared wavelength range. The coupling between free-space electromagnetic waves and spatially confined plasmonic modes in the designed structure is systematically investigated. As demonstrated in the calculation and experiment, the free-standing plasmonic filters have more than 90% transmission efficiency and superior angular tolerance. The experimental results are in good agreement with the theoretical calculations. These artificial nanostructured filtering devices may find potential applications in the extremely compact device architectures.

Broadband enhancement of photoluminance from colloidal metal halide perovskite nanocrystals on plasmonic nanostructured surfaces

Metal halide perovskite nanocrystals (NCs) as a new kind of promising optoelectronic material have attracted wide attention due to their high photoluminescence (PL) quantum yield, narrow emission linewidth and wideband color tunability. Since the PL intensity always has a direct influence on the performance of optoelectronic devices, it is of vital importance to improve the perovskite NCs’ fluorescence emission efficiency. Here, we synthesize three inorganic perovskite NCs and experimentally demonstrate a broadband fluorescence enhancement of perovskite NCs by exploiting plasmonic nanostructured surface consisting of nanogrooves array. The strong near-field optical localization associated with surface plasmon polariton-coupled emission effect generated by the nanogrooves array can significantly boost the absorption of perovskite NCs and tailor the fluorescence emissions. As a result, the PL intensities of perovskite NCs are broadband enhanced with a maximum factor higher than 8-fold achieved in experimental demonstration. Moreover, the high efficiency PL of perovskite NCs embedded in the polymer matrix layer on the top of plasmonic nanostructured surface can be maintained for more than three weeks. These results imply that plasmonic nanostructured surface is a good candidate to stably broadband enhance the PL intensity of perovskite NCs and further promote their potentials in the application of visible-light-emitting devices.

An ultra-flexible plasmonic metamaterial film for efficient omnidirectional and broadband optical absorption

An omnidirectional and broadband optical absorber has long been pursued for its wide application in optics, sensing and energy fields. The recent development of flexible and non-planar optoelectronic devices, however, poses a great challenge to fabricate an optical absorber with excellent mechanical flexibility. Here, based on a facile solution method, we demonstrate an ultra-flexible plasmonic metamaterial film (PMF), which is a composite of gold nanoparticles (Au NPs) and aramid nanofibers, to achieve omnidirectional and broadband optical absorption. Due to the comprehensive contributions of the anti-reflection effect of the PMF surface, localized surface plasmon resonances of the Au NPs, and non-resonant decay of light inside the nanocomposite, the PMF exhibits highly efficient omnidirectional and broadband absorption at visible and near-infrared frequencies. In addition, it also presents exceptional mechanical and fast collective light-heating properties, which makes it promising to be applied on flexible and non-planar photo-thermal devices.

Hybrid metasurface for broadband enhancing optical absorption and Raman spectroscopy of graphene

A wafer-scale, hybrid metasurface consisting of randomly-distributed silver nanoparticles and silver substrate separated by a dielectric spacer is reported to realize the enhancement of optical absorption and Raman spectroscopy of graphene. Induced by the surface plasmon resonance, strong localized optical field and broadband absorption from the designed hybrid metasurface are achieved at visible frequencies. As a result, with graphene sheet transferred on the top of the metasurface, the absorption of graphene is remarkably improved across the entire visible region and its signal of surface enhanced Raman scattering is boosted more than 50-fold than that of single layer silver nanoparticles without silver substrate. We envision this lithography-free, hybrid metasurface would be a promising candidate as a platform for the optoelectronic applications of ultrathin two-dimensional materials.