Pei Ru Wu

Integrated plasmonic metasurfaces for spectropolarimetry

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500-700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.

Plasmon-induced hyperthermia: hybrid upconversion NaYF4:Yb/Er and gold nanomaterials for oral cancer photothermal therapy

Nanocomposites consisting of upconversion nanoparticles (UCPs) and plasmonic materials have been widely explored for bio-imaging and cancer photothermal therapy (PTT). However, several challenges, including incomprehensible efficiency of energy transfer processes and optimization of the conditions for plasmon-induced photothermal effects, still exist. In this study, we fabricated NaYF4:Yb3+/Er3+ nanoparticles (NPs) conjugated with gold nanomaterials (Au NMs), such as Au NPs and gold nanorods (Au NRs). NaYF4:Yb3+/Er3+ NPs were used as photoconverters, which could emit green and red light under excitation of a 980 nm laser; Au NPs and Au NRs were also prepared and used as heat producers. The silica shell was further coated around UCPs to improve biocompatibility and as a bridge linking UCPs and the Au NMs. Most importantly, the thickness of the silica shell was tuned precisely to investigate the effective distance of the plasmonic field for heat induction. Energy transfer was confirmed by the declining UCL photoluminescence and emission decay time after connecting to the Au NMs. Moreover, a simulative model was built using the finite element method to assess the differences in heat generation between UCP@SiO2-NPs and UCP@SiO2-NRs. The surfaces of the hybrid nanocomposites were modified with folic acid to improve the specific targeting to cancer cells. The performance of the modified hybrid nanocomposites in PTT for OECM-1 oral cancer cells was evaluated.

Optical Anapole Metamaterial

The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments.

Material-assisted metamaterial: a new dimension to create functional metamaterial

A high Q-value reflective type metasurface consisting of 1D Au nanorods, a SiO2 spacer and a Au back reflector is demonstrated. It is shown that the sideband of the resonant mode can be suppressed as the resonant wavelength close to the phonon absorption of SiO2. By combining both designed structured resonance and inherent property of the based materials, a low angle-dependent metasurface with a Q-value of 40 has been demonstrated. The proposed structure will be useful for high sensitivity sensing and narrow band thermal emitter.

Plasmonic toroidal excitation with engineering metamaterials

Natural toroidal molecules, such as biomolecules and proteins, possess toroidal dipole moments that are hard to be detected, which leads to extensive studies of artificial toroidal materials. Recently, toroidal metamaterials have been widely investigated to enhance toroidal dipole moments while the other multipoles are eliminated due to the spacial symmetry. In this talk, we will show several cases on the plasmonic toroidal excitation by engineering the near-field coupling between metamaterials, including their promising applications. In addition, a novel design for a toroidal metamaterial with engineering anapole mode will also be discussed.

Horizontal toroidal response in three-dimensional plasmonic(Conference Presentation)

The toroidal dipole moments of natural molecules are hard to be detected so the artificial toroidal materials made by metamaterial attract more attentions. Metamaterial, the sub-wavelength artificial structures, can modulate reflection or transmission of light. The toroidal metamaterial can not only amplify the toroidal moment but also repress the electric and magnetic dipole so it can be used to study the properties of toroidal dipole moment. However, there are many limitations for the experiments, such as the lateral light is necessary to excite the toroidal response. Most of the toroidal dipole moments oscillate perpendicularly to the substrate, therefore it is difficult to couple it with other dipole moments and could be only excited in the microwave region. In this paper, we design a toroidal metamaterial consisting of dumbbell-shaped aperture and vertical split ring resonator (VSRR) vertically. The toroidal dipole moment of our metamaterial is excited in the optical region. The arrangement of our nanostructures is vertical instead of planar annular arrangement to reduce the size of the unit cell and increase the density of the toroidal dipole moment. Moreover, the direction of toroidal dipole moment is parallel to the substrate which can be used for the study of the coupling effect with other kinds of dipolar moments.

Fabrication of three-dimensional plasmonic structure and multilayer metamaterials by femtosecond laser-induced forward transfer

Plasmonic metamaterials, composed by arti?cial micro or nanostructures, exhibit extraordinary optical properties such as negative refraction, which cannot be observed in nature. The focused ion beam (FIB) and electron beam lithography (EBL) technique are frequently adopted to fabricate multilayer and three-dimensional plasmonic structures, but the alignment error, high cost and time consuming cannot be avoided. In contrast, the laser direct writing (LDW) technique is one of the best candidates to fabricate plasmonic structure owing to its low cost, high throughput and simple experimental setup. Here, we fabricate the plasmonic structures to manipulate the scattering of SPP waves by LDW technique. Both the backward and forward scattering of surface plasmonic waves can be generated by the arranged gold (Au) nanobumps on an Au thin film. Upon controlling the geometry of the plasmonic structures, the height, position, and pattern of scattered light are modified as desired. It provides a simple and efficient way to project a specific light pattern into free space, and demonstrate the capability of three-dimensional light manipulation. Furthermore, we demonstrated that, with tightly contacted donor-receiver pair, multilayer plasmonic structures such as multilayer split resonant rings and square-shaped multilayer resonant cavities, can be fabricated on donor by laser-induced forward transfer (LIFT) process as a kind of LDW. By using the contact-mode LIFT, the illuminated materials of sputtered multilayer thin films are ablated and transferred to the receiver, which would leave the uniform metamolecules with multilayer structures on the donor. Finally, we have analyzed their plasmonic modes by both measurement and simulated results. These properties may facilitate many applications in integrated optics, optical nonlinearities, and luminescence enhancement, etc..

Novel applications of plasmonic metamaterials

Metamaterials are the artificial materials composed of multiple individual elements in specially designed periodic subwavelength patterns. The properties of these metamaterials strongly depend on the featured shape or size of the unit of metamaterials rather than the original properties of the composition of materials. Their precise shape, geometry, size, orientation and arrangement can affect the light in an unconventional manner, creating material properties which are unachievable with conventional materials, such as negative refractive index, abnormal reflection, and so on. Due to these fascinating properties, we have designed and fabricated various plasmonic metamaterials with localized energy, electromagnetic field enhancement, and nonlinearity by the state of the art methods, such as e-beam lithography with multi-exposure technique, focus ion beam drilling, ultrafast laser direct writing, phase-change materials etching process, etc..

Plasmonic metadevices by vertical split ring resonator

Split-ring resonator (SRR), one kind of building block of metamaterials, has attracted wide attentions due to the resonance excitation of electric and magnetic dipolar response. Fundamental plasmon properties and potential applications in novel three dimensional vertical split-ring resonators (VSRRs) are designed and investigated. The resonant properties arose from the electric and magnetic interactions between the VSRR and light are theoretically and experimentally studied. Tuning the configuration of VSRR unit cells is able to generate various novel coupling phenomena in VSRRs, such as plasmon hybridization and Fano resonance. The magnetic resonance plays a key role in plasmon coupling in VSRRs. The VSRR-based refractive-index sensor will be demonstrated, as shown in Figs. 1. Due to the unique structural configuration, the enhanced plasmon fields localized in VSRR gaps can be lifted off from the dielectric substrate, allowing for the increase of sensing volume and enhancing the sensitivity. We perform a VSRR based metasurface for light manipulation in optical communication frequency, as shown in Fig. 1 as well. By varying the prong heights, the 2π phase modulation can be achieved in VSRR for the design of metasurface. Because the phase shift is changed via the upright configuration rather than the one parallel to the substrate, it can be used for high areal density integration of metal nanostructures and opto-electronic devices. Fabrication of three dimensional VSRR by stress-driven assembly method for uniaxial-isotropic metamaterials is demonstrated as well.

Time-resolved phase-change recording mark formation with zinc oxide near-field optical active layer

In this paper, an optical active thin film of zinc oxide (ZnOx) nano-composites exploited for the enhancement of optical signals in an ultra-high density recording scheme has been demonstrated. Via the electron microscope investigation, the results display randomly distributed crystalline nanograins in the ZnOx thin films. Optical disks with the ZnOx nanostructured thin films show that the carrier-to-noise ratio (CNR) above 25 dB can be obtained at the mark trains of 100 nm, while the optimal writing power is reduced as a function of the increasing thickness of the ZnOx films. Furthermore, by conducting a series of the optical pump–probe experiments, the optical responses of recording marks on as-deposited phase-change Ge2Sb2Te5 (as-GST) recording layers present that the highly contrast bright recording bits can be acquired with the existence of the ZnOx nanostructured thin films, providing prospective potentials in future data storage and optoelectronic devices.