We demonstrate a new plasmonic approach to high-density optical data storage; using
dual-color plasmonic nano-pixels to encode two information sets into the same unit
area using single arrays of two-state metal nano-apertures.
The ability to effectively separate discrete colors from white-light lies at the heart
of how we record and view optical information; whether that be the arrangement of
colored inks in painting and printing applications, or the spectral filters that enable
many modern image display and recording technologies. In each case, color
separation is typically provided by organic compounds; dyes and pigments that absorb
and scatter particular wavelengths of light, leading to their distinct color profiles.
Recently, structural color systems based on engineered nanophotonic materials have
emerged as an appealing alternative to absorptive dyes [1]. Among these examples are
color filters based on plasmonics. Plasmonic filters hold several dimensional and
stability advantages over their micro-scale, dye-based counterparts. As a result, they
have been positioned as new technological solutions for sub-wavelength color
printing [1], RGB splitting for image sensors [2], anti-counterfeiting measures [3], and
optical data storage [4]; thus representing one of the most promising, commercially
relevant areas of current plasmonic research activity.
Here, we demonstrate a method for patterning full-color images and codes that
exhibit polarization-dependent information states. Our individual pixels are comprised
of asymmetric cross-shaped nano-apertures in a thin film of aluminum; each aperture
engineered to exhibit 2 independent plasmonic color resonances that can be
individually tuned across the sRGB spectrum. This enables us to encode 2 arbitrary
information sets into the same unit area using the same array of nano-pixels. We show
that using a standard optical microscope, color separation can be controlled down to
2x2 nano-pixels while retaining polarization selectivity. This defines our maximum
data storage capability; each 2x2 pixel area acting as a 2-state data bit that can be read
optically. The maximum data density we can achieve using this technique is
approximately 1.46 Gb/cm2, with the added ability to further encode each of those
pixels using the full visible-color spectrum.

Graphene-combined devices enable a convenient and robust way for terahertz(THz) wave manipulation due to its plasmonic behavior and tunability via electrical biasing [1, 2]. Here we propose a type of graphene metasurfaces consisting of rectangular graphene patches for efficient THz wave manipulation. The metasurfaces can convert both linear and circular incident wave to its cross-polarized component with perfect polarization conversion. Moreover, the phase of the cross-polarized wave can be tuned in a wide range over 180° via electrical biasing, so we can develop several functional devices such as a switchable anomalous deflection device and a tunable dual-polarity focusing mirror.

The basic structure is composed of a graphene patch with size L₁ = 23 μm and L₂ = 18 μm  on a metal ground, with a 20 μm-thick silica spacer inserted in between, demonstrated in Fig. 1(a). The size of each unit cell is P_x = P_y = 40 μm and the chemical potential μ_c is 0.5 eV. The plasmonic resonance modes along both sides  L₁ and L₂ are coupled with the dielectric cavity, leading to a halfwave-plate behavior, shown in Fig. 1(b). When illuminated by an x-polarized incident wave, both resonance modes can be excited, leading to a polarization conversion ratio ( PCR = R_xy/(R_xx+R_xy)) higher than 99% in a range from 217 μm  to 237 μm. The same principle applies to circular incidence, thereby leading to a straightforward design of high-efficiency metasurfaces based on Pancharatnam–Berry phase, see in Fig. 2(a). The efficiency is higher than previous work using graphene cut-wires [3] or nano-crosses [4].

 The cross-polarization response of a unit cell versus μ_c is shown in Fig. 2(b), with incidence wavelength λ = 230 μm. Varying μ_c from 0.21eV to 0.68eV can result in a phase change of 180° while keeping the normalized amplitude larger than 0.6. Taking mirror image of the structure, an additional 180° phase shift is added, leading to a full 360° phase modulation. Utilizing these mirror-imaged unit cells, we can design a switchable beam deflection device whose reflection angle can be tuned from −46° to 0° and 46° simply by changing μ_c of each patch, as shown in Fig. 3. Another functional metasurface demonstration is a tunable focusing mirror whose focal spot can be tuned in both longitudinal and transversal direction. Besides, it can also be switched from a concave mirror to a convex one, shown in Fig. 4. It is worth emphasizing that these devices work under both linear and circular incidences.

In conclusion, we have proposed a type of graphene metasurfaces which can convert incident wave to its orthogonal counterpart with nearly 100% PCR. Moreover, the response of each unit cell can be tuned individually, leading to a dynamic manipulation of the wavefront. The proposed metasurfaces may have great potential in various THz applications.

This paper reports a new design of electrically tunable superconducting niobium nitride metamaterials device. The maximum transmission coefficient at 0.507 THz is 0.98 and decreases to 0.19 when the applied voltage increases to 0.9 V. A relative transmittance change of 80.6% is observed, making this device an efficient narrowband THz switch. Additionally, the frequency of the peak is red shifted from 0.507 to 0.425 THz, which means that the device can be used to select the frequency. This study offers an alternative tuning method to existing optical, thermal, magnetic-field, and electric-field tuning [1-4], delivering a promising approach for designing active and miniaturized THz devices.

  The geometry and dimensions of the unit cell are shown in Fig. 1(a). In the periodic array of CSRs, each row of adjacent FSRs is connected not only as resonators but also as continuous wires in the circuit. When a bias voltage is applied to the ends of the metamaterial as shown in Fig. 1(b), a current loop is naturally formed in this system.

  At a fixed temperature of 4.5 K, we measured the THz response of the NbN switch device with different applied voltages (currents). Figure 2(a) shows the measured transmission spectra through the sample as the voltage is varied from 0 to 1.3 V. For this switch, when the applied current increased, the volume of the normal state in the SC resonator increased. This was because of an increase in the resistance, leading to a significant increase in Ohmic loss. At about 0.8 V, the currents got the peak value of about 180 mA, corresponding to a critical current density of 610⁵ A/cm² for a wire cross section of 100 nm5 µm (this metamaterial has 60 shunt branches). When the applied current exceeded the critical value, the electromagnetic response of the NbN film approached the normal state and the transmission peak disappeared. A red shift of the peak in frequency from 0.507 to 0.425 THz was observed. And a relative transmittance change of 80.6% was obtained when the applied voltage was 0.9 V. We also measured this sample under different temperatures from 4.5 to 18 K, shown in Fig. 2(b). It was found that the two tuning methods resulted in similar transmission behavior.

  At fixed temperatures of 4.5 K, 7 K, 9 K, 11 K, and 13 K (as shown in Fig. 3–4), both the transmission amplitude and peak frequency had a similar variation with the increase in applied voltage. However, the dynamic range of the tuning dramatically decreased with increasing temperature. This is because the Ohmic losses at higher temperature increase and lead to the lower transmission at zero voltage. The temperature-dependent relationship of this device is in accordance with the electrical tuning at a fixed temperature of 4.5 K. Combining the temperature dependent and electrical effects, we can control the dynamic range of the variation of the NbN switching device under different working conditions.

  This study delivers a promising approach for designing active and miniaturized THz devices.

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures around Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. One can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of optical pump compensate the dissipative loss from metal shell, thus enable the strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces origin from the reversal of electric field at Fano resonance, which lead to pulling force on bound currents and charges. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.

Light is an electromagnetic wave composed of both electric and magnetic fields. Light-matter interaction results in complicated and vectorial field distribution of light in nanoscale. Hence, imaging the vector field of light in near field has become very important for both fundamental science and applications in nanophotonics and optofluidics. This paper reports a new design of near-field probe with silicon nanoparticle(SiNP) and gold nanoparticle(GNP) that is respectively sensitive to magnetic and electric components of light, and full vector field imaging of light in near field would be achieved by this kind of probe with nanoparticles.

Electric vector field imaging in near field was first reported by a GNP probe [1], but the GNP has no response to the weak magnetic components of light. In order to achieve full vector field imaging of light, probes that are both sensitive to the in-plane magnetic components(Hx and Hy) and out-of-plane magnetic components(Hz) should be designed and fabricated. Recently, several novel probes [2,3] have been demonstrated with SNOM/NSOM system for near-field optical magnetism detection; However, magnetic vector imaging still remains several challenges, such as complexity of design and fabrication, limitation of sensitivity for only one magnetic component(either in-plane or out-of-plane) and weak response for visible light. On the other hand, according to Mie theory, silicon nanopariticles have strong magnetic dipole resonance in visible band. Hence, our research utilizes SiNP probes for magnetic vector imaging, in association with GNP probes for electric vector imaging, and our method differs from previous study in the design and working principles. 

The design of probes with nanoparticles is illustrated in Fig. 1. Parabolic bare fibers by heat-pull and HF etching procedure are used for support and distance control in near field; GNPs with size about 50nm are prepared by chemical synthesis, and SiNPs with size almost from 100nm to 200nm are prepared by laser ablation method [4]; SiNP and GNP are attached to the fiber probe by APTES/APTMS. In the fabrication of SiNP probes, the most important procedure is the particle selection with a specific size. Therefore, dark-field microscopy and spectrometry are utilized for this selection, and Fig.2 shows the dark-field spectrum of SiNP and GNP that are used for 633nm laser. In this research, an evanescent standing wave with TE mode is generated by total internal reflection of a prism, and the SiNP and GNP probe are used for full vector field imaging of this distribution in near field. Furthermore, FDTD simulations were conducted in combination with reciprocity theory in near field [5,6], to interpret the electromagnetic response of the SiNP and GNP probes. The results in Fig. 3 concludes that the GNP is sensitive to the electric vector field (projection of Ey), and the SiNP is sensitive to the magnetic vector field (Hx and Hz).

Subwavelength interference in ultra-thin bilayer metamaterials is investigated, where both near- and far-field interactions play important roles. A hybrid coupling model is established, which clearly shows the different contributions of near- and far-field couplings to electromagnetic responses: the former is only responsible for the energy level splitting, while the latter mainly reshapes the profile of the resonance spectra. Accordingly, the reflection spectrum exhibits a very sharp subradiant resonance within the envelope of a superradiant resonance and interference between them leads to a sharp Fano resonance. Under two-antisymmetric-beam illumination, the subradiant mode can be selectively excited, while the superradiant mode is highly suppressed, attributing to their different mode symmetry. A sharp coherent perfect absorption (CPA) peak with well-defined Lorentzian lineshape is achieved under critical conditions, relating to the completely excitation of subradiant mode. The peak frequency can be tuned by tailoring the near-field coupling, which alters the energy-level splitting. Ultrathin bilayer terahertz metamaterials with flexible polyimide substrate and interlayer are fabricated and tested by THz time domain spectroscopy, showing very good agreement with the theoretical and numerical results. This work provides a method of how to extract narrow subradiant resonance from an asymmetric Fano lineshape, which may enlighten the way for selective mode excitation via coherent illumination.

Achieving broadband total light absorption of unpolarized light within a subwavelength ultrathin film is critical for optoelectronic applications such as photovoltaics, photodetectors, thermal emitters and optical modulators. Here we experimentally demonstrate a low-cost and scalable multilayer graphene ultra-broadband total light absorber of record-high 90% of unpolarized light absorption at near infrared wavelengths with a bandwidth of more than 300 nm. The thin metamaterial consists of alternating monolayer graphene and dielectric material prepared by a low-cost wet chemical layer-by-layer method. A simple grating is fabricated using flexible femtosecond laser writing that simultaneously removes the graphene in the ablated regions and converts the remaining graphene oxide to graphene via photo-reduction. Our results open a novel, flexible and viable approach to practical applications of nanostructured photonic devices based on 2D materials, which require strong absorption.

Metasurfaces have been successfully applied to realize cloaking in the reflection geometry recently[1, 2]. We propose a transmission cloaking scheme by using a cloaking shell consisting of a metasurface and a thin layer of zero-index medium. The metasurface is designed to impose a suitable amount of phase change on the incident waves and radiation waves, so as to avoid the total reflection on the surface of zero-index medium and to restore the propagating wave front. The energy flux is tunneled through the zero-index medium. Full-wave simulation results show good cloaking effects for cloaks of almost arbitrary shapes. We design and fabricate a rhombus cloaking shell for operation in the microwave region and the experimental results agree well with the numerical simulations.

In this talk, i would present some of our recent results on ultrasensitive meta-sensing in the Terahertz, Infrared and the Optical regime of the electromagnetic spectrum. The presented work would consist of vibrational sensing with high quality factor resonances, biosensing with plasmonic structures, and microfludic remote sensing mediated by the metamaterial resonators.

The ability to control resonant properties of individual metamolecule in a metamaterial structure on the sub-wavelength scale will offer an ultimate freedom for dynamically shaping wavefronts of electromagnetic radiation for applications such as variable aberration corrected planar lenses, dynamic holograms and spatial intensity and phase modulators with sub-wavelength pixelation. Here we report tunable metamaterials where the tunabilities are enabled by microfluidic technology. The resonant properties of every individual metamolecule can be continuously controlled at will without limited by the nonlinear properties of the materials. Therefore microfluidic technologies offer ultimate freedom in achieving a dynamic control of metamaterials optical properties as well as electromagnetic wavefront with wavelengths ranging from THz to GHz region. In this paper, we briefly review the pioneer works on MEMS and Microfluidic metamaterials and their applications on tunable lens and absorbers.