Artificial metasurfaces are capable of completely manipulating the phase, amplitude, and polarization of light with high spatial resolutions. The emerging design based on high-index and low-loss dielectrics has led to the realization of novel metasurfaces with high transmissions, but these devices usually operate at the limited bandwidth, and are sensitive to the incident polarization. Here, for the first time we report experimentally the polarization-independent and highefficiency dielectric metasurfaces spanning the visible wavelengths about 200 nm, which are of importance for novel flat optical devices operating over a broad spectrum. The diffraction efficiencies of the gradient metasurfaces consisting of the multi-fold symmetric nano-crystalline silicon nanopillars are up to 93% at 670 nm, and exceed 75% at the wavelengths from 600 to 800 nm for the two orthogonally polarized incidences. These dielectric metasurfaces hold great potential to replace prisms, lenses and other conventional optical elements.

The rapid developments in nanotechnology and plasmonics allow the manipulation of light at nanometer scales, such as light propagation and resonances. Differing from the symmetric Lorentzian‐like profiles in the conventional resonances, Fano resonances, which originate from the interference of different resonant modes, exhibit obviously asymmetric spectral profiles. Based on lineshape engineering, the Fano resonances with sharp asymmetric profiles exhibit a small linewidth and a high spectral contrast by exploiting different mechanisms and designing various metallic nanostructures. Both of the above properties in the sharp Fano resonances have significant applications in nanoscale plasmonic sensors and modulators. This review summarizes the underlying mechanism of the Fano resonances in various metallic nanostructures. Then, practical applications of the Fano resonances in nanoscale plasmonic sensing and modulation are reviewed. At last, the development and challenges of plasmonic sensing and modulation based on Fano resonances are discussed.

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The integration of on-chip dielectric lasers and subwavelength plasmonic waveguides has attracted enormous attention because of the combination of both the advantages of the high performances of the small dielectric lasers and the subwavelength plasmonic waveguides. However, the configurable integration is still a challenge owing to the complexity of the hybrid structures and the damageability of the gain media in the multistep micro/nanofabrications. By employing the dark-field optical imaging technique with a position uncertainty of about 21 nm and combining the high-resolution electron beam lithography, the small colloidal quantum dot (CQD) lasers without any damages are accurately aligned with the silver nanowires. As a result, the integration of the CQD lasers and the silver nanowires can be flexibly configured on chips. In the experiment, the tangential coupling, radial coupling, and complex coupling between the high-performance CQD lasers and the subwavelength silver nanowires are demonstrated. Because of the subwavelength field confinements of the silver nanowires, the deep-subwavelength coherent sources (multimode, one-color single-mode, or two-color single-mode) with a mode area of only 0.008λ are output from these hybrid structures. This configurable on-chip integration with high flexibility and controllability will greatly facilitate the developments of the complex functional hybrid photonic-plasmonic circuits.

so the SPPs could be actively tuned by varying the refractive index of the adjacent medium. To achieve the tunable plasmonic devices, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] various tuning methods were proposed to vary the refractive index in the experiment, including the electro-optical, thermo-optical, magneto-optical, and all-optical tuning schemes. By utilizing the electro-optical tuning method, the double-slit array, [11] the nanohole array, [12] and the metamaterial [13] were fabricated to tune the SPPs. The modulation depth was about 6 dB for an equivalent index variation of about Δn = 0.05 in the nanohole array covered with the liquid crystal, and the corresponding wavelength shift was about Δλ = 28 nm. [12] By utilizing the thermo-optical tuning scheme, the metallic photonic crystals, [14] the plasmonic waveguide, [15] and the hole array [16] were designed to realize the modulators. The modulation depth could reach up to about 13 dB for the index variation of about Δn = −0.06 in the hole array, and the corresponding wavelength shift was about Δλ = −33 nm. [16] Utilizing the magneto-optical tuning scheme, the plasmonic modulators were achieved in the metal/ferromagnetic/metal structures [17][18][19] and magneto-plasmonic crystal. [20] The modulation depth was up to about 17 dB, and the corresponding wavelength shift was about Δλ = 24 nm for an equivalent index variation of about Δn = 0.06 in the plasmonic crystal. [20] Herein, the values of Δn, Δλ, and dB are calculated based on the shift of the experimental spectra in these papers, and the largest values are chosen for each tuning method. The all-optical tuning scheme, where the signal beam is controlled by the pump beam, is preferred by the alloptical active devices and all-optical plasmonic circuits. However, because of the weak light-matter interactions in natural materials, the index variation induced by the pump beams is very limited. As a result, various nanostructures, including the prism structure, [21] nanohole array, [22] asymmetric nano-slit, [9] grating, [23] T-shape slit, [24] and the micro-ribbon structure, [10] were designed to enhance the light-matter interactions. [4] For example, the asymmetric slit was designed to enhance the light-matter interaction based on the cavity effect. [9] Consequently, the modulation depth was as high as 20 dB under an index variation of about Δn = −0.05, and the corresponding wavelength shift was about Δλ = −30 nm. [9] Besides the variations of the real part of the refractive index, the tunable The wide-range tunable all-optical plasmonic devices are of importance for the multifunctional photonic circuits of high densities. Here, the laserinduced bubbles are utilized to widely tune the surface plasmon polaritons (SPPs) in the metallic hole array. Because of the bubbles generation under a pump beam, the equivalent refractive index of the dielectric medium adjacent to the metal surface decreases. This decrease blue-shifts the transmission spectrum of the hole array. Experimental...

The plasmonic waveguide is a key element in the nanophotonic devices and highly integrated photonic circuits. The single‐mode waveguide greatly limits the degrees of freedom in the filed manipulating and information encoding. Here, a multimode metallic double‐strip waveguide, comprising two metallic strips placed on the metal substrate, is proposed to realize field manipulating and polarization splitting. This waveguide design is quite different from that of the previous waveguides. As a result, both of the symmetric and the antisymmetric surface‐plasmon‐polariton (SPP) modes supported by the proposed waveguide have long propagation lengths (ten times greater than the plasmon wavelengths). Moreover, the field distributions of the two SPP modes are located in different areas in the metallic double‐strip waveguide. Hence, these two SPP modes can be easily manipulated independently. The metallic double‐strip waveguide is fabricated on the metal film and is then measured. As an example, the polarization manipulation is demonstrated in the experiment with a structure size as small as 0.7 × 4.5 µm2. The multimode metallic double‐strip waveguide with long propagation lengths and different field distributions provides more degrees of freedom to manipulate fields and encode information, and thus it has important applications in multifunctional nanophotonic devices and integrated photonic circuits.

Polarization beam splitters (PBSs) are one of the key components in the integrated photonic circuits. To increase the integration density, various complex hybrid plasmonic structures have been numerically designed to shrink the footprints of the PBSs. Here, to decrease the complexity of the small hybrid structures and the difficulty of the hybrid micro-nano fabrications, the radiation losses are utilized to experimentally demonstrate an ultra-small, broadband, and efficient PBS in a simple bending hybrid plasmonic waveguide structure. The hybrid plasmonic waveguide comprising a dielectric strip on the metal surface supports both the transverse-magnetic (TM) and transverse-electric (TE) waveguide modes. Because of the different field confinements, the TE waveguide mode has larger radiation loss than the TM waveguide mode in the bending hybrid strip waveguide. Based on the different radiation losses, the two incident waveguide modes of orthogonal polarization states are efficiently split in the proposed structure with a footprint of only about 2.2 × 2.2 μm2 on chips. Since there is no resonance or interference in the splitting process, the operation bandwidth is as broad as Δλ = 70 nm. Moreover, the utilization of the strongly confined waveguide modes instead of the bulk free-space light (with the spot size of at least a few wavelengths) as the incident source considerably increases the coupling efficiency, resulting in a low insertion loss of <3 dB.

Electrical and optical properties of sputtered amorphous vanadium oxide thin films