We investigate the optical properties of gold nanoring (NR) dimers in both simulation and experiment. The resonance peak wavelength of gold NR dimers is strongly dependent on the polarization direction and gap distance. As the gold NR particles approach each other, exponential red shift and slight blue shift of coupled bonding (CB) mode in gold NR dimers for longitudinal and transverse polarizations are obtained. In finite element method analysis, a very strong surface plasmon coupling in the gap region of gold NR dimers is observed, whose field intensity at the gap distance of 10 nm is enhanced 23% compared to that for gold nanodisk (ND) dimers with the same diameter. In addition, plasmonic dimer system exhibits a great improvement in the sensing performance. Near-field coupling in gold NR dimers causes exponential increase in sensitivity to refractive index of surrounding medium with decreasing the gap distance. Compared with coupled dipole mode in gold ND dimers, CB mode in gold NR dimers shows higher index sensitivity. This better index sensing performance is resulted form the additional electric field in inside region of NR and the larger field enhancement in the gap region owing to the stronger coupling of collective dipole plasmon resonances for CB mode. These results pave the way to design plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.

We investigate the index sensing characteristics of plasmonic arrays based on square lattice slablike gold nanorings ͑NRs͒ with different ring widths. The gold NR arrays exhibit two extinction peaks in the visible and near-infrared corresponding to antibonding and bonding modes. Redshift and blueshift in antibonding and bonding modes when broadening the average ring width are observed. We experimentally demonstrate the sensitivity of bonding mode can be tuned by varying the average ring width. However, the sensing performance is limited because the enhanced fields concentrate inside the shell-like structures. The nanoring ͑NR͒ structure surmounts this limitation due to the extension of inside field to the environment. This leads to significant increase in the detection sensitivity. 20 The NR plasmonic properties can be seen as the electromagnetic interaction between the nanodisc and the nanohole plasmons. [21][22][23] This results in the splitting of the plasmon mode into two resonance modes, which are the low energy "bonding" mode and the high energy "antibonding" mode. The resonance frequencies of both resonance modes can be easily tuned in the visible ͑VIS͒ and NIR range by adjusting the geometry of NR, which is useful for various optical sensing applications.In this letter, we report our experimental investigation of bonding and antibonding modes in plasmonic arrays based on square lattice slablike gold NRs with different ring widths. The refractive index sensing experiments of such structures are performed by immersing the samples in index matching liquids. We show that the sensitivities of NR arrays exhibit a linear relationship with ring widths for the bonding mode in the NIR regime. These results are crucial in designing LSP resonance ͑LSPR͒ sensors based on periodic metallic nanostructures.The scheme of square lattice gold NR array with ring thickness t, periodicity p, diameter d, and average width w is shown in Fig. 1͑a͒. The periodicity, diameter, and thickness are fixed at 1 m, 500 nm, and 50 nm. In fabrication, the ITO glass is utilized as the substrate to avoid the charge accumulation effect during electron beam lithography ͑EBL͒. First, the ITO glass was spin-coated a 150 nm polymethylmethacrylate ͑PMMA͒ layer. The NR patterns with area of 300ϫ 300 m 2 and different ring widths were defined on the PMMA layer by EBL. After the development process, a gold thin film with thickness of 50 nm was deposited by thermal evaporation. Then the liftoff process is applied by rinsing the sample in acetone for a few hours. The periodicity p of array is 1 m, the ring diameter is 500 nm, and the ring thickness t is 50 nm.APPLIED PHYSICS LETTERS 98, 153108 ͑2011͒

Traditional electrochromic devices change the color of electrochromic materials by mainly transforming the absorption band of the materials electrically, which leads to low schedulable color selection and color performance of such materials after electrochromism. Although the addition of an interference‐enhanced nanocavity can improve this issue, achieving full‐color controllability on a single electrochromic device is still a huge challenge. This study first demonstrates a near‐full‐color tunable chiroptical electrothermochromic device using a supramolecular chiral photonic material called ferroelectric liquid crystal (FLC)‐doped cholesteric liquid crystal (CLC) (FLC‐CLC). Experimental results show that the pitch of the CLC can be elongated significantly by doping a low concentration of FLC (≈4 wt%) such that the photonic bandgap (PBG) redshifts from blue to the shortwave near‐infrared region at near room temperature. Based on this fascinating feature, the PBG of the FLC‐CLC can be tuned electrically over the entire visible region with high color performance at near room temperature in a low‐voltage range (≤3 V) via the efficient electrothermal effect of the indium‐tin‐oxide‐coated substrate of the sample. Two potential low‐voltage tunable applications based on electrothermochromic FLC‐CLC materials, namely, a broadband tunable laser and a near‐full‐color tunable coaxial microfibric textile, are demonstrated in the study.

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