Zongqiang Chen

Independently tunable double Fano resonances in asymmetric MIM waveguide structure

In this paper, an asymmetric plasmonic structure composed of a MIM (metal-insulator-metal) waveguide and a rectangular cavity is reported, which can support double Fano resonances originating from two different mechanisms. One of Fano resonance originates from the interference between a horizontal and a vertical resonance in the rectangular cavity. And the other is induced by the asymmetry of the plasmonic structure. Just because the double Fano resonances originate from two different mechanisms, each Fano resonance can be well tuned independently by changing the parameters of the rectangular cavity. And during the tuning process, the FOMs (figure of merit) of both the Fano resonances can keep unchanged almost with large values, both larger than 650. Such, the transmission spectra of the plasmonic structure can be well modulated to form transmission window with the position and the full width at half maximum (FWHM) can be tuned freely, which is useful for the applications in sensors, nonlinear and slow-light devices.

Tunable Resonances in the Plasmonic Split-Ring Resonator

A nanoscale resonator composed of two metal-insulator-metal (MIM) waveguides and a split ring is investigated numerically. The multipolar plasmonic resonance modes can be excited, weakened, or even cut off by adjusting the split angle. These novel phenomena are due to the electric polarization in the split area. Odd modes exhibit when the electric field is polarized perpendicular to the split. The resonator acts as a LC circuit for the electric field polarized parallel to the split, in which even modes are excited. The capacitance diminished when the split depth is increased, and the resonance wavelengths of even modes exhibit blue shift. Our results imply an extensive potential for tunable multichannel filters and biosensor devices in integrated nano-optics.

Simulation of Nanoscale Multifunctional Interferometric Logic Gates Based on Coupled Metal Gap Waveguides

A novel design of ultrasmall multifunctional Boolean logic gates is proposed in this letter. This interferometric logic device is based on the coupled metal waveguides. This structure is a theoretical analysis, and logic performances are proved by using the finite-difference time-domain method. The single device can perform individually four different kinds of basic functions: and, or, xor, and not operations. The device with an extremely small feature size is an attractive candidate for high density nano-photonic integrated circuits.

Plasmonic analog of electromagnetically induced transparency in planar metamaterials: manipulation and applications

In this article, we have investigated an analog of electromagnetically induced transparency (EIT) in planar metamaterials by a metallic regular triangle (RT) embedded in split ring (SR) nanostructure. It is demonstrated that, depending on the different coupling ways between the RT and the SR, the EIT-like spectral response can be actively modulated by simply adjusting the incident light polarization angle. Based on this observation, an on-to-off active modulation of the EIT-like transparency window can be realized, and it can serve as the base for an optical switching with the switching efficiency exceeding 95%. The size of the RT finely controls the coupling strength between the RT and the SR. It is shown that the EIT-like transparency window can be enhanced or suppressed by adjusting the size of the RT, which shows the big modulation to the EIT-like spectral response. Furthermore, the transmission spectra show potential applications in sensing due to high sensitivity of about 600 nm/RIU with figure of merit exceeding 36 to the surrounding environment.

Giant circular dichroism of large-area extrinsic chiral metal nanocrecents

In this work, we demonstrate the strong extrinsic chirality of the larger-area metal nanocrescents by experiments and simulations. Our results show that the metal nanocrescent exhibits giant and tunable circular dichroism (CD) effect, which is intensively dependent on the incident angle of light. We attribute the giant extrinsic chirality of the metal nanocrescent to the excitation efficiencies difference of localized surface plasmon resonance (LSPR) modes for two kinds of circularly polarized light at a non-zero incident angle. In experiment, the largest CD of 0.37 is obtained at the wavelength of 826 nm with the incident angle of 60°. Furthermore, the CD spectra can be tuned flexibly by changing the metal nanocrescent diameter. Benefitting from the simple, low-cost and mature fabrication process, the proposed large-area metal nanocrescents are propitious to application.

Ultra-high quality factor metallic micro-cavity based on concentric double metal-insulator-metal rings

We propose and numerically investigate a novel ultra-high quality (Q) factor metallic micro-cavity based on concentric double metal-insulator-metal (MIM) rings (CDMR). In this CDMR cavity, because of the angular momentum matching, the strong coupling occurs between the same order modes of the inner and outer rings with huge resonance frequency difference. Consequently, the energy distribution between in the inner and outer rings presents enormous difference. Especially, for the quasi-in-phase CDMR modes, the energy is confined in the inner ring mainly, which suppresses the radiation loss greatly and results in ultra-narrow resonance dips and ultra-high Q factors. The full width at half-maximum (FWHM) of this CDMR cavity can be less than 2 nm and the Q factor can be higher than 300. Moreover, the character of this CDMR metallic micro-cavity can be modulated by varying the gap width between the two MIM rings. Our CDMR metallic micro-cavity provides a new perspective to design the advanced optical cavity with high Q factor and small mode volumes.

Adjustment of extinction spectra of Au nanocuboidal dimer by modulation of conductive connection

In conductively connected nanoparticles, the bonding dimer plasmons (BDP) mode and the charge transfer plasmon (CTP) mode are allowed at different wavelengths. We present a connected nanocuboidal shaped system with huge coupling surface areas. The red shift of extinction spectra occurs for increasing gap and decreasing conductive junction cross sectional area. With the change of the electric field distribution brought by the connection, we also get a CTP red shift over 250 nm by moving the connection outward less than 50 nanometers horizontally. Besides, the full width at half maximum of the red-shifting CTP peak also decreases by over 50 nm. The CTP peak also red shifts when the connection moves outside vertically.

Polarization-insensitive plasmonic-induced transparency in planar metamaterial consisting of a regular triangle and a ring

In this article, we demonstrate plasmonic-induced optical transparency (PIT) in a planar metamaterial consisting of a metallic regular triangle (RT) embedded in a ring nanostructure. The interference between the bright dipole mode of the RT and the dark quadrupole mode of the ring leads to the emergence of a transparent window in the visible regime. By combining nanostructures with different degrees of symmetry, the PIT transmission properties of our metamaterial remain stable with respect to the incident polarization, showing polarization-insensitivity to the incident wave. The transmission efficiency of the PIT peak for different polarizations can be maintained at greater than 95.77% with a fluctuation range of 0.01% in our calculation accuracy.

Narrow-band filter based on cascaded nanodisks embedded in a metal-insulator-metal waveguide

Narrow-band filter based on three cascaded nanodisks embedded in a plasmonic waveguide is proposed. Resonant mode caused by the coupling among cascaded nanodisks makes the filter have high transmittance and high Q factor simultaneously.