Chunzhen Fan

Double Fano resonances due to interplay of electric and magnetic plasmon modes in planar plasmonic structure with high sensing sensitivity

Double Fano resonant characteristics are investigated in planar plasmonic structure by embedding a metallic nanorod in symmetric U-shaped split ring resonators, which are caused by a strong interplay between a broad bright mode and narrow dark modes. The bright mode is resulted from the nanorod electric dipole resonance while the dark modes originate from the magnetic dipole induced by LC resonances. The overlapped dual Fano resonances can be decomposed to two separate ones by adjusting the coupling length between the nanorod and U-shaped split ring resonators. Fano resonances in the designed structure exhibit high refractive-index sensing sensitivity and figure of merit, which have potential applications in single or double-wavelength sensing in the near-infrared region.

Tunable broad-band perfect absorber by exciting of multiple plasmon resonances at optical frequency

A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.

A novel planar metamaterial design for electromagnetically induced transparency and slow light

A novel planar plasmonic metamaterial for electromagnetically induced transparency and slow light characteristic is presented in this paper, which consists of nanoring and nanorod compound structures. Two bright modes in the metamaterial are induced by the electric dipole resonance inside nanoring and nanorod, respectively. The coupling between two bright modes introduces transparency window and large group index. By adjusting the geometric parameters of metamaterial structure, the transmittance of EIT window at 385 THz is about 60%, and the corresponding group index and Q factor can reach up to 1.2 × 10³ and 97, respectively, which has an important application in slow-light device, active plasmonic switch, SERS and optical sensing.

A giant localized field enhancement and high sensitivity in an asymmetric ring by exhibiting Fano resonance

The optical properties of asymmetric ring structures are investigated theoretically by using the discrete dipole approximation method. The numerical results revealed that this kind of structure can achieve a giant localized field enhancement (LFE, 264) and a high LSPR sensitivity (corresponding FOM, 8.28) in the visible spectrum by Fano resonance, whose origin is discussed based on plasmon hybridization theory. Furthermore, the dependence of the Fano resonance on the polarization states of the incident light is also demonstrated. Giant LFE and high LSPR sensitivity enable this structure to be promising for surface enhanced Raman spectroscopy and sensing applications.

Ultra-narrow band perfect absorbers based on plasmonic analog of electromagnetically induced absorption

A novel plasmonic metamaterial consisting of the solid (bar) and the inverse (slot) compound metallic nanostructure for electromagnetically induced absorption (EIA) is proposed in this paper, which is demonstrated to achieve an ultra-narrow absorption peak with the linewidth less than 8 nm and the absorptivity exceeding 97% at optical frequencies. This is attributed to the plasmonic EIA resonance arising from the efficient coupling between the magnetic response of the slot (dark mode) and the electric resonance of the bar (bright mode). To the best of our knowledge, this is the first time that the plasmonic EIA is used to realize the narrow-band perfect absorbers. The underlying physics are revealed by applying the two-coupled-oscillator model. The near-perfect-absorption resonance also causes an enhancement of about 50 times in H-field and about 130 times in E-field within the slots. Such absorber possesses potential for applications in filter, thermal emitter, surface enhanced Raman scattering, sensing and nonlinear optics.

Low-threshold surface plasmon amplification from a gain-assisted core–shell nanoparticle with broken symmetry

We report that the gain threshold of a core–shell nanoparticle-based spaser can be reduced significantly by offsetting the gain-doped dielectric core within the metallic shell. By investigating the optical cross sections of the reduced symmetry core–shell nanoparticle with different levels of gain, we determined the gain threshold of the asymmetric nanoparticle-based spaser fueled by different plasmon modes. The calculation results indicate that when multipolar plasmon oscillations excited in asymmetric core–shell nanostructures are used as lasing modes, the gain threshold of an asymmetric spaser particle can drop by 30% as compared to the case of a perfect or symmetric particle. The underlying physics of low-threshold surface plasmon amplification is explained by investigating the Q factor and the optical field confinement and enhancement associated with the lasing mode.

Realization of high sensitive SERS substrates with one-pot fabrication of Ag–Fe3O4 nanocomposites

Ag-Fe3O4 nanocomposites were synthesized by the redox reaction between Ag2O and Fe(OH)2 in the absence of additional reductant at moderate temperature and atmospheric condition. The as-synthesized Ag-Fe3O4 nanocomposites are assembled into an orderly arrayed SERS substrate holding clean and reproducible properties with an applied external magnetic field. 4-mercaptobenzoic acid (4-MBA) is chosen as the probe molecule to test the enhancement factors (EF), uniformity and reproducibility of the SERS substrate. Experimental results indicate that the EF of 4-MBA on our proposed SERS substrate is up to 5.2×10(6) and the detection limit is down to ∼10(-10) M. The SERS spectra of 4-MBA molecules ranging from 200 cm(-1) to 2000 cm(-1) were randomly collected from a number of positions on the substrate and six Ag-Fe3O4 nanocomposites substrates are measured with the same procedure. It is shown that the SERS substrate have the good uniformity and reproducibility with low standard deviation, indicating our proposed Ag-Fe3O4 nanocomposites with external magnetic field control abilities have potential applications in the fields of magnetic separation and SERS techniques.

Near-field engineering of Fano resonances in a plasmonic assembly for maximizing CARS enhancements

Surface enhanced coherent anti-Stokes Raman scattering (SECARS) is a sensitive tool and promising for single molecular detection and chemical selective imaging. However, the enhancement factors (EF) were only 10~100 for colloidal silver and gold nanoparticles usually used as SECARS substrates. In this paper, we present a design of SECARS substrate consisting of three asymmetric gold disks and strategies for maximizing the EF by engineering near-field properties of the plasmonic Fano nanoassembly. It is found that the E-field “hot spots” corresponding to three different frequencies involved in SECARS process can be brought to the same spatial locations by tuning incident orientations, giving rise to highly confined SECARS “hot spots” with the EF reaching single-molecule sensitivity. Besides, an even higher EF of SECARS is achieved by introducing double Fano resonances in this plasmonic nanoassembly via further enlarging the sizes of the constituent disks. These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes.

Optical Switching Based on Polarization Tunable Plasmon-Induced Transparency in Disk/Rod Hybrid Metasurfaces

Dynamical control of plasmon-induced transparency (PIT) in metamaterials promises essential application opportunities. In this paper, we design a novel disk/rod hybrid metasurface to investigate an actively controlled PIT spectral response through polarization-dependent near-field coupling between the disk (bright) and the rod (dark) resonator. It is found that an on-to-off amplitude modulation of the PIT transparency window is achieved by the rotation of polarization, allowing for dynamically tunable group delays of incident waves. An analytic model based on the dipole-dipole interactions is developed for the proposed configuration, which agrees well with the numerical results. The disk/rod metasurface offers great promise for optical switching and compact slow light devices.

Theoretical investigation of a multi-resonance plasmonic substrate for enhanced coherent anti-Stokes Raman scattering

The development of new substrates for surface-enhanced spectroscopy is primarily motivated by the ability to design such substrates to provide the maximum signal enhancement. In this paper, we theoretically design and investigate a crisscross dimer array as a plasmonic substrate for enhancing coherent anti-Stokes Raman scattering (CARS). The plasmonic film-crisscross dimer array system can excite multiple resonances at optical frequencies. By properly designing structure parameters, three plasmon resonances with large field enhancements and same spatial hot spot regions can spectrally match with the pump, Stokes and anti-Stokes beams, respectively. The CARS signals are strongly enhanced by multi-resonance plasmon field enhancements. The estimated CARS factor can reach as high order as ~1016 over conventional CARS without the plasmonic substrate.