Enhanced effect of local fields in subwavelength metallic series nanocavities from surface plasmon polaritons.

The enhancement and confinement characteristics of the local field in the two-dimensional (2D) subwavelength-size series cavities structure are investigated numerically by using the boundary integral method. The series cavities are built of two pieces of finite silver thin slabs with subwavelength corrugations on their inner boundaries, set in a face-to-face arrangement with a separating space, and the central part is a narrow channel (NC). We calculate the average amplitude of the local field in the NC as a function of the wavelength for exploring the influence of the structural parameters and demonstrate the amplitude distribution of the magnetic field in the structure and the cross-section distributions of the local field in the NC region along both the longitudinal axis direction and the transverse directions. The simulations show that the local field in the NC has significant enhancement, up to 2 orders of magnitude, of the incident light field, and the local light field is confined to a small region less than one fifth of the resonant wavelength in the longitudinal direction and one twentieth of the resonant wavelength in the lateral direction. Replacing the metallic material of the cavity walls with the semiconductor germanium leads to the complete disappearance of the enhancement of the local field. It is clearly shown that surface plasmon polaritons on the metal play a critical role for this enhancement phenomenon. The influences of various geometric parameters on the resonant wavelength and the peak value of the average amplitude of the local field are extensively investigated.

Rigorous electromagnetic analysis of the common focusing characteristics of a cylindrical microlens with long focal depth and under multiwavelength illumination.

The common focusing characteristics of a cylindrical microlens with a long focal depth and under a given multiple-wavelength illumination are analyzed based on the boundary element method (BEM). The surface-relief profile of a finite-substrate-thickness microlens with a long focal depth is presented. Its focusing performances, such as the common extended focal depth (CEFD), the spot size, and the diffraction efficiency, are numerically studied in the case of TE polarization. The results show that the CEFD of the microlens increases initially, reaches a peak value, and then decreases with increasing preset focal depth. Two modified profiles of a finite-substrate-thickness cylindrical microlens are proposed for enlarging the CEFD. The rigorous numerical results indicate that the modified surface-relief structures of a cylindrical microlens can successfully modulate the optical field distribution to achieve longer CEFD, higher transverse resolution, and higher diffraction efficiency simultaneously, compared with the prototypical microlens. These investigations may provide useful information for the design and application of micro-optical elements in various multiwavelength optical systems.