Methods The numerical design of the field probe selleck compound shown in Figure 1 was performed
by the Fourier Modal Method (FMM), which is a standard algorithm for rigorous electromagnetic analysis of diffractive structures . The FMM is directly applicable to periodic structures only, but non-periodic devices such as the one shown in Figure 1 can be treated by adding a perfectly matched layer (PML) between each ‘superperiod’ as shown schematically in Figure 2; the PML acts as an artificial infinite space between the adjacent superperiods . The superperiod (selleck kinase inhibitor length D) contains the slit aperture surrounded on both sides by a finite grating with period d and N/2 grooves, as well as the PML with thickness q. Figure 2 Computational model. A schematic illustration of the computational cell with superperiod D containing the slit, N grooves, and a perfectly matched layer with thickness q. Since a HeNe laser with wavelength λ = 632.8 nm was to be used in the experiments, the refractive indices of the materials were taken in the design eFT508 manufacturer to correspond
to this wavelength. We used the following values: 1.38 + 7.62i for Al, 2.37 for TiO2, 1.56 for the optical adhesive NOA-61, and 1.46 for SiO2. The medium on the entrance side was assumed to be either air or water, and the NOA-61 on the exit side could be assumed to extend to infinity because its thickness is several tens of micrometers. The thin TiO2 layers (thickness h t = 10 nm) shown in Figure 1 have no operational functionality but are introduced only to facilitate the fabrication process as Org 27569 described below. The width w of the slit was fixed to 50 nm in order to obtain high spatial resolution and to keep the transmitted signal on a reasonable level for the experimental measurements.
Hence, the variables left for the FMM-based design are h, h m , d, and f. The choice of these parameters will be discussed in the next section. A TM-polarized cylindrical Gaussian wave with its waist located at the entrance plane of the probe was assumed in the numerical simulations: the non-vanishing magnetic field component was taken to be of the following form: (1) with the value W = 200 nm being assumed in all numerical simulations. In the FMM calculations, this field was represented using its sampled angular spectrum of plane waves, as usual, when dealing with incident fields of finite spatial extent. Figure 3 shows the fabrication process flow. First a 180-nm-thick aluminum film was deposited by electron beam evaporation (Leybold L560, Oerlikon Leybold Vacuum GmbH, Cologne, Germany) on a 2-in diameter Si (100) wafer. A 10-nm-thick titanium dioxide film was added on top of the aluminum by atomic layer deposition to work as an etching mask and to cover the aluminum film against oxidation.