纳米粒子表面等离激元效应仿真说明

/cn/fdtd/sp_single_device_on_a_substrate.html
/cn/fdtd/sp.html
进入上面的网页下载仿真程序。

按照下面的仿真方法得到下面的图片。

然后改变纳米粒子直径（宽、长），得到不同纳米粒子参数下的散射截面图。

Simulation setup
The file mie_example_3d.fsp contains an example of the Mie problem in 3D, using a total-field scattered-field source(TFSF) that surrounds a gold particle. There are two analysis groups, each of which consist of a box of power monitors: one in the total field region and one in the scattered field region. These analysis groups can be used to calculate the absorption and scattering cross sections, as well as the angular distribution of scattered radiation. In addition, 3 frequency profile monitors are added in the total field region to calculate the electric field enhancement. The total scattered field source covers a wavelength range of 300 to 600 nm.
The gold material is a copy of the "Au (Gold) - Johnson and Christy" material that is contained in the default material database. Since the default number of coefficients is not sufficient to provide a good fit to the sampled data over the source bandwidth, the maximum coefficients is set to 10. The theoretical Mie scattering cross sections (absorption and scattering) have been calculated for both the material fit and the sampled data and are stored in the
files mie_au_jc_fdtd.txt and mie_au_jc_raw.txt, respectivelty.
With the default settings (simulation span is 1x1x1 um3 and a mesh accuracy of 3, plus an override region of 5nm at the sphere) the simulation will require about 234 MB of memory.
Once the simulation is finished, mie_analysis_3d.lsf will perform the following analysis.
Absorption and Scattering cross sections
The absorption cross section (the rate at which energy is removed from the incident plane wave by absorption)is calculated by an analysis group located inside the TFSF source. The analysis group calculates the net power flow into the particle and hence the absorption cross section using the optical theorem. Similarly, the scattering cross section is calculated by an analysis group located outside the TFSF source. This group measures the net power scattered from the particle.
The above plots compare the FDTD results with the results from theory which were computed using the Gold data from the material fit. In the script file, change the name of the file from which to plot the theoretical results from mie_au_jc_fdtd.txt to mie_au_jc_raw.txt to compare the FDTD results with the results from theory that were computed using the raw Gold data. Running the script file results in the following two plots for the cross sections.
Higher accuracy results
The default settings for this simulation are designed to give reasonably accurate results while minimizing the simulation time. If higher accuracy is desired, make the following modifications. These modifications will increase the memory requirements to around 1.7 GB.
Mesh refinement Set the mesh refinement to 'conformal variant 1' to achieve sub-cell resolution for the gold particle boundary. Care must be taken when selecting this setting if the mesh is coarse and there is a large difference in permittivity between the metal and surrounding medium at the frequencies of interest. It is best to perform some convergence testing. A more detailed discussion on convergence testing can be found on the convergence testing page.
Mesh size Set the mesh override mesh size to 0.8nm
Simulation span Set the simulation span to 2um in all directions.
When the simulation region is too small, evanescent tails of the resonant surface plasmon modes will interact with the PML boundary conditions.
PML reflections Double the number of PML layers.
Any light reflecting from the PML boundary conditions may affect the results. More PML layers will reduce reflections.
Symmetry Set the X min boundary condition to Symmetric
Set the Z min boundary condition to Anti-Symmetric
We can take advantage of the symmetry of the simulation to reduce the simulation
memory and time by a factor of 4.
The following figures show the cross sections from the higher accuracy simulation. Agreement between the FDTD and theoretical results is clearly much better.
Far field angular scattering
In the majority of scattering experiments, measurements of the scattered field (radiation pattern) are made far away from the scatterer. We will, therefore, examine the behavior of the scattered field in the far zone in the X-Y plane and the Y-Z plane. The image on the right shows how the numerical angles are oriented in 3D space.
Field enhancement
We use a series of profile monitors to image the field profile |E|^2 in the YZ, XZ and XY planes through the center of the particle. Notice that the edge of the TFSF source is visible in the plots. These figures were created with the higher accuracy settings.。