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Design and Analysis Guides
A full three-dimensional model in Lumerical FDTD of a focused beam interacting with gold posts on a DVD disc surface. Near-field images and far-field radiation patterns are compared with and without the post, and a dimension sweep finds the smallest gold post that still gives a usable modulation signal.
This example builds a complete three-dimensional model of a focused light beam interacting with the structured gold surface of a typical DVD disc. The aim is to determine the smallest gold post that still produces a large modulation signal, so that as much information as possible can be stored on the disc surface.
To reproduce these simulation results you need to download the simulation file from the application gallery in the Ansys Optics software or from the Ansys website. Note that the download location may differ depending on the instructions you are following.
Open the downloaded simulation file in Lumerical FDTD. For guidance on operating Lumerical FDTD, see the knowledge base.
With the simulation file open in Lumerical FDTD, run the attached script file from within Lumerical FDTD.
The script runs two simulations, one with the gold post and one without.
It first calculates and plots the near-field images with and without the gold post.
Without the gold post (above left), the reflected beam looks almost the same as the incident beam. Once the post moves under the beam (above right), the reflected beam picks up a great deal of structure. The plotted near-field profiles show that only the central part of the beam interacts strongly with the gold post.
Next, look at the far-field radiation patterns from the same two simulations.
Without the gold post, most of the reflected power can be collected by an objective lens with the same divergence angle as the Gaussian source. With the metal post present there is a large scattered field in the y direction, and the amount of power collected by the objective lens drops sharply.
To quantify this, use the “signal” analysis group. This group returns a result called “signal”, calculated by integrating the far field over a cone corresponding to a given divergence angle. Note that the setup script of the “model” element reads the divergence angle of the Gaussian source automatically and sets the divergence angle of the analysis group to the same value. The script on the “analysis” tab of the analysis group (below) calculates the fraction of reflected power that falls within the defined divergence angle.
e2 = farfield3d("reflection",1,200,200,1,1,1,1);
ux = farfieldux("reflection",1,200,200,1);
uy = farfielduy("reflection",1,200,200,1);
# calcualtes portion of e field within the divergence angle of the lens (or the Gaussian source)
signal=farfield3dintegrate(e2,ux,uy,divergence_angle)/farfield3dintegrate(e2,ux,uy);Finally, to find the conditions that return the minimum signal, you can set up a parameter sweep over the dimensions of the gold post. Fix the post length and calculate the signal as a function of post width. Click the sweep project and choose Visualize -> signal to plot the result below in the visualizer.
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