In this example, we construct a complete three-dimensional model of the interaction of a focused optical beam and the structured, gold
surface of a typical DVD disc. The goal is to determine the minimum feature size of the gold post that
results in a strong modulated signal, such that maximum information can be stored on the surface of the DVD.
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Step 1: Construct the model of the DVD surface in the layout editor and simulate
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The layout editor shows all of the simulation objects.
Objects can be moved and resized with simple mouse movements. The windows show the
top, side, end and perspective views to assist with visualization.
- use a built-in dispersive gold material valid for the visible
- orange box shows the extent of the computation volume and the PML boundaries
- yellow planes show transmission monitors
- use focused Gaussian beam with 0.6 numerical aperture (blue arrow is the polarization, purple arrow the direction of propagation)
- window at bottom shows script window, where user-customized commands and analysis is performed
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Step 2: Measure the reflection from the DVD surface in the near-field
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Integrated analysis routines facilitate data analysis and visualization. Choose from drop-down menus which
monitor you wish to analyze, and the field component of interest. Use frequency-domain monitors to directly
measure the steady-state or continuous-wave response at a specified frequency.
- image plot shows the near-field profile of the beam after it has reflected from the surface
- without the gold post, the reflected beam looks almost identical to the incident beam
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- once the post moves under the beam, the reflected beam contains a lot of structure
- from the near-field profile plotted, we can see that only the central part of the beam interacts with the
gold post strongly
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Step 3: Project the near field results into the far field
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Use built-in analysis routines to render complicated analysis simple. Defined functions project near-field
profiles into the far-field, and integrate the profile over a specified angular cone.
- image plots depict the far-field profile of the reflected signal without the gold post present
- far field visualization shows angular divergence of signal
- scripting functions show >90% of the light is collected by the
NA=0.6 lens when the gold post is not present
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- once the gold post is present, note that the scattering from the nanostructured DVD surface results in
significant scattered field in the y direction (i.e. to the sides of the post)
- for this geometry, 'transmission()' indicates that only 3X% of the incident light is collected
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Step 4: Automate simulation and analysis - determine optimum gold post length
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The built-in scripting environment can be used to perform parameter sweeps, customize analysis, or automate both simulation and analysis to optimize device performance.
- five automated simulations calculate collected power as a function of the
post length
- the resulting data shows that for posts longer than 270nm, the collected signal falls to one half of the
incident signal
- based on collection system sensitivity, adjust the post length to provide
required modulation depth
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Step 5: Automate simulation and analysis - determine optimum gold post width for maximum modulation
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- here we calculate collected power as a function of the
post width for a fixed post length
- the resulting data shows that an opimum minimum collection occurs for widths from 300 to 420nm, including the industry-standard width of 320nm
- in the limit that the post width becomes very small or very large with respect to the spot size, the collected
light approaches that of the flat interface, as expected
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