Nanowire grid polarizer design and optimization with FDTD Solutions

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High-contrast polarization control devices composed of sub-wavelength metal gratings - nanowire grid polarizers - are replacing bulk optical elements. Nanowire grid polarizers offer improved extinction ratio contrast, minimal absorption to address high brightness illumination, and compact form factors to facilitate mass manufacture and integration within small optical assemblies. However, nanowire grid polarizers are challenging components to design, especially if manufacturing imperfections are taken into account. In this application example, we show how FDTD Solutions can be used to maximize the contrast ratio of a nanowire grid polarizer at any angle, while maintaining high transmission.

Step 1. Construct the FDTD Solutions model of the nanowire grid polarizer

FDTD Solutions model of an aluminum grating from which the nanowire grid polarizer is constructed.

The layout editor shows the positioning of all of the simulation objects. Here we show the aluminum grating that forms the basis of nanowire grid polarizer.

Different classes of objects (physical primitives, radiation sources, monitors) are color coded for easy identification. Objects can be moved and resized with simple mouse movements.

•	Materials with different optical properties are displayed with different colours for easy identification.

•	The orange boundary outlines the simulation region.

•	A single beam source is shown incident on the nanowire grid polarizer. The blue arrow shows the polarization direction of the incident radiation.

•	The yellow line shows the detector region above and below the wire.

Step 2. Simulate the contrast ratio as a function of pitch for the aluminum nanowire grid polarizer

Contrast ratio of the aluminum nanowire grid polarizer as a function of the grating period. The contrast ratio decreases rapidly as the grating pitch increases.

Using a broadband illumination source in FDTD Solutions allows for one to measure optical responses across a broad wavelength range, or at multiple discrete wavelengths, in a single simulation.

As simulated by FDTD Solutions, the plot to the left shows that the contrast ratio of the nanowire grid polarizer decreases rapidly as the pitch increases for the three wavelengths simulated.

Step 3. Measure the contrast ratio of the nanowire grid polarizer as a function of grating duty cycle

The transmission of p-polarized light for the aluminum nanowire grid polarizer as a function of the grating duty cycle.

This curve shows that the transmission of p-polarized light decreases as the duty cycle increases. Based on these results, an aluminum grating with a duty cycle of 50% has a transmission of about 85%. With an s-polarized transmission of 2ž10-6 (results not shown), an ideal 50% duty-cycle aluminum grating with no manufacturing errors can achieve a contrast ratio of approximately 4ž106.

Step 4. Simulate the response of the nanowire grid polarizer for non-normal incidence illumination.

The aluminum grating wiregrid polarizer has a TE transmission of approximately 85% for a normally-incident plane wave. Now, with a source angle of 45 degrees, the transmission drops to approximately 83%.

Magnitude (top figures) and phase (bottom figures) for the x- (left figures) and y- (right figures) component of the electric field.

The top figures show the magnitude of the Ex (left) and Ey (right) field components. The ripples in the region above the aluminum metal grating result from interference between the incoming light and that reflected from the top surface of the nanowire grid polarizer.

In the bottom figures, which show the phase of the Ex (left) and Ey (right) field components, the change in angle of the wavefront results from the higher refractive index of the silicon substrate relative to the air region above the aluminum grating nanowire grid polarizer.

These results were generated by simulating a single period of the wiregrid polarizer, and then using the sophisticated scripting environment within FDTD Solutions to concatenate the response from a single grating tooth to compose the response from a multi-tooth section of the aluminum grating.

Step 5. Visualize the operation of the nanogrid wire polarizer - watch the movie

The movies shown to the left illustrate the behaviour of the nanowire grid polarizer to p- (left) and s-polarized (right) light. As shown in the movie, the aluminum grating is effective at transmitting the p-polarized light and reflecting the s-polarized light. By separating these two polarizations from each other, the nanowire grid polarizer considered is able to achieve large contrast ratios if manufacturing imperfections can be minimized.

If the movies above are not visible in your browser, you can download a copy of the p-polarization transmission movie or s-polarization transmission movie.

See how easily FDTD Solutions can assist you with your design efforts! Download a free 30 day trial and request that a technical expert contact you.


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