Nanowire grid polarizer as compact photonic polarization control elements - design and optimization with FDTD Solutions

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

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.  The aluminum wiregrid polaizer model is easily constructed with the array function combined with the broadband optical materials contained in the material database.

Three-dimensional model of an aluminum nanowire grid polarizer on a quartz substrate.  The polarizer is excited by a fixed polarization state plane wave at a specified angle of incidence.

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

Using a broadband illumination source in FDTD Solutions allows for one to measure optical responses across a broad wavelength range in a single simulation.  By performing two simulations - one with s polarization, and another with p polarization - and taking the ratio of the frequency-domain transmission curves that result, the contrast ratio can be simulated for a specified wiregrid polarizer geometry.

With the parameter sweep and optimization framework within FDTD Solutions, it is straight-forward to measure the polarizer contrast ratio as a function of grating period.  That calculation is performend here at normal incidence:

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.

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

Likewise, using the same parameter sweep framework, how the polarizer contrast ratio varies with duty cycle can be easily simulated.  The resulting curve below 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 2X10^-6 (results not shown), an ideal 50% duty-cycle aluminum grating with no manufacturing errors can achieve a contrast ratio of approximately 4X10^-6.

Variation in p polarization tranmissions versus wiregrid duty cycle.

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%.

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.

		Plots of the real part of the x- and y-components of the electromagnetic field through the wiregrid polarizer.
		Plots of the phase of the x- and y-components of the electromagnetic field through the wiregrid polarizer.

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

FDTD Solutions allows you to visualize the field dynamics in the simulation, facilitating analysis and aiding in intuition of device performance and behavior.  Here, two movies have been generated: one for the s polarization and another for the p polarization.   As shown in the movies, 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.