Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

A protocol for the fabrication and optical characterization of dielectric metasurfaces is presented. This method can be applied to the fabrication of not only beam splitters, but also of general dielectric metasurfaces, such as lenses, holograms, and optical cloaks.

Transcript

This protocol provides a detailed fabrication method to realize high efficiency metasurfaces, one of the most important issues in metasurface research area. Compared with other methods to test atomic layer deposition, this technique is a low cost and fast fabrication method to realize high efficiency metasurfaces working at visible wavelengths. This protocol can be applied to fabricate general metasurfaces such as lenses, holograms, and optical clocks just by changing the pattern configuration.

This technique can provide insight into silicon photonics research field which utilizes silicon micro and nano structures. The substrate for the metasurface is fused silica. Prepare a clean double side polished square fused silica two centimeters on a side.

Take the substrate to load into the PECVD system. There in the load lock chamber, position the substrate on the jig. Close the chamber and prepare to operate the system.

In the control software, set up the deposition process by setting the temperature, RF power, gas flow rates, and process pressure. After the deposition, which takes about 300 seconds, recover the sample. Take the sample with deposited hydrogenated amorphous silicon to a spin coater.

Load the sample on the spin coater sample holder. Next, get a filter mounted five milliliter syringe containing PMMA with 2%anusol. Coat the sample with the PMMA before starting rotation at 2, 000 RPM for one minute.

When done, transfer the sample to a hot plate. Bake the sample at 180 degrees Celsius for five minutes. Next, remove the sample to cool it for one minute before continuing.

Then return the sample to the spin coater. Use a one milliliter pipette to release a conducting polymer solution on its surface. Coat the sample at 2, 000 RPM for one minute.

When the coating is complete, take the sample for electron beam lithography. Have the sample fixed on the jig for the machine. Then put the jig in the machine's chamber and complete the loading process.

Move on to work with the console and prepare the electron beam lithography system for the procedure. After preparing the system, work at the computer connected to the console. In this system, use the command line to convert a GDS file to a CEL file.

When the file is converted, enter job to start the EBL software. Use the command line to check that the desired pattern is in a CEL format file in the current directory. Enter job to run the software.

In the software, click the chip size modification menu. Select 600 micrometers by 600 micrometers. Next, select 240, 000 dots.

Save the changes. Then exit this screen. Now click on the pattern data creation menu.

In the command window, enter PS to load the pattern's CEL file. Enter I in the command window. Then click the pattern to magnify the image.

Now enter ST0 in the command window to set the dose time to three microseconds. Enter SP11 to set the exposing pitch to a normal condition. Create a CCC file by entering PC and a filename.

When done, click the center of the pattern. To apply the exposing conditions, enter CP in the command window and click the pattern. Enter SV and a filename to create a CON file.

Exit this pattern data creation menu by entering Q.Move on to click the exposure menu. Enter I and the chosen CON filename. Set the dose value to 2.4.

Push the escape button to complete the schedule. Then enter E and click the exposure button to start the exposure process. When the exposure process finishes, return to the EBL console.

Turn the isolation button to off. Push the EX button to move the stage. Then unload the sample from the chamber.

Next, prepare to remove the conductive polymer. Do this by immersing the sample in 50 milliliters of deionized water for one minute. Then move the sample to a 10 milliliter solution of methyl isobutyl ketone and isopropyl alcohol surrounded by ice.

After 12 minutes, remove the sample and rinse it with isopropyl alcohol. Dry it with blown nitrogen gas. The next step requires an electron beam evaporator.

Have the sample fixed on the holder of the evaporator and mount the holder inside the evaporation chamber. Now get the chromium for use in the evaporator. Prepare piece type chromium in a graphite crucible for evaporation onto the sample surface.

Load the crucible into the chamber. Next, work with the software for the electron beam evaporator. Click the chamber pumping button to create a vacuum in the chamber.

In the material section, select chromium. Then click the material button to apply the selection. Click the E beam shutter button to open the source shutter.

Next, click high voltage. Follow this by clicking source. Use the upward arrow to slowly increase the beam power.

Stop at the target deposition rate. To reset the thickness gauge, click the zero button. Click the main shutter button to open that shutter.

Monitor the thickness gauge. When the gauge reaches 30 nanometers, click the main shutter button to close the shutter. Click E beam shutter to close the source shutter.

Use the downward arrow to slowly decrease the beam power to zero. Once at zero, click source followed by high voltage. Allow the chamber to cool for 15 minutes then click vent.

Remove the sample from the chamber and holder. Next, take it for a liftoff process. First, immerse it in 50 milliliters of acetone for three minutes.

Follow this by sonication for one minute at 40 kilohertz. Rinse the sample in isopropyl alcohol and dry it with nitrogen gas. At this point, the sample is ready for etching.

Get thermal glue and spread it on the back of the sample before attaching the sample to the etching system's jig. Load the jig into the etching system. At the computer, set the chlorine gas and hydrogen bromide gas flow rates, the source power and the bias before etching for 100 seconds.

After the etching, unload the sample. With a dust free wipe, remove the thermal glue. Immerse the sample in 20 milliliters of chromium etchant for two minutes.

Then transfer it to 50 milliliters of deionized water for one minute. Rinse the sample with deionized water and blow it dry with nitrogen gas. This is a scanning electron microscopy image of the top of the metasurface.

Each of the cells has a base that is 150 nanometers by 80 nanometers. The cell height is 300 nanometers. Additional details of the cells are visible in this perspective view.

An experiment that measures the beam power when a 532 nanometer laser beam is incident on the metasurface demonstrates the polarization-independent functionality of the device. For right circularly polarized beams, linearly polarized beams, and left circularly polarized beams, the power at the plus and minus one diffraction orders are equal. Experiments with a 635 nanometer laser beam yield similar results.

The develop method is the most important step because we can precisely control the development process due to slow reaction speed. Most failures occur during drying steps. One should keep in mind that strong blowing is better than weak blowing in general.

This procedure can be applied to not only general dielectric metasurfaces, but also silicon photonics and micro electron mechanical systems. Generally, we can make delicate nano structures by this technique so it paves the way to address how light interacts with some wavelength structures. PMMA and development solution vapor are both hazardous so the processes involving them must be performed in fume hoods.

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Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

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