Metamaterial AR Coatings for Alumina

Joey Golec4/1/20

Method and Design

• Precision dicing of sub-wavelength structures creates a metamaterial that simulates dielectric layers which act as an AR coating

• This method has been used for all of the metamaterial AR coatings for the silicon optics

• Since alumina is much harder than silicon, constraints get placed on blade thickness

Alumina

Vacuum

At mm-wavelengths

Alumina

Vacuum

n1_eff

n2_eff

Coating Designs

• Only the MF coating has been fabricated and only on a 6 inch diameter wafer

• The UHF and LF designs are very preliminary and are subject to blade constraints, etc.

*Not to scale

Layer Kerf(K) Depth (D)1 0.687 mm 1.45 mm2 0.182 mm 0.946 mm

K1

K2

D1

D2

Pitch (P) = 1.554 mmLF (20/40 GHz)

P

PK1

K2

D1

D2

Layer Kerf(K) Depth (D)1 0.220 mm 0.425 mm2 0.07 mm 0.289 mm

Pitch (P) = 0.522 mm

P

KD

MF (90/150 GHz)

Layer Kerf(K) Depth (D)

1 0.110 mm 0.162 mm

Pitch (P) = 0.300 mmUHF (220/280 GHz)

Prototype Fabrication• We fabricated the AR coating

on a 6-inch diameter wafer

• Blade wear was the main complication of the fabrication process and resulted in the thick cut not being full depth

• If we implement automatic compensation for blade wear the fabrication procedure is simplified drastically but blade changes are inevitable

• I estimate it would take ~16 days to fabricate a 40 cm diameter optic

1.044 mm

Prototype Performance Forecast

• The 6-inch diameter prototype is at UChicago right now and measurements of the reflection performance are going to be made as soon as the labs open back up

• I modeled what the “imperfections” of the coating we fabricated

• The bottom plot to the right is the simulated performance of the fabricated coating

• The model with the rounded edges can be optimized to perform as well as the flat bottom cut model

Future Steps and Conclusions

• Measure the performance of the 6-inch diameter MF prototype and confirm it matches simulations

• AR coat the other side of the 6-inch prototype and measure to confirm percent level reflection

• In parallel, write code infrastructure to compensate for blade wear

• Investigate the viability of the coatings for other frequencies

• Barring significant speedups, it takes a couple weeks to completely fabricate a 40 cm diameter optic