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Optical properties of injection molded subwavelength gratings

Jacob C. Jonsson and Fredrik Nikolajeff

Open Access Open Access

Abstract

We have fabricated injection molded subwavelength gratings for anti-reflection purposes superimposed upon a blazed grating structure in polycarbonate. The gratings are initially formed by electron-beam lithography and subsequently replicated using injection molding. There are several problems when trying to optically characterize a component such as a blazed transmittance grating. Standard spectrophotometers are not well suited for measuring transmittance in the different diffraction orders individually. Our sample size of 0.8×0.8 mm2 is also a problem for standard instruments. First order transmittance has been measured for blazed gratings with single and double-sided AR-treatment and is transmittance is compared with with higher diffraction orders. Double-sided AR-treatment not only increase the total transmittance but also widens the wavelength range with high effectiveness of the first order diffraction.

©2004 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. a) The principle for blazed grating with superimposed AR-structure. The backside of the sample is flat without any structure. b) AFM pictograph taken from the master. The peak at the edge of each blaze is for proximity compensation of the rounding at the bottom of the next blaze. The depth as seen as the distance between the two triangle markers is 1 μm.

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Fig. 2.
Fig. 2. The transmittance at normal angle of incidence for the uncoated PC, one-side dip-coated PC, one-side subwavelength grating, and two-side dip-coated PC versus wavelength. The sol-gel reduces the surface reflections, hence increase the transmittance. The wavelength dependence of the AR-coating is not as dominating in the single-side coating as in the double-side coating. The strong decrease in transmittance at lower wavelength for the subwavelength grating is due to scattering into higher orders from the structure, this effect is expected to occur when the wavelength of the illuminating light approaches the grating period.

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Fig. 3.
Fig. 3. The measurement set-up for measuring transmittance of the individual diffraction orders. The monochromatic incident light is limited in size by the pinhole. Light is diffracted through the blazed grating. All the ports of the order shield but the 1st order port are covered. The detector is positioned to collect light regardless of the wavelength of the incident light.

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Fig. 4.
Fig. 4. Transmittance of the untreated PC substrate measured both with and without the pinhole. The measurements are in good agreement, even though the measurement with pinhole shows somewhat more noise towards the shorter wavelengths.

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Fig. 5.
Fig. 5. The transmittance of the dip-coated blazed grating for the first order compared to the sum of the transmittances of orders -2, -1, 0, +1, and +2. The effectiveness is high and rather constant around 633 nm.

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Fig. 6.
Fig. 6. Transmittance through the grating in the first diffraction order. The sample with dip-coated back surface shows a higher transmittance and wider wavelength range of effectiveness.

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