Broad band antireflection coating on silicon from 1.5 to 6um spectral band

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Broad band antireflection coating on silicon from 1.5 to 6um spectral band
A. Ghosh *, P.K. Bandyopadhyay
4 H/ U% g5 I2 c) g2 Z* [1 K5 [( X* PInstruments Research and Development Establishment, Raipur Road, Dehra Dun, Uttaranchal 248008, India
+ z, }3 o- i; R8 l4 p/ e! NReceived 24 June 2004
Available online 12 October 2004
Abstract
Silicon is one of the easily available, low cost and stable material transmitting in 1.5–6lm spectral band. Silicon
s
4 W! ]8 o7 y8 T8 L8 z$ Qoptical application includes thermal imaging, motion sensors and forward looking infrared (FLIR) technology. In this
% B1 N, k) y6 P. fwork a broad band antireflection coating on silicon substrate for 1.5–6lm spectral band is reported. In the design, a
, |) e. z$ ~2 y" M( j0 f/ F6 k( _gradient index approach is used which is a very useful tool to achieve broad band antireflection coating. To realize this
gradient index profile, the refractive index is assumed to increase from 1.45 (thorium fluoride chosen as low index material)
7 n1 R- e; }3 J! ]% cto 3.42 of silicon in five discrete optimized steps. The design was conceived with thorium fluoride as first layer next
4 ^, i5 `& _7 ]: B' c9 j% f  bto air and each other four gradient layers were replaced by two equivalent layer combination of silicon and thorium
! o# d( ]5 G9 kfluoride. This nine layer coating deposited by e-beam evaporation system has a total thickness of only 879nm. Experimental
results shows average transmission 96% in 1.5–6lm region on a 5mm thick silicon substrate. This coating also
has 96.3% transmission in the 1.54lm eye safe laser and 96.5% average transmission in 3–5lm wave bands. The coating
was environmentally stable, tested as per MIL standard.

2004 Elsevier B.V. All rights reserved.
& b9 t6 u: Q) eKeywords: Broad band antireflection coating; 1.5–6lm spectral band; Gradient index profile; Two equivalent layer combination;
E-beam evaporation; Eye safe laser; Environmentally stable coating

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