Using films of gold and silver for optical filter

Baumeister, P. W.

English

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THE UNIVERSITY OF ROCHESTER 
THE INSTITUTE OF OPTICS 
ROCHESTER, NEW YORK 
PROPERTIES OF MULTILAYER FILTERS 
Covering the Period 
September 1, 1969 to February 28, 1970 
Research Grant No. NGL 33 019 003 
with 
Washington 25, D.C. 
Principal Investigator: P.W. Baumeister 
Interim Report 
National Aeronautics and 
Space Administration 
CASE F I L E  
COPY 
https://ntrs.nasa.gov/search.jsp?R=19700020811 2020-03-12T02:31:08+00:00Z
PROPERTIES OF MULTILAYER FILTERS 
In te r im Report 
Covering t h e  Period 
September 1, 1969 t o  February 28, 1970 
ABSTRACT 
TABLE OF CONTENTS 
I Introduction 
The Properties of Metals with Low 
Reflectance 
I11 Tandem Arrays of Filters with Large 
Attenuation on the Long-wavelength 
-Side of the Passband 
IV Personnel 
V References to the Literature 
VII 
Captions to the Figures 
Figures 
b 
I. Introduction 
During previous periods of this Grant, we designed. - 
interference filters for the spectral region from 1200 X i I
tr\ mnn 9 The iIegipn method is now well understood and i C U  J V V V  &a. A&&- ---- mu -------- - -- - - -  
the principles of design have been 
hl emn r e m a i n  r 
----..- - - 
(1) The manufacture of such coatings. 
. .
Pil This Includes an analysis of the 
tolerance data -- i.e. the effect- 
L 
of changes in the optical constang 
and errors in the thicknesses of 
the layers. 
(2) The - techniques of combining such 
filters to attenuate in the long- 
wave portion 
i.e. the addition of blocking filq 
ters . 
In this report, we focus attention upon the second - 1 
problem, that of producing suitable blocking filte , .  
- the previous report,' we carefully investigated the block- 
11 .- 
. . a ing of 1-M filters by adding two or more blocking filters 
ength side of the passband. These block- 
ing filters were composed of quarterwave stacks of di- 



at wavelength of 25008, - T is 35%,R is 28%, and - A is 37%. 'I 
by 13% to 48%, and the absorptance to 52%. This peak % 
transmittance is barely acceptable. The problem with- 
'II 
using silver for the blocking in the 300021 to 34002 pa'rt 
of the spectrum, is that it has a considerably higher 
transmittance in this region than at shorter wavelengths, 
notwithstanding the antireflection coatings which are - 
' - % 
applied at these shorter wavelengths. Thus we conclude 
that the use of silver in these filters in the 25002 
region is marginal. That is it can possibly be used, I r 
but we should first look for other materials. 
mitt: 
As shown in Figs. 5 and 6, the reflectance and trans- 
does not display the troublesome transmittance leak at! 
32008, as does the silver. However, the 21 (L-g) curv 3 
(shown in Fig. 7) is not very encouraging. The thinner 
I 
gold film has a C ~/(1-R) -  of only 25% at 25002. The TI{ 
- 1 is still lower for the thicker film. The transmittanc 
of the gold blockinn filter is too low to be considered 
t described some tandem arrays - 
1-M filters and multilayer layer blocking filters. The 
4 obiective was to add attenuation to a 1-M bandpass filter 
can be used with a solar blind CsTe photocathode. I 
I 
A previous report discussed the method adding the 
blocking filters in series with the one-M filter, as 
shown in Fig. 1. The filters are all at normal 
compact, but has the disadvantage that the peak attenua- 
~d 
tion is not very large. 
Another method is to arrange the filters at an angle 
of 30°, as shown in Fig. 1, so that the multiply reflected 
beam "walk off" and are reflected out of the system. In 4 
this case, it is patent that the offband attenuation in- I 
w 
w 
creases substantially because the optical densities of = 
the individual filters directly add. The disadvantage ' 
a is that the filter assembly is now rather bulky. It also! 
has some incl-i-ned surfaces which introduce astigmatism 1 
-.- - - r - 
k and other aberrations, when used in an imaging system. 
In measuring the transmittance ot these stacks at 
E l %  care was taken to correct for any residual polal 
ization produced by the spectrophotometer.,That is,, 
flux emerging from the Cary model 14 spectrophotometer 
is a mixture of unpolarized and linearly polarized flux;' 
This effect of the polarization can be taken into accoun 
by rotating the filter assembly about an azimuthal axis 
which is coincident with the beam. For example, Fig. 8 
shows the transmittance of a one-M filter (curve - a) at 
normal incidence. The other (curve b) is at an angle of 
mittance at 30", with the azimuthal angle of the filter 
stack rotated by go0. The transmittance actually changes 
slightly at all wavelengths, but is only appreciable in a 
the   ass band re~ion. as shown bv the dashed curve. In 
incidence, the same care was taken to insure that the effeci 
w 
of the volarization of the svectro~hotometer was taken into 
nognal incidence (curve - a). Transmittance curves were ru 
4 on the other blocking filters, but theyare so similar to 
b t h e  curve shown in Figure 9, that there is no point 
pinure 10 shows the measured transmittance of t h ~  

Douglas Harr'son* lilnll 
Gary DeBell: 
Graduate Research Assistant 
No salary charged 
Graduate Research Assistant 
One half time 
Two months, 50% 8 ,. 1 
References to the Literature 
1. Properties of Multilayer Filters 
Interim report of NGL 33 019 003 
March 1, 1969 to August 31, 1969 
2. P. W. Baumeister, "Radiant Power 
Flow and Absorptance in Thin 
Films", Appl. Opt. 8, 423, 1969 
VI. Captions to the Figures 
1. The arrangement of a tandem array of filters 
I 
in which 
(a) the surfaces are parallel and hence the 
multiply reflected beams are collected 
I 
by the detector, and 
(b) The surfaces are inclined and hence the 
reflected beams are deflected out of the 
' system. 
. . 
I- ,  . 
2. The measured spectral transmittance of silver 
fused quartz substrate. 
silver films, as described in the caption to 
h. 
. b 
Fig. 2. The dotted portion of the curves is . 
' I 
extrapolated. . 
. ' - 7  
4 .  The ra-diant power flow ratio, T/ (1-R) of the 
- .A 
silver films described in the caption to Fig. 2. r' I Y 
' 
I 
The dotted portion of the curve uses the extra- ; 
polated reflectance values shown in Fig. 3. 
3 -3 
5. The measured spectral transmittance of gold 
films of thicknesses approximately 1502 (curve - a) - - p E '  
4 -'? 
and 2502 (curve - b) , deposited on a quartz sub- t 
. t 
strate. , . .- 
6. The measured spectral reflectance of the same ii 
gold films, as described in the caption to - 
c.l 
Fig. 4. 
7. The radiant powe 
gold films described in the caption to Fig. 5. : .  
- 
. . 
i 
8. The measured spectral transmittance of one-M - I 
'?I 
. . 
0 . r filter at normal incidence (curve - a) and at 30 . 
d 
. . 
: ii? 
incidence (curve - b). The dashed portion of 
r 
tb-. - curve - b is with the filter rotated by an 
. * I -  - Ei 
1,- azimuthal angle of 90° about an axis parallel 
- 
with the res~ect to beam of the s~ectror>hotometer.. ,. 

incident 
flux I 






Wavelength ( P )  
Fig. 8 
I I I I I I 1 
- - 
- )\): a - 
aa 
0 
C 
0 
= 16' - 
. - 
E 
U) 
C 
f - 
t- 
- - 
ld2 I I I I I I I I I 
0.20 0.30 0.40 
Wavelength ( P )  
Fig. 9 
I 1 1 1 1 ~ 1 1 1 ' 1  
- 
- 
- - 
lo-' - - 
lo-2 - 
8 - 
0 
- 
- 
.- \ g - - 
s 
16' - I - 1 
- - 
- - 
1 6 ~  - - 
- - 
- - 
1650.2~' ' I I 0.30 I I I I '  0.40 
Wavelength ( p )  
Fig. 10 
Fig. I I  


Properties of multilayer filters Interim report, 1 Sep. 1969 - 28 Feb. 1970

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