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OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
Jean-Marie MACKOWSKI
Université Claude Bernard Lyon 1
SMA-VIRGO
Bât 213
22, Bd Niels Bohr
69622 Villeurbanne Cedex
[email protected]
Phone : + 33 04 72 43 26 69
Fax : + 33 04 78 89 19 36
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
TODAY MENU
COATING DEFINITION
DIELECTRIC COATINGS
Quaterwave Rule
Some Useful Designs
METALLIC COATINGS
Silver, Aluminum, Gold Reflectors
Passivation Layers
Antireflection of a Metal
Enhanced reflectors by Dielectric Layers
TOMORROW MENU
Coatings Deposition techniques
Performances & limitations
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
OPTICAL COATINGS
Incident
2.
Reflected
Coating
Surface
Transmitted
Optical coating consist of a layer or series of layers
of different materials,
that are deposited over the surface to be treated.
The desired properties of the coating
are achieved by a mixture of
interference and intrinsic properties of the materials that are used
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
COATINGS
INCLUDING
DIELECTRIC LAYERS ONLY
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
WHY THIN AND NOT THICK FILMS ?
Using interference properties means creating and changing
the shape of interference fringes
Transmittance (%)
Thick materials
Give fringes too
Closely spaced
To be useful
97.5
97.0
Glass 100 µm
96.5
96.0
95.5
400
500
700
Wavelength (nm)
Transmittance (%)
Thin films Give
The desirable
Broad fringes
That we need
600
97.5
97.0
Glass 1 µm
96.5
96.0
95.5
400
500
600
700
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
BASIC DESIGN PRINCIPLES
Quaterwave Layers give Maximum interference Effect
Half Layers are Absentee Layers-They have No Effect
Dielectric Layers Become Weaker whit Increasing Wavelenght
Metal Layers become Stronger with Increasing Wavelenght
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
THE QUATERWAVE RULE
A quarterwave:
L,H 
0
4n f
transforms the surface
n2f
following the rule :
nt 
ns
A quarter stack with x layers of H and (x-1) layers of L:
has reflectance
n0 HLHLLH ns
(nH) 2x
no 
(n L) 2(x1)n S
R
(nH) 2x
no 
(n L) 2(x1)n S
 0 is the working wavelenght
n f,n t and n s are the refractive indexes of film, transformed surface, and substrat, respectively
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
THE QUATERWAVE RULE ...
Interference calculations for two waves are very simple when the waves are combined
Are exactly in phase or exactly out of phase. In the former case the resultant amplitude
Is simply given by the sum of individual amplitudes while in the latter it is the difference
of amplitudes.
All others cases are intermediate.
The phase shift on reflection at a simple interface between two dielectric media is either
Zero or 180° (  / 2).
The phase shift suffered by a wave traveling through thickness d of a thin film is given by
 n / 
The minus sign indicates a phase lag.
This is such an important quantity that its magnitude is given the symbol :
- 2
 =2  n d / 
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
REFLECTION OF SINGLE FILM
:
Amplitude reflectance
of light:
no
n1 > n
n1-n2
n1+n2 > 0
n2 < n
o
/2 = /4+0+/4
air
/2
n0-n1
n0+n1 < 0
/2
0
1
Thin film
Substrate
Reflected light:
Beams interfere
constructively
Film Thickness is Quaterwave
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
ANTIREFLECTION OF SINGLE FILM
:
/2
 =  /4+  / 2 +  /4
Thin film thickness =  /4
Amplitude reflectance
of light:
no
/2
n0-n1
n0+n1 < 0
n1 > n
n1-n2
n1+n2 < 0
n2 > n
o
/2
1
air
Beams interfere
destructively
Thin film
Substrate
Reflection = 0 if no / n1 = n1 / n2  n1 = (no n2)1/2
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
DIELECTRIC MIRRORS
:
no
nH > n
o
nL < n
H
nH > n
L
nL < n
H
nH > n
L
 /2  /2 3 /2 3 /2 5 /2 5 /2
Coatings
Principles
Beams interfere
constructively
air
/2
high index
0
low index
/2
high index
0
low index
/2
0
nS < n
H
NATO/ASI and Euro Summer School September 16-27, 2002
high index
Substrate
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
QUATERWAVE STACK IS A BASIC BUILDING BLOCK
23-Layer quaterwave stack centered on 800 nm
Ripple
Transmittance (%)
100
80
Notch Filter
60
40
20
0
200
High
Reflectance
Longwave pass
or Dichroic Filter
Shortwave pass
or Dichroic Filter
400
600
800
1000
1200
1400
Wavelength (nm)
The ripple is usually removed by adding several layers at each end
and refining them into a matching structure
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
RIPPLE CONTROL
23-Layer quaterwave stack centered on 800 nm
Without Ripple control
Transmittance (%)
100
80
60
40
20
0
200
400
600
800
Wavelength (nm)
1000
1200
1400
23-Layer quaterwave stack centered on 800 nm
With Ripple control (Matching Layers)
I.M
Quaterwave stack L (HL)^11
(2L.1H)^2
NATO/ASI and Euro Summer School September 16-27, 2002
E.M
(.1H2L)^2
E.M
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
BROADBAND REFLECTOR
Reflectance (%)
100
80
60
(HL)^5 1.2L (1.4H 1.4L)^5 1.4H
40
20
0
350
400
450
500
550
Reflectance (%)
L index : 1.35
Cryolite :Na3ALF6
600
650
700
750
800
850
900
950 1000
Wavelength (nm)
100
H index : 2.35
ZnS
98
96
94
23 Layers
92
90
350 400 450 500 550 600 650 700 750 800 850 900 950 1000
NATO/ASI and Euro Summer School September 16-27, 2002 Wavelength (nm)
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
MULTIPLE-CAVITY FILTER
Transmittance (%)
100
80
Three-cavity
60
40
Two-cavity
Single cavity
20
0
990
995
1000
1005
1010
Wavelength (nm)
A simple cavity consists of a half wave layer surrounded by to reflectors
This gives a narrow band of transmission.
Better pass band can be achieved by coupling cavities into multiple-cavity filters
Here a three cavity: {(HL)^5 HH (LH)^5}^3 giving a band pass of 1.2 nm.
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
OBLIQUE INCIDENCE
At oblique incidence the path difference between the beams is reduced and
their amplitudes for s-polarized light is increased and for p-polarized light decreased.
Characteristics move
Transmittance (%)
To shorter
Wavelenght and become
100
@ 45° of incidence :
Stronger
80
for s-polarization
And weaker
60
for p-polarization
40
The green curve is given
20
B
at
Normal incidence.
0
300
R
G
400
500
600
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
WIDE-ANGLE ANTIREFLECTION COATING
Reflectance (%)
s-Polarization p-Polarization
5
4
3
2
1
0
0
10
20
30
40
50
60
70
Incident Angle (deg)
Here an antireflection coating on glass for a single wavelenght (510 nm)
At angles of incidence up to 5O° and both polarizations.
9 layers of MgF2, Al2O3 and TiO2
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
NON-POLARIZING BEAM SPLITTER
Reflectance (%)
60
40
20
0
500
600
Wavelength (nm)
The design of dielectric coatings to have equal p- and s-polarization over a large spectral
region is exceptionally difficult.
Here a simple 8 layers 45° beam splitter for 500 to 600 nm
using TiO2, Al2O3 and SiO2
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
Lx, Hz (HL)^10 Stack Performances
Transmittance (%)
100
80
L (HL)^10
60
H (HL)^10
40
LL (HL)^10
20
0
400
500
600
700
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
ELECTRIC FIELD DISTRIBUTION
ALL LAYERS
Electric Field (V/m)
60
L (HL)^10
40
H (HL)^10
20
LL (HL)^10
-1
0
1
2
3
4
5
6
Design2:
Parallel
Electric
Field
Optical Distance
from Medium
THE FIRST
LAYERS OF ALL DESIGNS
Electric Field (V/m)
60
40
20
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Optical Distance from Medium
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
COMPONENT ABSORPTION VS DESIGN
0 = 800 nm
Total Layer Absorptance (%)
0.0005
0.0004
L(HL)^10
0.0003
H(HL)^10
LL(HL)^10
0.0002
0.0001
0.0000
1
2
3
4
5
6
7
Total Layer Absorptance (%)
8
9
10
11
12
13
14
15
16
17
18
19
20
Layer number
0 = 550 nm
0.00008
0.00007
0.00006
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
1
2
3
4
5
6
7
8
9
NATO/ASI and Euro Summer School September 16-27, 2002
10
11
12
13
14
15
16
17
18
19
20
Layer number
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
L(HL)^10- R=800 nm, C=550nm
L(HL)^10
R=800 nm, C=550 nm
Electric Field (V/m)
100
80
60
40
20
-1
0
1
2
3
4
5
6
Optical Distance from Medium
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
H(HL)^10B=800
B=800 nm,
C=550
nm
H(HL)^10
nm,
C=550
nm
Electric Field (V/m)
100
80
60
40
20
0
0
1
2
3
4
5
6
Optical Distance from Medium
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
LL(HL)^10 G=800 nm, C=550 nm
Electric Field (V/m)
100
80
60
40
20
-1
0
1
2
3
4
5
6
Optical Distance from Medium
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
MULTIDIELECTRIC MIRRORS
 Advantages :

- High reflectance (> 99.9 %)
- Low absorption loss (Visible, IR : < 10 ppm)
Drawbacks :
- Multilayers (HL) x HLL (> 30 layers, deposition time long)
- High reflectance over a short wavelength domain ( = 250 nm)
100
2x
H
H
L
L
. n 2H /n S
2x
. n 2H /n S
)
)
80
2
nH  nL
arcsin
 =  . 0 .
nH  nL
70
Reflectance (%)
( 1 - (n /n )
R
( 1  (n /n )
6 layers
14 layers
26 layers
90
2
60
50

40
30
20
0
10
0
700
NATO/ASI and Euro Summer School September 16-27, 2002
800
900
1000
1100
Wavelength (nm)
1200
1300
1400
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
COATINGS
INCLUDING
METALLIC LAYERS
The most popular in Astronomy
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
100 nm of Silver (R), Aluminum (G), Gold (B) on glass
Reflectance (%)
100
80
60
40
20
0
0
2000
4000
6000
8000
10000
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
100 nm of Silver (R), Aluminum (G), Gold (B) on glass
Reflectance (%)
100
80
60
40
20
0
0
200 400 600 800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
100 nm of Silver (R), Aluminum (G), Gold (B) on glass
Reflectance (%)
100.0
99.5
99.0
98.5
98.0
97.5
6000
7000
NATO/ASI and Euro Summer School September 16-27, 2002
8000
Wavelength (nm)
9000
10000
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
REFLECTANCE OF A METAL FILM WITH PASSIVATION LAYER
b
a
BULK
METAL
Reflectance (%)
100
SiO2-QW/Ag-100 nm/SIO2-Qw
Ag-100 nm
80
60
40
20
DIELECTRIC Layer (QW)
0
0
NATO/ASI and Euro Summer School September 16-27, 2002
1000
B
2000
3000
4000
Wavelength (nm)
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
ANTIREFLECTION OF A METAL FILM
a
b
Reflectance (%)
Cr - 10 nm on glass
60
Reflector
40
Dielectric phase
Matching layer
20
0
400
Metal layer
500
600
700
Wavelength (nm)
Cr 10 nm / MgF2 100 nm / Al 3 nm
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
Coatings
Principles
OPTICS IN ASTROPHYSICS
SMA
INDUCED TRANSMISSION IN A METAL FILM
Metal layer
High index
V
Low index
Reflectance (%)
100
80
(HL)^3-M-(LH)^3
60
40
20
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
Enhanced 100 nm of aluminum protected by dielectric layers
L (low index) : SiO2 or MgF2 & H (high index) : TiO2
Reflectance (%)
100
98
Al+(H L)^3
96
Al+(H L)^3
94
Al+(H L)^2
92
Al+H L
90
Al
88
86
340
360
380
400
420
440
460
480
500
520
540
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
Coatings
Principles
SMA
Enhanced 100 nm of aluminum protected by dielectric layers
L (low index) : SiO2 , H (high index) : TiO2
Reflectance (%)
100
95
90
Al+(HL)^20
T=99,54%
85
80
Al+(HL)^3
R=99,57%
75
70
340
360
380
400
420
440
460
480
500
520
540
Wavelength (nm)
NATO/ASI and Euro Summer School September 16-27, 2002
JM.M
OPTICS IN ASTROPHYSICS
SMA
Coatings
Principles
METALLIC MIRRORS
 Advantages :

High reflectance (> 90 %)
- over a large range of incident angles
- over a wide band of wavelength (UV, Visible, IR)
Drawback : High absorption loss
Al : good for U.V. (R > 90 %), adhere
on most substrates, passivation
necessary (oxidation)
Al
Ag : most popular, easy to deposit,
highest reflectance in visible
and I.R., tarnish rapidly,
protection necessary
Au : best material in I.R. (> 700 nm),
high reflectance, does not tarnish
NATO/ASI and Euro Summer School September 16-27, 2002
Au
(1 - n) 2  k 2
R
(1  n) 2  k 2
Ag
JM.M