Multilayer optical component

A multilayer optical component is provided in thin-film technology. The component comprises a monocrystalline substrate 1, for example a garnet substrate, which supports a stack 2 of monocrystalline layers 11-16, for example garnet layers, provided epitaxially on the substrate. The layers have alternately a high and a low refractive index and as regards thickness and refractive index are optimized to minimally or maximally reflect electromagnetic radiation of a given wavelength in the infrared or optical range of the spectrum. Said optical component is suitable in particular for use in high-power lasers.

FIELD OF THE INVENTION 
The invention relates to a multilayer optical component in thin-film 
technology for handling electromagnetic radiation in the visible and/or 
infrared spectral range which is incident transversely to the plane of the 
component, having a substrate which supports a stack of thin-film layers 
with alternately a high and a low refractive index. 
BACKGROUND OF THE INVENTION 
An optical component for handling electromagnetic radiation which is 
incident transversely to the plane is to be understood to mean herein a 
system of layers having reflecting or anti-reflecting properties with 
regard to electromagnetic radiation of a given wavelength. 
A system of layers having reflecting properties and constructed as a 
multilayer thin-film component is known from U.K. Patent Application No. 
GB-A 2,020,842. The reflector described in said Application comprises a 
number (7-9) of dielectric layers having alternately a high and a low 
refractive index and thicknesses equal to 1/4.lambda., where .lambda. is 
the wavelength of the radiation to be reflected. The layers have been 
vapour-deposited and consist alternately of kryolite and zinc sulphide, or 
alternately of thorium fluoride and zinc sulphide. The disadvantage of 
this known reflector is that it exhibits physical defects which are 
inherent in layers formed by vapour-deposition. They locally have an 
insufficiently low absorption in the wavelength range of the radiation to 
be reflected and are insufficiently homogeneous and so have scattering or 
absorption centres As a result of this they are unfit notably for use in 
mirrors for high-power lasers. 
It is the object of the invention to provide a multilayer optical component 
of the type mentioned in the opening paragraph which is well fitted for 
use as a mirror for high-power lasers. 
According to the invention a multilayer optical component is characterized 
in that the substrate is a monocrystalline substrate having a lattice 
constant a.sub.o and that the stack of thin-film layers consists of a 
number of monocrystalline layers which have been grown epitaxially on the 
substrate and have a lattice constant which is substantially equal to 
a.sub.o. 
A practical embodiment of the optical component in accordance with the 
invention is characterized in that the monocrystalline layers which, taken 
from the substrate, have an even number consist of the same material as 
the substrate. The phrase "taken from the substrate" as used herein means 
counting from the substrate with the substrate being numbered zero. 
Within the scope of the invention the substrate may, for example, consist 
of monocrystalline gallium phosphide. Monocrystalline layers of 
alternately silicon and gallium phosphide may have been deposited thereon 
by means of hetero-epitaxy. 
Alternatively, the substrate may consist of monocrystalline gallium 
arsenide. Monocrystalline layers of alternately silicon and gallium 
arsenide may have been deposited thereon by means of hetero-epitaxy. 
According to a preferred form of the invention the substrate and the 
monocrystalline layers grown epitaxially on the substrate consist of a 
monocrystalline material having a garnet structure. 
Epitaxial growth, for example from the liquid phase, of monocrystalline 
garnet layers physically speaking leads to substantially perfect layers as 
compared with vapour-deposited layers and which therefore have very few 
absorption centres and a minimum number of scattering centres. Various 
types of garnets can form combinations of layers having at least 
substantially equal lattice constants but different values of refractive 
index. Dependent on the difference in refractive index a smaller or a 
larger number of layers with alternately a high and a low refractive index 
may be used to realize a desired reflection. 
A practical embodiment of the invention is characterized in that the 
substrate is of gadolinium gallium garnet (GGG). GGG can be obtained with 
a very high optical quality, i.e. a minimum of physical defects and 
negligible optical inhomogeneities. 
For handling electromagnetic radiation in the infrared spectral range a 
further embodiment of the invention is characterized in that the 
monocrystalline garnet layers having an odd number when taken from the 
substrate consist of a material based on yttrium iron garnet and the 
layers having an even number when taken from the substrate consist of 
gadolinium gallium garnet. 
For example, when alternate layers of yttrium iron garnet (YIG) (n=2.2) and 
GGG (n=2.0) are grown on a GGG substrate in thicknesses optimised for 
reflection, it is found that the reflection with an overall number of 2 
layers is 18%, with an overall number of 10 layers is 49%, and with an 
overall number of 16 layers is 69%. 
For handling electromagnetic radiation in the optical spectral range a 
further embodiment of the invention is characterized in that the 
monocrystalline garnet layers having an odd number when taken from the 
substrate consist of a material based on Y.sub.3 Al.sub.3 Sc.sub.2 
O.sub.12 and the monocrystalline garnet layers having an even number when 
taken from the substrate consist of gadolinium gallium garnet. 
Besides being useful as a reflecting or antireflecting element in lasers, 
the optical component according to the invention is also suitable for use 
in combination with an epitaxial magneto-optically active garnet layer. 
This may be present on the side of the stack of epitaxial monocrystalline 
garnet layers remote from the substrate or between the substrate and the 
stack of epitaxial monocrystalline garnet layers In these cases the 
thicknesses of the epitaxial monocrystalline garnet layers of the stack 
are chosen to be so that optimum reflection occurs for the wavelength of 
the electromagnetic radiation to be used. 
The epitaxial magneto-optically active garnet layer, however, may also be 
provided between a first and a second sub-stack of epitaxial layers, the 
thicknesses of the layers of the first stack being chosen to be so that 
optimum reflection occurs for the wavelength of the electromagnetic 
radiation to be used, the thicknesses of the layers of the second 
sub-stack being chosen to be so that (optimum) anti-reflection occurs for 
the wavelengths of the radiation to be used. In this case the first 
substack may, for example, adjoin the substrate and the electromagnetic 
radiation to be treated may be incident via the second sub-stack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a monocrystalline substrate 1, for example, a gadolinium 
gallium garnet (GGG). A stack 2 of monocrystalline garnet layers 11-16 is 
deposited epitaxially hereon, for example, from the liquid phase. 
Substrate 1 with stack 2 constitutes an optical component according to the 
invention. As regards thickness and refractive index the layers 11-16 are 
optimised so as to obtain maximum reflection (they may hence be considered 
as 1/4.lambda. layers) and alternately have a first refractive index 
n.sub.1 (the layers 11, 13, 15) and a second refractive index n.sub.2 (the 
layers 12, 14, 16), with n.sub.2 &lt;n.sub.1. In the case in which the layers 
12, 14, 16 consist of GGG (n.sub.2 =2.0) and the layers 11, 13, 15 of YIG 
(n.sub.1 =2.2), the reflection is 34% (.lambda.=1000 nm). 
The reflection measured directly at the substrate 1, i.e. without 
1/4.lambda. layers, is 11%. The stack 2 may consist of more than six 
layers or less than six layers. The reflection as a function of the number 
of 1/4.lambda. layers is: 
______________________________________ 
number of 1/4 .lambda. layers 
reflection in % 
______________________________________ 
0 11 
2 18 
4 26 
6 34 
8 42 
10 49 
12 57 
14 63 
16 69 
______________________________________ 
The lattice constants of GGG and YIG are sufficiently close to each other 
(a.sub.o =12.383 .ANG. and a.sub.o =12.377 .ANG., respectively to permit 
epitaxial growth. 
In FIG. 1 I is the incident radiation, R is the reflected radiation, and T 
is the transmitted radiation. 
In the case where maximum reflection is not desired, but rather maximum 
transmission (hence a system of layers having anti-reflecting properties), 
the thicknesses and the refractive indices of the layers 11-16 may be 
optimized for that purpose. 
In order to obtain an optital component which is suitable for handling 
electromagnetic radiation in the optical spectral range, GGG may also be 
used for the material of the substrate 1 and alternate layers 11, 13, 15 
of Y.sub.3 Al.sub.3 Sc.sub.2 O.sub.12 (n=1.8) and layers 12, 14, 16 of GGG 
(n=2.0) can be deposited hereon epitaxially. The lattice constants of GGG 
(a.sub.o =12.383 .ANG.) and of Y.sub.3 Al.sub.3 Sc.sub.2 O.sub.12 (a.sub.o 
=12.38.+-.0.01) are sufficiently matched to each other to enable epitaxial 
growth. 
FIG. 2 shows the substrate of FIG. 1 with the stack 2 of epitaxial 
1/4.lambda. layers. In this case an epitaxial garnet layer 3 having 
magneto-optically active properties is grown on the stack 2. This is, for 
example, a magnetic bubble garnet layer having a composition based on 
yttrium iron garnet or on bismuth yttrium iron garnet. 
Substrate 1 having layers of stack 2 serves as a reflector for the 
magneto-optically active layer 3. A magnetic bubble 4 in the layer 3 can 
be detected by means of a detector. system consisting of a laser source 5, 
a lens system 6, 7 to focus the radiation of the laser source 5, a 
polarizer 8, an analyzer 9, a semi-transparent mirror 10 and a detector 
20. 
In this case detection is carried out from the front of the substrate 1. 
However, it is also possible to perform the detection from the rear side 
of the substrate, i.e. through the substrate. An optical component which 
is suitable for that purpose is shown diagrammatically in FIG. 3. 
Substrate 1 and stack 2 of layers 11-16 are the same as in FIGS. 1 and 2, 
only in this case an epitaxial monocrystalline garnet layer 3 having 
magneto-optically active properties is provided between the substrate 1 
and the stack 2. Electromagnetic radiation I is incident via substrate 1, 
passes through layer 3, is reflected partly by the stack 2 of 1/4.lambda. 
layers dependent on the number of layers of the stack 2, passes again 
through layer 3 and can be detected on the free side of the substrate. The 
magneto-optically active layer 3 may optionally be present between a 
sub-stack 2 having reflecting properties and a sub-stack 2' having 
anti-reflecting properties. 
FIG. 4 shows a first type of laser arrangement in which a discharge tube 21 
which comprises a Brewster angle window 22, 23 at each end is placed 
between two mirrors 24, 25. The mirrors 24 and 25 are optical components 
according to the invention and each consist of a monocrystalline substrate 
1 on which a stack 2 of monocrystalline epitaxial garnet layers of 
alternately a high and a low refractive index is grown. The thicknesses 
and the refractive indices are optimized for maximum reflection of the 
radiation of the discharge tube 21. 
FIG. 5 shows a second type of laser arrangement. In this case a discharge 
tube 26 is placed between two mirrors 27, 28 each formed by reflecting 
optical components according to the invention. In this case a resonant 
cavity 29 with a magneto-optically active element 31 surrounded by a coil 
30 is present between the discharge tube 26 and the mirror 28. Since the 
highest energies of the present laser system are handled in the resonant 
cavity 29 it is of great importance for the ends of the magneto-optically 
active element to have anti-reflecting properties for the radiation of the 
discharge tube 26. For this purpose the ends of element 31, which is 
based, for example, on the use of a monocrystalline garnet substrate, each 
comprise a stack 2.sup.1 consisting of monocrystalline epitaxial garnet 
layers having refractive indices and thicknesses which are optimized to 
minimally reflect radiation of the discharge tube 26.