Multilayer Optical Component

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.

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.

A few embodiments of the invention will be described in greater detail, by way of example, with reference to the drawings, in which:

FIG. 1 is a cross-sectional view of an optical component according to the invention having reflecting properties;

FIGS. 2 and 3 are diagrammatic elevations of two different combinations of an optical component according to the invention with a magneto-optically active layer;

FIG. 4 shows diagrammatically a laser arrangement with two reflecting optical components according to the invention;

FIG. 5 shows diagrammatically a laser arrangement having two reflecting and two antireflecting components according to the invention.

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 <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:

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.