WO1988001909A1

WO1988001909A1 - Optical coatings

Info

Publication number: WO1988001909A1
Application number: PCT/GB1987/000637
Authority: WO
Inventors: Ian Robert Peterson; Peter Vincent Kolinsky; Roger Michael Wood
Original assignee: The General Electric Company, P.L.C.
Priority date: 1986-09-15
Filing date: 1987-09-11
Publication date: 1988-03-24
Also published as: GB8622158D0; GB2196271A; GB2196271B; GB8721367D0

Abstract

A method of providing a damage-resistant optical coating on a component (10) comprises depositing on the component one or more transparent or semi-transparent layers (5 - 9) of predetermined thickness by forming a required number of monomolecular layers of a material of a predetermined refractive index, using the Langmuir-Blodgett process. By use of this process, the resulting transparent or semi-transparent layers are damage-resistant, particularly to large laser output pulses. Furthermore, the thickness of the layers can be very accurately controlled, and, by adjustment of the constituents of the material in the Langmuir-Blodgett trough, the refractive index of each layer can be accurately determined. The transparent or semi-transparent layers may be alternately of high and low refractive index, and the optical thickness can be controlled to provide an anti-reflective or reflective coating.

Description

Optical Coatings

This invention relates to optical coatings, and particularly to damage-resistant optical coatings and components provided with such coatings.

With the use of high power lasers there is a requirement that optical coatings, for example anti-reflection coatings, carried by optical components shall be damage resistant. Such coatings may be damaged at optical flux density thresholds which are much less than those of bulk components.

Conventional methods of fabrication of such coatings involve thermal or electron beam evaporation. The laser induced damage thresholds of these coatings are limited by hollow pores, defects and substrate replication defects and are typically of the order of 1 to 5 Jcm"^ for Q-switched pulse irradiation. At present, the most favoured technique for the production of damage-resistant optical coatings is the sol-gel process. In such a process a liquid alkoxy compound of a high refractory metal oxide is hydrolysed to form a hydrated gel, which is then fired to give a uniform glassy layer of the metal oxide. Coatings produced by such a process exhibit damage thresholds typically of the order of 10 Jcrrf^. This threshold is, however, limited by the number of small hollow pores produced during the deposition process. Recently, damage-resistant optical coatings have been produced by molecular beam epitaxy. Unfortunately, however, this technique is extremely slow, and employs very sophisticated and expensive equipment.

It is an object of the present invention to provide an improved method forming a damage-resistant optical coating on a component.

According to the invention, a method of providing a damage- resistant optical coating on a component comprises depositing on the component at least one transparent or semi-transparent layer of predetermined thickness by forming a required number of monomolecular layers of a material of predetermined refractive index using the Langmuir-Blodgett process.

If the component comprises a substrate of a material of refractive index n_s and a single transparent or semi-transparent layer is formed thereon, the refractive index of that layer is preferably

and the thickness is preferably 4, whereby an antireflective coating is formed.

The coating may comprise a plurality of said transparent or semi-transparent layers, each formed from a material having a refractive index which is different from that of the material forming the or each layer adjacent thereto.

For an antireflective coating, the method then preferably comprises depositing a relatively small number (e.g.2-5) of alternate layers of respectively high and low refractive indices. Preferably the thickness of each layer multiplied by its refractive index is approximately 0.7/V4, where is the wavelength of the incident light.

Alternatively, the optical coating may be a reflective coating, in which case the method comprises depositing a relatively large odd number (e.g. 11-81) of alternate layers of respectively high and low refractive indices and of substantial lyΛ/4 thickness.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 and 2 show schematic cross sections of damage-resistant optical coatings in accordance with the invention, which coatings are respectively anti -reflective and reflective; and Figure 3 shows a schematic cross section of an alternative coating.

The coatings are produced by using the Langmuir-Blodgett technique to deposit a chosen number of monomolecular layers (monolayers) on a surface of an optical component, by which technique the thickness of the coating may be defined with great accuracy, and will be uniform over the whole surface. Defects within the coating may be removed while each monolayer is on the surface of the water within the Langmuir-Blodgett trough prior to deposition on the component. One way of achieving this is to apply pressure to the monolayer trapped at the air-water interface in the manner described in Sci. Prog. Oxf. volume 69, 1985, pages 533-550.

Referring to Figure 1, a damage- resistant ant i -reflective coating 1 is formed on a glass substrate 2 of refractive index 1.5. The coating comprises a first layer 3 which has a refractive index of 2.5 in the visible band, formed by depositing successive monolayers of hemicyamine, using the Langmuir-Blodgett (LB) process. The thickness d of the layer 3 is preferably such that d x 2.5 = 0.7.ty4. For example, if ^"^ is lOOOnm, then 2.5d = 175nm and d = 70nm.

Assuming that a single LB layer has a thickness of 1.5nm, the number of LB layers required will be 70/1.5 = 47. A monomolecular layer of the material is deposited on the surface of the liquid in the Langmuir-Blodgett trough and the substrate is passed through the layers forty-seven times to achieve the required thickness, ensuring, throughout, that a monomolecular layer of the material is maintained on the surface. Since each LB layer is a single molecule thick, the thickness of the layer 3 can be controlled to within 1.5nm.

A second layer 4 of refractive index 1.5 is then deposited over the layer 3 by the LB process. This layer can be formed of 22 tricosenoic acid and would be of 175/1.5nm = 117nm thickness. If a single monomolecular layer of the material has a thickness of 3nm, the number of such layers required will be 117/3 = 39.

The refractive indices of the layers can be controlled accurately during deposition by slight adjustment of the constituents of the materials in the LB trough, for example by the incorporation of additives therein. Figure 2 shows schematically a section through a reflective coating. In this case, layers 5, 6 and 7 of refractive index 2.5 interleaved with layers 8 and 9 of refractive index 1.5 are deposited on a substrate 10 of refractive index 1.5. The layers are deposited by the LB process as described above, but each will be of A/4 optical thickness. The number of passes through the monomolecular layer in the LB trough will be determined accordingly, as in the previous embodiment.

Any desired number of layers of alternately high and low refractive indices may be deposited, depending upon the required reflective or antireflective characteristics of the coating.

An alternative method of controlling the refractive index of a layer is provided by skeletonisation. This involves the deposition of a mixture of the required coating material with a soluble material. After deposition of the layer, the soluble material is at least partially removed, leaving a skeleton of the coating material. The refractive index is reduced by an amount depending upon the degree of skeletonisation. The limit is reached when the layer only just remains continuous. One material which an be used in this way is cadmium eicosanoate, which would normally have a refractive index of 1.526. By use of skeletonisation, a refractive index of, say, 1.23 may be obtained. Such materials as 22-tπ^'cosenoic acid may be partially polymerised after skeletonisation. If the skeletonised layer is formed on a layer of unskeletonised cadmium eicosanoate, a good antireflective coating will be formed, since 1.23² = 1.526. Similarly, skeletonised cadmium eicosanoate on glass of refractive index 1.5 will produce a good ant i -reflective coating.

Figure 3 shows an example of a structure in which the thickness of successive high refractive index layers 11, 13, 15, 17, 19 decreases, and the thickness of successive low refractive index layers 12, 14, 16, 18 increases, with distance from the substrate. For the sake of explanation, the layers are shown divided by dotted lines which are spaced apart by a notional unit thickness. In the figure, the high refractive index layers 11, 13, 15, 17 and 19 are respectively 5, 4, 3, 2 and 1 units thick, and the low refractive index layers 12, 14, 16 and 18 are respectively 1, 2, 3 and 4 units thi ck. This arrangement gives a graded opti cal characteristi c, whi ch is particularly advantageous for handl ing hi gh power flux densities e. g. for laser output windows where the laser pulse passes through the coating. The graded index gives mi nimal electrical field discontinuities.

The refractive index of any indvi^'dual layer of a coating may be graded through the thickness of the layer by slight changes in the composition of the material in the LB trough during the course of the passes of the substrate through the material .

Examples of other material s which may be deposited by the LB process are eicosanoates of other heavy metals besi des cadmi um, eicosenoic acid, which has a refracti ve index of 1.425 at a wavelength of 632 nm, and 22-gedye, whi ch has a refractive index of 4.2 at that wavelength.

Measurements have shown that optical coatings comprising monol ayers of hemacya ine deposited by the LB process may withstand fflluuxxeess in excess of lOOJ/cπr for 25ns pulses of a 1.0464 _^μm wavelength l aser.

It wil l be apparent that the present invention provides a simple method of providing damage-resistant coatings on opti cal components , by whi ch method the thi ckness and the refracti ve indi ces of the vari ous layers can be very accurately control led.

Claims

1. A method of providing a damage- resistant optical coating on a component, comprising depositing on the component at least one transparent or semi -transparent layer of predetermined thi ckness by forming a requi red number of monomolecular layers of a material of predetermined refractive index using the Langmuir-Blodgett process.

2. A method as cl aimed in Claim 1, comprising depositi ng a plural ity of said transparent or semi -transparent l ayers, each formed from a material having a refracti ve index which is different from that of the material forming the or each layer adjacent thereto.

3. A method as cl aimed in Cl aim 2, wherein the transparent or semi-transparent l ayers are alternately of materials of hi gh and l ow refractive index values .

4. A method as claimed in Claim 2 or Claim 3 , wherein the number of monomolecular layers deposited by the Langmu r-Bl odgett process is such that the thi ckness d of each t ransparent layer or semi -transparent l ayer is given by

d = 0.7 A/4n

where n is the refractive index of the particular layer and is the wavelength of the incident li ght.

5. A method as claimed in Claim 2 or Claim 3 , wherein the number of monomolecular layers deposited by the Langmui r-Bl odgett process is such that the thickness of each transparent or sem -transparent layer is substantial ly equal to A/4, where as the wavel ength of the incident li ght.

6. A method as cl aimed in Claim 2 or Claim 3 , wherein the number of monomolecular layers deposited by the Langmuir-Bl odgett process is such that the thi ckness of the transparent or semi -transparent layers of hi gh refractive index value decreases with distance from the substrate and the thickness of the layers of low refractive index increases with distance f rom the substrate.

7. A method as claimed in any preceding claim, wherein the material of at least one of the of the transparent or sem -transparen layers comprises a fatty acid.

8. A method as claimed in Claim 7 wherein the fatty acid is 22-tπ^'cosenoic acid or the eicosanoate of a heavy metal such as cadmium.

9. A method as claimed in any one of Claim 1-6, wherein the material of at least one of the transparent layers or semi-transparen l ayers is hemi cyanine.

10. A method as cl aimed in any preceding cl aim, wherein the refracti ve index of at least one of the tansparent or semi -transparen l ayers is reduced by skeletonisati on of the material of the layer.

11. A method as claimed in Cl aim 10, wherein the material is at l east partial ly polymerised after skeletonisati on thereof.

12. A method of provi ding a damage^ resist ant opti cal coating on component , substantial ly as hereinbefore described with reference to the accompanying drawing.

13. A component provided with a coating by a method as cl aimed i any preceding claim.