This effort was sponsored by the Air Force Weapons Laboratory, Air Force Systems Command, United States Air Force, Kirtland AFB, New Mexico 87117.

6. References

[1] Harper, James M.E. Ion Beam Deposition, chapter II-5 in Thin Film Processes, J.L. Vossen and W. Kern, eds. New York: Academic Press; 1978.

[2] Wehner, G.K.; Anderson, G.S. The nature of physical sputtering, chapter 3 in Handbook of Thin Film Technology, L.I. Maissel and R. Glang, eds. New York: McGraw-Hill; 1978. [3] Sites, J.R.; Gilstrap, P.; Rujkorakarn, R. Ion beam sputter deposition of optical coatings. Opt. Eng. 22: 447-449; 1983.

[4] Demiryont, H.; Sites, J.R. Oxygen threshold for ion-beam sputter deposited oxide coatings. Nat. Bur. Stand. (U.S.) Spec. Publ. 727: 180-186; 1984.

[5] Allen, T.H. Reactive ion beam sputtered optical coatings. Proceedings of the 30th Annual Technical Conference of the Society of Vacuum Coaters: 27-41; 1987.

[6] Pond, B.; Schmell, R.A.; Carniglia, C.K.; Raj, T. Comparison of the optical properties of some high-index oxide films prepared by ion-beam sputter deposition with those of electron beam evaporated films. NIST Spec. Publ. 752: 410-417; 1986.

[7] Varitimos, T.E.; Tustison, R.W.

Films 151: 27-33; 1987.

Ion beam sputtering of ZnS thin films. Thin Solid

[8] Allen, T.H.; Lehan, J.; McIntyre, L.C. Ion beam sputtered magnesium fluoride. Optical Interference Coatings, 1988 Technical Digest Series, Vol. 6, 1988 April 12-15; Tucson, AZ. 293.

[9] Mouchart, J.; Lagier, G.; Pointu, B. Determination des constants optiques net k de materiaux faiblement absorbents. Appl. Opt. 24: 1808-1814; 1985.

[10] Manifacier, J.C.; Gasiot, J.; Fillard, J.P. Determination of the optical constants n, k and the thickness of a weakly absorbing thin film. J. of Phys. E9: 1002-1004; 1976. [11] Arndt, D.P. et al. Multiple determination of the optical constants of thin film coating materials. Appl. Opt. 23: 3571-3596; 1984.

[12] Campbell, D.S. Mechanical properties of thin films, chapter 12 in Handbook of Thin Film Technology, L.I. Maissel and R. Glang, eds. New York: McGraw-Hill; 1970.

[13] Guenther, K.H.; Wierer, P.G.; Bennett, J.M. Surface roughness measurements of low-scatter mirrors and roughness standards. Appl. Opt. 23: 3820-3836; 1984.

[14] Heavens, 0.S.; Smith, S.D. Dielectric thin films. J. Opt. Soc. Am. 47: 469-472; 1957. [15] Pulker, H.K. Characterization of optical thin films. Appl. Opt. 18: 1969-1977; 1979. [16] Plyler, E.K.; Griff, N. Absolute absorption coefficients of liquid water at 2.95μ, 4.7μ, and 6.1μ. Appl. Opt. 4: 1663-1662; 1965.

[17] Willmott, J.C. 1950.

The infrared spectrum of magnesium oxide. Proc. Phys. Soc. 63: 389-402;

[18] Malitson, I.H. A redetermination of some optical properties of calcium fluoride. Appl. Opt. 2: 1103-1107; 1963.

[19] Handbook of Chemistry and Physics, R.C. Weast, M.J. Astle, W.H. Beyer, eds. Boca Roton, FA: Chemical Rubber Company Press; 1988.

[20] Heavens, 0.S. Optical properties of thin films. Rep. Prog. Phys. 23: 1-65; 1960.

Manuscript Received
1-18-89

Localization of Absorption Losses in Optical Coatings

Wu Zhouling Fan Zhengxiu

Shanghai Institute of Optics and fine Mechanics

Academia Sinica, P.0. Box 8211, Shanghai, PRC

Absorption measurements performed by means of photothermal deflection technique in suitably prepared samples permit a localization of absorption losses in optical coatings.

For single layers, wedge-shaped ZrO,, NgF., ZnS, Tio,, Ta, 0, and Si0, films are measured. The experimental results show that in Zr0,, MgF, and ZnS films investigated, the film-substrate interface absorption and the air-film interface absorption are nearly the same, while in Ti0,, Ta, 0, and Si0, films, the film-substrate interface absorption dominates over the air-film interface absorption, being the main source of the total absorption loss.

For multilayers, a seperate evaluation of the volume and interface absorption of Tio,/Sio, laser mirrors was carried out by means of an appropriate variation in the thickness of the high and low refracting components, and the measured results show a predominance of interface absorption over volume absorption in the multilayer system under investigation.

Key Words, bulk and interface absorption; optical coating; photothermal deflection.

1. Introduction

For many systems in modern optics, the reduction of losses in dielectric thin films is extremely important. The absorption loss causes laser-induced damage in dielectric layers, thus lints the rediation intensity of many lasers. To reduce the absorption loss in dielectric thin films, it is necessary to understand the physical origin of absorption in the concerned coatings.

This paper reports our recent local characterization of absorption losses in dielectric thin films. Total absorption measurement was carried out by transverse photothermal deflection technique [1-3], while the seperate evaluation of the bulk and interface absorption portions was realized with the help of suitably prepared samples [4, 5].

2. Preparation of the samples

All of the samples were produced in conventional high vacuum evaperation plants equipped with oil diffusion pumps. For single layers, the samples were made wedge-shaped [4], as shown in fig. 1. For multilayers, an appropriate variation in the thickness of the high and low refracting components was introduced [5], as shown in fig. 2. The deposition methods and the related characterizations of the samples under investigation is given in table 1.

3. Experimental procedure and related theory

3.1 Total absorption measurement

Total absorption measurement of the samples investigated was carried out by transverse photo

thermal deflection technique. The physical basis for this technique [1, 2] and a detailed description of our apparatus [3] have been published earlier. Here we introduce the measurement by simply giving

A=A, S/S.

where A. and S. are the absorptance and the photothermal deflection amplitude signal of the caliberating sample, while A and S those of the sample being measured.

3.2 Seperate evaluation of volume and interface absorption of single layers

To investigate the absorption of single-layer films deposited on glass-substrates we need to seperate four absorption portions, the air-film interface (af), the bulk of the film (f), the filmsubstrate interface (fs), and the bulk of the substrate (s). Hence, the measured total absorption A of the sample investigated consists of an air-film as well as a film-substrate interface tern and bulk absorption of the film as well as of the substrate [5],

A=A ̧, +A, +A, ̧ +A,

= Paf Auf + Pƒãƒ df + Pfs Ofs + Pjäs ds

where a...,(a,,)- the specific absorption at the air-film (film-substrate) interface; f(s) the spatially averaged film (substrate) absorption coefficient,

(1)

(2)

Paffs) - the relative light power densitry at the air-film (film-substrate) interface; PP) the spatially averaged relative light power density inside the film (substrate).

In our experiment, to seperate A...,, A,, A,, and A., we employ the following procedure,

First, measures. This can be carried out by direct measurement of the bare substrate absorptance in the wedge-shaped sample, as shown in fig. 1.

Second, seperate ƒ, a,. and a... Here, in dependence of the light beam position on the layer, and, hence, of the actual film thickness, the relative power densities at both interfaces and within the film volume will be changing in a characteristic pattern, ranging from a quarterwave to a half wave optical thickness, labled by indices (1) and (2), respectively. Measuring wedge-shaped film up to an optical thickness at least of 2, the rise in A'' or A'*' versus the optical thickness permits a determination of

(3)

The specific interface absorption a,, and a., are yielded from measured data extrapolated to zero thickness. By rewriting equation (2) for quaterwave as well as halfwave thickness we have

Third, calculate A.,, A,, A,. and A, by comparing eq. (1) and eq.(2). This can be realized in a direct way.

3.3 Seperate evaluation of volume and interface absorption for multilayer samples

where A indicates total absorptance while A with indices indicate related absorptance portions.

To simplify the analysis, we take quater-wave-stack HR coatings as examples, thus

where d, is the geometrical thickness of the i-th layer. Then we have [6]

where

is the spatially averaged absorption coefficient of low (high) refracting compo

nent and a, the specific absorption at the H-L interface.

To make a seperate evaluation of the volume and interface absorption of the HR coatings, an appropriate variation in the thickness of the high and low refracting components is neccessary [5]. Thus, if we have

where A(q, p) is the total absorption corresponding (q,p), A ̧(q) and A ̧(p) are the respective volume absorptions. By measuring A(q,p) with different (q, p) and then fitting the results to eq. (10), we can get a, and 4, and hence A, and A. (q,p).

4. Experimental results and discussion

4.1 Single-layer Samples

L

Table 2 shows our experimental results of single layers. From these results we can see, (1). A seperate evaluation of bulk and interface absorption of single-layer optical thin films has been realized by means of the combination of transverse photothermal deflection tech

nique and wedge-shaped samples.

(2). For samples No. 1~No. 3 (ZrO,, MgF., ZnS), the three absorption portions are nearly the same (A,,~A., ~A,), whereas for samples No. 4~No.6 (Ti0,, Ta,0,, Sio,), the film-substrate interface absorption A,, dominates over the air-film interface absorption A.,, being the main soure of the total absorption loss.

(3) Though deposited under the same conditions, the localized absorption losses of samples No. 1~3 and No. 4~6 show a great difference. This implies that the kind of material plays an inportant role in the distribution of absorption losses in optical coatings.

4.2 Multilayer Samples

Table 3 shows our calculated,

and A./A, (q, p) of the investigated Tio, /Si0, coat

ings from experimental data and their comparison with those reported previously [5]. From this table we can see,

L

(1) For the Ti0,/Si0, coatings investigated we have A., A. (q, p) and H)) de. Thus, in the coatings the absorption of the H-L interface and that of the high refracting components make most contributions to the total absorption loss.

(2). Though no consideration of the "intra-interface" [6] absorption was taken into account during processing the experimental data, our calculated results show a good agreement with those reported previously with 0.75 [5]. with 0.75 [5]. This indicates that the "intra-interface" absorption is negligible for the coatings investigated in this paper.

5. Conclusions

By means of the conbination of photothermal deflection technique and a suitable preparation of the samples, a seperate determination of volume and interface absorption is realized for both single-layer and multilayer optical coatings. Experimental results show that in Zr0,, NgF, and ZnS single layers investigated, the film-substrate interface absorption and the air-film interface absorption are nearly the same, whereas in Ti0,, Ta, 0, and Si0, thin films, the film-substrate interface absorption dominates over the air-film interface absorption, being the main source of the total absorption loss.

For the Ti0,/Si0, multilayer samples under investigation, the H-L interface is the main source of the absorption loss, though the high refracting components also show a considerable comtribution.

The authors are grateful to Prof. Zhou J.L. and Shi B. X. for their helpful advices and fruitful discussions.

6. References

[1] Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier D. Photothermal deflection spectroscopy and detection. Appl. Opt. 20(8), 1333; 1981.

[2] Murphy, J. C.; Aamodt, L. C. Photothermal spectroscopy using optical beam probing, mirage effect J. Appl. Phys. 51(9), 4580;1980.