Optical constants of polymer coatings in the infrared

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physics& Technology36 (1995) 1125-1129

Research Note

Optical constants of polymer coatings in the infrared M. Saito, T. Gojo, Y. Kato, M. Miyagi Department of Electrical Communications, Tohoku University, Sendai 980, Japan

Received 17 June 1994

Abstract The refractive indices and absorption coefficients of fluorocarbon, polymethyl methacrylate, and polyimide were measured in the wavelength range of 2-12 ~tm. The reflection losses of KRS-5 and CsI windows were reduced successfully by coating fluorocarbon polymers on the surfaces.

1. Introduction Recently various optical polymers have been developed and used for fabricating optical components [1-3]. In particular, polymers are useful in thin-film technology, since they can be coated on windows and lenses by such a simple method as dip-coating and spin-coating. In the fabrication of an optical coating, the refractive index of a polymer is an important design parameter. Although the optical properties of polymers have been studied in the visible wavelength region, few data are available in the infrared region. The evaluation of the absorption coefficients is also important, since most polymers have their characteristic absorption bands in the infrared region. In this study, we measure the complex refractive indices n - j x of polymers in the mid-infrared region. Being based on the measured data, we

design and form anti-reflection coatings for infrared windows.

2. Sample preparation Three polymers were chosen as coating materials; i.e., fluorocarbon (Asahi Glass, C Y T O P ' ) , polymethyl methacrylate (PMMA; Tokyo Ohka Kogyo, OEBR-1000), and polyimide (Toray, # 3000). BaF 2 plates of 1 mm thickness were used as substrates. The transmittance of a BaF 2 substrate is 93-94% below 11 ~tm wavelength and decreases gradually to 87% at 12 tam. A solution of a polymer was spin-coated on one side of a BaF2 substrate. Film thickness was varied between 0.4 and 4 tam by changing the spinning speed between 500 and 7000 rpm. The coated sample was dried at room temperature for ~ 0 . 5 h , and then it was baked in an open furnace. Baking temperature and

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M. Saito et al. / Infrared Physics & Technology 36 (1995) 1125-1129

1126

time were 120°C and 5 h for fluorocarbon, 170°C and 0.5h for PMMA, and 180°C and 1 h for polyimide.

10 o 2

3. Extinction coefficients

10 .2

The transmittances of the samples were measured for normal incidence by using a grating monochromator and a HgCdTe sensor. Fig. 1 shows the examples of measured transmission spectra. Several absorption bands are seen in the spectra. To evaluate the thicknesses of the polymer films, transmission spectra were also measured in the visible wavelength range by using a spectrophotometer. Since the refractive indices of the polymers are known in the visible spectral range, film thickness can be evaluated from the wavelengths of optical interference peaks. The infrared transmittances were measured for three samples of different film thicknesses and were plotted as a function of thickness. An absorp-

100 2 ~ ,

0 100

6

,

8

f iiilrocaiio~ i i i i n n

I PMMA

~.~ I--

4

l

100

l

l

n

l

l

J

10

i

i

i

l

6

(b)

i

(c) 8,,,

10

Wavelength (pm)

X~ lO_3

/ ~ /

•~5

'

8

10

12

.

E~ 10 - 4

(a)

, :" . . . .

,

,

,

,

8'°°FP MA ...... I°-I I UJ 10 -2L100

1

i '

'E ,

'° | '

" . . . .

--I

(b)

. . . . . . . . .

I Polyimide

10-11 lO- ,

t l

A

--

!TvwV

. . . . . . .

8

lO

Wavelength (pm)

12

(c)

(a)

i

. . . . . . . . .

. . .4. .

6

Fig. 2. Extinction coefficients ~ of (a) fluorocarbon, (b) PMMA, and (c) polyimide. K was evaluated from the difference of transmittances that were measured for the films of different thicknesses. Film thicknesses are (a) 1.23, 2.88, and 4.19 p.m (large dots beyond 7 pm wavelength); 9, 44, and 91 l.tm (small dots below 7 pm wavelength); (b) 0.40, 0.65, and 1.01 lam; (c) 1.01, 1.64, and 2.12 pm.

12

Polyimide 0

4

12

Fig. 1. Transmission spectra o f (a) f l u o r o c a r b o n , (b) P M M A , and (c) p o l y i m i d e films t h a t were coated on B a F 2 substrates. F i l m thicknesses are (a) 1.23, (b) 0.65, and (c) 2.12 lam. The thickness o f the substrates is 1 ram.

tion coefficient ~ and an extinction coefficient rc = (2/4n)~ were evaluated from the slope of the plots by using the least-squares fitting method. Fig. 2 shows the extinction coefficients K at various wavelengths. Fluorocarbon has a prominent absorption peak around 8 pm wavelength. PMMA and polyimide exhibit many absorption peaks. The extinction coefficient of fluorocarbon is smaller than those of PMMA and polyimide at most wavelengths except for 8-10 pm. In the present experiments, the measurement errors of transmittance and film thickness are _+1% and -t- 0.02 lain, respectively. By taking into account the measurement errors, the accuracy of the evaluated extinction coefficients x is assumed to be _+0.01 or _+0.02. Therefore we can not evaluate

M. Saito et al. / Infrared Physics & Technology 36 (1995) 1125-1129

K accurately in a shorter wavelength range in which ~¢ is less than 10 2. For the evaluation of a smaller extinction coefficient, thicker films are required. Fluorocarbon films of 9, 44, and 91 lam thicknesses were prepared by an extrusion method. Film thickness was measured by a micrometer with an accuracy of _+ 1 ~tm. Smaller dots below 7 lam wavelength in Fig. 2a show the data that were evaluated from the transmittances of thick films. The evaluation error of ~: is of the order of 10 -4 in this case. In the wavelength range beyond 7 ~tm, in which K is larger than 10 2, the transmittances of such thick films are too low to measure accurately.

4. R e f r a c t i v e

indices

The power reflectance and transmittance at the air/BaF2 boundary are nO __ n s 2

R l = n0+n~ '

Tl=l-Ri

(1)

for normal incidence, where n o and ns denote the refractive indices of air and BaF2, respectively [4]. If the surface of a BaF2 substrate is coated with a film of complex refractive index ~ = n - j x , the power reflectance and transmittance become no - fi R2 =

h - n~,

//

.4~rJd'~

1 -no- + fi + -n +- n~,exp ~--J--~--) no -- fi h -- n~ [ + + no+~-~+nseXp(-J~z.

2,

. 47zhd~ )

T: = 1 - R2,

(2)

where d and 2 denote film thickness and light wavelength, respectively [4]. In the current experiment, only one side of a BaF2 substrate was coated with a polymer. By taking into account the multiple reflection inside a BaF2 substrate, the power transmittance of a coated BaF2 substrate is calculated as T=

T IT2+ TI(RzRI)T2+ TI(R2RI)2T2+...

TI T2

(3)

1 - Ri R2

In the derivation of Eq. (3), we did not consider the optical interference inside a BaF: substrate, since

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0.002

Polyimide O -

>

a

0.001

PMMA Fluorocarbon I

J

I5

~

1'3 14 1 1'6 17 Refractive Index n o f calculated transmittances T¢(n, .;'~) and

Fig. 3. Deviation a measured transmittances T(,;o). a takes the minimum value at n = 1.35 for fluorocarbon, 1.51 for P M M A , and 1.59 for

polyimide. These values are assumed to be the refractive indices of the polymers.

the substrate is thicker than the coherent length of probe light. In Eqs. (1)-(3), no(= 1), ns, [5] ~c, d, 2, and T are known or measurable parameters, and n is the only unknown parameter. We assumed a tentative value for n, and calculated theoretical transmittances T~(n, ).) by using Eqs. (!)-(3). For an assumed value of n, the deviation of theoretical transmittances T~(n, 2) from the measured transmittances T(2) is

where N denotes the number of data. Fig. 3 shows the deviation a as a function of n. For a fluorocarbon coating, cr takes the minimum value at n = 1.35. If T and d change within their measurement errors, i.e., +_1% and +0.02 gm, the minim u m point of a shifts by _+0.05. Therefore the refractive index of fluorocarbon is assumed to be 1.35 _+ 0.05. For P M M A and polyimide coatings, the refractive indices are assumed to be 1.51 -4- 0.05 and 1.59 +0.05, respectively. In the evaluation above, we obtain a better fitting (a smaller value of a) if we assume that the refractive index decreases slightly toward a longer wavelength (dispersion). However, the wavelength dependence of the refractive index is of the order of 0.01, which is smaller than the evaluation error. Hence we assumed a constant value for n in the calculation of Fig. 3. Fig. 4 summarizes the transparent (• < 10 -') spectral ranges and the refractive indices of the polymers. Fluorocarbon, PMMA, and polyimide

M. Saito et al. / Infrared Physics & Technology 36 (1995) 1125-1129

1128

X < 1 0 -2

Fluoro~ 1 carbon . . . . .

I

PMMA

1.35-+0.05

ml iii I

,.5-+oo5

Polyimie m i r a il 2

4

5. Anti-reflection coating

n

159±005

6

8

Wavelength (pm) Fig. 4. Transparent (~c < 10-2) spectral ranges and refractive indices of fluorocarbon, PMMA, and polyimide.

have low, moderate, and high refractive indices, respectively. Hence we can choose a suitable polymer depending on the reqmrement for the refractive index. Fluorocarbon seems more useful than the other polymers, since it has a wide transmission range that extends to 7 Ixm wavelength. (a) 100

i

i

B

~

90

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80

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J

i

,

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,

C

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c-

~

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6

itl

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10

,

12

Wavelength (pm) (b) 100 C

~

90

c

80

D

,~_ E 70 e--

~

60

F-50

Csl 10

12

Wavelength (pm) Fig. 5. Transmission spectra of (a) KRS-5 and (b) Csl windows. The thickness of the windows is 2 mm. (A) Thin solid curves show the spectra of uncoated windows. (B) Thick solid, (C) dashed, and (D) dotted curves show the spectra of the windows with the fluorocarbon films of ~0.6, ~0.9, and ~2.011m thicknesses, respectively.

Anti-reflection (AR) coating for optical components is of particular importance in the infrared region, since most infrared transmitting materials yield a large reflection loss because of thei'r high refractive indices [6]. Also, a polymer coating is effective for the protection of infrared optical components, since infrared materials do not have sufficient durability for humidity. We coated fluorocarbon polymers on KRS-5 (T1Br-TII) and CsI windows to reduce a reflection loss. We designed the AR coatings for 3, 5, and 10.6 ~tm wavelengths. By using the measured refractive index n = 1.35, suitable film thicknesses are calculated to be ~0.6, ~0.9, and ~ 2 . 0 # m , respectively. To obtain the desired film thicknesses, the speed of spin-coating was set at 4200, 2300, or 1200 rpm. The windows are 15 mm in diameter and 2 mm in thickness. Both sides of the windows were coated with fluorocarbon. Fig. 5a shows the transmission spectra of KRS-5 windows. About 30% of incident light is lost in an uncoated window (thin solid curve), since the refractive index of KRS-5 is 2.4 [5]. By coating a fluorocarbon polymer, the reflection loss decreased to less than 10% at desired wavelengths. Since the refractive index of fluorocarbon is smaller than the optimum value x/2--A for AR-coating of KRS-5 [6], the highest theoretical transmittance is 92%. The theoretical value is attained at around 3 and 5~tm wavelengths in the measured spectra. In the wavelength range around 10~tm, however, the maximum transmittance is lower because of the absorption by fluorocarbon. Nevertheless polymer coating is useful for CO2 laser transmission if laser power is not so high as to cause heat damage. Fig. 5b shows the transmission spectra of CsI windows. The refractive index of CsI is 1.74 [7]. Fluorocarbon is appropriate for AR coating of CsI, since the refractive index is close to x/]---~= 1.32. The transmittance of CsI windows increased to 97-98% at around 3 and 5 p.m wavelengths by fluorocarbon coatings.

M. Saito et al. / Infrared Physics & Technology 36 (1995) 1125-1129

6. Conclusion The complex refractive indices of fluorocarbon, P M M A , and polyimide were measured in the mid-infrared region. Fluorocarbon is superior to the other polymers since it has a lower absorption coefficient than the others in a wide spectral range. By coating fluorocarbon films on KRS-5 and CsI windows, the reflection loss decreases notably at a desired wavelength. Since these polymer films can be formed by a simple method, they are useful for fabricating optical interference coatings and protection coatings for infrared components.

Acknowledgements The authors acknowledge Asahi Glass Co., Ltd. for preparation of thick fluorocarbon films. They

1129

also acknowledge Mr. H. Yahagi of Fujitoku Co. for useful discussion.

References [1] J.I. Thackara, G.F. Lipscomb, M.A. Stiller, A.J. Ticknor and R. Lytel, Appl. Phys. Lett. 52 0988) 1031. [2] K. Takizawa, H. Kikuchi, H. Fujikake, Y. Namikawa and K. Tada, Opt. Eng. 32 (1993) 1781. [3] M. Saito, M. Shibasaki, S. Nakamura and M. Miyagi, Opt. Lett. 19 (1994) 710. [4] M. Born and E. Wolf, Principles of Optics (Pergamon Press, Oxford, 1987) pp. 61~6. [5] K. Kudo, Kiso Bussei Zuhyo (Table of Fundamental Properties of Materials) (Kyoritsu Shuppan, Tokyo, 1972), in Japanese. [6] T. Miyata, Proc. SPIE, 650 (1986) 131. [7] E.D. Palik, Ed., Handbook of Optical Constants of Solids II (Academic Press, Boston, 1991).