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Optical properties of arachidate monolayers by integrated optical techniques
Volume
24, number
OPTICAL PROPERTIES OF ARACHIDATE BY INTEGRATED
J.D. SWALEN, K.E. RIECKHOFF
11 October
1978
MONOLAYERS
OPTICAL TECHNIQUES
IBM Research Laboratory, Received
January
OPTICS COMMUNICATIONS
1
and M. TACKE
San Jose. CalifOrnia 9.5193, USA
1977
The refractive indices of thin dielectric films on the surface of an optical waveguide were determined from the mode patterns, i.e., effective indices in the guide as a function of selected visible laser wavelengths. These more accurate results are compared with previous measurements and illustrate the advantages of this method.
The optical properties of cadmium-arachidate (Cd-A) film in the visible region of the spectrum are of interest because of the ability of this material to be prepared as successive monolayers on suitable substrates. As such in this layered form it is widely used in many investigations of surface phenomena by optical techniques [l] and frequently used as a matrix to hold in a specific orientation other molecules of interest [2]. Since the molecules in the monolayer all have their long molecular axis oriented perpendicular to the substrate surface, the index of refraction is highly anisotropic. Measurements of the optical properties of Cd-A layers have been reviewed by Drexhage [ 11, who reported literature values for the ordinary (in the plane of the film) and extraordinary (perpendicular to the plane of the film) index of refraction obtained at specific wavelengths by what may be called conventional albeit sophisticated and difficult techniques using transmission and/or reflection of light. No consistent set of measurements of the dispersion of the refractive indices over the visible region, however, has been reported and none of the methods reviewed by Drexhage were able to give highly precise values for the extraordinary index of refraction (i.e., accuracy significantly better than 1%). We have applied the method for the determination of optical constants of thin films by integrated optical techniques developed by Swalen and Co-workers [3,4] to determine the ordinary and extraordinary indices of Cd-A between 457.9 nm and 632.8 nm. An optical 146
waveguide structure consisted of two films; one is either a Corning glass film of thickness - 1.4 pm deposited by rf sputtering or a doctor bladed film of poly(methylmethacrylate) (PMMA) from chlorobenzene solution onto a fused silica substrate and the other consists of n monolayers of Cd-A placed on top of the glass film (n = 29,37 or 4 1 depending on the particular run). Laser light of appropriate wavelength 37
1 IF----1 Degree
TM
37 1 layer
.l
Fig. 1. Mode patterns for TM (upper curves) and TE (lower curves) modes of a typical film showing the angular shift with 36 layers of cadmium arachidate.
Volume
24, number
January
OPTICS COMMUNICATIONS
1
1978
I
Table
Previous
Cd-A
Substrate
Cd-A
h(nm)
nTM
“TE
“TM
nTE
“TM
“TE
nTM
632.8
glass PMMA
1.5424 1.4868
1.5422 1.4869
1.525 1.517
1.565 1.550
1.522a) 1.517b)
1.59a) 1.544b)
514.5
glass PMMA
1.5483 1.4924
1.5481 1.4925
1.525 1.533
1.568 1.571
1.526c) 1.54d)
457.9
glass
1.5527
1.5526
1.536
1.616
PMMA
1.4966
1.4967
1.544
1.589
1.60e)
___Measured values are given. In view of uncertainties in the film thickness better than 50.01. c, Green light A - 530 nm (81. b)h=632.8nm[7]. a) h=612nm[6].
1.62-
\
1.60 .o
1.58
l
\
-
i? n 5 ,;
1.56 -
*\
;
l
E a”
Extraordinary TM-p
C 1.54
-
1.52
-
A
m------*0
Ordinary TE-s
a
1.5otli 400
450
500 Wavelength
550
600
650
(nm)
Fig. 2. Dispersion curves for refractive indices of cadmium arachidate. A [6]; q [7];m 181;~ [9]; o [lo]. Our data points l and a smooth average curve.
was coupled into and out of the waveguide through an air interface by high refractive index prisms. The angle of incidence was scanned and the resulting mode pattern for both TE and TM modes obtained with an angular resolution of kO.0 lo. The optical constants and the thickness of the glass and PMMA films were first obtained independently (i.e., without the presence of the film of Cd-A). The mode structure (see fig. 1) of the combined films, with the known thickness of both films and the refractive indices of the glass of PMMA film as constants, was used to calculate, with the help of an APL computer program for nonlinear least squares fitting, the
the actual
accuracy
is conservatively
d, Green
light [9].
e, h = 405 nm
estimated
to be
[ lo].
TE (ordinary, i.e., perpendicular to the plane of incidence - s polarization) and TM (extraordinary, i.e., parallel to the plane of incidence - p polarization) refractive indices of the Cd-A film. The thickness of the Cd-A film was obtained from the known number of monolayers employed and the best known value of the single monolayer thickness of 26.8 A [ 1,231. A computer analysis showed that a thickness error in the third digit will be reflected in the fourth decimal place of the refractive index only. Thus the accuracy in our refractive indices is conservatively estimated to be +O.Ol. Our results and a comparison with published results are presented in table 1 and a plot of dispersion of the refractive indices is presented in fig. 2. There is reasonable agreement with previously published data except for a few points. Our results do provide, however, further evidence for the power and precision of the integrated optics technique for the determination of the optical properties of very thin anisotropic films. We wish to thank R. Santo for making the arachidate films. References
Ill
K.H. Drexhage, Interactions of light with monomolecular dye layers, in: Progress in optics, Vol. XII, ed. E. Wolf (North-Holland, Amsterdam, 1974). of 121 H. Kuhn, D. Mobius and H. Bticher, Spectroscopy monolayer assemblies, in: Techniques of Chemistry, Vol. 1, Chapter VII, Physical methods of chemistry, Part IIIB, eds. A. Weissberger and B.W. Rossiter.
147
Volume
24, number
1
OPTICS COMMUNICATIONS
[3] J.D. Swalen, M. Tacke, R. Santo and J. Fisher, Optics Commun. 18 (1976) 387. [4] S.D. Swalen, R. Santo, M. Tdcke and J. t’ischer, IBM J. of Research and Development 21 (1977) 168. [5] G.L. Gaines, Insoluble monolayers at liquid-gas interfaces (Interscience, New York, 1966).
148
January
1978
[6] M. Fleck, Dissertation, University of Marburg, Germany, 1966. [7] D. den Engelscn, J. Opt. Sot. Amer. 61 (1971) 1460. [S] K.II. Drexhage, Habilitation-Schrift, University of Marburg, Germany, 1966. [9] K.D. Blodgett, Phys. Rev. 55 (1939) 391. [lo] H. Foster, Diplomarbeit, University of Marburg, Germany 1967.
24, number
OPTICAL PROPERTIES OF ARACHIDATE BY INTEGRATED
J.D. SWALEN, K.E. RIECKHOFF
11 October
1978
MONOLAYERS
OPTICAL TECHNIQUES
IBM Research Laboratory, Received
January
OPTICS COMMUNICATIONS
1
and M. TACKE
San Jose. CalifOrnia 9.5193, USA
1977
The refractive indices of thin dielectric films on the surface of an optical waveguide were determined from the mode patterns, i.e., effective indices in the guide as a function of selected visible laser wavelengths. These more accurate results are compared with previous measurements and illustrate the advantages of this method.
The optical properties of cadmium-arachidate (Cd-A) film in the visible region of the spectrum are of interest because of the ability of this material to be prepared as successive monolayers on suitable substrates. As such in this layered form it is widely used in many investigations of surface phenomena by optical techniques [l] and frequently used as a matrix to hold in a specific orientation other molecules of interest [2]. Since the molecules in the monolayer all have their long molecular axis oriented perpendicular to the substrate surface, the index of refraction is highly anisotropic. Measurements of the optical properties of Cd-A layers have been reviewed by Drexhage [ 11, who reported literature values for the ordinary (in the plane of the film) and extraordinary (perpendicular to the plane of the film) index of refraction obtained at specific wavelengths by what may be called conventional albeit sophisticated and difficult techniques using transmission and/or reflection of light. No consistent set of measurements of the dispersion of the refractive indices over the visible region, however, has been reported and none of the methods reviewed by Drexhage were able to give highly precise values for the extraordinary index of refraction (i.e., accuracy significantly better than 1%). We have applied the method for the determination of optical constants of thin films by integrated optical techniques developed by Swalen and Co-workers [3,4] to determine the ordinary and extraordinary indices of Cd-A between 457.9 nm and 632.8 nm. An optical 146
waveguide structure consisted of two films; one is either a Corning glass film of thickness - 1.4 pm deposited by rf sputtering or a doctor bladed film of poly(methylmethacrylate) (PMMA) from chlorobenzene solution onto a fused silica substrate and the other consists of n monolayers of Cd-A placed on top of the glass film (n = 29,37 or 4 1 depending on the particular run). Laser light of appropriate wavelength 37
1 IF----1 Degree
TM
37 1 layer
.l
Fig. 1. Mode patterns for TM (upper curves) and TE (lower curves) modes of a typical film showing the angular shift with 36 layers of cadmium arachidate.
Volume
24, number
January
OPTICS COMMUNICATIONS
1
1978
I
Table
Previous
Cd-A
Substrate
Cd-A
h(nm)
nTM
“TE
“TM
nTE
“TM
“TE
nTM
632.8
glass PMMA
1.5424 1.4868
1.5422 1.4869
1.525 1.517
1.565 1.550
1.522a) 1.517b)
1.59a) 1.544b)
514.5
glass PMMA
1.5483 1.4924
1.5481 1.4925
1.525 1.533
1.568 1.571
1.526c) 1.54d)
457.9
glass
1.5527
1.5526
1.536
1.616
PMMA
1.4966
1.4967
1.544
1.589
1.60e)
___Measured values are given. In view of uncertainties in the film thickness better than 50.01. c, Green light A - 530 nm (81. b)h=632.8nm[7]. a) h=612nm[6].
1.62-
\
1.60 .o
1.58
l
\
-
i? n 5 ,;
1.56 -
*\
;
l
E a”
Extraordinary TM-p
C 1.54
-
1.52
-
A
m------*0
Ordinary TE-s
a
1.5otli 400
450
500 Wavelength
550
600
650
(nm)
Fig. 2. Dispersion curves for refractive indices of cadmium arachidate. A [6]; q [7];m 181;~ [9]; o [lo]. Our data points l and a smooth average curve.
was coupled into and out of the waveguide through an air interface by high refractive index prisms. The angle of incidence was scanned and the resulting mode pattern for both TE and TM modes obtained with an angular resolution of kO.0 lo. The optical constants and the thickness of the glass and PMMA films were first obtained independently (i.e., without the presence of the film of Cd-A). The mode structure (see fig. 1) of the combined films, with the known thickness of both films and the refractive indices of the glass of PMMA film as constants, was used to calculate, with the help of an APL computer program for nonlinear least squares fitting, the
the actual
accuracy
is conservatively
d, Green
light [9].
e, h = 405 nm
estimated
to be
[ lo].
TE (ordinary, i.e., perpendicular to the plane of incidence - s polarization) and TM (extraordinary, i.e., parallel to the plane of incidence - p polarization) refractive indices of the Cd-A film. The thickness of the Cd-A film was obtained from the known number of monolayers employed and the best known value of the single monolayer thickness of 26.8 A [ 1,231. A computer analysis showed that a thickness error in the third digit will be reflected in the fourth decimal place of the refractive index only. Thus the accuracy in our refractive indices is conservatively estimated to be +O.Ol. Our results and a comparison with published results are presented in table 1 and a plot of dispersion of the refractive indices is presented in fig. 2. There is reasonable agreement with previously published data except for a few points. Our results do provide, however, further evidence for the power and precision of the integrated optics technique for the determination of the optical properties of very thin anisotropic films. We wish to thank R. Santo for making the arachidate films. References
Ill
K.H. Drexhage, Interactions of light with monomolecular dye layers, in: Progress in optics, Vol. XII, ed. E. Wolf (North-Holland, Amsterdam, 1974). of 121 H. Kuhn, D. Mobius and H. Bticher, Spectroscopy monolayer assemblies, in: Techniques of Chemistry, Vol. 1, Chapter VII, Physical methods of chemistry, Part IIIB, eds. A. Weissberger and B.W. Rossiter.
147
Volume
24, number
1
OPTICS COMMUNICATIONS
[3] J.D. Swalen, M. Tacke, R. Santo and J. Fisher, Optics Commun. 18 (1976) 387. [4] S.D. Swalen, R. Santo, M. Tdcke and J. t’ischer, IBM J. of Research and Development 21 (1977) 168. [5] G.L. Gaines, Insoluble monolayers at liquid-gas interfaces (Interscience, New York, 1966).
148
January
1978
[6] M. Fleck, Dissertation, University of Marburg, Germany, 1966. [7] D. den Engelscn, J. Opt. Sot. Amer. 61 (1971) 1460. [S] K.II. Drexhage, Habilitation-Schrift, University of Marburg, Germany, 1966. [9] K.D. Blodgett, Phys. Rev. 55 (1939) 391. [lo] H. Foster, Diplomarbeit, University of Marburg, Germany 1967.