Langmuir 1990,6, 1403-1407
1403
Optical Waveguides from Novel Polymeric Langmuir-Blodgett Multilayer Assemblies Werner Hickel,? Gisela Duda,? Mark Jurich,x Thomas Krohl,? Kent Rochford,e George I. Stegeman,e J. D. Swalen,* Gerhard Wegner,? and Wolfgang Knoll’*? Max-Planck-Institut fur Polymerforschung, Ackermannweg 10,D-6500 Mainz, FRG, IBM Almaden Research Center, 650 Harry Road, Sun Jose, California 95120-6099, and Optical Sciences Center, University of Arizona, Tucson, Arizona 85721 Received January 4, 1990 Multilayer assemblies of various partially long alkyl chain substituted polyglutamates have been prepared by the Langmuir-Blodgett dipping technique ranging in thickness from a double layer (3.5 nm thick) to more than 500 layers (-1 pm thick). Surface plasmon spectroscopy,photothermal interferencespectroscopy, and waveguide spectroscopies have been employed to characterize the thickness and the optical properties including the anisotropies of the index of refraction of these films as a function of the number of layers. Waveguide mode-loss measurements of the first all-polyglutamate waveguide sample yielded values in the range 2.5-5.5 dB/cm.
Introduction The successful implementation of the LangmuirBlodgett (LB) deposition technique’ for the engineering of supramolecular structures with purposefully designed properties2 for various device applications3still suffers from insufficient structural features on most of the “classical” LB materials. These molecules derive their surface activity (which allows them to be spread, compressed, and thereby organized a t a water-air interface) from their amphiphilic character; Le., they are composed of a polar headgroup and a hydrophobic tail and are, in general, waterin~oluble.~ Such a monolayer, which is the basic unit for the building-up of multilayer assemblies, is composed of densely packed molecules oriented more or less parallel to the surface normal. In most cases, these layers are quasitwo-dimensional polycrystalline systems with a rich variety of lateral structures as can be seen by fluorescence microsc~py.~-~ Consequently, any attempts to use these thin films, e.g., as electrically insulating coatings for semiconductor device fabrication, almost never show the desired results because the performance is limited by their high defect d e n ~ i t y . ~ Another ?~ example for a possible use of LB multilayers is the field of integrated optics which involves, e.g., planar waveguide structures.10 Here, the light scattering grain boundaries of the domain structure of the + Max-Planck-Inatitut t
fur Polymerforschung.
IBM Almaden Research Center.
8 University of Arizona. (1) Kuhn, H.; MBbius, D.; Bijcher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Part IIB,
Chapter VII. (2) Stroeve, P.; Franeee, E. I. Thin Solid Films 1987, 152, 405. (3) Roberta, G. G. Contemp. Phys. 1984,25,109. (4) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces: Wiley: New York, 1966. (5) Tscharner, V. v.; McConnell, H. M. Biophys. J. 1981,36,409. ( 6 ) LBsche. M.: Sachmann.. E.:. MBhwald. H. Ber. Bunsen-Ges. Phvs.
them. 1983, io,848.
(7) Duschl, C.; Kemper, D.; Frey, W.; Meller, P.; Ringsdorf, H.; Knoll, W . J. Phys. Chem. 1989,93,4587. (8)Lesieur, P.; Barraud, A.; Vandevyver, M. Thin Solid Films 1987, 152, 155. (9) Bibo, A. M.; Peterson, J. R. Thin Solid Films 1989,178,81. (10)Swalen, J. D. J. Mol. Electron. 1986,2, 155.
0743-7463/90/2406-1403$02.50/0
polycrystalline layers allow for the fabrication of devices with losses no better than 10 dB/cm.ll Numerous attempts have been reported to improve the structural quality of these layers, e.g., by annealing expansion-compression cycles9 or by heat treatment of the monolayer,12 reminiscent of the zone melting in threedimensiwal single-crystal growth. The aim of these manipulationswas to produce large singlecrystalline mone domain samples. A completely different approach has been suggested recently with the introduction of a novel type of LB material, namely, stiff rod-like polymers with flexible side chains.13 Examples given so far a r e phthalocyaninatopolysiloxane,14c e l l ~ l o s e and, , ~ ~ in particular, polyglutamates.16 Here the idea is to introduce by the aliphatic side chains a highly disordered fluidlike moiety which gives these systems properties similar to liquid-crystalline polymen. The formation of crystalline domains with grain boundaries should be largely avoided this way. Many of these new materials can be processed by the LB technique with all the well-known advantages like the preparation of ultrathin very homogeneous coatings or more sophisticated superlattices, to mention only a few. In this paper we report on the optical properties of polyglutamate layers ranging in thickness from a mere bilayer (ca. 3.5 nm thick) to thick multilayers capable of carrying guided light modes (ca. 0.6 Fm). We first determined the anisotropic index of refraction of these layers by surface plasmon spectroscopy,17photothermal (interference) spectroscopy,18 and (hybrid) waveguide s p e c t r o s ~ o p y .Finally, ~~ we give results on the first prepared all-polyglutamate waveguides and show that very (11) Tredgold, R. H. Thin Solid Film 1987,152,223. (12) Kasuga, T.; Kumehara, H.; Watanabe, T.; Miyata, S. Thin Solid Films 1989, 178, 183. (13) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988,159,221. (14) Orthmann, E Wegner, G. Angew. Chem. 1986,98,1114. (15) Ritcey, A.; Wenz, G. unpublished results. (16) Duda, G. Ph.D. Thesis, Johannea-Gutenberg-Univeraitit Mainz, 1989. (17) Raether, H. In Physics of Thin Films; Hass, G., Francombe, M.
H., Hoffmann, R. W., Eds.; Academic: New York, San Francisco,London,
1977; Vol. 9, pp 145-261. (18) Knoll, W.; Coufal, H. Appl. Phys. Lett. 1987,51,892. (19) Swalen, J. D.; Tacke, M.; Santo, R.; Rieckhoff, K.E.; Fischer, J. Helu. Chim. Acta 1978, 61,960.
0 1990 American Chemical Society
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1404 Langmuir, Vol. 6, No. 8, 1990 a)
layer transfer onto hydrophobic substrates could be performed as a Y-type deposition a t a lateral pressure of II = 20-25 mN/m with a sample lift speed of 20-25 mm/min. The polymer rods have been shown to align t o some degree on the water surface and then during transfer parallel t o the dipping direction, which gives rise t o a pronounced s t r u c t u r a l anisotropy t o t h e multilayers.l3 A schematic drawing of a double layer on a hydrophobic substrate (either evaporated silver or hexamethyldichlorodisilazane-treated quartz/glass slides) is shown in Figure Ib. Hydrocarbon stands for the disordered alkyl chains. The inset shows the nomenclature of the three different indices of refraction used in the discussion.
Measurements Surface Plasmon Spectroscopy. Plasmon surface POlaritons (surface plasmons or PSP for short) are surface electromagneticmodes that travel along a metal-dielectric interface as bound, nonradiative waves with their field amplitudes decaying exponentially perpendicular to the i n t e r f a ~ e . ' ~Their . ~ ~ dispersion relation is given (for an interface separating two half-infinite spaces, namely, metal and dielectric) by
Hydrocarbon
Polyglutomote H@dmcllfbCll
hydrophobic Substrate
Figure 1. (a) Pressure-area ( P A ) isotherm of polyglutamate GD 10 on pure water a t T = 20 "C. The area is scaled to one repeat unit of the polymer. The inseb are schematic cross sections of the helical rods with the alkyl side chains on the water surface a t various lateral pressures. Transfer for the buildup of LB multilayer assemblies is performed at A = 20-25 m N / m . (b) Schematic representation of a double layer of polyglutamate deposited onto a hydrophobic substrate. Shown are the helical polymer rods only; hydrocarbon stands for the disordered fluidlike alkyl moiety. The coordinate system in the inset gives the definition of the three different indices of refraction used in the analysis of the various optical experiments. Note the high structural in-plane anisotropy.
low loss structures can be obtained with these materials with considerably improved performance for LB waveguides. In passing, we note that these studies complement investigations aimed a t characterizing the structural properties of these new materials13 as well as their elastic20 and heat conduction properties.21
where kXmis the longitudinal component of the complex PSP wave vector, w is the real frequency, c is the speed of light, tm(w) = c,,(w) + itmi(o)is the complex frequencydependent dielectric constant of the metal, and Q ( W ) = €&(a)+ & ( W ) that of the dielectric material. Photons cannot be converted directly into PSP modes (the square root in eq 1 is always >1) but require a so-called plasmon coupler, which in our case was a prism in the Kretschmann c ~ n f i g u r a t i o n(Figure ~ ~ 2a). Laser light of frequency WL is coupled to PSPs through the thin (ca. 50 nm) Ag layer evaporated directly onto the base of a 90' BK-7 glass prism in an attenuated total internal reflection geometry. The optical behavior of such a thin-film architecture is slightly modified relative to the first case described by eq 1 but can still be treated quantitatively within Fresnel theories. Resonant excitation occurs at an angle 00 where the parallel momentum of the photon wave vector matches that of the PSP mode sin 8, = k, kljhoton= -tprismli2 WL C
Experimental Section Sample Preparation. The synthesis of a whole variety of polyglutamates with different side chains, degrees of polymerization, or degrees of substitution as well as monolayer behavior a t the water-air-interface have been described already in detail.13J6 In this study, we concentrate on samples with octadecyl substituents: poly[ (y-methyl L-glutamate)-co-(y-octadecy1 kglutamate)], GD 10 for short. This random copolymer contains 32 mol P1 octadecyl-bearing repeat _units as obtained from its viscosity-average molecular weight M, = 19 400. For some experiments, a corresponding polyglutamate with eicosyl side chains was employed for comparison, poly[(y-methyl L-glutamate)-co-(y-eicosyl g glutamate)], GD 14 for short. Typically, a solution containing 0.1 mg of polymer/mL of CHC13 was spread on a pure water subphase (Milli-Q quality) in two different Langmuir troughs (Joyce Loebl and homebuilt) both equipped with a Wilhelmy plate for the surface pressure measurements. No systematic differences in the various isotherms were found. A typical example is given in Figure l a ) together with schematic pictures of the cross sections of the rodlike molecules on the water surface a t different stages of the pressure-area ( E A ) isotherm. As discussed before,16 mono(20) Nizzoli, F.; Hillebrands, B.; Lee, S.;Stegeman, G. I.; Duda, G.; Weg ner, G.; Knoll, W. Phys. Reo. E: Condens. Matter 1989,40, 3323. (21) Fischer, T.; Krohl, T.; Knoll, W., unpublished results.
Any thin dielectric coating causes an angular shift of this resonance condition. This has been shown to become a sensitive tool for the optical characterization of LB monoand multilayer^.^^ For Ag at XL = 633 nm (the employed HeNe laser wavelength), the technique is primarily sensitive to n, because the PSP field components are mostly E, polarized with only a negligable contribution of E , ( E , / E , = 5.2). Given a starting value for the thickness per double-layer polyglutamate, we fitted the experimental reflectivity scans (8 - 28) w i t h theoretical curves calculated according to Fresnel's formulas (allowing for thickness deviations of not more than fl nm/double layer) and obtained n, as a function of the overall coating thickness. P h o t o t h e r m a l I n t e r f e r e n c e Spectroscopy. A schematic of this technique is given in Figure 3a. The front metal electrode, Ag in our case, of a pyroelectric thinfilm detector (polyvinylidene fluoride, PVFdZ5acts as a (22) Burstein, E.; Chen, W. P.; Hartstein,A. J. Vac.Sci. Techno!.1974,
11, 1004.
(23) Kretschmann, E. Opt. Commun. 1972,6, 185. (24) Gordon, J. G., 11; Swalen, J. D. Opt. Commun. 1977,22, 374. (25) Coufal, H. Appl. Phys. Lett. 1984, 44, 59.
Optical Waveguides from Polymeric Assemblies
Langmuir, Vol. 6, No. 8, 1990 1405
a1 Photodiode
Prism 50nm pig
bl x ;d
I
:o
I4
bl
Wdcg
Figure 2. (a) Schematicof the experimental setup used for reflectivity-wangle(k228) seans in the Kretachmann confmration. Laser light IO of frequency OL is totally internally reflected at the base of a glass (BK-7) prism and the reflected intensity monitored by a photodiode. Theca. 50-nm-thick Ag layer was evaporated ontothe prism and could be mted by an even number of polyglutamate layers. (b) Reflected intensity (AI. = 633 nm) as a function of the (external) angle 8 for various film architectures: 0 ,bare Ag (ea, = -16.35 + i0.6,thickness s = 50.8 nm); A, 2 layers of GD 10 on top; 0,4;X, 6; V, 10; 0,20 layers; see also inset. The solid lines are Fresnel calculations with parameters given in Table I.
mirror so that a standing electromagnetic wave is excited by the incoming light, Zi. Its spatial intensity distribution for any wavelength A is given by
Z(d) = 1 + R,,
+ ZR,,"'
cos
[4snd - 6 1
Figure 3. (a) Schematic of the detector architecturefor photothermal (interference)spectroscopy. The thin-film calorimeter (a 20-pm-thickpyroelectric poly(viny1idenefluoride),PVF., foil coated with two Ag electrodes) is coated by a certain number, m, of spacer layers of polyglutamate GD 14 and then by a single dye monolayer. The latter absorbs energy from the incoming light, ti., part of which is transformed into heat by radiationless transitions which then in turn is detected by the calorimeter. (h) Photothermalsignal, S,as a function of the spacer layer thickness, d. The solid line is a fit describing the standing light wave in front of a Ag mirror. etched into it, the angles for coupling into the various guiding modes were measured. The thickness and index of refraction were determined by solutions to the eigenvalue equation tan-'
(3)
where R- is the reflectivity of the Ag surface and 6 is the phase change t h a t t h e light wave experiences upon reflectionJs Both quantities can be calculated by Fresnel's theory provided the dielectric function, em(*) = cm,(w) + icmi(w), of the mirror material is known. The heat that is generated by absorption and subsequent thermalization in a dye monolayer placed at a distance d in front of the mirror by a spacer layer, e.g., a certain number of LB multilayers of polyglutamate, generates a photothermal signal, S ( d ) ,that varies according to eq 3.26 The only free parameter to fit the data is then the index of refraction of the spacer layers, n. Given the experimental accuracy, we limit the analysis to the isotropic case, which is justified in view of the small refraction index anisotropies found for polyglutamates by other techniques (see below). The dye layer used in our study was a J-aggregate monolayer of a cyanine dye (l-methyl-l'-octad~l-Z,Z'-~anine iodide) with a peak absorption at X = 580 11111.~ Waveguide Spectroscopy. On a glass (Corning 7059) optical waveguide on a Pyrex substrate containing a grating (26) Kr6hl, T.PbD. Thesis, JohsnnesGutenberg-Uni"~~i~t Maim, 1989.
PI2+ tan? Pa2+ m s = kZzd2
(4)
where for &polarization (TE)
k: =
- :k
and for p-polarization (TM)
k: = c,k: - czk:/cz Here dz is the film thickness, ko = ZR/X = w / c is the propagation vector in free space, k, is the propagation vector along the surface of the film, and m is an integer and the mode index ( m = 0, 1,2, ...). Now k, = %/A + kosin 8, where A is the grating constant and 8 is the angle of incidence off the normal. With the known indices of refraction for the superstrate (air) and the substrate (Pyrex), n2 and dz can be calculated for each mode. By a computer program n2 is varied in a systematic way until each mode gives approximately the same thickness dz with the smallest deviation from the average thickness.
1406 Langmuir, Vol. 6, No. 8, 1990
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After a solution for the glass waveguide was obtained, two multilayers of 100 monolayers each of polyglutamate GD 10 were deposited in two orthogonal directions, that is, with the grating direction and perpendicular to it. Shifts in the coupling angles resulted from the added film thickness with its refractive index. The thickness was then varied in values around the expected value from the monolayer thickness and the number of layers to give the smallest error among the modes. Another setup for waveguide spectroscopic characterization of relatively thin LB multilayer assemblies is based on the Kretschmann ATR configuration. On top of the metal layer (Cr and Au in this case. Cr: t' + 2' = -6.3 i9.8, thickness 1 = 4.5 nm. Au: -12.2 i1.45,l = 40 nm. X = 633 nm) evaporated onto the glass prism, 150 layers of GD 10 were deposited. This first all-polyglutamate waveguide could carry one T E and one TM mode. Since both polarizations could be measured with the mode propagating either parallel or perpendicular to the dipping (and hence PG-rod axis) direction, all three indices of refraction (see Figure lb) together with the overall multilayer thickness could be derived from the calculations. Loss Measurements. Waveguide losses were estimated by measuring the intensity of out-of-plane scattering from the guided wave. A coherent fiber bundle was placed close to the waveguide and positioned along the guided streak.27 A stepper motor scanned a slit along the streak image at the opposite end of the bundle, and the scattered intensity was quantified with a photomultiplier tube (see Figure 4a). The intensity was recorded as a function of slit position a t 200-pm intervals over about 10 mm. Assuming a uniform distribution of scattering sites, this measured intensity is proportional to the energy in the waveguide a t each position. This data are fitted to a Beer's law exponential decay to yield a value for waveguide loss.
+
+
Results and Discussion The aim of the present work was (i) to measure the optical parameters of polyglutamate LB multilayer assemblies of various thicknesses, (ii) to look, in particular, for any anisotropy of the index of refraction and its relation to the structural anisotropy, and (iii) to demonstrate the possible use of these novel polymeric materials as lowloss planar waveguides in integrated optics. We started out with surface plasmon spectroscopy of few-layer samples because this technique has the sensitivity to monitor thin dielectric coatings of even monolayer thickness. Some reflectivity-versus-angle (8-28) scans obtained with GD 10 are shown in Figure 2b. Given are the data for bare Ag and for 2 , 4 , 6 , 10, and 20 layers of GD 10, respectively. The full lines are Fresnel fits with parameters given in Table I. As can be seen from these data, there exists only a minor tendency for an increased optical thickness per monolayer for thicker samples, although this trend is almost within the sample-tosample variations. Note t h a t i t is n, t h a t can be determined with this technique, provided the geometrical thickness of the layer is known. By X-ray diffraction it was found that for thick samples the layer thickness is do = 1.75 nm for GD 10, and so this value was also chosen as a first approximation for the reflectivity calculations of these thin films. For thicker films of GD 10 and GD 14, Brillouin scattering experiments had given some hints that the indices of refraction of these two slightly different materials are almost identical ( n = 1.48), and, in addition, no (27) Himel, M.
subst rat e-
D.;Gibson, U. J. A p p l . Opt.
1986, 25,4413.
O
k
-02
00
02
OL
06 08 Posi tion/cm
10
12
Figure 4. (a) Schematic of the experimental setup used for the loss measurement. Light of a HeNe laser is coupled to the grating in the substrate of the polyglutamate waveguide (consisting of 322 layers of GD 14, thickness d = 0.6 rm)so as to excite guided modes in the LB film. The scattered light, out-coupled along the streak in the waveguide, is transmitted through a coherent fiber bundle and read by a stepper motor driven photomultiplier tube with a slit diaphragm placed in front of it. (b) Scattered intensity (on log scale) along the streak as a function of the slit aperture position. From the slope of the straight line (leastsquares fit), the loss of the waveguide can be derived. Table I. Parameters Used To Fit the Reflectivity Scans Presented in Figures 2,5, and 6, Respectively, for LB Multilayers of Various Thicknesses of Polyglutamate GD l W n. d , nm do, nm layers nl nil 2 1.480 3.5 1.75 4 1.487 7.1 1.775 6 1.485 10.5 1.75 10 1.488 17.7 1.77 20 1.490 35.5 1.775 100 1.52f 0.01 1.53f0.01 1.50f 0.2 168.8 1.688 1.500 1.486* 253.2 150 1.490 1.688 " X ~ = 6 3 3 nm. Substrate metal was either Ag ( e & = -16.35 + i0.6, thickness SA^ = 50.8 nm) for the thin coatings (120 layers) or Cr/Au ( C C=~ -6.3 f i9.8, scr = 4.5 nm/eA, = -12.2 + i1.45, SA^ = 40 nm) for the 150-layer sample. Average of the 2-20-layer samples.
pronounced anisotropy had been found. This result was confirmed in this study by photothermal interference spectroscopy. The experimental data for GD 14 are shown in Figure 3b for more than two oscillations of the standing light wave in front of the Ag mirror. If a thickness per monolayer for GD 14 of do = 1.85 nm is assumed, one obtains n = 1.498 from the theoretical fit according to eq 3 (solid line in Figure 3b). This result did not depend on the polarization of the exciting light wave relative to the dipping direction of the detector during the spacer layer deposition. A small anisotropy was determined by the waveguide
Langmuir, Vol. 6, No. 8, 1990 1407
Optical Waveguides from Polymeric Assemblies 100
I
r
I
p-pol.
.2 1 1
I1
100
,
I
s-pol
t
1
Figure 6. Reflectivity-vs-angleof incidence (0-20) scans taken for a thick (150 layers of GD 10) polyglutamate LB film on top of a C r / A u layer in a Kretschmann configuration with p-polarization (TM,upper frame) and s-polarization(TE, lower frame). Both scans were performed with the mode propagation to the rod axes direction being parallel (11) or perpendicular (1) (dipping direction). Different symbols are experimentalpoints; solid lines are Fresnel calculations with parameters given in Table I. The employed laser wavelength was XL = 633 nm.
that despite the clear structural anisotropy which is found also, e.g., in the elastic properties of these films by Brillouin spectroscopy, no major difference exists between the indices of refraction in the three space coordinates. While this might be understandable for n , and n,, it is yet somewhat surprising that the polarization parallel to the rod long axis is comparable. The last, but in view of possible applications, most important experiment is presented in Figure 4b. Here the light scattered off the streak of a guided T E mode is plotted as a function of the lateral distance from the grating in the quartz substrate that coupled the X = 633 nm light from a HeNe laser into the polyglutamate waveguide structure. The sample consisted of 322 layers of GD 14 with an estimated thickness of d = 0.6 pm (based on the X-ray data, Le., 1.85 nm/monolayer) and could support two modes. Most measurements showed losses between 5.5 and 2.6 dB/cm, with an estimated accuracy of the technique of better than 1 dB/cm. These values are already significantly better than the lowest, -10 dB/ cm, so far quoted for classical LB layers but there is some evidence that it can be further improved by chemically engineering better materials and optimizing the preparation techniques. The result is, however, already a convincing justification for looking further for liquid-(crystalline) analogue LB materials where t h e residual director fluctuations cause some scattering losses in waveguides which are by far below the scattering a t grain boundaries in polycrystalline films.
