Z-type Langmuir-Blodgett film structures: surface plasmon resonance, second harmonic generation and fibre optic devices Geoffrey J. Ashwell,"" Gary Jefferies," Christopher D. George," Rakesh Ranjan," Robert B. Chartersband Ralph P. Tatamb "Centrefor Molecular Electronics, Cranfield University, Cranfield, UK MK43 OALT boptical Sensors Group, School of Mechanical Engineering, Cranfield University, Cranfield, UK MK43 OALT Second harmonic generation (SHG)from Langmuir-Blodgett (LB) films of the iodide (I) and octadecylsulfate (11)salts of E-N-octadecyl-4-[ 2-(4-dibutylaminophenyl)ethenyl]quinolinium and from films of a related pyridinium dye (111)increases with the number of layers. The LB film structures are non-centrosymmetric (Z-type) and have high second-order susceptibilities, x(')zzz,of 120 pm V-' for dye 1(20 layers), 80 pm V-' for dye I1 (100 layers) and 30 pm V-' for dye I11 (160 layers) at Am= 1.064 pm.The values are resonantly enhanced but for dye I11 the absorbance is very weak at the harmonic wavelength. The real and imaginary parts of the dielectric permittivity of dye I, derived from the surface plasmon resonance (SPR), are 3.14 f0.06 and 0.66 f0.03, respectively, at 532 nm and, with the exception of the monolayer, the thickness is 3.0 f0.3 nm layer-'. Waveguiding overlays of dye I, evanescently coupled to side-polished optical fibres, have resulted in useful in-line wavelength-selective elements. The LB technique allows the necessary control of film thickness for such devices. Langmuir-Blodgett (LB) films have been extensively studied for second harmonic generation (SHG) in thr~ugh-planel-~ and guided-wave configuration^.'-^ Pitt and Walpita' first established that LB multilayers of fatty acids could be utilised as planar waveguides and numerous methods of guiding in both passiveg*10 and active overlay^^-^ have appeared.For SHG it is necessary for the structure to be non-centrosym- metric. This can be realised by interleaving the layers with compatible spacers'-4 and, as predicted by theory, the second- harmonic intensity increases quadratically with the number of active layers if long-range order is maintained. The interleaved films include bilayer structures with interlocking geometries ('molecular zips'),' those stabilised by interlayer hydrogen bonding' and polymeric LB films.3 A four-fold improvement in the SHG is feasible if each layer is active, but there have been few examples of homomolecular films showing quadratic enhancement to greater than ten layers.''-'6 The first reported example, 2-docosylamino-5-nitropyridine(DCANP), has a Y-type structure in which the molecular layers pack head-to- head and tail-to-tail." Such arrangements are usually centro- symmetric but, in this case, the layers adopt a non-centrosym- metric herringbone arrangement with the charge-transfer axis parallel to the plane of the substrate.Another example is the quinolinium zwitterion, Z-P-(N-hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (C16H33-Q3CNQ), which adopts a Z-type structure with a head-to-tail packing arrange- ment displaying a quadratic SHG dependence to 200 layers.'2 The structure is stabilised by the negative charge on the terminal dicyanomethanide group which would repel in a Y-type film.The use of unconventional two-legged cationic dyes with hydrophobic chains at opposite ends of a hydrophilic chromophore has recently provided additional examples for SHG.13-16Such materials invariably form Z-type structures and quadratic SHG enhancement has been observed to more than 100 layers in three instance^.'^-'^ In this work we report that the Z-type deposition and associated non-linear optical properties of some unconven- tional cationic dyes may be improved by the addition of a t Both groups are affiliated to the Centre for Photonics and Optical Engineering.third alkyl chain and the use of an amphiphilic anion (Fig. 1). The SHG from films of one of these dyes has been shown to increase quadratically with the number of LB layers to thick- nesses of cu. 0.6 pm;this is the thickest Z-type structure to date, although thicker non-centrosymmetric films have been obtained by interleaving the layers with inactive spacer^.^ We also report the application of such multi-legged dyes in wave- guiding overlays, evanescently coupled to optical fibres, for use as in-line channel dropping filters. Our current work has demonstrated the value of the LB technique in controlling the thickness to satisfy the phase-matching condition for coupling optical power from the fibre to the LB film.Q X-L==/ 'C4H9 Fig. 1 Molecular structures of (a) dye I (X-=iodide) and dye I1 (X-=octadecylsulfate), and (b) dye I11 J. Muter. Chem., 1996,6(2), 131-136 131 Experimental The dyes were synthesised using the procedure reported in ref. 17, recrystallised from methanol and purified by column chromatography before use. Dye I was spread from dilute chloroform solution (0.1mg ml-') onto the pure water sub- phase of one compartment of an alternate-layer LB trough (Nima Technology, model 622), left for 5 min at 20 & 2 "C,and then compressed at 0.5 cm2 s-l (0.1% s-' of area). The films were transferred at 30mN m-' by passing a glass slide (for SHG) or a silver-coated slide (for SPR) via the second compart- ment of the trough, under the fixed surface barrier separating the compartments, to deposit on the upstroke at a rate of 5 mm min-'.Films were also transferred to annularly side- polished optical fibres, at 5 mm min-' for the first ten passes and then subsequently at a rate of 25 mm min-', by lowering and raising the fibre through the floating layer compressed to 30 mN m-'. The device transmission was measured in situ as deposition proceeded using the method previously described." Dyes I1 and I11 were obtained by the metathesis of their iodide salts and sodium octadecylsulfate at the air-water interface of the LB trough. The materials were spread in a 1:1 mole ratio from dilute chloroform-methanol solution onto the subphase, causing the water soluble ions, Na+ and I-, to dissolve and leaving octadecylsulfate salts of the dyes at the surface.The Langmuir films were compressed at 0.5 cm2 s-' and then transferred on the upstroke to a glass slide at 40 mN m-' (dye 11) and 35 mN m-l (dye 111) in the manner described for the iodide salt above. Results and Discussion Is0therms The pressure-area isotherm of dye I is featureless and collapses at 32 mN m-' [Fig. 2(a)]. Grazing incidence synchrotron X-ray diffraction studies on tbe floating monolayer have provided an in-plane area of 46.6 A2 molecule-' at 30 mN mo-' and 20.7 "C,compared with the corresponding value of 62 A2 from the isotherm. The discrepancy is not unusual and is readily explained.The value from X-ray diffraction only arises from the ordered domains whereas the isotherms aver!ge over the entire film including the voids. Furthermore, 46.6 A2 is in close agreement with the cross-sectional van der Waals area of the dibutylamino group and thus, the molecules probably adopt a vertical alignment at higher pressures with the hydrophobic dibutylamino group adjacent to the water subphase. This unusual arrangement has been reported for two-legged dyes with hydrophobic end-groups and has been confirmed by X- ray synchrotron diffraction studies at the air-water interfa~e.'~ The isotherms of the two octadecylsulfate salts show similar trends with the onset of plateau-like regions at 23 mN m-' (dye 11) and 19mN m-' (dye 111) and with corresponding areas of 92 and 100 A2 molecule-', respectively [Fig.2(b)]. The areas are consistent with those obtained from the van der Waals dimensions of the chromophores, whereas at collapse the areas reduce to 35 and 40A2 molecule-' and match the cross-sections Ff the dibutylamino group and the two octadecyl chains (ca. 40 A2). Thus, the plateau regions may be attributed to structural rearrangements which involve a change from horizontal to vertical alignment of the chromophore. The octadecylsulfate counter-ion improves the stability of the film and for the quinolinium dye the collapse pressure increases to above 50 mN m-', compared with 32 mN m-l for the iodide. Spectra The spectra of films of the quinolinium dye are similar but the broad absorption maximum of the charge-transfer band is shifted from 530 nm for the iodide to 515 nm for the octadecyl- sulfate salt (Fig.3). In both cases the films show negligible 132 J. Muter. Chern., 1996, 6(2), 131-136 t -10 0 '"1'"' 0 25 50 75 100 125 150 175 200 area/A* molecule-' Fig. 2 Pressure-area isotherms at 2042 "C for (a) dye I; (b) dye I1 (-) and dye I11 (---) 0.4 1 wavelengthhm Fig.3 UV-VIS spectrum of a 100 layer LB film of dye I1 (Lax=515 nm, A,,, =0.0037 layer-'). The spectrum of dye I is very similar but the broad maximum is shifted to 530 nm with A,,, =0.007 layer-'. absorbance above ca. 750nm but absorb strongly at the harmonic wavelength. In contrast, the weaker pyridinium acceptor causes the charge-transfer maximum to be shifted to 425 nm (Fig.4). In this case, the absorbance at the harmonic wavelength is very weak (6 x lop4 layer-') compared with 7 x lop3for dye I and 3.5 x lop3for dye 11. Thus, the pyridin- ium dye is an ideal candidate for SHG because there is still a degree of resonant enhancement, although the absorbance is meagre. wavelengthhm Fig. 4 UV-VIS spectrum of a 160 layer LB film of dye I11 (~,,,= 425 nm, A,,, = 0.0040 layer -') Surface plasmon resonance SPR studies were carried out using an attenuated total reflec- tion geometry in the Kretschmann configuration.'* Silver was vacuum-deposited onto clean glass substrates to a thickness of approximately 46nm and the substrate index matched to one face of a 60" BK7 crown glass prism.Reflectivity data were collected as a function of incidence angle using a p-polarized frequency-doubled Nd:YAG laser beam (A = 532 nm) and subsequently corrected for reflections at the entrance and exit faces of the prism prior to analysis by using the Fresnel reflection f~rmulae.'~ The real (E,) and imaginary (E~)compo-nents of the relative permittivity and film thickness (1) obtained for freshly deposited silver were used in the subsequent analysis of the glasslAg structures coated, in turn, with one to seven LB layers of the iodide salt (dye I). The experimental data for the silver and the first five LB layers and the theoretical fit to the experimental data for the three layer film are illustrated in Fig. 5 and 6.The plots show characteristic broadening and decreasing minima as the thickness increases beyond the bilayer, whereupon absorption contributes significantly to increased losses within the LB overlay. Analysis of the SPR data gave real and imaginary compo- nents of the dielectric permittivity of 3.14 f0.06 and 0.66 f0.03, respectively, the imaginary part being in close agreement with the value calculated from the absorbance at 532nm. The derived thickness is 2.47 nm for the first layer and 3.0 f0.3 nm 1.0 0.8 .-0 .-> CI0 0.6 a L -0 3 0.4 E 8 c 0.2 0.0 angle of incidenceldegrees Fig. 5 Normalized reflectivity data at 532 nm for a glass slide overlaid with a 46 nm thick silver film (far left) and progressively, from left to right, overlaid with one to five layers of dye I.For clarity the theoretical fits for all films and the experimental data for six and seven LB layers have been omitted from the figure. angle of incidenceldegrees Fig. 6 Experimental (+) and theoretical (-) reflectivity data for the glasslAgltrilayer structure of dye I at 532 nm. The theoretical fit corresponds to E, = 3.08, E~ =0.64 and I = 8.23 nm. layer-' for all subsequent layers. This suggests that the mono- layer adopts a more tilted arrangement on silver, whereas the larger value for the bulk film is consistent with the mean thickness of 3.0 nm layer-' from ellipsometry for 20 LB layers on glass. For comparison, ellipsometry has provided a thickness of 3.5 nm layer-' for I11 and a refractive index of ca.1.5 at 670 nm. The thicker film is consistent with a different molecular tilt of 30+ 1" for I11 compared with 41 f5" for I (see below). Second harmonic generation SHG measurements were performed in transmission using a Nd:YAG laser (A= 1.064 pm) with the beam at 45" to the film, as described in ref. 4. The SHG from films comprising from one to twenty layers of the iodide salt have been reported previously and show quadratic SHG enhancement with increasing thi~kness.'~ The octadecylsulfate salt, reported in this work, shows similar behaviour to 20 layers but for greater thicknesses the SHG deviates from the quadratic dependence, L(N) =LJ(l)N2 (1) where N is the number of LB layers.The second-harmonic intensity from a freshly deposited 100 layer film is 5 x lo3times the monolayer signal, but has been found to increase with time to an upper limit of ca. 7 x lo3 (Fig. 7). The film absorbs at LVV 160 + + + t + n 2 100 t t + 60 t+ *O *++;++++ number of layers Fig. 7 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye I1 when first deposited (+) and four weeks later after reaching a constant value (0) J. Muter. Chem., 1996, 6(2), 131-136 133 the harmonic wavelength and the expected dependence approximates to: j=N 12w(N)=IZW(1) 1 N*TN-j j= 1 where T, the mean layer transmittance, is 0.992 at 532nm. Eqn. (2) predicts 12w(N)/12w(1)M 6.9 x lo3 for N =100 and this corresponds to the upper limit observed for the 100 layer film. Allowing for absorbtion losses, the SHG dependence is consist- ent with theory and, therefore, the alignment must be non- centrosymmetric (Z-type) and reasonably ordered throughout the 100 layers.The SHG polarization dependence, 12J p-+p)/12Js-+p), is similar for the iodide and octadecylsulfate salts and, using the method of Kajikawa et ~l.,~'the data correspond to chromo- phore tilt angles of 4 =41 f5" from the substrate normal. The validity of this approach is not clear as intermolecular charge transfer as well as intramolecular processes can give rise to strong SHG.21 Nonetheless, the product of molecular length and COS~closely matches the thickness obtained from SPR and ellipsometry.Using the data obtained from these tech- niques the second-order susceptibility, x(2)zzz,of dye I is 120 pm V-'. Unfortunately, the ellipsometry studies on films of dye I1 have been unsuccessful to date. However, as the chromophore tilt angle is the same for both dyes it may be assumed that the thickness of the octadecylsulfate salt is similar and for 1= 3.0 nm layer -'the susceptibility, corrected for absorbance by the 100 layer film, is 80 pm V-' (dye 11).Although these values are resonantly enhanced, the films may be applicable to frequency doubling at more suitable wavelengths where they are transparent. Importantly, we have demonstrated that the presence of the hydrophobic dibutylamino end-groups causes the structures to be non-centrosymmetric (Z-type).To overcome the problem of absorbance at 532nm (AJ our recent work has concerned a novel pyridinium analogue (dye 111). The SHG has been investigated to 160 layers and the intensity varies by no more than &4% along a 30 mm length of the film. The normalised intensity, 120(N)/N2,is higher for the LB monolayer and this may be attributed to a slightly different tilt angle for molecules adjacent to the glass substrate than in the bulk film. However, quadratic SHG enhancement has been realised from 4 to 160 layers and this is clearly indicative of an ordered Z-type arrangement (Fig. 8). The chromophore tilt angle from the SHG polarization dependence and the second-order susceptibility are 4 =30 f1"and x(2)zzz= 30 pm V-l.The film absorbance is 6 x lop4layer-' at 532 nm and thus, the transparency/efficiency trade-off is very favour- able. Moreover, the Z-type structures of dye I11 are stable and 100 80 + tt t + t + 20 '0 20 40 60 80 100 120 140 160 number of layers Fig. 8 Variation of the square root of the second-harmonic intensity with the number of LB layers of dye I11 134 J. Muter. Chem., 1996, 6(2), 131-136 in the year since the 160 layer film was first fabricated the SHG has shown no sign of deterioration. Fibre optic studies We have studied the possibility of evanescent coupling to the LB film using side-polished single mode optical fibres to realise in-line channel dropping filters.The fibre cladding was removed using an annular polishing process'' to access the propagating optical field and an LB film of dye I was deposited onto the polished fibre over an interaction length of 9.6k0.3 mm (Fig. 9). By assuming weak coupling the device may be viewed as an asymmetric directional coupler; the strong differential waveguide dispersion between the fibre-guided mode and the LB-guided mode results in a bandstop or channel dropping spectral response centred on the synchronous wavelength (A,) at which phase matching between the two guided modes occurs. This simple description yields accurate predictions for A, as a function of the LB film thickness," but the true device geometry of cylindrical fibre and planar overlay must be taken into account to determine the form of the spectral res~onse.~~.~~ The LB film was deposited onto the optical fibre during 188 passes through the Langmuir film (upstroke and downstroke) and the normalised spectral responses from this device are shown in Fig.10 (see also Table 1). It is difficult to assign polarization states to the resonances at 744 and 820 nm since the optical nature of the films is not well enough established. Indeed, with an assumed tilt angle of ca. 41",taken from the SHG polarization dependence of films deposited onto micro- scope slides, it may well be that the normal modes of the film are hybrid.24 In terms of device performance this would manifest itself as TE/TM (s/p) polarization cross-coupling.In Interaction Fibre Region Fibre Core \ t-l "Trig\ I I Langmuir-Blodgett Film Fig. 9 Schematic representation of the structure used for the in-line fibre optic device 700 740 780 820 860 900 wavelengthhn Fig. 10 Spectral responses of the fibre optic device of dye I for 188 passes (94 each on the upstroke and downstroke) through the floating monolayer. Each trace corresponds to a different polarization state. Table 1 Spectral characteristics of a polished fibre device overlaid with an LB film of the iodide salt (dye I). The LB film was deposited during 188 passes through the floating monolayer (94 each on the downstroke and upstroke) .synchronous modulation wavelength/nm depth/dB 3 dB linewidthlnm 744 11.7 29 820 14.7 42 this respect, the use of a highly birefringent polarization- maintaining fibre would be beneficial allowing an accurate TE-like ( LPOly) and TM-like (LP,,,) polarization state to be established in the device before dep~sition.'~ This would allow the magnitude of such cross-coupling effects to be ascertained.Wavelength scans in the range 700 to 950nm revealed no polarization-dependent responses other than those shown in Fig. 10 and thus, it is likely that the normal modes are predominantly TE and TM. The effects of absorption upon the bandstop responses are evident in the lower-wavelength trace with an increased insertion loss of 0.69k0.05 dB at 700 nm compared to 0.20 & 0.05 dB at il>880 nm.By compari- son with previously reported data for o-tricosenoic acid" the occurrence of the resonance at 820 nm from 188 passes through the floating layer arises from deposition on the upstroke only, i.e. with 94 deposited layers. If this is the case, then consider- ation of the SHG data suggests that the LB film structure is also likely to be Z-type and, as such, the device should exhibit an electro-optic response. Consequently, we are currently investigating the addition of a suitable electrode structure to the device. By assuming an all-electronic contribution to the second- order non-linearity of dye I the high susceptibility ( x(z)zzz= 120pm V-') implies that the molecule should also exhibit a strong electro-optic coefficient (r33).However, since the second- harmonic wavelength lies within an electronic charge-transfer resonance (Table 2) the data are unsuitable for modelling bandstop responses at longer wavelengths and, in view of the significant tilt angle, it is difficult to model the device response with any degree of accuracy.Nonetheless, with reference to its absorption spectrum it is clear that by operating above 800 nm one would expect, by the Kramers-Kronig relations,26 a sig- nificant material dispersion with negligible optical absorption. Broberg et aLZ7 have already shown that this can augment the differential waveguide dispersion between the fibre and the LB film guided modes, resulting in linewidth narrowing and poss- ibly even a degree of resonance enhancement of the susceptibility. Fibre optic overlays could be assumed to be a possible alternative to prism coupling but the geometrical structure of the cylindrical fibre and planar overlay must be taken into account since the lack of lateral confinement in the latter results in losses in this dimension." A theoretical description of the experimentally observed spectral responses, although possible, is complicated even for optically isotropic overlays and, as such, precludes the use of this method as a reliable Table 2 Spectra, non-linear optical properties, chromophore tilt angle and thickness dye nlnaxl nm AmI1 layer- p'ZZZI pm V-' #/degrees l/nm layer-' I 530 0.007 120 41 3.0 I1 515 0.0037" 80 41 3.0b I11 425 0.0040" 30 30 3.5 "Absorbance derived from thick LB films comprising 100 layers of dye I1 and 160 layers of dye 111.Thickness assumed in the calculation of the susceptibility. thin film characterisation tool. In certain cases, however, it could be used to augment existing methods such as SPR. Conclusion The use of chromophores with hydrophobic end-groups has been shown to facilitate Z-type deposition to thicknesses suitable for waveguiding and the non-centrosymmetric align- ment has resulted in high second-order non-linearities, albeit resonantly enhanced, for dyes I and 11. The SHG from films of dye I11 has been shown to increase quadratically with the number of LB layers to thicknesses of 0.56 pm, this being the thickest homomolecular Z-type structure obtained to date.In this case the use of a pyridinium acceptor in place of quinolin- ium has resulted in negligible absorbance at the harmonic wavelength. The LB technique has also been shown to provide precise thickness control for the formation of in-line fibre optic channel dropping filters and our current work on such overlays involves the development of suitable electrode structures to demonstrate intensity modulation. We are grateful to Dr. I. R. Gentle and Professor C. H. L. Kennard (The University of Queensland) for the X-ray synchrotron diffraction data and acknowledge the EPSRC (UK) for support of the non-linear optics programme at Cranfield. The EPSRC are also acknowledged for providing an assistantship (to G.J.) and studentships (to R. B. C. and R. 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