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We report the observation and measurement of second-harmonic generation in self-assembled ultra thin film nonlinear optical materials using a femtosecond high repetition rate laser system. Second-harmonic intensity, as a function of the incident angle in these films, has been measured using incident p-polarized and s-polarized optical beam components. The second-order nonlinear optical susceptibilities of the thin films have also been determined. Using a curve fitting method and a crystal reference material, we have obtained second-order susceptibilities χ_{333} = 6.17 ± 0.18 pm/V and χ_{311} = 0.68 ± 0.02 pm/V at a fundamental wavelength of 1,200 nm. Based on linear molecular model approximation, we have also used the fitted data to investigate the average orientation distribution of the chromophore dipoles in the self-assembled film. The result indicates that the average tilt angle of the chromophore dipoles away from the substrate normal line is 25.2° ± 0.8°.

Recent interest in conjugated organic molecular and polymeric materials has been stimulated in part due to their intrinsic large optical nonlinearities. The strong second-harmonic (SH) generation in organic molecules originates from the fact that they possess electron donor and acceptor groups attached to a π-conjugated system. Since the first external electric field poled second order nonlinear optical (NLO) polymers were reported in 1982 [

Electrostatically self-assembled (ESA) processes have recently been developed, which allow detailed structural control at the molecular level with ease of manufacturing [

For the fabrication of samples used in these measurements, poly S-119 and poly (diallyl dimethyl ammonium chloride) (PDDA) were purchased from Sigma and Aldrich, respectively.

Theoretical and experimental investigations of SH generation in thin films have been proposed and developed [_{∞}_{v}_{311} and χ_{333} are only the non-zero components of the second-harmonic susceptibilities.

Using field boundary conditions at z = 0 and z = 1(1 is the thickness of the film) and neglecting the refractive index dispersion of air, after some manipulations, the intensity I_{2ω} of the SH wave in air can be written as [^{g}_{2ω} is the refractive index of the glass at 2ω, θ^{g}_{2ω} is the refractive angle of the second-harmonic signal in the glass, n_{ω} and n_{2ω} are refractive indices of the film at the fundamental and harmonic frequencies, n°_{ω}, n^{e}_{ω}, n°_{2ω}, and n^{e}_{2ω} are ordinary and extraordinary refractive indices of the film at fundamental and harmonic frequencies, respectively, and t_{ω} is the product of the transmission coefficients at the air-glass and glass-film interfaces. For the incident s-polarized component
^{g}_{ω} is the refractive index of the glass at ω, θ^{g}_{ω} is the refractive angle in the glass and θ^{f}_{ω} is the refractive angle of the fundamental beam at the glass-film interface given by Snell's law
_{2ω}_{333}/χ_{311}. Note that T_{2ω} includes r = χ_{333}/χ_{311} only for p-polarized incidence while I_{2ω} only includes χ_{311} for s-polarized incidence. Therefore, if one can experimentally measure I_{2ω} as a function of the incident angle for the p-polarized incidence, one should be able to obtain r using a computational fitting method. Moreover, if one can obtain χ_{311} from the s-polarized incidence, χ_{333} and χ_{311} can be determined.

Because absolute measurements of fundamental and SH intensities are always difficult, for more accurate determination of nonlinear optical coefficients of the thin films, researchers now often use inorganic crystals as reference materials since careful measurements for these crystals have been made. In most cases, quartz is widely used as a reference material [_{311} for the s-polarized incidence, we used a Y-cut quartz platelet with a thickness of 1 mm as a reference. The SH intensity from this reference was measured using the same fundamental intensity as that used for our sample films. The rotation axis is the x-axis. The fundamental beam polarized along the x-axis was incident upon the quartz along the Y direction. The SH intensity from the quartz platelet is [_{111} is one of the nonlinear optical coefficients of quartz, n′_{ω} and n′_{2ω} are refractive indices of the quartz at the fundamental and the SH wavelengths, respectively, and
^{q}_{ω} and θ^{q}_{2ω} are the refractive angles at the fundamental and the SH wavelengths, respectively. Using _{311} now can be calculated using χ_{111} and experimentally measured values for I_{2ω} and I′_{2ω}.

Experiments were performed using a Coherent femto-second ultrafast laser system as shown in

Using the p-polarized fundamental incidence, we measured the p-polarized SH intensity (p-p configuration) as a function of the incident angle in the poly S-119/PDDA film. The measurements show that the maximum SH intensity occurred at θ = 60°. As mentioned above, due to the difficulty of accurately measuring absolute intensities of both fundamental and SH waves, to obtain d_{r} more accurately, we have used normalized signal to remove I^{2}_{ω} from

_{2ω} = a(I_{ω})^{b}, where I_{2ω} and I_{ω} are the second-harmonic and fundamental intensities, respectively. The fits yields a value of b = 1.98 and a = 0.053, which is in good agreement with the quadratic dependence on fundamental intensity.

_{2ω} = n°_{2ω} = n^{e}_{2ω} = 1.639, n_{ω} = n°_{ω} = n^{e}_{ω} = 1.488, refractive indices of the optical glass n^{g}_{ω} = 1.513, n^{g}_{2ω} = 1.516 [_{333}/χ_{311} = 9.07 ± 0.45 after using the normalization method to eliminate I^{2}_{2ω} from

To separate χ333 and χ_{311}, as mentioned above, we have to compute χ311 first by using the s-polarized fundamental beam to obtain the p-polarized SH intensity from the film in comparison with that from the quartz platelet under the same conditions. We used the s-polarized fundamental beam to measure the p-polarized SH intensity as a function of the incident angle from the film (s-p configuration). Because only χ311 can contribute to the SH intensity of this polarization configuration, the SH signal is much weaker than that of the p-p configuration. To obtain a more accurate value of χ311, we measured the SH signal as a function of the incident angle over a wide range (10°–70°) instead of only at one point. For comparison, the Y-cut quartz sample was measured under the same intensity of the fundamental beam. The quartz sample was rotated to the maximum SH orientation at near normal incidence. The rotation axis is the X-axis and the fundamental beam polarized in the X-axis was incident upon the quartz along Y direction (corresponding to the effective second-order susceptibility χ_{111}). Then, we treat the signal of the quartz sample as unity to normalize the SH signal from the film. Using _{311}. _{111} = 0.68 pm/V [_{2ω} = 1.547 and n′_{ω} = 1.534 [_{311} = 0.68 ± 0.02 pm/V. Using χ_{333}/χ_{311} = 9.07 ± 0.45 yields χ_{333} = 6.17 ± 0.18 pm/V.

Second-harmonic generation is an important tool to determine molecular dipole tilt angle [_{zzz} is the only dominant component of the second-order optical susceptibility, χ_{333} and χ_{311} can be written as [_{1}> and <P_{3}> are order parameters related to the Legendre polynomials, N is the number density of molecules, and β^{*}_{zzz} is the local-field-corrected β_{zzz}. From Equation (20)/_{333}/χ_{311}. From Equations (20) and _{3}> = 0 gives the minimum r = 3. This corresponds to isotropic materials [_{3}> increases. As a result, the value of r is increased as well. In an extreme case when the order parameters are unity, one has r ∼ ∞. In our case, the computer fit gives r = 9.07 ± 0.45, which yields <P_{3}>/<P1>∼0.55. This indicates that the molecular orientation in our films has been significantly improved compared to isotropic materials. _{311}/χ_{311} = 9.07 ± 0.45 yields δ = 25.2° ± 0.8°.

We have shown that thin films fabricated using the ESA technique can spontaneously assemble into a noncentrosymmetric structure with substantial second-harmonic generation effect, without the need of external electric field poling. Devices made using this approach may have potential applications in electro-optic and frequency doubling waveguide devices [

The electrostatically self-assembled (ESA) process used to fabricate thin films: (

The structures of poly S-119 and poly(diallyl dimethyl ammonium chloride) (PDDA).

Absorbance

The structure of the sample and the incident geometry in our experiments used: G, glass substrate; F, film; p-p, both SH and incident beam are p-polarized; s-p, incident beam is s-polarized and SH is p-polarized.

Experimental setup to measure SH generation: PL, pump laser; FL, femtosecond laser pumped by the pump laser; OPO: optical parameter oscillator pumped by the femtosecond laser; C: chopper to provide a reference signal to the lock-in amplifier; HF, half-wave plate at 1,200 nm; P1: polarizer; F′s: interference bandpass filters; S: sample; P2: analyzer; M: monochromator; PMT: photomultiplier; OSC: oscilloscope; LA: Lock-in amplifier.

Dependence of the SH intensity at 600 nm on the fundamental intensity at 1,200 nm in the p-p incidence configuration.

Normalized SH intensity as a function of the incident angle in the p-p configuration. The symbols and solid-curve correspond to the measured and fitted results, respectively.

Normalized SH intensity as a function of the incident angle in the s-p configuration. The symbols and solid-curve correspond to the measured and fitted results, respectively.

The average tilt angle of the poly S-119 molecular dipoles from the substrate normal. The poly S-119 molecule has been simplified by a linear model approximation [

The authors would like to thank Fajian Zhang, You-Xiong Wang, Richard O. Claus and David Wood at NanoSonic Incorporated for their assistance in fabrication of these films.

_{3}thin films

_{3}thin films