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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 13 — Jun. 22, 2009
  • pp: 10642–10647
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Molecular orientation in self-assembled multilayers measured by Second Harmonic generation using femtosecond pulses.

Catalina von Bilderling, Mario Tagliazucchi, Ernesto J. Calvo, and Andrea V. Bragas  »View Author Affiliations


Optics Express, Vol. 17, Issue 13, pp. 10642-10647 (2009)
http://dx.doi.org/10.1364/OE.17.010642


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Abstract

We present measurements of the optical second-harmonic generation in self assembled multilayer films of PAZO/PAH polymers with the aim to investigate molecular order in the layer-by-layer architecture. The experiments are performed in transmission, using a femtosecond Ti:Sa pulsed laser, which allows a more accurate determination of the amplitude of the second harmonic signal, without interference fringes usually present in nanosecond experiments. We found that the first bilayer, in contact with the substrate, presents a broad distribution of the orientation of the molecules, while the addition of successive bilayers (up to 12) produces ordering of the molecules with a small tilt angle respect to the surface normal. This result, together with the monotonic increment of the second harmonic signal with the number of layers indicates that the molecules grow orderly assembled in these films.

© 2009 Optical Society of America

1. Introduction

1.1 Brief review of the Second Harmonic Generation in molecular films

SH signal comes from the nonlinear electronic susceptibility of materials, χ (i). These higher order terms in the response of the material become evident in presence of intense external fields E, being the polarization:

P=χ(1)E+χ(2)E·E+χ(3)E·E·E+
(1)

At the microscopic level, the polarization can be described as a power series as well, where the term of interest for second harmonic generation is proportional to the molecular hyperpolarizability tensor β (2). While the χ (2) tensor describes the second-order nonlinear response of the material, the β (2) tensor describes the SH response of an isolated chromophore.

pi=μi+αij(1)Ej+βijk(2)EjEk+
(2)

D=cos3θZZcosθZZ,
(3)

where θ ZZ’ is the tilt angle between the long molecular axis (z’) and the surface normal (z), and the brackets indicate an average value. If the distribution about the mean orientation angle is narrow, then the average tilt angle of the chromophores away from the surface normal can be determined considering D≅cos2θ z'z〉 For rod-like chromophores (i.e., for molecules with a dominant second-order β Z’Z’Z’ molecular nonlinear polarizability tensor element) specific relations [12

12. B. Dick, “Irreducible tensor analysis of sum- and difference- frequency generation in partially oriented samples,” Chem. Phys. 96, 199–215 (1985). [CrossRef]

] between the elements of χ (2) and orientation averages gives:

D=χZZZ(2)χZZZ(2)+2χZXX(2).
(4)

Therefore, to determine the angle θ ZZ’, one should extract the value of the second-order NLO tensor χ (2) from the SH experiment and replace it in (4). The nonzero elements of χ (2) can be evaluated from the SH intensities polarized parallel (p) or perpendicular (s) to the plane of incidence, as a function of the polarization of the fundamental beam. For a system with a single unique axis (typical of most thin organic film systems), only three of the possible 27 elements of the χ(2) tensor are nonzero; namely, χ ZZZ, χ ZXX=χ ZYY and χ XXZ=χ XZX=χ YYZ=χ YZY. If one assumes that the surface layer thickness d is much smaller than the wavelength of the SH, then the specific relationship between the transmitted s- and p-polarized SH intensities and the χ (2) tensor elements is given by [13

13. B. Dick, A. Gierulski, and G. Marowsky, “Determination of the Nonlinear Optical Susceptibility χ(2) of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 38, 107–116 (1985). [CrossRef]

]

Is2ω(γ)=Cd2s1sin(2γ)χXXZ2(Iω)2,
(5a)
Ip2ω(γ)=Cd2cos2(γ)[s2χXXZ+s3χZXX+s4χZZZ]+s4χZXX2(Iω)2,
(5b)

where I is the SH intensity, the subscripts indicate the polarization, Iω is the intensity of the incident beam, γ is the polarization angle of the fundamental beam (γ=0° corresponds to p-polarized light), C is the instrument response, and sn are coefficients dependent on the angle of incidence, θi, and linear and nonlinear Fresnel factors [13

13. B. Dick, A. Gierulski, and G. Marowsky, “Determination of the Nonlinear Optical Susceptibility χ(2) of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 38, 107–116 (1985). [CrossRef]

,14

14. P. Guyot-Sionnest and Y. R. Shen, “Comments on “Determination of the Nonlinear Optical Susceptibility χ(2 of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 42, 237–238 (1987). [CrossRef]

].

The refractive index of the polymer film and the substrate at ω and defines the position of the maximum of I when plotted as a function of θ i, through the Fresnel factor dependence on these parameters. Reasonable values for the refractive indexes give the angle at the maximum between 60 to 80 degrees [13

13. B. Dick, A. Gierulski, and G. Marowsky, “Determination of the Nonlinear Optical Susceptibility χ(2) of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 38, 107–116 (1985). [CrossRef]

,14

14. P. Guyot-Sionnest and Y. R. Shen, “Comments on “Determination of the Nonlinear Optical Susceptibility χ(2 of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 42, 237–238 (1987). [CrossRef]

]. Transmission experiments have the advantage that measurements as a function of the incident angle are simpler to perform, compared to reflection experiments. However, signal becomes more complex when the SH signal generated from one side of the substrate interferes with the SH generated at the other side [10

10. F. Kajzar, J. Messier, J. Zyss, and I. Ledoux, “Nonlinear Interferometry in Langmuir-Blodgett multilayers of polydiacetylene,” Opt. Commun. 45, 133–137 (1983). [CrossRef]

]. In these cases, equation (5) describes only the envelope, although the detected signal is modulated by interference fringes. Fringes appear if the coherence length of the laser is larger than the width of the substrate, which is on the order of 150 µm for a coverslip and 1mm for a microscope slide. Nanosecond lasers have typically tens of centimeters of coherence length while femtosecond lasers have only few microns, justifying why there are no fringes in our experiment. In this paper we measure always at the incident angle of the maximum signal, adjusting the focus of the beam if necessary.

2. Experiment

2. 1 Multilayer fabrication

2.2 SH measurements

The experiments were performed in transmission, using a modelocked Ti:Sa laser (50 fs pulse width, 400 mW average power, 80MHz repetition rate, 780-800 nm) focused by a glass lens (2.5cm focal length) down to a spot size of about 5µm diameter. Transmission polarized or unpolarized signal enters into a monochromator (Metrolab 250AA), after which the SH is detected by a photomultiplier tube (Hamamatsu 1P28) with photon counting sensitivity. The polarization response of the monochromator has been taken into account. The samples were rotated around an axis parallel to the optical table to adjust the angle of incidence.

3. Results and discussion

3.1 Characterization of Multilayer Films

UV/VIS absorption spectra of films with different number of layers are shown in Fig. 1(a). All spectra have the absorption peak at 370 nm, which is attributed to the absorption of PAZO.

Fig. 1. (a) UV/Vis spectra of the (PAH/PAZO)n assemblies. Curves, from down to top, correspond to n=1, 4, 6, and 12. (b) Absorbance at the maximum (black squares) and ellipsometric thickness (open circles) as a function of n, solid lines are linear fits. The thickness increases about 1 nm per bilayer.

The linear increase of the absorbance at this wavelength (Fig. 1(b)) with the number of deposited bilayers confirms that an equal amount of PAZO was adsorbed in each deposition cycle. The absorption peak of the azobenzene chromophore showed a considerable red-shift in the film as compared to that in water solution. Formation of J-aggregates of azobenzene chromophores has been suggested to be responsible for the observed red-shift [8

8. E. Kang, T. Bu, P. Jin, J. Sun, Y. Yang, and J. Shen, “Layer-by-Layer Deposited Organic/Inorganic Hybrid Multilayer Films Containing Noncentrosymmetrically Orientated Azobenzene Chromophores,” Langmuir , 23, 7594–7601 (2007). [CrossRef] [PubMed]

,9

9. Y. Lvov, S. Yamada, and T. Kunitake, “Non-linear optical effects in layer-by-layer alternate films of polycations and an azobenzene-containing polyanion,” Thin Solid Films 300, 107–112 (1996) [CrossRef]

]. The linear growth of the films is also confirmed by the thickness dependence on the number of bilayers, shown in Fig. 1(b). A refractive index of 1.51 was obtained for the films from these ellipsometry measurements.

3.2 Second Harmonic Generation

Figure 2(a) shows a typical SH spectrum obtained for a multilayer (PAH/PAZO)12 film, taken at the γ that maximizes the signal. The peak intensity corresponds to half of the incident wavelength. Note that the signal, even after the monochromator, is well beyond the thousand of counts per second, while the background signal is below a hundred of counts. Spectrum and power dependence of the SH intensity is always checked, as in Fig. 2(b) which shows the quadratic increment of the SH at the maximum as a function of the input power.

Fig. 2. (a) Spectral response of (PAH/PAZO)12 film proves that the observed signal is SH. (b) Excitation power dependence of the SH signal at λ=398nm, following a quadratic law.

3.3 Orientation measurements

By construction of our experimental setup, horizontal and vertical components of the signal respect to the lab system of coordinates, have a small rotation angle of 15 degrees respect to the p- or s- polarized components of the SH at the sample. Then, we call s’ and p’ the rotated coordinates. We measure the s’- and p’-polarized SH response for 1, 6, 9, and 12 bilayers films. As an example, Fig. 3 shows the dependence of the s’-, p’-polarized and total SH signal for of (PAH/PAZO)6. Full lines in Fig. 3 are the fittings with Eq. (5) taking into account the setup rotation of 15 degrees. All the parameters used for the fits are taken from experimental values measured for these specific films (n, d and θi which, in turn, are used to calculate the sn parameters for each measurement). The average chromophore orientation angles with respect to the substrate normal obtained are: 17° for the 1 bilayer film and 0° for films of 6, 9 and 12 bilayers, with an uncertainty of 3°.

Fig. 3. (a) s’- and (b) p’-polarized SH response (rotated 15 degrees from the s- or p- polarized signals) of (PAH/PAZO)6 as a function of the polarization angle of the fundamental beam (90° corresponds to s- polarized incident beam). (c) is the total signal. Full lines are fittings following Eq (5). (d) Square root of second-harmonic intensity of PAH/PAZO films as a function of the number of deposited bilayers. Squares correspond to the maximum total signal measured, triangles correspond to the same quantity obtained from the fitting function. Dotted line is a guide to the eye.

Figure 3(d) shows the relationship between the square root of the SH signal (considered as the maximum of the total signal, Fig. 3(c)) and n, in which a monotonic increment of the signal can be seen. As expected from equation (5), the signal should be quadratic with n if (i) d is proportional to n and (ii) χ iii are constant along the bilayers. Even assuming that d is proportional to n, we do not expect an exact quadratic behavior, since the distribution width has been shown to be broader for the first bilayer and, therefore, the χ iii values may change from the first to the subsequent bilayers. The increment of the SH indicates that the chromophores are not disordered even for a higher number of bilayers, and that the film grows regularly along. This assumption reinforces the results obtained for the polarization dependence measurements.

4. Conclusions

In this paper we show that second harmonic measurements in self-assembled multilayers can be easily performed with non-amplified femtosecond pulses on the order of the nJ per pulse, avoiding interference fringes, which may increase the level of difficulty on the retrieval of the relevant information. We also show that PAH/PAZO films grow uniformly, and that the cromophores are oriented close to the surface normal up to the 12 bilayers tested. The presence of the chromophores does not perturb the ordered growth of the layer by layer architecture. The first bilayer copies somehow the roughness of the substrate giving an orientation angle compatible with a small angle and a broad distribution around it.

Acknowledgments

This work was supported by the University of Buenos Aires under Grant X010 and by ANPCYT under Grant PICT 14209.

References and links

1.

G. Decher and J. B. Schlenoff, Multilayer Thin Films: Sequentially Assembly of Nanocomposite Materials (Wiley-VCH, 2002).

2.

A. Garg, R. M. Davis, C. Durak, J. R. Heflin, and H. W. Gibson, “Polar orientation of a pendant anionic chromophore in thick layer-by-layer self-assembled polymeric films,” J. App. Phys. 104, 053116 1–8 (2008). [CrossRef]

3.

S. S. Shiratori and M. F. Rubner, “pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes,” Macromol. 33, 4213–4219 (2000). [CrossRef]

4.

Soo-Hyoung Lee, S. Balasubramanian, D. Y. Kim, N. K. Viswanathan, S. Bian, J. Kumar, and S. K. Tripathy, “Azo Polymer Multilayer Films by Electrostatic Self-Assembly and Layer-by-Layer Post Azo Functionalization,” Macromol. 33, 6534–6540 (2000). [CrossRef]

5.

Y. R. Shen, “Optical Second Harmonic Generation at Interfaces,” Annu. Rev. Phys. Chem. 40, 327–350 (1989). [CrossRef]

6.

G. J. Simpson and K. L. Rowlen, “Measurement of Orientation in Organic Thin Films,” Acc. Chem. Res. 33, 781–789 (2000). [CrossRef] [PubMed]

7.

J. R. Heflin, C. Figura, D. Marciu, Y Liu, and R. O. Claus, “Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers,” Appl. Phys. Lett. 74, 495–497 (1999). [CrossRef]

8.

E. Kang, T. Bu, P. Jin, J. Sun, Y. Yang, and J. Shen, “Layer-by-Layer Deposited Organic/Inorganic Hybrid Multilayer Films Containing Noncentrosymmetrically Orientated Azobenzene Chromophores,” Langmuir , 23, 7594–7601 (2007). [CrossRef] [PubMed]

9.

Y. Lvov, S. Yamada, and T. Kunitake, “Non-linear optical effects in layer-by-layer alternate films of polycations and an azobenzene-containing polyanion,” Thin Solid Films 300, 107–112 (1996) [CrossRef]

10.

F. Kajzar, J. Messier, J. Zyss, and I. Ledoux, “Nonlinear Interferometry in Langmuir-Blodgett multilayers of polydiacetylene,” Opt. Commun. 45, 133–137 (1983). [CrossRef]

11.

P.D Maker, R.W Terhune, M. Nisenoff, and C. M. Savage, “Effects of dispersion and focusing on the production of optical harmonics,” Phys. Rev. Lett. 8, 21–22 (1962).

12.

B. Dick, “Irreducible tensor analysis of sum- and difference- frequency generation in partially oriented samples,” Chem. Phys. 96, 199–215 (1985). [CrossRef]

13.

B. Dick, A. Gierulski, and G. Marowsky, “Determination of the Nonlinear Optical Susceptibility χ(2) of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 38, 107–116 (1985). [CrossRef]

14.

P. Guyot-Sionnest and Y. R. Shen, “Comments on “Determination of the Nonlinear Optical Susceptibility χ(2 of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,” Appl. Phys. B 42, 237–238 (1987). [CrossRef]

OCIS Codes
(190.4400) Nonlinear optics : Nonlinear optics, materials
(310.6860) Thin films : Thin films, optical properties
(320.7110) Ultrafast optics : Ultrafast nonlinear optics

ToC Category:
Nonlinear Optics

History
Original Manuscript: April 2, 2009
Revised Manuscript: May 8, 2009
Manuscript Accepted: May 31, 2009
Published: June 10, 2009

Citation
Catalina von Bilderling, Mario Tagliazucchi, Ernesto J. Calvo, and Andrea V. Bragas, "Molecular orientation in self-assembled multilayers measured by Second Harmonic generation using femtosecond pulses.," Opt. Express 17, 10642-10647 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-13-10642


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References

  1. G. Decher and J. B. Schlenoff, Multilayer Thin Films: Sequentially Assembly of Nanocomposite Materials (Wiley-VCH, 2002).
  2. A. Garg, R. M. Davis, C. Durak, J. R. Heflin, and H. W. Gibson, "Polar orientation of a pendant anionic chromophore in thick layer-by-layer self-assembled polymeric films," J. App. Phys. 104, 053116 1-8 (2008). [CrossRef]
  3. S. S. Shiratori and M. F. Rubner, "pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes," Macromol. 33, 4213-4219 (2000). [CrossRef]
  4. Soo-Hyoung Lee, S. Balasubramanian, D. Y. Kim, N. K. Viswanathan, S. Bian, J. Kumar, and S. K. Tripathy, "Azo Polymer Multilayer Films by Electrostatic Self-Assembly and Layer-by-Layer Post Azo Functionalization," Macromol. 33, 6534-6540 (2000). [CrossRef]
  5. Y. R. Shen, "Optical Second Harmonic Generation at Interfaces," Annu. Rev. Phys. Chem. 40, 327-350 (1989). [CrossRef]
  6. G. J. Simpson and K. L. Rowlen, "Measurement of Orientation in Organic Thin Films," Acc. Chem. Res. 33, 781-789 (2000). [CrossRef] [PubMed]
  7. J. R. Heflin, C. Figura, D. Marciu, Y Liu, and R. O. Claus, "Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers," Appl. Phys. Lett. 74, 495-497 (1999). [CrossRef]
  8. E. Kang, T. Bu, P. Jin, J. Sun, Y. Yang, and J. Shen, "Layer-by-Layer Deposited Organic/Inorganic Hybrid Multilayer Films Containing Noncentrosymmetrically Orientated Azobenzene Chromophores," Langmuir,  23, 7594-7601 (2007). [CrossRef] [PubMed]
  9. Y. Lvov, S. Yamada, and T. Kunitake, "Non-linear optical effects in layer-by-layer alternate films of polycations and an azobenzene-containing polyanion," Thin Solid Films 300, 107-112 (1996) [CrossRef]
  10. F. Kajzar, J. Messier, J. Zyss, and I. Ledoux, "Nonlinear Interferometry in Langmuir-Blodgett multilayers of polydiacetylene," Opt. Commun. 45, 133-137 (1983). [CrossRef]
  11. P.D Maker, R.W Terhune, M. Nisenoff, and C. M. Savage, "Effects of dispersion and focusing on the production of optical harmonics," Phys. Rev. Lett. 8, 21-22 (1962).
  12. B. Dick, "Irreducible tensor analysis of sum- and difference- frequency generation in partially oriented samples," Chem. Phys. 96, 199-215 (1985). [CrossRef]
  13. B. Dick, A. Gierulski, and G. Marowsky, "Determination of the Nonlinear Optical Susceptibility χ(2) of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission," Appl. Phys. B 38, 107-116 (1985). [CrossRef]
  14. P. Guyot-Sionnest and Y. R. Shen, "Comments on "Determination of the Nonlinear Optical Susceptibility χ(2 of Surface Layers by Sum and Difference Frequency Generation in Reflection and Transmission,"Appl. Phys. B 42, 237-238 (1987). [CrossRef]

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