**Author details**

slow cooling. This behavior is similar to what happens in the spontaneous drying assembly, since as the film cools the chains are losing mobility, but slowly enough for them to recover the best configuration induced by electrostatic interaction, thus recovering the order and restoring the SHG signal. For the films fabricated at pH 3.5, the SHG signal as a function of the azimuthal angle has the same isotropic profile before and after heating. On the other hand, the same was not verified for films fabricated at pH 10, where we can observe that after heating the ordering is no longer isotropic, as shown in Figure 17 for a 1-bilayer PAH/PS-119 film. This suggests that the films fabricated at this pH value have larger mobility than those at pH 3.5 or 7, which allows the rearrangement of chains to form macroscopic domains (~ hundreds of

profile before and after heating. On the other hand, the same was not verified for films

fabricated at pH 10, where we can observe that after heating the ordering is no longer

isotropic, as shown in Figure 17 for a 1-bilayer PAH/PS-119 film. This suggests that the

films fabricated at this pH value have larger mobility than those at pH 3.5 or 7, which

allows the rearrangement of chains to form macroscopic domains (~ hundreds of

 SS SP

**Figure 17: SHG signal in SS and SP polarization combination for a one-bilayer PAH/PS-119 film** 

**Figure 17.** SHG signal in SS and SP polarization combination for a one-bilayer PAH/PS-119 film fabricated at pH 10,

SHG signal / arb. u.

In this chapter we have discussed how nonlinear optical methods, and in particular

second-harmonic generation (SHG), can be used to investigate the molecular order in

In this chapter we have discussed how nonlinear optical methods, and in particular secondharmonic generation (SHG), can be used to investigate the molecular order in polyelectrolyte layer-by-layer films containing azopolymers. After a brief outline of the basic theory of SHG for interface studies, we have shown how its polarization dependence can be used to obtain quantitative information about the orientational distribution function of azo-groups in these thin films. However, even a qualitative analysis of the SHG signal can give important infor‐ mation about the film structure. For example, the SHG dependence on the azimuthal rotation of the sample has shown that the way the films are dried has a marked influence of their molecular arrangement, which is isotropic for slow (spontaneous) drying, while it becomes

polyelectrolyte layer-by-layer films containing azopolymers. After a brief outline of the

basic theory of SHG for interface studies, we have shown how its polarization dependence

can be used to obtain quantitative information about the orientational distribution function

of azo-groups in these thin films. However, even a qualitative analysis of the SHG signal

can give important information about the film structure. For example, the SHG dependence

We have also investigated how the molecular ordering depends on the film thickness and fabrication conditions, especially the pH of the assembling/rinsing solutions. In contrast to previous reports in the literature, we did not find that all layers keep the same relative

on the azimuthal rotation of the sample has shown that the way the films are dried has a

marked influence of their molecular arrangement, which is isotropic for slow (spontaneous)

thickness and fabrication conditions, especially the pH of the assembling/rinsing solutions.

In contrast to previous reports in the literature, we did not find that all layers keep the same

relative orientation, leading to a linear increase of the optical nonlinearity with thickness.

Instead, we find that for films fabricated at low or high pH, the nonlinearity tends to

decrease for thick films (~10 bilayers). Films fabricated at neutral pH generate an SHG

We have also investigated how the molecular ordering depends on the film

drying, while it becomes anisotropic and inhomogeneous with nitrogen-flow drying.

20 40 60 180

150

210

0

 SS SP

330

30

60

300

90

(PAH/PS-119)1 After slow cooling

270

120

240

micrometers) with preferential orientation along the substrate plane.

0

330

30

micrometers) with preferential orientation along the substrate plane.

60

300

anisotropic and inhomogeneous with nitrogen-flow drying.

90

(PAH/PS-119)1 Before heating

270

**fabricated at pH 10, before and after heating to 190C.** 

120

240

before and after heating to 190°C.

SHG signal / arb. u.

**5. Conclusions** 

180

150

54 Advanced Electromagnetic Waves

210

**5. Conclusions**

Heurison S. Silva1\*, Irismar G. Paz1 and Paulo B. Miranda2

\*Address all correspondence to: heurison@ufpi.edu.br

1 Universidade Federal do Piauí, Departamento de Física, Campus Universitário Ministro Petrônio Portella, Teresina, PI, Brazil

2 Universidade de São Paulo, Instituto de Física de São Carlos, Departamento de Física e Ciência dos Materiais, São Carlos, SP, Brazil
