**4. Anisotropic optical transmission of femtosecond laser-induced periodic surface nanostructures on indium-tin-oxide films**

This section focuses on the anisotropic optical properties of the nanostructures, including nanodots and nanolines, on ITO films fabricated by fs laser irradiation [12]. **Figure 6a** and **b** shows the surface morphology of nanodots and nanolines on ITO films. The solid line in **Figure 6c** presents the optical transmission of an

as-deposited ITO film in the visible range of nonpolarized light. The transmission spectra of the fs laser-treated ITO films were measured using polarized light with L//P or L⊥P, as shown in **Figure 6e**. The transmission of L//P (*T*L//P) for visible light was lower than that of L⊥P (*TL*⊥*<sup>P</sup>*) in the ITO films with structures of nanodots and nanolines in which the nanoline structure exhibited larger difference between the transmittances of *TL*//*P* and *TL*⊥*<sup>P</sup>* in visible range than that of nanodot structure. Indeed, the extinction ratio (*TL*⊥*<sup>P</sup>*/*TL*//*P*) in the nanoline film was enhanced by 42% at a wavelength of 400 nm (**Figure 6d**). The laser-induced periodic nanostructures on the film surfaces induced this dichroic or anisotropic transmission property for the fs laser-treated ITO films.

A schematic illustration for the dichroic optical property of the nanoline film is shown in the inset of **Figure 6c**. The vertically polarized light (PV), which is parallel to the long axis of the nanoline structure (L), was significantly blocked by the nanostructured ITO films. Meanwhile, the horizontally polarized light (PH) can pass through the nanoline structure, which is perpendicular to the long axis of the nanoline structure (L). These results demonstrate that anisotropic optical transmission can be simply induced and manipulated by using ITO films treated under fs laser irradiation. This property may have potential in applications such as optical devices for polarization control in the visible range [26]. We note that the anisotropic reflection property of the nanostructured ITO films was not evident because the films had a relatively low reflectance compared to the transmittance.

#### **Figure 6.**

*(a and b) SEM images of nanodots and nanolines on ITO films. The arrows indicate the polarization direction of the irradiated fs laser. (c) The optical transmission spectra of as-deposited ITO and fs laser-treated films with the structures of nanodot and nanoline; the inset in (c) shows a schematic illustration of the two forms of polarized light (PV and PH) passing through a fs laser-treated ITO film. L: the directions of nanolines. (d) The extinction ratio (T <sup>L</sup>*⊥*P/T L//P) for the fs laser-treated films with structures of nanodot and nanoline. T <sup>L</sup>*⊥*<sup>P</sup> (T L//P) is the transmittance in the configuration, L*⊥*P (L//P), as shown in (e) [12].*

#### *Nanostructuring Indium-Tin-Oxide Thin Films by Femtosecond Laser Processing DOI: http://dx.doi.org/10.5772/intechopen.82790*

For the case of L//P in **Figure 6e**, the metallic and periodic nanoline structure on the ITO films reflects or absorbs the incoming electromagnetic (EM) wave due to the movement of electron along the metallic nanolines and Joule heating loss, consequently leading to blocking of the EM wave. However, the movement of electrons along the metallic nanolines is not in the same manner for the case of EM wave with L⊥P (**Figure 6f**). The loss caused by Joule heating and reflection is limited, and thus, the EM wave is transmitted highly through the ITO films with periodic nanodot or nanoline structures (**Figure 6f**).

To get an insight into the responsible mechanism for the anisotropic optical properties, the compositions of fs laser-treated ITO films were investigated using AES. **Figure 7a** shows the first derivative (*dN*/*dE*) AES peaks (i.e., In(MNN) at 410 eV, Sn(MNN) at 433 eV, and O(KLL) at 519 eV [27]) of an as-deposited film and a nanoline film. The point A (outside a nanoline) and point B (inside a nanoline) are shown by the SEM image in **Figure 7b**. Obviously, for the fs lasertreated ITO film, the *dN*/*dE* signals of In(MNN), Sn(MNN), and O(KLL) slightly reduced at point A as compared with those of as-deposited ITO film. Meanwhile, the *dN*/*dE* signals (top-pink line in **Figure 7a**) reduced remarkably at point B (inside a nanoline, see **Figure 7b**). The reduction in AES signals indicates that the composition of fs laser-treated ITO films is modified, especially at the nanoline locations. According to the results in the previous section and further examination [12], the surface compositions at nanodot and nanoline structures deviated from the stoichiometry of ITO and are metal-like area in particular. Consequently, the ITO films with a nanoline structure functioned as a grid of regular metallic wires, which induced the movement of electron along the metallic nanolines and Joule heating loss to block the EM wave for the case of L//P. However, the energy loss caused by Joule heating and reflection is limited for L⊥P because an EM wave cannot induce electron movement along the metallic nanolines in the same manner to result in high EM wave transmission. Since the total area of nanoline is larger than that of nanodot, the anisotropic optical property of the former is more critical than the latter [12].

In summary, the fs laser-treated ITO films with the structures of nanodot and nanoline on the surfaces possessed anisotropic optical transmission properties (i.e., *TL*⊥*<sup>P</sup>* <sup>&</sup>gt; *TL*//*P*). The fs laser-treated ITO films may be applied to optoelectronics as the polarizing optical elements and smart window technology in the visible spectroscopy.

#### **Figure 7.**

*(a) The first derivative (dN/dE) of AES peaks, In (MNN), Sn (MNN), and O (KLL), measured for an as-deposited ITO film and a fs laser-treated ITO film with a nanoline structure. The point A (outside a nanoline) and point B (inside a nanoline) correspond to the arrows marked in (b), the SEM image of a fs laser-treated ITO film [12].*
