**5. Femtosecond laser-colorized indium-tin-oxide films for blue light attenuation and image screening**

This section further demonstrates the development of various nanostructuring patterns on ITO films by fs laser annealing. **Figure 8** presents the surface morphology of an untreated ITO film and five laser-annealed ITO films. **Table 1** summarizes the experimental conditions and the nanostructures on the film surfaces. Differ from the flat surface of as-deposited ITO film in **Figure 8a**, the UDL-1 presents a densely cotton-like structure on the surface when the fluence of 646.1 mJ/cm2 is employed (**Figure 8b**). By decreasing the laser fluence to 216.7 mJ/cm2 , the film has a brick-like structure. At a low fluence of 59.6 mJ/cm<sup>2</sup> , a regular ripple structure (composed of nano-bricks) is obtained on the surface (**Figure 8f**). **Figure 8c**–**f** shows all of the ripple structures (formed by nano-bricks), which were fabricated by scanning a laser spot along the y-axis and parallel to the polarization of laser beam. The spatial period along the y-axis is much smaller than the wavelength of the radiation, which is the so-called high spatial frequency LIPSS (HSFL). Usually, in transparent materials, HSFLs are generated under hundreds to thousands of ultra-short laser pulses [28] perpendicular to the direction of laser polarization with a fluence below the damage threshold of materials [29]. With a scanning speed of ~12.5 pulses/μm and a spot size of 21 μm in width, the ITO films were irradiated by about 260 pulses at a single point. All laser-annealed ITO films obtained surface nanostructures with HSFLs.

For sample UDL-5 in **Figure 8f**, the nano-bricks exhibited a length of ~250 nm (along the x-axis) and a width of ~70 nm (along the y-axis), and they are regularly separated by around 500 nm along the x-axis. This 500-nm-period structure, so-called low spatial frequency LIPSS (LSFL), is parallel to the polarization of laser beam. Usually, LSFL is observed on the surface of dielectric materials. The spatial period can be estimated by *Λ*parallel = *λ*/*n*, where *n* is the refractive index of

#### **Figure 8.**

*SEM images showing the morphology of the ITO films before and after fs laser annealing. (a) The surface morphology of an untreated ITO film. (b) A laser-annealed ITO film (UDL-1, fluence = 646 mJ/cm2 ) with scanning along the y-axis and polarization (blue arrow) parallel to the laser line-spot (red). (c–f) Laser-annealed ITO films [UDL-2 (fluence = 217 mJ/cm<sup>2</sup> ), UDL-3 (fluence = 197 mJ/cm2 ), UDL-4 (fluence = 68 mJ/cm<sup>2</sup> ), and UDL-5 (fluence = 60 mJ/cm2 )] with scanning along the y-axis and polarization (blue arrow) perpendicular to the laser line-spot (red) [14].*


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

#### **Table 1.**

*Laser fluence and the structure formation on the colorized ITO films.*

the dielectric material, when the photon energy is smaller than the bandgap of the material [30]. The LIPSS theory of Sipe et al. for transparent materials has also predicted the spatial period of *Λ*parallel = *λ*/*n* that is attributed to radiation remnants [31]. Radiation remnants could be generated at the solid/air interface by absorbing photons from the incident radiation and then transfer to the materials at the associated spatial frequencies. Additionally, when the photon energy is smaller than the bandgap of the material, electrons can be excited to higher energy levels by absorbing multiple photons. In other words, laser provides a sufficient strong electric field to drive the electrons to tunneling out and consequently induces the ablation on the surface of materials. The bandgap for ITO is around 3.7 eV [32], and refractive index *n* is ~1.60 at 800 nm [33]. Here the photon energy of the irradiated laser is 1.55 eV, so it is estimated that *Λ*parallel = 500 nm, agreeing well with the experimental result for sample UDL-5.

**Figure 9a** shows the laser fluence dependence of the colors of laser-annealed ITO films. The untreated ITO film exhibits purple color, while laser-annealed ITO films become cyan, yellow, or orange, depending on the nanostructures on their surfaces. The reflectance and transmittance spectra of all ITO films in the visible region were measured to clarify the origin of laser-colorized ITO films. For an untreated ITO film, the reflectance is relatively high for wavelength ranges below 425 nm and above 650 nm (**Figure 9b**). Thus, the untreated ITO film is purple. Intriguingly, the reflectance spectra for the laser-annealed ITO films are significantly different. For high fluence (UDL-1), the reflectance spectrum in a range of 450–600 nm is substantially higher than that of the untreated ITO film, with a 2.2-fold increase at around 550 nm; meanwhile, the reflectance spectra below 400 nm and above 650 nm reduce remarkably. Therefore, the laser-colorized ITO film of UDL-1 is cyan. When the laser fluence reduces, the broad main peak in **Figure 9c** gradually shifts from 550 to 575 nm, and thus, the color changes from cyan to yellow and then to orange.

As shown in **Figure 9d**, all of the transmittance spectra for ITO films are reduced after fs laser annealing. Interestingly, the transmittance reduction in the region of 390–480 nm is significantly greater than that in long-wavelength region. It is worthy of mentioning that blue light is dominated in the region of 390–480 nm, and it has been demonstrated to cause damage to eyes [34]. Among the treated samples, UDL-5 sample treated under low fluence offers a larger reduction in the blue-light region and a smaller suppression in the red-light region. This study has demonstrated that the fs laser-colorized ITO films can be directly used for LCD displays in which the films maintain their original function as an electrode and act as a blue light filter to protect eyes.

When laser-colorized ITO films are used in the LCD displays, the image displayed on the LCD can be selectively screened by varying the view angle. **Figure 10a** shows the schematics for testing this concept. If the view angle *θ* is close to 0 (i.e.,

watching the panel normally), the image on the panel is normal. However, the image on the panel is blocked by the strong reflected light with colors that depend on view angle *θ* as watching the panel obliquely (**Figure 10b**–**e**). As a result, due to strong reflection of certain colors, the image on the panel cannot be seen. This finding is potential for information security and the protection of privacy [14].

**Figure 9.**

*(a) The colors of ITO films before and after fs laser annealing. (b) The reflectance (R0) and transmittance (T0) spectra for an ITO film before fs laser annealing. (c) The ratio of the reflectance spectra for the ITO films before (R0) and after (R) fs laser annealing. (d) The ratio of the transmittance spectra for the ITO films before (T0) and after (T) fs laser annealing. The blue-shaded area covers the wavelengths that cause damage to eyes [14].*

#### **Figure 10.**

*(a) The schematics for capturing the images in (b–e), where θ is the view angle. (b–e) observing the image on a screen through an untreated ITO film and a laser-colorized ITO film (UDL-5) at various view angles [14].*

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