**2.3. Deposition procedure**

**Figure 8.** XRD spectrum of the upconversion phosphor powder baked for 1 h at 400°C with the diffraction peaks

136 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

Aluminum-doped ZnO (AZO) compound is an optically transparent electric conductor with the surface plasmon resonance (SPR) enhancement of local optical field similar to the noble metals, but without significant optical losses attributed to them [65–68]. It can thus be expected that adding AZO nanoparticles to the upconversion phosphor in a polymer nano-composite film will bring the local enhancement of the pumping IR optical field in the vicinity of the phosphor nanoparticles and consequently increase the intensity of the upconversion emission. PLD target was a pellet of Zn0.98Al 0.02O where the aluminum fraction was 2% of the total by weight as compared with zinc, not counting the oxygen. The AZO pellet had 20 mm in diameter and 3 mm in thickness. The pellet was prepared by spark plasma sintering (SPS), also referred to as pulsed electric current sintering (PECS). In SPS pulsed DC current passed through a graphite die, as well as the AZO powder compact. Joule heating is the key mechanism in the densification of the powder compact. The densification resulted in near theoretical density at a lower sintering temperature than in conventional sintering. The heat is produced internally. This differs from the regular hot pressing with the heat generated by external source. Due to high heating/cooling rate (up to 1000 K/min) sintering was very fast (within few minutes). High speed of the process ensured it densified the powder without coarsening, which accompanied standard densification routes. While the term "spark plasma sintering" is commonly used, a literal interpretation of the term may be misleading since neither a spark nor a plasma is present in the process. **Figure 9** presents the X-ray diffraction spectrum of the AZO target taken with Bruker D2 Phaser X-ray diffractometer. All the observed diffraction peaks can be attributed

attributed to the hexagonal *β*-phase of NaYF4.

*2.2.2.2. Electro-optic AZO compound*

to ZnO.

The PMMA solution in chlorobenzene was poured in a copper cup of the MAPLE target assembly (**Figure 5**) and frozen with liquid nitrogen. The first PLD target was the pellet of the upconversion phosphor. The AZO pellet was the second PLD target. Two pulsed laser sources were used. The first was a Spectra Physics Quanta Ray Nd:YAG Q-switched Pro-250-50 laser with a pulse repetition rate of 50 Hz, 750-mJ energy per pulse (1064-nm wavelength) and 400 mJ energy per pulse at the 532-nm second harmonic. The second laser source (synchronized with the first laser) was a Spectra Physics Quanta Ray Nd:YAG Q-switched Lab-170-10 laser with a pulse repetition rate of 10 Hz, 850-mJ energy per pulse (1064-nm wavelength) and 450 mJ energy per pulse at the 532-nm second harmonic. The MAPLE target was evaporated with the 1064-nm beam from the first laser source and exposed to the fluence ranging from 0.84 to 2.4 J/cm2 per pulse. The first PLD target was concurrently ablated with the 532-nm frequency doubled Nd:YAG beam from the same source. The fluence was tuned up between 0.8 and 1.0 J/cm2 to keep the proportion of the upconversion material in the polymer film at approximately 5% by weight. The second PLD target was concurrently ablated with the 532-nm frequency doubled Nd:YAG beam from the second source. The fluence was tuned up between 1.0 and 2.2 J/cm2 to keep the proportion of the upconversion material in the polymer film at approximately 5% by weight. The polymer films were deposited on preoxidized Si (100) substrates with a SiO2 layer thickness of 2.0 μm. The deposition time was ∼3.0 min (until PLD targets started showing the signs of erosion). The thickness of the deposited films was between 180 and 200 nm as measured with an atomic force microscope.
