**2.2. Target materials**

The MBPT-MAPLE/PLD method was used to deposit thin films of a polymer nano-composite film with two inorganic additives: upconversion phosphor and the electro-optic enhancer of the upconversion emission.

### *2.2.1. MAPLE target*

The MAPLE target material was the solution of poly(methyl methacrylate) known as PMMA in chlorobenzene at a proportion of 1.0 g solids per 10 mL. The solution was filtered through a 0.2-micron filter.

### *2.2.2. Inorganic PLD targets*

### *2.2.2.1. Upconversion phosphor*

The first inorganic PLD target was made by compressing a powder of efficient upconversion phosphor NaYF4:Yb3+, Er3+ with a 25-ton hydraulic press. Upconversion phosphor is a material that absorbs low-energy photons, such as infrared (IR) ones, and re-emits high-energy photons of visible or ultra-violet light. The compounds of the rare earth (RE) elements are particularly attractive as upconversion phosphors with high efficiency of converting IR radiation to upconversion emission. The efficiency depends significantly on the host material for the RE ions. There is a group of efficient upconversion phosphors based on the hexagonal crystalline fluoride NaYF4 (*β*-NaYF4) as a host. This is due to the low phonon energy of its crystalline lattice that keeps at minimum the rate of the non-radiative multi-phonon relaxation of the excited RE dopant ions. The powder of phosphor NaYF4:Yb3+, Er3+ with the doping rates of Yb3+ and Er3+ 10 and 2%, respectively (10 ytterbium ions and 2 erbium ions per 100 ions of sodium), was synthesized using the wet method [55] and baked for 1 h at 400°C in open air to convert host NaYF4 to its hexagonal crystalline *β*-phase and maximize the upconversion efficiency. **Figure 6** presents the energy diagram of the phosphor and **Figure 7** its absorption/ fluorescence spectra [60]. The energy of the laser photons is mostly absorbed by the ions of Yb3+ acting as synthesizers and then transferred via the energy transfer process to the ions of Er3+, which generated the upconversion, higher energy photons due to the two-photon mechanism. The phosphor powder produced intense visible upconversion emission with two green spectral peaks at 515 and 535 nm and one red spectral peak at 653 nm being pumped with infrared (980 nm) radiation from a laser diode as presented in **Figure 7**. **Figure 8** presents the X-ray diffraction spectrum of the phosphor powder taken with Bruker D2 Phaser X-ray diffractometer. All the observed diffraction peaks can be attributed to *β*-NaYF4, thus indicating that this is the dominating crystalline phase of the powder.

**Figure 6.** Energy diagram of upconversion phosphor NaYF4: Yb3+, Er3+ [60].

**Figure 5.** View of the MAPLE target assembly removed from the vacuum chamber. The target is cooled with liquid

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

The MBPT-MAPLE/PLD method was used to deposit thin films of a polymer nano-composite film with two inorganic additives: upconversion phosphor and the electro-optic enhancer of

The MAPLE target material was the solution of poly(methyl methacrylate) known as PMMA in chlorobenzene at a proportion of 1.0 g solids per 10 mL. The solution was filtered through

The first inorganic PLD target was made by compressing a powder of efficient upconversion phosphor NaYF4:Yb3+, Er3+ with a 25-ton hydraulic press. Upconversion phosphor is a material that absorbs low-energy photons, such as infrared (IR) ones, and re-emits high-energy photons of visible or ultra-violet light. The compounds of the rare earth (RE) elements are particularly attractive as upconversion phosphors with high efficiency of converting IR radiation to upconversion emission. The efficiency depends significantly on the host material for the RE ions. There is a group of efficient upconversion phosphors based on the hexagonal crystalline fluoride NaYF4 (*β*-NaYF4) as a host. This is due to the low phonon energy of its crystalline lattice that keeps at minimum the rate of the non-radiative multi-phonon relaxation of the excited RE dopant ions. The powder of phosphor NaYF4:Yb3+, Er3+ with the doping rates of Yb3+ and Er3+ 10 and 2%, respectively (10 ytterbium ions and 2 erbium ions per 100 ions of sodium), was synthesized using the wet method [55] and baked for 1 h at 400°C in open air to convert host NaYF4 to its hexagonal crystalline *β*-phase and maximize the upconversion efficiency. **Figure 6** presents the energy diagram of the phosphor and **Figure 7** its absorption/ fluorescence spectra [60]. The energy of the laser photons is mostly absorbed by the ions of

nitrogen (LN).

**2.2. Target materials**

*2.2.1. MAPLE target*

a 0.2-micron filter.

the upconversion emission.

*2.2.2. Inorganic PLD targets*

*2.2.2.1. Upconversion phosphor*

**Figure 7.** Spectra of optical absorbance (dash-dotted line) and upconversion fluorescence (solid line) of phosphor NaYF4: Yb3+, Er3+ [60].

**Figure 8.** XRD spectrum of the upconversion phosphor powder baked for 1 h at 400°C with the diffraction peaks attributed to the hexagonal *β*-phase of NaYF4.

### *2.2.2.2. Electro-optic AZO compound*

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 to ZnO.

**Figure 9.** XRD spectrum of the prepared AZO PLD target with the diffraction peaks attributed to ZnO.
