**4. Conclusions**

upconversion phosphor NaYF4: Yb3+, Er3+ in the literature [69]. QE of the polymer nanocomposite film containing only the nanoparticles of the upconversion phosphor was ∼12 times less than that of the bulk phosphor. On the other side, the nano-composite film deposited under similar conditions, but also containing AZO nanoparticles, had QE ∼1.6 times greater. This could be attributed to the plasmonic enhancement effect of the AZO nanoparticles on the local optical pump IR field. Since the upconversion emission is a twophoton process, QE could be increased proportional to approximately square of the pump power. In this experiment, the maximum pump power was limited to 150 mW, less than the

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

**Figure 13.** Spectrum of upconversion emission of the PMMA nano-composite film with NaYF4: Yb3+, Er3+ and AZO ad-

The reason for the nano-composite film having an order of magnitude weaker upconversion fluorescence than that of the bulk phosphor powder, besides a limited concentration of the phosphor nanoparticles in the polymer host, could be related to the size effect. Based on the doping rate of the RE ions in the phosphor (see Sub-Section 2.2.2.1) and the computational approach in [70], the average number of the Er3+ and Yb3+ ions in the particles of an average diameter of 10 nm could be estimated as 128 and 628, respectively. The particles of 100-nm and

involved two types of RE ions with the Yb3+ ion acting as a captor of the pump IR photons that later excited the Er3+ ion through the energy transfer process involving two IR photons, but not one. The more the RE ions contained in a phosphor particle, the stronger the upconversion emission would be. Accordingly, the nano-composite film including the nanoparticles of the upconversion phosphor of the size not exceeding ∼200 nm should expectedly have upconversion QE less than that of the bulk powder with significant presence of 1-μm and greater particles. Adding the nanoparticles of AZO compound to the polymer nano-composite film helped to partially compensate the drop of upconversion QE due to the plasmon enhancement of the local pump IR optical field. As an illustration of possible applications for upconversion fiber illuminators, **Figure 14** presents the photograph of the tip of a single-mode fiber coated

times more RE ions. The upconversion emission

and 106

damage threshold of the nano-composite film.

ditives excited with a 980-nm laser diode.

1-*μ*m diameter would contain 103

The multi-beam multi-target MAPLE/PLD method as a new variation of the film deposition via laser ablation makes possible to transfer concurrently several inorganic additives, such as an efficient upconversion RE inorganic phosphor and AZO compound, in nano-composite polymer films preserving crystalline structure of the additives and the upconversion emission properties. This is due to much better control of the deposition process of the materials of different nature from separate targets with different laser beams. The basic components of the new triple-beam triple-target MAPLE/PLD apparatus and the major process steps have been designed, built, and tested. The preliminary results indicated that adding AZO nanoparticles improved by a factor of 1.6 the upconversion quantum efficiency of the upconversion emission from the films possibly due to the plasmon enhancement of the local optical pump IR field in the vicinity of AZO nano-particles. The MBMT-MAPLE/PLD technique can find its applications in fabrication of multi-component polymer-inorganic composites with functionalities spanning emitting light, sensing bio- and chemical agents, energy harvesting, and other applications.
