**3.2. Morphology**

**3. Properties of the deposited films**

crystalline structure during the PLD process.

Er3+ with the diffraction peaks attributed to the hexagonal *β*-phase of NaYF4.

Er3+and AZO.

**3.1. Crystalline structure of the inorganic additives in the polymer nano-composite film**

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

In order to conduct X-ray diffraction spectroscopy, the deposited films were separated from the substrates and placed in a Bruker D2 Phaser X-ray diffractometer. A reference sample of the PMMA nano-composite film containing only one upconversion phosphor additive was also made. The X-ray diffraction spectrum of this sample is presented in **Figure 10**. The spectrum has all the signatures of the hexagonal *β*-phase NaYF4 that was initially present in the first PLD target made of the upconversion phosphor. **Figure 11** shows the X-ray diffraction spectrum of the polymer nano-composite film including nanoparticles of both inorganic additives: the upconversion phosphor and AZO. The observed spectral peaks include those that can be attributed to both *β*-phase NaYF4 and AZO. It thus can be concluded that the two inorganic additives have been transferred to the polymer film without modification of their

**Figure 10.** XRD spectrum of the two-component composite film made of PMMA and the nanoparticles of NaYF4: Yb3+,

**Figure 11.** XRD spectrum of the three-component composite film made of PMMA and the nanoparticles of NaYF4: Yb3+,

**Figure 12** presents the high-resolution scanning electron microscope (SEM) image of the produced nano-composite film with a magnification of ×60 K. For the sake of convenience, the images of some exemplary nanoparticles are marked with arrows. The size of the nanoparticles varied widely in the range of the order of 10–200 nm. The nearly uniform distribution of the nanoparticles in the polymer film was occasionally disrupted by much larger particles or clusters (limited resolution of SEM prohibited distinguishing between large particles and the clusters of nanoparticles) of the order of 500–1000 nm.

**Figure 12.** Scanning electron microscopy (SEM) image of the three-component composite film made of PMMA and the nanoparticles of NaYF4: Yb3+, Er3+and AZO taken with magnification ×60,000. White arrows point to exemplary nanoparticles of various sizes (from ∼10 to 200 nm) embedded in the polymer matrix.

### **3.3. Fluorescence**

The deposited nano-composite films demonstrated visible upconversion fluorescence being pumped with a 980-nm IR radiation from a laser diode (PL980P330J from Thorlabs; 330-mW maximum power; quantum well laser chip, pigtailed with a wavelength stabilizing fiber Bragg grating). The spectrum of the upconversion emission was measured with a Princeton Instruments 500-mm focal-length Spectra Pro (SP–2500i) imaging spectrometer/monochromator equipped with 1200 gr/mm (blazed at 500 nm) holographic diffraction grating. The spectrum presented in **Figure 13** had peaks at 522, 540, and 656 nm and resembled the spectrum of the bulk phosphor target (**Figure 7**).

The upconversion fluorescence of the films was quantitatively characterized by the quantum efficiency (QE) *η*. QE is the ratio of the number of the photons of the upconversion radiation generated per unit of time *n*up to the number of the photons of the infrared pump radiation *η* = (*nup/npump*) × 100% [69]. The quantum efficiencies of green (at 522- and 540-nm spectral peaks combined) upconversion emission of the deposited three-component films (PMMAphosphor-AZO), two-component films (PMMA-phosphor), and the bulk phosphor target were measured to be of 0.072, 0.045, and 0.56% respectively. QE of the bulk phosphor PLD target compared well against the highest 3% quantum efficiency reported for similar 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 damage threshold of the nano-composite film.

**Figure 13.** Spectrum of upconversion emission of the PMMA nano-composite film with NaYF4: Yb3+, Er3+ and AZO additives excited with a 980-nm laser diode.

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 1-*μ*m diameter would contain 103 and 106 times more RE ions. The upconversion emission 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 using the above-described MAPLE/PLD method with a nano-composite film of PMMA + NaYF4: Yb3+, Er3+ + AZO pumped with a 980-nm laser diode (125-mW power). The tip of the fiber illuminated the white back side of a business card with visible upconversion light. The picture was taken with an iPhone 6 digital camera at dimmed room light.

**Figure 14.** Photograph of the tip of a single-mode fiber coated with a nano-composite film of PMMA + NaYF4: Yb3+, Er3+ + AZO pumped with a 980-nm laser diode (125-mW power) illuminating white back side of a business card.
