Anti-escaping of incident laser in rare-earth doped fluoride ceramics with glass forming layer

Adaptive fluoride ceramic with glass forming layer (GCZBL-Er) used in laser anti-escaping has been prepared by one-step synthesis, and the thickness of glass layer is identified as ~0.41 mm. Blue, green and red emissions of Er3+/Yb3+ codoped fluoride ceramic (CZBL-Er) and glass layer (GZBL-Er) have been investigated under ~980 nm laser pumping. With the forming of thin glass layer on ceramic surface, the absorption intensities on diffuse reflection of GCZBL-Er at 974 nm and 1.53 μm increase by 48% and 53% than those of CZBL-Er. Excited by a 979 nm laser, the presence of the glass layer increases the absolute absorption rate in spectral power from 75% in CZBL-Er to 83% in GCZBL-Er, which is consistent with the improvement in the absorbed photon number. In addition, the quantum yield of GCZBL-Er complex is raised by 28.4% compared to the case of ceramic substrate by photon quantification. Intense absorption-conversion ability and efficient macroscopical anti-escaping effect confirm the superiority of ingenious structure in the fluoride ceramics with glass forming layer, which provides a new approach for developing the absorption-conversion materials of anti-NIR laser detection.


Discussion
Structure and morphology property. To reveal absorption-conversion efficiency for NIR detecting laser, 1.0 wt% ErF 3 and 2.0 wt% YbF 3 as dopants are introduced into fluorozirconate matrix and denoted as GC ZBL -Er, and individual glass and ceramic phase are labeled as G ZBL -Er and C ZBL -Er, respectively. XRD pattern of as-synthesized G ZBL -Er powder exhibits a broad diffuse scattering at lower angles rather than the narrow diffraction peaks for crystal phase, and the amorphous nature of the G ZBL -Er glass layer is well identified, as exhibited in Fig. 1(a). In addition, the detected diffraction peaks of C ZBL -Er are in accordance well with the standard BaZrF 6 (JCPDS 76-1699), and the derived cell parameters (a = 7.744 Å, b = 11.691 Å, c = 5.404 Å, α = β = γ = 90°) of C ZBL -Er are coincide with those of BaZrF 6 phase (a = 7.681 Å, b = 11.357 Å, c = 5.511 Å, α = β = γ = 90°), indicating the formation of the pure BaZrF 6 phase in upper layer of GC ZBL -Er composite. Meanwhile, a little difference in cell parameters is attributed to the crystalline environment variation and the lattice deformation with the introduction of Er 3+ and Yb 3+ ions to some extent.
The microstructure and morphology of as-synthesized GC ZBL -Er composite is explored, and the SEM image of the glass-ceramic transition region is displayed in Fig. 1(b). The interface between the glass and ceramic phase is obvious and uniform, and the thickness of the glass layer is measured to be ∼0.41 mm under the optical microscope. In addition, the grain size of the ceramics phase in Fig. 1(c,d) is identified to be 550 × 120 nm, and the crystalline phase of the rectangular structure with neat arrangement is further judged as the aggregation of several BaZrF 6 crystallites.
Under 980 nm laser excitation, the emission spectra of G ZBL -Er and C ZBL -Er powders with sundry pumping powers are depicted in Fig. 2(a,b), and four emission bands centered at 408, 522, 544 and 653 nm are attributed to f-f transitions 2 H 9/2 → 4 I 15/2 , 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 , respectively. As the excitation power increases, the intensity of each peak increases exponentially, and the upconversion emission intensity I lumin is proportional to the nth power of the 980 nm excitation intensity I excit , which can be simply expressed as ∝ I I n lumin e xcit , where I lumin is fluorescence intensity, I excit is excitation power and n is the number of 980 nm photons absorbed per visible photon emitted. In addition, the intense upconversion green and red emissions are confirmed to be two-photon absorption processes as indicated in Fig. 2(c,d), besides, rare 408 nm blue emission is identified as three-photon absorption process in the low phonon energy material. In fluorozirconate glass system, the polyhedral network structure is formed mainly through Zr−F−Zr bridging, promoting the G ZBL -Er sample with low phonon energy ∼570 cm −1 60,61 . The characteristic can reduce the probability of non-radiative transition, which reflects intuitively in 544 nm emission of Er 3+ . The fluorescence decay curves of the 4 S 3/2 level for G ZBL -Er and C ZBL -Er layer monitored at 544 nm are exhibited in the inset of Fig. 2(a,b). In addition, the fluorescent lifetimes (τ exp-avg ) of G ZBL -Er and C ZBL -Er are up to be 461.9 and 558.5 μs, respectively, which far exceed to 58 μs in Li 2 B 4 O 7 glass 62 , 26 μs in SrF 2 nanocrystals 63 , 333 μs in NaYF 4 64 and are close to 490 μs in oxyfluoride tellurite 65 , indicating that the low phonon-energy material contributes to photon releasing effectively.
Enhanced absorption effect and principle analysis of GC ZBL -er. Although the optical transition capability of the glass fluorescent material to NIR laser are not as intense as crystal materials, when a glass forming layer is compounded on fluoride ceramics, the situation will be reversed. As exhibited in Fig. 3(a), the absorption intensities of GC ZBL -Er at 974 and 1532 nm are ∼1.48 and ∼1.53 times higher than those in C ZBL -Er, meanwhile. Correspondingly, the derived reflectance curve shown in Fig. 3(b) more clearly indicates that the reflectivity is reduced from 50% to 36% with the existence of thin glass layer. The reflected laser intensity including the Fresnel reflection is uniformly distributed and fully recorded in the integrating sphere, and the intensity radio of the reflected laser to incident laser can analyze the reflection ability of the material more macroscopically. Besides, the molar absorption coefficient α OH can be used to evaluate the residual OH content in glass samples and is derived to be 0.91 cm −1 in 75TeO 2 -10ZnO-10Na 2 O-5GeO 2 glasses 66 , while the value in this work glass is as low as 0.57 cm −1 . The FT-IR spectrum of glass layer is shown in Fig. 3(c), and the low OH content is beneficial for anticipated photon emitting of this material. The apparent improvement can be attributed to the complex surface morphology of ceramic matrix and the intense dispersion effect of glass layer, and the schematic diagram of the absorption mechanism is shown in Fig. 3(d). When the detection laser is incident into the composite material GC ZBL -Er, it will be absorbed by the glass phase in the reflection process of the glass-ceramic transition region. Then the residual laser is re-reflected to the ceramic boundary owning to the specular effect of the glass layer, forming a multiple-cycle effect, which heightens the absorption ability effectively of GC ZBL -Er for NIR laser. In addition, the interface of the glass to ceramic region in schematic diagram is a rough outline, while the actual www.nature.com/scientificreports www.nature.com/scientificreports/ surface topography is more complicated in fact. So the absorption effect of NIR incident laser is greatly increased with the complexity of the reflection process in the GC ZBL -Er composite material. Just as the inset of Fig. 3(d), the facula area of GC ZBL -Er is bigger than C ZBL -Er and the luminous intensity is brighter at the same condition, showing that the GC ZBL -Er composite increases the absorption intensity effectively to the NIR incident laser with the forming of thin glass layer on ceramic surface. Besides, the surface of formed glass is quietly smooth and further can effectively solve the follow-up cleaning problems in application. These results indicate that the complex structure of GC ZBL -Er can be employed to enhance the absorption efficiency of ∼980 nm and ∼1.53 μm wavelengths, further exhibiting a laser anti-escaping effect.
Photon quantification on absorption-conversion potential of c ZBL -Er and GC ZBL -er. In order to quantitatively characterize on absorption-conversion behavior of C ZBL -Er and GC ZBL -Er for laser beam, integrating sphere coupled with a CCD detector is applied to measure the absolute spectral parameters, which provides external quantum yield (QY) to evaluate luminescence and laser materials. Figure 4 presents the spectral power distributions as a function of 979 nm laser pumping power in C ZBL -Er and GC ZBL -Er samples, and the measured excitation powers are selected as 33, 106, 264, 400, 549 and 701 mW, respectively. Here, to ensure the laser fully diverged in integrating sphere, each sample is placed obliquely at the same angle and keeps a distance from the laser head. Besides, the tilt angle of the sample, the divergence angle of the laser and the distance of the laser head to the sample are measured to derive the area of the laser spot. Based on the above, the laser excitation power densities of the sample are further determined to be 16,52,129,196, 269 and 344 mW/mm 2 , respectively. Taking the high-power 701 mW and low-power 106 mW incident laser as an example, the 980 nm incident laser, the residual lasers on the excited glass surface and ceramic surface are measured as shown in Fig. 5. Furthermore, the absorption ratio of C ZBL -Er to the incident laser is as high as 75.1% and 75.0% under 979 nm laser with 701 mW and 106 mW powers, respectively. Surprisingly, when the glass layer of GC ZBL -Er sample faces laser head, the absorption rates further rise to 83.4% and 82.9%, which is attributed to the intense dispersion of laser beam between glass and ceramic, proving that the special structure of composite glass layer is more suitable for the absorption of NIR laser light.
As a clear resolution of the photon number cumulative conversion effect, the photon number distribution can further elaborate the up-conversion emission law of the sample. The photon quantization is adopted to explain the enhanced absorption-conservation ability and anti-escaping effect of glass layer to NIR lasers. Based on the net spectral power distribution, the photon number distribution can be derived by , where λ is the wavelength, ν is the wavenumber, h is the Planck constant, c is the vacuum light velocity, and P(λ) is spectral power distribution. The net absorption and emission photon distribution curves of C ZBL -Er and GC ZBL -Er are derived as presented in Fig. 6, and   www.nature.com/scientificreports www.nature.com/scientificreports/ of Er 3+ , respectively. In addition, the intense UC 848 nm emission is not easy to obtain, which provides more sufficient approaches for the conservation of incident laser. Since the ceramic substrate composites the glass layer, the emission intensity of GC ZBL -Er at wavenumber is stronger than that in C ZBL -Er material. When laser power density is selected to be 129 mW/mm 2 , the net emission photons of four emissions at 522, 543, 665 and 848 nm are as high as 2.31 × 10 14 , 11.49 × 10 14 , 10.38 × 10 14 and 6.62 × 10 14 cps of C ZBL -Er, respectively. Moreover, with the formation of the glass layer, emission photons further improve and reach to be 3.56 × 10 14 , 16.48 × 10 14 , 13.24 × 10 14 , 8.75 × 10 14 cps in GC ZBL -Er, respectively. In addition, the enhanced percentage of total emitted photon number shows a trend of improving first and then decreasing slightly with the increase of laser pumping power. Figure 7(a,b) show the emission photon number of C ZBL -Er and GC ZBL -Er under the 979 nm laser with different excitation power density, where the rising tendency of the above four emission photons become dramatically severe, manifesting that the two-photon-excited luminescence has a positive dependency on the excitation power density.
The photoluminescence quantum yield (QY) is conducive to judge the luminous characters of optical materials, which provides a direct evaluation for the laser absorption-conversion efficiency. Thus the absolute fluorescence parameter of QY for C ZBL -Er and GC ZBL -Er are carried out based on = QY N N / em abs , where the N abs and N em represent the net absorption photon number and net emission photon number The QYs for green, red and NIR UC emission of C ZBL -Er and GC ZBL -Er under 979 nm NIR laser with different pumping power densities are listed in Table 1 and illustrated in Fig. 7(c,d). As listed in Table 2, the total QYs of C ZBL -Er and GC ZBL -Er reach to be 0.86 × 10 −4 ∼11.74 × 10 −4 and 0.96 × 10 −4 ∼14.67 × 10 −4 , respectively. The QY of the GC ZBL -Er up to 7.96 × 10 −4 is solved under the excitation of 129 mW/mm 2 power density, which is 28.4% more than that of C ZBL -Er. The QYs of green and red emissions from Er 3+ and Ho 3+ in different glass matrices are listed in Table 3. As can be seen from the data, the high quantum yield in GC ZBL -Er sample is over ten times higher than the values of BALMT glass, NMAG glass, BZYTLE glass and other oxide glasses [67][68][69][70] . However, the quantum yield in fluoride glass exceeds than that of GC ZBL -Er 67 , which is attributed to the superior absorption capacity of the composite with  www.nature.com/scientificreports www.nature.com/scientificreports/ special structure. With the enhancing incident laser power densities, the number of photons emitted increases exponentially, and the quantum yield improves continuously, which indicates that both absorption and emission of C ZBL -Er and GC ZBL -Er for NIR laser are still not saturated. Taken together, these results manifests that the forming of glass layer on ceramic substrate not only improves the absorption for NIR laser by the multi-reflection process, but also greatly enhances the optical-conversion ability, which confirms the GC ZBL -Er complex processes a potential applied in anti-escaping of incident laser. www.nature.com/scientificreports www.nature.com/scientificreports/

Conclusion
Multi-photon-excited blue, green, red and NIR emissions have been quantified in Er 3+ /Yb 3+ doped fluoride ceramic (C ZBL -Er) and glass layer (G ZBL -Er). The fluorescent lifetimes of G ZBL -Er and C ZBL -Er are up to be 461.9 and 558.5 μs, which indicates the fluorozirconate system can achieve effective photon releasing due to low maximum phonon energy. The absorption intensity of GC ZBL -Er at 974 and 1532 nm are determined to be ∼1.48 and ∼1.53 times higher than those in C ZBL -Er, and the absorption enhancement is attributed to the reflection of complex surface morphology on ceramic substrates and the diffusion absorption of glass layers. With the forming of  thin glass film on ceramic surface, net absorption power and net absorption photon number of GC ZBL -Er exhibit an increase of ~10% by photon quantization in the integrating sphere. Corresponding the emission photon number and quantum yield enhance by 40% and 28%, respectively, and the higher photon release efficiency further implies the superiority of the special composite structure in light conversion. The high absorption-conversation efficiency attributed to the complex structure of transition layer confirms the macroscopical anti-escaping effect in GC ZBL -Er, which provides a reliable approach for anti-NIR laser detection.

Methods
Prototype design and fabrication of C ZBL -Er and GC ZBL -er. The fluoride ceramic-based composite glass layers were prepared based on the molar host composition of 60ZrF 4 −30BaF 2 −10LaF 3 (ZBL) via the meltquench method in reducing atmosphere. In addition, 1.0 wt% ErF 3 and 2.0 wt% YbF 3 as dopants were introduced into ZBL matrix and denoted as GC ZBL -Er, and the individual glass phase and ceramic phase were labeled as G ZBL -Er and C ZBL -Er, respectively. The high-purity fluoride raw materials were melted at 900 °C for 5 min in a platinum crucible, and then the molten glasses were poured into a metal mold in a dry air atmosphere. Here, the lower liquid contacting with aluminum plate rapidly formed an ultrathin glass layer owing to the process of efficient heat conduction, where the metal mold quickly was taken away a lot of heat. Correspondingly, the upper liquid itself provided the energy needed for glass crystallization, which greatly promoted the formation of crystals and the adhesion of glass layers. Subsequently, all samples were annealed at 260 °C for 2 h, and then cooled down slowly to room temperature inside the furnace. For optical measurements, the annealed samples were sliced into pieces and polished into pieces with parallel sides.

Measurement and characterization.
The amorphous nature of G ZBL -Er and the crystal structure of C ZBL -Er were identified utilizing a Shimadzu XRD-7000 diffractometer with Cu-Kα radiation (λ = 1.5406 Å) operated at 40 kV and 30 mA. The morphological behaviors for the section of GC ZBL -Er were observed by a field-emission scanning electron microscope (SEM instrument, JEOL JSM-7800F). The transmittance spectra of glass were recorded by a Perkin-Elmer FTIR/NIR Spectrometer (FTIR). The thickness of the glass layer of the GC ZBL -Er was measured by a fluorescence microscope (Imaging system CK-500). Visible fluorescence spectra and fluorescence decay curves were determined by a Hitachi F-7000 fluorescence spectrophotometer equipped with an R928 photomultiplier tube (PMT) as a detector and a commercial Xe-lamp as an excitation source. Diffuse reflectance spectra of samples were recorded by Shimadzu corporation UV3600 spectrophotometer, and