Nanocrystalline (Ho 0.03 Y 0.97 ) 2 Hf 2 O 7 luminophore for near-and mid-infrared

. In recent years, a great scientific effort was dedicated to extending the operating range of lasers and amplifiers beyond the conventional 1.5 – 2.0 μ m. Lasers operating in the mid-infrared range 2 – 5 μ m find various applications as LIDARs, sensors, medicine, etc. However, the commonly used silica glass is unsuitable for emission above 2 μ m due to the high phonon energy of the silica lattice, which completely quenches emission at longer wavelengths. The materials based on crystalline structure are perspective low-phonon materials for laser operation in the mid-infrared rage. In this contribution, we present the preparation and properties of a novel laser-active material based on holmium-doped yttrium-hafniate (Ho 0.03 Y 0.97 ) 2 Hf 2 O 7 . The emission around 3 μ m is successfully demonstrated.


Introduction
Lasers and amplifiers are one of the major scientific breakthroughs of the 20 th century.In recent years, a great scientific effort was focused on expanding the operating range of lasers and amplifiers beyond the commonly used 1.5 -2.0 μm window.Lasers operating in the mid-infrared (MIR) region, 2.5 -5.0 μm find applications as LIDARs, sensors or in medicine [1].However, the commonly used silica glass is unsuitable for use in the MIR region due to low transparency and high phonon energy, which leads to quenching of emission above 2 μm [2].The intensive material research brought forward a wide range of lowphonon, highly transparent materials, such as chalcogenide, telluride, or germanate glass systems [3,4].However, these so-called "soft" glasses suffer from various drawbacks, such as low thermal stability or tendency towards chemical corrosion.Crystalline pyrochlores with the general formula A2X2O7, where A = RE, Y, and X = Ti, Zr, Hf, represent a perspective material for lasers operating in MIR region owing to the low phonon energy, excellent thermal stability and resistance against chemical corrosion [5,6].In this work, we report on the preparation of Ho-doped yttrium-hafniate, (Ho0.03Y0.97)2Hf2O7.The material was prepared by the solgel method, the structure was characterized by XRD, and the luminescence properties of Ho 3+ ions around 2 μm and 3 μm were measured.

Experimental
The nanocrystalline samples were prepared by sol-gel synthesis followed by thermal treatment similar to a previously described procedure [7].The starting compounds, hafnium butoxide, yttrium chloride hexahydrate YCl3.6H2O and holmium chloride hexahydrate HoCl3.3H2O were dispersed in propanol and stirred at 120 °C for 3 hours.The gel-like powders were obtained by vacuum evaporation of propanol, and heat treated at 1000 or 1500 °C for 1 hour.
X-Ray Diffraction (XRD) analysis was performed on Bruker D2 Phaser using standard Bragg-Brentano geometry and Co-Kα radiation.The emission spectra were measured on a FTIR spectrometer Nicolet 8700, the fluorescence decay curves were measured on fluorolog Horiba JobinYvon.The samples were irradiated by a 1.2W diode emitting at 450 nm.

Results
Following the heat treatment at 1000 °C, the powder remained unsintered.After heat treatment at 1500 °C, the powder sintered in a compact bulk, the XRD spectrum is shown in Fig. 1.The XRD peaks correspond to a cubic phase of hafniate, Y2Hf2O7.Fig. 2 shows the emission spectrum recorded in the NIR range around 2 µm.A typical emission band can be seen in the spectra, which can be ascribed to the 5 I7→ 5 I8 transition of Ho 3+ ions.The intensity of the emission increased several-fold from 1000 °C to 1500 °C, which suggests an improved incorporation of Ho 3+ ions in the sintered bulk.However, it must be noted that the emission band exhibited a distinct lack of Stark splitting compared to other pyrochlores, e.g.Ho-doped Y2Ti2O7, which suggests the placement of Ho 3+ ions in a lower symmetry environment [6].A possible explanation lies in the preferential incorporation of Ho 3+ ions at the grain boundaries or residual amorphous phase, rather than the crystalline grains of Y2Hf2O7 themselves.The decay curves of the 2 µm emission are shown in Fig. 3.The curve of the 1000 °C powder is multiexponential and the decay is fast due to the imperfect incorporation of Ho 3+ ions in the matrix.In the 1500 °C bulk, the decay curve is single exponential, which suggests a homogenous distribution of Ho 3+ ions.The sample exhibits long fluorescence lifetime of 4.2 ms, several times higher than achieved in silicate glass (typically 0.8 -1.5 ms [2]).The material is thus perspective for lasers operating around 2 µm.The bulk sample heat treated at 1500 °C exhibited intense emission around 2.8 µm, owing to the 5 I6→ 5 I7 transition of the Ho 3+ ion.The hafniate pyrochlore material is thus perspective for application in lasers or amplifiers operating in the MIR region around 3 µm.

Conclusion
A highly crystalline (Ho0.03Y0.97)2Hf2O7was prepared by the sol-gel method.The material exhibited intense emission around 2.0 and 2.8 μm with long fluorescence lifetime, making it perspective for near-and mid-infrared lasers.The hafniate pyrochlore represents a suitable alternative to conventionally used silica glass, or "soft" glass systems such as chalcogenide, telluride or fluoride.

Fig. 4
Fig. 4 shows the emission spectrum recorded in the MIR range around 3 µm.Only a weak emission can be