Transparent ceramics for high-power infrared lasers operating above 2 um

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IEEE ICTON 2024* The final version is available at: https://doi.org/10.1109/ICTON62926.2024.10647861 Transparent ceramics for high-power infrared lasers operating above 2 m Jan Mrázek*, Ondřej Podrazký, Petr Vařák, Ivo Bartoň, Jana Proboštová Institute of Photonics and Electronics of the Czech Academy of Sciences, Chaberska 57, 182 57 Prague 8, Czech Republic Tel: (0420) 266 773 558, Fax: (0420) 284 680 222, e-mail: [email protected] ABSTRACT Solid-state lasers are a key part of advanced devices and great efforts has devoted to improving their performance, including the research of novel materials as a powerful alternative to conventional glass or single-crystals. We demonstrated a versatile sol-gel approach to nanocrystalline (Ho0.03La0.97)2Zr2O7 thin films. We proved Ho3+ ions regularly substituted La3+ ions in the host matrix. The residual -OH groups were presented in the samples bonded on La3+ ions. The highest phonon energy was 653 cm-1 allowing the luminescence up to 3.06 m. The film of the thickness of 380 nm showed a refractive index of 2.28 at a wavelength of 632 nm. The sample showed luminescence spectra characteristic for Ho3+ ions. The emission maximum was observed for the electronic transition 5I7→ 5I8 at 2.1 m. The luminescence intensity of the electronic transition 5I6→ 5I7 at 2.9 m reached 7% of the maxima. The luminescence decay time recorded at 2.1 m and 2.9 m was 6.814 ms and 0.565 ms, respectively. High thermal and chemical stability of proposed materials, together with advanced processing methods open new horizons in technology of the mid-infrared active waveguides and fibers with improved lasing efficiency and stability. Keywords: nanocrystals, rare-earth, holmium, luminescence, infrared, thin films. 1. INTRODUCTION Solid-state lasers are a key part of advanced devices and great efforts has devoted to improving their performance, including output power, operation wavelength and efficiency. Special attention has been paid to the research of novel materials that allow to shift the laser wavelength into the infrared region and to use lasers as a powerful source in the construction of LIDARs, tracking systems and sensors [1, 2]. Luminescence of rare earth ions distributed in a host matrix has been widely used for a construction of lasers. However, lack of suitable host materials limits the application in infrared. The conventional silica glass exhibit a continuous absorption band above 2.6 mm [3]. The chalcogenide or fluoride glass provide a suitable alternative allowing to tailor the transmission region by modifying the chemical composition of glass [4, 5]. However, their low temperature stability limits their use in high-power lasers where overheating can occur and damage the laser [6]. Nanocrystalline ceramic luminophores represents a promising alternative that can overcome these shortcomings. The common technology allow to produce single-crystal fibers [7]. However, this method is time consuming and the range of usable materials is limited by the equilibrium processes involved in single crystal growth [7]. Transparent ceramics represents a promising alternative to glass or single-crystals that attracted a huge scientific attention. It can be used as a highly temperature and chemically stable host matrix for rare earth elements and applied as active medium in solid-state lasers [8]. Therefore, the synthesis and evaluation of new active ceramic materials with improved luminescence properties to shift the laser wavelength towards the infrared region represents a challenge in current materials research. We present a versatile sol-gel approach to nanocrystalline films (Ho0.03La0.97)2Zr2O7. We studied the films’ structural and optical properties and we evaluated the steady-state and time-resolved luminescence above 2 m. The results can be applied for a construction of active waveguides and fibers operating above 2 m with improved lasing efficiency and stability. 2. EXPERIMENTAL To prepare a colloidal solution we used a colloidal approach established for the synthesis of pyrochlores [9]. A total of 1.683 g Ho(NO3)3·5H2O (99.9% Alfa Aesar) and 52.832 g La(NO3)3·6H2O (99.9% Alfa Aesar) were dissolved in a solution prepared by dissolving a total of 57 g Zirkonium(IV) butoxide (80% w/w solution in butanol, Alfa Aesar ) in 2 l of ethanol (99.9%, BC-Chemservis). The solution was boiled for 3 hours and then allowed to cool. The sol was dip-coated on the silica slide (TGP Ohio) heat-treated in a furnace at 1100 °C for 5 minutes under an oxygen flow of 50 sccm. X-ray diffraction (XRD) structural analysis was performed on a Bruker D8 diffractometer with BraggBrentano geometry using Cu-K radiation (=1.54060Å, operating voltage 10 kV, current 10 mA, integration time 0.2 s). Raman spectra were recorded on a Renishaw Invia apparatus. The samples were excited by a built-in laser emitting at 473 nm with an output power of 200 mW. The transmittance spectra were recorded on the set of fiber optic spectrometers USB 2000 and NIR 256-2.1 (Ocean Optics). To excite the luminescence, the samples were illuminated by a 1.2W laser diode emitting at 450 nm (Osram). Steady-state luminescence spectra were measured by a Nicolet 8700 FTIR spectrometer (Thermo Scientific) equipped with TEC InGaAs detector for the spectral range 1000-2600 nm and liquid nitrogen-cooled MCT detectors for the spectral range 2500-3000 nm. To collect the time-resolved luminescence a SWIR Longpass filter 880-2500 nm or a bandpass optical filter centred at 2950 nm (Edmund Optics) were used to filter the signal collected by a liquid nitrogen-cooled InAs detector P7163 (Hamamatsu). The 15 Hz LD excitation pulse was generated by the LD controller ITC4001 (Thorlabs) and the synchronized detector response was collected using a WaveAce 1000 oscilloscope (LeCroy). 3. RESULTS AND DISCUSSION Preciously tailored nanocrystal structure and Ho3+ ions uniformly distributed in the host lattice are a key factor for a successful synthesis of highly efficient luminophores [9]. Fig. 1a demonstrates the XRD record showing well-pronounced diffraction pattern that corresponded to La2Zr2O7 according to ICDD datafile N° 04-008-6353 crystallizing in a pyrochlore crystal lattice. The presence of side-formed phase was not observed in the sample. The diffraction peaks were slightly shifted to higher diffraction angles, indicating an increase in the crystal lattice size which can be explained by regular substitution of La3+ by Ho3+ ions. The nanocrystal nature of the samples caused the XRD peak broadening and diffractometer built-in software using the Scherrer equation determined a nanocrystal size of 40 nm. To confirm the crystal structure and to estimate the phonon energy of nanocrystalline (Ho0.03La0.97)2Zr2O7 we recorded the Raman spectrum, that is shown in Fig. 1b. The particular peaks correspond to the Raman spectra of materials crystallizing in pyrochlore crystal lattice [10]. The most intensive peak at 295 cm-1 was assigned to the F2g bending modes, the peak at 512 cm-1 was assigned to A1g stretching mode. The weakest peak around 738 cm-1 corresponded to the 2nd order scattering effects caused by high laser beam intensity. In addition to the peaks representing the pyrochlore phase, five peaks were observed in the spectrum at 392, 490, 567, 603 and 653 cm-1. The origin of these peaks can be found in a presence of distorted La-O bonds that are usually caused by the presence of residual -OH groups [11]. To prevent a non-radiative quenching caused by the phonon interactions between the RE manifolds, the phonon energy must be sufficiently small, typically less than 1/5 of the energy gap between the particular manifolds [12]. The presented value of 653 cm-1 is equal to a phonon 15 20 25 30 35 40 45 50 55 60 65 a) Intensity (a.u.) Diffraction Angle (deg) (444) (622) (440) (331) (400) (222) 200 300 400 500 600 700 800 La-O distorsion 2nd order scattering A1g Intensity (a.u.) Wavelength (nm) F2g La-O distorsion b) Fig. 1. Results of the structural analysis. a) X-ray diffraction record with denoted hkl indices proving the formation of (Ho0.03La0.97)2Zr2O7 nanocrystals. b) Raman spectra showing the formation of distorted La-O bonds in the pyrochlore lattice. energy of 0.08097 eV and fifth times higher value is 0.4049 eV. Thus, the (Ho0.03La0.97)2Zr2O7 lattice can show the luminescence up to 3.06 m. Transparent film technology is required for potential use in active optical waveguides. Fig. 2a shows the transmittance spectrum of the prepared film of a thickness of 380 nm. The spectrum showed several diffraction fringes providing the transmittance maximum of 92.71% at 1.504 m and minima of 63.81% and 66.0% at 0.601 and 0.976 m, respectively. The decreasing transmittance at the maxima of the interference fringes can be attributed to the contribution of Rayleigh scattering [9]. According to Swanepoel’s method [13] The minima of the interference fringes give the information about the refractive index of the film, which was 2.28 at a wavelength of 632 nm. Fig. 2b shows the luminescence spectrum recorded under excitation at 450 nm. Three emission bands were found out in the spectrum. First one localized around 1.2 m was the less intensive and corresponded to the electronic transition 5I6 → 5I8. The second one at 2.0 m was the most intensive and corresponded to the electronic transition 5I7 → 5I8. This wavelength is widely exploited for the construction of fiber lasers [14]. According to the observed low-phonon energy of (Ho0.03La0.97)2Zr2O7 lattice, the third emission band corresponding to the electronic transition 5I7 → 5I8 was found at 2.9 m. The time-resolved luminescence curves recorded for 5I7→5I8 and 5I6→ 5I7 electronic transitions at 2.1 m and 2.9 m showed a single-exponential course. The lifetime,  , was calculated according to equation (1). y= 𝐴 ∙ 𝑒− 𝑡 𝜏+ 𝑦0 , (1) 0 5 10 15 20 25 30 0.01 0.1 15I7→5I8 transition Normalized Intensity (log scale) Time (ms) Emission wavelength: 2.1 m Experimental data Single-exponential fit a) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.01 0.1 1 b) Normalized Intensity (log scale) 5I6→5I7 transition Emission wavelength: 2.9 m Experimental data Single-exponential fit Time (ms) Fig. 3. Experimental data and single-exponential decay curves recorded for a) 5I7→5I8 transition at 2.1 m and b) the 5I6→ 5I7 transition at 2.9 m. Fig. 2. a) Transmittance spectra of the reference SiO2 substrate and (Ho0.03La0.97)2Zr2O7 thin film. b) Steady-state luminescence spectra under 450nm excitation with inserted energy diagram of selected Ho3+ electronic transitions. 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 0 20 40 60 80 100 Transmittance (%) Wavelength (nm) blank SiO2 substrate film processed at 1100 °C a) 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 0.0 0.2 0.4 0.6 0.8 1.0 5I6→5I7 5I7→5I8 Intensity (a.u.) Wavelength (nm) 5I6→5I8 b) where t is a time, A is a pre-exponential factor and y0 is the intercept. The fitted parameters are summarized in Table 1. The radiative lifetime recorded at 2.1 m reached the value of 6.814 ms that is about 47% higher than the value of 4.6 ms reported for (Ho0.03Y0.97)2Ti2O7 [9]. The explanation can be found in lower phonon energy of lanthanum-doped matrix that support the radiative transitions. Extending the lifetime and increasing the luminescence efficiency can be achieved by further reducing the phonon energy of the matrix; for example, by replacing the La3+ with Lu3+ ions. The very same effect can be used for improving the luminescence at 2.9 m. In this case, the removing of residual -OH which strongly absorb in this region can be beneficial as well. High thermal and chemical stability of proposed materials, together with advanced processing methods that can be used to prepare highly transparent films [9] open new horizons in technology of the mid-infrared active waveguides and fibers with improved lasing efficiency and stability. Table. 1 Parameters of the single-exponential fits of time-resolved luminescence recorded at 2.1 m and 2.9 m. Transition Wavelength (m) A  (ms) y0 R2 5I7→5I8 2.1 1.052 6.814 -0.021 0.9996 5I6→ 5I7 2.9 0.997 0.565 0.003 0.9895 4. CONCLUSIONS We demonstrated a versatile sol-gel approach to nanocrystalline (Ho0.03La0.97)2Zr2O7 thin films. We proved Ho3+ ions regularly substituted La3+ ions in the host matrix. But, the residual -OH groups were presented in the samples bonded to La3+ ions. The highest phonon energy was 653 cm-1 allowing the luminescence up to 3.06 m. The sample showed luminescence spectra characteristic for Ho3+ ions. The emission maximum was observed for the electronic transition 5I7→ 5I8 at 2.1 m. The luminescence intensity of the electronic transition 5I6→ 5I7 at 2.9 m reached 7% of the maxima. The luminescence decay time recorded at 2.1 m and 2.9 m was 6.814 ms and 0.565 ms, respectively. High thermal and chemical stability of proposed materials, together with advanced processing methods open new horizons in technology of the mid-infrared active waveguides and fibers with improved lasing efficiency and stability. ACKNOWLEDGEMENTS The authors acknowledge the financial support of the Czech Science Foundation, project number 22-14200S dee. REFERENCES [1] Y. Liu, J. Q. Liu, and W. B. 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