Temperature-Dependent Stimulated Emission Cross-Section in Nd3+:YLF Crystal

materials Article Temperature-Dependent Stimulated Emission Cross-Section in Nd3+:YLF Crystal Giorgio Turri 1, Scott Webster 2, Michael Bass 2 and Alessandra Toncelli 3,4,* 1 Mathematics and Science Department, Full Sail University, 3300 University Blvd, Winter Park, FL 32792, USA; [email protected] 2 CREOL, The College of Optics and Photonics, University of Central Florida, 4304 Scorpius St., Orlando, FL 32816, USA; [email protected] (S.W.); [email protected] (M.B.) 3 Physics Department, University of Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy 4 Italian National Research Council (CNR), Institute of Nanoscience (NANO), 56127 Pisa, Italy * Correspondence: [email protected] Abstract: Spectroscopic properties of neodymium-doped yttrium lithium fluoride were measured at different temperatures from 35 K to 350 K in specimens with 1 at% Nd3+ concentration. The absorption spectrum was measured at room temperature from 400 to 900 nm. The decay dynamics 4 of the F3/2 multiplet was investigated by measuring the fluorescence lifetime as a function of the sample temperature, and the radiative decay time was derived by extrapolation to 0 K. The 4 4 4 4 stimulated-emission cross-sections of the transitions from the F3/2 to the I9/2, I11/2, and I13/2 levels were obtained from the fluorescence spectrum measured at different temperatures, using the Aull–Jenssen technique. The results show consistency with most results previously published at room temperature, extending them over a broader range of temperatures. A semi-empirical formula for the magnitude of the stimulated-emission cross-section as a function of temperature in the 250 K 4 4 to 350 K temperature range, is presented for the most intense transitions to the I11/2 and I13/2 levels. Keywords: Nd:YLF; stimulated-emission cross-section; thermal effects; solid-state laser; rare-earth Citation: Turri, G.; Webster, S.; Bass, doped crystal; absorption; decay dynamics M.; Toncelli, A. Temperature- Dependent Stimulated Emission Cross-Section in Nd3+:YLF Crystal. Materials 2021, 14, 431. https:// 1. Introduction doi.org/10.3390/ma14020431 3+ Nd-doped Yttrium Lithium Fluoride (Nd :LiYF4) is one of the most commonly used Received: 30 November 2020 solid-state laser elements for its long fluorescence decay time at room temperature, weak Accepted: 6 January 2021 thermal lensing, and natural birefringence [1–4]. 4 4 Published: 16 January 2021 It is usually optically pumped to the F5/2 level, or directly to the F3/2 upper laser level, by 808 nm or 880 nm wavelength light, respectively, though recently laser operations Publisher’s Note: MDPI stays neu- have also been achieved with 908 nm wavelength pump [5]. 4 4 4 tral with regard to jurisdictional clai- The three main decay channels lead to the I13/2, I11/2, and I9/2 levels, with the ms in published maps and institutio- emission of light at ~1.3 µm; ~1.0 µm, and ~0.9 µm wavelengths, respectively. The most nal affiliations. common Nd:YLF lasers, by far, operate at 1047 nm (π-polarization) and 1053 nm (σ- polarization) because of the stronger intensity, but laser operation has also been achieved for the other two final laser levels [4,6–8]. Figure1 depicts the energy levels of interest for laser applications, as well as the transitions that will be investigated in this work. We Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. are reporting the Stark multiplet energies as published by Zhang [9], other authors have This article is an open access article reported slightly different values [10,11]. distributed under the terms and con- Most of the previous investigations of the spectroscopic properties of Nd:YLF have 3+ 4 ditions of the Creative Commons At- focused on the excitation and the decay of the Nd F3/2 level at room temperature. tribution (CC BY) license (https:// Although the decay dynamics of that level are in general considered well understood, there creativecommons.org/licenses/by/ is some inconsistency among the published values of the stimulated emission cross-section, 4.0/). especially when estimated with different techniques [12,13]. There are also disagreements Materials 2021, 14, 431. https://doi.org/10.3390/ma14020431 https://www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 12 of that level is in general considered well understood, there is some inconsistency among the Materials 2021, 14, 431 2 of 12 published values of the stimulated emission cross-section, especially when estimated with different techniques [12,13]. Also, there are disagreements about the values of the absorption coefficient, the absorption cross-section, and the general shape of the absorption spectrum in the 800 - 900 nm range, likelyabout caused the values by low of resolution, the absorption inaccurate coefficient, estimates the absorptionof the dopant cross-section, concentration, and or the the general presence of contaminantsshape of the [13-15].– absorption− spectrum in the 800–900 nm range, likely caused by low resolution, inaccurate estimates of the dopant concentration, or the presence of contaminants [13–15]. Energy [cm−1] −1 4 4 F7/2 S3/2 13,408–13,656 cm ~740 nm −1 4 2 12,535–12,829 cm F5/2 H9/2 ~800 nm −1 4 11,536–11,595 cm F3/2 ~870 nm 4 ~1350 nm −1 I13/2 3947–4240 cm −1 4 ~1060 nm 1997–2262 cm I11/2 ~870 nm 4 −1 I9/2 0–527 cm Figure 1. Partial energy level diagram of Nd:YLF crystal. Fig. 1. Partial energy level diagram of Nd:YLF crystal from [9]. At higher or lower than room temperature, the thermal expansion [16], the thermal lensing [17,18], and the wavelengths shift and width [10,11] have been modeled theoretically or measured experimentally. Still, very few investigations reported the emission spectrumAt higher variation or lower with than the temperatureroom temperature, and only the over thermal a limited expansion temperature [16], the range thermal [15,19 lensing]. [17,18],Knowledge and the of wavelengths the temperature shift dependenceand width [10,11 of the] have absorption, been modeled decay dynamics,theoretically and or emis-measured experimentally.sion of a rare-earth-doped Still, very few crystal investigations or glass, reported carries significantthe emission information spectrum variation for practical with the applications. By controlling its operation temperature, one can tune the emission wave- temperature and only over a limited temperature range [15,19]. Knowledge of the temperature length and intensity, or optimize the pump efficiency, of a solid-state laser [20,21]. Some dependencestudies have of alsothe absorption, shown the possibilitydecay dynamics of obtaining and em temperature-independentission of a rare-earth doped lasers crystal [22 ,23or ].glass, carriesFor laser significant using birefringentinformation for or triaxialpractical crystals applications. as a gain By controlling medium, one its operation can look fortemperature, tem- oneperatures can tune wherethe emission the emission wavelength at different and intensity, polarizations or optimize occurs the pump with efficiency, the same of intensity. a solid-state laserFollowing [20,21]. aSome 1969 studies pioneering have work also byshown Harmer the possibility [14], recently of obtaining Cho et al. temperature-independent [15] obtained simultaneous laser emission with the same intensity, orthogonally-polarized, at 1047 nm and lasers1053 [22,23]. nm by properlyFor lasercooling using birefringent the Nd:YLF or crystal triaxial at cryogeniccrystals as temperature. a gain medium, Optimization one can look of for temperaturesthe simultaneous where laser the performanceemission at wasdifferent accomplished polarizations by finely occurs tuning with the the operating same tem-intensity. Followingperature ofa 1969 the crystal.pioneering Still, work no detailed by Harmer investigation [14], recently of the Cho main et spectroscopical. [15] obtained parameters simultaneous laser(cross-section emission with and the lifetime same intensity, of the observed orthogonally-polarized, transition) has been at 1047 investigated. nm and 1053 nm by properly In this work, we present the emission spectrum and the stimulated emission cross- cooling the Nd:YLF crystal at cryogenic temperature.4 Optimization3+ of the simultaneous laser section of the three main decay channels of the F3/2 state of Nd in LiYF4, at different performancetemperatures was from accomplished ~35 K to ~350 by finely K. Moreover, tuning the we operating present semi-empiricaltemperature of the formulas crystal. that Still, no detailedallow oneinvestigation to estimate of the the stimulated main spectroscopic emission cross-sectionparameters (cross-section as a function and of the lifetime sample of the observedtemperature transition) for a fewhas been selected investigated. wavelengths of possible use for laser purposes. Finally, the absorption and the emission spectra collected in this work at room temperature will be used to discuss some of the partial inconsistencies reported in the literature. The stimulated emission cross-sections were obtained from the measured emission spectra using the Aull–Jenssen technique [24]; a detailed description of this method can be found in the investigations published by these authors for Nd:YAG [20] and Nd:YVO4 [25]. Materials 2021, 14, 431 3 of 12 2. Materials and Methods Two Nd:YLF samples were provided by VLOC (New Port Richey, FL, USA) and by ACMaterials (Tarpon Springs, FL, USA). Both were about 1 × 2.5 × 5 mm, with a nominal Nd3+ concentration of 1 at%, and were oriented with the optical c axis along the longest dimension. The absorption spectrum was measured at room temperature using a Cary 500 spectrometer (Agilent, Santa Clara, CA, USA). The resolution of the spectrometer was set to 0.5 nm full width at half maximum (FWHM), and the wavelength was changed at 0.125 nm intervals. The fluorescence emission spectra and lifetime measurements were performed in a pressurized helium cryostat combined with a heater and a temperature controller.

Load more