## 2 Materials and Methods  
Eutectic rods of Co-doped MgO-MgAl\({}_{2}\)O\({}_{4}\) were obtained by directional solidification by using the LFZ method. There is only one eutectic point in the Al\({}_{2}\)O\({}_{3}\)-MgAl\({}_{2}\)O\({}_{4}\) system with a molar composition of 67.43%MgO-32.57%Al\({}_{2}\)O\({}_{3}\) and a melting temperature of 1995 \({}^{\circ}\)C [21]. Ceramic powders were prepared using a mixture of commercial powders of MgO (Sigma-Aldrich, Burlington, MA, United States, >99%), Al\({}_{2}\)O\({}_{3}\) (Sigma-Aldrich, Burlington, MA, United States, 99.99%) and Co\({}_{3}\)O\({}_{4}\) (Alfa Aesar, Ward Hill, MA, United States, 99.7%) with the compositions given in Table 1. Cylindrical feed-rods were fabricated by isostatically pressing the powder for 3 min at 200 MPa and sintering at 1500 \({}^{\circ}\)C during 12 h.  
Ceramic rods were directionally solidified in air using a continuous-wave CO\({}_{2}\) laser as a heating source. To eliminate the feed-rod porosity, different densification stages were applied at a low pulling rate (in the range 25-200 mm/h). The final directional solidification was always performed with the grown crystal travelling downwards to obtain bubble-free samples. Eutectic rods were grown using a processing rate of 50 and 750 mm/h at the final stage without rotation. The solidified rods had a final diameter in the range 2-2.5 mm. For comparison purposes, Co-doped MgAl\({}_{2}\)O\({}_{4}\) single crystals and undoped MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic rods were also grown in air using the same procedure and a growth rate of 50 mm/h and 100 mm/h, respectively. Due to some cobalt evaporation during the laser processing, the final dopant level could significantly vary from the nominal composition, being strongly dependent on the number and solidification rate of the densification stages. From now, the different processed ceramics will be referred to using the acronym Cox-y and Spinelx-y, for the MgO-MgAl\({}_{2}\)O\({}_{4}\) eutectic and MgAl\({}_{2}\)O\({}_{4}\) single crystal, respectively, where x is the cobalt content (%at Co) estimated by Energy-dispersive X-ray spectroscopy (EDS) and y is the processing rate (mm/h). Table 1 presents all the samples used in this research and their cobalt content, as determined by EDS, in the composite and in each eutectic phase. A cobalt-doped MgO single crystal (MgO:Co, courtesy of Prof. [PERSON]) was also used in the experiments. Its cobalt content was measured by EDS and is included in Table 1.  
Microstructural characterization was performed in polished transverse cross sections of rods by means of back-scattered electron images obtained in a Field Emission SEM (FE-SEM, model Carl Zeiss MERLIN). Specimens for this characterization were prepared using conventional metallographic methods. Semiquantitative compositional characterization was carried out by means of the EDS detector coupled to the FESEM. Thermal emission spectra were measured on the as-grown rods by heating the samples with the CO\({}_{2}\)-laser focused annularly on the sample surface. Emitted light was collected using an optical fiber and the emission spectrum was measured in the 850-2400 nm range using an NIR 256-2.5 spectrophotometer from Ocean Optics. The spectral sensitivity of the spectrometer was calibrated using a halogen lamp with a brightness temperature of 2968 K. The temperature of the samples was measured using a two-colour pyrometer (Impac, ISR12-LO MB33) and varied from 1000 \({}^{\circ}\)C to 1500 \({}^{\circ}\)C by changing the laser power. The optical absorption of the doped crystals, MgAl\({}_{2}\)O\({}_{4}\) and MgO, was measured with a Cary5000 UV-VIS-NIR spectrophotometer from Agilent Technologies. Polished slices of