Fig. 1. Morphological and spectral characterizations. (a) SEM image to show large grains with micrometer sizes. (b) Optical absorption spectrum (inset: band edge with PL). (c) Up-conversion ASE under 700 nm excitation wavelength (threshold pumping fluence: 1.19 mJ/cm²). Inset: spectral narrowing with emission intensity simultaneously increasing phenomena. (d) Threshold dependence on excitation wavelength. Inset: luminescence above (the upper left) and below (the bottom right) threshold.

Fig. 2. Multi-photon up-conversion PL and intensity dependence. (a, d) 2-photon process under 900 nm excitation wavelength. (b),(e) 3-photon process under 1200 nm excitation wavelength. (c, f) 4-photon process under 1500 nm excitation wavelength.

Fig. 3. ASE measured with and without magnetic field in both up- and down-conversion regions. Magnetic field: 1.2 T. (a) Up-conversion ASE under 800 nm excitation wavelength. (b) Down-conversion ASE under 480 nm excitation wavelength. (c) Orbital order: orbitals are aligned through magnetic dipoles to form cooperative light-emitting states towards developing an ASE.

Fig. 4. Angle and polarization dependences for ASE and PL. (a) Setup for angle dependence measurement. (b) Spatial distributions of ASE and PL outputs. (c) Setup for polarization measurement. (d) Polarizations of ASE and PL outputs.