Nanooptics and photonics

Laser induced anti-Stokes white emission nature from Yb-doped YAG nanopowders
Chaika M., Tomala R., Strek W.
1 Institute of Low Temperature and Structure Research PAS. Okolna 2, Wroclaw, 05-422, Poland. 
E-mail: m.chaika@intibs.pl
The laser Induced Broadband Emission (LIBE) has recently been detected by several groups as a source of sun-like emission spectra [3,4,5,6]. LIBE is a characteristic threshold phenomenon that occurs under a focused beam of continuous infrared laser. This phenomenon was detected in vacuum ambient; a decrease in ambient pressure below a certain point leads to disappearance of LIBE [4]. Several possible mechanisms of LIBE in different materials have been proposed. Due to the lack of experimental data, the mechanism behind this process is still not entirely understood. 
The laser induced broadband emission (LIBE) of (YbxY1-x)3Al5O12 nanocrystals was investigated in vacuum using the focused infrared laser beam. The samples were prepared via Pechini's technique. The different shapes of LIBE spectra were observed. The LIBE characteristics including an order of the process, rise and decay times, were independent on the type of emission spectra and concentration of Yb3+ ions. Intensity of the emission spectra decreased/increased with time after switching off/on the laser excitation, while the shape of the emission spectra remained approximately the same. This is consistent with the colour of the emission, which remains white even after significant decrease in intensity. The color of the sample in the emitting spot changes from white to dark blue caused by formation of Yb2+ ions. The generation of broadband emission begins after reaching a certain “time delay” in range 0-10 seconds.
Beforehand, several possible LIBE mechanisms have been proposed for different materials, however they don’t cover all aspects of this phenomenon. All of these mechanisms are based on multiphoton absorptions with further propulsion of an electron into the conduction band/upper state where the electron emit high energy photons. The most popular explanation of LIWE in RE-doped materials is Inter Valence Charge Transfer (IVCT) process in Yb2+/Yb3+ mixed valence pair for example. IVCT mechanism may be responsible for multiphoton absorptions of laser light and, as result, promotion of electrons to the conduction band with further radiative recombination.
This work was supported by Polish National Science Centre, grant: PRELUDIUM-18 2019/35/N/ST3/01018.