Fig. 1. Emission spectra of the LiF crystal sample under focused femtosecond laser irradiation and 448 nm monochromatic light excitation (using a 600 nm IR cut filter). Inset: The sample irradiated by a focused femtosecond laser in a darkroom.

Fig. 2. Absorption spectra of the LiF crystal before (black curve) and after (red curve) femtosecond laser irradiation. Inset: Spectral differences after and before femtosecond laser irradiation.

Fig. 3. Excitation and emission spectra for the femtosecond laser induced CCs in LiF crystals at room tempertaure. (λexcitation=448  nm, λemission=540  nm).

Fig. 4. (a) Illustration of upconversion luminescence of LiF by femtosecond laser irradiation. (b) Energy level diagram for F3+ CCs in LiF and the radiative (—), nonradiative (– – –), and relaxation (⋯) transitions by femtosecond laser irradiation.

Fig. 5. (a) Luminescence intensity of the LiF crystals as a function of femtosecond pump power. (b) Dependence of the upconversion luminescence efficiency on excitation power.