  In this work, dot-sized upconversion nanocrystals (UCN-dots) with diameter
c.a. 3.4-0.15 nm have been synthesized. These UCN-dots exhibit visible emission
(at 497, 527 and 545 nm) under the excitation with 980 nm CW-NIR laser.
Further, these UCN-dots exhibit high energy upconversion emission (UV region,
206 to 231 nm) with Ti-Sapphire Femtosecond laser of 140-femtoseconds duration
at 80 MHz repetition rate at different excitation, which has never been
reported. This is interesting to report that the generation of high energy
UV-Vis emission and their shifting from 206 to 231 nm for the UCN-dots by
tuning the excitation wavelength ranging from 950 nm to 980 nm irradiated from
Ti: sapphire Femtosecond laser observed. We have demonstrated the generation of
high energy upconversions with change in energy band gaps as well as number of
absorbed photons per photon emitted under the Femtosecond-laser excitation
power. Additionally, we report the photo luminescence of UCN-dots in visible
range with 450 nm excitation wavelength exhibiting blue and red emission
(visible to visible). The generation of high energy up-conversion in UV-Vis
region could be useful for designing optoelectronic and biomedical devices for
therapeutic application.
