Table 1.

UCNP-based functional systems for optical storage applications.

UCNP-Based Systems	Methods of Preparation	Properties	Ref.
UCNPs-1	Sol–gel pyrolysis technique	Stimulated absorption mechanism and emission in the range of visible light	[54]
UCNPs-2	Solvothermal method	Reducing the requirement of optical depletion for laser intensity	[55]
UCNPs-3	Oxygen-free hydrothermal protocol	Background-free and ultrahigh-sensitivity imaging	[56]
NaYF4 nanocrystals	Solvothermal method and crystal growth	Precise control of phase, size, and optical emission features	[57]
Different phases of NaYF4:Yb,Er	Thermal decomposition of metal oleate precursors	Defect-reduction strategies of preparing small and brighter UCNPs	[58]
UCNPs-4	Solvothermal method	Realizing the 28 nm super high-resolution	[30]
Lifetime-coded microspheres	Electrostatic interaction	Enlarging the optical multiplexing range by increasing the time dimension	[59]