Recently, upconverting nanoparticles (UCNP) have been used in various applications like bioimaging, therapy, sensors, etc. It is preferred over conventional fluorophores in the biological applications because of the longer wavelength absorption for emitting in the visible region. Trapping of the UCNP from the non-invasive technique using the optical tweezers has brought versatility to its applications. Lanthanide based rare-earth doped NaYF4:Yb,Er UCNP of two different morphologies and sizes are synthesized for performing various experiments. It includes hexagonal and spherical shaped UCNP of sizes 4 µm and 120 nm respectively.

Our works involve development of techniques related to the fields of optical manipulation and absorption spectroscopy. Firstly, the hexagonal shaped UCNP is rotated in pitch sense (out-of-plane rotation) with the help of a thin film of gold coated cover slip. Stable and controlled pitch rotation is exhibited by the UCNP near the vicinity of the hot spot along the directions of the generated convection currents. The simplicity and generality of this technique make it a practical approach for rotating the nanoparticles and cells. In the second work, we found that the optically trapped UCNP exhibits Hot Brownian Motion (HBM) along the axial direction. Mean square displacement and position distribution show the deviation from the diffusive behavior for the active particle. In the third work, we have used a subdiffractive spherical shaped UCNP as the light source for absorption spectroscopy. The UCNP emits in narrow bandwidth at several wavelengths in the visible region. The process of resonance energy transfer has been demonstrated for rhodamine B dye, gold layer and soft oxometallate.​