Study On The Mechanism Of Dielectric Superlens Enhancing Optical Temperature Sensitivity Of Upconversion Nanocrystals

Temperature measurement technology plays an important role in ensuring product quality,saving energy,promoting national economic development and providing basic diagnostic standards for biomedical field.Recently,the fluorescence intensity ratio(FIR)technique based on the thermally coupled energy levels of rare earth ions has been considered as a reliable and promising optical temperature sensing method due to its high accuracy and high stability.The main research content of this paper is the optical temperature sensing technology based on upconversion luminescent nanoparticles(UCNPs),using fir as the optical temperature measurement scheme.The main purpose of the research is to design multifunctional nanocomposites based on UCNPs and improve the sensitivity of optical temperature measurement.The main contents of this paper are as follows:(1)We spin coated UCNPs on the aluminum flake,and then covered the top with a layer of dense polystyrene microspheres(PS spheres)to form a composite film.The near-field distribution is simulated by the finite-difference time-domain(FDTD)method with an incident angle of 45°.The wavefront of the plane wave in the ultraviolet region can be effectively transformed into a convergent wave after passing through the PS sphere,and the convergent upconversion(UC)emission can be significantly enhanced in the far field after passing through the PS spheres superlens.The UC fluorescence spectra of the composite films irradiated by 980 nm near-infrared laser with different incident angles(?=30°,35°,40°and 45°)show that all UV emission peaks are significantly enhanced by the dielectric superlensing effect when?exceeds40°.(2)We report the enhancement of the optical temperature sensitivity based on UCNPs by dielectric superlensing modulation.First,UCNPs were spin coated on the aluminum flake,and then covered with a layer of PS spheres to form a composite film.Under 980 nm near infrared laser excitation,the FIR of ~2H_11/2?~4I_15/2(522 nm)and~4S_3/2?~4I_15/2(542 nm)transitions of Er³⁺ions are very sensitive to temperature.When the temperature is 363 K,the temperature sensitivity of the composite film is 43%higher than that of UCNPs film.In Tm³⁺doped UCNPs,the FIR of ~1I₆?~3H₆(310 nm)and ~1D₂?~3H₆(360 nm)transitions of Tm³⁺ions are very sensitive to temperature.When the temperature is 350 K,the temperature sensitivity of the composite film is more than70%higher than that of UCNPs film.This is due to the modulation of UC emission peak wavefront by dielectric superlensing effect.(3)In order to improve the sensitivity of optical thermometer of UCNPs,we designed composite nanomaterials based on the dual modulation of local surface plasmon resonance(LSPR)effect and dielectric superlensing effect.PS spheres array as a superlens significantly enhances the photon upconversion process of UCNPs.LSPR effect makes silver nanoparticles(Ag NPs)absorb the photon energy of UCNPs.Through the dielectric superlensing effect and LSPR effect,the FIR of ~1I₆?~3H₆(310nm)and ~1D₂?~3H₆(360 nm)transitions of Tm³⁺ions are very sensitive to temperature.When the temperature is 363 K,the temperature sensitivity of Ag NPs/UCNPs/PS spheres composite film is higher than that of UCNPs film by more than 424%.