Study On The Enhancement Of Luminescence Properties Of Doped Nanocrystals Using Whispering Gallery Mode

Rare earth ions in doped nanocrystals can continuously absorb multiple photons to achieve upconversion luminescence from near infrared light to visible light.This luminescence has the advantages of large Stokes shifts,sharp emission peaks,long fluorescence lifetimes,high photochemical stability and low toxicity,making it suitable for applications in biomedicine,temperature sensing and lasers.However,the quantum efficiency of doped nanocrystal luminescence is often below 1%due to the small absorption cross section and the harsh energy leap conditions,which severely limits its application.In order to increase the luminescence efficiency and intensity of doped nanocrystals,many methods have been proposed at home and abroad,such as ion co-doping,core-shell structures,surface plasma enhancement and whispering gallery mode enhancement.Whispering gallery mode enhancement,with its advantages of high quality factor,high energy density and long photon lifetime,is an emerging enhancement method with great potential in recent years.Among the many studies using whispering gallery mode enhancement of doped nanocrystals,most of them seek higher quality factors,narrower and more pronounced mode linewidths,and thus microenvironmental temperature,humidity,p H,and particle sensing through mode shifts,spreading,and cleavage phenomena.However,the greatest advantage of doped nanocrystals lies in their upconversion luminescence properties rather than in sensing,which is only one major application area based on their upconversion luminescence properties.The reason for the current focus on sensing is that the sensing field does not require high luminescence intensity,but rather a sensitive response and a high signal-to-noise ratio.This study differs from other doped nanocrystal enhancement studies in the sensing field that focus on whispering gallery fundamental modes and lower order modes,but instead focuses on the entire whispering gallery mode cavity,using the abundance of modes in it to maximize the light coupled into the microsphere,converting it to upconversion light and increasing the overall upconversion luminescence intensity of the microcavity.The main works include:1)theβ-Na YF₄:Yb³⁺/Er³⁺doped nanocrystals were prepared by high temperature co-precipitation method,their luminescence mechanism was analyzed by energy level diagrams,their morphology was characterized by scanning electron microscopy and X-ray diffraction patterns,and their luminescence properties were tested by devices such as spectrometers and photodetectors;2)a process is proposed for the preparation of whispering gallery mode microsphere cavities in which doped nanocrystals can be introduced uniformly throughout;3)theoretically,the enhancement effect of whispering gallery mode microsphere cavities on the luminescence properties of doped nanocrystals was analyzed through the derivation of equations and simulations,and a test system for tapered fiber coupling excitation of the whispering gallery mode was built to measure and analyze the light intensity gain effect and spectral modulation effect of whispering gallery mode on the fabricated doped nanocrystals,and the enhancement effect was tested for sustainability.The experimental results show that:1)the fabricated whispering gallery mode microsphere cavities can significantly increase the upconversion luminescence intensity of the doped nanocrystals,where the green light at 541 nm is enhanced by a factor of 25 and the red light at 654 nm is enhanced by a factor of 72;2)the whispering gallery mode microsphere cavity has the ability to spectrally modulate the free spectral range of 7.6 nm for a 14-diameter microsphere;3)the proposed whispering gallery mode microsphere is capable of sustained upconversion luminescence for more than 1 min at a pump power of 2 W and is able to dynamically adjust the color of the upconversion luminescence from green to yellow depending on the pump power.The results could expand the application of doped nanocrystals in areas such as micro-environmental upconversion lighting and color-tunable lighting devices.