| Fluorescent materials have been widely applied in nanophotonics,optoelectronics and biomedicines,etc.,where high efficiency and stability are of importance.However,agglomeration and photocorrosion in low-dimensional fluorescent materials limit the development of highly-effieent fluorescence-based micro-/nano-optoeletronic devices.Therefore,it is essential to explore a universal enhancement technique to improve the luminescence efficiency and weaken the quenching effect via near-field optical modulation.In this thesis,we proposed a dielectric microsphere cavity based optical regulation structure for enhancement of photoluminescence with low energy losses in broad spectra.Firstly,we investigated the effect of substrate and film thickness on photoluminescence（PL）enhancement of wide bandgap semiconductor（i.e.Zn O）by dielectric microsphere array/luminescence film/substrate（MLS）sandwich structures.The enhancement ratio of intensity（ERI）for Zn O UV-PL from free-exciton recombination in the sandwich structure was found to be strongly dependent upon the refractive index of substrate and luminescence film thickness.In order to achieve high ERI for PL emission,the refractive index of substrate should differ from luminescence film and the film thickness needs to be chosen to support whispering gallery modes（WGMs）in the sandwich structure.The maximum ERI beyond one order of magnitude for Zn O UV-PL was therefore predicted theoretically and validated experimentally,where 11.25-fold UV-PL enhancement ratio was achieved in～650-nm-thick Zn O film grown on Si C substrate and capped with 5.06-μm-diameter microsphere array.The PL enhancement channels in the sandwich structure were revealed,for the first time,including the focusing property of microsphere array distinctly enhancing free-exciton recombination,anti-reflection effect of MSA increasing excitation cross-section area,MLS supported travelling wave-/standing wave-WGMs（TW-/SW-WGMs）inducing ASE and Purcell’s effect,and optical directional antenna effect for high equivalent NA of objective lens as well as out-coupling efficiency.Then,a microsphere coupled Cs Pb Br3 perovskite microcrystal（Pe MC）vertical structure was proposed for amplification of spontaneous radiation.The regulation of exciton dynamics and exciton-phonon（EP）interactions by the microsphere cavity were understood by temperature-dependent steady-state and time-resolved PL spectra.Purcell effect of microsphere-cavity-supported WGMs was able to weaken the strength of EP interaction in Pe MCs for suppression of nonradiative recombination and boosting of radiative recombination,leading to PLQY promotion from 85.5%to 91.4%.Over two orders of magnitude spontaneous radiation amplification was realized in the 5.06μm diameter microsphere coupled Cs Pb Br3 Pe MC vertical structure.Furthermore,a straightforward approach was presented for a selective microsphere array capped on a Pe MC film as a vertical structure for patterned PL enhancement.The excitation-power-and excitation-observation-angle-dependent luminescence contrast inspire a novel anticounterfeiting strategy.Finally,the broadband response of the dielectric microsphere cavities was explored by constructing a microsphere-cavity array capped three primary color QDs/PDMS film vertical structure.As the results,the microsphere cavities presented giant fluorescence enhancement ratios for all three-primary color QD/PDMS films,the ERIs for blue,green and red QD/PDMS were 420,2900,and 1400 folds,respectively.Furthermore,the enhancement in bright white-light unidirectional emission from the wearable flexible QD/PDMS film was demonstrated.The results of this thesis pave new ways for the design of high-performance optoelectronic devices,with promising applications in energy saving,flexible lighting/displaying,fluorescence anticounterfeiting. |
