An Investigation On Continuum White Light Generation Driven By Near-infrared Laser

Efficient white light sources are widely used in diverse fields including lighting,display and precision spectroscopy,et al.In recent years,near-infrared(NIR)constant wave(CW)laser driven broadband white light generation has stimulated wide interest due to the high efficiency and the peculiar properties.This WL has been observed in a wide range of organic and inorganic materials with similar spectral characteristics,while the given mechanistic explanations differ from material to material.Therefore,a general strategy for the design of efficient white light embittering materials under NIR CW laser pumping has not yet developed.The research in the present thesis concentrates on the white light generation from a series of rare earth(RE)activated oxide phosphors.Based on theoretical modeling and experimental study,the mechanisms,as well as the controlling factors for the white light generation,are investigated,and an efficient strategy for enhancing white light generation has been demonstrated.The detailed contents of this thesis include:(1)To understand the photophysical process involved in the white light generation process,we first studied the white light generation in a typical oxide phosphor Y2O3:x%Yb3+(x=0,10,40,70,100)under a CW 980 nm laser excitation,and examined the power-dependence the time evolution of the emission spectra and the sample temperature.We observed strong nonlinear dependence of the white light intensity on the excitation laser power and a step-wise increase of intensity with laser power density.The entire process for white light generation can be divided into three stages:?)For laser power density P<61 W·cm-2,the cooperative luminescence of Yb3+dimers is dominating;?)when 61<P<153 W·cm-2,thermal accumulation occurs which is accompanied by slow growth of intensity;?)when P>153 W·cm-2,strong white light emission prevails,and at the same time,we observed rapid temperature increase accompanied by white light emission,indicating the strong temperature dependence of the emission intensity.Based on these results,we suggest that the thermal emission may not be the mechanism,and a direct photon-to-photon conversion might dominate the emission process.(2)To confirm the existence of a similar mechanism for materials of a rather different character in the white light generation process,we studied this process in graphene(G),an oxide phosphor BaGd2ZnO5:1%Er3+,4%Yb3+(BGZO),and their mixtures.In the meantime,the doping of Er3+ allows for use as a temperature probe for ratiometric recording the temperature change during white light generation.We observed the increase in white light intensity with the rise of the graphene content in the mixture,while the step-wide growth of white light intensity is not observed.This result is interpreted in terms of the high thermal conductivity of graphene that facilitate heat dissipation around the laser focus.In addition,the increase in absorption with the rise in graphene content also contributes to the accelerated temperature rise of the sample.On the other hand,the temperature derived by fitting with the black-body radiation theory is far higher than that obtained from the temperature probe(Er3+)for samples with different compositions,and the emission spectra differ from sample to sample even at the same temperature.Those results suggest that the white light emission is temperature-dependent,but it is clearly different from black-body radiation.It may be better described by a thermally-assisted photon-to-photon conversion process.(3)To suppress the heat dissipation and enhance white light intensity,we employed a host of low thermal conductivity based on porous silica(activated with Yb3+).Through the modulation of the microstructure,we are able to control the white light emission process.We observed a nearly 100 times increase in intensity for the porous silica as compared with fully densified counterparts.In addition,the presence of pores also reduces threshold power density for white light emission,from 437 W/cm2 for dense silica to 215 W/cm2 for porous silica.Combined with the observation made with a thermal camera and the numerical simulation of the heat conduction,we suggest that enhancement of white light emission can be ascribed to the suppressed heat dissipation around the laser focus.This observation points to a new strategy for the search of high efficient white light emitting materials.