Understanding Near Infrared Laser Driven Continuum White Light Emission by Graphene and Its Mixture with an Oxide Phosphor

Abstract

Recent observations suggest that continuum white light generation driven by a near infrared (NIR) laser is not limited to rare earth (RE) doped phosphors, but is rather a general photophysical process of light–matter interaction as long as the NIR laser energy is efficiently absorbed. Since the proposed explanations seem to be material‐dependent, a comparative investigation on the white light generation in graphene, and its mixture with a typical RE‐doped oxide phosphor is reported here for a general understanding of this process. In contrast to the steady increase of white light intensity in graphene, the white light generated from the oxide phosphor and their mixtures with graphene exhibits a stepwise growth of intensity with laser power. By using the temperature‐dependent upconversion luminescence as ratiometric temperature sensing, it is observed that this emission is closely correlated with the jumps in temperature with the rise of excitation power. Since the black body temperatures are much higher than the temperatures derived by the ratiometric method, it is suggested that this process can be understood by a general thermally assisted direct photon‐to‐photon conversion process, rather than by pure thermal radiation. The results of this work may have important implication for the understanding and design of efficient continuum white light sources.

Boosting Continuous‐Wave Laser‐Driven Nonlinear Photothermal White Light Generation by Nanoscale Porosity

“…After an initial slow rise of intensity with excitation power, rapid growth of intensity is observed at laser power greater than the threshold excitation power. Similar to the previous observations, [5,10] the excitation power dependence of WL intensity can be divided into three regions, which is manifested by the stepwise increase of sample temperature. [10] Based on the relation of I ∝ P n with I the emission intensity and P the excitation power, the calculated n values (Table S1, Supporting Information) are below 0.5 for all the examined samples at low excitation density (region I).…”

“…As shown in Figure 2c, the P th values for these samples show a clear dependence on the samples' porosity, which increases from 215 W cm -2 for S 600 to 437 W cm -2 for S 1250 . Considering a photothermal origin of the WL emission, [5,10,12] these results could be rationalized by the accelerated heat accumulation around the laser focus in samples with high porosity due to strong localization of optical energy around the laser focus due to blocked heat dissipation. By further increasing the porosity (for the sample with higher porosity (S 600 ) made from 7 nm silica NPs), higher white light emission intensity can be obtained under the same excitation condition (Figure S7, Supporting Information).…”

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the white light (WL) contributed from discrete emissions, generation of thermal WL featuring a continuum spectrum has been observed in a wide range of materials under excitation by near-infrared (NIR) continuous-wave (CW) lasers, stimulating growing efforts in unraveling the involved mechanisms and exploring for potential applications. The NIR-laser-induced emission has been observed in diverse materials including doped and undoped inorganics with lanthanides, [2] carbon nanotubes, [3] graphene foam, [4,5] semiconductors, [6] and organometallics materials. [7,8] In recent years, an increasing number of investigations have been devoted to search for more efficient materials and to unravel the involved photophysics, [9][10][11] and this broadband emission independent of the type of emission centers has been generally associated with a photothermal origin with strong optical nonlinearity.

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Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

“…As summarized in Table 1, previous studies have largely reported various types of ratiometric UCL nanoprobes for temperature sensing. [168][169][170][171][172][173][174][175][176]193,194 Most of the nanoprobe systems consist of lanthanide-doped UCNPs, other upconverting luminescent inorganic materials, 29,36,40,43,45,47,48,54,[56][57][58][59][60][61]65,[73][74][75][76] and the derivatives with functional modifications, including the core@shell, 37,38,41,68,72,78,79 hollow/porous, 38 complexes, 46,49,51 hybrid, 32,52 mixture, 33,35 assembly, 34,55 or engineering structures.…”

“…As summarized in Table 1, previous studies have largely reported various types of ratiometric UCL nanoprobes for temperature sensing. 28–65,168–176,193,194 Most of the nanoprobe systems consist of lanthanide-doped UCNPs, other upconverting luminescent inorganic materials, 29,36,40,43,45,47,48,54,56–61,65,73–76 and the derivatives with functional modifications, including the core@shell, 37,38,41,68,72,78,79 hollow/porous, 38 complexes, 46,49,51 hybrid, 32,52 mixture, 33,35 assembly, 34,55 or engineering structures. 28,31,39,62–64,66,67,77 Usually, temperature sensing mechanisms of upconverting luminescent materials in nanoprobe systems refer to thermally/non-thermally coupled energy levels, thermal-linked energy levels, thermal-quenching effects, thermal-sensitive luminescence, thermal-enhanced emission, etc .…”