Broadband anti-Stokes white emission of Sr2CeO4 nanocrystals induced by laser irradiation

Broad Luminescence Generated by IR Laser Excitation from CsPbBr3:Yb3+ Perovskite Ceramics

This paper demonstrates the generation of broadband emission in the visible and infrared ranges induced by a concentrated beam of infrared radiation from CsPbBr₃ ceramics doped with Yb³⁺ ions. The sample was obtained by the conventional solid-state reaction method, and XRD measurements confirmed the phase purity of the material crystallizing in the orthorhombic system. Spectroscopic measurements required further sample preparation in the form of ceramics using a high-pressure press. The research showed that as the excitation power increases, the emission intensity does not increase linearly from the beginning of the experiment. Irradiation of the material results in the accumulation of the delivered energy. Absorption of a sufficient number of photons triggers avalanche emission. It was found that the most intense luminescence is produced in a vacuum. Changes in conductivity were also observed, where the excitation was able to lower the resistivity of the material and it was highly dependent on the excitation power. The mechanism responsible for the generation of the observed phenomenon involving intervalence charge transfer (IVCT) transitions has been postulated.

Laser induced white emission generation from La1-xNdxAlO3 nanocrystals

The synthesis and structural characteristics of nanocrystalline LaAlO₃ perovskites doped with different concentrations of Nd³⁺ ions are reported. Their excitation and Stokes emission spectra, in which significant concentration quenching was observed, were recorded and characterized. The measurements of anti-Stokes laser induced white emission (LIWE) spectra were also performed. It was found that the LIWE intensity increased exponentially with the excitation laser power above the excitation threshold. In particular, the concentration and pressure dependences of the LIWE spectra of La_1-xNd_xAlO₃ were investigated. Despite the pronounced concentration quenching observed in the Stokes emission, no such effect could be observed in the anti-Stokes one. A possible inter-valence charge transfer (IVCT) mechanism for the white emission, in which different bands that were observed correspond to different valences of Nd, is proposed.

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

The irradiation of an optically absorptive medium by a continuous-wave (CW) near-infrared (NIR) laser can result in a spectral continuum emission covering both the visible and NIR regions, which is attractive for applications as continuum light sources in diverse fields. It is shown here that this NIR-laser-driven light emission can be effectively modulated with nanoscale architecture in the medium. By using porous silica as the model matrix and Yb³⁺ ions as the photothermally active centers, up to 100 folds increment in NIR-laser-induced emission intensity and dramatic decrease in threshold excitation density are demonstrated. It is observed that the emission intensity exhibits a strong nonlinear dependence on the power of the NIR excitation laser, featuring clear excitation power thresholds. Based on combined numerical simulation and spectral and temperature measurements, the improved broadband emission and photothermal nonlinearity are interpreted by enhanced optical energy localization around the laser spot that results in boosting the photon-to-photon conversion efficiency. The use of the nonlinear photothermal emission process as a broadband NIR light source, which could be exploited for applications including NIR spectroscopy, imaging, and sensing, is further demonstrated as a proof-of-concept.

Laser induced visible and infrared emission of a tungsten filament

The measurements of laser induced emission (LIE) of a tungsten filament upon irradiation with the focused beam of a CW IR laser diode are reported. It was found that the emission occurred in visible and infrared range. The influence of the applied DC electric field significantly affected the intensity of LIE of the tungsten filament. The origin of LIE is discussed in terms of multiphoton ionization of tungsten W⁺ atoms assisted by light emission due to the intervalence charge transfer in the tungsten hybrid domain (W, W⁺).

Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence

Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu²⁺ and Tb³⁺ dopants in CaS. This peculiar emission overlaps with the red Eu²⁺ emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is efficiently excited with visible light. Experimental evidence for metal-to-metal charge transfer (MMCT) luminescence is collected, comprising data from luminescence spectroscopy, microscopy and X-ray spectroscopy. State-of-the-art multiconfigurational ab initio calculations attribute the IR emission to the radiative decay of a metastable MMCT state of a Eu²⁺-Tb³⁺ pair. The calculations explain why no MMCT emission is found in the similar compound SrS:Eu,Tb and are used to anticipate how to fine-tune the characteristics of the MMCT luminescence. Finally, a near-IR LED for versatile spectroscopic use is manufactured based on the MMCT emission.

Broadband near-infrared (IR) light-emitting diodes (LEDs) are desirable for smart devices and bio-imaging applications, but the efficiency is limited by the phosphor materials. The authors report broadband emission in Eu-Tb co-doped CaS due to metal-to-metal charge transfer between dopants, and build a broadband near-IR LED with output surpassing the state of the art.

Studies of graphene influence on the laser induced white emission spectra of Sr2CeO4/graphene flake composites

The Stokes and anti-Stokes emission spectra generated from Sr2CeO4/graphene flake composites were investigated. The excitation and emission spectra, decay profiles and quantum efficiency of the studied materials were collected. It was found that the addition of graphene flakes (GFs) significantly affects spectroscopic properties. In particular, the anti-Stokes laser induced white emission spectra were analyzed as a function of excitation laser power, and ambient atmospheric pressure. The influence of graphene flakes concentration on laser induced photocurrent was investigated. The color of their emission significantly differs from the color of other tested composites. In particular, the impact of graphene concentration on the investigated features will be presented and mechanisms responsible for the observed effects will be discussed.

Near-infrared laser driven white light continuum generation: materials, photophysical behaviours and applications

The pursuit of efficient light sources has stimulated continued effort in the search of materials and methods for generating white light emission. In addition to the white light produced by light-emitting diodes (LEDs) and fluorescent lamps that involves spectral conversion of high energy to low energy emission, recent studies showed that it was also possible to produce white visible light by irradiating different active materials with near-infrared (NIR) constant-wave (CW) lasers. In this review, we begin by introducing and categorizing different materials that exhibit NIR laser driven white light emission, including normal inorganic phosphors, organometallic compounds, graphene, etc. We then discuss the photophysical behavior of this process in terms of optical spectra, temperature evolution and photoelectric response. Different mechanisms of while light generation are analyzed afterwards, and the possibility of a more general physical picture of this process is discussed. This review is concluded with a summary of the current understanding and discussion on potential applications and future perspectives.

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu²⁺/Eu³⁺ pairs in Eu-doped CaF₂, SrF₂, and BaF₂. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions. Their assignment as IVCT states is provided by state-of-the-art multiconfigurational ab initio calculations. Although IVCT states of lanthanide-doped materials have largely been overlooked so far, they can cause luminescence quenching and even complete luminescence excitation loss. Their direct observation and independent assignment in classical dopant (Eu) and hosts (CaF₂, SrF₂, BaF₂) are very significant: They suggest that the occurrence of IVCT states in other lanthanide-activated materials is very likely overlooked and their impact is ignored.

Laser induced white emission generated by infrared excitation from Eu3+:Sr2CeO4 nanocrystals

The laser induced white emission (LIWE) was observed from Eu³⁺:Sr₂CeO₄ nanocrystals. The samples were obtained in form of powders by the modified sol-gel route. The structure and morphology of the phosphors were investigated by powder X-ray diffraction and transmission electron microscope techniques. The intense LIWE occurred under reduced pressure and focused beam of near infrared laser excitation. The power and pressure dependencies exhibit evident threshold character typical for the avalanche effect. The photoconductivity of the Eu³⁺:Sr₂CeO₄ nanocrystals measured as a function of different powers of excitation source was analyzed.

Laser induced white lighting of graphene foam

Laser induced white light emission was observed from porous graphene foam irradiated with a focused continuous wave beam of the infrared laser diode. It was found that the intensity of the emission increases exponentially with increasing laser power density, having a saturation level at ca. 1.5 W and being characterized by stable emission conditions. It was also observed that the white light emission is spatially confined to the focal point dimensions of the illuminating laser light. Several other features of the laser induced white light emission were also discussed. It was observed that the white light emission is highly dependent on the electric field intensity, allowing one to modulate the emission intensity. The electric field intensity ca. 0.5 V/μm was able to decrease the white light intensity by half. Origins of the laser-induced white light emission along with its characteristic features were discussed in terms of avalanche multiphoton ionization, inter-valence charge transfer and possible plasma build-up processes. It is shown that the laser-induced white light emission may be well utilized in new types of white light sources.