R. Tomala

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.

Synthesis and up-conversion luminescence of Er3+ and Y b3+ codoped nanocrystalline tetra- (KLaP4O12) and pentaphosphates (LaP5O14)

The up-converting nanocrystals of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 were prepared using co-precipitation method. The spectroscopic properties of these materials were investigated in a function of Y b(3+) concentration. The up-conversion emission, power dependence of emission intensities, and the luminescence decay times were investigated. It was found that the green to red and (2)H11/2 → (4)I15/2 to (4)S3/2 → (4)I15/2 emission intensity ratio were strongly affected by the Y b(3+) concentration. Moreover, the order of up-conversion emission and threshold power rises up with Y b(3+) concentration for (4)S3/2 → (4)I15/2 transition. The luminescence decay time of the (4)S3/2 → (4)I15/2 emission increases with Y b(3+) concentration while the (4)F9/2 → (4)I15/2 emission is independent of dopant concentration. The influence of the Y b(3+) concentration on the up-conversion emission intensities was discussed in terms of concentration dependent hetero looped photon avalanche process. A comparison of the up-conversion properties of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 nanocrystals was presented.

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

The laser induced white emission (LIWE) from Sr2CeO4 nanocrystals upon irradiation with a focused IR laser beam was investigated. It was observed to be a threshold phenomenon with its intensity increasing exponentially with the excitation power density. This process was investigated under double laser beam simultaneous excitation in the UV range leading to Stokes emission in the visible range and in the IR range leading to anti-Stokes LIWE. With increasing LIWE intensity, the Stokes emission intensity strongly decreased. The LIWE is accompanied by efficient photocurrent generation depending on laser excitation density followed by multiphoton absorption and ionization processes. Photoimpedance measurements showed a sharp increase of the dielectric constant by several orders of magnitude in the Sr2CeO4 nanocrystals during the LIWE process demonstrating a metallic-like behaviour. The mechanisms of LIWE include multiphoton absorption and ionization that lead to the creation of a coupled pair of Ce3+ and Ce4+ ions that allow for the intervalence charge transfer (IVCT) emission transitions in the white light range. A strong decrease of absorption band intensity of Sr2CeO4 with increasing LIWE intensity confirms the creation of (Ce3+, Ce4+) pairs.

Modulation of thulium upconversion in potassium tetraphosphate (KLaP4O12) nanocrystals by co-doping with Yb3+ ions

The influence of sensitizer concentration on the up-conversion properties of KLa0.99−xTm0.01YbxP4O12 nanocrystals was investigated in a wide range of Yb3+ concentrations (x = 0.05–0.5). This impact was manifested by Yb3+ concentration dependent variation of up-conversion emission spectra, blue to NIR emission ratio variation, luminescence decay times as well as on the order of upconversion processes. The observed processes of energy transfer between Yb3+ and Tm3+ were discussed in terms of reduction of the average distance between dopants, Tm3+ → Yb3+ back energy transfer and energy migration to surface quenchers over a Yb3+ sublattice.

Laser induced broad band anti-Stokes white emission from LiYbF4 nanocrystals

Abstract Spectroscopic properties of tetragonal LiYbF 4 nanocrystals under high dense NIR excitation at vacuum condition were investigated. White, broad band emission covering whole visible part of the spectrum from LiYbF 4 nanocrystals was observed. Its intensity strongly depended on the excitation power, excitation wavelength and ambient pressure. Temperature of the nanocrystals under 975 nm excitation was determined as a function of excitation power. Strong photo-induced current was observed from LiYbF 4 pallet. The emission kinetic was analyzed. The mechanism of the anti-Stokes white emission was discussed in terms of the laser-induced charge transfer emission from Yb 2+ states.

Glass photonic structures fabricated by sol-gel route

To emphasize the scientific and technological interest in the silica-based glassy systems and the versatility of the sol-gel route, nano and micrometer scale structures are discussed focusing the attention mainly on the rare-earth-activated materials and their spectroscopic characterization. We have demonstrated that various SiO2-based binary systems can be successfully employed for the fabrication of amorphous planar waveguides, glass–ceramic waveguides, and tapered rib waveguide laser. Different technological processes allowed to realize Er3+ -activated microspheres that can be exploited as microresonator and it has been evidenced that through specific coating it is possible to modify modal free spectral range and/or modal dispersion of the microresonator and achieve laser action. In the case of rare-earth-doped 3-D colloidal crystals in different configurations (direct and inverse one), it has been shown that the relaxation dynamics of the electronic states can be engineered.

The bright white emission of µ-diamonds

The synthetic µ-diamonds powder was synthesized using high pressure high temperature technique. The metal-catalyst effect was used to reduce the pressure and temperature of diamonds synthesis. The irradiation of diamonds by focused beam of 975 nm laser diode lead to obtain intense broadband white emission with maximum at 620 nm. This type of white emission generation were described earlier for graphene ceramic and graphene foam. The emission of μ-diamonds is characterized by low threshold of power excitation and strong dependence of ambient pressure. It was suggested that the white light emission observed from graphene samples may originate from the laser-induced sp2↔sp3 hybridization change as a result of the mechanism analogous to the intervalence charge transfer in Sr2CeO4 nanocrystals. Similar sp2↔sp3 hybridization change can occur in µ-diamonds resulting in intense, bright white light emission.

Graphene for white lighting

In the matter of white light (WL) generation, considerable scientific interest has been recently devoted to the studies of carbon materials where laser light or electrical current are used for incandescence or light emission. Experiments have been performed on a number of structures, including carbon nanotubes, fullerenes, graphene, and graphene QDs, which are being considered as materials for next-generation light sources. The studies on WL emission of graphene-based materials (foam and ceramic) are presented. The intense broad band white lighting was observed in vacuum under a focused beam of laser irradiation at a number of different excitation wavelengths, from the VIS to the NIR range. It was observed that the WL emission from graphene materials was a threshold process exhibiting supralinear behavior. The intensity was highly dependent on the ambient pressure. Relatively low temperature of the sample was measured that did not exceeded 600 °C. The luminescence process was accompanied by efficient photoconductivity that increased with excitation power of the laser. The possible mechanism of white light generation is discussed as a photoinduced, transient, domain-like sp2→sp3phase transitions.