M. Mortier

Mapping and quantifying electric and magnetic dipole luminescence at the nanoscale.

We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a Eu^{3+}-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated with two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weights of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of single emitter luminescence.

Laser performance of diode-pumped Yb:CaF 2 optical ceramics synthesized using an energy-efficient process

Due to their natural toughness, and also their flexible and cheap production process, laser ceramics could provide an interesting alternative to single crystals for meeting the challenges posed by the demanding domain of ultrafast and high-power laser operations. We investigated the laser performance of a Yb:CaF2 ceramic developed with a simple and green synthesis process. A 4 at. %-doped ceramic sample was diode-pumped to deliver output power of 1.6xa0W with optical-to-optical efficiency reaching 25%, a slope efficiency of 43%, a gain of 1.4, and wavelength tunability from 1015 to 1060xa0nm. Results are comparable to the typical single Yb:CaF2 crystal performance, thereby enabling the applications of these greener and low-cost Yb:CaF2 ceramics in high-power diode-pumped lasers.

Time-gated luminescence bioimaging with new luminescent nanocolloids based on [Mo6I8(C2F5COO)6](2-) metal atom clusters.

Bioimaging and cell labeling using red or near infrared phosphors emitting in the therapeutic window of biological tissues have recently become some of the most active research fields in modern medical diagnostics. However, because organic and inorganic autofluorophores are omnipresent in nature, very often the background signal from fluorochromes other than targeted probes has to be eliminated. This discrimination could be available using a time-gated luminescence microscopy (TGLM) technique associated with long lifetime phosphorescent nanocomposites. Here, we report new SiO2 nanostructured particle (50 nm in diameter) embedded luminescent nanosized [Mo6I8(C2F5COO)6]2- metal atom clusters (1 nm in diameter), successfully prepared by the microemulsion technique. This combination provides new physical insight and displays red emission in biological based solution under UV-Vis excitation with long lifetimes of around 17 and 84 μs. Moreover, the nanoparticles can be internalized by cancer cells after surface functionalization by transferrin protein and clearly imaged by TGLM under excitation at 365 nm. The nanocomposites have been mainly characterized by scanning and transmission electron microscopies (SEM and HAADF-STEM), UV-Vis and photoluminescence (PL) spectroscopies.

Near-Field Imaging of Surface Plasmon Polaritons Excited by Chains of Gold Nanodiscs

We present a study of the near-field pattern created by chains of gold nanodiscs situated on a gold thin film and illuminated at oblique incidence. Each disc generates surface plasmon polaritons that propagate on the gold surface. The created waves interfere between them and with the illuminating beam. We observed that when the discs are separated by a distance smaller than the half wavelength, the chain behaves like a continuous ridge. When the discs separation increases, a complex periodic pattern appears and extends up to several wavelengths from the chain. For some specific separation distances, a directional emission of surface plasmon is also observed. The experimental results are in good agreement with numerical simulations performed by considering each disk as an independent dipole-like surface plasmon source.

Thermal conductivity measurements of Yb:CaF2 transparent ceramics using the 3 ω method

This paper reports on the measurement of the thermal conductivity of ytterbium doped CaF2 transparent ceramics using the 3ω method. Samples with different doping levels were studied in a temperature range of 10u2009K to 310u2009K. The results for doped samples are very similar to those obtained for Yb:CaF2 single crystals with the same doping levels, which demonstrates an important improvement in the thermal properties of transparent ceramics. However, the results obtained for non-doped ceramics and single crystals differ for the maximum conductivity due to the presence of grain boundaries inside the ceramic. Theoretical calculations were also conducted using Callaways model and are in a very good agreement with experimental results. The fitting parameters found tend to indicate that the introduction of a doping element in the lattice has more impact on thermal conductivity than the presence of grain boundaries in the material.

Near-field observation of surface plasmon polaritons launched by V-shaped nanorods on a gold surface.

By scanning near-field optical microscopy, we study the propagation of surface waves created by V-shaped nanorods deposited on a gold thin film. The nanorods launch surface plasmon polaritons that interfere with the incident light, producing interference patterns. The angle of the V-shaped rods varies from 110° to 180° (straight rod). We observe that the near-field distribution strongly depends on the angle of the V. For angles close to straight rods, a hot spot is visible, whereas for a narrower angle, the surface plasmon waves are launched in specific directions. The experimental results are in good qualitative agreement with numerical simulations performed with a simple analytical model that considers the rods as a sum of isolated surface plasmon sources.

Imaging current paths in complex conductors by scanning fluorescence microscopy

Using a fluorescence-based scanning thermal microscope, we observed the Joule heating of a complex circuit made of submicron-wide rectangular stripes interconnected between them. The thermal images reveal the presence of localized hot spots which allow to visualize the main current paths in the networks. By suppressing a single elementary conductive element in the device, we observed a different hot spot repartition showing the new electrical current paths. The experimental results are in good agreement with simulations and open the way to directly characterize the electrical and thermal behaviour of complex devices and metal-dielectric composites.

Nanoscale thermal imaging of active devices by fluorescent SThM

We describe a scanning thermal microscope that uses a fluorescent nanocrystal as a temperature probe. The nanocrystal is made of an inorganic fluoride material doped with erbium ions. The temperature is determined by measuring the fluorescence intensity ratio between two adjacent fluorescence lines. We first visualized the heating of a Cr stripe, and observed two different heat transfer channels, by direct contact between the tip and the device and by conduction through the air gap. We then measured the temperature map of a Joule heated submicron wide Pt wire and observed that the temperature elevation is uniform all along the wire. The measured images are obtained with a submicron lateral resolution and demonstrate the good reliability of the technique for characterizing the thermal properties of nanoscale devices and structures.