Luc Museur

Defect-related photoluminescence of hexagonal boron nitride

Photoluminescence of polycrystalline hexagonal boron nitride (hBN) was measured by means of time- and energy-resolved spectroscopy methods. The observed bands are related to donor-acceptor pair transitions, impurities, and structural defects. The excitation of samples by high-energy photons above 5.4 eV enables a phenomenon of photostimulated luminescence (PSL), which is due to distantly trapped conduction band electrons and valence band holes. These trapped charges are metastable and their re-excitation with low-energy photons results in anti-Stokes photoluminescence. The comparison of photoluminescence excitation spectra and PSL excitation spectra allows band analysis that supports the hypothesis of Frenkel-type exciton in hBN with a large binding energy.

Photoluminescence properties of pyrolytic boron nitride

We report on spectroscopic study of pyrolytic hBN (pBN) by means of time- and energy-resolved photoluminescence methods. A high purity pBN samples (though low crystallinity) allow complementary information about excited states involved into the luminescence process. We affirm our recent conclusions about the impurity-related nature of most of fluorescence bands in microcrystalline hBN. In addition, a broad band centred at 3.7xa0eV previously not considered because of its superposition with an intense structured impurity emission is attributed to the radiative recombination of deep DAPs.

Femtosecond UV laser non-ablative surface structuring of ZnO crystal: impact on exciton photoluminescence

The ultraviolet (UV) laser irradiation (248 nm) of monocrystalline wurtzite ZnO with 450 fs pulses results in surface modification. A formation of two orthogonal ripple structures with a period of 400-500 nm was observed oriented parallel and perpendicular to the laser beam polarization. The UV exciton emission obtained on the irradiated domains is found greatly enhanced locally up to ~103 times. The photoluminescence band is redshifted by 2-3 nm and 40% narrower (full width at half-maximum), while at the same time the E2(439 cm−1) Raman band intensity increases up to ~50 times. The process is found irreversible with the threshold fluence of 11 mJ/cm2, which is considerably lower than the ablation threshold 115 mJ/cm2. Fine surface nanostructuring on the scale of ~10 nm may be responsible for the observed effect.

Photoluminescence and electronic transitions in cubic silicon nitride

A spectroscopic study of cubic silicon nitride (γ-Si3N4) at cryogenic temperatures of 8u2009K in the near IR - VUV range of spectra with synchrotron radiation excitation provided the first experimental evidence of direct electronic transitions in this material. The observed photoluminescence (PL) bands were assigned to excitons and excited and centers formed after the electron capture by neutral structural defects. The excitons are weakly quenched on neutral and strongly on charged defects. The fundamental band-gap energy of 5.05u2009±u20090.05u2009eV and strong free exciton binding energy ~0.65u2009eV were determined. The latter value suggests a high efficiency of the electric power transformation in light in defect-free crystals. Combined with a very high hardness and exceptional thermal stability in air, our results indicate that γ-Si3N4 has a potential for fabrication of robust and efficient photonic emitters.

Surface modification of monocrystalline zinc oxide induced by high-density electronic excitation

Strong modifications of semiconductors can be provoked by high-density electronic excitation. We report on surface structuring of monocrystalline wurtzite O-face (0001) ZnO excited by UV femtosecond laser pulses (248 nm) below the ablation threshold. At fluences above 11 mJ/cm2, nanoholes of D=10 nm diameter appear quasi-periodically separated by a distance ∼30 nm (=3 D). Dual-pulse (pump-pump) experiments permit estimation of the electronic excitation lifetime responsible for this nanostructuring, which is in agreement with the electron-hole plasma lifetime 220 ps. The nanostructuring results in a smaller monocrystalline domain of ∼0.1 μm size and increases the crystalline interplane c-distance by 0.11%. The excitonic luminescence of the irradiated sample is found to increase by about 10 times. The nanostructuring remains stable in a limited range of laser fluences: above 40 mJ/cm2 the surface melts, which accelerates the photoinduced bonds breaking leading to surface erosion. We tentatively ascribe the ...

A New Route for High-Purity Organic Materials: High-Pressure-Ramp-Induced Ultrafast Polymerization of 2-(Hydroxyethyl)Methacrylate.

The synthesis of highly biocompatible polymers is important for modern biotechnologies and medicine. Here, we report a unique process based on a two-step high-pressure ramp (HPR) for the ultrafast and efficient bulk polymerization of 2-(hydroxyethyl)methacrylate (HEMA) at room temperature without photo- and thermal activation or addition of initiator. The HEMA monomers are first activated during the compression step but their reactivity is hindered by the dense glass-like environment. The rapid polymerization occurs in only the second step upon decompression to the liquid state. The conversion yield was found to exceed 90% in the recovered samples. The gel permeation chromatography evidences the overriding role of HEMA2•• biradicals in the polymerization mechanism. The HPR process extends the application field of HP-induced polymerization, beyond the family of crystallized monomers considered up today. It is also an appealing alternative to typical photo- or thermal activation, allowing the efficient synthesis of highly pure organic materials.

High-pressure cell for studies of ultra-short laser impulses action on materials

A high-pressure cell has been developed for in situ studies of the behavior of materials under ultra-short impulses on a sub-picosecond timescale. This set-up allows performing experiments under controlled atmosphere between a primary vacuum up to 100 MPa. We present some primarily results on hBN.

Linear and nonlinear refractive index modifications in pHEMA-oxo-TiO 2 hybrids studied via cw-UV laser excitation and self-action of picosecond laser pulses

Linear and nonlinear optical properties of pHEMA-oxo-TiO2 nanohybrids were studied. The reduction of refractive index with the increase of UV irradiation dose was observed. The TiO2 nanoparticles concentration strongly affects the nonlinear optical response of the composite system.