E. Moser

Low-voltage onset of electroluminescence in nanocrystalline-Si/SiO2 multilayers

Thin film metal-oxide-semiconductor light emitting devices (LEDs) based on nanocrystalline silicon multilayer structure were grown by plasma-enhanced chemical vapor deposition. Room temperature electroluminescence was studied under direct current and time-resolved pulsed-current injection schemes. Multilayer LEDs operating at voltages below 5 V and electroluminescence turn-on voltage of 1.4–1.7 V are demonstrated. The turn-on voltage is less than 3.2 V which corresponds to the barrier height at the silicon oxide interface for electrons. Electrical injection in the multilayer LED is controlled by direct tunneling of electrons and holes among silicon nanocrystals. This injection regime is different than the Fowler–Nordheim tunneling that controls the electron injection in single thick layer LED operating at high voltages. A comparison of the power efficiency for the multilayer based LED and a similar single thick layer LED shows larger power efficiency for the former than for the second. Our results open ne...

Low-loss optical Er3+-activated glass-ceramics planar waveguides fabricated by bottom-up approach

A low-loss optical erbium activated silica-hafnia glass-ceramic planar waveguide was fabricated using a bottom-up approach. Hafnia nanoparticles were first prepared by colloidal route and then mixed in a silica-hafnia:Er3+ sol. The resulting sol was deposited on SiO2 substrate. Analysis of the photoluminescence has demonstrated that erbium ions are embedded in a crystalline phase. A lifetime enhancement of the I13∕24 metastable level was observed. Losses measurements at 1542nm highlight a very low attenuation coefficient (0.3dB∕cm) making this nanostructured material suitable for single band waveguide amplifier in the C band of telecommunications.

Er-doped silica-based waveguides prepared by different techniques: RF-sputtering, sol-gel and ion-exchange

Erbium-activated silica-based planar waveguides were prepared by three different technological routes: RF-sputtering, sol–gel and ion exchange. Various parameters of preparation were varied in order to optimize the waveguides for operation in the NIR region. Particular attention was devoted to the minimization of the losses and the increase of the luminescence efficiency of the metastable 4I13/2 state of the Er3+ ion. Waveguide properties were determined by m-line spectroscopy and loss measurements. Waveguide Raman and luminescence spectroscopy were used to obtain information about the structure of␣the prepared films and about the dynamical processes related to the luminescence of the Er3+ ions.

Nucleation of titania nanocrystals in silica titania waveguides

SiO2(1 − x)-TiO2(x) monomode waveguides at 632.8 nm, with x in the range 0.07–0.2 and thickness of about 0.4 μm, were deposited on silica substrates by a dip-coating technique. Nucleation of TiO2 nanocrystals and the growth of their size by thermal annealing up to 1300°C were studied by waveguided Raman scattering in the SiO2(0.8)-TiO2(0.2) composition. In the low frequency region (5–50 cm−1) of the VV and HV polarized Raman spectra the symmetric and quadrupolar acoustic vibrations are observed. The mean size of the titania particles are obtained from the frequencies of the Raman peaks. The results are compared with those obtained from the measure of the linewidths in the X-ray diffraction spectra. Nanocrystals with a mean size in the range 4–20 nm are obtained by thermal annealing in a corresponding range of 700–1300°C.

High quality factor Er3+-activated dielectric microcavity fabricated by rf sputtering

The authors report on one-dimensional dielectric photonic crystals activated by Er3+ ion and fabricated by rf-sputtering deposition. The cavity was constituted by an Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of six pairs of SiO2∕TiO2 layers. Near infrared transmittance spectra evidence the presence of a stop band from 1350to1850nm and a cavity resonance centered at 1537nm. Intensity enhancement and narrowing of the I13∕24→I15∕24 emission band of Er3+ ion, due to the cavity effect, were observed. A cavity quality factor of 171 was achieved.

Structure and dynamics of hydrogenated silica xerogel by Raman and Brillouin scattering

Abstract Structure and dynamics of silica xerogel samples are compared with melt quenched amorphous silica by means of infrared, Raman and Brillouin spectroscopes. In particular the attention was concentrated on a family of hydrogenated sol-gel systems whose nominal stoichiometric formula is Si 2 O 3 H 2 , Si 8 O 13 H 6 , Si 4 O 7 H 2 , and Si 8 O 15 H 2 . Thermal treatment of the samples were made at 500°C and at 800°C for 24 h. Samples were cooled at a rate of 0.3°C/min. Densities were measured by pycnometer technique. The densification of the sol-gel network occurs at about 500°C depending on the samples. At about 800°C the structure and the dynamics together with physical properties, such as sound velocities and elastic constants, become indistinguishable from those of glasses derived from melting.

Er3+ activated silica-hafnia glass-ceramics planar waveguides

Silica-hafnia glass-ceramics waveguides activated by Er3+ ions were fabricated by sol-gel route. X ray diffraction and optical spectroscopy showed that after an adapted heat treatment, the resulting materials showed a crystalline environment. Analysis of the luminescence properties has demonstrated that erbium ions are, at least partially, trapped in a crystalline phase. Losses measurements at different wavelength highlight a very low attenuation coefficient indicating that this nanostructured material is suitable for a single band waveguide amplifier in the C band of telecommunication.

Rare-earth-activated fluoride and tellurite glasses: optical and spectroscopic properties

In this work we report on spectroscopic properties of Er3+-doped aluminum fluorophosphate glasses of molar composition 29.8AlF3:3.5 MgF2: 19.8 CaF2: 10.9 SrF2: 12.8 BaF2: 8.4 YF3:9.8 ZrF4:4 NaPO3:1 ErF3 and 30 AlF3:3.5 MgF2:20 CaF2: 11 SrF2: 13 BaF2: 8.4 YF3: 10 ZrF4: 4 NaPO3: 0.1 ErF3, and Er3+-doped tellurite glasses of molar composition 75 TeO2: 12 ZnO: 10 Na2O: 2 PbO: 1 Er2O3 and 75 TeO2: 12 ZnO: 10 Na2O: 2 GeO2: 1 Er2O3. Absorption and Stokes luminescence upon visible excitation were measured. Emission in the third telecom window was observed upon excitation at 514.5 nm and the lifetime of the 4I13/2 level was measured. IR-to-visible up conversion emission under continuous-wave laser excitation at 976 nm was observed. The up conversion results in a strong green emission and a weaker red emission, whose intensity shows a quadratic dependence on the excitation power, indicating that two photons are involved in the process. Lifetime measurements were performed in order to study the dynamical behavior of the up conversion emission and to clarify the contributions of excited state absorption and energy transfer to the up conversion process.

Raman and luminescence studies of Tb3+ doped monolithic silica xerogels

Abstract Monolithic silica xerogels were doped with Tb3+ concentrations ranging from 200 to 40 000 ppm. Structural differences, such as the degree of densification or the different OH-contents were quantified by Raman spectroscopy. The Tb3+ luminescence properties were studied as a function of the temperature of heat-treatment, the time of heat-treatment, and the Tb3+ concentration. Samples densified at 900°C or above and with a Tb3+ content lower than 20 000 ppm show emission from both the 5D3 and the 5D4 state. Emission spectra and decay curves indicate that Tb3+ ions have a strong tendency to form clusters also at a very low Tb3+ concentration. This clustering results in a very efficient quenching of the 5D3 emission due to energy transfer processes among Tb3+ ions. Moreover, the luminescence of the 5D3 state is quenched by non-radiative transitions induced by OH vibrations.

Erbium-activated silica–titania planar waveguides on silica-on-silicon substrates prepared by rf sputtering

Abstract Erbium-activated silica–titania planar waveguides were prepared by radio-frequency (rf) sputtering technique. Silica-on-silicon substrates obtained by plasma-enhanced chemical vapor deposition (PECVD) and rf sputtering (RFS) were employed. The refractive indices, the thickness and the propagation losses of the waveguides were measured. The refractive index and the roughness of the silica substrates produced by RFS appear to be dependent on the thickness. Thermal annealing, which is a necessary condition to obtain light propagation, induces a decrease of the refractive index in the silica substrates. The waveguide deposited on PECVD substrate exhibits several propagating modes with an attenuation coefficient 1.7 dB/cm compared with 12.2 dB/cm measured for the waveguide deposited on silica substrate produced by RFS technique. Emission of the 4 I 13/2 → 4 I 15/2 transition with a 53 nm bandwidth was observed.