Photoluminescence in fullerene-doped porous glasses

We report enhanced and broad-band photoluminescence (PL) behavior of C₆₀ doped in porous silica glasses. By carrying out photoluminescence and photoluminescence-excitation spectroscopy of C₆₀ doped in surface-unpassivated and -passivated porous glasses, we show that the interaction of C₆₀ with the silica surface is quite strong and that the modified C₆₀ molecules contribute only in the orange-red region (∼1.9 eV) of the observed PL spectrum. The PL intensity in the blue-green region (∼2.3 eV) observed in doped glasses is found to originate in the porous glass itself. Received: 13 July 2000 / Revised version: 24 January 2001 / Published online: 9 May 2001     

Study on size-dependent optical property of 4,4′-dibromobiphenyl nanocrystals

4,4^′-dibromobiphenyl nanocrystals with different sizes in the range from 20 nm to 300 nm were prepared by the reprecipitation method. It was found that their absorption peaks experienced a red shift while the size of the nanocrystals increased. Through analyzing these spectra of the nanocrystals with different sizes, it is suggested that this size-dependent optical property is contributed by two factors, the size effect and the J-aggregate formation. Received: 28 August 2000 / Accepted: 2 March 2001 / Published online: 23 May 2001     

Cathodoluminescence from nanocrystalline silicon films and porous silicon

Received: 4 May 1998 / Accepted: 30 October 1998     

Measurement of nonlinear properties and optical limiting ability of Rhodamine6G doped silica and polymeric samples

The third order nonlinear optical properties of Rhodamine6G (Rh6G) doped silica and polymeric samples have been investigated using single beam z-scan technique under excitation by the second harmonic of Nd:YAG laser beam (532 nm). The nonlinear refractive index, nonlinear absorption coefficient, real and imaginary parts of third order nonlinear susceptibility in the samples of silica and poly-methylmethacrylate (PMMA) matrices are measured. Thermal contribution to the nonlinear refractive index in case of undoped silica samples has been calculated in order to have better accuracy of the material response contribution to third order nonlinearity. The comparative study of the optical limiting performance of Rh6G doped silica and polymeric samples show that Rh6G doped silica is relatively superior for optical limiting applications.     

Photoluminescence of Er3+ ions in amorphous silicon layers under intensive laser excitation

Transients of the photoluminescence (1.54 μm) of Er³⁺ ions embedded in an amorphous silicon matrix excited with intensive laser pulses are simulated using a phenomenological model which takes into account both the defect-related excitation mechanism and stimulated optical transitions in the ions. The simulated transients are compared with the experimental ones observed in Er-doped amorphous silicon layers under pulsed laser excitation. The modeling and the experimental results demonstrate a possibility to realize a regime of superradiance in the system of Er³⁺ ions pumped via an electronic excitation of the amorphous matrix. Received: 7 August 2001 / Revised version: 1 November 2001 / Published online: 17 January 2002     

High refractive index and transparent heavy metal oxide glassy thin films

Heavy metal oxide thin films of the ternary system Nb₂O₅–GeO₂–PbO have been prepared by pulsed laser deposition in an O₂ environment from either glassy or crystalline bulk samples. The range of ([Pb]+[Nb]) content in which the films are optically homogeneous and transparent is much broader (0.5–1.0) than that of the bulk samples considered in the present work (0.55–0.62). The imaginary part of the refractive index is very low in all cases (k<10^-3), whereas the real part increases linearly with the ([Pb]+[Nb]) content up to values as high as 2.35. The optical energy gap has been found to be strongly dependent on [Pb], whereas it is almost independent of [Nb]. This dependence is discussed in terms of the role of Pb and Nb as network modifiers or formers. Received: 5 August 2002 / Accepted: 8 August 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: j.gonzalo@io.cfmac.csic.es     

The relationship between the macroscopic properties of PECVD silicon nitride and oxynitride layers and the characteristics of their networks

Silicon nitride and oxynitride films have been deposited on silicon wafers using plasma-enhanced chemical vapour deposition. Various amounts of ammonia, silane and nitrous oxide gases were applied at fixed total gas flow and at the same deposition temperature. The dependence of the macroscopic properties of the layers such as refractive index, internal stress and etch rate on the reaction atmosphere during deposition has been demonstrated. The chemical structure of amorphous layers was studied using infrared spectroscopy. The network was found to be characterised by SiN_xO_yH_z tetrahedra, joined to each other by common corners. The characteristic vibrational bands due to species that join tetrahedral units (N(-Si≡)₃, ≡Si-N-Si≡, ≡Si-O-Si≡) and species that stop this interconnection (Si-H, N-H) were determined and discussed with reference to the corresponding species available during deposition. The analysis resulted in the determination of the relationship between the chemical structure of the network and the layer’s refractive index, internal stress and etch rate. Received: 24 July 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

The characterization of plasma-polymerized C60 thin films

Plasma-polymerized C₆₀ thin films were investigated in the form of a field effect transistor (FET) structure. In the FET device, the C₆₀ polymer acts as a p-type semiconductor, whereas C₆₀ is an n-type semiconductor. Its conduction mechanism can be described as due to variable range hopping. As a sensing device, the C₆₀ polymer can behave as a gas sensor for electrophoric gases and can also be operated as a photo-sensing device in air. Received: 21 April 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Multilayer optical memory using femtosecond-laser induced fluorescence in rare-earth ion doped glass

We report three-dimensional fluorescent memory by recording optical bits with irradiation of femtosecond laser pulses at 800 nm and by reading photoluminescence change in Eu³⁺ doped glass. We produced multi-layered micro-bit patterns and read the blue emission from the 405 and 325 nm excitations due to permanent reduction of Eu³⁺ to Eu²⁺ in sodium borate glass by scanning the irradiated region in multilayers.     

Experimental access to doped fullerene polymers

60 compound could be obtained by slow cooling the high temperature fcc phase and by quenching with subsequent annealing. The various phases after quenching and during annealing were studied in detail. No evidence for a direct doping of undoped C₆₀ to the polymeric AC₆₀ phase was found. Due to the local character of the doping process the formation of A₃C₆₀ clusters is observed. The same results were obtained from doping experiments performed with undoped polymeric structures like phototransformed and pressure polymerized C₆₀. Received: 6 October 1996/Accepted: 13 December 1996     

Size distribution and optical properties of self-assembled Ge on Si

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ge islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 °C), therefore the diffusion length of adatoms is high (∼100 μm) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38–1.55 μm. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up to 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Formation and removal of multi-layered fluorescence patterns in gold-ion doped glass

We report the formation of fluorescence patterns inside gold-doped glass medium by femtosecond-laser fabrication. Strong fluorescence images appeared from the irradiated multi-layered region after low temperature annealing. We removed the images by exposing the glass to an electric furnace or a CO₂ laser beam for high temperature annealing. The method was also applied to recording, reading, and erasing of fluorescence data by a femtosecond laser, a 405-nm laser diode, and a CO₂ laser respectively.     

CdTe epilayers for uses in optical waveguides

CdTe epilayers have been grown by vapor phase epitaxy (VPE) on glass, MgO, sapphire, LiNbO₃ and mica substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies show the good structural quality of the epilayers. In these epilayers, a few optical modes were excited with a 1.33-μm laser. The measured propagation losses were in the range between 5 dB/cm and less than 0.5 dB/cm. From dark-mode m-lines, the epilayer thickness was found to be in the 1–3 μm range, in good accord with that obtained by SEM measurements. The refractive index obtained from the fitting is also in good accord with that of bulk CdTe. Received: 7 October 1999 / Accepted: 13 March 2000 / Published online: 5 July 2000     

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

Reverse saturable absorption of a novel rhodamine-B cation fulleride salt at 532 nm

A novel rhodamine-B cation fulleride salt was synthesized by metathetical reaction. Its tetrahydrofuran solution was prepared and the fluence-dependent transmission was measured with 10-ns 532-nm laser pulses in a collimated optical setup. With increases in the input fluence, its transmittance decreased significantly, which indicated reverse saturable absorption behavior. For comparison, the input fluence-dependent transmittance of fullerene C₆₀ and rhodamine-B solution were also measured. The nonlinear optical limiting effect of the fulleride salt in tetrahydrofuran is slightly lower than that of C₆₀ in toluene and the explanations are provided. Received: 25 August 2000 / Accepted: 6 November 2000 / Published online: 3 April 2001     

Design of optical polarization splitters in a single-section deeply etched MMI waveguide

The design of a single-section polarization splitter in a deeply etched semiconductor MMI waveguide is presented. Numerically simulated results indicate that the semiconductor MMI exhibits considerable polarization dependence and, utilizing this effect, a compact 1.6-mm-long polarization splitter may be fabricated to yield more than 8-dB polarization separation and only 0.11-dB optical loss, using a very simple design-approach. Received: 16 May 2001 / Revised version: 14 August 2001 / Published online: 2 November 2001     

Growth and characterization of oriented cadmium sulphide nanocrystals under Langmuir-Blodgett monolayer of arachidic acid

Cadmium sulphide nanocrystals were grown at room temperature (20 °C) under arachidic acid monolayers floating over an aqueous solution of CdCl₂ inside an enclosed Langmuir-Blodgett set-up, through slow infusion of H₂S gas. X-ray diffraction spectra suggest an oriented growth of the crystallites. The particle sizes were found to increase with duration of exposure to the H₂S gas. Atomic force microscopy indicated that the particles were nearly circular pellets with uniform morphology throughout. In Raman spectra, the FWHM of the LO phonon was found to be large (≈20 cm^-1) for all the films grown with different exposure times in H₂S gas, and was found to reduce to 8 cm^-1 after annealing a typical sample at 500 °C for 45 min. Received: 30 September 1998 / Accepted: 29 March 1999 / Published online: 11 August 1999