Μm Luminescence Research Articles

Broadband sensitization of 1.53μm Er3+ luminescence in erbium-implanted alumina

Experimental evidence of an efficient broadband sensitization mechanism in erbium-implanted alumina is presented. Alumina thin films were deposited by plasma-enhanced chemical vapor deposition using trimethyl-amine alane and nitrous oxide. The as-grown films, together with sapphire crystals, were implanted with erbium. Photoluminescence excitation spectra showed that erbium-implanted sapphire crystals exhibit characteristic Er3+ luminescence at 1.53μm only when pumped resonantly. In contrast, erbium-implanted alumina thin films exhibit 1.53μm luminescence even when pumped at wavelengths outside Er3+ absorption bands. We postulate that the sensitizing species is either small nanoclusters of aluminum or pairs of aluminum ions.

Infrared (1.2–1.6 μm) luminescence in Cr 4+:Yb 3Al 5O 12 single crystal with 940 nm diode pumping

Infrared (1.2–1.6 μm) luminescence in a ytterbium aluminium garnet (YbAG) crystal, doped with Cr (0.05 at.%) ions, was investigated under CW laser diode pumping ( λ = 940 nm). The Cr 4+ emission band was observed with its peak at 1.34 μm and measured to be about 1.3 times with respect to Yb 3+ IR luminescence ( λ = 1.03 μm). We demonstrate that for the excitation wavelength of 940 nm Yb 3+ ions act as sensitizers of the 3B 2( 3T 2)– 3B 1( 3A 2) emission of Cr 4+ ions. This crystal is promising as a high-efficient system for tunable laser (1.2–1.6 μm) output.

Time dependence and excitation spectra of the photoluminescence emission at 1.54μm in Si-nanocluster and Er co-doped silica

Abstract The risetime of the 1.54 μm luminescence emission in Er:Si-nc co-doped glasses provides information on the mechanisms leading to population of the luminescent level, i.e. on the energy transfer time. In this paper we present a detailed study of this risetime in silica glasses after excitation in the wavelength region 360–860 nm by a continuously tunable pulsed Ti:Al 2 O 3 laser. The emission risetime turns out to be dependent on the sample fabrication method used, i.e. implantation or co-sputtering and on the annealing temperature used to precipitate the silicon aggregates. The fastest energy transfer time observed in our samples is τ tr ⩽ 2 μs. The risetime was found to be weakly dependent on the excitation wavelength up to around 700 nm where a sudden increase is observed. This increase is more pronounced in samples where the annealing temperature was such to produce amorphous aggregates rather than crystalline ones. The difference in the nature of electronic states excited by near-infrared pumping in the two types of samples containing crystalline or amorphous aggregates is also evidenced by CW photoluminescence excitation (PLE) measurements which reflect the absorption coefficient of the sensitizing centers. In samples containing amorphous aggregates we observe an exponential dependence of the PL emission intensity versus excitation energy, which is indicative of an Urbach tail due to the disordered structure of aggregates. In the sample containing crystalline aggregates we observe an indirect interband transition behavior, with an energy gap of 1.56 eV.

Strong 1.3–1.5 μm luminescence from Ge/Si self-assembled islands in highly confining microcavities on silicon on insulator

We report dramatic enhancement of 1.3–1.5 μm room-temperature emission from self-assembled Ge/Si islands in highly confining microcavities on silicon on insulator. The microcavities are fabricated either by creating defects in two-dimensional silicon-based photonic crystals or by etching the silicon layer in order to form isolated micropillars. The optical emission is characterized by nonlinear evolution with pump power, the nonlinearity being more pronounced as the microcavity size is reduced. Both the nonlinearity and luminescence extraction are enhanced in photonic crystals with large air filling factors. The results are interpreted in terms of carrier localization. The luminescence extracted is more than two orders of magnitude higher than that of the unprocessed sample while it is 1% that of a single InGaAs quantum well. This system appears to be a promising alternative for microsources on silicon at telecommunication wavelengths that are fully compatible with silicon-based processing technologies.

An upper limit of concentration for erbium photoluminescence from silica-based thin films formed by MEVVA implantation

SiO 2 thin films containing nanocrystalline Si (nc-Si) and Er ions have been fabricated by using a metal vapor vacuum arc (MEVVA) ion source. Photoluminescence (PL) properties of the films were studied as the function of the Er ion dose. The PL intensity was found to be strongly enhanced by incorporating nc-Si in the films. The intensity of the 1.54 μm luminescence peaks firstly increases with the increasing of the Er concentration, arriving at a maximum with Er concentration of 8.0 at.% at room temperature (RT). Then PL yields decreased with further increase in Er ion dose. A mechanism about energy transferring between the photogenerated carrier in the nc-Si and Er 3+ in SiO 2 has been investigated.

Enhanced spectroscopic properties at 1.5 μm in Er3+/Yb3+-activated silica–titania planar waveguides fabricated by rf-sputtering

Two Er 3+ /Yb 3+ -codoped 94SiO 2 -6TiO 2 planar waveguides were fabricated by the rf-sputtering technique. One sample was doped with 1 mol% Er and the other was also doped with ytterbium, with a molar ratio Yb/Er = 2. The thickness and the refractive index of the waveguides were measured by an m-line apparatus operating at 632.8 and 543.5 nm. The losses, for the TE 0 mode, were evaluated at 632.8 and 1300 nm. The structural properties were investigated with several techniques, such as secondary ion mass spectrometry, energy dispersive spectroscopy and Raman spectroscopy. All waveguides were single-mode at 1300 and 1550 nm. An attenuation coefficient of 0.5 dB/cm at 632.8 nm and a lifetime of the 4 I 13/2 metastable state of 7.2 ms were measured. Upon cw excitation at 978 nm, an increase of the 1.53 μm luminescence intensity larger than two-fold was detected in the Yb 3+ /Er 3+ -codoped waveguide with respect to the waveguide doped only with Er 3+ .

1.54 μm luminescence quenching of erbium‐doped hydrogeated amorphous silicon deposited by D.C. magnetron sputtering

Erbium was doped onto hydrogenated amorphous silicon using the D.C. magnetron sputtering technique. After annealing under oxygen atmosphere at 350 °C, a-Si:H<Er> films with variable optical gaps exhibits efficient room temperature at 1.538 mm. The intensities of the 1538 and 1549.6 nm Er3+ lines at room temperature were 15 and 12%, respectively, of those at 15K. This result suggests that DC sputtered a-Si:H is a suitable host for efficient Er3+ emission at room temperature. Amorphous silicon has an erbium solubility higher than 1020 cm−3 and a greatly reduced thermal quenching of the erbium PL due to a less efficient excitation energy transfer. The observation of efficient room temperature luminescence from erbium-doped a-Si:H films involves impurities such as oxygen. We found that the increase in the optical gap and the electrical resistivity of the a-Si:H<Er> films produced at low substrate temperature (high defect density, low gap) is more important than that in material deposited at 240 °C. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Increase of 1.5 µm luminescence from Cryogenic Temperature to Room Temperature from Er-doped SiO2 Films with Si Nanocrystallites Fabricated by Laser Ablation

Er-doped SiO2 thin films including Si nanocrystallites were fabricated by laser ablation of Si targets covered with Er thin films in O2 gas and subsequent thermal annealing. Photoluminescence measurements were performed at temperatures from 10 K to 295 K. The optimum condition was found to be an O2 gas pressure range of 40–50 mTorr. We found that the temperature dependence of Er photoluminescence intensity is governed by power density of excitation laser light. Furthermore, Er photoluminescence is more intense at 295 K than that at 10 K at power densities higher than 480 mW/cm2. These results confirm that Er atoms are excited by energy transfer from photo-excited Si nanocrystallites. On the other hand, photoluminescence intensity of Er-doped Si nanoparticles grown in gas phase gradually decreases with increasing temperature.

Platelet-derived exosomes: a new vascular redox signaling pathway

Cells release microparticles following apoptosis (apoptotic bodies) or for signaling purposes (exosomes). In contrast to the bigger apoptotic bodies (> 400 nm), exosomes (diameter ~100 nm) do not present phosphatidylserine on their surface and expose major histocompatibility complex components, CD9 and CD63. Prognosis of some thrombotic-inflammatory diseases has been related to microparticle release. Sepsis is an abnormal immuno-inflammatory response to an infection, including dysregulation of apoptotic mechanisms in vascular cells. The major redox signaling pathway in vessels involves the enzymatic complex superoxide generating NADPH oxidase. In previous work we showed that, in sepsis, there is augmented platelet release of microparticles when compared with healthy controls. Those septic microparticles also possess greater NADPH oxidase activity, which can be responsible for vascular cell apoptosis. Our objective was to better characterize those microparticles obtained from septic patients and determine possible pathways related to their release. Through sequential filtration and centrifugation we separated microparticles from septic plasma (n = 16, 24 hours of diagnosis accordingly to ACCP/SCCM 1992 criteria) or from healthy controls (n = 6). Apoptotic bodies were obtained from the medium of cultured endothelial cells exposed to ultraviolet light for 30 min. Laser light scattering revealed particles with diameter between 82 and 112 nm. In contrast to the microparticles, apoptotic bodies do not have NADPH oxidase activity as assayed by lucigenin 5 μM luminescence. Western blot analysis revealed greater NADPH oxidase subunit expression in septic particles when compared with healthy controls, and none on apoptotic bodies. Flow cytometry disclosed positive phosphatidylserine exposure on apoptotic bodies, while microparticles were positive to CD9 and CD63. Washed, fresh platelets from single donors were stimulated with thrombin (0.1 U/ml), tumor necrosis factor alpha (10 μg/ml), lipopolysaccharide (LPS) (0.1 μg/ml) and with the nitric oxide (NO) donor sodium nitroprusside (2 mM) for 30 min. Particles obtained from the tumor necrosis factor-stimulated and thrombin-stimulated platelets were similar to the apoptotic bodies, while those obtained from NO-stimulated or LPS-stimulated platelets shared characteristics with the septic microparticles, with low phosphatidylserine exposure, high CD9 and CD63 expression, and oxidase activity. In conclusion, in sepsis there is a platelet release of exosomes, which possess a vascular pro-apoptotic NADPH oxidase activity. Furthermore, we showed that LPS and NO, agents related to the pathophysiology of severe sepsis, induce similar superoxide producing exosome release from healthy platelets, suggesting the existence of a new redox signaling pathway.

Optimum silicon ion dose for 1540 nm photoluminescence from erbium-doped silica thin films formed by MEVVA implantation

In this work, SiO 2 thin films containing Er and Si nanocrystals were prepared by implantation with a metal vapor vacuum arc (MEVVA) ion source. Photoluminescence (PL) properties of the films at 77 K and at room temperature (RT) were studied as functions of the Si ion dose. The PL intensities were strongly enhanced by the incorporation of nanocrystalline Si (nc-Si) in the SiO 2 films. The intensity of the 1.54 μm luminescence peak first increased with increasing Si dose, arriving at a maximum at a Si dose of 5×10 17 cm −2, then decreased with further increase in Si dose. These results are discussed and understood in terms of the role of nc-Si in Er luminescence mechanisms in silica containing nc-Si.

Improvement of 1.54 μm luminescence in erbium-doped a-SiO x:H alloys

Erbium-doped amorphous silicon films have been deposited using magnetron-assisted silane decomposition (MASD) technique. The increase of partial concentration of O 2 in the gas phase above 6.5 mol% during the deposition enhances the intensity of the 1.54 μm Er photoluminescence (PL) at room temperature. The films exhibit an inhomogeneous structure containing oxygen and Er enriched nanoclusters, where the Er ions reside in optically active but electrically passive configuration.

P22phox-containing microparticles from plasma of septic patients are related to reactive oxygen species production and apoptosis of vascular cells in culture

Reactive oxygen species (ROS) are related to vascular dysfunction in several pathologies and may be related to the pathophysiology of sepsis and multiple organ dysfunction. To investigate their role and sources we evaluated the effects of plasma of septic patients (SPP) on reactive ROS production and apoptosis of rabbit endothelial (REC) and aortic smooth muscle (RASM) cells in culture through NAD(P)H (0.3 mM)-driven lucigenin (5 μM) luminescence and annexin V assay respectively. SPP alone showed intrinsic luminescence when compared to plasma from healthy controls (HCP) (51.4 ± 9.5 cpm/min/mg protein vs 7.0 ± 1.2, n = 5, P < 0.05). After incubation with REC and RASM cells SPP induced a two-fold increase in ROS production (n = 5, P < 0.05) when compared to incubation with HCP. This enhancement of ROS production by REC and RASM cells were paralleled by increase in apoptosis rates induced by SPP. The fraction responsible for ROS production and apoptosis induction was determined through filtration and ultracentrifugation to be over 50 kD and to contain microparticles (MP), similar to those from activated platelets. MP showed similar luminescence (60.6 ± 4.4 vs 32.9 ± 3.1, n = 11), blocked by DPI (60%), a flavoenzyme inhibitor, and by SOD (80%). Addition of MP from SPP to REC and RASM also induced increased luminescence when compared to the effect of MP from HCP; These signals were also inhibitable by SOD and DPI. Incubation of REC and RASM with MP from SPP doubled apoptosis rates when compared to incubation with MP from HCP. Apoptosis was also inhibited by addition of SOD or DPI. After lipid extraction, western-blot analysis of the MP was positive for the p22phox and gp91 subunits of the NAD(P)H oxidase. Thus, SPP has intrinsic and enhances REC and RASM ROS production, due to NAD(P)H oxidase activity. It causes apoptosis of REC and RASM, also inhibited by SOD and DPI. These effects may be attributable to MP containing the p22phox and gp91 subunits of NAD(P)H oxidase. ROS derived from microparticles may represent a new signaling pathway involved in the pathophysiology of severe sepsis.

Electrical and optical properties of erbium-related centers in 6H silicon carbide

Implants of Er, Al and N with energies ranging from 120 keV to 2 MeV were carried out in n- and p-type 6H-SiC. The aim is to identify electrically active defects in the bandgap which can be associated with the 1.54 μm Er 3+ luminescence in 6H-SiC. Measurements are conducted by means of deep level transient spectroscopy (DLTS) as well as low temperature photoluminescence (LTPL). In n-type 6H-SiC implanted with Er, we find well known DLTS damage levels whereas strong characteristic lines of Er 3+, near 1.54 μm, are seen in the photoluminescence. In p-type 6H-SiC implanted with Er two DLTS peaks can be uniquely associated with the Er 3+, on the other hand, no luminescence is observed. Conversion of such p-type 6H-SiC into n-type, by nitrogen implantation, recovers the photoluminescence. We conclude that the Er-associated defect levels in the lower half of the gap are likely not involved in the energy transfer mechanism which is responsible for the 1.54 μm luminescence.

Optimisation of MBE growth conditions for InAs quantum dots on (0 0 1) GaAs for 1.3 μm luminescence

We present a study of the optimised growth conditions for InAs quantum dots (QDs) grown on GaAs substrates by solid source molecular beam epitaxy (SSMBE). Growth conditions for best luminescence intensity and linewidth were found within narrow windows of substrate temperature (500–520°C) and nominal InAs layer thickness (3.3–3.7 monolayers). The emission wavelength of such InAs QDs capped by GaAs was around 1.24 μm. However, this is red-shifted to 1.3 μm or more by capping the InAs QDs with a thin layer of In x Ga 1− x As. The results show that both In content and thickness of the capping layer can be used to tune the emission wavelength. Atomic force microscopy images show that the surface recovers to two-dimensional when depositing In 0.2Ga 0.8As while remaining three-dimensional when depositing In 0.4Ga 0.6As.

Sensitization of the 1.54 μm luminescence of Er 3+ in SiO 2 films by Yb and Si-nanocrystals

A systematic study of excitation of the 4 f–4 f luminescence of Er 3+ ions near 1.5 μm in SiO 2 layers codoped with Yb and containing Si-nanocrystals (nc-Si) is presented. Layers of SiO 2 were implanted with Er + and Yb + ions at the energies of 800 and 830 keV, respectively, at a wide range of doses (0.5–4.0×10 15 cm −2). Photoluminescence (PL) and PL excitation spectroscopy were applied to study excitation of Er 3+ ions. The Yb 3+ ions were found to play a key role in sensitizing the PL of Er 3+ for pump wavelengths ∼500 nm and 1μm. Models of sensitization processes involving energy transfer from Yb-centers are discussed. Silicon nanocrystals were produced by high dose Si implantation into SiO 2 (excess Si contents ∼7%) followed by annealing at 1100°C. An evidence is presented that transfer of energy from nc-Si is not the dominant mechanisms of excitation of Er 3+ ions. Defect mediated mechanisms seem to be more efficient. It is also found that excess Si in SiO 2 prevents clustering of rare earth atoms.

Inhibition of vascular NADH/NADPH oxidase activity by thiol reagents: lack of correlation with cellular glutathione redox status

Vascular NAD(P)H oxidase activity contributes to oxidative stress. Thiol oxidants inhibit leukocyte NADPH oxidase. To assess the role of reactive thiols on vascular oxidase, rabbit iliac/carotid artery homogenates were incubated with distinct thiol reagents. NAD(P)H-driven enzyme activity, assessed by lucigenin (5 or 250 μM) luminescence, was nearly completely (> 97%) inhibited by the oxidant diamide (1mM) or the alkylator p-chloromercuryphenylsulfonate (pCMPS, 0.5mM). Analogous inhibition was also shown with EPR spectroscopy using DMPO as a spin trap. The oxidant dithionitrobenzoic acid (0.5mM) inhibited NADPH-driven signals by 92% but had no effect on NADH-driven signals. In contrast, the vicinal dithiol ligand phenylarsine oxide (PAO, 1μM) induced minor nonsignificant inhibition of NADPH-driven activity, but significant stimulation of NADH-triggered signals. The alkylator N-ethyl maleimide (NEM, 0.5mM) or glutathione disulfide (GSSG, 3mM) had no effect with each substrate. Coincubation of N-acetylcysteine (NAC, 3mM) with diamide or pCMPS reversed their inhibitory effects by 30–60%, whereas NAC alone inhibited the oxidase by 52%. Incubation of intact arterial rings with the above reagents disclosed similar results, except that PAO became inhibitor and NAC stimulator of NADH-driven signals. Notably, the cell-impermeant reagent pCMPS was also inhibitory in whole rings, suggesting that reactive thiol(s) affecting oxidase activity are highly accessible. Since lack of oxidase inhibition by NEM or GSSG occurred despite significant cellular glutathione depletion, change in intracellular redox status is not sufficient to account for oxidase inhibition. Moreover, the observed differences between NADPH and NADH-driven oxidase activity point to complex or multiple enzyme forms.

1.5μm luminescence characteristic of erbium in B, P doped a-SiO:H films

Hydrogenated amorphous silicon films co-doped with oxygen (O), boron (B) and phosphorus (P) were fabricated using PECVD technique. The erbium (Er) implanted samples were annealed in a N-2 ambient by rapid thermal annealing. Strong photoluminescence (PL) spectra of these samples were observed at room temperature. The incorporation of O, B and P could not only enhance the PL intensity but also the thermal annealing temperature of the strongest PL intensity. It seems that the incorporation of B or P can decrease the grain boundary potential barriers thus leading to an easier movement of carriers and a stronger PL intensity. Temperature dependence of PL indicated the thermal quenching of Er-doped hydrogenated amorphous silicon is very weak.

Room temperature 1.55 μm emission from InAs quantum dots grown on (0 0 1)InP substrate by molecular beam epitaxy

Self-assembled InAs nanostructures on (0 0 1)InP substrate have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). It is found that the morphologies of InAs nanostructures depend strongly on the underlying alloy. Through introducing a lattice-matched underlying InAlGaAs layer on InAlAs buffer layer, the InAs quantum dots (QDs) can be much more uniform in size and great improvement in PL properties can be attained at the same time. In particular, 1.55 μm luminescence at room temperature (RT) can be realized in InAs QDs deposited on (0 0 1)InP substrate with underlying InAlGaAs layer.

Temperature effect on the radiative and non-radiative processes in LLGG:Cr 3+, Nd 3+

Effect of temperature on the radiative and non-radiative processes in LLGG:Cr 3+, LLGG:Nd 3+, and the Cr-to-Nd energy transfer processes in LLGG:Cr 3+, Nd 3+ crystals were investigated in detail. Data collected includes absorption, luminescence, excitation and response to pulsed excitation measurements. The radiative lifetime of Cr 3+ ion in both LLGG:Cr 3+ and LLGG:Cr 3+, Nd 3+ crystals was found to be temperature dependent, hence the Cr-to-Nd energy transfer in the latter crystal was also temperature dependent. The lifetime and the integrated luminescence intensity of 1.06 μm luminescence show the same temperature dependent indicating that radiative lifetime of Nd 3+ ion in LLGG crystal is temperature independent. However the temperature dependence of 1.06 μm emission of Nd 3+ ion in LLGG:Cr 3+, Nd 3+ crystal was found to be different than that of LLGG:Nd 3+ when this emission was excited via Cr 3+ ions. A model that takes into account the variations of both radiative and non-radiative decay probabilities of Cr 3+ ion with temperature was presented to explain these phenomena and the temperature dependence of Cr 3+-to-Nd 3+ energy transfer processes in LLGG:Cr 3+, Nd 3+.

Charge collection mapping of the back-transfer process in Er-doped silicon

Charge collection analyses have been carried out on Er-doped silicon to investigate the excitation and de-excitation mechanisms of the Er3+ ion related to the λ=1.54 μm luminescence. Carrier recombination and trapping at the defective states induced in the material by the presence of Er play a significant role in the excitation of the Er3+ ion and in its nonradiative decay by the energy back-transfer process. We have obtained a two-dimensional map of the lattice sites where the back-transfer process occurs, and provided experimental proof of the cooperation of two different defects in the excitation and de-excitation processes.