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The spectroscopic characteristics of highly Nd3+-doped lead phosphate glasses (xNd:Pb3(PO4)2) have been investigated. The X-ray spectra show that the matrices are glassy up to 25 wt% of Nd3+ doping. From the Judd–Ofelt analysis we observe that while the Ω(2) parameter remains constant indicating that the 4fN and 4fN−15 d1 configurations are not affected by the Nd3+ doping, the behavior of both Ω(4) and Ω(6) changes for 15 wt% of Nd3+ doping. The reduction of the Ω(6) parameter is related to the increase of the covalence bonding between the ligands and the Nd3+ ions. At this particular concentration, the radiative lifetime has a four-fold enhancement. Such behaviors are likely to be related to a modification in the glass structure for high Nd3+ concentrations.

In this work we investigate the effect of replacing Na2O by TiO2 on the structural and spectroscopic characteristics of the Nd2O3-doped 60B2O3–(20−x)Na2O–10PbO–10Al2O3:xTiO2:yNd2O3 borate glass matrix. Measurements of X-ray patterns, glass temperatures (Tg), vibrational (Raman and infrared (IR)) and optical (absorption and emission) spectra were carried out. The trend of Tg, the Judd–Ofelt parameters and Nephelauxetic ratio as the TiO2 and Nd2O3 concentration increases was interpreted quantitatively and qualitatively in terms of the network bonds and coordination numbers. Measurements of Tg across the Nd-doped glass series for different levels of Ti suggests changes in the rigidity of the matrix due to change of coordination of boron oxide resulting from the BO4–BO3 back conversion effect. The changes observed in the Raman and IR spectra are related to the BO3 → BO4 conversion effect. The variation of the Judd–Ofelt parameters Ω ( 2 ) and Ω ( 6 ) indicates that the average rare earth-ligand radius decreases establishing an electronic density distribution when Nd3+ concentration increases. This effect is interpreted in terms of Judd–Ofelt parameters and by the bonding parameter b 1 / 2 due to Nephelauxetic effect.

Optical and vibrational studies have been carried out on 60B{sub 2}O{sub 3}.(20-x)Na{sub 2}O.10PbO.10Al{sub 2}O{sub 3}:xTiO{sub 2} (x=0, 1, 2, 3, 4, and 5 mol%) glasses, in order to understand the role of TiO{sub 2} in the 60B{sub 2}O{sub 3}.20Na{sub 2}O.10PbO.10Al{sub 2}O{sub 3} glass matrix. The X-ray patterns reveal homogeneous glasses over the entire compositional range. The absorption spectra show that the energy of the optical band gap ({Delta}E{sub opt}) and Urbach's energy (E{sub U}) decreases as TiO{sub 2} content increases. The changes observed in the Raman and IR spectra are related to the BO{sub 4}{yields}BO{sub 3} back conversion effect and the appearance of 'loose' BO{sub 4}{sup -} groups. The data indicate that titanium ions act as a network modifier. - Graphical abstract: Behavior of the relative integrated intensity of the IR bands, Ar=A{sub 4}/A{sub 3}, as TiO{sub 2} content increases. A decrease of 45% of Ar indicates that the number of BO{sub 4} groups is decreasing due to the BO{sub 4}{yields}BO{sub 3} back conversion. Highlights: > The XRD data show the amorphous character of all measured glasses. > The optical absorption results evidenced possible change of the glass structure. > The Raman data have revealed some vibrational modifications. > Themore » overall results indicate that titanium acts as a network modifier.« less

We investigate non-Gaussian statistical properties of stationary stochastic signals generated by an analog circuit that simulates a random multiplicative process with weak additive noise. The random noises are originated by thermal shot noise and avalanche processes, while the multiplicative process is generated by a fully analog circuit. The resulting signal describes stochastic time series of current interest in several areas such as turbulence, finance, biology and environment, which exhibit power-law distributions. Specifically, we study the correlation properties of the signal by employing a detrended fluctuation analysis and explore its multifractal nature. The singularity spectrum is obtained and analyzed as a function of the control circuit parameter that tunes the asymptotic power-law form of the probability distribution function.

We investigate the first-passage-time statistics of the integrate–fire neuron model driven by a sub-threshold harmonic signal superposed with a non-Gaussian noise. Here, we considered the noise as the result of a random multiplicative process displaced from the origin by an additive term. Such a mechanism generates a power-law distributed noise whose characteristic decay exponent can be finely tuned. We performed numerical simulations to analyze the influence of the noise non-Gaussian character on the stochastic resonance condition. We found that when the noise deviates from Gaussian statistics, the resonance condition occurs at weaker noise intensities, achieving a minimum at a finite value of the distribution function decay exponent. We discuss the possible relevance of this feature to the efficiency of the firing dynamics of biological neurons, as the present result indicates that neurons would require a lower noise level to detect a sub-threshold signal when its statistics departs from Gaussian.

In this paper we investigate the energy transfer processes in Tm 3+ /Er 3+ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er 3+ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm 3+ ions. This is evidence of the relevant contribution of the route Er 1 ( 4 I 11/2 ) + Er 2 ( 4 I 13/2 ) → Er 1 ( 4 I 15/2 ) + Er 2 ( 4 F 9/2 ) to the process. Energy migration among pumped 4 I 9/2 level reducing the efficiency of the upconversion emission rate ( 3 H 11/2 , 4 S 3/2 , and 4 F 9/2 ) is observed for Er 3+ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.

In this work, we develop an analog circuit to generate a stochastic signal with stationary distribution exhibiting a tunable power-law tail. The proposed circuit design is a variant of a recently introduced one based on a differential equation with both multiplicative and additive noises. Here, the circuit is carefully designed in order to ensure that all components operate within their specific regimes. We provide a detailed characterization of the output signal, including the power-law exponent dependence on the tunable component. We apply the present circuit to study the first passage time problem in a simple integrate-fire model of neural dynamics driven by a non-Gaussian noise.

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb 3+ /Yb 3+ -codoped tellurite glasses excited at 1.064 μm is investigated. Bright luminescence emission around 485, 550, 590, 625 and 655 nm, identified as due to the 5 D 4 → 7 F J ( J =6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the 5 D 4 emitting level of the Tb 3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300–500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb 3+ -sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb–Yb pair and Tb 3+ ground-state depopulation via multiphonon excitation of the 7 F J excited states describes quite well the experimental results.

Temperature investigation of infrared-to-visible frequency upconversion in erbium-doped tellurite glasses excited by CW laser radiation at 1540 nm and under cryogenic temperatures is reported. Intense upconversion emission signals around 530, 550 and 660 nm corresponding to the 2H11/2, 4S3/2, and 4F9/2 transitions to the 4I15/2 ground state were generated and studied as a function of the laser intensity and temperature. The upconversion excitation mechanism of the Er3+ ions emitting energy levels was accomplished via stepwise multiphoton absorption. The green upconversion luminescence exhibited a fivefold intensity enhancement when the temperature of the sample was varied in the range between 5 and 300 K. A maximum green upconversion intensity was attained around 120 K and a steady decreasing behavior for higher temperatures up to 300 K was observed. A model based upon conventional rate equations was used to model the observed temperature evolution of the upconversion luminescence.

Red, green, and blue emission through frequency upconversion and energy-transfer processes in tellurite glasses doped with Tm 3+ and Er 3+ excited at 1.064 μm is investigated. The Tm 3+ /Er 3+ -codoped samples produced intense upconversion emission signals at around 480, 530, 550 and 660 nm. The 480 nm blue emission was originated from the 1 G 4 → 3 H 6 transition of the Tm 3+ ions excited by a multiphoton stepwise phonon-assisted excited-state absorption process. The 530, 550 nm green and 660 nm red upconversion luminescences were identified as originating from the 2 H 11/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 transitions of the Er 3+ ions, respectively, populated via efficient cross-relaxation processes and excited-state absorption. White light generation employing a single infrared excitation source is also examined.

Upconversion luminescence and thermal effects in Pr3+/Yb3+- and Er3+/Yb3+-codoped 60TeO2–10GeO2–10K2O–10Li2O–10Nb2O5 tellurite glasses excited by CW infrared radiation at 1.064 μm is reported. Generation of intense green and red fluorescence emission in Er3+/Yb3+-codoped samples and appreciable upconversion luminescence in the wavelength region of 450–680 nm in Pr3+/Yb3+-codoped samples is observed. Temperature-induced enhancement of ×12 in the upconversion efficiency in Er3+/Yb3+- and ×10 in the Pr3+/Yb3+-doped samples is demonstrated.

Efficient and thermally enhanced frequency upconversion of cw infrared radiation at 1.064 μm into blue (480 nm), red (660 nm) and near-infrared (800 nm) light in integrated Tm 3+ /Yb 3+ -codoped phosphosilicate silica-on-silicon buried waveguides is reported. Thermally induced frequency upconversion enhancement is observed for the first time in Tm 3+ /Yb 3+ -codoped systems excited off-resonance at 1.064 μm. A times seven thermally induced upconversion efficiency enhancement was obtained when the waveguide's temperature was varied in the 20–120°C range. The thermal effect is believed to be associated with a temperature dependent effective absorption cross-section for the Yb 3+ -sensitizer, which depends upon the phonon occupation number in the host matrix.

We present an accurate, fast, and easily implemented procedure for modeling erbium-doped waveguide amplifiers (EDWAs) with high concentration doping level. The model is shown to be in a very good agreement when compared with experimental results, and is used in a detailed analysis of a waveguide amplifier with 980-nm pumping.

We experimentally investigate the ghost resonance phenomenon in the electronic circuit of Chua operating in the chaotic regime. The circuit can be stimulated to jump between two single-scroll attractors by an external periodic signal with an amplitude above an intrinsic threshold. For subthreshold signals, jumps between the chaotic attractors can be promoted by a superposed white noise. We show that the circuit output can exhibit a well-defined ghost resonance signature, i.e., a resonance on a frequency that is absent in a multicomponent input signal, when the amplitudes of the input components are properly related. Further, we show that ghost resonance can be induced by the Chua's circuit's own chaotic dynamics when it is driven by a suprathreshold multicomponent signal without the need of an external noise source.

Infrared-to-visible upconversion luminescence emission in Nd 3+ -doped 60TeO 2 –10GeO 2 –10K 2 O–10Li 2 O–10Nb 2 O 5 tellurite glasses under cw excitation around 800 nm is investigated. Blue (430, and 475 nm), green (530 nm) and yellow-orange (590 nm) energy upconversion emission owing to the 2 P 1/2 → 4 I j ( j =9/2, 11/2, 13/2 and 15/2) transitions of the Nd 3+ ions, respectively, was recorded. The dependence of the upconversion intensity upon the excitation wavelength and pump power is also studied. The upconversion excitation mechanism responsible for the observed emission signals is attributed to stepwise multiphoton absorption.

Efficient energy upconversion of cw radiation at 1.064 μm into blue, red, and near infrared emission in Tm3+-doped Yb3+-sensitized 60TeO2-10GeO2-10K2O-10Li2O-10Nb2O5 glasses is reported. Intense blue upconversion luminescence at 485 nm corresponding to the Tm3+ 1G4→3H6 transition with a measured absolute power of 0.1 μW for 800 mW excitation power at room temperature is observed. The experimental results also revealed a sevenfold enhancement in the upconversion efficiency when the sample was heated from room temperature to 235 °C yielding 0.7 μW of blue absolute fluorescence power for 800 mW pump power. High brightness emission around 800 nm (3F4→3H6) in addition to a less intense 655 nm (1G4→3H4 and 3F2,3→3H6) fluorescence is also recorded. The energy upconversion excitation mechanism for thulium emitting levels is assigned to multiphonon-assisted anti-Stokes excitation of the ytterbium-sensitizer followed by multiphonon-assisted sequential energy-transfer processes.

In this work, bright visible upconversion emission excited through cooperative energy-transfer and thermal effects in Tb/sup 3+//Yb/sup 3+/-codoped 60TeO/sub 2/-10GeO/sub 2/-10K/sub 2/O-10Li/sub 2/O-10Nb/sub 2/O/sub 5/glasses excited at 1.064 /spl mu/m is investigated. Bright upconversion luminescence around 485, 550, 590, 625 and 655 nm, identified as due to the /sup 5/D/sub 4//spl rarr//sup 7/F/sub J/(J= 6, 5, 4, 3, and 2) transitions of the terbium-ions, respectively, was generated.

The spectroscopic characteristics of Er3+-doped lead phosphate glasses have been investigated, and Judd?Ofelt analysis was used to evaluate the effect of increasing the Er3+ content on the glass matrices. The intensity-dependent Judd?Ofelt parameters, ?(4) and ?(6), remained constant while ?(2) decreased. The concentration quenching effect on the lifetime of the Er3+?:?4I13/2 ?4 I15/2 (1530?nm) transition is also evaluated as a result of the addition of Er3+ to the lead phosphate glass composition. The observed relatively large reduction in the lifetime reflects the significant effects of non-radiative processes in this system. The potential use of these glasses as photonic devices is also discussed.

Stable and intense second-order nonlinearity in soda lime glass is investigated tailoring the induced electric current. This procedure allows the determination of the relative contributions of the dipole orientation as well as the ionic contributions to the poling process. The experiments are developed in the light of multiple-carrier models controlling the output power supply applied current to tailor the frozen-in induced electric field — Edc. This method permits the induction of the stable nonlinearity for applied electric fields above ∼5kV∕cm and temperatures ∼250°C. It is also possible to reach higher temperatures than the ones used in normal poling procedures avoiding the electric current breakdown. The controlled Edc formation enables it to participate in essential chemical reactions that determine the intensity and stability of the nonlinearity. The induced d33 of ∼0.41pm∕V measured 20 days after poling reduced only ∼50% during the next seven months.

We report the experimental observation of up-conversion in an integrated silica-on-silicon waveguide. Blue light at 455 nm and 475 nm was observed. In addition to the strong blue emission near infrared (∼ 800 nm), visible (648 nm) and UV (366 nm) lines were also detected. We investigated the excitation mechanisms by using different pumping wavelengths.