Excited State Absorption Process Research Articles

Spectroscopy and visible frequency upconversion in Er3+–Yb3+: TeO2–ZnO glass

The UV–Vis–NIR absorption studies of the Er3+/Er3+–Yb3+ doped/codoped TeO2–ZnO (TZO) glasses fabricated by the melting and quenching method has been performed. The spectroscopic radiative parameters viz. radiative transition probabilities, branching ratios and lifetimes have been determined from the absorption spectrum by using Judd–Ofelt theory. The near infrared (NIR) to visible frequency upconversion (UC) have been monitored by using an excitation of 976nm wavelength radiation from a CW diode laser. The effect of codoping with Yb3+ ions on the intensity of the UC emission bands from the Er3+ ions throughout visible region has been studied. The mechanism responsible for the observed upconversion emissions in the prepared samples have been explained on the basis of excited state absorption and efficient energy transfer processes.

Preparation and upconversion luminescence of monodisperse Gd2O3:Ho3+,Yb3+ nanocrystals

Gd2O3:Ho3+,Yb3+ nanocrystals were synthesized via solvothermal method. X-ray diffraction (XRD), transmission electron microscopy (TEM), absorption and upconversion spectra were employed to characterize the synthesized nanocrystals. The results of XRD and TEM showed that obtained Gd2O3:Ho3+,Yb3+ nanocrystals were cubic in crystal structure and uniform spherical in morphology. The average crystallite size was calculated to be 7.5 nm. Green and red up-conversion emissions corresponding to (5F4, 5S2)→ 5I8 and 5F5→ 5I8 transition were observed upon 980 nm excitation at room temperature. The results indicated that both green and red luminescence were based on the two-photon processes. Laser power and doping concentration dependence of the upconverted emissions were studied to understand the upconversion mechanisms. Excited state absorption and energy-transfer processes were discussed as the possible mechanisms for the visible emissions.

Remarkable optical limiting and stabilization effects of a malononitrile derivative on 1064 nm pulsed laser

A malononitrile derivative, 2-[2,5-bis-(4-diethylamino-benzylidene)-cyclopentylidene]-malononitrile (S2), is reported to exhibit a much more remarkable optical limiting and stabilization behaviors on 1064 nm pulsed laser compared with a typical stilbazolium dye, 4-[2-(4-diethylamino-phenyl)-vinyl]-1-ethyl-pyridinium (R1). The optical limiting mechamisms of S2 and R1 under ns pulsed laser are both due to a two-photon absorption induced excited state absorption process. A superior large excited state absorption cross section of 2.6 × 10−15 cm2 at 1064 nm is found as the main reason to the remarkable optical limiting effect of S2. Meanwhile, S2 presents higher thermal stability and equivalent photochemical stability compared with R1.

Influence of Yb3+ doping concentration on upconversion luminescence of Er3+ in Lu2O3 phosphor

Cubic phase Lu2O3:Er3+/Yb3+ nanocrystal phosphors were prepared by sol–gel method. Fourier transform infrared (FT-IR) spectra were measured to evaluate the vibrational feature of the samples. Green and red radiations were observed upon 980nm diode laser excitation. Laser power and Er3+ or Yb3+ doping concentration dependence of upconversion luminescence were studied to understand upconversion mechanisms. Excited state absorption, cross relaxation and energy transfer processes are the possible mechanisms for the visible emissions.

Photophysical Properties and Ultrafast Excited-State Dynamics of a New Two-Photon Absorbing Thiopyranyl Probe

Comprehensive linear photophysical and photochemical characterization, two-photon absorption (2PA) properties, and femtosecond excited-state dynamics of a symmetrical fluorene derivative 2-(2,6-bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)vinyl)-4H-thiopyran-4-ylidene)malononitrile (1) are reported. The linear one-photon absorption (1PA), steady-state fluorescence, excitation, and excitation anisotropy spectra of 1 were investigated in organic solvents of different polarities at room temperature, exhibiting rather complex absorption and emission behavior. The relatively strong 2PA of thiopyranyl 1 was investigated by the open aperture femtosecond Z-scan technique in the main long wavelength 1PA contour with maxima cross sections up to 600–700 GM. Femtosecond dynamics of the excited-state absorption (ESA) and gain processes in 1 exhibited fast complicated relaxation phenomena with a strong dependence on solvent polarity and a weak dependence on excitation wavelength. The nature of the observed transient absorption kinetics was explained based on the short-lived ESA bands of 1 and solvate relaxation phenomena. Quantum chemical calculations, based on density functional theory, were employed for additional analysis of the 1PA and 2PA properties of 1.

Excited-State Absorption of Conjugated Polymers in the Near-Infrared and Visible: A Computational Study of Oligofluorenes

Excited-state properties of conjugated polymers play a central role in applications ranging from organics-based photovoltaics to nonlinear photonics. From a theoretical and computational point of view, however, an accurate first-principles description poses a formidable task. Typical molecule sizes go well beyond the size limits for which highly reliable wave function based electronic-structure methods can be applied. In the present work, we demonstrate that nonlinear-response density functional theory can be used to accurately model the excited state absorption process in an important class of conjugated materials. We compute transitions between up to 100 excited states for fluorene oligomers containing up to about 100 conjugated atoms. Furthermore, we demonstrate that this approach can explain the nature of absorption bands in the ESA in near-infrared and visible spectral range. These systems are large enough that we approach the polymer limit in terms of electronic properties of excited states. The res...

Near-infrared emission and upconversion in Er3+-doped TeO2–ZnO–ZnF2 glasses

We have investigated the near infrared emission and upconversion of Er3+ ions in two different compositions of glasses based on TeO2, ZnO, and ZnF2 for different ErF3 concentrations (0.5, 1, 2, and 3wt%). Judd–Ofelt intensity parameters have been determined and used to calculate the radiative transition probabilities and radiative lifetimes. The infrared emission at around 1532nm corresponding to the 4I13/2→4I15/2 transition is broader by nearly 30nm if compared to silica based glasses. The stimulated emission cross section is higher for the glass with the lowest content of ZnF2 which also shows higher values of the figure of merit for bandwidth. On the other hand, the lifetimes of the excited states are longer for the glass with the highest content of ZnF2. Green and red emissions corresponding to transitions (2H11/2,4S3/2)→4I15/2 and 4F9/2→4I15/2 have been observed under excitation at 801nm and attributed to a two photon process. The temporal evolution of the green emission suggests the presence of excited state absorption and energy transfer upconversion processes to populate the 4S3/2 level. In the case of the red emission, its increase as ErF3 concentration increases together with its temporal behavior indicate that for ErF3 concentrations higher than 0.5wt%, level 4F9/2 is populated by multiphonon relaxation from level 4S3/2 and energy transfer processes.

Photoluminescence study of Y2O3:Er3+-Eu3+-Yb3+ phosphor for lighting and sensing applications

The Er3+, Eu3+, and Yb3+ codoped Y2O3 phosphors have been synthesized by combustion synthesis process. For the structural information, the XRD analysis of the developed phosphor has been done. The frequency upconversion (UC) emissions in the codoped Y2O3 phosphor on excitation with 980 nm diode laser in the visible region have been performed and explained on the basis of excited state absorption and energy transfer process. The mechanism responsible in UC emissions was observed to involve two photon absorption and efficiency of the UC luminescence is significantly enhanced by introducing the Yb3+ ions. The tunability in colour of emitted radiation has been visualized by chromaticity diagram on increasing power of excitation source. The temperature sensing behaviour of developed phosphor material has been investigated using fluorescence intensity ratio technique.

Structural and luminescence studies on Er3+/Yb3+ co-doped boro-tellurite glasses

The present work reports structural and optical behaviour of the Er3+/Yb3+ co-doped boro-tellurite glasses 50TeO2+15H3BO3+(5−x−y)La2O3+15MgCO3+15SrCO3+xEr2O3+yYb2O3 (where x=1, y=0, 0.5, 1 and 2 in wt%) studied through Fourier transform infrared (FTIR), Raman, absorption, luminescence, upconversion luminescence and lifetime measurements for green laser applications. The TeOTe, O3BOBO3 bond bending vibrations and the fundamental stretching vibration of OH groups are confirmed through FTIR and Raman spectra. Through the absorption spectra, bonding parameters, oscillator strengths and Judd–Ofelt (JO) parameters were calculated and reported. From the luminescence spectra, radiative parameters have been calculated for the 2H9/2→4I15/2,4S3/2→4I15/2 and 4I13/2→4I15/2 excited state transitions. Upconversion luminescence spectra of the prepared glasses have been recorded and the excited state absorption (ESA) and Energy transfer (ET) processes are discussed. The luminescence decay curves for the 4I13/2→4I15/2 transition of the title glasses under 980 excitation wavelength have also been studied and reported.

Theoretical Modeling of Ho-Doped Fiber Lasers Pumped by Laser-Diodes Around 1.125 $\mu$m

A theoretical model to describe Holmium-doped fiber lasers pumped by Laser-diodes around 1.125 μm is presented. With considerations of important energy transfer processes, such as laser re-absorption, cross-relaxation (CR), energy transfer upconversion (ETU), and excited state absorption (ESA), good agreement is achieved between the simulations and experimental measurements. The ESA process is found playing an important role to the fiber laser performance, and the CR and ETU processes make a negative contribution to the lasers. On the basis of the model, optimization of the lasers threshold and slope efficiency as well as output power on the length of active media, the output coupling, pump wavelength, and Ho3+ ions concentration are investigated. It is found that lower doping concentration (about 5000 ~ 10000 ppm) for the fiber lasers is preferred for efficient operation.

Synthesis and luminescence properties of La2Ti2O7:Er3+ nanocrystals with pyrochlore structure

Er3+-doped La2Ti2O7 nanocrystals with pyrochlore structure are prepared by sol–gel method and examined by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV–vis spectrophotometer. An intense green emission along with a red emission is observed under the excitation at 980nm. La2Ti2O7:Er3+ exhibits the intense green emission bands around 525 and 549nm, and the weak red emission bands around 660nm. The optimal annealing temperature is 1000°C and the appropriate doping concentration of Er3+ is 13at.%. Power dependence analysis reveals that the green and red emissions originate from a two-photon absorption process. Excited state absorption and energy transfer processes are discussed as the possible upconversion mechanisms.

Structural, thermal, and optical properties of Pr3+/Yb3+ co-doped oxyhalide tellurite glasses and its nano-crystalline parts

Nano-crystalized glass-ceramics and ceramics co-doped with Pr3+/Yb3+ have been successfully prepared by heat treatment of the precursor oxyhalide glass sample synthesized by melt-quench method. Thermal measurement of glass sample shows its glass transition temperature (Tg), and onset crystallization temperature (Tc). Formation of nano-crystals was verified by X-ray diffraction patterns and transmission electron microscopy (TEM) images. Values of optical band gap (Eopt) were found to be 2.85 eV and 1.54 eV for direct and indirect transitions, respectively. Up and downconversion emission spectra recorded with 976 and 532 nm laser wavelengths show emission from Pr3+ ions. Emission of Pr3+ enhances approximately thirty times when Yb3+ ion is added with the matrix. The Stark splitting and the intensity of different emission bands increase effectively when we approach to ceramic via glass-ceramic sample. Intense upconversion emission observed by naked eyes has been quantified in terms of standard chromaticity diagram (CIE). Power dependence study shows that the upconversion of NIR radiation to visible takes place mainly via photon avalanche (PA) process. Upconversion mechanisms have been discussed in terms of excited state absorption (ESA) and energy transfer (ET) processes.

Energy transfer and frequency upconversion in Er3+-Yb3+ codoped oxy-fluoro-tungstosilicate glasses

Er3+-Yb3+ codoped oxy-fluoro-tungstosilicate glasses with infrared-to-visible frequency upconversion luminescence were prepared by melting quenching in air. The effects of Er3+ doping on the optical properties of the samples were measured by means of techniques such as optical absorption spectra and photoluminescence spectra. The results showed that intense green and red signals centered at 546 and 665 nm, corresponding to the 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ by a multiphoton stepwise phonon-assisted excited-state absorption process, respectively, were simultaneously observed by exciting the samples with a diode laser operating at 980 nm at room temperature. The upconversion process was found very sensitive to Er3+ content at a constant Yb2O3 content of 5 mol.%. With the increase of Er3+ content from 0.5% to 1.5%, the upconversion intensity increased gradually. Further increasing of Er3+ content to 3.0% resulted in a significant fluorescence quenching. Moreover, the possible upconversion mechanisms were discussed based on the energy-matching conditions and the quadratic dependence on excitation power.

Energy level decay and excited state absorption processes in erbium-doped tellurite glass

The fundamental excited state decay processes relating to the 4I11/2 → 4I13/2 transition in singly Er3+-doped tellurite (TZNL) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 4I11/2 energy level at 970 nm and selective laser excitation of the 4I13/2 energy level at 1485 nm has established that energy transfer upconversion by way of a dipole-dipole interaction between two excited erbium ions in the 4I13/2 level populates the 4I11/2 upper laser level of the 3 μm transition. This upconversion has been analyzed for Er2O3 concentrations between 0.5 mol. % and 2.2 mol. %. The 4I13/2 and 4I11/2 energy levels emit luminescence with peaks located at 1532 nm and 2734 nm, respectively, with radiative decay efficiencies of 65% and 6.8% for the higher (2.2 mol. %) concentration sample. The low 2.7 μm emission efficiency is due to the non-radiative decay bridging the 4I11/2 → 4I13/2 transition and energy transfer to the OH− groups in the glass. Excited state absorption was observed to occur from the 4I13/2 and 4I11/2 levels with peak absorptions occurring at 1550 nm and 971 nm, respectively. The decay time of the 4I11/2 excited state decreased with an increase in the Er3+ concentration, which related to energy transfer to OH− ions that had a measured concentration of 6.6 × 1018 cm−3. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼80 kW cm−2 for a cw laser pump at 976 nm if [Er3+] ≥ 1.2 × 1021 cm−3 (or [Er2O3] ≥ 2.65 mol. %) without OH− impurities being present.

Tunable upconversion luminescence and energy transfer process between Yb3+ and Er3+ in the CaY4F14

Yb3+/Er3+ codoped CaY4F14 materials with tunable upconversion (UC) emission were prepared by a high temperature solid-state method. Based on the UC luminescence properties investigations, it was found that, under 980nm excitation, CaY4F14:Yb3+,Er3+ showed green–yellow–red upconversion luminescence by changing the composition of the Yb3+-Er3+ dopants. It was also found that the green and red emissions originated from the two photon processes, and the ground-state absorption (GSA), excited-state absorption (ESA) and energy transfer (ET) processes between Er3+/Yb3+ ions were involved in the upconverted emission mechanism. The measured lifetimes show a strong dependence on Yb concentration.

Enhanced Melanin Fluorescence by Stepwise Three‐photon Excitation

The fluorescence of eumelanin (from Sepia officinalis and black human hair) was activated and enhanced by almost three orders of magnitude by exposure to near-infrared radiation. No activation or enhanced emission was observed when the samples were heated up to 100°C. The near-infrared irradiation caused obvious changes to the eumelanin and could be seen by fluorescence and bright field imaging. The area of enhanced emission appeared to originate from a region with changes in the morphology of the eumelanin's granule and increased with exposure time. At least two different components with enhanced fluorescence were activated and could be distinguished by their excitation properties. One component could be excited efficiently with wavelengths in the visible region and exhibited linear absorption dependence with respect to the laser power level. The second component could be excited efficiently using near-infrared wavelengths by a nonlinear process and exhibited a third-order dependence on the excitation. The third-order dependence is explained by a step-wise excited-state absorption process since the same third-order dependence was present when either continuous wave or femtosecond pulsed laser, with similar average-power levels, was used.

Synthesis and upconversion luminescent properties of Er 3+ doped and Er 3+–Yb 3+ codoped GdOCl powders

Er 3+ doped and Er 3+–Yb 3+ codoped GdOCl phosphors were prepared by modified solid state reaction. X-ray diffraction, scanning electron microscope, and Raman spectrum of the samples were studied. The result of Raman spectrum shows that the cutoff phonon energy of GdOCl is only 505 cm −1, which is beneficial for upconversion luminescence. Infrared-to-visible upconversion emissions were observed under 980 nm diode laser excitation. It was found that the ratio of green to red upconversion emission intensity varies with concentration of the Er 3+ or Yb 3+ ion. Laser power and doping concentration dependence of upconversion luminescence were studied to understand the upconversion mechanisms. Excited state absorption and energy-transfer processes are proposed to be the possible mechanisms for the visible emissions.

Plasmonic Enhancement of Nonradiative Charge Carrier Relaxation and Proposed Effects from Enhanced Radiative Electronic Processes in Semiconductor−Gold Core−Shell Nanorod Arrays

Plasmonic field enhancement of nonradiative exciton relaxation rates in vertically aligned arrays of high aspect ratio CdTe−Au core−shell nanorods was investigated by transient absorption spectroscopy, computational electromagnetics, and kinetic modeling. Increasing shell thickness in the high aspect ratio nanorods was found to result in dramatic differences in polarization-dependent nonradiative exciton relaxation rates, which we attribute to differing mechanisms of plasmonic field enhancement associated with predominant ground- or excited-state absorption processes. These results are compared with previous investigations of low aspect ratio CdTe−Au core−shell nanorods, and overall conclusions regarding plasmonic enhancement of nonradiative relaxation rates in this system are presented. We propose that when the resonantly coupled dipolar plasmon field of the shell is polarized parallel to the ground-state absorption transition moment of the core, Auger recombination dominates carrier relaxation and slower second-order decay kinetics are observed. When contributions of the resonantly coupled plasmon field are nondipolar or orthogonal to the ground-state absorption transition moment of the core, excited-state absorption processes are believed to dominate and increasingly rapid first-order relaxation kinetics are observed. We find that these processes can vary greatly, depending on shell thickness and the orientation of the array, but are insensitive to aspect ratio. These investigations have significant implications in the design of photovoltaic and optoelectronic devices incorporating anisotropic plasmonic elements.

Model and Characteristics of Self-Pulsing in ${\rm Tm}^{3+}$-Doped Silica Fiber Lasers

In this paper, a theoretical model, taking into account the phonon-assisted excited-state absorption process ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ), has been constructed to explain the self-pulsing (SP) phenomenon in the Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers. Both numerical simulation and experimental investigation on the SP characteristics of ~2-μ.m Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers have been carried out. The numerical simulation shows agreement with the experimental results. Furthermore, the impacts of output coupling, pump intensity, and Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -ion doping concentration on the SP characteristics of the Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers have been studied explicitly, and optimization of the SP features has been discussed. In addition, the potential applications of SP in fiber lasers or solid-state lasers have also been proposed.

Nonsaturable absorption in alumino-silicate bismuth-doped fibers

We report an experimental and theoretical investigation of fluorescence decay and transmission coefficient at ∼1 μm pumping for a series of bismuth (Bi) doped alumino-silicate fibers with different concentrations of Bi centers. By modeling the experimental data, we show that the excited-state absorption (ESA) and up-conversion (UC) processes are responsible for a growth of nonsaturable absorption and deviation from exponential the fluorescence decay in the fibers with an increase in Bi centers content. Assuming that Bi centers ensemble is composed of subsystems of single and paired centers, and an increase in partial weight of the latter with a Bi centers concentration growth, we can successfully explain the experimental data. Also, these assumptions allow us to estimate the constants characterizing the ESA and both homogeneous and inhomogeneous UC processes in the fibers.