Excited State Absorption Measurements Research Articles

Excited state absorption and energy transfer in Ho3+-doped indium fluoride glass

This investigation examines in detail the rates of energy transfer relevant to the 5I5 → 5I6 transition (at 3930 nm) in Ho3+-doped InF3 glass as a function of the Ho3+ concentration. The decay times, branching ratios and rate parameters for energy transfer were measured in this investigation for Ho3+ (x)-doped InF3 glass with x = 2, 4 and 10 mol.% and they were used as the input parameters for a rate equation analysis. Excited state absorption (ESA) initiating from the lower laser level is included in the study. Numerical simulation of CW laser emission at 3.9 μm was performed using two pump wavelengths, one for upper laser level excitation (i.e., 5I8 → 5I5 = λP1) and the other for lower laser level de-excitation (i.e., 5I6 → 5S2 = λP2). The pump wavelength λP2 = 962 nm was chosen based on the measurements of ESA and the application of the McCumber method. Critically, the estimated ESA cross section at λP2 = 962 nm (σESA = 7.1 × 10−21 cm2) is approximately sixteen times larger than ground state (5I8) absorption cross section (σGSA = 4.3 × 10−22 cm2) and ESA does not overlap with any ground state absorption process. Our calculations suggest that even for high Ho3+ concentration in which cross relaxation has been shown in a previous study to quench the 5I5 level, ESA is nevertheless strong enough to allow a sufficient population inversion required for practical CW emission.

Designing Small Molecule Organic Solar Cells with High Open‐Circuit Voltage

AbstractThree extended 2,5‐dithienylthiazolo[5,4‐d]thiazole‐based small molecule chromophores are prepared via a sustainable direct arylation approach and their physicochemical and opto‐electrical material characteristics are analyzed toward integration in solution‐processed bulk heterojunction organic photovoltaics. Efficient charge separation and high values of the charge transfer state energy are derived from sensitive ground and excited state absorption and photoluminescence measurements on blends of the thiazolo[5,4‐d]thiazole‐based electron donor components with the PC71BM fullerene acceptor. Upon implementation in organic solar cells, a maximum power conversion efficiency of 2.7% and particularly high open‐circuit voltages (0.93−0.98 V) are observed, which are correlated to the charge transfer state energies as derived from photoluminescence, Fourier transform photocurrent spectroscopy and combined electrochemical and photophysical data. Furthermore, several loss processes at the origin of the modest short‐circuit current densities and fill factors are elucidated.

Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.

Relaxation dynamics of Au25L18 nanoclusters studied by femtosecond time-resolved near infrared transient absorption spectroscopy

The relaxation dynamics of electronically excited [Au(25)(SR)(18)](q), where q = 0 or -1 and SR = S(CH(2))(2)Ph, were studied using femtosecond time-resolved transient absorption spectroscopy. Nanoclusters excited by 400 nm light were probed using temporally delayed broad-bandwidth continuum probe pulses. Continuum pulses were generated in both the visible and near infrared (NIR) spectral regions, providing access to a wide range of transient spectral features. The use of NIR probe pulses allowed the relaxation dynamics of the excited states located near the HOMO-LUMO energy gap to be monitored in the probe step via the sp ← LUMO and sp ← LUMO+1 transitions. These NIR measurements yielded excited state absorption (ESA) data that were much less congested than the typical visible transient spectrum. For the neutral nanocluster, the time-domain data were composed of three components: (1) a few-picosecond decay, (2) a slower decay taking a few hundred picoseconds and (3) a non-decaying plateau function. Component 1 reflected energy relaxation to semi-ring ligand states; component 2 was attributed to relaxation via a manifold of states located near the HOMO-LUMO energy gap. Component 3 arose from slow radiative recombination. The dynamics of the anion depended upon the identity of the excited state from which the particle was relaxing. The LUMO+1 state of the anion exhibited relaxation dynamics that were similar to those observed for the neutral nanocluster. By comparison, the time-domain data observed for the LUMO state contained only two components: (1) a 3.3 ± 0.2 ps decay and (2) a 5 ± 1 ns decay. The amplitude coefficients of each component were also analyzed. Taken together, the amplitude coefficients and lifetimes were indicative of an activation barrier located approximately 100 meV above the HOMO-LUMO energy gap, which mediated a previously unobserved excited state decay process for [Au(25)(SR)(18)](0). These data suggested that NIR ESA measurements will be instrumental in describing the relaxation processes of quantum-confined nanoclusters.

Excited state absorption measurement in the 900-1250 nm wavelength range for bismuth-doped silicate fibers

The feasibility of direct laser diode pumping of Bi-doped fiber lasers at the wavelengths of 915 and 975 nm was examined by measuring excited state absorption in Bi-doped silicate fibers for the wavelength range of 900-1250 nm. When the Bi-doped fibers were pumped at 1047 nm a strong excited state absorption was found at 915 and 975 nm, whereas no significant excited state absorption was observed in the 1080 nm pumping band nor in the emission band, approximately 1160 nm, of Bi-doped fiber lasers.

Initial state-resolved excited state absorption spectroscopy of ZBLAN:Ho3+ glass

Phase-sensitive and frequency-resolved detection techniques are used for the initial state-resolved excited state absorption (ESA) measurements in ZBLAN:Ho3+ glass. Both experimental techniques were applied simultaneously in a broad spectral range (550–1750 nm) for the first time. Estimated results are compared and discussed in detail. A simple kinetic model, used for qualitative considerations, is presented and successfully compared with the experimental data. The measured spectra will be useful for identifying new up-conversion excitation channels in the considered system, where ESA transitions originating from several excited levels are observed.

Upconversion in Nd3+-doped glasses: Microscopic theory and spectroscopic measurements

In this work, we report a systematic investigation of upconversion losses and their effects on fluorescence quantum efficiency and fractional thermal loading in Nd3+-doped fluoride glasses. The energy transfer upconversion (γup) parameter, which describes upconversion losses, was experimentally determined using different methods: thermal lens (TL) technique and steady state luminescence (SSL) measurements. Additionally, the upconversion parameter was also obtained from energy transfer models and excited state absorption measurements. The results reveal that the microscopic treatment provided by the energy transfer models is similar to the macroscopic ones achieved from the TL and SSL measurements because similar γup parameters were obtained. Besides, the achieved results also point out the migration-assisted energy transfer according to diffusion-limited regime rather than hopping regime as responsible for the upconversion losses in Nd-doped glasses.

Excited state absorption measurements as a powerful tool for advanced spectroscopy

Examples of the excited state absorption (ESA) spectra measurements along with interpretations are presented and concerned as a tool which can provide answers to questions arising from other spectroscopic data. Author presents several cases where the data taken from the ESA spectra had been successfully working in the above context. The ESA characteristics for Cr3+, Cr4+, Cr6+ or Pr3+ ions in the glass, ceramic and crystalline matrices are demonstrated and discussed.In case of Cr3+ and Cr4+ ions in very different matrices the ESA spectra allowed to determine character of incident and terminal states taking part in the transitions, in case of Cr6+ ions in two different glasses the ESA spectra gave rise for answer to the question on [CrO4]2− group and its luminescence efficiency, and finally, in case of Pr3+ ions in the fluoroaluminate glass the ESA measurements revealed multiple-line gain and its growth with temperature whereas in the LiNbO3 crystal the ESA spectra allowed to identify a trapped exciton taking part in the deexcitation and luminescence process.

Signal excited-state absorption in the L-band EDFA: Simulation and measurements

Signal excited-state absorption (ESA) influences the performance of erbium-doped fiber amplifier (EDFA) in the L band. The influence is twofold: 1) ESA is responsible for signal absorption, and thus for noise figure increase in the long wavelength range of the L band and 2) ESA decreases inversion of erbium ions, and as a result, amplifier gain is decreased. Two methods of signal ESA measurement are discussed. It is shown that reliable data can be obtained through measurement and later used for accurate simulation of amplifier performance.

Experimental and theoretical investigation of4f3↔4f25dinterconfigurational transitions inNd3+:LiYF4crystals

A complete spectroscopic investigation of the ${4f}^{3}\ensuremath{\leftrightarrow}{4f}^{2}5d$ optical transitions of the ${\mathrm{Nd}}^{3+}$ ion is performed in the case of ${\mathrm{N}\mathrm{d}:\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_{4},$ based on emission, excitation, ground-state absorption as well as excited-state absorption measurements. The global shape of all the experimental spectra is well reproduced by means of crystal-field calculations of the ${4f}^{2}5d$ sublevels, and the differences observed between polarized spectra on one hand, and between those recorded at low and room temperatures on the other hand, are successfully accounted for. It is also found that the absorption barycentre is shifted toward higher ${4f}^{2}5d$ energies when the chosen initial ${4f}^{3}$ sublevel itself is shifted toward higher energies. This behavior is assigned to a total or partial ``memory'' during the optical transition for the expectation values of different terms of the hamiltonian, especially the electrostatic ones. It is demonstrated that this effect should not be specific of ${\mathrm{Nd}}^{3+}:{\mathrm{LiYF}}_{4}$ and that it should occur, on the contrary, in every crystalline matrix and for any rare-earth ion from ${\mathrm{Pr}}^{3+}$ ${(4f}^{2})$ to ${\mathrm{Tm}}^{3+}$ ${(4f}^{12}).$

Spectroscopic characterisation of the upconversion avalanche mechanism in Pr3+,Yb3+:BaY2F8

Abstract The paper presents the spectroscopic investigation of Pr 3+ ,Yb 3+ :BaY 2 F 8 and the characteristics of the upconverted emission from the 3 P 0 level due to an avalanche mechanism under infrared excitation around 820 and 870 nm. Absorption and emission measurements at 12 K were performed in order to refine the Stark level scheme. Room temperature absorption, emission, excitation spectra, and excited state absorption measurements were performed and analysed in order to investigate the properties of the avalanche mechanism and to determine the spectroscopic parameters of this system. Based on the determined spectroscopic parameters a system of differential equations was established in order to describe the observed avalanche mechanism.

Multiple binding sites of fluorescein isothiocyanate moieties on myoglobin: photophysical heterogeneity as revealed by ground- and excited-state spectroscopy

Fluorescein isothiocyanate (FITC)–myoglobin conjugates were synthesized with a binding stoichiometry of one to three fluorophores per protein. FITC binding sites were determined by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). Five lysine residues and the N-terminal amino group were identified as preferential binding sites. The ground and excited-state absorption spectra and the fluorescence decay of the conjugates in the native and denatured state of the carrier protein were analyzed. For comparison, unbound FITC and FITC covalently bound to a polysaccharide (dextran) were studied. For FITC, FITC–dextran and the FITC–myoglobin conjugates, only one FITC absorption peak was obtained in the ground state spectrum. Similarly, the excited state absorption (ESA) spectra of unbound FITC and of FITC–dextran showed only one single maximum whereas two maxima were detected for the native FITC–myoglobin conjugates. One of these sub-bands disappeared following urea treatment of the conjugate. We conclude that ESA measurements of extrinsic fluorophores on proteins can be used to monitor different micro-environments of the fluorophore and to distinguish between different conformational states of the labeled protein. This method can be a useful tool for analysing coexisting protein conformations.

Optical spectroscopy and multiple-line gain in Pr3+-activated fluoroaluminateglass

We demonstrate the results of excited-state absorption measurements that revealoptical gain at several lines due to triplet–triplet transitions in Pr3+ ions in afluoroaluminate glass host. Gain around 589 nm is especially attractive for laserapplications in the area of satellite communication. The multiple-line character ofthe gain as well as the broadening of these lines is very promising because it canmake possible the creation of a widely tunable solid-state laser. The gainmeasurements are complemented by systematic measurements of the basicspectroscopic characteristics of Pr3+ in the fluoroaluminate glass host, where wedemonstrate the role of energy transfer and nonradiative transitions in thedeexcitation process. The latter is associated with a defect state in the glass host.

Excited state absorption in materials containing Cr3+ and Cr4+ ions

The ground state absorption (GSA), the excited state absorption (ESA) and fluorescence properties (low- and room temperature emission spectra) of Cr3+, Cr4+-doped single crystals: Y3Al5O12 (YAG) and SrGdGa3O7 (SGG) have been studied at various temperatures in a broad spectral range (300–1700 nm). In the measured characteristics of those crystals we have observed features which could be ascribed to both Cr3+ and Cr4+. In this work, we put a special stress upon the ESA measurements. The measurements were performed by the continuous wave pump-and-probe phase-sensitive technique using an Ar-laser (excitation directly into the ion) and tungstate lamp, and by pulsed technique using an excimer laser (UV excitation) and Xe-flash lamp. Relations between spectral responses of the Cr3+ and Cr4+ ions in different matrices under various excitations are observed and discussed.

Spectroscopic and colour centre studies of Pr-doped LuAlO3

We present the results of classical 4f 2 intra-configurational absorption spectroscopy, 3 P 0 and 1 D 2 fluorescence dynamics and 4 f 2 ( 1 D 2 )→4 f5d inter-configurational excited state absorption (ESA) measurements in LuAlO 3 :Pr 3+ crystals. As optical excitations with blue or ultraviolet (UV) photons lead to colour centre formation, these centres are studied in variously Pr-doped LuAlO 3 crystals after Xe-lamp irradiation.

Spectroscopic investigation of the Nd 3+ 4f 25d states in chloroelpasolite crystals

New spectroscopic results obtained with Nd 3+-doped Cs 2NaYCl 6 samples, as well as with the pure Cs 2NaNdCl 6 crystal, are presented. Static and dynamical properties of the Nd 3+ 4f 3 4D 3/2 state located in the ultraviolet spectral domain are investigated using time resolved spectroscopy and fluorescence decay time measurements. The Nd 3+ 4f 25d bands are studied via ground state absorption as well as by excited state absorption measurements.

Systematics of4felectron energies relative to host bands by resonant photoemission of rare-earth ions in aluminum garnets

The energies of trivalent rare earth ions relative to the host valence band were measured for a series of rare-earth-doped yttrium aluminum garnets, RxY3-xAl5O12 (R=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and 0<x<3), using ultraviolet photoemission spectroscopy. The 4f photoemission spectra were acquired using synchrotron radiation, exploiting the 4d to 4f “giant resonance” in the 4f electron photoemission cross-section to separate the 4f contribution. Theoretical valence band and 4f photoemission spectra were fit to experimental results to accurately determine electron energies. The measured 4f n ground state energies of these ions range from 700 meV above the valence band maximum for Tb 3+ to 4.7 eV below the valence band maximum for Lu 3+ , and all ground state energies, except for Tb 3+ , are degenerate with valence band states. An empirical model is successful in describing the relative energies of the 4f n ground states for rare earth ions in these materials. This model is used to estimate the positions of the lighter rare earth ions, giving good agreement with published excited state absorption and photoconductivity measurements on Ce 3+ in yttrium aluminum garnet. It is shown that the energies of the 4f electrons relative to the valence band can be estimated from the photoemission spectrum of the undoped host, providing a simple method for extending these results to related host crystals. The success of this model suggests that further studies of additional host compounds will rapidly lead to a broader picture of the effect of the host lattice on the 4f electron binding energies.

Excited state absorption in chromium doped Li2B4O7 glass

Excited state absorption (ESA) measurements of the Cr:Li2B4O7 glass(Cr:LBO-glass) along with preliminary interpretation are presented. Thepresence of chromium in its tri- (d3) and hexa- (d0) valence states isobserved. Both Cr3+ and Cr6+ ions appear to contribute in the de-excitationprocesses and can be attributed in the ESA spectra under excitationwavelengths at 308 nm, 488 nm, 515 nm and 610 nm. The ESA spectra detectedwith UV excitation have been interpreted in terms of transitions in theframework of the Cr5+O- centre, which forms after charge-transfer-type absorptionin the [CrO4]2- group. Assumption of the double-electron state of the 3d22p4electronic configuration together with crystal-field-split states of the3d12p5 configuration allowed us to reproduce the obtained ESA spectra. The ESAspectra of the Cr3+ ions have different characteristics and are related totransitions to the conduction band.

Mechanism of carrier generation and recombination in conjugated polymers

Transient excited-state absorption measurements in the spectral region spanning the infrared active vibrational active (IRAV) modes in prototypical luminescent polymers, poly(phenylene vinylene) (PPV), and poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV), reveal charge carrier generation within 100 fs after photoexcitation. The photocarrier quantum efficiency in MEH-PPV is φ 0≈0.1 in zero applied electric field. There is no correlation between the temporal behavior of the photoinduced IRAV signals and the exciton lifetime. Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation. Our data indicate that the carrier recombination rate is sensitive to the strength of the interchain interaction.

Transient excited-state absorption measurements in chromium-doped forsterite

The nonradiative transition dynamics between the excited ${}^{3}{A}^{\ensuremath{''}}{[}^{3}{E(}^{3}{T}_{1a})]$ state and the metastable ${}^{3}{A}^{\ensuremath{''}}{[}^{3}{E(}^{3}{T}_{2})]$ storage state of the tetravalent chromium ion $({\mathrm{Cr}}^{4+})$ in forsterite is investigated using femtosecond excite-and-probe technique. Following excitation to ${}^{3}{T}_{1a}$ with pump pulses at 750 or 773 nm the induced excited state absorption was probed with femtosecond pulses in the ultraviolet (306 nm), visible (650 nm), and infrared (1500 nm). The nonradiative decay time from ${}^{3}{T}_{1a}$ to ${}^{3}{T}_{2}$ was estimated from the rise time of the excited state absorption at 1500 nm using a rate equations model and its value at room temperature is only 3\ifmmode\pm\else\textpm\fi{}1 ps.