Excited State Absorption Spectra Research Articles

Excited-state absorption and up-conversion losses in the Nd-doped glasses for high-power lasers

Excited-state absorption spectra of two Nd-doped glasses, a silicate LSG91H and a phosphate LHG8 from Hoya, are reported. An adequate model based on up-conversion energy transfers is then used to account for the shortening of the infrared fluorescence decays and the losses on the stored energies observed or predicted in this type of materials at high pump fluences.

The up-conversion of near-infrared excitation radiation in Ho 3+-doped LiYF 4

The up-conversion processes due to the continuous wave excitation of the holmium 5 I 4 multiplet in LiYF 4 crystal are investigated. Despite a very low absorption cross-section for the transition from the holmium 5 I 8 ground state to the 5 I 4 multiplet, a bright green up-converted emission originating from the 5 S 2 multiplet is observed. The excitation and excited state absorption spectra indicate that the up-conversion process is based mainly on excited state absorption from the metastable 5 I 7 multiplet. Some features of the up-converted green emission are characteristic for the photon avalanche process.

Experimental and theoretical investigations of the 4ƒ^2→ 4ƒ5d ground-state and excited-state absorption spectra of Pr^3+ in LiYF_4

Near-UV excited-state absorption spectra obtained with a pulsed pump–probe technique were recorded in LiYF4:Pr3+ at 77 and at 8 K and revealed a detailed structure of the 4f2 1D2→4f5d and 4f2 (3P0+1I6)→4f5d optical transitions. These experimental results, together with those previously obtained at room temperature, are compared with the numerical predictions of a full calculation of the 4f5d sublevels and of the electric dipole transitions toward these sublevels from different states of the 4f2 configuration. The agreement is good, provided that the four adjustable parameters of the theory are taken as B20(5d)=7290 cm-1,B40(5d)=-14 900 cm-1,F0=60 557 cm-1, and 〈r〉fd=0.245 A. The level structure of the 4f5d configuration of LiYF4:Pr3+ is shown to be dominated, as expected, by the crystal field coupling of the 5d electron but also, to a noticeable extent, by the spin–orbit coupling of the 4f electron and the Coulomb repulsion of both electrons. The former of these two interactions is somewhat more efficient than the latter in the low-energy part of the 4f5d configuration, i.e., in the spectral region where the largest amount of experimental data is currently available.

Spectroscopy and diode-pumped laser oscillation of Yb3+, Ho3+-doped yttrium scandium gallium garnet

We report on the laser oscillation of Yb3+, Ho3+-doped yttrium scandium gallium garnet around 1.2 and 3 μm under laser diode excitation. At 1208 nm output powers of up to 135 mW and slope efficiencies up to 7.6% with respect to the incident pump power were achieved at room temperature under continuous wave excitation. Quasicontinuous wave excitation led to laser emission at 2844 nm with pulse energies up to 10 m.J. The laser relevant spectroscopic data, i.e., lifetimes, absorption, and emission cross sections are given and the excited state absorption spectrum is presented.

Absorption Spectra of C60-Excited States in Various Solvents: Their Dependence on the Ionization Potential of Solvent Molecules

Abstract Using a picosecond laser photolysis system, the absorption spectra of excited states of C60 in twelve solvents were measured in the 590—1190 nm wavelength range. The absorption spectra observed immediately after a 532 nm picosecond laser excitation and those in the late-time region depended upon the solvents. In particular, the former dependence was much larger than the latter one. Their spectral shapes did not correlate with either dielectric constant of the solvents or their refractive index, but with their ionization potential (Ip). The early-gated absorption spectra of C60 in solvents with a small Ip were extraordinary broad compared with those in solvents with a large Ip. In addition, the absorption changed to those observed in the late-time region both with a decay time constant of 1.2±0.05 ns and with keeping the isosbestic points. This time constant was the same as the lifetime of the C60-singlet excited state (1C60*) reported previously, and the presence of the isosbestic points was also the same as that observed during spectral evolution from Sn←S1 absorption of C60 to its Tn←T1 one. Combining the results of the resonance Raman spectra by Gallagher et al. with the present ones suggested that the solvent-dependent early-gated absorption spectra of the C60-excited state are ascribed to 1C60* with a distortion induced by specific interactions with solvents molecules through the donation of their π-electron density without forming obviously excited-state complexes. The transient absorption spectra observed immediately after the direct excitation of ground-state charge-transfer complexes formed between C60 and solvent molecules with a small Ip were not due to ionic species, but to 1C60*, suggesting that the photogenerated ionic species undergo a very rapid charge-recombination reaction to result in 1C60* and the ground-state solvent molecule. The absorption spectra in the late-time region, which showed a slight dependence on Ip of the solvent molecules, as aforementioned, were assigned to the Tn←T1 absorption of C60; the dependence was considered to be also ascribed to the same reason as that for 1C60*.

Excited-state absorption of Er 3+ in the Ca 2Al 2SiO 7 laser crystal

The excited-state absorption (ESA) and stimulated-emission (SE) of the Er 3+ ion in the crystal Ca 2Al 2SiO 7 have been measured between 425 and 1800 nm. The measurements were performed by a continuous wave pump and probe technique. The calibrated ESA and SE cross-sections spectra are presented and the incidence of ESA on the laser performance around 1.55 μm (transition 4 I 13/2→ 4 I 15/2) is discussed. Finally, the Judd–Ofelt analysis, usually applied from the ground-state manifold, is used to estimate the ESA-integrated cross-sections and this formalism is found to be reliable in its predictions with an accuracy of about 20%.

Spectroscopic investigations of the 4ƒ5d energy levels of Pr^3+ in fluoride crystals by excited-state absorption and two-step excitation measurements

Excited-state absorption spectra obtained by a pulsed pump–probe technique specially designed for the near-UV spectral domain were recorded in three Pr3+-doped fluoride crystals (KY3F10,LiYF4, and BaY2F8). These excited-state absorption spectra correspond to optical transitions from the 3P0 or the 1D2 metastable levels of the 4f2 configuration up to the first energy levels of the 4f5d electronic configuration. The position, the anisotropic behavior, and the line shape of the observed excited-state absorption bands are discussed and compared with ground-state absorption (from level 3H4) or direct excitation spectra available in the literature. Subsequently, two-step excitation into the 4f5d high-lying energy levels of Pr3 by means of 3P0 and 1D2 is shown to be an efficient way to produce broadband UV fluorescence by use of pump beams in the blue and the near UV.

Nonlinear spectroscopy and passive Q-switching operation of a Co^2+ :LaMgAl_11O_19 crystal

A pump–probe technique with picosecond resolution was used to record and derive the differential and excited-state absorption (ESA) spectra of a Co2+:LaMgAl11O19 (LMA) single crystal in the visible and near-infrared spectral regions. Time resolution allowed us to observe ESA bands that can be assigned to a 4T2(4F)→4T1(4P) optical transition and to transitions from the thermally populated 2E(2G) excited state to doublet levels that arise from the 2F free-ion level of the tetrahedrally coordinated Co2+ ion. Intensity-dependent transmission measurements were also carried out at 1.34 and 1.54 µm. Passive Q switching of Nd3+:YAlO3 (1.34-µm) and of Er3+:glass (1.54-µm) lasers by use of the Co2+:LMA crystal as a saturable absorber was demonstrated. The pulse durations (energies) of the Q-switched Nd3+:YAlO3 and Er3+:glass lasers were found to be 75 ns (3.8 mJ) and 50 ns (4.5 mJ), respectively.

Excited state absorption study and judd-ofelt analysis of Er3+-states in Ti: LiNbO3 channel-waveguides

The Ti: LiNbO3 channel-waveguides with Er3+ ions incroporated by diffusion-doping were studied. Excited state absorption spectra under pumping at 1.5 μm and 980 nm were measured by means of a double modulation lock-in technique. The evaluated line strengths for several absorption transitions were analyzed by the Judd—Ofelt theory taking into account the crystal anisotropy. On this basis the pumping power dependence of relative populations of Er3+ -states was determined which is important in view of possible 3 μm and 550 nm up-conversion laser operation.

Excited-state absorption at 1.3 μm in Nd 3+-doped fluoride and sulfide glasses

Excited-state absorption (ESA) and emission spectra are recorded from the 4F 3/2 level of Nd 3+ ions in Zr–Ba–La–Al–Na (ZBLAN) and Ba–In–Ga (BIG) fluoride glasses and in Ge–Ga–S–Cs–Cl (GGSC) chlorosulfide glass. The respective contributions of stimulated emission (SE) and ESA are determined in the 1.3 μm region, a spectral domain that corresponds to the second telecommunication window. The pump–probe technique utilized in this study predicts the gain profile as a function wavelength in optical fibers. Gain is expected at wavelengths greater than 1.31 μm with ZBLAN and BIG, and 1.35 μm with GGSC. It is shown that the relative intensities of SE and ESA processes can be evaluated with consistency by Judd–Ofelt calculations.

Some optical properties of Cr 4+-doped crystals

We summarize briefly some of our past studies, and report of preliminary recent new results concerning the optical properties of Cr 4+-doped crystals that are important for utilization as passive Q-switching devices in Nd:YAG laser systems. The host crystals involved are YAG, YSGG, GGG, LuAG and forsterite. Excited-state lifetimes of 4.0, 1.0 and 1.7 μs were measured by the Cr 4+ fluorescence decay (1.3–1.7 μm) following pulsed excitation at 1064 nm in YAG, YSGG and GGG, respectively. The ground- and excited-state absorption (ESA) cross-sections at λ=1064 nm were estimated from transmission saturation measurements. For [Cr 4+,Mg 2+]:YAG the respective results were σ gs=(3.25±0.15)×10 −18 cm 2 and σ es=(6.25±0.5)×10 −19 cm 2. In the orthorhombic forsterite, the cross-sections were polarization dependent. We got σ gs<(3.3±1)×10 −19 cm 2 for E‖ a, σ gs=(23±2)×10 −19 cm 2 and σ es=(9.0±0.7)×10 −19 cm 2 for E‖ b, and σ gs=(16±1)×10 −19 cm 2 and σ es=(5.7±0.4)×10 −19 cm 2 for E‖ c. Polarised ESA spectra were measured between 680 and 960 nm using the pulsed pump/probe technique. At 750 nm we got σ gs=(110±10)×10 −19 cm 2 and σ es=(25±3)×10 −19 cm 2 for E‖ b. Passive Q-switching performance of a flashlamp-pumped Nd:YAG laser using an intracavity [Cr 4+,Ca 2+]:GGG sample is also demonstrated.

Ultrafast Dynamics in the Metal-to-Ligand Charge Transfer Excited-State Evolution of [Ru(4,4‘-diphenyl-2,2‘-bipyridine)3]2+

The transition metal complexes [Ru(dmb)3]2+ and [Ru(dpb)3]2+, where dmb is 4,4‘-dimethyl-2,2‘-bipyridine and dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been studied by femtosecond visible electronic absorption spectroscopy. Spectroelectrochemical measurements in conjunction with nanosecond time-resolved absorption spectroscopy allow for the assignment of various features in the excited-state differential absorption spectra as both ligand-based π* ← π* and ligand-to-metal charge transfer (LMCT) in nature. A unique absorptive feature centered at ∼ 530 nm in [Ru(dpb)3]2+ was identified as an optical marker for the thermalized (and hence fully intraligand delocalized) excited state. Single wavelength and full spectrum transient absorption data were obtained on both molecules in CH3CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excitation at 400 nm. Data on [Ru(dmb)3]2+ at 532 nm, a region of net excited-state absorption, revealed biphasic decay kinetics (∼120 fs and 5 ps) attributed to a combination of 1MLCT → 3MLCT intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb)3]2+ under identical conditions revealed biphasic rise times in the region of the ligand-based π* ← π* absorption at λprobe = 532 nm. Although the origin of the fast component (∼ 200 fs) is not yet clear, the ca. 2 ps rise is assigned to rotation of the peripheral aryl ring and thus corresponds to the time scale for intraligand electron delocalization.

4f2 to 4f5d excited state absorption in Pr3+:YAlO3

Excited-state absorption spectra, from the 3PJ-1I6 or 1D2 manifold to 4f5d levels, recorded in the 23000 - 42000 cm-1 range and calibrated in units of cross sections, are reported and discussed in the case of Pr3+-doped YAlO3. The 4f5d4f2Pr3+ emission between 30000 and 44000 cm-1 is then observed, showing the possibility of UV laser operation following two-step excitation pumping using near-UV-visible pump photons.

Optical spectroscopy and excited state absorption of the ZAS (ZrO 2–Al 2O 3–SiO 2) glass doped with chromium

We describe the general spectroscopic characteristics of the zirconia–alumino–silicate glass doped with chromium. The material displays all the features typical of trivalent chromium occupying weak field sites with spectral properties characteristic of the glass phase. In the excited state absorption (ESA) spectra features typical of chromium doped materials known from previously reported data are observed, although not all of them can be attributed to the Cr 3+ ions. The ESA spectrum, partially overlapping the emission range, appears to preclude lasing in the range 11300–14400 cm −1, but for the energies lower than 11300 cm −1 a net gain can occur. Based on absorption and emission data a single configuration coordinate diagram is created which allows a predicted ESA spectrum to be compared with the experimental data.

Signatures of excitons and polaron pairs in the femtosecond excited-state absorption spectra of phenylene-based conjugated polymers and oligomers

We present results of broadband femtosecond transient absorption measurements in thin films of several phenylenebased onjugated materials. We compare results for unsubstituted poly(phenylene vinylene) (PPV), polyflourene, and a substituted five-ring oligomer of PPV. We observe universal spectral features in all materials, implying a common origin for the photoexcited species in all members of this family. In each material, we observe two photoinduced absorption (PA) features in the visible region in addition to a stimulated emission (SE) feature. The first PA feature shows strong dependence of the dynamics on excitation intensity, and correlates precisely with the SE feature, proving that this feature is due to the primary emissive exciton. The second PA feature shows dynamics independent of intensity. In the oligomers, this feature grows quadratically with respect to the first PA feature, as a result of nonlinear interactions of the primary excitons. In the polymers, however, the second feature grows linearly with intensity, and indicates an ultrafast decay channel to nonemissive interchain species due to defects.

Excited-state absorption of Er3+-doped LiNbO3

The excited state absorption spectra of an Er:LiNbO3 single crystal are reported and analyzed versus polarization in both visible and near infrared spectral regions between 500 and 1800 nm. These spectra are completed with excited state excitation (ESE) spectra of the bright green anti-Stokes fluorescence observed in this material. These ESE spectra are recorded more particularly around 800 and 980 nm, and the results are analyzed in view of up-conversion laser operation after diode laser pumping around these two wavelengths.

Excited state absorption in Yb 3+–Er 3+-codoped phosphate glasses (ZnO–Al 2O 3–La 2O 3–P 2O 5) around the 4I 13/2→ 4I 15/2 emission spectral range

We are involved in Yb 3+–Er 3+-codoped phosphate glass research for eye-safe laser source near 1540 nm. Undoped and doped glasses of the composition: ZnO–Al 2O 3–La 2O 3–P 2O 5 have been synthesised. Absorption from the 4I 15/2 ground state, emission spectra and decay time measurements have been performed at room temperature on these materials. In addition to the usual spectroscopic characterisations, the excited state absorption 4I 13/2→ 4I 9/2 Er 3+ transition located at the long wavelength edge of the 4I 13/2→ 4I 15/2 emission transition have also been investigated at room temperature. The excited state absorption spectra were obtained with a pump and probe technique employing a double modulation scheme in the infrared wavelength region from 1300 to 1900 nm. Furthermore, the line strength of the excited state absorption transition have been calculated in order to compare it with the experimental.

Systematic synthesis and photochemistry of tetraaryl porphyrins mono-substituted with a transition metal carbonyl: characterisation of a zinc porphyrin–rhenium carbonyl complex †

A systematic synthesis designed to connect a metalloporphyrin to a metal carbonyl moiety is demonstrated through a zinc tetraphenyl porphyrin substituted with Re(CO)3(4′-methyl-2,2′-bipyridyl)Br via an amide bond; the emission and excited state absorption spectra are dominated by porphyrin transitions.

Single State Absorption Spectra of Novel Nonlinear Optical Materials

AbstractDuring the absorption of a laser pulse of moderate length, the leading edge can experience excited state absorption out of the first singlet state. The measurement of this excited state absorption spectrum can only be accurately probed using short pulse pump-probe techniques. Specifically, we examine the excited state absorption of AF-380 in THF using ultrafast transient white light absorption spectroscopy (TWLA). This material has been the focus of several investigations due to it's purported large two-photon absorption cross-section, the discrepancies between long and short pulse measurements, and it's use in holographic twophoton induced photopolymerization. It is believed that a substantial excited state absorbance can account for the difference in two-photon cross section measurements. It is also possible that this excited state exhibits coherence for time scales that can affect further absorption of longer pump pulses. We examine the transient absorption of this species, as well as polarization and free carrier effects and discuss the possible implications with regards to measurement techniques.

Excited state absorption in the gahnite glass ceramics and its parent glass doped with chromium

We report on comparison of the basic spectroscopy and first of all excited state absorption (ESA) characteristics measured for the chromium doped pink gahnite glass ceramics and its green parent glass doped with chromium. The ESA spectrum of the ceramics is rather predictable and reveals peaks which can be ascribed to doublet–doublet transitions, typical for the Cr 3+ ions in the high crystal field, and is additionally affected by the ground state absorption bleaching. Unlike the ceramics, the parent glass, being a low-field material, reveals quartet–quartet ESA, typical for the Cr 3+ ions in the low crystal field. The ESA spectrum apart of chromium-related features and bleaching of the chromium ground state absorption, is connected with other possible centers of the host. Based on basic spectroscopic data we create configuration coordinate diagrams for both cases, predict the shape of the expected ESA spectra and compare them with measured ones. A reasonable agreement has been achieved.