Excited State Absorption Transition Research Articles

Determining Excited-State Absorption Properties of a Quinoid Flavin by Polarization-Resolved Transient Spectroscopy.

As important naturally occurring chromophores, photophysical/chemical properties of quinoid flavins have been extensively studied both experimentally and theoretically. However, little is known about the transition dipole moment (TDM) orientation of excited-state absorption transitions of these important compounds. This aspect is of high interest in the fields of photocatalysis and quantum control studies. In this work, we employ polarization-associated spectra (PAS) to study the excited-state absorption transitions and the underlying TDM directions of a standard quinoid flavin compound. As compared to transient absorption anisotropy (TAA), an analysis based on PAS not only avoids diverging signals but also retrieves the relative angle for ESA transitions with respect to known TDM directions. Quantum chemical calculations of excited-state properties lead to good agreement with TA signals measured in magic angle configuration. Only when comparing experiment and theory for TAA spectra and PAS, do we find deviations when and only when the S0 → S1 of flavin is used as a reference. We attribute this to the vibronic coupling of this transition to a dark state. This effect is only observed in the employed polarization-controlled spectroscopy and would have gone unnoticed in conventional TA.

Upconversion-pumped femtosecond thulium laser at 2309 nm mode-locked by a GaSb-based SESAM

We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.

Benchmarking DFT functionals for photophysics of pyranoflavylium cations

An intense absorption, phosphorescence, a long triplet excited state lifetime and singlet oxygen generation capabilities are characteristics of pyranoflavylium cations, analogues to pyranoanthocyanidins originated in the maturation process of red wine. Such properties make these compounds potential photosensitizers to be applied in photodynamic therapy. In this context, the photophysical processes underlying that treatment critically depend on the electronic structure of the pyranoflavylium molecules. When employing density functional theory to describe the electronic structure of molecules, the choice of the most suitable functional is not trivial, and benchmark studies are needed to orient practitioners in the field. In this work, a benchmark of seven of the most commonly used density functionals in addressing the photophysical properties of a set of eight pyranoflavylium cations is reported. Ground and excited state geometries, molecular orbitals, and absorption, fluorescence and phosphorescence transition energies were calculated using density functional theory approaches, and evaluated and compared to experimental data and monoreferential wave function-based methodologies. Statistical analysis of the results indicates that global-hybrid functionals allow an excellent description of absorption and emission energies, with errors around 0.05 eV, while range-separated variants led to somewhat larger errors in the range 0.1–0.2 eV. In contrast, range-separated functionals display excellent phosphorescence energies with errors close to 0.05 eV, in this case global-hybrids showing increased discrepancies around 0.5–0.1 eV.

Manipulating Excited State Properties of Iridium Phenylpyridine Complexes with "Push-Pull" Substituents.

We have prepared a series of complexes of the type [IrIII(ppy)2(L]n+ complexes (1-4), where ppy is a substituted 2-phenylpyridine and L is a chelating phosphine thioether ligand. The parent complex (1) comprises an unsubstituted phenylpyridine ligand, whereas complex 2 contains a nitro substituent on the pyridine ring, complex 3 features a diphenylamine group on the phenyl ring, and 4 has both nitro and diphenylamine groups. Crystallographic, 1H NMR, and elemental analysis data are consistent with each of the chemical formulae. DFT (density functional theory) computational results show a complicated electronic structure with contributions from Ir, ppy, and the PS ligand. Ultrafast pump-probe data show strong contributions from the phenylpyridine moieties as well as strong panchromatic excited state absorption transitions. The data show that nitro and/or diphenylamine substituents dominate the spectroscopy of this series of compounds.

Excited-state absorption in thulium-doped materials in the near-infrared.

Excited-state absorption (ESA) is a key process for upconversion pumping schemes of thulium (Tm3+) doped laser materials. We have systematically studied two ESA transitions in the near-infrared spectral range, namely 3F4 → 3F2,3 (at ∼1 µm) and 3F4 → 3H4 (at ∼1.5 µm), in various Tm3+-doped fluoride (ZBLAN glass, cubic KY3F10 and CaF2, tetragonal LiYF4 and LiLuF4, monoclinic BaY2F8 crystals) and oxide (cubic Y3Al5O12, orthorhombic YAlO3 crystals) laser materials, using a pump-probe method with a polarized light. An approach to calculate the constants of energy-transfer upconversion (ETU) is also presented.

Vibrational Dynamics of the Intramolecular H-Bond in Acetylacetone Investigated with Transient and 2D IR Spectroscopy.

Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm-1 with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω21 excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.

Optical Transitions and Excited State Absorption Cross Sections of SrLaGaO4 Doped with Ho3+ Ions.

The spectroscopic properties of SrLaGaO4 (SLO) crystal doped with Ho3+ ions were studied in this work. Absorption, emission spectra and decay dynamics of excited states have been measured and discussed using the Judd–Ofelt model. Photoluminescence emissions were attributed to transitions from the excited 3D3, 5S2, 5F5, 5I6 and 5I7 multiplet manifolds. The experimental lifetimes for five excited states have been compared to the theoretical values, calculated using Judd–Ofelt theory, allowing for the determination of the multiphonon relaxation rates (WnR) of the respective states. The experimental data were approximately on a line expressed by WnR = W0 exp(−αΔE) with W0 = 0.5 × 107 s−1 and α = 2.6 × 10−3 cm. To discuss the excited state absorption (ESA) pathways, that originated from several excited levels, we used the Judd–Ofelt formalism allowing determination of the integrated cross section for ESA transitions.

Modeling and Analysis of a Pulsed Yb-Tm Fiber Laser System

Taking a comprehensive consideration of related excited state absorption, upconversion emission, and cross relaxation processes, a theoretical model concerning a pulsed Yb-Tm fiber laser is established. The criterion for stable Q-switching is derived with both excited state absorption processes and mode field area mismatch taken into account. And, detailed population transitions of major energy levels are illustrated. Impacts of pump power, mode field area ratio and saturable absorber length on the output characteristics of the pulsed laser system are investigated and analyzed. Simulations show that the excited state absorption transition 3F4→3F2,3 is the predominant factor supporting the passive Q-switching of Yb-Tm fiber lasers. Besides the pump power, the mode field area ratio and the length of the saturable absorber are also important factors affecting the output characteristics of the pulsed laser system. Proper choice of these parameters not only brings significant improvements to the pulse performance, with regards to both the pulse energy and the pulse duration, but also expands the stable operation range of the laser system. And, in practice, an effective mode field area ratio bigger than 1.65 should be exploited for the stable Q-switching operation of a pulsed Yb-Tm fiber laser.

Highly Conjugated, Fused-Ring, Quadrupolar Organic Chromophores with Large Two-Photon Absorption Cross-Sections in the Near-Infrared.

The two-photon absorption (2PA) properties are investigated for two series of organic, π-conjugated, fused-ring, quadrupolar A-π-D-π-A chromophores of the type originally developed as nonfullerene acceptors for organic photovoltaics. These molecules are found to exhibit large nondegenerate two-photon absorption (ND2PA) cross-sections (ca. 6-27 × 103 GM) in the near-infrared (NIR). In the first series, involving molecules of varying core size, ND2PA spectra and cross-sections characterized by femtosecond ND2PA spectroscopy in chloroform solutions reveal that increases in core size, and thus conjugation length, leads to substantially red-shifted and enhanced 2PA. In a second series, variation of the strength of the terminal acceptor (A) with constant core size (seven rings, indacene-based) led to less dramatic variation in the 2PA properties. Among the two core types studied, compounds in which the donor has a thieno[3,2-b]thiophene center demonstrate larger 2PA cross-sections than their indacene-centered counterparts, due to the greater electron-richness of their cores amplifying intramolecular charge transfer. Excited-state absorption (ESA) contributions to nonlinear absorption measured by open-aperture Z-scans are deduced for some of the compounds by analyzing the spectral overlap between 2PA bands and NIR ESA transitions obtained by ND2PA and transient absorption measurements, respectively. ESA cross-sections extracted from transient absorption and irradiance-dependent open-aperture Z-scans are in reasonable agreement, and their moderate magnitudes (ca. 10-21 m2) suggest that, although ESA contributions are non-negligible, the effective response is predominantly instantaneous 2PA.

Thulium laser at ∼2.3 μm based on upconversion pumping.

We report on novel upconversion (UC) pumping schemes for 2.3μm thulium (Tm) lasers (the H43→H53 transition) based on a photon avalanche mechanism populating the intermediate metastable level (F43) acting as an effective ground state. The proposed pump wavelengths are ∼1 and ∼1.5 μm, each one corresponding to a resonant excited-state absorption transition F43→F2,33 and F43→H43, respectively. UC pumping at 1040, 1055, and 1451nm of 2.3μm Tm:LiYF4 lasers is demonstrated. In the former case, the laser generates 102mW at 2302nm with a slope efficiency of 14.6% (versus the incident pump power). The laser dynamics is studied. UC pumping is promising for reaching high efficiencies in 2.3μm Tm lasers.

Investigation of allowed and forbidden electronic transitions in rare earth doped materials for laser cooling application by thermal lens spectroscopy

In this work Thermal Lens Spectroscopy (TLS) is used for sensitive characterization of absorption in optical materials with particular application for solid-state laser cooling (optical refrigeration). TLS is used to characterize key parameters involved in 4f-4f transition in Yb3+:YLF and 4f-5d transition in Ce3+:YAG systems. The results confirm the excellent laser cooling results found in previous works on Yb3+:YLF crystal. In addition, although no cooling effect was observed in Ce3+:YAG sample, the TLS showed that this material exhibits promising characteristics towards laser cooling applications, such as high external quantum efficiency (>92%), and low parasitic background absorption of (1.2 ± 0.2) × 10−4 cm−1 at 885 nm. The key mechanisms that appear to inhibit net cooling in Ce3+:YAG are attributed to the intrinsic excited state absorption and/or 4f-4f transitions between the two lowest levels of Ce3+. This investigation was turned possible due to the high sensitivity of TLS for very low absorptions.

Recent Advances in 3.5 μm Erbium-Doped Mid-Infrared Fiber Lasers

The performance of mid-infrared Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers has dramatically improved in the last few years. In this paper, we present an overview of the progress in 3.5 μm fiber lasers based on the dual-wavelength pumping approach. The cross section of the excited state absorption transition used by the 1973 nm second pump is found experimentally. A numerical simulation of the expected Q-switched behavior of this fiber laser is presented. This shows that increasing the power of the 977 nm pump is important to achieve high-peak power pulses.

Quenching of $3.4~\mu \text{m}$ Dual-Wavelength Pumped Erbium Doped Fiber Lasers

We report on a quenching behavior of the $3.4~\mu \text{m}$ dual-wavelength pumped fiber laser that can be accounted by the adverse contribution from a new excited state absorption transition in Er3+:ZrF4 glasses. The transition occurs between levels 4F9/2 and 4F7/2 and partially overlaps with the 1976 nm pump wavelength. We show that this upconversion process causes strong quenching of the lasing power at $3.4~\mu \text{m}$ under insufficient 974 nm pumping. Intracavity fluorescence at 550 nm from levels (2H11/2, 4S3/2) is also monitored as a witness of this excited state absorption mechanism. Through numerical modeling, we estimate the cross section value of the process and show that it is essential to reproduce both power and fluorescence curves. This paper also emphasizes the critical role played by the pump wavelengths.

Energy-Looping Nanoparticles: Harnessing Excited-State Absorption for Deep-Tissue Imaging.

Near infrared (NIR) microscopy enables noninvasive imaging in tissue, particularly in the NIR-II spectral range (1000-1400 nm) where attenuation due to tissue scattering and absorption is minimized. Lanthanide-doped upconverting nanocrystals are promising deep-tissue imaging probes due to their photostable emission in the visible and NIR, but these materials are not efficiently excited at NIR-II wavelengths due to the dearth of lanthanide ground-state absorption transitions in this window. Here, we develop a class of lanthanide-doped imaging probes that harness an energy-looping mechanism that facilitates excitation at NIR-II wavelengths, such as 1064 nm, that are resonant with excited-state absorption transitions but not ground-state absorption. Using computational methods and combinatorial screening, we have identified Tm(3+)-doped NaYF4 nanoparticles as efficient looping systems that emit at 800 nm under continuous-wave excitation at 1064 nm. Using this benign excitation with standard confocal microscopy, energy-looping nanoparticles (ELNPs) are imaged in cultured mammalian cells and through brain tissue without autofluorescence. The 1 mm imaging depths and 2 μm feature sizes are comparable to those demonstrated by state-of-the-art multiphoton techniques, illustrating that ELNPs are a promising class of NIR probes for high-fidelity visualization in cells and tissue.

Structural Heterogeneity in the Localized Excited States of Poly(3-hexylthiophene).

Transient hole-burning and resonantly enhanced Raman spectroscopies are used to probe heterogeneities among localized singlet excitons of poly(3-hexylthiophene) in solution. Transient hole-burning spectroscopy facilitated by population dumping through wavelength-selective stimulated emission exposes inhomogeneous broadening of the exciton absorption band in the near-infrared, as reflected by correlations between stimulated emission and excited-state absorption transition energies. Dump-induced spectral diffusion of the exciton absorption band reflects structural fluctuations in the locally excited polymer. This diffusion is observed to occur slightly faster or slower than the nonequilibrium relaxation that follows direct excitation of the polymer (8-9 ps), with the time scale for diffusion varying with subpopulation: dumping across small vs large band gaps results in diffusion over 5 vs 35 ps, respectively. Furthermore, incomplete spectral relaxation of transient holes reflects that subsets of locally excited structural motifs prepared through photoexcitation cannot interchange through structural fluctuations that occur over the singlet-exciton lifetime. Raman spectra of the C═C/C-C stretching region collected in resonance at energies across the exciton absorption band exhibit frequency and intensity trends (Raman "dispersion") ascribed to variation in the local effective conjugation length. Together, results explicitly reveal heterogeneities among excitonic states associated with variations and fluctuations in local conformational order.

Photodynamic Processes and Lasing in Ce,Yb:LiYXLu1-XF4Crystals

Measurements of photoconductivity were conducted in LiY X Lu 1-X F 4 :RE (RE = Ce(1%), Yb (1%); x = 0..1) crystals. It was found that the excited state absorption transitions of Ce3+ ions are intracenter and terminate at 6s state of cerium ions. Lasing at room temperature was achieved, differential gain (up to ~22%) and tuning range were determined. By lowering the temperature of the active element and using additional antisolarant pumping at 532 nm lasing differential gain efficiency was increased (up to ~ 35%), and the tuning range was expanded.

Excited-state absorption in erbium-doped calcium lithium niobium gallium garnet

Excited-state absorption (ESA) spectra of Er3+ in the partially disordered calcium lithium niobium gallium garnet (CLNGG), recorded in the spectral range 400–900nm, are presented and discussed. The use of a pump wavelength of 973nm enables a complete separation of the ESA spectra from ground-state absorption (GSA) and stimulated emission. For the identification of various ESA transitions, the Er3+ energy levels above S3/24 in CLNGG are determined using GSA spectra at low temperature. Excitation spectra for the population of (S3/24,H211/22) levels by two-step sequential absorption with I13/24 as an intermediate level are generated using the recorded ESA and GSA spectra.

Spectroscopy of upper energy levels in an Er^3+-doped amorphous oxide

A spectroscopic study of the population mechanisms in erbium-doped amorphous aluminum oxide up to the H11/22/S3/24 levels is performed. Via luminescence decay measurements, absorption and emission spectra, and a Judd&#x2013;Ofelt analysis, we determine luminescence lifetimes, radiative and nonradiative decay-rate constants, and branching ratios of the Er3+ intermanifold transitions. With a continuous-wave pump-probe technique, the excited-state absorption (ESA) spectrum is recorded between 900 and 1800&#xA0;nm and the cross sections of the ESA transitions I13/24&#x2192;I9/24, I13/24&#x2192;F9/24, and I11/24&#x2192;F7/24 are determined. The microparameters and efficiencies of resonant and phonon-assisted energy-migration and energy-transfer upconversion (ETU) processes among Er3+ ions occurring from the first and second excited states are evaluated. From the ratio of the S3/24 and F9/24 luminescence intensities as a function of Er3+ concentration, we prove the existence and quantify the macroscopic ETU coefficient of the two-phonon-assisted ETU process (I13/24,I11/24)&#x2192;(I15/24,F9/24).

Ultrafast Relaxation Dynamics of Rod-Shaped 25-Atom Gold Nanoclusters

We report a femtosecond spectroscopic investigation on the electronic structure and relaxation dynamics of a rod-shaped, 25-atom (Au25) nanocluster capped by organic ligands. Broadband femtosecond transient absorption spectra of the cluster show overlapped excited state absorption and ground state bleach signals. Two lifetimes (i.e., 0.8 ps fast component and a 2.4 μs long component) are identified, with the 0.8 ps component attributed to the fast internal conversion process from LUMO+n to LUMO and the long component to electron relaxation to the ground state. The rod shape of the cluster induces a strong anisotropic response in the transient absorption spectra, from which we deduce that the transition moment is oriented with the long axis of the prolate-shaped cluster. In addition, coherent phonon emission at 26 cm−1 was observed and results in the modulation of the excited state absorption transition energy.

Electronic Double-Quantum Coherences and Their Impact on Ultrafast Spectroscopy: The Example of β-Carotene

The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated β-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S2→Sn2). The signatures of this transition in pump−probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence.