Strong Excited-state Absorption Research Articles

Dimension-Dependent Nonlinear Optical Properties of Stanene Nanosheets.

Stanene (Sn)-based materials, with a graphene-like hexagonal structure, are now attracting considerable interest for potential applications in photoelectricity. However, the nonlinear optical properties of stanene remain largely unexplored. In this work, we prepared different sizes of stanene by liquid phase exfoliation and differential centrifugation methods, including two-dimensional (2D) Sn nanosheets (NSs) and zero-dimensional (0D) Sn nanodots (NDs). Z-scan measurements reveal that 2D Sn NSs exhibited high saturable absorption behavior, while the 0D Sn NDs led to reverse saturable absorption. Femtosecond ultrafast transient absorption spectra revealed that the reverse saturable absorption of Sn NDs is derived from a stronger excited state absorption and faster exciton relaxation dynamics. This research expands the potential applications of stanene in laser shielding and other ultrafast photonics technologies.

Giant excited-state absorption in resonant absorption band and intensity-depend ultrafast sign switching of nonlinear absorption in azonaphthalene compound

The ultrafast optical nonlinearity of an azonaphthalene derivative, hydroxy naphthol blue (HNB) disodium salt, is investigated using transient absorption spectra (TAS) and Z-scan technique. Reverse saturable absorption (RSA) is observed within the resonant absorption band, which is usually dominated by ground-state bleaching (GSB) and stimulated emission (SE) in most organic compounds. This observation indicates a remarkably strong excited-state absorption (ESA) in HNB. Global analysis of TAS suggests an ultrafast conversion from saturable absorption (SA) to RSA, which is assigned to the internal conversion from locally excited (LE) state to the first singlet (S1) state. Further Z-scan study verifies the above results and reveals another intensity-depend SA-RSA transition, indicating increased S1 population under strong laser irradiation. The calculated value of third-order nonlinear absorption coefficient at 650 nm, 190 fs is determined as 1.23 × 10−12 m/W. These studies demonstrate significant potential of azonaphthalene in ultrafast laser protection if further modulation of excited-state absorption can be achieved.

UV-laser-diode-pumped visible Tb3+:LiLuF4 lasers.

We investigate the dependence of the visible laser performance of Tb3+:LiLuF3 (Tb:LLF) on the ultraviolet (UV) pumping wavelength and present the first, to the best of our knowledge, UV-laser-diode-pumped Tb3+-based laser. We find an onset of thermal effects already at moderate pump power for UV pump wavelengths with strong excited-state absorption (ESA), which vanishes at pump wavelengths with weak ESA. Pumping with a UV laser diode emitting at 378.5 nm enables continuous wave laser operation in a 3-mm short Tb3+(28 at.%):LLF crystal. Slope efficiencies of 36% at 542/544 nm and 17% at 587 nm are obtained with a minimum laser threshold as low as 4 mW.

Tunable photoluminescence emission and third order optical nonlinearity in Sm2/3Cu3Ti4O12 ceramics for optical limiting applications

A combined investigation of the photoluminescence and third order optical nonlinearity of polycrystalline samarium copper titanate, Sm2/3Cu3Ti4O12 (SmCTO), is presented in the current work. SmCTO ceramic has been synthesized using a cost-effective solid state reaction method and investigated by means of spectral absorbance (UV-Vis), photoluminescence (PL), X-ray diffraction (XRD), Field Emission Secondary Emission Microscopy (FESEM), energy dispersive X-ray spectrometry (EDAX) and X-ray Photoelectron Spectroscopy (XPS). From the PL spectra, it is clear that the blue band is due to the TiO6 cluster with oxygen vacancy of the Cu+ type and is assigned to the shallow defects. The green band obtained is due to the reduction in the density of trapped charges as well as particle size. The orange to red emission band corresponds to the charge transfer to the TiO6 cluster with oxygen vacancy of doubly ionized ions. Thus, we can infer the presence of defects like oxygen vacancies (Vox,Vo•,andVo••) in SmCTO. The third order nonlinear absorption coefficient measured by employing the open aperture Z-scan technique shows high values of the order of 10−8 cm/W and 10−4 cm/W, for pulsed and continuous wave lasers, respectively. This is primarily due to the effect of strong excited state absorption, which leads to excellent optical limiting behaviour. Therefore, these materials can be used to make optical limiting devices which can protect human eyes and sensitive optical detectors from dangerous laser beams. The PL and nonlinear optical studies reveals that SmCTO ceramics are practically useful for photonic applications.

Watt-Level High-Efficiency Deep-Red Ho3+:ZBLAN Fiber Laser

To date, Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fibers have been investigated as a viable route for deep-red fiber lasers. However, high output performance deep-red fiber lasers around 750 nm have been proven challenging to achieve due to limitations in both the excitation source and pumping mechanism. In this study, we use a home-made 640 nm pump source to develop a deep-red laser with a high pumping rate, and obtain a compact watt-level high efficiency deep-red laser output. Population inversion is achieved by strong excited state absorption in a Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped ZBLAN fiber. We realize a maximum 1.46 W deep-red fiber laser operating at 752.6 nm, and its slope efficiency is as high as 53%. The improved rate equations are used to describe the quasi-four-level system, and the experimental results agree well with the simulations. Moreover, with the development of blazed grating, we demonstrate a tunable deep-red fiber laser with a tuning range of 13.7 nm, from 746.5 to 760.2 nm, which is the widest tuning range of Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber laser at deep-red wavelengths.

Excitation-Dependence of Excited-State Dynamics and Vibrational Relaxation of Lutein Explored by Multiplex Transient Grating.

Multiplex transient grating (MTG) spectroscopy was applied to lutein in ethanol to investigate the excitation-energy dependence of the excited-state dynamics and vibrational relaxation. The transient spectra obtained upon low (480 nm) and high-energy (380 nm) excitation both recorded a strong excited-state absorption (ESA) of S1 → S n as well as a broad band in the blue wavelength that was previously proposed as the S* state. By means of Gaussian decomposition and global fitting of the ESA band, a long-time component assigned to the triplet state was derived from the kinetic trace of 480 nm excitation. Moreover, the MTG signal with a resolution of 110 fs displayed the short-time quantum beat signal. In order to unveil the vibrational coherence in the excited-state decay, the linear and non-linear simulations of the steady spectrum and dynamic signals were presented in which at least three fundamental modes standing for C-C stretching (ν1), C=C stretching (ν2), and O-H valence vibrations (ν3) were considered to analyze the experimental signals. It was identified that the vibrational coherence between ν1 and ν3 or ν2 and ν3 was responsible for quantum beat that may be associated with the triplet state. We concluded that upon low- or high-energy excitation into the S2 state, the photo-isomerization of the molecule and structural recovery on the time-scale of vibrational cooling are the key factors to form a mixed conformation in the hot-S1 state that is the precursor of a long life-time triplet.

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.

Transition metal complexes with strong and long-lived excited state absorption: from molecular design to optical power limiting behavior

Abstract Transition metal complexes (TMCs) with strong and long-lived excited state absorption (ESA) usually exhibit high-performance optical power limiting (OPL) response. Several techniques, such as transmission vs. incident fluence curves and Z-scan have been widely used to assess the OPL performance of typical TMCs. The OPL performance of TMCs is highly molecular structure-dependent. Special emphasis is placed on the structure-OPL response relationships of Pt(II), Ir(III), Ru(II), and other metal complexes. This review concludes with perspectives on the current status of OPL field, as well as opportunities that lie just beyond its frontier.

Upconversion pumping of lasers via excited-state absorption: an analytical rate-equation formulation

Coupled power-gain equations were used to model lasers where optical pumping occurs as a result of a weak ground-state absorption followed by strong pump excited-state absorption (ESA). Inspired by the gain dynamics and energy-level structure of upconversion-pumped 2.3 µm T m 3 + lasers and 3.4 µm E r 3 + lasers, the model assumes that population in the lower laser level relaxes via a fast nonradiative decay to an intermediate storage level, which typically has a considerably slower decay rate than that of the upper laser level. After obtaining a description of the rate equation model, the continuous-wave regime was investigated by deriving analytical expressions for the power-dependent absorption, threshold pump power for lasing, output laser power, and laser slope efficiency. Results indicate that a slow decay rate of the lower level is necessary, so that sufficient population buildup can lead to enhanced nonlinear absorption via ESA. Furthermore, the asymptotic slope efficiency of the ESA pumped laser is similar to that of the ideal four-level laser, where the linear absorption is replaced by the effective nonlinear absorption.

Ultrafast dynamics of dual fluorescence of 2-(2′-hydroxyphenyl) benzothiazole and its derivatives by femtosecond transient absorption spectroscopy

The dual fluorescence behaviors of 2-(2′-Hydroxyphenyl) benzothiazole (HBT) and 2-(2′-hydroxyphenyl) benzoxazole (HBO) molecules are studied by femtosecond transient absorption (fs-TA) spectroscopy. The steady-state fluorescence (515 nm) of HBT appears in the stimulated emission region in fs-TA spectroscopy, and the other fluorescence (363 nm) is covered by the strong excited state absorption signal. The large energy barrier (25.06 kcal/mol) of conformational torsion from O–H⋯N to O–H⋯S form show that the torsion is hardly to occur in the S1 state, which is also supported by the inconsistent spectra behaviors with the experimental results. Meanwhile, the nonadiabatic dynamics of HBT shows that the proton transfer (PT) from Enol to Keto occurs in 115 fs, which is consistent with the result of fs-TA in 117.2 fs. The fluorescence (493 nm) of the Keto form of HBT is consistent with experimental result (515 nm), and the other fluorescence (363 nm) comes from the Enol form of HBT. Together with favorable energy barrier (1.68 kcal/mol), these results demonstrate that the PT occurred in fs timescale is the main factor for dual fluorescence of HBT. Compared to HBT, HBO has higher energy barrier (3.74 kcal/mol) and longer PT time (164.7 fs). The combination of fs-TA spectroscopy and theoretical simulation provides insight to understand the fluorescent dynamics of HBT and HBO.

Synthesis, characterization and third-order nonlinear optical behaviour of three novel ethyne-linked donor/acceptor chromophores

Three novel ethyne-linked donor/acceptor chromophores, BAB, BAS and SAS were synthesized. The results of BAB in femtosecond and nanosecond Z-scan at 532 nm reveal that the transition from saturable absorption (SA) to reverse saturable absorption (RSA), while that of BAS and SAS display RSA. The femtosecond Z-scan results for all compounds at 600 nm–800 nm indicate the obvious RSA, which can be mainly attributed to the two-photon absorption (TPA). The TA results demonstrate that all compounds display strong excited-state absorption and long lifetime, and the evolution of TA spectra for these compounds reveals the relaxation process from the local excited-state (LE) to charge transfer state (CTS). With different conjugation extent and intramolecular charge transfer (ICT), the nonlinear response and excited-state dynamics of these chromophores could be dramatically modulated. The threshold of BAS/DCM solution excited at 532 nm with nanosecond pulse was 0.631 J/cm2. The results indicate that these compounds may be the potential candidates for future application in optical limiting.

Cyclometalated Pt(II) complexes with tetradentate Schiff base ligands: Synthesis, photophysics, electrochemical studies and optical power limiting performance

Pt(II) complexes with tetradentate Schiff base ligands (Pt-1–Pt-5) were prepared, and their detailed photophysics were studied using steady-state, transient spectroscopies and density functional theory systematically. Pt-1–Pt-5 all exhibit 1π,π* transitions and metal-to-ligand/intraligand charge transfer transitions in the UV–vis region. Meanwhile, these complexes are emissive in solution at room temperature (λem = 400–800 nm, Φem = 0.008–0.196). Triplet transient absorption (TA) demonstrated that except Pt-3, all complexes exhibit broad and strong triplet excited-state absorption from 400 to 750 nm. What's more, nonlinear transmission experiment demonstrated that Pt-1–Pt-5 show robust reverse saturable absorption (RSA) at 532 nm ns laser pulses. Among them, Pt-5 shows the strongest nonlinear absorption performance, which could potentially be used as optical power limiting (OPL) materials.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4'-R-2,2':6',2″-terpyridine (4'-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured 1π,π* absorption bands at <400 nm and broad charge transfer (1CT)/1π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the 1CT/1π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the 3π,π*/3CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Strong band filling induced significant excited state absorption in MAPbI3 under high pump power

Perovskite has been considered as promising material for solar cell application. Understanding the carrier dynamics in perovskite material plays a vital role for further improvement of the power conversion efficiency of solar cells. In this article, carrier dynamics of MAPbI3 under different pump powers have been investigated by ultrafast pump-probe transient absorption spectroscopy. Complete processes of carrier cooling and recombination under different pump powers have been characterized and compared carefully. It is found that a significant carrier band filling happened during cooling process under high pump power, which results in a strong excited state absorption at the photon energy of 2.353 eV. High carrier density induced strong Auger effect appears not only in the band edge but also the higher conduction band. And subsequently, a large part of carriers keeps hot for more than 5 ns indicating that MAPbI3 is meaningful for hot carrier extraction in solar cells. A critical carrier concentration of 9.32 × 1017 cm−3 has been determined at which Auger effect starts to work. It could provide some guidance for perovskite solar cell application.

Ultrafast broadband nonlinear optical properties and excited-state dynamics of two bis-chalcone derivatives.

The development of organic nonlinear optical (NLO) chromophores is vital for various fields such as two-photon biomedical imaging, optical limiting, etc. In this work, two bis-chalcone molecules 1,4-bis[3-(2,4-dimethoxyphenyl)-2-acryloyl]benzene (C1) and 4,4′-bis[3-(2,4-bimethoxy phenyl)-2-acryloyl]biphenyl (C2) were synthesized and characterized. The excited-state dynamics of these two chromophores were studied using femtosecond transient absorption (TA) measurements. And their broadband nonlinear absorption properties and optical limiting (OL) response were investigated by femtosecond open-aperture Z-scan and intensity-dependent transmittance measurements in the wavelength range from 515 nm to 800 nm, respectively. The TA results demonstrate that C2 has strong excited-state absorption behavior and longer lifetime. In addition, the nonlinear absorption response of C2 was found to be superior to that of C1 in the visible range after 500 nm, which is attributed to a two-photon-absorption induced excited-state absorption mechanism. These results indicate that the nonlinear optical response and excited-state dynamics in bis-chalcone compounds could be enhanced via intramolecular charge-transfer.

An Optical Power Limiting and Ultrafast Photophysics Investigation of a Series of Multi-Branched Heavy Atom Substituted Fluorene Molecules

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)–alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt–alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump–probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Cyclometalated iridium(III) complex containing indolinyl-based phenanthroline ligand: Synthesis, structure and photophysical properties

A heteroleptic cationic Ir(III) complex [(X)2Ir(L)]+PF6− was designed and synthesized, where L = 2-(1,3,3-trimethylindolin-2-yl)oxazolo[4,5-f] [1,10]phenanthroline, X = 2-phenylpyridine (ppy). Its structural and photophysical properties have been systematically investigated via a series of spectroscopic methods. This complex exhibits ligand-localized 1π,π* transitions below 350 nm, broad and structureless metal-to-ligand/ligand-to-ligand charge transfer (1MLCT/1LLCT) transitions between 350 and 450 nm, and spin-forbidden 3MLCT/3LLCT/3π,π* transitions above 450 nm. Upon excitation at its absorption maximum, the complex exhibits orange phosphorescence in solution. In addition, this complex exhibits strong excited-state absorption in the visible spectral region, good anti-photobleaching and electrochemical stability. Reverse saturable absorption (RSA) of this complex was demonstrated at 532 nm using 4.1 ns laser pulses, which would be useful for a rational design of nonlinear optical materials.

Designing Hybrids of Graphene Oxide and Gold Nanoparticles for Nonlinear Optical Response

Nonlinear optical absorption of light by materials are weak due to its perturbative nature, although a strong nonlinear response is of crucial importance to applications in optical limiting and switching. Here we demonstrate experimentally and theoretically an extremely efficient scheme of excited state absorption by charge transfer between donor and acceptor materials as the new method to enhance the nonlinear absorption by orders of magnitude. With this idea, we have demonstrated strong excited state absorption (ESA) in reduced graphene oxide that otherwise shows increased transparency at high fluence and enhancement of ESA by one orders of magnitude in graphene oxide by attaching gold nanoparticles (AuNP) in the tandem configuration that acts as an efficient charge transfer pair when excited at the plasmonic wavelength. To explain the unprecedented enhancement, we have developed a five-level rate equation model based on the charge transfer between the two materials and numerically simulated the results. To understand the correlation of interfacial charge-transfer with the concentration and type of the functional ligands attached to the graphene oxide sheet, we have investigated the AuNP-graphene oxide interface with various possible ligand configurations from first-principles calculations. By using the strong ESA of our hybrid materials, we have fabricated liquid cell-based high-performance optical limiters with important device parameters better than that of the benchmark optical limiters.

Novel N6 trisbidentate ligand coordinated Ir(iii) complexes and their Ru(ii) analogs

Novel N6-coordinated Ir(iii) complexes bearing polypyridyl ligands were synthesized and characterized. In comparison to their Ru(ii) analogs, these Ir(iii) complexes showed blue-shifted UV-vis absorption and emission spectra, but dramatically increased triplet lifetimes with much broader and stronger triplet excited-state absorption.

Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption.

A series of 2-aryloxazolo[4,5-f][1,10]phenanthroline ligands (N^N ligands) and their cationic iridium(III) complexes (1-11, aryl = 4-NO2-phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh2-phenyl (4), 4-NH2-phenyl (5), pyridin-4-yl (6), naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9), anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of π-conjugation (6-11) on oxazolo[4,5-f][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f][1,10]phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized 1π,π*/1ILCT (intraligand charge transfer) transitions, mixed with 1LLCT (ligand-to-ligand charge transfer)/1MLCT (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong 1ILCT/1LLCT/1MLCT transitions at 350-450 nm, and broad but very weak 3LLCT/3MLCT absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO2 or NPh2 substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.