Excited-state Absorption Cross Sections Research Articles

Effective photodarkening suppression in Yb-doped fiber lasers by visible light injection

Al-silicate fibers have excellent manufacturing quality. Unfortunately, high-Yb doping concentration may be limited by severe losses induced by photodarkening phenomenon. In this paper we demonstrate for the first time that Al-silicate Yb-doped fibers with high-inversion and doping concentration above 1 wt% can be successfully used by implementing a simple optical bleaching scheme. A co-injection into the active fiber of a few mW of light at around 550 nm wavelength successfully eliminates almost all photodarkening induced losses. We demonstrate operation at above 90% of the pristine output power level in several lasers with up to 30% Yb ions in the excited state. These results may allow using Yb-doped Al-silicate fibers with doping level increased by one order of magnitude. Finally, we provide a comprehensive picture of main parameters affecting photobleaching performance and, to the best of our knowledge, we report the first quantitative measurement of the Ytterbium excited state absorption cross-section in the visible range.

Effects of Extended π-Conjugation in Phenanthroline (N∧N) and Phenylpyridine (C∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes

Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R′-phenyl)pyridine)2 (1, R = fluoren-2-yl, R′ = H; 2, R = 7-benzothiazolylfluoren-2-yl, R′ = H; 3, R = H, R′ = fluoren-2-yl; 4, R = H, R′ = 7-benzothiazolylfluoren-2-yl; 5, R = R′ = 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (C∧N) and/or phenanthroline (N∧N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV–vis absorption, emission, and transient difference absorption spectroscopy. Time-dependent density functional theory (TDDFT) calculations were performed to complement the experimental data and aid in our understanding of the characters of optical transitions. In addition, reverse saturable absorption was demonstrated at 532 nm for all complexes using nanosecond laser pulses. All complexes possess a weak low-energy tail that is attributed to 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1–5 arise from the 1π,π* and intraligand (1ILCT) transitions within the N∧N or C∧N ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents on N∧N ligand results in stronger red-shifts of the main absorption band to 400 and 408 nm for 1 and 2, respectively, as compared to 321 and 361 nm in complexes 3 and 4 with the same substituents on the C∧N ligands. In contrast, the 1,3MLCT/1,3LLCT transitions in 1 and 2 are just slightly red-shifted as compared to those in 3 and 4. The emission of complexes 1 and 2 is attributed to the N∧N ligand-centered 3π,π* state with some admixture of 3MLCT/3ILCT/3LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the 3MLCT/3LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the C∧N and the N∧N ligands, the emission predominantly arises from the 3MLCT/3LLCT states with a small portion of N∧N ligand-localized 3π,π* character. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, with the major absorption bands bathochromically shifted in 1 and 2 as compared to those in 3 and 4. The stronger excited-state absorption leads to dramatic reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses. The RSA strength decreases as 5 ≈ 2 > 4 ≈ 1 > 3, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground state. Extended π-conjugation in the N∧N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with π-conjugated substituents only on the C∧N ligands (3 and 4).

Effect of electron acceptor type on nonlinear optical absorption properties in the chiral polymers based on polybinaphthyls

Nonlinear optical absorption properties of two chiral polybinaphthyls with the same backbone but different electron acceptor hydroxyl and octoxy were studied in tetrahydrofuran solution. When excited by 100 fs 800 nm laser pulses, the up-converted fluorescence were observed and the intensities related to the different input irradiances were measured to confirm the two-photon excited fluorescence. The two-photon absorption coefficients were obtained as 4.82 cm GW−1 and 12.36 cm GW−1 with 100 fs pulses at 800 nm for two polymers, respectively. Under the excitation of 532 nm 38 ps pulse, it was found that the polymer with the side-chain of hydroxyl showed saturable absorption while the polymer with the side-chain of octoxy presented reverse saturable absorption. Their different nonlinear absorption behavior resulted from the fact that the ground-state absorption cross-section in the latter polymer was about 10 times larger than that of the former polymer while the excited-state absorption cross-section represented similar.

Development and saturation investigation of MALO saturable absorber

The paper describes the investigation of saturation effect in newly developed MgAl2O4:Co saturable absorber with the aim of thermally bounding it with the active media such as erbium glasses. On the basis of the experimental results the most important parameters, from the point of view of laser generation, such as ground state absorption cross section, excited state absorption cross section and dissipative losses were calculated. The comparison of this parameters with the parameters presented in the literature was done showing the advantages of the newly developed saturable absorber over commercially available ones.

Ultrafast Laser Energy Density and Retinal Absorption Cross-Section Determination by Saturable Absorption Measurements

Laser pulse nonlinear transmission measurements through saturable absorbers of known absorption parameters allow the measurement of their energy density. On the other hand, nonlinear transmission measurements of laser pulses of known energy density through absorbing media allow their absorption parameter determination. The peak energy density w0P of second harmonic pulses of a mode-locked titanium sapphire laser at wavelength λP = 400 nm is determined by nonlinear energy transmission measurement TE through the dye ADS084BE (1,4-bis(9-ethyl-3-car-bazovinylene)-2-methoxy-5-(2’-ethyl-hexyloxy)-benzene) in tetrahydrofuran. TE(w0P) calibration curves are calculated for laser pulse peak energy density reading w0P from measured pulse energy transmissions TE. The ground-state absorption cross-section σP and the excited-state absorption cross-section σex at λP, and the number density N0 of the retinal Schiff base isoform RetA in pH 7.4 buffer of the blue-light adapted recombinant rhodopsin fragment of the histidine kinase rhodopsin HKR1 from Chlamydomonas reinhardtii were determined by picosecond titanium sapphire second harmonic laser pulse energy transmission measurement TE through RetA as a function of laser input peak energy density w0P. The complete absorption cross-section spectrum σ

Pump-probe studies of absorption saturation and optical gain in Ce:LiCaAlF6 ultraviolet active medium

Aim of this work was to investigate nonlinear absorption of pump radiation in UV spectral range of Ce3+:LiCaAlF6 crystalline active medium. We were able to evaluate excited state absorption cross-section at pump wavelength (266 nm) and recombination probabilities by means of computer simulation.

Resonant excited state absorption and relaxation mechanisms in Tb^3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.

Investigation of ground and excited state photophysical properties of gadolinium phthalocyanine

In this present work, we investigated the singlet excited state absorption (ESA) spectrum and intersystem crossing dynamics of gadolinium phthalocyanine (GdPc) in toluene. For that, we employed the femtosecond wavelength-tunable Z-scan and picosecond Pulse Train Fluorescence (PTF) techniques to determine, respectively, the singlet ESA cross-section on a wide spectral range (from 460 up to 740 nm) and the intersystem crossing time (singlet–triplet transition). We observed that the ESA spectrum presents distinct absorption behaviors, i.e., saturable (SA) and reverse saturable absorption (RSA). We have also been able to identify regions in which the excited state absorption cross-section has the same magnitude as the ground state one (at 600 and 720 nm). The RSA effect was observed for wavelengths shorter than 600 nm and longer than 720 nm, while SA occurs in the Q-band region, located between 600 and 720 nm. To describe the main singlet–singlet transitions, we measured the fluorescence decay time and employed a rate equation model considering a three-energy-level-diagram. Ratios between excited and ground state absorption cross-section from 0.4 to 4 were observed along the ESA spectrum. From these results, we calculated the transition dipole moment between the excited states for the main electronic transitions of GdPc. To obtain the intersystem crossing time, we incorporated an additional level in our model and used the PTF technique to probe the population transfer to the triplet state. Our results pointed out that the intersystem crossing time is higher than 300 ns, which corresponds to a maximum triplet quantum yield of 1.5%.

Nonlinear optical behavior of metal octaphenoxy phthalocyanines: effect of distortion caused by the central metal

In this work, we investigated the nonlinear optical absorption properties of chloroaluminum, chlorogallium, chloroindium, zinc and lead octaphenoxy phthalocyanines in a dimethyl sulphoxide solution using 5 ns pulses at 532 nm. Using the Z-scan technique, the nonlinear optical properties and the excited state absorption cross-sections were determined by fitting the Z-scan data. The k value was the highest for Pb derivative at 211, making it the best nonlinear optical material. In terms of hyperpolarizability, Pb derivative (containing a larger central metal) also gave the largest value followed by aluminum derivative (containing a small central metal). The distortion of the ring caused by the central metal is used to explain the hyperpolarizability values. Large nonlinear absorption coefficient values, β eff , in the range of 3.558–4.763 × 10-9 cm.W-1 and low saturation fluence values, F sat between 21.5–38.5 mJ.cm-2, were obtained for these samples.

Nonlinear optical properties of Bi2O3-GeO2 glass at 800 and 532 nm

The nonlinear (NL) optical properties of glassy xBi2O3-(1−x) GeO2 with x = 0.72 and 0.82 were investigated. The experiments were performed with lasers at 800 nm (pulses of 150 fs) and 532 nm (pulses of 80 ps and 250 ns). Using the Kerr gate technique, we observed that the NL response of the samples at 800 nm is faster than 150 fs. NL refraction indices, |n2|≈ 5 × 10−16 cm2/W, and two-photon absorption coefficients, α2, smaller than 0.03 cm/GW, were measured at 800 nm. At 532 nm, we measured the NL transmittance of the samples. From the results obtained, we determined α2 ≈1 cm/GW and excited-state absorption cross-sections of ≈10−22 cm2 due to free-carriers.

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

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

Platinum Chloride Complexes Containing 6-[9,9-Di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine Ligand (R = NO2, CHO, Benzothiazol-2-yl, n-Bu, Carbazol-9-yl, NPh2): Tunable Photophysics and Reverse Saturable Absorption

Six new platinum(II) chloride complexes 1-6 containing a 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine (R = NO2, CHO, benzothiazol-2-yl (BTZ), n-Bu, carbazol-9-yl (CBZ), NPh2) ligand were synthesized and characterized. The influence of the electron-donating or electron-withdrawing substituent at the 7-position of the fluorenyl component on the photophysics of these complexes was systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). Electron-withdrawing or -donating substituents exert distinct effects on the photophysics of the complexes. All complexes feature a low-energy, broad (1)MLCT (metal-to-ligand charge transfer)/(1)ILCT (intraligand charge transfer)/(1)π,π* absorption band (tail) above ca. 430 nm and a major absorption band(s) between 320 and 430 nm, which admix (1)MLCT, (1)π,π*, (1)ILCT, and/or (1)LLCT (ligand-to-ligand charge transfer) characters. The contributions of different configurations to the major absorption band(s) vary depending on the nature of the substituent. Strong electron-donating or -withdrawing substituents (NPh2 and NO2) and the aromatic substituent BTZ cause a pronounced red-shift of the absorption spectra of 1, 3, and 6. All complexes are emissive at room temperature and at 77 K. The emitting excited state is dominated by (3)π,π* character in 1-3, with some contributions from (3)MLCT in 1 and 2, while the emission is predominantly from the (3)MLCT state for 4 and 5 but with some (3)π,π* character. For 6, the emitting state is (3)ILCT in nature. With the increased electron-donating ability of the substituent, the (3)π,π* character diminishes while charge transfer character increases. All complexes exhibit broad and strong triplet excited-state absorption (TA) from the near-UV to the near-IR spectral region. The TA band maxima are red-shifted for complexes 1-3 (which possess the electron-withdrawing substituents) compared to those of 4-6 (which contain electron-donating substituents). All complexes manifest strong reverse saturable absorption (RSA) for a nanosecond laser pulse at 532 nm, which originates from the much stronger triplet excited-state absorption than the ground-state absorption of 1-6 in the visible spectral region. The strength of RSA follows this trend: 4 ≈ 5 < 1 ≈ 3 < 2 < 6, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground-state absorption (σex/σ0) at 532 nm. The σex/σ0 ratios (116-261) of 1-6 at 532 nm are much larger than those of most of the reverse saturable absorbers reported in the literature, with the ratio of 6 (σex/σ0 = 261) being among the largest values reported to date. This makes complexes 1-6, especially 6, very promising reverse saturable absorbers.

Nonlinear optical and all-optical switching studies of novel ruthenium complex

We report the experimental investigation on third-order nonlinear optical parameters of a novel (2-thioxo-1,3-dithiole-4,5-dithiolato) triphenylphosphine Ru (III) [RuL] both in solution and solid form, by using Z-scan technique at the measurement wavelength of 532nm. The films were prepared by spin-coating technique on glass substrate. The magnitude and sign of the third-order nonlinear absorption and refractive index n2 of ruthenium complex were determined. The results reveal that ruthenium complex exhibits large negative nonlinear refractive index (n2) of the order of 10−9esu. The third-order nonlinear optical susceptibility (χ(3)) values were of the order 10−10esu. The nonlinear optical properties were found to vary with concentration. The χ(3) value is confirmed through degenerate four wave mixing (DFWM) experiment at the same wavelength. The effective excited-state absorption cross section was found to be larger than the ground state absorption cross section indicating that the operating nonlinear mechanism is reverse saturable absorption (RSA). It was found that the ruthenium complex exhibits good optical power limiting and also all-optical switching of nanosecond laser pulses. This new compound has potential application as photonic and optoelectronic devices.

Enhanced reverse saturable absorption and optical limiting properties in a protonated water-soluble porphyrin

Nonlinear optical properties of water-soluble porphyrin 5,10,15,20-Tetrakis (1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate) (TMPyP), protonated TMPyP (), TMPyP film with the gelatin matrix (TMPyP/Gelatin) were investigated by the open-aperture Z-scan technique in the nanosecond regime. Results show that exhibits a larger ratio of excited state absorption cross-section to that of the ground state and enhanced reverse saturable absorption properties compared with TMPyP and TMPyP/Gelatin. also shows the superior optical limiting performance, even better than the benchmark material C60 and the multi-walled carbon nanotubes (MWNTs) dispersion.

Observation of Two-Photon Induced Excited-State Absorption Phenomena in C60 and C70 Derivatives

Four fullerene dyads with different cabazole units, named a, b, c, d, were studied by using a femtosecond laser with 780 nm wavelength using Z-scan methods. Interestingly, all of them show strong two-photon induced excited-state absorption behaviors, which were observed firstly in femtosecond timescale. The values of their excited-state absorption cross-section were up to 6.07 × 10−18, 7.27 × 10−18, 3.86 × 10−18 and 4.51 × 10−18 cm−1 for a, b, c, and d, respectively.

Reverse saturation absorption spectra and optical limiting properties of chlorinated tetrasubstituted phthalocyanines containing different metals

A highly non-aggregated series of peripherally tetra(2,6-diisopropylphenoxy) substituted phthalocyanines (Pcs) containing different metals (Al, Ga, In and Zn) were prepared and their nonlinear optical properties (NLO), including the optical limiting behavior were investigated. This study explores the effect of the implanted metals along with the peripheral chlorine atoms on the NLO of the prepared complexes, thereby improving their optical limiting performance. The best wavelength at which the Pc complex exhibits the strongest nonlinear effect was determined over the low absorption window region (420–590nm) comprised between the Q and B bands using a nanosecond pulsed laser with the aid of a Z-scan technique. The excited state absorption cross sections (σeff) have been found to be wavelength dependent with common double peaks at 505nm and around 485nm and the highest value was calculated for Pc1–In (1.1×10−16cm2). The figure of merit spectra show different spectral structure when compared to the excited state absorption spectra and the largest figure of merit value was found to be around 107 for Pc1–In derivative. In full agreement with reverse saturation results, the optical limiting measurements, carried out at 485nm and 532nm for each derivative, show very low optical limiting thresholds at the former wavelength. In the continuous wave excitation regime, the Z-scan measurements, which were carried out at 632.8 and 532nm, showed strong refractive and absorptive nonlinear effects.

Saturable absorption of multi-walled carbon nanotubes/hybrid-glass composites

The saturated optical absorption of multi-walled carbon nanotube (MWNT)-doped hybrid-glass composites was studied at 1064 nm with nanosecond long pulses using the Z-scan technique. Increased transmission was demonstrated for high intensity pulses. The results were modeled for the limiting cases of a slow or a fast saturable absorber, both referring to a three-level system. The ground-state absorption cross section was estimated as 2.3x10−18 cm2, in good agreement with that of a single carbon atom. The excited state absorption cross section for carbon nanotubes (CNTs) was estimated for the first time as 6.9x10−19 cm3. The absorber's density of 2.8x1018 cm−3 is smaller by approximately a factor of 5 compared to the nominal density of carbon atoms incorporated into the glass. This could be a result of the broad band-like states formation in the MWNTs. The above figures were obtained by assuming that the excited state decay time was longer than the pulse duration of ~1 ns.

Optimization of the Double Pump–Probe Technique: Decoupling the Triplet Yield and Cross Section

The double pump-probe technique (DPP), first introduced by Swatton et al. [Appl. Phys. Lett. 1997, 71, 10], is a variant of the standard pump-probe method but uses two pumps instead of one to create two sets of initial conditions for solving the rate equations, allowing a unique determination of singlet- and triplet-state absorption parameters and transition rates. We investigate the advantages and limitations of the DPP theoretically and experimentally and determine the influence of several experimental parameters on its accuracy. The accuracy with which the DPP determines the triplet-state parameters improves when the fraction of the population in the triplet state relative to the ground state is increased. To simplify the analysis of the DPP, an analytical model is presented, which is applicable to both the reverse saturable and the saturable absorption regimes. We show that the DPP is optimized by working in the saturable absorption regime. Although increased accuracy is in principle achievable by increasing the pump fluence in the reverse saturable absorption range, this can cause photoinduced decomposition in photochemically unstable molecules. Alternatively, we can tune the excitation wavelength to the spectral region of larger ground-state absorption, to achieve similar accuracy. This results in an accurate separation of triplet yield and excited-state absorption cross section. If the cross section at another wavelength is then desired, a second pump-probe experiment at that wavelength can be utilized given the previously measured triplet yield under the usually valid assumption that the triplet yield is independent of excitation wavelength.

Photophysics and nonlinear absorption of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings

The photophysical properties, nonlinear absorption characteristics and the influence of the substituted aromatic rings on the nature of excited states of a series of 4,4′-diethynylazobenzene derivatives ( 1a – 1f ) were systematically investigated. All compounds exhibit reverse saturable absorption at 532 nm and could potentially be used as nonlinear absorbing materials. ► Photophysics of 4,4′-diethynylazobenzene derivatives was systematically investigated. ► Excited state properties can be tuned by alternation of the terminal substituents. ► Compounds exhibit varied reverse saturable absorption at 532 nm for ns laser pulses. The photophysical properties of a series of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings ( 1a : R = phenyl; 1b : R = 4-(diphenylamino)phenyl; 1c : R = 4-(9 H -carbazol-9-yl)phenyl; 1d : R = 9 H -fluoren-2-yl; 1e : R = biphenyl-4-yl; 1f : R = naphthalen-2-yl) were systematically investigated. All compounds exhibit strong 1 π , π * absorption bands in the UV region; and a broad, structureless charge-transfer band/shoulder in the visible region (except for 1a ), which systematically red-shifts when electron-donating substituents are introduced to the terminal phenyl rings, but blue-shifts when π -conjugation of the terminal aromatic ring increases. All compounds are emissive in solution at room temperature and at 77 K. When excited at the low-energy absorption band, the compounds emit fluorescence between 369 and 419 nm, which can be attributed to 1 π , π */ 1 ICT (intramolecular charge transfer) state. Density functional theory (DFT) calculations on 1a – 1f in gas phase were also performed to gain insight into the nature of the ground electronic state and the low-lying excited electronic states. 1d – 1f exhibit strong triplet transient absorption band(s) in the visible spectral region, which are mainly attributed to the 3 π , π * state. Reverse saturable absorption (RSA) of these compounds was demonstrated at 532 nm using ns laser pulses. The degree of RSA follows this trend: 1b > 1c ≈ 1a > 1e > 1f > 1d , which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield.

Studies of third-order optical nonlinearities and optical limiting of 2, 3-butanedione dihydrazone

2, 3-butanedione dihydrazone (BDDH) synthesized via chemical route and nonlinear optical parameters, such as nonlinear refractive index (n2), nonlinear absorption coefficient (β), third-order nonlinear optical susceptibility (χ(3)), second hyperpolarizability (γ), and optical-limiting behavior were investigated by single beam z-scan technique for different concentrations. Synthesized sample were irradiated by a Q-switched, frequency doubled Nd:YAG laser and found that n2 and β increases linearly with increasing concentration and hence γ decreases linearly. The excited-state absorption cross sections of BDDH were found to be larger than ground-state absorption cross sections, and it leads to reverse saturable absorption (RSA). The experimental results are well in agreement with the theory and also establish BDDH as one of the potential candidate materials for optical limiting at 532 nm.