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Optical birefringence in polymeric films containing azo-chromophores is an important feature related to the development of several technologies such as electro-optic modulators, optical switching, and optical gates, to cite a few. Therefore, it is essential to understand the main underlying mechanisms describing dynamic switching. In this context, we have investigated the optical birefringence performance of a guest–host film produced from a poly(methyl methacrylate) (PMMA) matrix containing a V-shaped azo-chromophore, which exhibited a larger optical response in comparison to the linear chromophores. The optical birefringence was induced by a linearly polarized diode laser (532 nm, writing laser), while a low-intensity HeNe (632.8 nm) laser and a tungsten-halogen lamp are employed, respectively, to monitor the optical storage and the absorption change during the photoinduced birefringence. Our results pointed out that the guest–host film presents maximum residual optical memory at around 50% and local optical birefringence at around 3.3 × 10−4 in the low concentration and intensity regimes. The high optical birefringence obtained in guest–host films was attributed to the considerable photoisomerization quantum yield in the solid-state (0.15 ± 0.02 for 532 nm). Besides, we have shown that the switching mechanism is driven by angular hole-burning during the first seconds after excitation, and, subsequently, molecular reorientation quickly rises, dominating the photochemical process. The latter mechanism is highly efficient in converting cis to trans molecules (100%), which is responsible for the high residual optical memory obtained. In order to better understand the isomerization mechanism of the azo-chromophore/PMMA film, we performed quantum chemical calculations within the DFT framework. The electronic transitions of the azo-chromophore isomers were determined using the TD-DFT method and potential energy curves (PECs) were constructed to investigate the possibility of the thermal-isomerization process of the V-shaped azo-chromophore through both rotation and inversion mechanisms. For both mechanisms, the amplitude of the energy barrier and activation energy for thermal isomerization are determined and the results are discussed.

Semiconductors thin films are the foundation of modern technology. While the nonlinear optical (NLO) properties of bulk semiconductors have been systematically studied in the last three decades, it is still a great challenge to obtain them for semiconductors thin films, as the high laser irradiance in NLO experiments tends to irreversibly damage the thin films. In addition, tuning the NLO response of semiconductor thin films by alloying and doping has not been explored yet. Here, we study the influence of the Aluminum content in AlxGa1-xN thin films and the n-type doping concentration in GaN thin films on their two-photon absorption (2PA) coefficients. For this, we investigate five different GaN-based thin films: an unintentionally doped one with no Al as a reference, two n-type doped GaN films with distinct concentrations of silicon impurities, and two AlxGa1-xN alloys with an aluminum content of 5.5% and 9.0%, respectively. The femtosecond 2PA spectra reveal that doping impurities reduce the non-linear coefficients (∼10%), while alloying with Al enhances the 2PA coefficient up to 30%. We use the model of Brandi and Araujo to determine Kane’s energy parameter related to the transition matrix element for each sample and compare them with recent theoretical studies based on the k·p theory where an excellent agreement is found.

International audience; In the present work, we report the multibranching effect on the dynamic first-order hyperpolarizability (β(−2ω; ω, ω)) of triphenylamine derivatives containing cyanopyridine one-branch (dipolar structure), two-branch (V-shaped structure), and three-branch (octupolar structure) structures. For this study, we used the hyper-Rayleigh scattering (HRS) technique involving picosecond pulse trains at 1064 nm. Our results show that βHRS increases from 2.02 × 10–28 to 9.24 × 10–28 cm5/esu when an extra branch is added to the molecule, configuring a change from a dipolar to a V-shaped (quadrupolar) molecular structure. When a third branch is added, leading to an octupolar structure, a decrease to 3.21 × 10–28 cm5/esu is observed. Such a significant decrease in βHRS is attributed to a negative contribution presented in the βHRS description by using a three-level energy approach due to their electronic structure and considering a specific combination of the angle between the dipole moments. On the other hand, the enhancement of βHRS found for the quadrupolar structure is associated with the cooperative enhancement due to the electronic coupling between the branches that increases considerably the transition dipole moment and permanent dipole moment change. To explain the βHRS results obtained for different molecules, we employed the HRS figure of merit, FOMHRS = βHRS/Neff3/2, in which Neff is the effective number of π-conjugated bonds, and the few-energy level approach for βHRS within the Frenkel exciton model. To shed more light on the experimental results interpretation, we performed time-dependent density functional theory calculations combined with a polarizable continuum model to confirm the energy and oscillator strength of the electronic transitions assumed in the Frenkel exciton model employed here.

The intramolecular cooperative effect in branched molecules is a consequence of the interaction and extent of electronic coupling among the different axes of charge transfer. Such an effect is the key to obtain remarkable nonlinear optical response in molecular systems. Here we show that triphenylamine derivative molecules containing only two branches present the strongest electronic interaction between them at the excited state, generating exponential enhancement of the 2PA cross section. The primary factor for such behavior was ascribed to the substantial extent and interaction of the π-electron delocalization promoted by the strong electron-donating and acceptor antisymmetrical groups present in each branch. However, for the three-branch molecules we observed an anticooperative effect, i.e., the 2PA cross section decreases as compared to the one-branch structure as we normalized the signal by the effective π-electron number in each molecule.

Nonlinear absorption of GaN and AlxGa1-xN thin films were studied by the Z-scan technique in the range of 0.53~0.63 Ephoton/Eg, with values ranging from 0.8 to 2.2 cm/GW depending of sample composition, indicating its influences on the nonlinearity.

This paper reports the synthesis and characterization of ternary CdTe:Zn2+ quantum dots (QDs) passivated with reduced L-glutathione (GSH) dispersed in water. The synthesis was performed through a one-pot approach and their size-dependent optical properties were investigated through steady-state absorption and emission spectroscopies along with the evolving factor analysis/multivariate curve resolution alternating least squares (EFA/MCR-ALS) method. Our results show that the incorporation of Zn2+ ions in CdTe:Zn2+ QDs, during the synthesis time, decreases the CdTe:Zn2+ QDs predominant diameter in the ensemble and increases their size dispersion. In addition, fluorescence quantum yield measurements suggest a reduction in the number of surface defects in CdTe:Zn2+ QDs as compared with CdTe QDs, when both are produced from the same synthesis route.

We have interpreted the two-photon absorption spectrum of water-soluble copper indium sulfide (CIS) QDs with stoichiometry 0.18 (Cu), 0.42 (In), and 2 (S) and an average diameter of approximately 2.6 nm. For that, we employed the wavelength-tunable femtosecond Z-scan technique and the parabolic effective-mass approximation model, in which the excitonic transition energies were phenomenologically corrected due to the stoichiometry of the nanocrystal. This model considers a conduction band and three valence sub-bands allowing excitonic transitions via centrosymmetric (Δl = ±1, where l is the angular momentum of the absorbing state) and non-centrosymmetric (Δl = 0) channels. In such case, this became relevant because the CIS QDs with chalcopyrite crystalline structure is a non-centrosymmetric semiconductor. Thus, our experimental results pointed out two 2 PA allowed bands located at 715 nm (2hv = 3.47 eV) and 625 nm (2hv = 3.97 eV) with cross sections of (6.3 ± 1.0) x 102 GM and (4.5 ± 0.7) x 102 GM, respectively. According to the theoretical model, these 2 PA bands can be ascribed to the 1P1/2(h3) → 1S3/2(e) (lower energy band) and 1P1/2(hheavy) → 1S3/2(e) (90%)/(10%)1P1/2(hsplit-off) → 1P3/2(e) (higher energy band) excitonic transitions. A good agreement (magnitude and spectral position) between the experimental and theoretical data were obtained. However, our experimental data suggest that the higher-energy 2 PA band may have other contributions due to the mixing between the heavy- and the light-hole bands, which the effective mass model does not take into consideration.

In the present work, we report on the positive effect of mercaptosuccinic acid (MSA) stabilizer agent on the optical features of colloidal CdTe quantum dots (QDs). With this aim, we performed some spectroscopic measurements such as steady-state absorption and fluorescence, fluorescence quantum yield and time-resolved photoluminescence for five MSA-capped CdTe QD samples with different synthesis times. The first general aspect to highlight is that the QDs' average size increased with synthesis time (from 30 to 150 min) while the size dispersion decreased due to the Ostwald ripening mechanism. Second, comparing the optical properties of CdTe QDs obtained from the same synthesis route, we show that MSA stabilizer agent enhanced the optical properties of CdTe QDs as compared with other widely used stabilizer agents such as GSH and TGA. We ascribe this outcome to reduction of the number of surface defects of the CdTe QDs because the MSA stabilizer agent decreases the growth rate of nanocrystals, causing an improvement in their surface quality. In the light of Fermi's golden rule, we observed that for longer synthesis time the optical properties of CdTe QDs increases due to the enhancement of the direct radiative recombination rate of electrons and holes and decrease in the decay rate for core states. Finally, we investigated the pH-dependent fluorescence and demonstrated the similar behaviour in acidic range between MSA-capped CdTe and mercaptocarboxylates-capped CdTe.

We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.

We report here on the direct observation of distinct two-photon transition channels in glutathione-capped (GSH) CdTe quantum dots (QDs) in a very strong confinement regime. CdTe-GSH QDs with different average diameters (2.5, 3.0, and 3.3 nm) were synthesized through the one-pot method and their two-photon absorption (2PA) spectrum determined by a femtosecond wavelength-tunable Z-scan. Our results show that the two lower-energy one-photon-allowed excitonic transitions, 1S3/2(h) → 1S(e) and 2S3/2(h) → 1S(e), are also accessed via 2PA. These results were ascribed to the relaxation of the parity selection rules due to the noncentrosymmetric structure of the CdTe QDs (zinc-blended structure), whose magnitude are determined by surface defects and structural irregularities present in CdTe-GSH QDs, in the strong confinement regime.

This paper report on the excited-state (ESA) and two-photon absorption (2PA) features of two novel salicylidene compounds: N,N′-bis(salicylidene)-1,2-phenylenediamine (salophen) and its Zn(II) coordination compound ([Zn(salophen) (OH2)]). The molecular structure of salophen is a tetradentated ligand with iminic and phenolic bonding sites, allowing the coordination of metal ions such as zinc(II) forming [Zn(salophen) (OH2)]. According to our data, the Zn(II) coordination complex modifies the ESA and 2PA spectrum of the salophen ligand, increasing significantly its hyperpolarizability and improving its optical properties due to the increase of the planarity and rigidity of the ligand framework. In order to obtain more information about optical properties of these molecules, theoretical calculations were performed at the DFT/TD-DFT level using aug-cc-pVDZ/CAM-B3LYP. Our results suggest that the molecular geometries of the ligand, corresponding to S0 and S1 states, are quite similar. However, the coordination...

In this Letter, we explored the use of polarized two-photon absorption (2PA) spectroscopy, which brings additional information when compared to methods that do not use polarization control, to investigate the electronic and molecular structure of two chromophores (FD43 and FD48) based on phenylacetylene moieties. The results were analyzed using quantum chemical calculations of the two-photon transition strengths for circularly and linearly polarized light, provided by the response function formalism. On the basis of these data, it was possible to distinguish and identify the excited electronic states responsible for the lowest-energy 2PA-allowed band in both chromophores. By modeling the 2PA circular-linear dichroism, within the sum-over-essential states approach, we obtained the relative orientation between the dipole moments that are associated with the molecular structure of the chromophores in solution. This result allowed to correlate the V-shape structure of the FD48 chromophore and the quantum-interference-modulated 2PA strength.

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

In this report, we investigate the polarization effect (linear, elliptical and circular) on the two-photon absorption (2PA) properties of a chiral compound based in azoaromatic moieties using the femtosecond Z-scan technique with low repetition rate and low pulse energy. We observed a strong 2PA modulation between 800 nm and 960 nm as a function the polarization changes from linear through elliptical to circular. Such results were interpreted employing the sum-over-essential states approach, which allowed us to model the 2PA circular-linear dichroism effect and to identifier the overlapping of the excited electronic states responsible by the 2PA allowed band.

International audience; This Article reports a combined experimental and theoretical analysis on the one and two-photon absorption properties of a novel class of organic molecules with a π-conjugated backbone based on phenylacetylene (JCM874, FD43, and FD48) and azoaromatic (YB3p25) moieties. Linear optical properties show that the phenylacetylene-based compounds exhibit strong molar absorptivity in the UV and high fluorescence quantum yield with lifetimes of approximately 2.0 ns, while the azoaromatic-compound has a strong absorption in the visible region with very low fluorescence quantum yield. The two-photon absorption was investigated employing nonlinear optical techniques and quantum chemical calculations based on the response functions formalism within the density functional theory framework. The experimental data revealed well-defined 2PA spectra with reasonable cross-section values in the visible and IR. Along the nonlinear spectra we observed two 2PA allowed bands, as well as the resonance enhancement effect due to the presence of one intermediate one-photon allowed state. Quantum chemical calculations revealed that the 2PA allowed bands correspond to transitions to states that are also one-photon allowed, indicating the relaxation of the electric-dipole selection rules. Moreover, using the theoretical results, we were able to interpret the experimental trends of the 2PA spectra. Finally, using a few-energy-level diagram, within the sum-over-essential states approach, we observed strong qualitative and quantitative correlation between experimental and theoretical results.

International audience; In this article, we investigate the linear and nonlinear optical properties of the thiophene/phenylene-based oligomer (SL128G) and polymer (FSE59) chemically modified with alquilic chains, which allow greater solubility and provide new optical properties. These compounds present a strong absorption in the UV visible region, providing a wide transparence window in visible-IR, ideal for applications in nonlinear optics. Employing the Z-scan technique with femtosecond pulses, we show that these compounds exhibit considerable two-photon absorption (2PA), with two 2PA allowed states located at 650 and 800 nm for SL128G and 780 and 920 nm for FSE59. Moreover, we observe the resonance enhancement effect as the excitation wavelength approaches the lowest one-photon-allowed state. By modeling the 2PA spectra considering a four-energy-level diagram within of the sum-over-essential states approach, we obtained the spectroscopic parameters of the electronic transitions to low-energy singlet excited states. Additionally, photoluminescence excited by femtosecond and picosecond pulses were performed to confirm the order of the multiphoton process and estimate the fluorescence lifetime, respectively.

Excited state dynamics, particularly intersystem crossing, of a set of meso-substituted porphyrins bearing different electron–donor and acceptor groups was studied by pulse train fluorescence technique. Triplet quantum yield was found to be critically dependent on the nature of meso-substituents in the porphyrin system. Porphyrins with meso methoxyphenyl groups were found to show high triplet quantum yields ([Formula: see text] between 0.70 and 0.81). Moreover, the quantity of methoxyphenyl groups and the substitution pattern directly influence [Formula: see text]. Alternatively, porphyrins attached to nitrophenyl group possess low triplet quantum yield values (~0.3). The observed structure-properties relationships suggest new ways for tuning the optical properties of porphyrins via chemical modification.