Nonlinear Absorption Spectroscopy Research Articles

Influence of matrix composition and its fluctuations on excitation relaxation and emission spectrum of Ce ions in (Gdx Y1-x)3Al2Ga3O12:Ce scintillators

Cerium-doped mixed garnet-type single crystals (GdxY1-x)3Al2Ga3O12 with different yttrium content have been fabricated and studied as a prospective scintillating material enabling improvement of scintillation properties by tuning the composition of the matrix-building crystal. The influence of the matrix composition on the emitting Ce ion is studied using linear and time-resolved nonlinear optical absorption and time-resolved photoluminescence spectroscopy. The study of photoluminescence at resonant excitation revealed a composite origin of Ce3+ emission band. This behavior is interpreted by the contribution of Ce3+ ions located in inequivalent positions expected due to the disorder caused in the garnet-type lattice of a mixed crystal by compositional fluctuations. The substitution of gadolinium by yttrium in the lattice results not only in an emission blue shift and in decreased splitting of the lowest doublet 5d state of Ce3+ but also in an increased separation between the lowest doublet level 5d1 and the lowest triplet 5d3 level, as well as in changing the rates of intracenter and extracenter energy relaxation.

Nonlinear refraction and absorption spectroscopy of tellurite glasses within telecom bands

In this work we determined the nonlinear refractive index (n2) and the two-photon absorption coefficient (β) spectra of seven different tellurite glasses by open-aperture nonlinear ellipse rotation (NER) measurements. In order to demonstrate the potential of these vitreous systems for telecommunication applications, their nonlinear responses were characterized in the range of 1200–1600 nm. Aiming all-optical switching applications, figures-of-merit (FOMs) were calculated, based on n2, α0 and β coefficients. The FOMs results indicate that the investigated tellurite glasses compositions are promising materials for all-optical switching within the telecom wavelength range.

Deep learning algorithms for temperature field reconstruction of nonlinear tomographic absorption spectroscopy

Nonlinear tomography can be combined with line-of-sight measurement techniques and take the full advantage of the nonlinear relationship between the target fields to be reconstructed and the measured projections. For example, it has been integrated with absorption spectroscopy for simultaneous imaging of temperature and species concentration, and the technique is referred to as nonlinear tomographic absorption spectroscopy (NTAS) which is especially suitable to applications where the optical access is extremely limited. However, the major drawback of nonlinear tomography is its high computational costs for the inversion process, which involves the solution of a large-scale nonlinear equation system. The situation becomes more severe when thousands of tomographic frames need to be processed. This limitation can be potentially overcome by applying a deep learning algorithm, which can build efficient mapping between the projections and the target fields for rapid reconstruction. Nevertheless, only preliminary study of convolutional neural networks was reported so far, and there are also other well-established methods which have not been investigated yet. In this work, we cite NTAS as an example and demonstrate the reconstruction of temperature field using deep brief network (DBN) and recurrent neural network (RNN) for the first time. This work also aims to provide systematic comparative studies between these representative algorithms. As expected, all the deep learning algorithms are very efficient in solving NTAS problems, and a typical reconstruction time is on the order of milliseconds. In addition, the results suggest that, in NTAS problem, RNN is superior to the other methods in terms of both accuracy and noise immunity, and it is more favorable for practical applications.

Probing the Strong Near-IR Two-Photon Transition in Supramolecular Triphenylamine-based Polymers by Nonlinear Absorption Spectroscopy.

Due to their capability of film formation and remarkable optical features, semiconductor polymers with high two-photon absorption (2PA) have been studied as potential candidates for the development of organic photonic platforms. Furthermore, there is a high demand for photonic devices operating in the near-infrared (IR) region. However, the magnitude of the nonlinear optical response of random coil polymers in the IR region is weak due to the loss of molecular structure caused by increasing the π-conjugated backbone. Thus, herein we aim to investigate the molecular structure and 2PA features relationship for four polymers with supramolecular (helical) rodlike structure. Such polymers have a rigid core based on triphenylamine groups connected to the chiral binaphthalene units and a strong electron-withdrawing group (EWG). This kind of structure allows a very high chromophore density, which was responsible for generating 2PA cross-section between 305 GM and 565 GM in the near-IR (900-1300 nm), depending on the EWG strength. in light of the two-level model within the sum-overstates approach, we estimated the degree of intramolecular charge transfer induced by 2PA in the IR region, and values as high as 50-70% were found. Such a critical outcome allows the 2PA cross-section in the IR region to remain high even though the ratio between the visible/IR-band 2PA cross-section increases as a function of EWG strength.

Simultaneous measurement of flame temperature and species concentration distribution from nonlinear tomographic absorption spectroscopy

This research introduces the evolution strategy based on covariance matrix adaption algorithm, which has been proven applicable for solving highly inseparable and nonlinear optimization problems, to resolve the nonlinear tomography absorption spectroscopy inverse optimization problem and simultaneously reconstruct a two-dimensional temperature and species concentration distribution fields in an absorption flame. Severe ill-posedness and crosstalk issues exist during multi-parameter field simultaneous reconstruction. To alleviate the ill-posedness in nonlinear tomography absorption spectroscopy equations, two regularization methods, Tikhonov regularization and the regularization based on the generalized Gaussian Markov random field, are applied to reconstruct the temperature distribution field. The impacts of different regularization factors (the temperature and the concentration regularization factors) are investigated as well, and optimal intervals are suggested for reference. Simulation results show that the evolution strategy based on covariance matrix adaption algorithm worked well in retrieving multi-parameter distribution fields on both symmetric and asymmetric flame distribution models. Moreover, the evolution strategy based on covariance matrix adaption algorithm alleviated the severe crosstalk issue between temperature and species concentration to improve the accuracy of the reconstructed species concentration distribution field.

Nonlinear Coherence Effects in Transient-Absorption Ion Spectroscopy with Stochastic Extreme-Ultraviolet Free-Electron Laser Pulses.

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2p-3d bound-bound transitions between the spin-orbit multiplets ^{3}P_{0,1,2} and ^{3}D_{1,2,3} of the transiently produced doubly charged Ne^{2+} ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4±0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time.

Compressing convolutional neural networks using POD for the reconstruction of nonlinear tomographic absorption spectroscopy

Nonlinear tomographic absorption spectroscopy (NTAS) is an imaging technique that inherits the advantages of both absorption spectroscopy and tomography such as species specificity, quantitative measurements, high sensitivity and capability in resolving non-uniformities of the target flow field. It has found many applications such as engine measurements where the flow field is highly turbulent. However, the solution of the NTAS problem is non-trivial as a set of nonlinear equations need to be solved. The inversion problem is typically converted into an optimization problem and solved with the simulated annealing algorithm which is criticized for its low computational efficiency. Convolutional neural networks havebeen successfully introduced to speed up the reconstructions of NTAS. However, due to the large amount of training data, CNN has a high demand for memory to store the network parameters, which also requires a large amount of time to be optimized during the learning process. In this work, we proposed a scheme to compress the output layer via dimension reduction using proper orthogonal decomposition. The simulative study performed here demonstrated that the number of network parameters and the training time can be reduced dramatically by a factor of ∼100 and ∼10 respectively. This improvement makes NTAS a more preferable technique for combustion diagnostics for which time and spatially resolved measurements are highly desired.

Single photon nonlinear absorption spectroscopy of cascade energy level systems

Theoretically discussed the third order nonlinear optical properties and absorption cross-sections in two and three level cascade systems by considering single photon resonant absorption at 671 nm of CW laser. The results indicate that while two level system producing Single Saturable Absorption (SSA), it is possible to obtain Double Saturable Absorption (DSA) and Reverse Saturable Absorption (RSA) with long range and modulation depth by tuning the spectroscopic parameters like absorption cross-section and lifetime of participating levels in three level system. Analytical expressions have been given for nonlinear coefficients of the electromagnetic wave equation by considering the propagation in two and three level cascade media. The gradient refractive index formed in single and double beam interaction with the material was discussed. The origin of large phase distortion in the z-scan closed aperture and formation of nonlinear grating in the degenerative four wave mixing are discussed. To emphasize the theoretical results, the discussion has been extended to explain the third order nonlinear experimental results of porphyrin molecules in the degenerative four wave mixing and z-scan techniques.

Reconstruction for limited-data nonlinear tomographic absorption spectroscopy via deep learning

Nonlinear tomographic absorption spectroscopy (NTAS) is an emerging gas sensing technique for reactive flows that has been proven to be capable of simultaneously imaging temperature and concentration of absorbing gas. However, the nonlinear tomographic problems are typically solved with an optimization algorithm such as simulated annealing which suffers from high computational cost. This problem becomes more severe when thousands of tomographic data needs to be processed for the temporal resolution of turbulent flames. To overcome this limitation, in this work we propose a reconstruction method based on convolutional neural networks (CNN) which can take full advantage of the large amount tomographic data to build an efficient neural networks to rapidly predict the reconstruction by feeding the sinograms to it. Simulative studies were performed to investigate how the parameters will affect the performance of neural networks. The results show that CNN can effectively reduce the computational cost and at the same time achieve a similar accuracy level as SA. The successful demonstration CNN in this work indicates possible applications of other sophisticated deep neural networks such as deep belief networks (DBN) and generative adversarial networks (GAN) to nonlinear tomography. © 2018 Elsevier Ltd.

Nonlinear absorption study in four and five energy level systems

Nonlinear absorption in four and five energy level systems have been studied with the aid of steady-state rate equation approach. We report on the tunability of saturable and reverse saturable absorption as a function of spectroscopic parameters (lifetimes and absorption cross-sections). Detailed information is given on the estimation of spectroscopic parameters in the nonlinear absorption spectroscopy. The advantage of four and five level cascade models over other models is that one can produce simultaneously saturable and reverse saturable absorption. Four and five level models can be potentially used as optical limiters and saturable absorbers due to their merit over two and three level models in terms of long tunability and high power control.

Measurement of non-equilibrium carriers dynamics in Ce-doped YAG, LuAG and GAGG crystals with and without Mg-codoping

Non-linear absorption spectroscopy in pump and probe configuration has been used to test the population of non-equilibrium carriers in Ce-doped Y3Al5O12 (YAG), Lu3Al5O12 (LuAG), and Gd3AlxGa(5-x)O12 (GAGG) crystals with and without codoping by Mg2+ ions. A faster rise time of the induced optical density has been observed in all crystals codoped with Mg with respect to that in Mg-free samples. A significant difference in the time evolution of the differential optical density in GAGG with respect to YAG and LuAG crystals has also been measured. In both GAGG:Ce and GAGG:Ce,Mg an absorption band with maximum in the blue-green range and a decay time of 1.4ps is present. This band is due to the absorption by free electrons before they are trapped or re-captured by Ce3+ ions. A broad absorption band in the yellow-red region with very short rise time and a decay time longer than 150ps has been observed in all the Ce-doped garnets under study and can be attributed to the absorption from the Ce3+ excited states.

Nonlinear Absorption Spectroscopy of Porphyrin J-aggregates in Aqueous Solution: Evidence for Control of Degree of Association by Light-Induced Force

Spectroscopic evidence was obtained for molecular aggregation states to be controlled by the irradiation of light, which is off-resonant below the peak absorption energies of both monomers and well-grown J-aggregates. In low (undersaturated)-concentration aqueous solutions of porphyrin molecules (tetraphenyl porphyrin tetrasulfonic acid; TPPS) where the monomer absorbance dominates, irradiation with a 532 nm laser induces a decrease in the monomer absorbance and an increase in the aggregate absorbance. The increase in the absorbance of J-aggregates occurs in a broad spectral range associated with the increase in the number of not only variously sized oligomer aggregates but also aggregates structurally different from well-grown stable J-aggregates. In high-concentration solutions where the J-aggregate absorbance dominates, a blue shift of the absorption peak of J-aggregates is induced at the same 532 nm irradiation, corresponding to a decrease in the aggregation number or in the association energy. By con...

Observation of confinement effects through liner and nonlinear absorption spectroscopy in cuprous oxide

Cuprous oxide nano clusters, micro cubes and micro particles were successfully synthesized by reducing copper (II) salt with ascorbic acid in the presence of sodium hydroxide via a co-precipitation method. The X-ray diffraction studies revealed the formation of pure single phase cubic. Raman spectrum shows the inevitable presence of CuO on the surface of the Cu2O powders which may have an impact on the stability of the phase. Transmission electron microscopy (TEM) data revealed that the morphology evolves from nanoclusters to micro cubes and micro particles by increasing the concentration of NaOH. Linear optical measurements show that the absorption peak maximum shifts towards red with changing morphology from nano clusters to micro cubes and micro particles. The nonlinear optical properties were studied using open aperture Z-scan technique with 532 nm, 6 ns laser pulses. Samples exhibited saturable as well as reverse saturable absorption. The results show that the transition from SA to RSA is ascribed to excited-state absorption (ESA) induced by two-photon absorption (TPA) process. Due to confinement effects (enhanced band gap) we observed enhanced nonlinear absorption coefficient (βeff) in the case of nano-clusters compared to their micro-cubes and micro-particles.

Optical nanofibers and spectroscopy

We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. In order to determine the waist diameter and to verify the fiber profile, we employ scanning electron microscope measurements and a novel accurate in situ optical method based on harmonic generation. We use our fibers for linear and nonlinear absorption and fluorescence spectroscopy of surface-adsorbed organic molecules and investigate their agglomeration dynamics. Furthermore, we apply our spectroscopic method to quantum dots on the surface of the fiber waist and to cesium vapor surrounding the fiber. Finally, toward dispersive measurements, we present our first results on building and testing a single-fiber bimodal interferometer.

All-fiber frequency-stabilized erbium doped ring laser

We present an all-fiber frequency-stabilized ring laser system with an integrated reference gas cell consisting of a hollow core fiber filled with acetylene. Through nonlinear absorption spectroscopy the laser frequency is stabilized to a specific absorption line of acetylene. Three different stabilization schemes are investigated and the minimum Allan deviation obtained after 100 s is 4.4 · 10(-11).

Probing Near-Infrared Photorelaxation Pathways in Eumelanins and Pheomelanins

Ultraviolet-visible spectroscopy readily discerns the two types of melanin pigments (eumelanin and pheomelanin), although fundamental details regarding the optical properties and pigment heterogeneity are more difficult to disentangle via analysis of the broad featureless absorption spectrum alone. We employed nonlinear transient absorption spectroscopy to study different melanin pigments at near-infrared wavelengths. Excited-state absorption, ground-state depletion, and stimulated emission signal contributions were distinguished for natural and synthetic eumelanins and pheomelanins. A starker contrast among the pigments is observed in the nonlinear excitation regime because they all exhibit distinct transient absorptive amplitudes, phase shifts, and nonexponential population dynamics spanning the femtosecond-nanosecond range. In this manner, different pigments within the pheomelanin subclass were distinguished in synthetic and human hair samples. These results highlight the potential of nonlinear spectroscopies to deliver an in situ analysis of natural melanins in tissue that are otherwise difficult to extract and purify.

Estimation of molar absorptivities and pigment sizes for eumelanin and pheomelanin using femtosecond transient absorption spectroscopy

Fundamental optical and structural properties of melanins are not well understood due to their poor solubility characteristics and the chemical disorder present during biomolecular synthesis. We apply nonlinear transient absorption spectroscopy to quantify molar absorptivities for eumelanin and pheomelanin and thereby get an estimate for their average pigment sizes. We determine that pheomelanin exhibits a larger molar absorptivity at near IR wavelengths (750 nm), which may be extended to shorter wavelengths. Using the molar absorptivities, we estimate that melanin pigments contain approximately 46 and 28 monomer units for eumelanin and pheomelanin, respectively. This is considerably larger than the oligomeric species that have been recently proposed to account for the absorption spectrum of eumelanin and illustrates that larger pigments comprise a significant fraction of the pigment distribution.

P7 Multidisciplinary assessment in elderly oncologic patients for treatment planning: final results of a prospective observational study in a General Hospital

Nonlinear tomographic absorption spectroscopy (NTAS) is an imaging technique that inherits the advantages of both absorption spectroscopy and tomography such as species specificity, quantitative measurements, high sensitivity and capability in resolving non-uniformities of the target flow field. It has found many applications such as engine measurements where the flow field is highly turbulent. However, the solution of the NTAS problem is non-trivial as a set of nonlinear equations need to be solved. The inversion problem is typically converted into an optimization problem and solved with the simulated annealing algorithm which is criticized for its low computational efficiency. Convolutional neural networks havebeen successfully introduced to speed up the reconstructions of NTAS. However, due to the large amount of training data, CNN has a high demand for memory to store the network parameters, which also requires a large amount of time to be optimized during the learning process. In this work, we proposed a scheme to compress the output layer via dimension reduction using proper orthogonal decomposition. The simulative study performed here demonstrated that the number of network parameters and the training time can be reduced dramatically by a factor of ∼100 and ∼10 respectively. This improvement makes NTAS a more preferable technique for combustion diagnostics for which time and spatially resolved measurements are highly desired.

Quantum limits and robustness of nonlinear intracavity absorption spectroscopy

We investigate the limits of intracavity absorption spectroscopy with nonlinear media. Using a common theoretical framework, we compare the detection of a trace gas within (i) an undriven cavity with gain near and above threshold; (ii) a driven cavity with gain kept just below threshold; and (iii) a cavity driven close to the saturation point of a saturable absorber. These phase-transition-based metrology methods are typically quantum limited by spontaneous emission, and we compare them to the empty-cavity shot-noise-limited case. Although the fundamental limits achievable with nonlinear media do not surpass the empty-cavity limits, we show that nonlinear methods are more robust against certain technical noise models. This recognition may have applications in spectrometer design for devices operating in nonideal field environments.

Nonlinear differential ultrafast laser absorption spectroscopy observation of charge carrier dynamicsof CdSxSe1−x nanocrystal doped glasses

Based on femtosecond transient absorption spectroscopy, the charge carrier dynamics ofCdSxSe1−x nanocrystal doped glasses was investigated by analyzing the nonlinear differential transientabsorption spectra. Time constants were extracted, which are different from the literatureresults using similar samples: they are pump laser intensity-dependent and decreasewith increasing pump laser intensity, whereas risetime constants are pump laserintensity-independent and high-order exciton recombination can be identified from thespectra. The threshold pump intensity was determined, which is lower than the literatureresult. Reasons for high-order recombination were analyzed, and broad size, statedistribution and high pump intensity are found as key factors. Auger recombination mayplay a key role in the multi-exciton recombination. Theoretical calculations andanalysis support this conclusion. A schematic model for electron transport and theformation of triexcitons in our samples are proposed to explain these results.