Time-resolved Reflectance Spectroscopy Research Articles

Quality Characteristics, Sensory Profiles and Ethylene Production of Stored ‘Abate Fetel’ Pears Sorted at Harvest by Time-Resolved Reflectance Spectroscopy

Time-resolved reflectance spectroscopy (TRS), a nondestructive technique, can help the industry to provide high-quality fruit to encourage pear consumption. The absorption coefficient measured by TRS at 670 nm (μa670) represents a maturity index for pear fruit, with less mature pears high μa670 and more mature low μa670. The aim of this work was to study the quality characteristics, the sensory profiles and the ethylene production of ‘Abate Fetel’ pears sorted at harvest in different TRS maturity classes and stored in different atmospheres. At harvest, 540 pears were measured by TRS for μa670, ranked by μa670 in three maturity classes (less-LeM, medium-MeM and more-MoM mature) and randomized in nine samples according to 1-MCP treatment (treated, control), storage time (4–6 months) and atmosphere (air-NA; CA: 8–12 kPa O2, 1 kPa CO2). Fruits were examined at harvest and after 7 days of poststorage shelf life for skin color, firmness, soluble solids, acidity and ethylene production and were submitted to sensory analysis. At harvest and after storage, MoM pears were less green and showed a higher SSC content than LeM ones. After storage, MoM pears produced less ethylene and were perceived to be firmer (especially in 1-MCP-treated pears), more astringent and less juicy (when stored for 6 months) than LeM ones.

Spin dynamics in the axion insulator candidate EuIn2As2

We report the investigation of spin dynamics in the antiferromagnetic metal ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ with the Zintl phase by using time-resolved magneto-optical Kerr effect (TR-MOKE) and transient reflectivity spectroscopy. In TR-MOKE measurement, we observe a spin precession mode with a frequency of about 18 GHz below 1.4 T and a new emerging coherent acoustic phonon mode near 35 GHz at a higher field where the latter shows a field-independent behavior. With temperature increasing, the spin precession disappears above N\'eel temperature ${T}_{N}$, whereas the acoustic phonon persists up to 25 K due to existence of magnetic-field-induced polarized paramagnetism. In contrast, the quasiparticle dynamics of ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ does not show dramatic change in the vicinity of ${T}_{N}$. By comparing the spin and quasiparticle dynamics, we attribute the appearance of an acoustic phonon in TR-MOKE to photoinduced transient perturbation of magneto-optical coefficient. Our paper provides an in-depth investigation on the dynamical magnetic properties and broadens the understanding of the ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ system.

Non-destructive determination of ripening in melon fruit using time-resolved spectroscopy

The aim of this work was to explore the feasibility of time-resolved reflectance spectroscopy (TRS) in determining the ripening degree and the quality of orange-fleshed melons. Sixty ‘Honey Moon’ melons were measured by TRS in the 540-1064 nm range and classified as less (LeM), medium (MeM), and more (MoM) mature according to increasing values of μa540. MoM fruit showed yellower peel color, slightly more orange pulp, higher juiciness and higher carotenoid contents than LeM ones. MoM fruit also showed higher internal ethylene concentration and lower firmness than LeM ones, even if the differences were not significant. The μa540 was positively related to internal ethylene, carotenoid accumulation, and juiciness, indicating that μa540 was linked to different ripening processes in melons. However, the relationship between μa540 and total carotenoid content was not as high as expected due to the low variability of pulp color and of carotenoid content. Changes in flesh color toward a more orange shade were accompanied by increased juiciness and ethylene production and by carotenoid accumulation, while changes in peel color were associated with changes in flesh firmness and juiciness. In conclusion, the absorption coefficient measured at 540 nm (μa540) by TRS could be used to sort melons in different ripening degrees; however, its applicability will need to be evaluated on a larger number of fruits and on other varieties.

Modeling mango ripening during shelf life based on pulp color nondestructively measured by time-resolved reflectance spectroscopy

Non-destructive techniques could help mango growers to pick fruit at the proper maturity degree and to monitor fruit quality during storage and marketing to satisfy the consumer expectations. The absorption coefficient non-destructively measured at 540 nm (μa540) by time-resolved reflectance spectroscopy (TRS) was shown to be a maturity index for mango fruit and was highly correlated with pulp color parameters and carotenoids content. The aim of this work was to model μa540 using the biological shift factor theory to verify if μa540 is able to assess the maturity degree of individual mango fruit and then to use μa540 to model pulp color in order to relate μa540 to an important index of mango ripening.Mango fruit (Mangifera indica L. cv Tommy Atkins) at commercial maturity for ship and air transport maturities, were measured by TRS, ranked according to decreasing μa540 (decreasing maturity), randomized into six batches per transport maturity and analyzed for μa540 and pulp color parameters after 0, 1, 2, 5, 6 and 7 days of shelf life at 20 °C.The μa540 as a function of biological shift factor increased during ripening following a logistic/exponential model (R2adj=99%) with a faster rate in less mature fruit than in more mature ones. The changes in pulp color during mango ripening depended on fruit maturity (i.e., μa540) and on time of shelf life at 20 °C. By converting the μa540 into the biological shift factor it was possible to model the increasing trend of a*, b*, C*, and IY and the decreasing trend of L* and h° during the shelf life period explaining 91.2–99.8% of the variation and to differentiate mango fruit according to their biological age. Similarly to μa540, color changes occurred earlier in more mature fruit and later in less mature ones with the same pattern in time.There is a synchronization between changes of μa540 and changes of a*, b*, C* and yellowness during ripening in mango fruit which allows to use μa540 to sort fruit according to their maturity degree and then to optimize fruit management along the supply chain.

Ultrafast coherent interlayer phonon dynamics in atomically thin layers of MnBi2Te4

The atomically thin MnBi2Te4 crystal is a novel magnetic topological insulator, exhibiting exotic quantum physics. Here we report a systematic investigation of ultrafast carrier dynamics and coherent interlayer phonons in few-layer MnBi2Te4 as a function of layer number using time-resolved pump-probe reflectivity spectroscopy. Pronounced coherent phonon oscillations from the interlayer breathing mode are directly observed in the time domain. We find that the coherent oscillation frequency, the photocarrier and coherent phonon decay rates all depend sensitively on the sample thickness. The time-resolved measurements are complemented by ultralow-frequency Raman spectroscopy measurements, which both confirm the interlayer breathing mode and additionally enable observation of the interlayer shear mode. The layer dependence of these modes allows us to extract both the out-of-plane and in-plane interlayer force constants. Our studies not only reveal the interlayer van der Waals coupling strengths, but also shed light on the ultrafast optical properties of this novel two-dimensional material.

Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule, and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration. Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy, we investigate the spin/valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy. With fine tuning of the photon energy of both pump and probe beams, the valley relaxation process for the neutral excitons and trions is found to be remarkably different—their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds, respectively. The observed long trion spin lifetime of > 2.0 ns is discussed to be associated with the dark trion states, which is evidenced by the photon-energy dependent valley polarization relaxation. Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-II Weyl semimetal Td−WTe2

The unique bulk crossing points in the band structure of topological Weyl semimetals have been shown to enhance nonlinear optical and optoelectronic properties, even for light at optical wavelengths. Such pronounced nonlinear optical effects have been studied in type-I Weyl semimetals, yet few studies have quantified these effects in type-II Weyl semimetals. We here present an optical experimental study with polarization resolution of two nonlinear optical effects in the type-II Weyl semimetal ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$. We begin by investigating the bulk optical second-harmonic response of this material using the rotational anisotropy of the second-harmonic generation. This technique allows us to simultaneously investigate the dependence of the second-harmonic response on the symmetry of the crystal and to quantify the size of that response, which we compare to other nonlinear crystals. We then use polarized time-resolved optical reflectivity spectroscopy to identify the nonlinear optical effect of impulsive stimulated Raman scattering as the origin of the coherent oscillations of the 0.25-THz shear mode. We find that the strength of this response in ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$ is enhanced compared with the observation of other modes excited through the displacive excitation of coherent phonons. Notably, both the second-harmonic response and the coherent excitations of the phonons demonstrate strong anisotropy, displaying a clear dependence on the polarization of the light used in the experiments. We use point-symmetry analyses of the material to guide our understanding of these observations and perform fluence-dependent measurements to investigate the electron-phonon coupling.

Thickness-dependent ultrafast charge-carrier dynamics and coherent acoustic phonon oscillations in mechanically exfoliated PdSe2 flakes.

Recently, palladium diselenide (PdSe2) has emerged as a promising material with potential applications in electronic and optoelectronic devices due to its intriguing electronic and optical properties. The performance of the device is strongly dependent on the charge-carrier dynamics and the related hot phonon behavior. Here, we investigate the photoexcited-carrier dynamics and coherent acoustic phonon (CAP) oscillations in mechanically exfoliated PdSe2 flakes with a thickness ranging from 10.6 nm to 54 nm using time-resolved non-degenerate pump-probe transient reflection (TR) spectroscopy. The results imply that the CAP frequency is thickness-dependent. Polarization-resolved transient reflection (PRTR) measurements reveal the isotropic charge-carrier relaxation dynamics and the CAP frequency in the 10.6 nm region. In addition, the deformation potential (DP) mechanism dominates the generation of the CAP. Moreover, a sound velocity of 6.78 × 103 m s-1 is extracted from the variation of the oscillation period with the flake thickness and the delay time of the acoustic echo. These results provide insight into the ultrafast optical coherent acoustic phonon and optoelectronic properties of PdSe2 and may open new possibilities for PdSe2 applications in THz-frequency mechanical resonators.

Femtosecond time-resolved diffuse reflectance study on facet engineered charge‐carrier dynamics in Ag3PO4 for antibiotics photodegradation

The contamination of antibiotics micro-pollutants in water bodies causes increasing concerns currently. Semiconductor-based photocatalysis has been considered as an efficient method for the removal of contaminations from aquatic environment, while its catalytic performance always depends on facets in semiconductor. To deeply understand the relation between facet and catalytic process, herein we take the different morphologies of Ag3PO4 as the cases to investigate the charge‐carrier dynamics in different facets of Ag3PO4 for the photodegradation of antibiotic ciprofloxacin (CIP), sulfamethoxazole (SMX), and tetracycline (TC). The degradation results show that Ag3PO4 tetrahedrons with dominated {111} facets exhibit 1.83 times higher CIP degradation rate than Ag3PO4 cubes with dominated {100} facets. Femtosecond time-resolved diffuse reflectance (fs-TDR) spectroscopy is used to give the detail charge-carrier dynamics in different facets of Ag3PO4 in accordance with the antibiotics pollutants degradation rate, which provides a more direct and visual proof.

Anthocyanin and carotenoid contents assessed by time-resolved reflectance spectroscopy in potato tubers (Solanum tuberosum L.) with different flesh colors

This work aimed at studying the relationships between the absorption spectra acquired by time-resolved reflectance spectroscopy (TRS) and the carotenoid (CAR) and/or the anthocyanin (ANT) contents in 9 potato genotypes with different flesh color (white, yellow, red, purple). Fifty whole and intact tubers/genotype were non-destructively measured by TRS in the 540-980 nm range; white- and yellow-fleshed were ranked according to increasing µa540, the red ones according to µa670 and the purple ones according to µa780. Then, 5 tubers/genotype, corresponding to the highest, the lowest and 3 intermediate values of each μa range, were analyzed for flesh color and CAR and ANT contents. In white- and yellow-fleshed genotypes, µa540 ranged from 0.078 to 0.207 cm-1, showing the highest value in ‘Melrose’ and in ‘ISCI 133/12-1’ and the lowest ones in ‘Romantica’ and in ‘CN 07.16.3’. In red-fleshed tubers, µa670 ranged from 0.049 to 0.146 with no significant differences between genotypes; in purple genotypes, µa780 ranged from 0.147 to 0.473, showing the highest values in ‘Bleuet’. CAR content ranged between 0.071 to 5.937 mg kg-1 FW, displaying the highest amounts in the deep yellow genotypes ‘Melrose’ and ‘ISCI 133/12-1’ and the lowest ones in the white ‘CN 07.16.3’ and in the dark purple ‘Bleuet’ tubers. ANT content ranged from 31.63 to 798.44 mg kg-1 FW in red-purple genotypes, having the highest values in ‘Bleuet’. By using TRS spectra and PLS analysis, it was possible to predict CAR (R2CV=0.79, RMSECV=0.89) and ANT (R2CV =0.81, RMSECV=95.53) contents and flesh color (h°) in yellow-fleshed genotypes (R2CV =0.93, RMSECV=0.67) and purple genotypes (R2CV =0.82, RMSECV=1.63).

Non-destructive detection of potato tubers internal defects: critical insight on the use of time-resolved spectroscopy

Aiming at investigating the feasibility of time-resolved reflectance spectroscopy (TRS) for the non-destructive detection of internal brown spot (IBS) and other defects in ‘El Beida’ potatoes, 90 tubers were measured in 8 points by TRS for the absorption coefficient at 730 nm (µa730) and then transversally cut open for recording presence and position of internal defects and IBS severity. The µa730 was lower in healthy tissue than in defected ones and increased with increasing IBS severity with no difference between healthy and slightly IBS tissues. Tubers having at least one out of the eight µa730 measures ≥ 0.04262 cm-1 were considered “defected”. Therefore, TRS tubers classification performance were: defected, 73.5%; healthy, 45.5%; slightly IBS, 57.1%; moderate IBS, 60%; and severe IBS, 100% of the cases. Misclassification could be due to the high variability in flesh color of ‘El Beida’ potatoes, as some healthy tubers showed L*, b* and C* color parameters very similar to that of defected ones, especially when IBS severity was slight or moderate, resulting in µa730 values not significantly different between healthy and IBS tissues. The feasibility of TRS in detecting internal disorders in potatoes must be investigated in other susceptible cultivar to see if flesh color can represent a real problem in the detection of defects linked to browning development.

Operando time-resolved diffuse reflection spectroscopy: The origins of photocatalytic water-oxidation activity of bismuth vanadate

Photocatalytic water splitting has been believed to be a key technology for the construction of a sustainable society based on a renewable energy source. Understanding the exact mechanisms of oxygen evolution by water-splitting reaction, which has been regarded as a bottleneck to realize the water splitting in practical basis, should help us to enhance solar-to-hydrogen energy conversion efficiency. Here, we present a detailed investigation based on two types of bismuth vanadate (BiVO4) crystals. One is a monoclinic scheelite crystal that forms a truncated tetragonal bipyramidal shape (MS), and the other is a tetragonal zircon crystal that forms an octahedral shape (TZ). MS, which has relatively higher photocatalytic activity of water oxidation, rarely adsorbed reduced mediator (Fe2+). Operando time-resolved diffuse reflection spectral measurement demonstrated, for the first time, that the surface holes of MS crystals show extremely high reactivity, while those of TZ crystals show poor reactivity. Our investigation would promote an essential understanding of the photocatalytic reaction mechanism of water oxidation.

Nondestructive Food Quality Monitoring Using Phase Information in Time-Resolved Reflectance Spectroscopy

A rapid and affordable method is desired by the food industry to monitor the quality of the food. A nondestructive and noninvasive testing is desired to determine the optimum harvesting time, monitor changes during storage, and asses the internal quality of the fruits. Time-resolved reflectance spectroscopy (TRS) is used for noninvasive simultaneous measurement of the scattering and the absorption properties of turbid media. In this article, a time-resolved phase reflectance-based nondestructive and noninvasive method is proposed to monitor the internal quality of the fruits. The time-resolved reflectance is determined by the number of internal scatterers based on the mean-free path length of the incident light. The change in the mean-free path due to change in the pigmentation of the internal pulp changes the order of the scatters and, therefore, affects the scattering profile of the reflected light. The scattering profile is monitored using the Stokes degree of linear polarization. As an example, the Red Star and Granny Smith apples are considered, and the degradation in the apples due to storage is correlated with the changes in the degree of polarization (DoP). The change in the DoP after seven days is 0.26% and 11.4% for a time resolution of 300 ps, while for 500 ps, it is 2% and 2.3% for Red Star and Granny Smith, respectively. The differential variations in the DoP help in monitoring the internal pulp quality of the two variants of the apples considered. This proposed method can also be used in all agro-based products for internal quality monitoring.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals. These measurements demonstrate the charge transfer time around 10 ps and long-lived charge-separated state over the nanosecond time scale in two-dimensional ligand-perovskite heterostructures. The efficient charge transfer processes coupled with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in two-dimensional perovskite solar cells.

Near Bandgap Excitation Inhibits the Interfacial Electron Transfer of Semiconductor/Cocatalyst.

Understanding the ultrafast interfacial electron transfer (IET) process is essential for establishing the structure-property relationship of the semiconductor/cocatalyst system for photocatalytic H2 evolution. However, the IET kinetics for the near bandgap excitation has not been reported. Herein, we investigate the IET kinetics of g-C3N4/Pt as a semiconductor/cocatalyst prototype by femtosecond time-resolved diffuse reflectance spectroscopy. We find that the near bandgap excitation of g-C3N4 inhibits the IET of g-C3N4/Pt due to electron deep trapping, resulting in a markedly decreased apparent quantum efficiency for photocatalytic H2 evolution. This work complements the kinetic understanding for the photocatalytic mechanism of the semiconductor/cocatalyst system in its whole light absorption range.

Shallow trap state-enhanced photocatalytic hydrogen evolution over thermal-decomposed polymeric carbon nitride.

We investigate the photogenerated electron kinetics of a thermal-decomposed polymeric carbon nitride (TCN) synthesized in air using femtosecond time-resolved diffuse reflectance spectroscopy. We find that the oxygen functional groups in TCN contribute to the formation of reactive shallow trap states for photogenerated electrons, leading to an enhanced activity for photocatalytic hydrogen evolution.

Shallow Trap State-Induced Efficient Electron Transfer at the Interface of Heterojunction Photocatalysts: The Crucial Role of Vacancy Defects.

Constructing vacancies has been demonstrated to be an effective way to modulate charge flow in semiconductor photocatalysts. However, the role of vacancies in the interfacial electron transfer (IET) of heterojunction photocatalysts remains poorly understood, which hinders the general design of heterojunction photocatalysts. Herein, by taking g-C3N4/MoS2 as a heterojunction photocatalyst prototype, we unravel that vacancies play a critical role in the IET of heterojunction photocatalysts. Theoretical simulations, combined with femtosecond time-resolved diffuse reflectance spectroscopy, give a clear physical picture that N vacancy states act as shallow trap states (STSs) for photogenerated electrons and thereby facilitate the IET process due to a large energy difference between STSs and charge separation states. Moreover, the excess electrons left by the loss of N atoms (producing N vacancies) could partially transfer to MoS2 to generate STSs in the forbidden band of MoS2, where the transferred photogenerated electrons could be further trapped to efficiently drive H2 evolution. This work suggests a promising strategy to tune IET of heterojunction photocatalysts for achieving highly efficient photocatalytic reactions.

Cerebral oxygenation and blood flow in normal term infants at rest measured by a hybrid near-infrared device (BabyLux).

The BabyLux device is a prototype optical neuro-monitor of cerebral oxygenation and blood flow for neonatology integrating time-resolved reflectance spectroscopy and diffuse correlation spectroscopy. Here we report the variability of six consecutive 30 s measurements performed in 27 healthy term infants at rest. Poor data quality excluded four infants. Mean cerebral oxygenation was 59.6 ± 8.0%, with intra-subject standard deviation of 3.4%, that is, coefficient of variation (CV) of 5.7%. The inter-subject CV was 13.5%. Mean blood flow index was 2.7 × 10-8 ± 1.56 × 10-8 (cm2/s), with intra-subject CV of 27% and inter-subject CV of 56%. The variability in blood flow index was not reduced by the use of individual measures of tissue scattering, nor accompanied by a parallel variability in cerebral oxygenation. The intra-subject variability for cerebral oxygenation variability was improved compared to spatially resolved spectroscopy devices, while for the blood flow index it was comparable to that of other modalities for estimating cerebral blood flow in newborn infants. Most importantly, the simultaneous measurement of oxygenation and flow allows for interpretation of the high inter-subject variability of cerebral blood flow as being due to error of measurement rather than to physiological instability.

In vivo label-free optical monitoring of structural and metabolic remodeling of myocardium following infarction.

Cardiac remodeling following myocardial infarction (MI) involves structural and functional alterations in the infarcted and remote viable myocardium that can ultimately lead to heart failure. The underlying mechanisms are not fully understood and, following our previous study of the autofluorescence lifetime and diffuse reflectance signatures of the myocardium in vivo at 16 weeks post MI in rats [Biomed. Opt. Express 6(2), 324 (2015)25780727], we here present data obtained at 1, 2 and 4 weeks post myocardial infarction that help follow the temporal progression of these changes. Our results demonstrate that both structural and metabolic changes in the heart can be monitored from the earliest time points following MI using label-free optical readouts, not only in the region of infarction but also in the remote non-infarcted myocardium. Changes in the autofluorescence intensity and lifetime parameters associated with collagen type I autofluorescence were indicative of progressive collagen deposition in tissue that was most pronounced at earlier time points and in the region of infarction. In addition to significant collagen deposition in infarcted and non-infarcted myocardium, we also report changes in the autofluorescence parameters associated with reduced nicotinamide adenine (phosphate) dinucleotide (NAD(P)H) and flavin adenine dinucleotide (FAD), which we associate with metabolic alterations throughout the heart. Parallel measurements of the diffuse reflectance spectra indicated an increased contribution of reduced cytochrome c. Our findings suggest that combining time-resolved spectrofluorometry and diffuse reflectance spectroscopy could provide a useful means to monitor cardiac function in vivo at the time of surgery.

In Situ Time-Resolved Redox Dynamics of Pd Catalysts Under Oscillating A/F Conditions

The redox dynamics of Pd catalysts under lean/rich perturbation conditions was analyzed by in situ time-resolved reflectance spectroscopy. The real-time redox of Pd/Al2O3 was monitored using the reflectance at 650 nm at every second under simulated exhausts (CO–C3H6–NO–O2), which were switched between air-to-fuel ratio of 14.1 (rich) and 15.0 (lean). Compared to the fast oxidation of metallic Pd (Pd0) upon rich-to-lean switching, the reduction of Pd oxide (Pd2+) upon reverse (lean-to-rich) switch started after a long induction period and proceeded slowly. Because the similar result was also observed for a Pd metal foil, the unequal redox rates under the lean/rich perturbation condition is characteristic of Pd. The temperature dependence of the redox rate demonstrated that the activation energy for the reduction of Pd oxide is greater than that of the Pd oxidation. The faster oxidation rate of Pd surface is considered as the primary reason for steep decrease of NO reduction efficiency in a slightly lean A/F region.