Pump-probe Absorption Research Articles

Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Qy band. Absorption changes of FRL-PSII were monitored at 278 K in the 400–800 nm spectral range on a timescale of 0.1–500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the ChlD1 position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify PD1+ChlD1− as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of ∼1.6 ps. Electron transfer to the secondary acceptor pheophytin PheoD1 has an apparent time of ∼7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, PD1+ChlD1− can be partially formed from the excited (PD1ChlD1)* state.

Ultrafast optical characteristics of Mn-doped CdS quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) doped with different concentrations of manganese (Mn = 0, 1, 3, 5, and 7 %) are synthesized via thermolysis of diselenocarbamato complexes of cadmium in the trioctylphosphine oxide (TOPO). The QDs show quantum size effects in the optical spectra and exhibit band-edge emission. The TEM images revealed that the QDs are spherical within the range of 3 to 5 nm. The nonlinear optical studies and optical limiting characteristics are studied. From open and closed aperture data, the nonlinear absorption coefficient (β) and refractive indices (n2) values are found to be in the range of 4–16 x10-9 cm2/w and 8-14x10-15 cm2/w. The cross relaxations and energy transfer mechanism of quantum dots are studied using the transient absorption pump–probe experiment.

Ab Initio Study of Charge Separation Dynamics and Pump-Probe Spectroscopy in the P3HT/PCBM Blend.

We develop a bottom-up computational method for excited-state dynamics and time-resolved spectroscopy signals in molecular aggregates, on the basis of ab initio excited-state calculations. As an application, we consider the charge separation dynamics and pump-probe spectroscopy in the amorphous P3HT/PCBM blend. To simulate quantum dynamics and time-resolved spectroscopy, the model Hamiltonian for single-excitation and double-excitation manifolds was derived on the basis of fragment-based excited-state calculations within the GW approximation and the Bethe-Salpeter equation. After elucidating the energetics of the electron-hole separation and examining linear absorption spectrum, we investigated the quantum dynamics of exciton and charge carriers in comparison with the pump-probe transient absorption spectra. In particular, we introduced the pump-probe excited-state absorption (ESA) anisotropy as a spectroscopic signature of charge carrier dynamics after exciton dissociation. We found that the charge separation dynamics can be probed by the pump-probe ESA anisotropy dynamics after charge-transfer excitations. The present study provides the fundamental information for understanding the experimental spectroscopy signals, by elucidating the relationship between the excited states, the exciton and charge carrier dynamics, and time-resolved spectroscopy.

Time-resolved pump–probe spectroscopic ellipsometry of cubic GaN II: Absorption edge shift with gain and temperature effects

We recently published a study concerning femtosecond pump–probe absorption edge spectroscopy of cubic GaN (fundamental bandgap: 3.23 eV), resulting in the transient dielectric function. In the present study, we continue our investigations of those pump–probe measurements by determining the time-dependent transition energy at the Fermi-vector between the conduction and valence bands. The generation of electron–hole pairs by the 266 nm pump-beam (4.66 eV) shifts the absorption edge by ≈500 meV within 1 ps due to many-body effects like band-filling and bandgap renormalization. Modeling this ultra-fast change is achieved by converting the transition energies into free-carrier concentrations, assuming the electron contributions to be dominant. We consider the relaxation, recombination, and diffusion of those free-carriers as well as either an additional gain-recombination or temperature effects. This allows for describing the transition energies on short time scales. Both models yield similar values for the characteristic relaxation time (≈0.21 ps), recombination time (≈25 ps), and diffusion coefficient (≈1 cm2/s).

Ultrafast electron transfer at the interface of gold nanoparticles and methylene blue molecular adsorbates.

Due to their unique property of possessing localized surface plasmon resonance (LSPR), metal nanoparticles (MNPs) have drastically impacted many applications. For instance, local field enhancement through LSPRs and plasmonic hot electron transfer are known to enhance the efficiency of MNP-based photoreactions. Here, we report on the ultrafast electron transfer from gold nanoparticles (Au-NPs) to methylene blue (MB) molecular adsorbate using femtosecond pump-probe and steady-state absorption and emission spectroscopy techniques. Although the energy band alignment of the interface allows both dipole-dipole Förster resonance energy transfer (FRET) and charge transfer, because the MB emission intensity at the Au-NPs/MB nanocomposite decreased by a factor of ∼3.6, the FRET process was ruled out. Selective excitation of LSPRs at the Au-NPs/MB nanocomposite sample in pump-probe experiments led to the formation of the MB ground-state depletion and a positive induced absorption at wavelengths shorter than ∼500 nm, which was attributed to the shoulder of the MB- anion absorption. Furthermore, despite the fact that the concentration of Au-NPs in the nanocomposite sample is the same as that in the Au-NPs solution, the initial intensity of the LSPR depletion signal was about six times weaker than that in the Au-NPs sample. These observations suggest that electron transfer from excited Au-NPs to MB adsorbates took place on a time-scale that is shorter than the ∼50 fs experimental temporal resolution.

Two-color, intracavity pump-probe, cavity ringdown spectroscopy.

We report a proof-of-principle demonstration of intracavity pump-probe, cavity ringdown (CRD) detection in a three-mirror, traveling-wave cavity. With cavity-enhanced pump power and probe absorption path length, the technique is a generally applicable, high-sensitivity, high-selectivity detection method. In our experiments, the pump radiation is switched off during every other probe ringdown, which allows uncorrelated measurements of analyte and background cavity decay rates. The net, two-color signal from the difference between the pump-on and pump-off decay rates is immune to empty-CRD drifts and spectral overlaps from non-target molecular transitions. The immunity to the ringdown drifts allows longer signal-averaging and, thus, higher detection sensitivity. The ability to compensate for the background absorption enhances the detection selectivity in spectrally congested regions. Our technique is well-suited for trace-detection in the mid-IR region, where pump-probe schemes based on strong rovibrational transitions can be applied. In this work, two-color CRD detection is implemented on a ladder-type, three-level system based on the N2O, ν3 = 1 ← 0, P(19) (pump) and ν3 = 2 ← 1, R(18) (probe), rovibrational transitions. By frequency-locking two-quantum cascade lasers to the p-polarization (pump, Finesse = 5280) and s-polarization (probe, Finesse = 67700) cavity modes, we achieve high intracavity pump power (36W) and high probe ringdown rates (>2 kHz). The observed two-color spectra are simulated by a density-matrix, three-level system model that is solved under the constraints of the cavity resonance conditions. In addition to its background compensation capability, experimental flexibility in the selection of pump-probe schemes and signal insensitivity to intracavity laser power are further features that enhance the utility of our technique for mid-IR trace-detection.

Investigation of Ultrafast Carrier Dynamics in InGaN/GaN‐Based Nanostructures Using Femtosecond Pump–Probe Absorption Spectroscopy

GaN‐based optoelectronic devices including light‐emitting diodes and lasers realized with quantum‐confined nanostructures, revolutionized the solid‐state lighting. Excited‐state ultrafast nonlinear dynamics of these nanostructures are crucial in determining the device performance in terms of luminescence efficacy, emission spectra, wall‐plug efficiency, turn‐on delay, lasing threshold current, and modulation bandwidth. Therefore, understanding the carrier and photon dynamics of these nanostructures is of utmost importance. A comparative investigation of ultrafast nonlinear carrier‐photon dynamics of 2D, 1D, and 0D InGaN/GaN nanostructures is carried out. Femtosecond pump–probe absorption spectroscopy is used uniformly herein to obtain the wavelength‐dependent ultrafast kinetics of these nanostructures, with a temporal resolution of ≈50 fs. Carrier thermalization and relaxation processes of all the samples are studied both experimentally and theoretically. Distinguished phenomena, such as fast carrier trapping and subsequent detrapping of the carriers from sub‐bandgap defect states are observed. Real‐time monitoring of quantum confined Stark effect, enhanced radiative recombination, and diffusion‐limited carrier kinetics are manifested. Carrier capture and recombination time constants are calculated theoretically, considering quantum properties. Theoretical values are in good agreement with the experimental observations. In addition, transient absorption spectroscopy is used to investigate and decouple the surface and the bulk properties of GaN surfaces.

Photoluminescence nonlinearity and picosecond transient absorption in an LYSO:Ce scintillator excited by a 266 nm ultraviolet laser.

Lutetium–yttrium oxyorthosilicate doped with cerium (LYSO:Ce) is a widely used scintillator, and the study of its nonlinear behavior under high excitation density is very significant owing to its direct influence on radiation measurements. Using a 266 nm ultraviolet laser to excite an LYSO:Ce crystal, the relationship between the photoluminescence (PL) light yield and excitation density was studied by Z scan experiments. The excitation threshold of the LYSO:Ce was obtained, which is about 2.3 J cm−3. Picosecond transient absorption of LYSO:Ce at 800 nm was obtained and used to analyze the dynamic process of carriers. The physical mechanism behind the nonlinearity was discussed and analyzed using the Förster dipole–dipole interaction model, and the interaction characteristic radius was obtained by fitting. This work can help us understand the nonlinearity phenomenon in scintillators and provide references for related radiation detection applications.

Pump-Probe Circular Dichroism Spectroscopy of Cyanobacteriochrome TePixJ Yields: Insights into Its Photoconversion.

The bilin-containing photoreceptor TePixJ, a member of the cyanobacteriochrome (CBCR) family of phytochromes, switches between blue-light-absorbing and green-light-absorbing states in order to drive phototaxis in Thermosynechococcus elongatus. Its photoswitching process involves the formation of a thioether linkage between the C10 carbon of phycoviolobilin and the sidechain of Cys494 during the change in state from green-absorbing to blue-absorbing forms. Complex changes in the binding pocket propagate the signal to other domains for downstream signaling. Here, we report time-resolved circular dichroism experiments in addition to pump-probe absorption measurements for interpretation of the biophysical mechanism of the green-to-blue photoconversion process of this receptor.

A Simple Time-Resolved Optical Measurement of Diffusion Transport Dynamics of Photoexcited Carriers and Its Demonstration in Intrinsic GaAs Films**Supported by the National Natural Science Foundation of China (Grant Nos. 11774438 and 61475195), the National Basic Research Program of China (Grant No. 2013CB922403), and the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2019A1515011572).

We develop a tightly focused pump-probe absorption technique to study diffusion dynamics of photoexcited carriers. It has many advantages including the simple setup and operations, higher detection sensitivity, an analytic descriptive model and fast data samplings. Diffusion dynamics are measured twice, separately using two different-sized probe spots, instead of many time-delayed diffusion profiles of a carrier pocket measured using spatially probe-spot scanning. An analytic model is derived to describe diffusion dynamics. Diffusion dynamics in GaAs are measured to demonstrate the feasibility of this technique. The diffusion coefficient is obtained and agrees well with the reported experimental and theoretical results.

Demonstration of femtosecond broadband X-rays from laser wakefield acceleration as a source for pump-probe X-ray absorption studies

We present X-ray absorption measurements near the K-edge of laser heated aluminum in a pump-probe configuration using X-rays generated in a laser wakefield accelerator. A 30 fs duration laser pulse from the Herculeslaser system was split into two beamlines, with one used to heat a 4 µm thickness Al foil and the second to drive a laser wakefield accelerator. The laser-heated plasma was probed at various pump-probe delays using the femtosecond duration X-rays generated by betatron oscillations of the electrons in the wakefield. We observe an apparent blue-shift of the K-edge occurring on a sub-picosecond timescale in the transmission spectra.

Measurement of asymmetric electron cloud in cluster nano-plasma

A nanocluster ionized by a femtosecond pulse of wavelength larger than the cluster size is known to set up an electron cloud that oscillates with respect to the ion cloud. Most theoretical frameworks on understanding cluster ionization are built on this dipole oscillation model. Here, we probe this dipole by pump-probe absorption spectroscopy. A linearly polarized pump pulse sets up the dipole and the polarization of the probe pulse that comes after a prescribed time is varied, and the differential changes in the absorption are measured. The polarization dependence of the probe pulse absorption gives a direct measure of the asymmetry and ellipticity in the dielectric permittivity. Our results show that orthogonally polarized pump-probe pulses absorb about 18% less compared to the parallel case at the peak of the linear plasma resonance. A dipole excitation model compared to the experimental observations is used to decipher the asymmetry in the transient electron cloud.

Impact of optically induced carriers on the spin relaxation of localized electron spins in isotopically enriched silicon

We investigate the influence of Auger electrons on the efficiency of optical spin pumping of donor bound electrons in isotopically enriched silicon by means of time-resolved pump-probe absorption spectroscopy. The experimentally observed drastic shortening of the spin relaxation time with increasing optical excitation results from inelastic scattering between free and localized electrons and an interconnected Orbach-type spin relaxation process. The maximal steady-state degree of polarization of the localized electron spins and the probe-beam-induced depolarization dynamics reveal that the fast, nonthermal energy dissipation of the conduction band electron distribution is detrimental for efficient optical spin initialization. In fact, each absorbed photon creates an Auger electron with enough energy to depolarize the spin of $\ensuremath{\approx}30$ donor bound electrons but the fast, nonthermal energy dissipation by phonons reduces this spin relaxation efficiency to approximately unity.

Reverse Photodynamics of the Noncanonical Red/Green NpR3784 Cyanobacteriochrome from Nostoc punctiforme.

In the companion paper (10.1021/acs.biochem.8b01274), we examined the forward Pr photodynamics of NpR3784 (UniProtKB B2J457 ), a representative member of a noncanonical red/green (R/G) cyanobacteriochrome (CBCR) subfamily. Here the reverse Pg → Pr photodynamics of NpR3784 was studied by broadband transient absorption pump-probe spectroscopy. Primary (100 fs to 10 ns) and secondary (10 ns to 1 ms) photodynamics were characterized over nine decades of time, which also were complemented with temperature-jump cryokinetics measurements. In contrast with canonical R/G CBCRs, the NpR3784 reverse photoconversion yielded two spectrally distinct primary photoproducts, Lumi-Go and Lumi-Gr, which decay on different time scales. The two primary photoproducts of NpR3784 equilibrate on the 40 ns time scale and subsequently propagate as a single intermediate population into Pr. Such heterogeneity could arise from differences in the direction of D-ring rotation, in chromophore protonation or hydrogen bonding, or in the mobility of protein residues or of solvent water nearby the chromophore or some combination therein. We conclude that the atypical photodynamics of NpR3784 reflects chromophore-protein interactions that differ from those present in the canonical R/G CBCR family.

Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin.

In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules), and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation is key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum and produce drastic changes in the transient absorption spectrum.

Direct Observation of Charge Injection From CH3NH3PbI3−xClx to Organic Semiconductors Monitored With sub‐ps Transient Absorption Spectroscopy

Thin films of the hybrid perovskite CH3NH3PbI3−xClx are prepared in contact to poly(3,4‐ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) or phenyl‐C61‐butyric acid methyl ester (PCBM) as model interfaces relevant in inverted perovskite solar cells (IPSCs) which provide some significant advantages over the established mesoporous cell concepts. Pump–probe absorption spectroscopy with femtosecond resolution is used to analyze injection of photo‐generated charge carriers from the perovskite to the PEDOT:PSS and PCBM layers by directly monitoring characteristic spectroscopic signatures of charge carriers in these contact phases. Such signal is identified for holes injected into the PEDOT:PSS layer, which appeared within the resolution of the measurement setup (<200 fs) proving ultrafast hole injection. Changes in the transient signals characteristic for the perovskite layer as reflected in the time constants obtained by global analysis confirmed this observation. Ultrafast injection of electrons into PCBM is also found based on the decay of signals characteristic for the excited state of the perovskite as well as by newly arising signals assigned to injected electrons into PCBM. Efficient injection of charge carriers following excitation of the perovskite film to both organic electron and hole conductors is thereby directly shown.

Soft X-ray backlighter source driven by a short-pulse laser for pump-probe characterization of warm dense matter.

Here we propose a pump-probe X-ray absorption spectroscopy temperature measurement technique appropriate for matter having temperature in the range of 10 to a few 100 eV and density up to solid density. Atomic modeling simulations indicate that for various low- to mid-Z materials in this range the energy and optical depth of bound-bound and bound-free absorption features are sensitive to temperature. We discuss sample thickness and tamp layer considerations. A series of experimental investigations was carried out using a range of laser parameters with pulse duration ≤5 ps and various pure and alloyed materials to identify backlighter sources suitable for the technique.

Nonlinear Optical Microscopy for Melanoma: Challenges, Tools and Opportunities.

The natural pigments known as melanins are thought to play a role in the etiology and progression of melanoma, but many of their roles are currently not well understood. While quantification of melanins have, up until now, have been performed in bulk tissue ex vivo, new imaging technologies have unlocked the means to visualize and quantify melanins at the sub-cellular scale. The nonlinear imaging methods known as pump-probe, coherent Raman, and sum-frequency absorption microscopies provide subcellular resolution imaging of melanins, enabling label-free, longitudinal quantification of both eumelanin and pheomelanin in situ and in vivo. These nonlinear imaging toolkits have been well proven in both animal models and human samples, moving them tantalizingly close to clinical application. Future efforts integrating these tools into practical, mobile imaging systems will provide immense benefit both to clinical research and practice.

Pump–probe and Four-wave Mixing Spectra Arising from Recoil-induced Resonance in an Operating Cesium Magneto-Optical Trap

We present experimental observation of recoil-induced resonance (RIR) in an operating cesium magneto-optical trap (MOT) by both pump–probe absorption and four-wave mixing spectra simultaneously. We investigate the dependence of amplitudes of these two spectra on pump beam intensity and frequency. The measurement results agree well with the recoil-induced theory with modifications of Raman transition effect and atomic number. The systematical study on RIR spectra is meaningful for the diagnostic measurement of cold atoms in an operating MOT.

Ultrafast Relaxation Dynamics in Zinc Tetraphenylporphyrin Surface-Mounted Metal Organic Framework

Ordered porphyrin-based metal organic frameworks (MOFs) may serve as a model for mimicking the natural photosynthesis with highly ordered chlorophylls, i.e., porphyrin-like chromophores. Study of light harvesting and energy transfer as the primary event of photosynthesis is of great importance leading to improvement of photovoltaics overall performance. Detailed characterization of ultrafast dynamics of zinc tetraphenylporphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various steady-state and time-resolved laser spectroscopic techniques, i.e., time-correlated single photon counting, fluorescence up-conversion and transient absorption pump–probe with 20 fs resolution. Obtained results in these nanoporous materials were compared with corresponding results for ZnTPP in ethanol measured under the same conditions. Dramatic quenching of both upper excited singlet state S2 and first excited state S1 was observed. Subpicosecond and picosecond lifetimes were detected in transi...