Transient Absorption Dynamics Research Articles

Charged biexciton polaritons sustaining strong nonlinearity in 2D semiconductor-based nanocavities

Controlling the interaction between light and matter at micro- and nano-scale can provide new opportunities for modern optics and optoelectronics. An archetypical example is polariton, a half-light-half-matter quasi particle inheriting simultaneously the robust coherence of light and the strong interaction of matter, which plays an important role in many exotic phenomena. Here, we open up a new kind of cooperative coupling between plasmon and different excitonic complexes in WS2-silver nanocavities, namely plasmon-exciton-trion-charged biexciton four coupling states. Thanks to the large Bohr radius of up to 5 nm, the charged biexciton polariton exhibits strong saturation nonlinearity, ~30 times higher than the neutral exciton polariton. Transient absorption dynamics further reveal the ultrafast many-body interaction nature, with a timescale of <100 fs. The demonstration of biexciton polariton here combines high nonlinearity, simple processing and strong scalability, permitting access for future energy-efficient optical switching and information processing.

Charge Transport Dynamics of Quasi-Type II Perovskite Janus Nanocrystals in High-Performance Photoconductors

CsPbBr3-Pb4S3Br2 Janus nanocrystals (NCs) are the only nanomaterial where the epitaxial structure of perovskite and chalcogenide materials has been realized at the nanoscale, but their exciton dynamics mechanism has not yet been thoroughly investigated or applied in photodetection applications. This work reports an attractive device performance of perovskite photoconductors based on epitaxial CsPbBr3-Pb4S3Br2 Janus NCs, as well as the carrier relaxation and transfer mechanism of the heterojunction. By a combination of transient optical absorption and quantum dynamics simulation, it is demonstrated that the photogenerated holes on CsPbBr3 can be successfully extracted by Pb4S3Br2, while the hole transfer proceeds about three times faster than energy loss and remains "hot" for about 300 fs. This feature has favorable effects on long-range charge separation and transport; therefore, the Janus NCs photoconductors exhibit an exceptional responsivity of 34.0 A W-1 and specific detectivity of 1.26 × 1014 Jones.

Application of graphdiyne oxide in photoelectrochemical-type photodetectors and ultrafast fiber lasers

Graphdiyne oxide (GDYO), the oxidized form of graphdiyne (GDY), has been widely explored for applications in various technological fields owing to its fascinating properties. However, the research of GDYO for applications in photodetectors (PDs) and mode-locker has been lagging. Herein, we present experimental demonstrations of promising applications of GDYO for narrow-band photoelectrochemical (PEC)-type PDs with tunable performance. Moreover, the measured excellent nonlinear optical absorption of GDYO makes it a suitable material for mode-locking in ultrafast fiber lasers. Ultrastable pulses as short as 1.5 ps could be obtained from an Erbium-doped fiber laser based on GDYO at 1559 nm. These findings have been supported and interpreted by density functional theory (DFT) calculations as well as transient absorption dynamics studies. The presented results provide pathways for the future utilization of GDYO in high-performance optoelectronic devices.

Exciton-Phonon Coupling of Chiral One-Dimensional Lead-Free Hybrid Metal Halides at Room Temperature.

The interaction between organic cations and inorganic metal halide octahedral units strongly affects the properties of organic-inorganic hybrid metal halides. The "soft" property of the lattice provides the possibility of its strong exciton-phonon interaction. Here we report one-dimensional (1D) lead-free chiral organic-inorganic hybrid metal halide single crystals of (R/S)-methylbenzylamine bismuth iodide (R/S-MBA)2Bi2I8, which exhibits a high level of octahedral bond distortion. The introduction of chiral amines leads to a strong chiroptical response in the range of 200-600 nm. The strong exciton-phonon coupling can be observed through the coherent oscillation spectrum of transient absorption dynamics at room temperature. The coherent phonon oscillation frequencies are ∼97 and ∼130 cm-1, corresponding to the symmetrical stretching or bending of the Bi-I octahedron. Our work provides new insights for the study of exciton-phonon coupling in 1D chiral hybrid metal halides.

The Synthesis and Photophysical Properties of Weakly Coupled Diketopyrrolopyrroles.

Three centrosymmetric diketopyrrolopyrroles possessing either two 2-(2′-methoxyphenyl)benzothiazole or two 2-(2′-methoxyphenyl)benzoxazolo-thiophene scaffolds were synthesized in a straightforward manner, and their photophysical properties were investigated. Their emission was significantly bathochromically shifted as compared with that of simple DPPs reaching 650 nm. Judging from theoretical calculations performed with time-dependent density functional theory, in all three cases the excited state was localized on the DPP core and there was no significant CT character. Consequently, emission was almost independent of solvents’ polarity. DPPs possessing 2,5-thiophene units vicinal to DPP core play a role in electronic transitions, resulting in bathochromically shifted absorption and emission. Interestingly, as judged from transient absorption dynamics, intersystem crossing was responsible for the deactivation of the excited states of DPPs possessing para linkers but not in the case of dye bearing meta linker.

Exploring Unusual Electrochemistry and Nlo Properties of Highly Electron-Deficient β-Functionalized Porphyrins

A series of β-tetracyanobutadiene(TCBD) appended porphyrins, M-TCBDP (M = 2H (2), Cu(II) (3), Ni(II) (4), Co(II) (5), Zn(II) (6)) were synthesized and characterized by various spectroscopic and electrochemical techniques. These porphyrins exhibit mutiredox process. Particularly, the unusual reduction potential in the range of -0.06 to -0.10 V is the consequence of TCBD group present at the β-position. The existence of enhanced polarization and intramolecular charge transfer in the absorption spectra are held responsible for nonlinear optical responses exhibited by the M-TCBD series as compared to the precursor (H2-PE2 (1)). Femtosecond Z-Scan experiments were performed to elucidate the third-order nonlinear optical properties and the temporal response of these porphyrin molecules are investigated using pump-probe spectroscopy to study the excited state transient absorption dynamics. Z-scan measurement reveals that Co-TCBD (5) exhibits better nonlinear optical response as compared to free base porphyrins. Two-photon absorption coefficient (β) and the imaginary part of third-order NLO susceptibility (χ(3) ) were procured from open aperture experiment whereas close aperture experiment delivers magnitude and sign of nonlinear optical refractive index (η2) and the real part of χ(3) . The excellent optical limiting threshold of synthesized porphyrins makes them adorable material for laser protection.

Slow Hot-Carrier Cooling Enabled by Uniformly Arranging Different-n-Value Nanoplates in Quasi-2D Perovskites through Long-Range Orbit-Orbit Interaction toward Enhancing Photovoltaic Actions.

This paper reports a slow hot-carriers cooling process enabled by uniformly arranging different-n-value nanoplates in quasi-2D perovskites (PEA2MAn-1PbnI3n+1). Here, the energetic carriers are slowly relaxing from 1.743 to 1.688 eV within the time window of 91.7 ps, shown in transient absorption (TA) dynamics. It verifies an unusual long-order electron-lattice coupling to slow down the relaxation of hot carriers, generating long-lived energetic carriers in quasi-2D perovskites. Furthermore, the long-range orbit-orbit interaction is established through orbital polarizations between excited carriers, identified by polarization-dependent photocurrent. This indicates that the long-range orbit-orbit interaction provides the necessary condition to enable the unusual long-order electron-lattice coupling, causing a slow hot-carrier cooling effect through the energy-momentum dispersion curvature within the crystalline lattice. Interestingly, with the slow hot-carrier cooling and long-range orbit-orbit interaction, the quasi-2D perovskite demonstrates superior carrier transport shown by high fill-factor values (>80%) with reduced photovoltaic loss in solar cells at all different carrier densities.

Long-lived charge separation following pump-wavelength-dependent ultrafast charge transfer in graphene/WS2 heterostructures.

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS2 following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS2 interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS2 to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.

Robustness and accuracy improvement of data processing with 2D neural networks for transient absorption dynamics.

To achieve the goal of efficiently analyzing transient absorption spectra without arbitrary assumption and to overcome the limitations of conventional methods in fitting ability and highly noised backgrounds, it is essential to develop new tools to achieve more accurate and robust prediction based on the intrinsic properties of a spectrum even under strong noise. In this work, Lasso regression and neural network were combined to achieve an effective fitting. Compared to the conventional global fitting method, our network could automatically determine the exponential form on each wave unit, in which the accuracy was as high as 97%. Thereafter, the lifetime with the corresponding amplitude ratio could be easily predicted by the neural network on each wave unit. This kind of prediction is difficult to achieve by global fitting due to the limitation of computational resources. Furthermore, more accurate fitting even under weak signals could be achieved for the mean square error (MSE) decreasing by more than 100 times on average compared to conventional global fitting methods. Attributed to its improved accuracy and robustness, our developed algorithm could be readily applied to analyze time-resolved transient spectra.

Direct Visualization and Determination of the Multiple Exciton Generation Rate.

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

Femtosecond Transient Absorption Spectra and Dynamics of Carrier Relaxation of Tin Perovskites in the Absence and Presence of Additives.

The effects of additives SnF2 (10%) and EDAI2 (1%) on the dynamics of carrier relaxation of formamidinium tin triiodide (FASnI3) perovskite were studied using femtosecond transient absorption spectra (TAS) with excitation at 600 and 870 nm. The TAS were analyzed according to a parallel sequential kinetic model with a global fit through singular-value decomposition. For excitation at 600 nm, two relaxation paths were found: one involved hot and cold carriers in the bulk state undergoing shallow bulk-defect-mediated charge recombination; the other involved trap carriers in the surface state undergoing deep surface-defect-mediated charge recombination. For excitation at 870 nm, only cold carriers were subjected to the bulk-state relaxation channel. Our spectral results indicate significant effects of the additives on retarding charge recombination in both bulk and surface states as well as decreasing the bandgap renormalization energy, the bandwidth of the photobleaching (PB) band, and the Stokes shift between the PB and photoluminescence bands, explaining how the device performance of FASnI3 solar cells became enhanced in the presence of SnF2 and EDAI2.

Ultrafast Flash Energy Conductance at MXene‐Surfactant Interface and Its Molecular Origins

AbstractWith the developments of energy science, biomedical therapy, and electronic technology, interfacial thermal transport has become a key issue for many applications. Furthermore, with the growing importance of thermal utilization of new 2D material MXene, capturing the interfacial thermal conduction of MXene‐soft material in solution is an urgent task. In this work, transient absorption dynamics is employed to investigate MXene (Ti3C2Tx)‐surfactant interfacial energy conductance in water solution. The sodium dodecyl sulfate molecules could not directly interact with Ti3C2Tx surface, and thus failed to inhibit the interfacial heat transfer of Ti3C2Tx. To the opposite, attributed to strong coulomb interaction, cetrimonium bromide (CTAB) molecules could form bilayers on Ti3C2Tx to isolate the energy dissipation from Ti3C2Tx to water and thus alter the heat dissipation rates of Ti3C2Tx. The interfacial energy conductance G ≈81 MW m−2 K−1 between CTAB and Ti3C2Tx is quantitatively obtained. Molecular dynamics simulation also presented the flash heating could disrupt the bilayer structures, showing that the finding contributed to the future engineering design for MXene thermal applications through tuning the attaching layer at MXene interface.

Influence of gadolinium doping on low- and high-order nonlinear optical properties and transient absorption dynamics of ZnO nanomaterials

Doping of materials by suitable additives can dramatically modify the physical characteristics of the complex systems. In this paper, we investigate the role of gadolinium concentration on the linear, nonlinear optical properties and transient absorption dynamics of ZnO and Al-doped ZnO. The characteristics of the mentioned materials with different dopant concentrations are evaluated using absorption spectroscopy, Z-scan and pump-probe technique with femtosecond laser pulses at 800 and 400 nm wavelengths. The nonlinear absorption is increased in Al-doped ZnO compared to pure ZnO at 800 nm wavelength. Gadolinium dopant demonstrated a significant reduction in nonlinear refractive index and absorption of Al-doped ZnO but could not influence pure ZnO. The transient absorption dynamics of ZnO and Al-doped ZnO illustrated the decrease of response time with dopant concentration. We also generated high-order harmonics using nanosecond heating pulses to generate laser plasmas and femtosecond driving pulses propagating through the plasmas. Laser plasma was generated from an ablated Gd-doped ZnO and the high-order harmonics were studied by varying the delay between heating and driving pulses.

Vibrational Relaxation Dynamics of the Core and Outer Part of Proton-Hydration Clusters.

We study the ultrafast relaxation dynamics of hydrated proton clusters in acetonitrile using femtosecond mid-infrared pump-probe spectroscopy. We observe a strong dependence of transient absorption dynamics on the frequency of excitation. When we excite the OH vibrations with frequencies ≤3100 cm–1, we observe an ultrafast energy relaxation that leads to the heating of the local environment of the proton. This response is assigned to the OH vibrations of the water molecules in the core of the hydrated proton cluster. When we excite with frequencies ≥3200 cm–1, we observe a relatively slow vibrational relaxation with a T1 time constant ranging from 0.22 ± 0.04 ps at νex = 3200 cm–1 to 0.37 ± 0.02 ps at νex = 3520 cm–1. We assign this response to water molecules in the outer part of the hydrated proton cluster.

Compressive imaging of transient absorption dynamics on the femtosecond timescale.

Femtosecond spectroscopy is an important tool used for tracking rapid photoinduced processes in a variety of materials. To spatially map the processes in a sample would substantially expand the method's capabilities. This is, however, difficult to achieve, due to the necessity of using low-noise detection and maintaining feasible data acquisition time. Here, we demonstrate realization of an imaging pump-probe setup, featuring sub-100 fs temporal resolution, by using a straightforward modification of a standard pump-probe technique, which uses a randomly structured probe beam. The structured beam, made by a diffuser, enabled us to computationally reconstruct the maps of transient absorption dynamics based on the concept of compressed sensing. We demonstrate the setup's functionality in two proof-of-principle experiments, where we achieve spatial resolution of 20 μm. The presented concept provides a feasible route to imaging, by using the pump-probe technique and ultrafast spectroscopy in general.

Ultrafast Excited-State Dynamics of Partially and Fully Functionalized Silsesquioxanes

A series of dimethylaminostilbene-substituted silsesquioxanes (SQs)—Me2NStilbeneSi(OSiMe3)3 {Corner}, [Me2NStilbeneSi(O)(OSiMe3)]4 {Half}, and [Me2NStilbeneSiO1.5]8 {Cube}—which contain partially and fully functionalized cages were investigated by ultrafast fluorescence upconversion and femtosecond transient absorption spectroscopy. These nonlinear techniques were used to study the mechanism of excitation energy transport and excited-state dynamics of these functionalized SQ structures. A strong fluorescence lifetime dependence was observed in all three systems. Depolarization of the fluorescence in the Half and Cube systems indicates excitation energy delocalization between the chromophores due to strong electronic coupling. The ultrafast rise time obtained from the upconversion experiment clearly depicts the fast transition taking place in the system after photoexcitation. Transient absorption dynamics and two-photon absorption measurements reveal the presence of ultrafast charge delocalization in the Half and Cube systems. Blue shifts in the excited-state absorption (ESA) observed in all three SQ systems correlate with the solvation process and reveal a solvent-stabilized state. The amount of ESA blue shift indicates an increase in the charge-transfer character of the excited state in the case of the Half and Cube systems. This explains their enhanced two-photon cross sections in comparison to that of the Corner SQ system.

Recombination Dynamics Study on Nanostructured Perovskite Light-Emitting Devices.

The field of organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs) has developed rapidly in recent years. Although the performance of PeLEDs continues to improve through film quality control and device optimization, little research has been dedicated to understanding the recombination dynamics in perovskite thin films. Likewise, little has been done to investigate the effects of recombination dynamics on the overall light-emitting behavior of PeLEDs. Therefore, this study investigates the recombination dynamics of CH3 NH3 PbI3 thin films with differing crystal sizes by measurement of fluence-dependent transient absorption dynamics and time-resolved photoluminescence. The aim is to find out the link between recombination dynamics and device behavior in PeLEDs. It is found that bimolecular and Auger recombination become more efficient as the crystal size decreases and monomolecular recombination rate is affected by the trap density of perovskite. By defining the radiative efficiency Φ(n), which relates to the monomolecular, bimolecular, and Auger recombination, the fundamental recombination properties of CH3 NH3 PbI3 films are discerned in quantitative terms. These findings help us to understand the light emission behavior of PeLEDs. This study takes an important step toward establishing the relationship between film structure, recombination dynamics, and device behavior for PeLEDs, thereby providing useful insights toward the design of better perovskite devices.

Solvent induced fluorescence enhancement of graphene oxide studied by ultrafast spectroscopy

Femtosecond transient absorption (TA) spectroscopy combined with picosecond time resolved fluorescence (TRF) were used to reveal the fluorescence kinetics of graphene oxide (GO) in water, ethanol and water-ethanol mixtures. Size-independent fluorescence of GO were observed in water, and pH-dependent fluorescence spectra could be fitted well by a triple emission relaxation with peaks around 440 nm, 500 nm, and 590 nm respectively. The results indicate that polycyclic aromatic hydrocarbons (PAHs) linked by oxygen-containing functional groups dominate GO’s fluorescence emission. GO’s fluorescence quantum yield was measured to be 2.8% in ethanol but 1.2% in water. The three decay components fluorescence decay, as well as the transient absorption dynamics with an offset, confirmed this solvent induced fluorescence enhancement. GO’s Raman spectral signals showed that GO in ethanol has a smaller average size of PAHs than that of GO in water. Therefore, besides other enhancement effects reported in literatures, we proposed that solvents could also change the size of PAHs, resulting in a photoluminescence enhancement. Our experimental data demonstrates that GO’s quantum yield could be up to 2.8% in water and 8.4% in ethanol and this observation may help ones to improve GO’s photoluminescence efficiency as well as its applications in solution.

Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer.

A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S1 → 1TT) in intramolecular singlet fission (SF). (Triisopropylsilyl)acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BT1 shows a bifurcation into both arm-localized (S1-loc) and dimer-delocalized (S1-dim) singlet exciton states. The S1-loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S1-dim. Emissive signatures of the S1-dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S1-dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor 1TT. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S1 → 1TT photoreaction.

Attosecond transient-absorption dynamics of xenon core-excited states in a strong driving field

Attosecond transient absorption spectroscopy of xenon atoms is performed using near-infrared fields well into the tunneling region, providing insight into the ultrafast control of material optical properties in the strongly driven regime.