Femtosecond Pump-probe Absorption Spectroscopy Research Articles

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.

Ultrafast charge and lattice dynamics in one-dimensional Mott insulator of CuO-chain compoundCa2CuO3investigated by femtosecond absorption spectroscopy

Charge dynamics in one-dimensional (1D) Mott insulators was investigated by femtosecond pump-probe absorption spectroscopy on ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{3}$. An irradiation of a femtosecond laser pulse gives rise to a Drude-like response due to the nature of spin-charge separation characteristic of 1D Mott insulators. The photoinduced metallic state decays with \ensuremath{\sim}30 fs, and photocarriers are localized as polarons via charge-phonon coupling which produce a broad midgap absorption. Calculations with the extended Hubbard-Holstein model suggest that the peak structure of the midgap absorption is a magnon sideband of the polaron absorption. This demonstrates that charge-spin coupling becomes effective via charge-phonon coupling in polarons.

UV photoexcitation of a dissolved metalloid Ge9 cluster compound and its extensive ultrafast response.

Femtosecond pump-probe absorption spectroscopy in tetrahydrofuran solution has been used to investigate the dynamics of a metalloid cluster compound {Ge9[Si(SiMe3)3]3}(-). Upon UV photoexcitation, the transients in the near-infrared spectral region showed signatures reminiscent of excess electrons in THF (bound or quasi-free) whereas in the visible part excited state dynamics of the cluster complex dominates.

Relaxation Dynamics of Electronically Excited C60 − in o-Dichlorobenzene and Tetrahydrofuran Solution

Abstract The ultrafast response of singly negatively charged C60 fullerene in solution has been investigated by femtosecond pump-probe absorption spectroscopy and transient anisotropy in the visible and near-infrared region. Pump excitation within the near-infrared band demonstrates that this spectral feature can be described as a vibrational progression associated with a single electronic transition. Relaxation of the first electronically excited state occurs primarily by internal conversion with a time constant of 3 ps, slightly depending on the solvents, tetrahydrofuran or o-dichlorobenzene, and also on the excitation wavelength. An excitation of the second electronically excited state around 530 nm leads to an ultrafast internal conversion to the first excited state with a pulse-limited time constant of less than 100 fs. As a minor channel, stimulated emission in the spectral regime of 1150–1300 nm was observed from the first electronically excited state both after near-infrared and visible excitation. After internal conversion to the electronic ground state, C60 − dissipates its excess internal energy into the solvent on a longer timescale of 40–70 ps. The transient anisotropy associated with directly populating the first excited state reveals an ultrafast component decaying within 100 fs, which is attributed to ultrafast vibrational motions, conceivably arising from excited state pseudorotation.

Photoinduced transition from Mott insulator to metal in the undoped cuprates Nd2CuO4and La2CuO4

Photoinduced transitions from a Mott insulator to a metal have been investigated in the undoped cuprates Nd${}_{2}$CuO${}_{4}$ and La${}_{2}$CuO${}_{4}$ by applying femtosecond pump-probe absorption spectroscopy to epitaxially grown thin-film samples. In Nd${}_{2}$CuO${}_{4}$, a metallic state is generated with low excitation photon density less than 0.01 photons/Cu and decays within 40 fs via the rapid photocarrier recombination. Residual photocarriers are localized by the effect of charge-spin coupling, exhibiting two midgap absorptions due to a particle and a hole. La${}_{2}$CuO${}_{4}$ also shows a photoinduced Mott insulator to metal transition, while it exhibits different charge dynamics from those of Nd${}_{2}$CuO${}_{4}$, that is, larger threshold excitation photon density for the formation of the metallic state, higher energies of the midgap absorptions, and slower recombination of polaronic carriers. These behaviors can be explained by the larger charge-phonon coupling strength in La${}_{2}$CuO${}_{4}$ than in Nd${}_{2}$CuO${}_{4}$, which is attributable to the presence of apical oxygen atoms in La${}_{2}$CuO${}_{4}$.

Femtosecond Two-Pulse Cross Correlation in Single-Walled Carbon Nanotubes: Evedence of Intersubband Transitions and Role of G-Band-Assisted Transitions

By femtosecond pump-probe absorption spectroscopy we demonstrated intersubband and phonon-assisted transitions in single-walled carbon nanotubes (SWNTs). After resonant excitations of specific tube types, electrons are promoted from the ground state to the second excitonic state (EX2) in PLV-prepared SWNTs or to vibrational excited levels of the first excitonic state (EX1) in HiPco-prepared SWNTs. At time-zero delay a [1 + 1] resonant cross-correlation due to short lifetimes of higher excited states or phonon modes can be observed. These results suggest that intersubband transitions between the excitonic states in SWNTs are allowed. The interpretation of the temporal profiles at pump/probe combinations implies that the G-band mode, which is known to have a high Raman intensity, plays an important role for the interpretation of both femtosecond cross-correlation measurements and photoluminescence spectra.

Femtosecond UV Excitation in Imidazolium-Based Ionic Liquids

Femtosecond pump-probe absorption spectroscopy was employed to investigate ultrafast dynamics in various room temperature ionic liquids (RTILs) based on imidazolium cations, i.e., 1,3-dimethylimidazolium iodide ([DMIM]I), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-hexyl-3-methylimidazolium iodide ([HMIM]I), 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl), and 1-methyl-3-octylimidazolium chloride ([MOIM]Cl). Immediately after photoexcitation, an induced absorption was observed at various probe wavelengths (555-1556 nm). Afterward, the decay of the induced absorption was found to be independent of the alkyl chain length and viscosity of the ionic liquids. Two alternative mechanisms were proposed to explain the dynamics. In a first scenario excess electrons are generated through one-photon photodetachment of halides analogous to aqueous halide photodetachment. The dynamics in this case were analyzed with the help of a competing kinetic model proposed for geminate recombination in aqueous chloride photodetachment. Alternatively, imidazolium cations may be subject to photoionization. The transient NIR absorption can then be assigned to imidazolium dimer radical cations and/or excess electrons which may be formed upon association of imidazolium radicals with their parent cations. Both scenarios suggest that a thorough explanation of the ultrafast dynamics probably requires the implication of cooperative effects in the ionic liquids upon photoexcitation.

Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide

Temperature-dependent investigations of excess electrons in molten solutions of cesium-doped cesium iodide (Cs-CsI) (mole fraction of Cs approximately 0.003) were performed applying femtosecond pump-probe absorption spectroscopy. The pulse-limited induced bleach observed at probe wavelengths from 600 to 1240 nm was attributed to the excitation of equilibrated excess electrons which were initially formed by melting a Cs-CsI mixture. The interpretation of the relaxation process is based on strongly localized polarons that constitute the majority of defect states in this melt. As expected, the bipolaron contribution was insignificant. The time constants (tau1) were found to be temperature dependent confirming our earlier findings in Na-NaI melts that ionic diffusion almost exclusively controls the dynamics of excess electrons in high temperature ionic liquids. Apart from this temperature dependence, the relaxation dynamics of excess electrons do not differ irrespective of the excitation regime (blue or red part of the respective stationary spectra).

Ultrafast Dynamics of Room Temperature Ionic Liquids after Ultraviolet Femtosecond Excitation

The photochemistry and relaxation dynamics of four room-temperature ionic liquids (RTILs) after ultraviolet (UV) photolysis were investigated by femtosecond pump-probe absorption spectroscopy. A pulse duration-limited rise of the induced absorption in halide-containing RTILs at various probe wavelengths was attributed to the generation of solvated electrons. With continuous irradiation (static conditions), di- and trihalide ion formation became apparent especially below 1000 nm. The formation of trihalide ions was further confirmed by steady-state UV absorption spectroscopy. All RTILs showed a rich photochemistry after UV photolysis leading to the build-up of various long-lived intermediate products as evidenced from the observation that ionic liquids turn yellow upon continuous irradiation. On the other hand, exposing RTILs to the excitation pulse for a short time (rapid-scan method) significantly suppressed the formation of halides. The results suggest that the development of flow-cell systems for highly viscous ionic liquids is urgently needed to quantitatively investigate their ultrafast dynamics.

Dynamics of charge pair generation in ladder-type poly(para-phenylene) at different excitation photon energies

Generation of charge pairs in a ladder-type poly(para-phenylene) was investigated by means of electric field modulated femtosecond pump-probe absorption spectroscopy upon excitation at 3.1 and $4.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Exciting at $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the optical ${S}_{0}\ensuremath{\rightarrow}{S}_{1}$ transition, the number of geminately bound electron--hole pairs rises smoothly within the entire exciton life time. Upon excitation at $4.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ an instantaneous generation of geminate pairs superimposed on the more gradual increase is observed. Both processes are field-assisted, albeit differently. The former is attributed to ultrafast dissociation of a hot Franck-Condon state, while the latter is assigned to dissociation of vibrationally relaxed singlet excitons.

Surface passivation in CdSe nanocrystal–polymer films revealed by ultrafast excitation relaxation dynamics

AbstractThe photoluminescence efficiencies and excitation relaxation dynamics in CdSe nanocrystals (NC) passivated with tri‐n‐octylphosphine oxide and embedded in two different polymer matrixes, poly(styrene) (PS) and poly(butylmethacrylate) (PBMA), are compared. Femtosecond pump‐probe absorption spectroscopy is used to clarify the influence of various transparent polymer matrixes on the electronic properties and excitation relaxation dynamics of quantum confined CdSe semiconductor nanocrystals of 5.0 nm diameter. The fluorescence intensity is reported to be ∼10 times higher for the NC‐PS sample compared to the NC‐PBMA film. This striking difference in fluorescence yield is shown to be related to markedly different rates of nonradiative excitation relaxation in the two samples, and is attributed directly to a role played by the polymer host. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of silicon nanocrystals.

Femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er(3+) dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping and the nature of the dopant architecture. In Si nanocrystals without Er(3+) dopant, a trapping component was identified to be a major electron relaxation mechanism. Addition of Er(3+) ions into the core or surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways, which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium. To gain additional insights on the relative depths of the trapping sites on the investigated nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate photoinduced electron transfer across the nanocrystal surface and subsequently assist in back electron transfer. The established reduction potential (-0.45 V versus SCE) of the electron acceptor helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling experiment also reveals that the back electron transfer seems not to recover completely to the ground state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are captured by deep trapping sites induced by erbium addition in the Si lattice.

Aggregation of phthalocyanine derivatives in liquid solutions and human blood

The aggregation of the copper (II) 4,4′,4′′,4′′′-tetrasulfonated phthalocyanine anion (Cu(tsPc) −4) has been studied in aqueous solutions, DMSO and in human blood by UV–VIS absorption spectroscopy and resonance Raman spectroscopy (RRS). The vibrational mode ν 4 (1530 cm −1) has been used as a probe in RRS. It has been shown that the dimerization equilibrium constant K is shifted significantly towards monomeric forms when human blood is added to the solution. The life-time of the singlet excited state S n of (Cu(tsPc) −4) in aqueous solution has been estimated to be shorter than 500 fs using femtosecond pump-probe absorption spectroscopy.

Picosecond and femtosecond studies of the energy redistribution in matrix-isolated C 60 molecules

The intramolecular energy relaxation processes of UV-excited C 60 molecules embedded in Ar crystals have been investigated by picosecond fluorescence studies and femtosecond pump–probe absorption spectroscopy. The various internal energy relaxation steps are clearly identified. The importance of intramolecular energy relaxation processes in solid C 60 films is underlined

Ultrafast electron trapping in an aqueous NaCl-solution

The generation of solvated electrons in an aqueous solution of sodium chloride is studied by femtosecond pump-probe absorption spectroscopy in the VIS and NIR with polarization resolution. Our data give evidence that the photo detachment of electrons occurs via two photon absorption of intense 200 fs pump pulses at 310 nm to an excited charge-transfer-to-solvent (CTTS) state of the halide anion. The subsequent relaxation dynamics involves the population of the CTTS ground state with a time constant of 120±30 fs which is somewhat longer-lived (lifetime 670±80 fs), followed by the formation of a complex state (Cl:e −), from which the solvated electrons are ejected with a time constant of 7±1.5 ps and large quantum yield.

Ultrafast electron transfer, recombination and spin dynamics

The quenching of excited singlet methylene blue ( 1MB +∗) by N,N-dimethylaminomethylferrocene (FcN) in acetonitrile at room temperature has been studied using femtosecond pump-probe absorption spectroscopy. At high FcN concentration static quenching via an intermolecular electron transfer mechanism constitutes the predominating decay channel for 1MB +∗. The time constants of the large amplitude components for the forward electron transfer from FcN donor to 1MB +∗ and the subsequent recombination process recovering the ground states have been determined to be 390 fs and 1 ps, respectively. Thus, the majority of radical pairs has recombined before paramagnetic relaxation induces spin mixing giving rise to triplet-phased radical pairs. These can escape geminate recombination as the system does not offer low-lying local triplet states to be occupied in the recombination process.