Ultrafast Exciton Dynamics Research Articles

Femtosecond excited-state dynamics in N,N-dimethylaminobenzylidene-1,3-indandione (DMABI) films

Ultrafast exciton dynamics in polycrystalline films of dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules was studied by femtosecond luminescence upconversion and transient absorption techniques. An enhanced nonradiative relaxation of the excitons, taking place on the subpicosecond time scale, was observed. It was attributed to the initial localisation and thermalisation of the self-trapped excitons. One-dimensional exciton–exciton annihilation was observed under high excitation conditions.

Ultrafast exciton dynamics in ternary II-VI semiconductor quantum wells

Ultrafast exciton dynamics is investigated in a high-quality ${\mathrm{Zn}}_{x}{\mathrm{Cd}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{e}\ensuremath{-}\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}$ multiple quantum well at low photoexcited carrier density by means of a two-color femtosecond absorption saturation technique. Broadening of the exciton lines due to carrier photocreation is found to dominate the observed nonlinear response. Measurements performed as a function of the initial energy of the photoexcited carriers show that the dynamics of the induced broadening strongly depends on the carrier distribution, suggesting that Pauli exclusion principle effects significantly influence exciton broadening in ternary quantum-well systems. Localized excitons are shown to modify the picosecond nonlinear response, inducing a frequency shift of the absorption change peak as the system approaches quasiequilibrium.

Ultrafast exciton and excited-exciton dynamics in J -aggregates of three-level porphyrin molecules

We observed the ultrafast response of exciton (S1-exciton) and excited-exciton (S2-exciton) in one-dimensional J-aggregates of three-level porphyrin molecules by femtosecond pump–probe spectroscopy. The decay profiles of the nonlinear response can be fitted to a sum of instantaneous response and two exponential decay components with time constants of 1.3±0.1 and 40±1 ps. The former and latter were found to correspond to the lifetimes of S2- and S1-excitons, respectively. The origins of the nonlinearity were attributed to the following three contributions: (1) coherent effects between the pump and probe via one-photon virtual S1-exciton, (2) induced absorption of real S2-excitons generated by two photons, and (3) induced absorption of real S1-exciton.

Giant magneto-optical effects in diluted magnetic semiconductor nanostructures

Nanostructures of diluted magnetic semiconductors show varieties of optical phenomena caused by the interaction of electrons and holes with magnetic ions in the confined electronic states. We present giant magneto-optical effects and ultrafast exciton dynamics in two-dimensional quantum wells of the Cd 1− x Mn x Te/ZnTe system and quantum dots of Cd 1− x Mn x Se.

Ultrafast self-trapping dynamics of excitons in quasi-one-dimensional halogen-bridged platinum complex

We present the first experimental results of femtosecond time-resolved luminescence measurements of a self-trapped exciton (STE) luminescence band in quasi-one-dimensional halogen-bridged platinum complex [Pt(en) 2][Pt(en) 2Cl 2](ClO 4) 4. The time evolution of luminescence has been investigated at room temperature using up-conversion technique. In high-energy tail of the band (from 1.8 to 2.1 eV), a very fast rise and decay are seen, while a slow rise of about 450 fs is observed around the luminescence peak energy. This rise becomes faster at the low energy side of the band. These behaviors are explained by the cooling of STE's just after the rapid self-trapping of photo-generated charge-transfer excitons.

Pump–probe spectroscopy of dissipative energy transfer dynamics in photosynthetic antenna complexes: A density matrix approach

The photoinduced ultrafast dynamics of singlet excitons in light-harvesting antennae is investigated using multilevel Redfield theory. Formulating the equations of motion for the reduced exciton density operator in terms of one- and two-exciton eigenstates we focus attention on the influence of dynamic exciton-vibrational coupling and static diagonal disorder on transient absorption spectra of peripheral antennae in photosynthetic purple bacteria. The simulations are discussed in view of recent experimental results obtained for the B850 absorption band of Rhodobacter sphaeroides. Further, we suggest a new way of estimating the size of the exciton coherence domain in these systems which puts emphasis on the dynamic character of exciton localization. For the B850 pigment pool we find that at room temperature the pump–pulse initially prepares a coherent superposition of one-exciton eigenstates which can be delocalized over the whole aggregate. With increasing delay time the exciton coherence domain shrinks to cover about four pigments in the asymptotic limit.

Polarization dependence of the ultrafast photoluminescence of oriented poly(p-phenylenevinylene)

The ultrafast dynamics of excitons in stretch-oriented poly($p$-phenylenevinylene) (PPV) are studied by femtosecond time-resolved photoluminescence spectroscopy. It is shown that the total luminescence intensity at 1 ps after excitation is the same for excitation either parallel or perpendicular to the stretch direction. Thus the number of intrachain excitons initially generated is independent of the polarization of the excitation light. This indicates that direct interchain photogeneration processes are not important in PPV. A small polarization ratio is measured at very early times after excitation, and is consistent with the localized excitons having appreciable off-axis transition dipole moments. The confinement of the excitons places constraints on the maximum possible anisotropy of the luminescence. Significant differences between the luminescence rise and decay times are observed for different excitation and detection polarizations. These results are explained by a combination of nonradiative decay at quenching sites, whose strength depends upon the polarization of the excitation pulses, and exciton migration processes.

Ultrafast coherent carrier control in quantum wells

We report the coherent control of excitons in quantum wells with pairs of 150-fs infrared pulses. Oscillations in the exciton density versus inter-pump-pulse separation, which correspond to the resonant transition energy, yield new insight into coherent exciton dynamics when their phase is analyzed. We demonstrate that these oscillations are caused when the second laser pulse destroys or enhances excitons that were created by the first pulse. The coherent carrier destruction generates ultrafast exciton dynamics that are not limited by the linewidth of the optical transition.

Ultrafast exciton dynamics in a polymeric heterostructure based on thienylene-phenylene sequences

We have investigated by femtosecond pump-and-probe measurements the exciton dynamics in a novel conjugated polymeric heterostructure consisting of regularly alternated thienylenic and phenylenic segments. A fast initial relaxation followed by a power law decay has been observed. The initial part of the decay is consistent with the self-trapped exciton relaxation process. The power law decay has been interpreted in terms of the modulation of the energy band gap along the polymeric chains. We have also studied the optical properties of a thienylene-based oligomer with the same well-barrier-well structure of the polymer in order to investigate the role of disorder on the polymer relaxation dynamics.

Ultrafast exciton dynamics in semiconductors: Effects of disorder and confinement

Abstract

Ultrafast exciton dynamics in highly oriented polydiacetylene films

The nonlinear optical response in uniaxially oriented films of poly[1.6-di(N-carbazolyl)-2, 4-hexadiyne], epitaxially grown on a potassium acid phthalate single crystal, has been measured by femtosecond pump and probe. A fast initial relaxation and the formation of long-lived species at high excitation intensities have been observed. A high third-order nonlinear optical susceptibility of 2×10−7 esu has been estimated at 627 nm. The effects of sample orientation on the relaxation dynamics have also been investigated.

Ultrafast adiabatic following in semiconductors.

The full semiconductor Bloch equations are solved to analyze the ultrafast exciton dynamics in semiconductors. Transient adiabatic following is obtained for large detunings and arbitrary intensities, leading to exciton Stark shift, bleaching, and recovery on the time scale of the pump pulse. The many-body Coulomb effects strongly influence the semiconductor response for resonant excitation conditions, almost doubling the effective Rabi frequency of the applied field