Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material

Abstract: The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott-Hubbard material V$_2$O$_3$ presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a$_{1g}$ orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A$_{1g}$ coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems.

Observation of optical Stark effect in InGaAs/InP multiple quantum wells

Abstract: We report an experimental observation of a blue shift in the n=1 heavy‐hole exciton line of In0.53Ga0.47As/InP multiple quantum wells resulting from a picosecond photoexcitation in the transparent spectral region. The temporal response of this shift follows the excitation and it is attributed to the optical Stark effect. The shift was measured to be 0.19 meV for an incident light with a photon energy 20 meV below the exciton peak and with a 10‐MW/cm2 intensity.

Magnetic field effect on excited-state spectroscopies of π -conjugated polymer films

Abstract: The magnetic field effect in organic light-emitting diodes, such as magnetoconductance and magnetoelectroluminescence, has been intensively explored in the last few years. Here, we demonstrate the magnetic field effect of two excited-state spectroscopies in films of a prototype \ensuremath{\pi}-conjugated polymer, i.e., a soluble derivative of poly(phenylene vinylene), [2-methoxy-5-(2\ensuremath{'}-ethylhexyloxy)-poly(p-phenylene vinylene)] (MEH-PPV); these are magnetophoto-induced absorption (MPA) and magnetophotoluminescence (MPL). We study these novel magnetic field effects in pristine MEH-PPV films, MEH-PPV films subjected to prolonged illumination, and blend of MEH-PPV with a fullerene derivative. Being spectroscopic, MPA and MPL are determined by the photoexcitation spin density and thus may unravel the occurrence of myriad spin-mixing processes in organic semiconductors that include hyperfine interaction in polaron pairs, spin-sublevel mixing in triplet excitons, triplet--triplet annihilation, and triplet--singlet collision. The recently observed ultrasmall magnetic field effect at $B$ \ensuremath{\sim}0.5 mT in organic diodes is also observed in the MPA response of MEH-PPV films that support polaron photoexcitations, thereby identifying the underlying mechanism as being due to spin mixing of polaron pairs by the hyperfine interaction.

Intramolecular Energy Transfer in Fullerene Pyrazine Dyads

Abstract: Excited-state properties of three different pyrazine derivatives 4−6 were probed by emission and transient absorption spectroscopy. They display emission maxima at 464 (4), 417 (5), and 515 nm (6) that are red-shifted with respect to their strong UV ground-state absorption and formed with overall quantum yields (Φ) of 0.156, 0.22, and 0.13, respectively. Once photoexcited, these triplet excited pyrazines undergo rapid intermolecular energy transfer to a monofunctionalized fullerene derivative (7) with bimolecular rate constants ranging from 3.64 × 109 M-1 s-1 (6) to 1.1 × 1010 M-1 s-1 (4). The product of these bimolecular energy-transfer reactions is in all cases the fullerene triplet excited state. Functionalization of pristine C60 with the investigated pyrazine derivatives promotes the UV−vis absorption characteristics and, in turn, improves the light-harvesting efficiency of the resulting dyads 1−3 relative to pristine C60. Photoexcitation of the pyrazine moieties in dyads 1−3 leads to the formation of...

Exciton and charge carrier dynamics in few-layer WS2

Abstract: Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.