Photoexcitation

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

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd 3 As 2

Abstract: The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals is vital towards their application in high-performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron-phonon coupling (1--4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2--9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonon relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the mixing of discrete optical phonon energies that assist the intraband transition enabled by photoexcitation of hot carriers.

Spectrally resolved intraband transitions on two-step photon absorption in InGaAs/GaAs quantum dot solar cell

Abstract: Two-step photon absorption processes in a self-organized In0.4Ga0.6As/GaAs quantum dot (QD) solar cell have been investigated by monitoring the mid-infrared (IR) photoinduced modulation of the external quantum efficiency (ΔEQE) at low temperature. The first step interband and the second step intraband transitions were both spectrally resolved by scanning photon energies of visible to near-IR CW light and mid-IR pulse lasers, respectively. A peak centered at 0.20 eV corresponding to the transition to virtual bound states and a band above 0.42 eV probably due to photoexcitation to GaAs continuum states were observed in ΔEQE spectra, when the interband transition was above 1.4 eV, directly exciting wetting layers or GaAs spacer layers. On the other hand, resonant excitation of the ground state of QDs at 1.35 eV resulted in a reduction of EQE. The sign of ΔEQE below 1.40 eV changed from negative to positive by increasing the excitation intensity of the interband transition. We ascribe this to the filling of higher energy trap states.

Broadband ultraviolet-visible transient absorption spectroscopy in the nanosecond to microsecond time domain with sub-nanosecond time resolution

Abstract: A combination of sub-nanosecond photoexcitation and femtosecond supercontinuum probing is used to extend femtosecond transient absorption spectroscopy into the nanosecond to microsecond time domain. Employing a passively Q-switched frequency tripled Nd:YAG laser and determining the jitter of the time delay between excitation and probe pulses with a high resolution time delay counter on a single-shot basis leads to a time resolution of 350 ps in picosecond excitation mode. The time overlap of almost an order of magnitude between fs and sub-ns excitation mode permits to extend ultrafast transient absorption (TA) experiments seamlessly into time ranges traditionally covered by laser flash photolysis. The broadband detection scheme eases the identification of intermediate reaction products which may remain undetected in single-wavelength detection flash photolysis arrangements. Single-shot referencing of the supercontinuum probe with two identical spectrometer/CCD arrangements yields an excellent signal-to-noise ratio for the so far investigated chromophores in short to moderate accumulation times.

Charge carrier dynamics and visible light photocatalysis in vanadium-doped TiO2 nanoparticles

Abstract: Vanadium-doped TiO2 nanoparticles (V-TiO2 NPs) with a V/Ti ratio of 3.0 at. % were prepared by gas-phase condensation and subsequent oxidation at elevated temperature. Both photocatalytic activity for -NO2 reduction and photoelectrochemical water splitting were induced by V-doping in the visible spectral range λ> 450 nm, where undoped TiO2 NPs are completely inactive. The photocatalytic properties were correlated with the ultrafast dynamics of the photoexcited charge carriers studied by femtosecond transient absorption (TA) spectroscopy with three different excitation wavelengths, i.e. λe = 330, 400, and 530 nm. Only in V-doped NPs, the photoexcitation of electrons into the conduction band by sub-bandgap irradiation (λe = 530 nm) was detected by TA spectroscopy. This observation was associated with electronic transitions from an intra-gap level localized on V4+ cations. The photoexcited electrons subsequently relaxed, with characteristic times of 200–500 ps depending on λe, into Ti-related surface traps that possessed suitable energy to promote -NO2 reduction. The photoexcited holes migrated to long-lived surface traps with sufficient overpotential for the oxidization of both 2-propanol and water. On the basis of TA spectroscopy and photocurrent measurements, the position of the dopant-induced intra-gap level was estimated as 2.2 eV below the conduction band minimum.

Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn1-xFex(bbtr)3](ClO4)2}∞: The Role of Cooperative Effects

Abstract: The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn1-xFex(bbtr)3](ClO4)2}∞ (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase tra...