Study Of Ultrafast Carrier Dynamics Research Articles

A comparative study of ultrafast carrier dynamics near A, B, and C-excitons in a monolayer MoS2 at high excitation densities

With the growing demand for monolayer MoS2 in diverse optoelectronic applications, it is more important than ever to understand carrier behavior under varied excitation conditions and at different excitonic levels. In this article, we show that band structure, excitation wavelength, and excitation density all have a significant impact on the carrier dynamics in a monolayer MoS2. At the A and B excitation levels, an initial bandgap renormalization is detected, while band bleaching is found to play a stronger role at the C-excitonic state. The exciton dissociation at the band edge near A and B-exciton causes the bandgap renormalization. On the other hand, band bleaching, which occurs at the C-exciton, is caused by the exciton formation due to the high availability of states. At this excitation energy, carrier relaxation and thermalization occur through lattice interaction by releasing a high number of hot phonons, which creates a bottleneck effect. Due to the hot phonon bottlenecking effect, carrier relaxation time is prolonged.

A comprehensive study of ultrafast carrier dynamics of LT-GaAs: Above and below bandgap regions

We report the study of ultrafast carrier dynamics of low temperature grown GaAs over wide spectral range (2.75 eV–0.88 eV) with above and below bandgap (Eg) optical excitations using ultrafast transient absorption spectroscopy. The presence of defects and their subsequent effect on the pathway and lifetime of carrier relaxation with probing at different energy levels is presented. The carrier relaxation pathways and their corresponding models are predicted. It is observed that for pumping (2.58 eV) and probing above Eg, the transient behavior is dominated by band filling while renormalization is in prominence when probed near to Eg. For below Eg pumping (0.88 eV) the optical behavior is mostly governed by defects. The calculated trap time is in the range of ~128 fs to ~807 fs while overall recombination time shows a wide variation from ~1.38 ps to ~1.25 ns depending on the position of traps/defects in the forbidden band.

Ultrafast two-dimensional terahertz spectroscopy of elementary excitations in solids

Recent experimental progress has allowed for the implementation of nonlinear two-dimensional (2D) terahertz (THz) spectroscopy in the ultrafast time domain. We discuss the principles of this technique based on multiple phase-locked electric field transients interacting in a collinear geometry with a solid and the phase-resolved detection of the THz fields after interaction with the sample. To illustrate the potential of this new method, 2D correlation spectra of coupled intersubband-longitudinal optical phonon excitations in a double quantum well system and a study of ultrafast carrier dynamics in graphene are presented.

Ultrafast carrier dynamics and radiative recombination in multiferroic BiFeO3

We report a comprehensive study of ultrafast carrier dynamics in single crystals of multiferroic BiFeO3. Using degenerate femtosecond optical pump-probe spectroscopy, we find that the photoexcited electrons relax to the conduction band minimum through electron-phonon coupling with a ∼1 ps time constant that does not significantly change across the antiferromagnetic transition. Electrons subsequently leave the conduction band and primarily decay via radiative recombination on a nanosecond timescale, as supported by photoluminescence measurements. We find that despite the coexisting ferroelectric and antiferromagnetic orders in BiFeO3, its intrinsic nature results in carrier relaxation similar to that observed in bulk semiconductors.

Study of ultrafast carrier dynamics in amorphous Ge2Sb2Te5 film by femtosecond-resolved reflectivity spectroscopy

Ultrafast dynamics and its excitation-energy-density dependence of photoexcited carriers in amorphous Ge2Sb2Te5 film were studied at room temperature by femtosecond-time-resolved pump-probe reflectivity spectroscopy. It was found that the reflectivity reduced down to a minimum in 05 ps after pump excitation, and then started to increase sharply up to a new maximum value larger than the initial reflectivity in several picoseconds. Furthermore, the decreased and increased amplitudes of the reflectivity with respect to its initial value as well as its rising rate from the minimum to the maximum both increased with the increasing excitation energy density. The sharp rising process of the reflectivity was explained quantitatively based on the model of Auger recombination of high density plasma and Auger-recombination-induced heating of cyrstal lattice, which shows that the heating effect induced by Auger recombination in high density plasma should be the main mechanism of ultra-short laser pulse-induced phase change.

A study of ultrafast carrier dynamics in laser-crystallized microcrystalline SiGe at highly excited density using time-resolved reflectivity measurement

Microcrystalline SiGe was obtained by using laser-induced crystallizing a-SiGe films deposited by plasma-enhanced chemical vapour deposition. The dynamics of dense electron–hole plasma in SiGe films, subsequent to photoexcitation by a 150 fs optical pulse, is studied by measuring time-resolved changes in the reflectivity of the films. The initial electron–hole pair densities are between 2.2 × 1019 and 2.6 × 1020 cm−3. Typical traces of the time-resolved reflectivity change are initiated with a rapid decrease, followed by a slower recovery of the reflectivity change, which can be well simulated by a double exponential curve. The fast component of relaxation time τf varies in the range of 0.75–0.79 ps and is almost independent of the initial carrier density. The slower component of relaxation time τs increases with the increase of the initial carrier density and varies in the range of 6.97–17.70 ps. The fast component τf could result from carrier cooling and the cooling rate may be due to carrier–phonon deformation potential scattering at the high excitation density. The recombination and trapping of the thermalized carriers are considered to be the possible origin for the slow component τs of the relaxation time of reflectivity recovery dynamics.

Study of ultrafast carrier dynamics of nanostructured TiO 2 films with and without CdSe quantum dot deposition using lens-free heterodyne detection transient grating technique

Ultrafast carrier dynamics of nanostructured TiO 2 films with and without CdSe quantum dot (QD) deposition were studied using a lens-free heterodyne detection transient grating (LF-HD-TG) technique. The nanostructured TiO 2 films were prepared with anatase TiO 2 nanoparticles with an average diameter of 27 nm. CdSe quantum dots with an average size of 5 nm were adsorbed onto the TiO 2 films by a chemical deposition (CD) technique. The relaxation processes of the photoexcited electrons and holes can be detected separately using the LF-HD-TG technique. For the TiO 2 film without CdSe deposition, it was found that the photoexcited holes relaxed within 2 ps. For the CdSe QD deposited nanostructured TiO 2 film, two decay processes with time constants of 2 ps ( τ 1) and 16 ps ( τ 2) were detected. The fast decay process was attributed to photoexcited hole relaxation due to trapping in CdSe QDs. The slow decay process mainly reflected photoexcited electron transfer from CdSe QDs to TiO 2.

Ultrafast carrier dynamics on the Si(100)2×1 surface

We present a study of ultrafast carrier dynamics on the clean Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface using time-resolved photoemission spectroscopy. A rapid thermalization inside the surface band is observed, and the carrier relaxation occurs on a time scale of a few hundred femtoseconds to a few picoseconds depending on the initial state energy. The relaxation time increases as the initial state energy decreases with respect to the band minimum.