Time-resolved Pump-probe Reflection Research Articles

Ultrafast coherent interlayer phonon dynamics in atomically thin layers of MnBi2Te4

The atomically thin MnBi2Te4 crystal is a novel magnetic topological insulator, exhibiting exotic quantum physics. Here we report a systematic investigation of ultrafast carrier dynamics and coherent interlayer phonons in few-layer MnBi2Te4 as a function of layer number using time-resolved pump-probe reflectivity spectroscopy. Pronounced coherent phonon oscillations from the interlayer breathing mode are directly observed in the time domain. We find that the coherent oscillation frequency, the photocarrier and coherent phonon decay rates all depend sensitively on the sample thickness. The time-resolved measurements are complemented by ultralow-frequency Raman spectroscopy measurements, which both confirm the interlayer breathing mode and additionally enable observation of the interlayer shear mode. The layer dependence of these modes allows us to extract both the out-of-plane and in-plane interlayer force constants. Our studies not only reveal the interlayer van der Waals coupling strengths, but also shed light on the ultrafast optical properties of this novel two-dimensional material.

Photo-excited carrier relaxation dynamics in two-dimensional InSe flakes

Carrier relaxation dynamics of InSe flakes is investigated by using time-resolved pump-probe reflectivity measurement. The photocarriers associated with the Pxy orbital band-edge transition at 2.40 eV, which is coupled to the in-plane polarized light, is observed to possess a lifetime of ∼19 ps at room temperature and ∼99 ps at 10 K. The temperature and power dependent carrier lifetime suggests that Shockley–Read–Hall process is the dominant nonradiative recombination mechanism responsible for the carrier relaxation. In addition, the electron scattering with a 14.5 meV optical phonon plays an active role in the carrier relaxation with increasing temperatures. A broad absorption around 1.65–1.90 eV is observed. The photocarriers associated with this broad transition show a long lifetime of ∼200 ps that is nearly independent of temperature and photon energy. This is indicative of bound carriers by defects. Our experimental results provide essential information for the characteristics of carrier dynamics and defects in InSe flakes. The experimental findings are fundamentally important for further development of microelectronics and optoelectronics based on InSe.

Ultrafast Carrier Dynamics of CdTe: Surface Effects

Cadmium telluride has been an important absorber material used in solar cells for decades given its near-optimum band gap and lower cost for device fabrication. However, the overall efficiency has been low compared to the theoretical limit. One major contributor to the problem of the relatively low open-circuit voltage is the high surface recombination of photogenerated carriers. In this contribution, time-resolved pump–probe reflectivity was used to study the carrier dynamics of CdTe(111) under the influence of selected surface conditions. It was found that surfaces after the processes of chemical etching and thermal oxidation for short durations exhibit bulk-like transient reflectivity and a clearly reduced surface recombination velocity, according to a model calculation. The comparative study indicates that the origins of carrier traps include the midgap defect states that are associated with the uncapped Te surface atoms, namely the Te antisites and Te interstitials, and subsurface damages resulting f...

Photoexcitation-induced carrier dynamics in an undoped InAs/GaSb quantum well

The carrier lifetime in an InAs/GaSb quantum well grown by molecular beam epitaxy has been investigated by the time-resolved pump–probe reflectivity (PPR) measurement. It is found that the PPR response shows a sign reversal when tuning excitation energy from 1.033 to 0.954 eV. A model is adopted, in which one takes into account the band gap renormalization and band filling effects, to simulate the temporal evolution of the PPR response. The carrier decay time is then extracted at different excitation energy. The carrier decay time of 98 ps obtained at a close resonant excitation of 0.827 eV is denoted as the carrier lifetime at room temperature. The temperature-dependent PPR is also examined to further investigate the dominant carrier decay mechanism. The short carrier lifetime and its very weak temperature-dependent behavior suggests that the carrier lifetime is mainly determined by the carrier capture process by defect levels in GaSb.

Electron spin relaxation in intrinsic bulk InP semiconductor

Electron spin dynamics in intrinsic bulk indium phosphide (InP) semiconductor is studied by time resolved pump probe reflectivity technique using the cocircularly and countercircularly polarized femtosecond pulses at room temperature and 70 K. The reflectivity change from bleaching into absorption is observed with increasing pump photon energy, which can be explained in terms of the spin sensitive band filling and band gap renormalization effects. Density dependence of electron spin relaxation time shows similar tendency at room temperature and 70 K. With increasing carrier density, the electron spin relaxation time increases and then decreases after reaching a maximum value. Our experimental results agree well with the recent theoretical prediction [Jiang and Wu, Phys. Rev. B 79, 125206 (2009)] and D’yakonov–Perel’ mechanism is considered as a dominating contribution to the electron spin relaxation in intrinsic bulk InP semiconductor.

Observation of a coherent optical phonon in the iron pnictide superconductorBa(Fe1−xCox)2As2(x=0.06and 0.08)

We report the observation of a coherent lattice oscillation in a pnictide superconductor. A coherent fully symmetric optical phonon was detected in $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ ($x=0.06$ and $x=0.08$) using time-resolved pump-probe reflectivity with 40 fs time resolution. The analysis of the phonon parameters for various excitation fluences reveals no evident difference below and above the critical temperature, suggesting that the ${A}_{1g}$ mode is not involved in the superconducting phase transition.

Ultrafast hot electron relaxation time anomaly in InN epitaxial films

Ultrafast carrier dynamics in InN epitaxial films was investigated by using femtosecond time-resolved pump-probe reflectivity measurements with a photon energy of 1.58eV. The hot electron relaxation time decreased with increasing electron density (n), measuring at n−0.5. The result was contradictory to what was expected from the hot phonon effect and the screening effect. The authors attributed this result to the important role played by electron-electron scattering in hot electron relaxation.

Optical Imaging of Defect Density Distribution in Ion-Implanted GaAs using Ultrafast Carrier Dynamics

By combining time-resolved pump-probe reflectivity measurements with scanning laser microscopy, we demonstrate microscopic two-dimensional (2D) imaging of defect density distribution on a GaAs surface. The carrier lifetime is significantly reduced by the recombination process through defect states introduced by 5-keV He+ ion implantation. Thus, when plotting the probe beam reflectivity at 1 ps after photoexcitation and scanning the probe beam across the GaAs surface at a spatial resolution of 2 µm with a scanning step of 10 µm, we obtain a clear image of defect distribution corresponding to a mask edge pattern placed on the sample during ion implantation. Clearer images can be obtained using longer wavelength probe light.

Retrieval of the dielectric function of thin films from femtosecond pump-probe experiments

The retrieval of the complex dielectric function from time-resolved pump-probe reflection and transmission experiments on thin films is investigated. A general approach is presented that takes into account interference effects of both the pump and the probe pulse. The influence of the pulse duration is investigated. The retrieval technique is exemplified with experimental data from a Ta2O5 film.

Theory for displacive excitation of coherent phonons.

We report femtosecond time-resolved pump-probe reflection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only ${\mathit{A}}_{1}$ modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with ${\mathit{A}}_{1}$ vibrational modes, the quasiequilibrium nuclear ${\mathit{A}}_{1}$ coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of ${\mathit{A}}_{1}$ symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with ${\mathit{A}}_{1}$ symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(${\mathrm{\ensuremath{\omega}}}_{0}$t) dependence, where t=0 is the time of arrival of the pump pulse peak, and ${\mathrm{\ensuremath{\omega}}}_{0}$ is the vibrational frequency of the ${\mathit{A}}_{1}$ mode. These predictions agree well with our observations in Bi, Sb, Te, and ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$. The fit of the experimental \ensuremath{\Delta}R(t)/R(0) data to the theory is excellent.