Ultrafast Time-resolved Reflectivity Research Articles

Fast Charge Diffusion in MAPb(I1- xBr x)3 Films for High-Efficiency Solar Cells Revealed by Ultrafast Time-Resolved Reflectivity.

Organometallic halide perovskite solar cells such as MAPbI3 have shown great promise as a low-cost, high-efficiency photovoltaic candidate. Recent studies demonstrated that by substituting an appropriate amount I ions of MAPbI3 with Br ions, the device performance parameters, such as moisture stability or power conversion efficiency, can be further optimized. In this study, using time-resolved optical reflectivity to track the carrier dynamics in MAPb(I1- xBr x)3 films with different Br contents, we found that photocarriers in MAPb(I1- xBr x)3 films with x = 0.01 and 0.02 diffuse much faster than those in films with other Br contents. We suggest that the faster charge carrier diffusion benefits from larger crystal grain size. As a result, this suppresses electron and hole recombination and increases the carrier extraction efficiency, in agreement with the higher power conversion efficiency reported previously.

Transient transition from free carrier metallic state to exciton insulating state in GaAs by ultrafast photoexcitation

We present systematic studies of the transient dynamics of GaAs by ultrafast time-resolved reflectivity. In photoexcited non-equilibrium states, we found a sign reverse in reflectivity change ΔR/R, from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a free carrier metallic state, while the latter is attributed to an exciton insulating state, in which the transient electronic properties is mostly dominated by excitons, resulting in a transient metal–insulator transition (MIT). Two transition temperatures (T1 and T2) are well identified by analyzing the intensity change of the transient reflectivity. We found that photoexcited MIT starts emerging at T1 as high as ∼ 230 K, in terms of a dip feature at 0.4 ps, and becomes stabilized below T2 that is up to ∼ 180 K, associated with a negative constant after 40 ps. Our results address a phase diagram that provides a framework for the inducing of MIT through temperature and photoexcitation, and may shed light on the understanding of light-semiconductor interaction and exciton physics.

The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors.

In the underdoped copper-oxides, high-temperature superconductivity condenses from a nonconventional metallic ”pseudogap” phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime. This CDW coexists and competes with superconductivity (SC) below the transition temperature Tc, suggesting that these two orders are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, τqp, as a function of temperature and magnetic field in underdoped HgBa2CuO4+δ (Hg-1201) and YBa2Cu3O6+x (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τqp(T ) exhibits a local maximum in a small temperature window near Tc that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that Tc marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.

Evidence for a photoinduced nonthermal superconducting-to-normal-state phase transition in overdoped Bi2Sr2Ca0.92Y0.08Cu2O8+δ

Here we report extensive ultrafast time-resolved reflectivity experiments on overdoped Bi$_{2}$Sr$_{2}$Ca$_{1-x}$Y$_x$Cu$_{2}$O$_{8+\delta}$ single crystals (T$_C$=78 K) aimed to clarify the nature of the superconducting-to-normal-state photoinduced phase transition. The experimental data show the lack of the quasiparticles decay time divergence at the fluence required to induce this phase transition, in contrast to the thermally-driven phase transition observed at T$_C$ and at variance with recently reported photoinduced charge-density-wave and spin-density-wave to metal phase transitions. Our data demonstrate the non-thermal character of the superconducting-to-normal-state photoinduced phase transition. The data have been analyzed using an ad-hoc developed time-dependent Rothwarf-Taylor model, opening the question on the order of this non-equilibrium phase transition.

Carrier dynamics of Mg-doped indium nitride

Recently, we have reported a significant enhancement (>500 times in intensity) in terahertz emission from Mg-doped indium nitride (InN:Mg) films compared to undoped InN. It was found that the intensity of terahertz radiation strongly depends on the background electron density. In this letter, we present the results on ultrafast time-resolved reflectivity measurement employed to investigate the carrier dynamics of InN:Mg. We find that the decay time constant of InN:Mg depends on background electron density in the same way as terahertz radiation does. The spatial redistribution of carriers in diffusion and drift is found to be responsible for the recombination behavior as well as terahertz radiation.

Coherent phonon excitation in bismuth

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A 1g optical phonon.

Ultrafast carrier dynamics in InP photonic crystals

Ultrafast time-resolved reflectivity investigations are performed on InP-based photoniccrystals with a wide range of structural parameters. It is found that the structure plays acritical role in determining the recombination dynamics of the photo-injected chargecarriers. For sufficiently large etched sidewall area densities the carrier lifetime is decreasedto a level below 100 ps.

Ultrafast dynamics in phosphorus-implanted silicon wafers: The effects of annealing

Carrier relaxation in phosphorus-implanted silicon wafers ${(10}^{16}{\mathrm{P}}^{+}/{\mathrm{cm}}^{2})$ annealed at different temperatures ranging from 350 to 1100 \ifmmode^\circ\else\textdegree\fi{}C is investigated by near-infrared ultrafast time-resolved reflectivity measurements. A kinetic model based on four couple differential equations describing the carrier density, electron, hole, and lattice temperatures is used to evaluate the expected changes in the time-resolved reflectivity. This model was used to fit the experimental data having the trap recombination time constant and the optical absorption coefficient as fitting parameters. Our measurements reveal a complicated recovery of the ion-implanted silicon samples to crystallinity following annealing with carrier lifetimes ranging from 1 psec for the nonimplanted sample and reaching \ensuremath{\sim}95 psec for the highest-annealed sample. Although this recovery directly effects the lifetime of the carriers, there are nontrivial mechanisms that activate recombination centers and traps throughout the annealing process, resulting in carrier lifetimes that are shorter than expected at higher annealing temperatures. Raman scattering data demonstrate the disorder and the recovery of ion-implanted silicon to its crystalline form, offering a direct comparison to the time resolved reflectivity data.

Spatial dependence of ultrafast carrier recombination centers of phosphorus-implanted and annealed silicon wafers

In this letter, the spatial dependence of the carrier recombination centers induced in phosphorus-implanted and annealed silicon wafers have been examined. Ultrafast time-resolved reflectivity measurements of a set of phosphorus-implanted annealed silicon wafers (1016 P+/cm2) as a function of position on the wafer have been carried out, and an x–y map of the carrier lifetime for each of the samples has been obtained. Measurements reveal distinct features of the distribution of carrier recombination centers for the nonannealed and annealed samples between 350 °C and 1100 °C in an area of 36×36 μm2 with resolution better than 3 μm. The presence of islands of clusters in ion-implanted and annealed samples is also discussed in this letter.