Photoinduced Carrier Dynamics Research Articles

Magnetization dynamics and theMn3+d−dexcitation of hexagonalHoMnO3single crystals using wavelength-tunable time-resolved femtosecond spectroscopy

We analyze the photo-induced carrier and magnetization dynamics in antiferromagnetic (AFM) ${\text{HoMnO}}_{3}$ single crystals, delineated by wavelength-dependent femtosecond spectroscopy. The emergence of long-range and short-range magnetic order are unambiguously revealed in association with an abnormal blueshift of ${\text{Mn}}^{3+}$ $3d$ level around the N\'eel temperature and the slope variation in the temperature-dependent transient reflectivity change $(\ensuremath{\Delta}R/R)$ near the N\'eel transition, respectively. Furthermore, the disturbance (in ps time scale) of the AFM ordering and its reordering time (in a few 100 ps time scale) are clearly identified in the negative $\ensuremath{\Delta}R/R$.

Photo-induced Carrier Dynamics in a Quasi-one-dimensional Material with Curved Morphology

Photo- or electron beam (EB) irradiation of C60 results in various cross-linkages with various dimensionalities. One characteristic example of such coalesced structures is the one-dimensional (1D) peanut-shaped C60 polymer reviewed by J.O. in another article. This EB-irradiated C60 polymer exhibits a Fermi-edge anomaly in the valence band spectrum, showing a similarity to those observed in quasi-1D conductors. In this article, we demonstrate the photo-induced carrier dynamics of EB-irradiated C60 polymer by utilizing an ultrafast optical spectroscopy. We first present an overview of the carrier dynamics in typical quasi-1D conductors, and then compare the properties with those of the EB-irradiated C60 polymer. We also compare the carrier dynamics in the C60 polymer and pristine samples, and show that the polymer sample exhibits an additional slow relaxation component below T∼60K. This component is similar to the interband relaxation observed in quasi-1D conductors below Peierls transition temperature, whereas the temperature dependence of the relaxation time exhibits no critical divergence that accounts for coherent gap formation. We will discuss the plausible origin.

Nanoscale dynamics probed by laser-combined scanning tunneling microscopy

With the miniaturization of functional devices, the difference in the electronic properties, for example, due to the structural nonuniformity in each element of nanoscale blocks has an ever more crucial influence on macroscopic functions. Laser-combined scanning tunneling microscopy, a potential method that enables us to probe the photoinduced carrier dynamics on the nanoscale, is reviewed with the latest results.

Ultrafast photoinduced carrier dynamics in GaNAs probed using femtosecond time-resolvedscanning tunnelling microscopy

The combination of scanning tunnelling microscopy (STM) with optical excitation usingultrashort laser pulses enables us, in principle, to simultaneously obtain ultimate spatialand temporal resolutions. We have developed the shaken-pulse-pair-excited STM(SPPX-STM) and succeeded in detecting a weak time-resolved tunnelling current signalfrom a low-temperature-grown GaNAs sample. To clarify the underlying physics inSPPX-STM measurements, we performed optical pump–probe reflectivity measurementswith a wavelength-changeable ultrashort-pulse laser. By comparing the results obtainedfrom the two methods with an analysis based on the nonlinear relationship between thephotocarrier density and tunnelling current, we obtained a comprehensive explanation thatthe photocarrier dynamics is reflected in the SPPX-STM signal through the surfacephotovoltage effect.

Investigation of nonlinear properties of CdS-doped glasses

The dynamics of photodarkening and transient absorption in commercial CdS-doped filter glass Asahi Y45 has been investigated at room temperature using pump and probe spectroscopy. The fresh spot of the sample was irradiated by pump pulse with the wavelength of 355 nm above the band gap and the probing was done at different wavelengths (1064, 760, and 469 nm). Influence of exposure as well as probe wavelength on dynamics of carriers has been investigated. The long lived photoinduced absorption dynamics corresponding to the higher probe photon energy illustrates the dominant contribution of the trapping states from the glass matrix. A simple model has been presented to understand the photoinduced carrier dynamics. The amplitude of differential absorbance as well as the photodarkening dynamics connected to the interior states are found to be in good agreement with the Drude's free electron model.

Photophysical and photoelectrochemical characteristics of multilayers of CdS nanoclusters.

Luminescence lifetime measurements in sub-ns time resolution and visible pump-IR absorption probe measurements to study photo-induced carrier dynamics using fs laser pulses were carried out for the CdS nanoclusters prepared by reverse micelle method, the nanoclusters modified with 2-mercaptoethanesulfonate (SO3-CdS) in water, and the SO3-CdS nanoclusters in a multilayer assembly. For a comparison, similar measurements were also carried out for a CdS nanocluster doped glass. The photoelectrochemical characteristics of the multilayer are also measured. The characteristics of nanocluster dispersion and those of multilayer assembly are compared. The sample dependent decay behaviors of luminescence and IR absorption are somewhat different and the importance of visible pump-IR absorption probe measurement is demonstrated.

Excitonic field screening and bleaching in InGaN/GaN multiple quantum wells

Photoinduced carrier dynamics in a sequence of InGaN/GaN multiple quantum wells (MQWs) are studied by employing steady state and ultrafast spectroscopy at room temperature. Time-resolved photoluminescence (PL) measured short carrier lifetimes of ∼140 ps at room temperature. Steady state differential transmission was used to measure the in-well field screening due to the photoinjected carriers. The observed offset in emission energy from excitonic screening energies is consistent with the emission of carriers through localized states slightly below the excitonic resonance energy. Furthermore, time-resolved differential transmission with amplified pulses, where significant carrier densities can be optically generated, provides evidence of both excitonic bleaching and field screening in these InGaN quantum wells (QWs). The comparison of the time-resolved differential absorption spectra at various carrier densities allows us to identify different carrier recombination dynamics in the InGaN well and to separate the field screening from the bleaching effects. Finally, the extreme prolongation of the carrier recombination lifetime up to ∼4 μs suggests the spatial separation between electrons and holes under the large in-well fields.

Spin transport driven by giant ambipolar diffusion

We report on the dynamics of photoinduced spin-polarized carriers in p–i–n–i–p doped structures. The lateral diffusion process along the doping layers in such structures is strongly enhanced due to the spatial separation of electrons and holes (giant ambipolar diffusion). We show that this fast diffusion process can be utilized to transport spin polarization within the short relaxation time over distances large compared to bulk semiconductors. Besides the fast diffusion process, the spin diffusion length is also strongly enhanced due to the fact that both internal recombination and spin relaxation due to electron–hole interaction (Bir–Aronov–Pikus mechanism) are negligible because of the spatial separation of electrons and holes.

Photo-induced carrier dynamics of high T c oxide superconductors

Photo-induced “transient thermoelectric effect (TTE)” has been measured for high T c superconductors (YBCO and BSCCO systems). The observed TTE signals show multiple relaxation processes with characteristic relaxation times. In the normal state we have found electron-hole recombination via some impurity centers and carrier diffusion processes for “two kinds” of holes in YBCO and “single holes” in BSCCO. In the superconducting state “quasiparticles” of photo-generated carriers undergo thermal diffusion to form “quasiparticle potential”. Furthermore anomalous behaviors in the TTE decay curves around 35 K for YBCO are observed.