Carrier Relaxations Research Articles

Research Progress on Rashba Effect in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites.

The Rashba effect appears in the semiconductors with an inversion-asymmetric structure and strong spin-orbit coupling, which splits the spin-degenerated band into two sub-bands with opposite spin states. The Rashba effect can not only be used to regulate carrier relaxations, thereby improving the performance of photoelectric devices, but also used to expand the applications of semiconductors in spintronics. In this mini-review, recent research progress on the Rashba effect of two-dimensional (2D) organic-inorganic hybrid perovskites is summarized. The origin and magnitude of Rashba spin splitting, layer-dependent Rashba band splitting of 2D perovskites, the Rashba effect in 2D perovskite quantum dots, a 2D/3D perovskite composite, and 2D-perovskites-based van der Waals heterostructures are discussed. Moreover, applications of the 2D Rashba effect in circularly polarized light detection are reviewed. Finally, future research to modulate the Rashba strength in 2D perovskites is prospected, which is conceived to promote the optoelectronic and spintronic applications of 2D perovskites.

Interband carrier recombination mechanism in Al-doped ZnO

Due to strong nonlinear optical effects and ultrafast response properties, transparent conductive oxides are promising candidate materials for next-generation THz modulation devices and other photonic applications. However, the mechanisms of photon-induced ultrafast carrier relaxations in those materials have yet to be fully understood. Here we investigated the interband carrier dynamics in a prototype of this family, Al-doped ZnO, with different excitation photon energies through femtosecond optical transmittance measurements. We found that the interband relaxation of the photon-induced carriers is mainly through second-order recombination, which is essentially the direct band-to-band recombination with the assistance of intraband electron relaxation. The high density of the doped electrons provides superior intraband relaxation channels through electron-phonon interaction and electron-impurity scattering, which contribute to the overall ultrafast response, especially under indirect and band tail transitions. This study provides a new insight into the ultrafast response mechanism of Al-doped ZnO and the analyzing method may also be generalized to other heavily doped semiconductors.

Ultrafast modulation of a high harmonic generation in a bulk ZnO single crystal.

Optical modulation of high harmonic generation (HHG) is of fundamental interest in science and technology, which can facilitate understanding of HHG generation mechanisms and expand the potential optoelectronic applications. However, the current established works have neither shown the advanced modulation performance nor provided a deep understanding of modulation mechanisms. In this work, taking wurtzite zinc oxide (ZnO) single crystal as a prototype, we have demonstrated an all-optical intensity modulation of high-order HHG with a response time of less than 0.2 ps and a depth of more than 95%, based on the pump-probe configuration with two different pumping wavelengths. Besides the achieved excellent modulation performance, we have also revealed that the modulation dynamics in ZnO single crystal highly depend on the excitation conditions. Specifically, the modulation dynamics with the near-bandgap or above-bandgap excitation are attributed to the non-equilibrium interband carrier relaxations, while for mid-gap excitation, the modulation dynamics are dominated by the nonlinear frequency mixing process. This work may enhance the current understanding of the HHG modulation mechanism and enlighten novel device designs.

Brightness evaluation of pulsed electron gun using negative electron affinity photocathode developed for time-resolved measurement using scanning electron microscope

Temporal changes in carrier relaxations, magnetic switching, and biological structures are known to be in the order of ns. These phenomena can be typically measured by means of an optical-pump & electron-probe method using an electron microscope combined with a pulsed electron source. A photoemission-type pulsed electron gun makes it possible to obtain a short-pulsed electron beam required for high temporal resolution. On the other hand, spatial resolution is restricted by the brightness of the pulsed electron gun used in electron microscopes when a low brightness electron source is used and an irradiation current larger than a certain value is required. Thus, we constructed a prototype pulsed electron gun using a negative electron affinity (NEA) photocathode for time-resolved measurement using a scanning electron microscope (SEM) with high spatiotemporal resolution. In this study, a high-speed detector containing an avalanche photodiode (APD) was used to directly measure waveforms of the pulsed electron beam excited by a rectangular-shape pulsed light with a variable pulse duration in the range of several ns to several μs. The measured waveforms were the same rectangular shape as incident pulsed excitation light. The maximum peak brightness of the pulsed electron beam was 4.2×107 A/m2/sr/V with a pulse duration of 3 ns. This value was larger than that of the continuous electron beam (1.6 × 107 A/m2/sr/V). Furthermore, an SEM image with image sharpness of 6.2 nm was obtained using an SEM equipped with a prototype pulsed electron gun at an acceleration voltage of 3 kV.

Role of different oxidation states of Crn+-TiO2 nanocomposites for the degradation of drugs under solar irradiation

The present work focuses on the effect of different oxidation states of impregnated Crn + ionic species over TiO2 surface. The resulting Crn + -TiO2 nanocomposites have been revealed to have improved optical response, charge carrier relaxation dynamics, adsorption and photocatalytic behaviour which is correlated to variable oxidation states of these composite materials. The optical absorption spectra (400–500 nm and 583–800 nm) covered the entire visible region after Crn + incorporation to have maximum photoresponse. These bands have been assigned to the 4A2g → 4T1g and 4A2g → 4T2g transitions, respectively. A remarkable quenching of photoluminescence peaks and further successive enhanced carrier relaxations from 3.62 ns (bare TiO2) to 3.73 and 4.09 ns for Cr3+ and Cr6+ loaded TiO2 catalysts were observed. The XPS analysis confirmed the presence of Cr3+ and Cr6+ oxidation states while a reduction in particle sizes with increased oxidation state (Cr3+; 16.9 nm and Cr6+; 8.6 nm) was witnessed by HRTEM images. The electrokinetic parameters such as surface charge demand, zeta and streaming potentials were also found to vary as a function of the oxidation state of the nanocatalysts. These Crn + -TiO2 catalysts were then analyzed for the photooxidation of aspirin and paracetamol drugs with some interesting behaviour. For aspirin, Cr6+-TiO2 with rate constant, k = 18.815 × 10−3 min−1 while in case of paracetamol, Cr3+-TiO2 catalyst with k = 8.052 × 10−1 h−1 showed maximum activity under solar irradiation. The observed photoactivity has been correlated to the different chemical nature of substrate molecules, catalyst oxidation state effects and catalyst-substrate associations.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).

Ultrafast carrier relaxation dynamics of photoexcited GaAs and GaAs/AlGaAs nanowire array.

Femtosecond optical pump-probe spectroscopy is employed to elucidate the ultrafast carrier nonradiative relaxation dynamics of bare GaAs and a core-shell GaAs/AlGaAs semiconductor nanowire array. Different from the single nanowire conventionally used for the study of ultrafast dynamics, a simple spin coating and peeling off method was performed to prepare transparent organic films containing a vertical oriented nanowire array for transient absorption measurement. The transient experiment provides the direct observation of carrier thermalization, carrier cooling, thermal dissipation and band-gap energy evolutions along with the carrier relaxations. Carrier thermalization occurs within sub-0.5 ps and proceeds almost independently on the AlGaAs-coating, while the time constants of carrier cooling and thermal dissipation are increased by an order of magnitude due to the AlGaAs-coating effect. The concomitant band-gap evolutions in GaAs and GaAs/AlGaAs include an initial rapid red-shift in thermalization period, followed by a slow blue and/or red shift in carrier cooling, and then by an even slower blue shift in thermal dissipation. The evolution is explained by the competition of band-gap renormalization, plasma screening and band-filling. These findings are significant for understanding the basic physics of carrier scattering, and also for the development of flexible optoelectronic devices.

Direct Observation of Surface Polarons in Capped CuInS2 Quantum Dots by Ultrafast Pump-Probe Spectroscopies.

Semiconductor nanocrystals are mostly prepared by colloid chemistry with organic surfactant molecules, and their surface polarization effect on the carrier relaxations arecritical to their optoelectronic applications. Until now, the surface polarization effect and detailed photophysical processes of these capped quantum dots (QDs) are still unclear. Here, we studied the dynamics of the photoinduced carriers and capping moleculevibrations of capped CuInS2 quantum dots by using the femtosecond pump-probe system in both visible and IR zones. It is identified that the capping molecular vibrations exhibit significant Fermion bleaching nature, whose relaxation profile is in good agreement with the radiative recombination dynamics of QDs in the visible region. These results demonstrate that the extrinsic surface polarons form by the coupling of photoinduced carriers and surface ligand vibrations, and take part in the photophysical processes of these QDs. This finding is helpful for the QD design and applications in photoelectronic devices.

Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect for the nonlinear optics of graphene are then discussed. Our analysis shows that the conventional perturbative treatment of the nonlinear optics, i.e. expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.

Ultrafast manipulation of topologically enhanced surface transport driven by mid-infrared and terahertz pulses in Bi2Se3

Topology-protected surface transport of ultimate thinness in three-dimensional topological insulators (TIs) is breaking new ground in quantum science and technology. Yet a challenge remains on how to disentangle and selectively control surface helical spin transport from the bulk contribution. Here we use the mid-infrared and terahertz (THz) photoexcitation of exclusive intraband transitions to enable ultrafast manipulation of surface THz conductivity in Bi2Se3. The unique, transient electronic state is characterized by frequency-dependent carrier relaxations that directly distinguish the faster surface channel than the bulk with no complication from interband excitations or need for reduced bulk doping. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., γBS/γSS ~3.80 in equilibrium. The ultra-broadband, wavelength-selective pumping may be applied to emerging topological semimetals for separation and control of the protected transport connected with the Weyl nodes from other bulk bands.

Coulomb correlations in quantum dots and their signatures in single dot femtosecond pump‐probe signals

AbstractWe present a theoretical study of femtosecond pump‐probe experiments on a negatively charged single quantum dot and analyze the influence of Coulomb correlation effects on the lowest trion absorption line. The Coulomb correlated states as well as their corresponding oscillator strengths are calculated by a full diagonalization approach. The dynamics induced by the laser pulses are determined numerically by solving the Liouville‐von Neumann equation in the basis of the calculated few‐particle states. In agreement with the experiment, we demonstrate that Coulomb interactions and carrier relaxations noticeably affect the lowest trion absorption line. For negative and short positive delay times between pump and probe pulse instantaneous Coulomb renormalizations dominate the signal while for longer delay times the signal is governed by carrier relaxations. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Relaxation of photoinduced spins and carriers in ferromagnetic InMnSb films

The authors report time resolved measurements and control of photoinduced spin and carrier relaxations in InMnSb ferromagnetic films with 2% Mn content (grown by low-temperature molecular beam epitaxy) using femtosecond laser pulses, and compare them to analogous measurements on InBeSb and InSb films. In this work, magneto-optical Kerr effect and standard pump-probe techniques provided a direct measure of the photoexcited spin and carrier lifetimes, respectively. They observe decrease in relaxations times in the high laser fluence regime and an absence of temperature dependence of the relaxation times.

Significance of dimensionality and dynamical screening on hot carrier relaxation in bulk GaAs and quantum wells

We have found theoretically that the reduced dimensionality from bulk GaAs to quantum wells has a strong effect on hot carrier relaxations at highly excited carrier densities. The distinct density of state and the dynamical screening cause hot carriers in quantum wells to relax significantly slower than in the bulk when the carrier density is above a critical value of 2×10 18 cm −3. With the random phase approximation, the dynamical screening in quantum wells appears to be much stronger than that in the bulk and as important as the hot phonon effect at high carrier densities. We also found that the dependence of the average energy-loss rates on the well width in quantum wells becomes more appreciable when Al compositions are high.

Amplification of picosecond optical pulses in midinfrared intersubband semiconductor optical amplifiers

A comprehensive theoretical model describing the amplification of picosecond optical pulses in midinfrared intersubband semiconductor optical amplifiers is developed, taking into account gain dispersion, short carrier relaxation time and ultrafast nonlinear refraction. It is shown that these factors are extremely important in determining the amplified optical pulse characteristics in both the time and frequency domains. The calculations also indicate that gain dispersion can be significantly enhanced by fast carrier relaxations, short pulse widths, and high pulse energies.

Near-field spectroscopy of single self-assembled InAs quantum dots: Observation of energy relaxation process

We have investigated carrier relaxations in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized PL spectroscopy of Zeeman splitting peaks with an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows suppression of spin flipping during the relaxation of carriers inside the quantum dots. The result also suggests different spin relaxation processes between in the quantum dots and in a wetting layer.

Inter- and intrasubband relaxation times in quantum wells measured by femtosecond time-resolved differential transmission

Time-resolved differential transmission measurements with pump and probe laser pulse durations of less than 100 fs were made on quantum wells with varying well widths. The results were analysed with a phenomenological model using detailed rate equations with five fitting parameters, and comparisons are made with quantum mechanical calculations for carrier relaxations. The times reported here are in good agreement with both theoretical calculations as well as some of the recent experimental results.

Photon-assisted avalanche spreading in reach-through photodiodes

We have investigated the spreading of the avalanche process over the area of reach-through avalanche photodiodes operated in Geiger mode. A comparison between the measurements and the results of a computer simulation suggests that photons emitted from hot carrier relaxations play the dominant role in the avalanche dynamics. It is the randomness of the photon-assisted process which impairs the performance of these detectors in timing measurements.