Time-resolved Differential Transmission Research Articles

Coherent Spin Dynamics of Electrons in CsPbBr3 Perovskite Nanocrystals at Room Temperature.

Coherent spin dynamics of charge carriers in CsPbBr3 perovskite nanocrystals are studied in a temperature range of 4-300 K and in magnetic fields of up to 500 mT using time-resolved pump-probe Faraday rotation and differential transmission techniques. We detect electron spin Larmor precession in the entire temperature range. At temperatures below 50 K, hole spin precession is also observed. The temperature dependences of spin-related parameters, such as Landè g-factor and spin dephasing time are measured and analyzed. The electron g-factor increases with growing temperature, which can not be described by the temperature-induced band gap renormalization. We find that photocharging of the nanocrystals with either electrons or holes depends on the sample cooling regime, namely the cooling rate and illumination conditions. The type of the charge carrier provided by the photocharging can be identified via the carrier spin Larmor precession.

Mid-infrared dielectric response of Fermi-degenerate electron-hole droplets in diamond

Exploring the properties of a highly quantum-degenerate electron-hole (e-h) state as a many-body system is an interesting problem in solid-state physics. Electron-hole droplets (EHDs) offer a potential platform for investigating such a state of Fermi-degenerate carriers at an extremely high density. Herein we present dielectric response measurements of carriers in the mid-infrared (MIR) region for a definitive evaluation of the dynamic properties of the e-h state of EHDs in diamond. With photoexcitation across the band-gap energy by using the two-photon absorption process, we performed time-resolved differential transmission spectroscopy by using MIR probe light and time-resolved photoluminescence spectroscopy in the deep-ultraviolet region. By linking the two temporal profiles, we overcame the problem of previous ambiguous PL analyses and elucidated the MIR dielectric response that reveals the surface plasmon resonance (SPR) of EHDs and the effect of the e-h Coulomb interaction on the dynamic dielectric response. Thus, we found that the carrier density inside EHDs remained constant at $8\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ up to 2 ns, whereas a decay time of 1 ns of the condensed volume was found and the AC conductivity relaxation rate of $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ maintained during this nanosecond timescale was the highest among group-IV semiconductors. We predicted a strong dependence of the e-h scattering rate on the probe frequency in the high-frequency region in which SPR appeared, suggesting that---when probed in the low-frequency region---dynamical relaxation does not interfere with the order formation expected in quantum-degenerate e-h systems at ultralow temperatures.

Terahertz photoconductivity and photocarrier dynamics in graphene–mesoporous silicon nanocomposites

We investigate charge transport and photocarrier dynamics in graphene--mesoporous silicon nanocomposites using optical-pump terahertz-probe measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial decay time of 5 ps and a longer decay time of about 25 ps. These decay times are significantly reduced compared to the values of ${\ensuremath{\tau}}_{1}\ensuremath{\sim}74$ ps and ${\ensuremath{\tau}}_{2}\ensuremath{\sim}730$ ps obtained for the uncoated mesoporous silicon membrane and this is attributed to the introduction of additional surface defects formed during the graphene deposition process. Based on the influence of the laser fluence on the time-resolved differential transmission curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz waveforms taken at different optical-pump THz-probe delays. These data curves are well reproduced using a modified Drude-Smith model taking into account diffusive-restoring currents. The $c$ parameter of this model, which describes the degree of carrier localization, is about $\ensuremath{-}0.73$ for the uncoated porous Si membrane and is approaching $\ensuremath{-}1$ for graphene--mesoporous Si nanocomposites formed at temperatures above $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For all the nanocomposites, the characteristics of the photoconductive material, in terms of photocarrier capture/recombination time and effective mobility, are of interest for the fabrication of pulsed terahertz devices.

Growth of Perovskite CsPbBr3 Nanocrystals and Their Formed Superstructures Revealed by In Situ Spectroscopy

Metal halide perovskites have attracted substantial interest because of their promising properties for optoelectronic applications. Despite much progress made in the field, the exact growth mechanism of perovskite nanocrystals (e.g., CsPbBr3) remains elusive and further improvement of their controllable synthesis is challenging. Herein, we point out different phenomena during the processes of growth, cooling, and purification of high-quality CsPbBr3 nanocrystals using in situ photoluminescence spectroscopy. The as-synthesized materials have been further characterized by time-resolved transient differential transmission and photoluminescence spectroscopies. Using X-ray scattering, we confirm that nanocrystals form superstructures during the process of cooling already in dispersion, which is frequently ignored. The purification process is explained using a proposed model based on the self-size-selection. On the one hand, such superstructures pave a potential pathway to the fabrication of high-quality devices such as light-emitting devices. On the other hand, the approach to reveal their formation process benefits the comparison and understanding of the difference between nanocrystals and supercrystals. The fact that superstructures form already during synthesis may also apply to the different perovskite systems and thus help to improve the quality of the as-prepared nanocrystals.

Spin Polarization Dynamics of Free Charge Carriers in CsPbI3 Nanocrystals.

Lead halide perovskites (LHPs) exhibit large spin-orbit coupling (SOC), leading to only twofold-degenerate valence and conduction bands and therefore allowing for efficient optical orientation. This makes them ideal materials to study charge carrier spins. With this study we elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI3 nanocrystals by employing time-resolved differential transmission spectroscopy (DTS). We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling toward the energetically favorable band edge. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). We propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier-phonon scattering, as described by the Elliott-Yafet mechanism.

Transfer of Direct to Indirect Bound Excitons by Electron Intervalley Scattering in Cs2AgBiBr6 Double Perovskite Nanocrystals

Lead-free halide double perovskites have emerged as a non-toxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Control of the ultrafast photo-electronic dynamics of a chemical-vapor-deposited-grown graphene by ozone oxidation

Diverse ultrafast dynamics have been reported on different graphene prepared by different methods. Chemical-vapor-deposited (CVD) growth is regarded as a very promising method for highly efficient production of graphene. However, CVD-grown graphene usually presents only one of the diverse ultrafast dynamics. Thus, control of the ultrafast photo-electronic dynamics of CVD-grown graphene is vital to present diversity for different photodetection applications of CVD-grown graphene. In this paper, we report on the first realization to our knowledge of control of the ultrafast dynamics of CVD-grown graphene and the manifestation of diverse ultrafast dynamics on sole CVD-grown graphene. We study the ultrafast photoelectronic dynamics of CVD-grown graphene with different degrees of oxidation caused by ozone oxidation using femtosecond time-resolved transient differential transmission spectroscopy, and we find that the ultrafast dynamics can evolve obviously with the time of ozone oxidation. The diverse ultrafast dynamics reported previously on different monolayer graphenes prepared by different methods are achieved on the sole CVD-grown graphene by controlling oxidation time. The mechanism for manipulation of the ultrafast dynamics by ozone oxidation is revealed by Raman spectroscopy as the control of the Fermi level of CVD-grown graphene. Simulations of dynamics based on the optical conductivity model of graphene and Fermi level change well reproduce the observed diverse ultrafast dynamics. Our results are very important for the diverse applications of graphene and open a new path toward the diverse ultrafast dynamics on sole graphene prepared by any method.

Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots.

Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, whereas holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation. Additionally, holes can be trapped at the core/shell interface, leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.

Probing the free-carrier absorption in multi-layer black phosphorus

We study the carrier relaxation dynamics in thin black phosphorus (bP) using time-resolved differential transmission measurements. The inter-band and intra-band transitions, relaxation, and carrier recombination lifetimes are revealed by tuning the mid-infrared probe wavelength above and below the bandgap of black phosphorus. When the probe energy exceeds the bandgap, Pauli blocked inter-band transitions are observed. The differential transmission signal changes sign from positive to negative when the probe energy is below the bandgap, due to the absence of inter-band transitions and enhancement in the free-carrier absorption. The minority carrier lifetime and radiative recombination coefficient are estimated to be 1.3 ns and 5.9 × 10−10 cm3/s, respectively. The overall recombination lifetime of bP is limited by radiative recombination for excess carrier densities larger than 5 × 1019 cm−3.

Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well

We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.

Photoluminescence features and carrier dynamics in InGaN heterostructures with wide staircase interlayers and differently shaped quantum wells

We present a comprehensive study of photoexcited carrier dynamics in differently grown InGaN/InGaN multiple quantum well (MQW) structures, modified by insertion of a wide interlayer structure and subsequent growth of differently shaped quantum wells (rectangular, triangular, trapezoidal). This approach of strain management allowed the reduction of dislocation density due to gradually increasing In content in the interlayer and shaping the smooth quantum well/barrier interfaces. A set of c-oriented MQW structures emitting at 470 nm were grown at Vilnius University, Institute of Applied Research, using a closed coupled showerhead type MOCVD reactor. Photoluminescence (PL) spectra of MQW structures were analysed combining continuous wave and pulsed PL measurements. Reactive ion etching of the structures enabled discrimination of PL signals originated in the InGaN interlayer structure, underlying quantum wells, and quantum barriers, thus providing growth-related conditions for enhanced carrier localization in the wells. Time-resolved PL and differential transmission kinetics provided carrier lifetimes and their spectral distribution, being the longest in triangular-shape QWs which exhibited the highest PL intensity. The light-induced transient grating (LITG) technique was used to determine the spatially averaged carrier lifetime in the entire heterostructure, in this way unravelling the electronic quality of the LED internal structure at conditions similar to device performance. LITG decay rates at low and high excitation energy densities revealed increasing with photoexcitation nonradiative recombination rate in the triangular and trapezoidal wells.

Combined influence of Coulomb interaction and polarons on the carrier dynamics in InGaAs quantum dots

Experimental results for the carrier capture and relaxation dynamics in self-organized semiconductor quantum dots are analyzed using a microscopic theory. Time-resolved differential transmission changes of the quantum-dot transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by quantum-dot polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of configuration-picture and single-particle-picture descriptions, both with nonperturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence.

Dynamics of photoexcited carriers and spins in InAsP ternary alloys

The recent rapid progress in the field of spintronics involves extensive measurements of carrier and spin relaxation dynamics in III-V semiconductors. In addition, as the switching rates in devices are pushed to higher frequencies, it is important to understand carrier dynamic phenomena in semiconductors on femtosecond time-scales. In this work, we employed time and spin resolved differential transmission measurements; to probe carrier and spin relaxation times in several InAsP ternary alloys. Our results demonstrate the sensitivity of the spin and carrier dynamics in this material system to the excitation wavelengths, the As concentrations, and temperature.

All-optical measurement of vertical charge carrier transport in mid-wave infrared InAs/GaSb type-II superlattices

Time-resolved differential transmission measurements were used to investigate vertical charge carrier transport in mid-wave infrared InAs/GaSb type-II superlattices (T2SLs). By optically generating excess carriers near one end of the mid-wave T2SL and measuring the transit time to a thin, lower-bandgap T2SL at the other end, the time-of-flight of vertically diffusing carriers was measured. Through investigation of both unintentionally doped and p-type T2SLs, the vertical hole and electron diffusion coefficients were measured to be 0.04 ± 0.03 cm2/s and 4.7 ± 0.5 cm2/s, corresponding to vertical mobilities of 6 ± 5 cm2/Vs and 700 ± 80 cm2/Vs, respectively, at a temperature of 77 K.

Time-resolved optical measurements of minority carrier recombination in a mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice

Measurements of carrier recombination rates using time-resolved differential transmission are reported for an unintentionally doped mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice. Measurements at 77 K yield minority carrier lifetimes of 3 μs and 9 μs for the InAsSb alloy and InAs/InAsSb superlattice, respectively. The un-optimized InAsSb-based materials also exhibit long lifetimes (>850 ns) at temperatures up to 250 K, indicating the potential use for these materials as mid-wave infrared photodetectors with improved performance over current type-II superlattice photodetectors at both cryogenic and near-ambient operating temperatures.

Probing ultrafast carrier dynamics, nonlinear absorption and refraction in core–shell silicon nanowires

We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time-resolved differential reflectivity and transmission spectroscopy at photon energies of 3.15 eV and 1.57 eV. The complex behavior of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photo-generated carrier density. Independent experimental results on crystalline silicon-on-sapphire help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single beam z-scan nonlinear transmission experiments at 1.57 eV in both open and close aperture configurations yield two-photon absorption coefficient \$beta$ (~3 cm/GW) and nonlinear refraction coefficient \$gamma$ (-2.5x10^-4 cm2/GW).

Time-resolved differential transmission in MOVPE-grown ferromagnetic InMnAs

Time-resolved differential transmission in MOVPE-grown ferromagnetic InMnAs

Two-probe study of hot carriers in reduced graphene oxide

The energy relaxation of carriers in reduced graphene oxide thin films is studied using optical pump-probe spectroscopy with two probes of different colors. We measure the time difference between peaks of the carrier density at each probing energy by measuring a time-resolved differential transmission and find that the carrier density at the lower probing energy peaks later than that at the higher probing energy. Also, we find that the peak time for the lower probing energy shifts from about 92 to 37 fs after the higher probing energy peak as the carrier density is increased from 1.5×1012 to 3×1013/cm2, while no noticeable shift is observed in that for the higher probing energy. Assuming the carriers rapidly thermalize after excitation, this indicates that the optical phonon emission time decreases from about 50 to about 20 fs and the energy relaxation rate increases from 4 to 10 meV/fs. The observed density dependence is inconsistent with the phonon bottleneck effect.

Epitaxial growth and picosecond carrier dynamics of GaInAs/GaInNAs superlattices

We study GaInAs/GaInNAs superlattice structures grown on InP substrate as potential candidates for photoconductive terahertz devices or saturable absorbers working at 1.55 μm wavelength. The N-rich GaInNAs layers are flat, with no three-dimensional islanding, and act as trapping layers where carriers can recombine rapidly. The carrier lifetime in GaInAs/GaInNAs superlattice was measured for various growth parameters using time-resolved differential transmission experiments at 1.55 μm wavelength. The carrier lifetime is found to depend strongly on N content and can be reduced down to 3.8 ps for samples with 14% of N. The mechanisms involved in the capture process of photocarriers are discussed.

Electron-Spin Beat Susceptibility of Excitons in Semiconductor Quantum Wells

Recent time-resolved differential transmission and Faraday rotation measurements of long-lived electron-spin coherence in quantum wells displayed intriguing parametric dependencies. For their understanding we formulate a microscopic theory of the optical response of a gas of optically incoherent excitons whose constituent electrons retain spin coherence, under a weak magnetic field applied in the quantum well's plane. We define a spin beat susceptibility and evaluate it in linear order of the exciton density. Our results explain the many-body physics underlying the basic features observed in the experimental measurements.