Photoinduced Reflection Research Articles

Transient and steady state magneto-optical studies of the CsPbBr3 crystal

We have studied the magneto-optical properties of photoexcitations in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ single crystal using the technique of picosecond time-resolved quantum beatings (QBs) in the circularly polarized photoinduced reflection as well as steady state magnetocircular dichroism (MCD) in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ film. In the Voigt geometry at magnetic field strength $B$ > 0, we observed fast and slow QB oscillations that we attribute to the Larmor precession frequency of electrons and holes, respectively. From the linear frequency dependence on $B$, we extract the carrier anisotropic Land\'e $g$ factors for applied B along [010] and [001]; for electrons $|{g}_{[001]}^{\mathrm{e}}|=1.95\ifmmode\pm\else\textpm\fi{}0.04$ and $|{g}_{[010]}^{\mathrm{e}}|=1.82\ifmmode\pm\else\textpm\fi{}0.04$, whereas for holes $|{g}_{[001]}^{\mathrm{h}}|=0.69\ifmmode\pm\else\textpm\fi{}0.02$ and $|{g}_{[010]}^{\mathrm{h}}|=0.76\ifmmode\pm\else\textpm\fi{}0.02$. These values are in excellent agreement with a k\ifmmode\cdot\else\textperiodcentered\fi{}p model calculation applied to $\mathrm{CsPb}{\mathrm{Br}}_{3}$. Surprisingly, at $B=0$, we still observed QB oscillation of \ensuremath{\sim}500 MHz that we interpret as due to the Overhauser field that originates from the spin-aligned nuclei caused by the Knight field. From the measured MCD spectrum vs $B$, we obtained the $g$ factor of the bright excitons ${g}_{\mathrm{ex}}=2.18\ifmmode\pm\else\textpm\fi{}0.08$ showing that the $g$ value of holes in $\mathrm{CsPb}{\mathrm{Br}}_{3}$ is positive. We also measured the temperature and magnetic field dependencies of the electron and hole spin dephasing times which support the Elliot-Yafet spin-relaxation mechanism.

Transient quantum beatings of trions in hybrid organic tri-iodine perovskite single crystal

Utilizing the spin degree of freedom of photoexcitations in hybrid organic inorganic perovskites for quantum information science applications has been recently proposed and explored. However, it is still unclear whether the stable photoexcitations in these compounds correspond to excitons, free/trapped electron-hole pairs, or charged exciton complexes such as trions. Here we investigate quantum beating oscillations in the picosecond time-resolved circularly polarized photoinduced reflection of single crystal methyl-ammonium tri-iodine perovskite (MAPbI3) measured at cryogenic temperatures. We observe two quantum beating oscillations (fast and slow) whose frequencies increase linearly with B with slopes that depend on the crystal orientation with respect to the applied magnetic field. We assign the quantum beatings to positive and negative trions whose Landé g-factors are determined by those of the electron and hole, respectively, or by the carriers left behind after trion recombination. These are {g}_{[001]}^{e} = 2.52 and {g}_{[1bar{1}0]}^{e},= 2.63 for electrons, whereas big|{g}_{[001]}^{h}big|,= 0.28 and big|{g}_{[1bar{1}0]}^{h}big|,= 0.57 for holes. The obtained g-values are in excellent agreement with an 8-band K.P calculation for orthorhombic MAPbI3. Using the technique of resonant spin amplification of the quantum beatings we measure a relatively long spin coherence time of ~ 11 (6) nanoseconds for electrons (holes) at 4 K.

Spin Transport in a Quantum Hall Insulator

A novel experimental optical method, based on photoluminescence and photo-induced resonant reflection techniques, is used to investigate the spin transport over long distances in a new, recently discovered collective state—magnetofermionic condensate. The given Bose–Einstein condensate exists in a purely fermionic system (ν = 2 quantum Hall insulator) due to the presence of a non-equilibrium ensemble of spin-triplet magnetoexcitons—composite bosons. It is found that the condensate can spread over macroscopically long distances of approximately 200 μm. The propagation velocity of long-lived spin excitations is measured to be 25 m/s.

Photoreflectance and photoinduced microwave reflectance studies of surface band bending in Mg-doped InN

Photoreflectance (PR) and microwave-reflectance photoconductivity-decay (μ-PCD) have been applied to study the surface band bending in Mg-doped InN layers with various Mg concentrations (∼1 × 1017 to ∼7 × 1019 cm−3) and thicknesses (∼400 to ∼3600 nm). A PR resonance related to the band-to-band transition at the Γ point below the surface electron accumulation is observed for moderately doped samples ([Mg]: 1.3 × 1017 cm−3 to 8.7 × 1017 cm−3). Samples with the Mg doping concentration in the range of ∼5 × 1018 ≤ [Mg] ≤ 7 × 1019 cm−3 are p-type and show very weak or no PR resonance. It is shown that the results correlate with the surface layer depletion thickness. The strongest PR signal is observed for thick depletion layers. The results are confirmed by μ-PCD measurements that show the longest carrier lifetimes for moderately doped samples with the thickest surface depletion layer.

Spin transport in the bulk of two-dimensional Hall insulator

Magneto-fermionic condensate under study is a Bose-Einstein condensate of cyclotron spin-flip magnetoexcitons in a quantum Hall insulator. This condensate features unique properties such as millisecond range lifetime and hundreds of micrometers of propagation length. In this study, utilizing the photo-induced resonant reflection technique, we measured the exciton escape time. Finally, we estimated the exciton condensate propagation velocity as 25 m/s, which is much higher than a single particle propagation velocity. We also proposed a mechanism of exciton condensation.

Long-lived magnetoexcitons in 2D-fermion system

The paper addresses the experimental technique that, when applied to a 2D-electron system in the integer quantum Hall regime with filling factor ν = 2 (the Hall insulating state), allows resonant excitation of magnetoexcitons, their detection, control of an ensemble of long-lived triplet excitons and investigation of their radiationless decay related to exciton spin relaxation into the ground state. The technique proposed enables independent control of photoexcited electrons and Fermi-holes using photoinduced resonance reflection spectra as well as estimate with a reasonable degree of accuracy the resulting density of photoinduced electron-hole pairs bound into magnetoexcitons. The mere existence of triplet excitons was directly established by inelastic light scattering spectra which were analyzed to determine the value of singlet-triplet exciton splitting. It was found that the lifetimes of triplet excitons conditioned by electron spin relaxation in highly perfect GaAs/AlGaAs heterostructures with highly mobile 2D electrons are extremely long exceeding 100 μs at T < 1 K. The paper presents a qualitative explanation of the long-spin relaxation lifetimes which are unprecedented for translation-invariant 2D systems. This enabled us to create sufficiently high concentrations of triplet magnetoexcitons, electrically neutral excitations following Bose–Einstein statistics, in a Fermi electron system and investigate their collective properties. At sufficiently high densities of triplet magnetoexcitons and low temperatures, T < 1 K, the degenerate magnetofermionic system exhibits condensation of the triplet magnetoexcitons into a qualitatively new collective state with unusual properties which occurs in the space of generalized moments (magnetic translation vectors). The occurrence of a condensed phase is accompanied with a significant decrease in the viscosity of the photoexcited system, which is responsible for electron spin transport at macroscopic distances, as well as with the effects of threshold enhancement of the system response to the external action of the electromagnetic field and emergence of a new intensive radiative recombination channel.

Role of Transient Reflection in Graphene Nonlinear Infrared Optics

Understanding the optical response of graphene at terahertz frequencies is of critical importance for designing graphene-based devices that operate in this frequency range. Here we present a terahertz pump–probe measurement that simultaneously measures both the transmitted and reflected probe radiation from multilayer epitaxial graphene, allowing for an unambiguous determination of the pump-induced absorption change in the graphene layers. The photon energy in the experiment (30 meV) is on the order of the doping level in the graphene, which enables the exploration of the transition from interband to intraband processes, depending on the amount of pump-induced heating. Our findings establish the presence of a large, photoinduced reflection that contributes to the change in sign of the relative transmitted terahertz radiation, which can be purely positive or predominantly negative depending on the pump fluence, while the change in absorption is found to be negative at all fluences. We develop a hot carrier...

Long-lived two-dimensional triplet magnetoexcitons in a Hall insulator

An experimental technique is developed to perform photoexcitation of an ensemble of translationinvariant triplet excitons, to manipulate this ensemble, and to detect the properties of its components. In particular, the influence of temperature on the radiationless decay during the relaxation of an exciton spin into the ground state of a Hall insulator at a filling factor ν = 2 is studied. The generation of photoexcited electrons and holes is controlled using photoinduced resonance reflection spectra, which makes it possible to estimate the density of light-generated electron–hole pairs and to independently control the self-consistent generation of electrons at the first Landau level and holes (vacancies) at the ground (zero) cyclotron electronic level. The existence of triplet excitons is established from inelastic light scattering spectra, which are used to determine the singlet–triplet exciton splitting. The lifetimes of triplet excitons, which are closely related to the relaxation time of an electron spin, are extremely long: they reach 100 μs in perfect GaAs/AlGaAs heterostructures with a high mobility of two-dimensional electrons at low temperatures. These long spin relaxation times are qualitatively explained, and the expected collective behavior of high-density triplet magnetoexcitons at sufficiently low temperatures, which is related to their Bose nature, is discussed.

Semiconductor interfacial carrier dynamics via photoinduced electric fields.

Solar photoconversion in semiconductors is driven by charge separation at the interface of the semiconductor and contacting layers. Here we demonstrate that time-resolved photoinduced reflectance from a semiconductor captures interfacial carrier dynamics. We applied this transient photoreflectance method to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) interfaces critically important to solar-driven water splitting. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. The data show that a field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces drives charge separation. Additionally, the charge recombination rate at the p-GaInP2/TiO2 interface is greatly reduced owing to its p-n nature, compared with the Schottky nature of the p-GaInP2/Pt interface.

Spin-dependent polaron formation dynamics inEu0.75Y0.25MnO3probed by femtosecond pump-probe spectroscopy

We present a femtosecond optical pump-probe study of the multiferroic manganite Eu$_{0.75}$Y$_{0.25}$MnO$_3$. The optical response of the material at pump energies of 1.55 and 3.1 eV is dominated by the $d$-$d$ and $p$-$d$ transitions of the Mn$^{3+}$ ions. The relaxation of photoexcited electrons includes the relaxation of the Jahn-Teller distortion and polaron trapping at Mn$^{2+}$ and Mn$^{4+}$ sites. Ultrafast switching of superexchange interactions due to modulated $e_g$ orbital occupancy creates a localized spin excitation, which then decays on a time scale of tens of picoseconds at low temperatures. The localized spin state decay appears as a tremendous increase in the amplitude of the photoinduced reflectance, due to the strong coupling of optical transitions to the spin-spin correlations in the crystalline $a$-$b$ plane.

In Situ Detection of Free and Trapped Electrons in Dye-Sensitized Solar Cells by Photo-Induced Microwave Reflectance Measurements

In order to study the behavior of photoinjected electrons in dye-sensitized solar cells (DSC), steady-state microwave reflectance measurements (33 GHz, Ka band) have been carried out on a working c...

Hybridization and Superconducting Gaps in the Heavy-Fermion SuperconductorPuCoGa5Probed via the Dynamics of Photoinduced Quasiparticles

We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconductivity. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario--evidence for a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the superconducting state is consistent with the heavy character of the quasiparticles that participate in Cooper pairing.

Generation and detection of coherent longitudinal acoustic phonons in the La0.67Sr0.33MnO3 thin films by femtosecond light pulses

The authors report on an anomalous first-to-zero sound crossover in the colossal magnetoresistance compound La0.67Sr0.33MnO3 (LSMO) using ultrafast time-resolved optical techniques. Two-color pump-probe setup was employed to record the photoinduced reflectance oscillations due to excitations of longitudinal acoustic phonons. By changing the sample thickness and probe wavelength, they determine the longitudinal sound velocities of both the thin film and substrate . The values of the sound velocity in LSMO at gigahertz frequencies are more than 20% larger than those from ultrasound experiments in the megahertz regime near TC. The results show the possible existence of dynamic polaron correlations in LSMO.

Exciplex dynamics in a blend ofπ-conjugated polymers with electron donating and accepting properties: MDMO-PPV and PCNEPV

The photophysical properties of a solution processed blend of two semiconducting polymers with electron donating and electron accepting properties, respectively, as used in polymer photovoltaic devices have been investigated. We show that in the binary mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) photoexcitation of either one of the polymers results in formation of a luminescent exciplex at the interface of the two materials. Photoinduced absorption spectroscopy shows that this exciplex can decay to the lowest triplet state $({T}_{1})$ of MDMO-PPV. Application of an electric field results in dissociation of the marginally stable exciplex into charge carriers, which provides the basis for the photovoltaic effect of this combination of materials. Spin allowed recombination of the charge carriers to the MDMO-PPV ${T}_{1}$ state is invoked to explain the field-enhanced quantum yield for triplet formation observed by photoinduced reflection measurements on photovoltaic devices made from the composite films. The field enhanced triplet yield is identified as loss mechanism for the photovoltaic performance of this combination of materials.

Long-lived coherent acoustic waves generated by femtosecond light pulses

We report on photoexcitation of coherent longitudinal acoustic phonons in single-crystal cuprate thin films on epitaxialy matched substrates. The photoinduced reflectance oscillations are unusually long lived; in some samples we could easily resolve them for hundreds of periods. We studied the effect of varying a number of parameters, including the film doping level, thickness, and temperature, as well as the pump and probe beam wavelength, power, polarization, and incidence angle. We account quantitatively for the oscillation period, dispersion, phase, and decay of amplitude with time.

Time-Domain Raman Measurement of Molecular Submonolayers by Time-Resolved Reflection Spectroscopy

Femtosecond time-resolved photoinduced reflectance change was measured on a spin-coated dye (cresyl violet) of submonolayer coverage (4 × 1013 cm-2) on a fused silica substrate. We found that the sensitivity of the reflection measurement was much higher than that of the absorption measurement for thin monolayers of dye. A prominent beating structure due to vibrational coherence of the dye was observed on the time-resolved reflection signal. A vibrational spectrum of the dye at submonolayer coverage, which corresponded to its resonance Raman spectrum, was obtained in the range 0−800 cm-1 by a Fourier transform analysis of the reflection signal.

Investigation of excited states in polymer/fullerene solar cells by means of photoinduced reflection-/absorption spectroscopy

The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes has led to intensive spectroscopic investigations of solid state composites of these materials. Based on this charge transfer reaction, high efficient solar cell devices have been developed using the concept of bulk heterojunctions, i.e. blending of the conjugated polymer and fullerene. In this work, we present a spectroscopic study on such devices under working conditions. These devices were investigated by means of photoinduced absorption spectroscopy in reflection geometry at different temperatures and different applied external electric fields. Significant changes in the photoinduced signal were observed upon changing the applied voltage. These changes can be attributed to persistent charge carriers, charge carrier transport and charge carrier injection. From this the mean polaron lifetime, the built-in potential and the fraction of persistent charge carriers was estimated.

Anisotropic Photoexcited Carrier Dynamics in (100)-, (001)-, and (110)-Oriented YBCO Films by Polarized Ultrafast Optical Spectroscopy

We report, the time-resolved ultrafast optical spectroscopy results obtained from a series of well characterized (001)-, (100)- and (110)-oriented YBa2Cu3O7−δ (YBCO) superconducting films prepared by pulsed laser deposition. The well-controlled film orientation has allowed us to probe the relaxation dynamics along various crystal orientations using respectively polarized pump and probe laser beams. The significant anisotropies in both the magnitude of the characteristic relaxation time and the temperature dependence of the photo-induced transient reflectance change indicate the nature of relaxation channel might be intrinsically different along different orientations. The implications of the results on the fundamental characteristics concerning superconducting gaps are discussed.

Quasiparticle dynamics and gap structure inHgBa2Ca2Cu3O8+δinvestigated with femtosecond spectroscopy

Measurements of the temperature dependence of the quasiparticle (QP) dynamics in Hg1223 with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature dependent Dc that closes at Tc, and another temperature independent ''pseudogap'' Dp. The zero-temperature magnitudes of the two gaps are Dc/kTc = 6 +/- 0.5 and Dp/kTc = 6.4 +/- 0.5 respectively. The quasiparticle lifetime is found to exhibit a divergence as T -> Tc from below, which is attributed to the existence of a superconducting gap which closes at Tc. Above Tc the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the ''pseudogap''. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a bi-particle recombination process.

Carrier-relaxation dynamics in intragap states: The case of the superconductorYBa2Cu3O7−δand the charge-density-wave semiconductorK0.3MoO3

The unusual slow carrier relaxation dynamics\char22{}observed in femtosecond pump-probe experiments on high-temperature superconductors and recently also in a charge-density-wave system\char22{}is analyzed in terms of a model for relaxation of carriers in intragap states. The data on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ near optimum doping and ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}$ are found to be described very well with the model using a BCS-like gap which closes at ${T}_{c}.$ From the analysis of the data we conclude that a significant intragap density of localized states exists in these materials, which can be clearly distinguished from quasiparticle states by the time-resolved optical experiments because of the different time and temperature dependences of the photoinduced transmission or reflection. Localized charges are suggested to be the most likely origin of the intragap states, while the similarity of the response in the two materials appear to exclude spin and vortex excitations.