Excitation Intensity Dependence Research Articles

Ultrafast magneto-photocurrents in GaAs: Separation of surface and bulk contributions

We induce ultrafast magneto-photocurrents in a GaAs crystal employing interband excitation with femtosecond laser pulses at room temperature and non-invasively separate surface and bulk contributions to the overall current response. The separation between the different symmetry contributions is achieved by measuring the simultaneously emitted terahertz radiation for different sample orientations. Excitation intensity and photon energy dependences of the magneto-photocurrents for linearly and circularly polarized excitations reveal an involvement of different microscopic origins, one of which is the inverse spin Hall effect. Our experiments are important for a better understanding of the complex momentum-space carrier dynamics in magnetic fields.

Measuring electron and hole mobilities in organic systems: charge selective CELIV

We report on the realization of charge selective CELIV (charge extraction by linearly increasing voltage) via the selective extraction of each charge carrier species in a SiO2 inserted MIS (metal-insulator-semiconductor) structure. The experiments are performed in a typical active layer of P3HT (poly (3-hexylthiophene)):PCBM (phenyl C61-butyric acid methyl ester) with a composition ratio of 1:1. The presence of double CELIV peaks indicates the involvement of deep trap states in the charge transport process. Furthermore the difference in excitation intensity dependence between photo-excited holes and electrons suggests a difference in population probabilities of electrons and holes for shallow and deep trap states. Charge selective CELIV experiments also show that the simultaneous extraction of holes and electrons is not a simple linear superposition of the cases where they are extracted separately, indicating that a fraction of photo-excited charges populating shallow trap states can escape from the device.

Investigation of internal electric fields in GaAs solar cell under highly-concentrated light

The temperature and the excitation-intensity dependences of the junction electric fields in the GaAs p-i-n solar cell structure have been investigated by using photoreflectance (PR) spectroscopy. In the p-i-n solar cell structure, two different electric fields are observed. The fast Fourier transform (FFT) analysis implies that the two electric fields can be assigned to the p-i and the i-n interfaces. The strengths of the electric fields at the p-i and the i-n interfaces are 38 and 44 kV/cm, respectively. The electric fields gradually increase due to the temperature-dependent photovoltage effect with increasing sample temperature. With increasing excitation intensity, the electric field at the p-i interface gradually decreases due to the photovoltage effect caused by carrier screening while that at the i-n interface is insensitive to the light’s intensity. This abnormal behavior can be explained by the anisotropy carrier dynamics at the p-i and the i-n interfaces., The relation between the open-circuit voltage (VOC) and the ideality factor in concentration photovoltaic (CPV) devices is discussed.

Spontaneous Defect Annihilation in CH3NH3PbI3 Thin Films at Room Temperature Revealed by Time-Resolved Photoluminescence Spectroscopy.

We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the film's fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached ∼30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence associated with the PL dynamics. We submit that this can be explained by the fact that a significant number of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temperature annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.

Study on photoluminescence properties of 1.05 eV InGaAsP layers grown by molecular beam epitaxy

The photoluminescence properties of InGaAsP films with a bandgap energy of 1.05 eV for quadruple-junction solar cells grown by molecular beam epitaxy (MBE) are investigated. We make the excitation intensity and temperature dependence of continuous-wave photoluminescence (cw-PL) measurements. The PL peak position is 1.1 eV at 10 K, and almost independent of the excitation power, but the integrated intensity of the PL emission peaks is roughly proportional to the excitation power. The shift of peak position with temperature follows the band gap shrinking predicted by the well-known Varshni's empirical formula. These results indicate that the intrinsic transition dominates the light emission of the InGaAsP material. In addition, we also make the time-resolved photoluminescence (TRPL) measurements to determine the carrier luminescence relaxation time in InGaAsP. PL spectra suggest that the relaxation time is 10.4 ns at room temperature and increases with increasing excitation power, which demonstrates the high quality of the InGaAsP material. However, the relaxation time shows an S-shape variation with increasing temperature: it increases at temperatures lower than 50 K, and then decreases between 50–150 K, and increases again when temperature is over 150 K. According to the effect of temperature and the non-radiative recombination center concentration on the carrier relaxation time, the recombination mechanism of S-shape variation can be explained by the carrier relaxation dynamics.

Excitation wavelength and intensity dependence of photo-spectral blue shift in single CdSe/ZnS quantum dots

The influence of excitation wavelength and intensity on core/shell CdSe/ZnS quantum dots (QDs) photo-spectral blue shift was investigated by spectral imaging. Analysis of the evolution of the distance between the zeroth-order spot and the first-order spectral streak, we found that the extent of blue shift strongly depends on the excitation wavelength and QDs sizes, but not on the excitation intensity. Converted the extent of blue shift into the decreased QDs volume at a series of time, the core oxidation kinetics of CdSe/ZnS QDs was uncovered that provided a quantitative comparison method for study the excitation wavelength and intensity dependence of single QDs blue shift. The core oxidation rate is almost proportional to the excitation intensity. These results are explained by a fact that higher energy excitation wavelength can accelerate individual exciton formation and higher excitation intensity can induce more amount of exciton formation per a unit time.

Excitation intensity dependence of lateral photocurrent in InGaAs/GaAs dot-chain structures

Abstract Optical properties of multi-layer In x Ga 1 − x As/GaAs dot-chain heterostructures are studied by means of lateral photoconductivity (PC). Effects of carrier generation and recombination on the photoresponsivity of deep defects and quantum dot arrays are considered. It is shown that the radiative recombination significantly affects the lateral PC spectra thus leading to a nonlinear dependence of photocurrent on excitation intensity. For ground state excitation of the quantum dots the photocurrent nonlinearities are determined by a competition of both generation and recombination processes which include thermal activation.

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = INBE/IDLE can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.

Excitation intensity dependence of plasmonic enhancement of energy transfer between quantum dots

We studied the impact excitation power on the plasmonic enhancement of Forster resonance energy transfer (FRET) between CdSe/ZnS colloidal dots. We showed that when such quantum dots (QDs) were in the vicinity of metallic nanoparticles (MNPs), with the increase of the excitation laser intensity the spectral width of their emission was initially increased and then decreased, reaching its original value when the laser intensity was low. Such a spectral lineshape recovery happened while the emission peak wavelength of the quantum dots underwent a significant amount of red shift. By tuning the exciton–plasmon coupling strength and including the effects of the heat generated by plasmonic absorption, we showed that these results could be explained in terms of transition from a donor-dominated spectrum at low laser intensity to an acceptor-dominated one at high laser intensity. This suggests that excitation intensity can increase the rate of FRET between colloidal QDs when they are in close proximity to MNPs.

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

In nanocrystal quantum dots (NQDs), generating multiexcitons offers an enabling tool for enhancing NQD-based devices. However, the photocharging effect makes understanding multiexciton kinetics in NQD solids fundamentally challenging, which is critically important for solid-state devices. To date, this lack of understanding and the spectral-temporal aspects of the multiexciton recombination still remain unresolved in solid NQD ensembles, which is mainly due to the confusion with recombination of carriers in charged NQDs. In this work, we reveal the spectral-temporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with different core sizes, revealing the strength tunability of the X-X interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.

Singlet Fission in Poly(9,9′-di-n-octylfluorene) Films

The dynamics of singlet fission in poly(9,9′-di-n-octylfluorene) (PFO) films was studied by transient absorption spectroscopy. Under a high excitation intensity, triplet excitons were rapidly interconverted from singlet excitons on a picosecond time stage. From the excitation intensity dependence of the generation yield, the rapid triplet exciton formation is attributed to the singlet fission from a higher singlet excited state Sn formed by the singlet–singlet annihilation (singlet fusion followed by singlet fission). The singlet fission from the Sn state was about 2 times more efficient in amorphous PFO films than in ordered β-PFO films, in which the conjugated main chains are planarly extended and aligned. On the other hand, about a half of triplet pairs were deactivated in the amorphous PFO film through the triplet–triplet annihilation with a time constant of 1.7 ns, while no triplet pair decay was observed in the β-PFO film. As a result, the overall triplet generation yield was comparable in these fil...

Excitonic emission of CuInS2 crystals using confocal microscopy system

AbstractPhotoluminescence (PL) spectra in the band‐edge region on bulk single‐crystals of CuInS2 grown by the traveling heater method have been investigated using a confocal microscopy system. The observed PL spectra are separated into two Lorentzian peaks which are assigned to be A and B free excitons, by the analysis of the excitation intensity dependence of the emissions. Consequently, we present the behaviour of B free exciton within a wide range of temperatures. The time‐resolved emissions of A free exciton have also been examined. The decay of the emissions is analyzed using a double exponential curve. Fast and slow components are attributed to nonradiative relaxation and radiative recombination, respectively. The decay‐time constant of the slow component corresponds to the radiative lifetime of A free exciton and is obtained over the wide temperature region until 300 K. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong excitation intensity dependence of the photoluminescence line shape in GaAs1−xBix single quantum well samples

A set of high quality single quantum well samples of GaAs1−xBix with bismuth concentrations not exceeding 6% and well widths ranging from 7.5 to 13 nm grown by molecular beam epitaxy on a GaAs substrate at low temperature is studied by means of photoluminescence (PL). It is shown that the PL line shape changes when the exciton reduced mass behavior changes from an anomalous increase (x < 5%) to a conventional decrease (x > 5%). Strongly non-monotonous PL bandwidth dependence on the excitation intensity is revealed and interpreted in terms of optically unresolved contributions from the saturable emission of bound free excitons.

Mechanistic Study of Bleach-Imaged Plasmon Propagation (BlIPP)

Bleach-imaged plasmon propagation, BlIPP, is a far-field microscopy technique developed to characterize the propagation length of surface plasmon polaritons in metallic waveguides. To correctly extract the propagation length from the measured photobleach intensity, it is necessary to understand the mechanism by which dye photobleaching occurs. In particular, 1- vs 2-photon bleaching reactions yield different propagation lengths based on a kinetic model for BlIPP. Because a number of studies have reported on the importance of 2-photon processes for dye photobleaching, we investigate here the nature of the photobleaching step in BlIPP. We are able to demonstrate that only 1-photon absorption is relevant for typical BlIPP conditions as tested here for a thin film of indocyanine green fluorescent dye molecules coated over gold nanowires and excited at a wavelength of 785 nm. These results are obtained by directly measuring the excitation intensity dependence of the photobleaching rate constant of the dye in the presence of the metallic waveguide.

Room-temperature lasing from fluorene thin-film crystals densely doped with anthracene

The potential for room-temperature lasing in densely doped organic crystals has been examined using (fluorene)0.8-(anthracene)0.2 thin crystals fabricated in a narrow gap between two quartz plates. An excitation-intensity dependence study confirms that the system shows spectral line narrowing, a super linear increase of the narrow line above threshold excitation intensity, and laser cavity modes. These results indicate that lasing has been observed in the densely doped organic crystal, which contains in-plane cavities with a length of approximately 100 μm.

Preparation and time-resolved fluorescence study of RGB organic crystals

In this work, large-size tetracene- and pentacene-doped 1,4-Bis(4-methylstyryl)-benzene (BSB-Me) crystals were prepared with physical vapor transfer technique. The photoluminescence emission can be adjusted from blue to green and even to red by changing the doping. Their fluorescence features are investigated by both the steady-state fluorescence spectra and time-resolved fluorescence measurements, which indicate highly efficient energy transfer from the host BSB-Me to the acceptor. The PL efficiencies of the tetracene-doped BSB-Me and pentacene-doped BSB-Me crystals are as high as 79% and 47%, the energy transfer efficiency is 49.7% and 73.1%, respectively. Excitation intensity-dependent ultrafast dynamics was further probed, which shows that the kinetics in doped crystals exhibit much less excitation intensity dependence than that of the pure BSB-Me crystals. Our results are relevant to the understanding of ultrafast energy-transfer dynamics in doped crystals and are expected to be of interest for organic light emitting transistors, diodes, and lasers.

Photoluminescence spectra of TlInSe2

AbstractPhotoluminescence spectra of TlInSe2 crystals with quasi one‐dimensional structure have been investigated. The observed broad spectra have been found to result from overlapping of two peaks centered at 1.05 and 0.97 eV. Following the results of the examination of the excitation intensity dependence of the observed photoluminescence, the last peaks have been attributed to free‐to‐bound radiative transitions. The activation energy of donor or acceptor levels associated with these emissions has been tentatively evaluated. Furthermore, two sharp peaks located at 1.17 and 1.23 eV have been observed at 10 K in the excitation spectra of the observed photoluminescence (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation intensity dependence of photoluminescence spectra of SiGe quantum dots grown on prepatterned Si substrates: Evidence for biexcitonic transition

The pumping intensity ($I$) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear $I$ dependence of the PL intensity. At approximately $I\ensuremath{\ge}3\phantom{\rule{0.28em}{0ex}}\text{W}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, additional bands with a nearly quadratic $I$ dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

Free-to-bound recombination in near stoichiometric Cu2ZnSnS4single crystals

Cu${}_{2}$ZnSnS${}_{4}$ single crystals prepared by the chemical vapor transport technique were investigated by x-ray diffraction, chemical microprobe analysis, and Raman and photoluminescence spectroscopies. Single-crystal x-ray measurements show that Cu${}_{2}$ZnSnS${}_{4}$ crystallizes in the kesterite structure with the space group $I\overline{4}$ and unit cell parameters a = 5.4174(2) and c = 10.7903(7) $\AA{}$ at 150 K. Photoluminescence measurements show one broad asymmetric band peaked at 1.29 eV at low temperature. Temperature and excitation intensity dependencies of the luminescence band indicate that the transition is due to a free-to-bound recombination with an activation energy of 140 meV. We attribute this activation energy to the intrinsic acceptor-type Cu${}_{\mathrm{Zn}}$ antisite defect in close agreement with results from density-functional theory. It is shown that by a detailed analysis of the temperature dependence of the free-to-bound luminescence transition the majority carrier density can be deduced, which in our case is estimated at $\mathrm{p}\ensuremath{\approx}{10}^{16}$ cm${}^{\ensuremath{-}3}$.

Properties of ultraviolet anti‐Stokes photoluminescence in ZnO single crystals

AbstractUltraviolet (UV) anti‐Stokes photoluminescence (ASPL) was observed in ZnO single crystals at low temperature. The ASPL spectrum was essentially the same as the normal photoluminescence (PL) spectrum under excitation in the band to band transition. By analogy to the ASPL of GaN, a two‐step two‐photon absorption process is suggested to occur in ZnO. The ratio of re‐excited electrons in the intermediate state is, however, smaller than that for GaN from the excitation intensity dependence of the ASPL. This difference is probably due to the carrier lifetime of the intermediate state for ZnO being shorter than that for GaN.