High Excitation Intensities Research Articles

Spectroscopic signature of strain-induced quantum dots created by buried InAs quantum dots in an InGaAs quantum well

Spectroscopic study of self-assembled InAs/GaAs quantum dots (QDs) capped with an InxGa1−xAs quantum well (QW) is carried out under variable excitation intensity and temperature. The QW reduces strain in the QDs, and it is demonstrated that the cap layer shifts the transition energy of the QD lower, while maintaining the strong quantization of the QD electronic states which develops in photoluminescence as a series of excited state transitions at higher excitation intensity. An additional spectral band is also revealed below the energy of the InGaAs QW exciton, which is unambiguously assigned to the emission of a strain-induced QD layer created by the InAs stressors within the InGaAs QW.

Electrically tunable hole tunnelling from a single self-assembled quantum dot embedded in an n-i-Schottky photovoltaic cell

We perform excitation-intensity-dependent measurements of the neutral exciton (X0) photocurrent (PC) peak amplitude from a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode. Since resonant laser-excitation of the X0 transition cannot occur until the comparatively slow hole tunnels out of the QD, we observe a saturation of the PC peak amplitude towards high excitation-intensities, allowing us to determine the hole tunnelling time by fitting with an appropriate theoretical model. By repeating this measurement for a range of bias voltages, we obtain the hole tunnelling time as a function of vertical electric field, showing that it can be tuned by nearly two orders of magnitude. Finally, we find that the hole tunnelling rate can be described accurately by a theoretical model based on a Wentzel-Kramers-Brillouin approximation to yield precise values for the QD height and hole confinement potential.

Enhanced magneto-optical oscillations from two-dimensional hole-gases in the presence of Mn ions

We investigated the effects of nearby Mn ions on the optical properties of two-dimensional hole-gases confined in InGaAs/GaAs quantum wells. We observed energy oscillations on both the averaged emission and the spin-splitting energies, whereas the first one presents maxima at all integer filling factors, and the second one, solely at odd-filling factors. The strength of the oscillations clearly increases with the Mn concentration. Furthermore, considering the relatively low-mobility of our structures, the oscillations are surprisingly strong and robust, persisting up to relatively high temperatures and excitation intensities.

Photoluminescence study of ZnO structures grown by aqueous chemical growth

ZnO micro-structures were deposited by aqueous chemical growth on Si (100) substrates, their morphology and size depending on the growth period. Characterization of the structures was performed using X-ray diffraction, scanning electron microscopy and Raman spectroscopy. Photoluminescence spectra recorded at 18 and 295 K for 325 nm CW excitation indicated that these are strongly affected by the morphology of the structures. Rods and tubes emit stronger UV radiation, in contrast to stronger yellow-green emission observed for flower-like structures. A red shift of the UV emission was found for increasing input power, while, thermal annealing of the samples induced stimulated emission for quite high excitation intensities.

Dynamics of Bound Exciton Complexes in CdS Nanobelts

Intrinsic defects such as vacancies, interstitials, and anti-sites often introduce rich luminescent properties in II-VI semiconductor nanomaterials. A clear understanding of the dynamics of the defect-related excitons is particularly important for the design and optimization of nanoscale optoelectronic devices. In this paper, low-temperature steady-state and time-resolved photoluminescence (PL) spectroscopies have been carried out to investigate the emission of cadmium sulfide (CdS) nanobelts that originates from the radiative recombination of excitons bound to neutral donors (I(2)) and the spatially localized donor-acceptor pairs (DAP), in which the assignment is supported by first principle calculations. Our results verify that the shallow donors in CdS are contributed by sulfur vacancies while the acceptors are contributed by cadmium vacancies. At high excitation intensities, the DAP emission saturates and the PL is dominated by I(2) emission. Beyond a threshold power of approximately 5 μW, amplified spontaneous emission (ASE) of I(2) occurs. Further analysis shows that these intrinsic defects created long-lived (spin triplet) DAP trap states due to spin-polarized Cd vacancies which become saturated at intense carrier excitations.

Efficient cerium-based sol–gel derived phosphors in different garnet matrices for light-emitting diodes

Efficiency of sol–gel derived Y 3− x Al 5O 12:Ce x 3+ (YAG:Ce) and Y 3− x Mg 2AlSi 2O 12:Ce x 3+ (YMASG:Ce) phosphors, which are prospective for application in white light emitting diodes (LED), is studied. Sets of samples containing different cerium amount x from 0.015 to 0.06 and sintered at different temperatures (1400–1550 °C) were investigated. Importance of absorption peculiarities in agglomerates of phosphor nanocrystals is demonstrated by studying the excitation wavelength dependence of quantum efficiency and by applying PL measurements in confocal mode. Emission saturation is demonstrated to occur at higher excitation intensities than those typical for operating white LEDs.

Quantitative TCSPC FRET-FLIM Applied to Donors Exhibiting Multi-Exponential Decays: Spatio-Temporal Interaction Between Calmodulin and the Kv7.2 Potassium Channel in Living Cells

The spatio-temporal study of the dynamics of molecular interactions using FRET-FLIM is generally compromised by the large number of photons required to fit the multiple-lifetime decay of the donor population in each pixel of an image. Long acquisitions prevent interacting dynamics to be detected in an image, while the use of high excitation intensities results in artifactual measurements due to bleaching. The computation of the minimal fraction of donor molecules (mfD) undergoing FRET allows quantitative imaging of molecular interactions with either single or multi-lifetime donors such as CFP, where the complexity associated to fitting a fluorescent decay with more than two components hampers quantification with traditional least-squares fitting strategies. This novel non-fitting analysis has been recently applied to wide-field time gated FLIM systems and we now extend it to more widely available TCSPC systems.The use of mfD analysis has allowed us to study the spatio-temporal dynamics of the interaction between CFP-tagged Kv7.2 channels (donor) and YFP-labeled calmodulin (acceptor) in HEK293 living cells on a TCSPC system. We show the existence of discrete interacting domains that can be followed as a function of time (every 12-15s), where the fraction of interacting KCNQ2 with calmodulin oscillates at least between 15% and 25%.In summary, we have shown that the mfD analysis allows quantitative study of the spatio-temporal dynamics of molecular interactions on TCSPC systems. Using this non-fitting strategy we have quantitatively imaged the interacting dynamics of CFP-tagged Kv7.2 channels and YFP-labeled calmodulin in living cells.

Stimulated emission from anthracene aggregates in fluorene crystals fabricated by cell method

AbstractIn fluorene(80%)–anthracene(20%) mixed crystals, mainly observed luminescence is broad D2 bands, which originates from an excitation state localized on two anthracene molecules and is accompanied by lattice relaxation. Under high intensity excitation, several sharp luminescence lines emerged in the high energy side of the D2 0–1 bands and their intensity increased superlinearly with excitation intensity. The decay time of the luminescence lines is 30 ps, which is two orders of magnitude smaller than decay time under low intensity excitation. These results suggest that sharp luminescence lines are stimulated luminescence from D2 centers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and Lasing Properties of ZnO Nanorods

In this study, we investigated the structures, photoluminescence (PL), and lasing characteristics of the ZnO nanorods prepared by using chemical bath deposition. The continuous-wave HeCd laserexcited PL spectra of the ZnO nanorods exhibited two emission bands, one in the UV region and the other in the visible region. The UV emission band has its peak at 3.25 eV with a bandwidth of 160 meV. However, the PL spectra under 355-nm, 35-ps pulse excitation exhibited a spectrally-narrowed UV emission band with a peak at 3.20 eV and a spectral width of 35 meV. The lasing phenomena were ascribed to the amplified spontaneous emission (ASE) caused by coupling of the microcavity eect of ZnO nanorods and the high-intensity excitation. Above the lasing threshold, the ASE peak intensity exhibited a superlinear dependence on the excitation intensity. For an excitation pulse energy of 3 mJ, the ASE peak intensity was increased by enlarging the length of the ZnO nanorods from 1 µm to 4 µm. In addition, the PL spectrum under 800-nm femtosecond pulse excitation exhibited second harmonic generation, as well as the multiphoton absorption-induced UV emission band. In this research, ZnO nanorods were grown on seed layers by using chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. The seed layers were prepared on conducting glass substrates by dip coating in an aqueous colloidal dispersion containing 50% 70nm ZnO nanoparticles. Scanning electron microscopy clearly revealed that ZnO nanorods were successfully grown on the seed layers.

Specific features of the mechanisms of excitation of erbium photoluminescence in epitaxial Si:Er/Si structures

The excitation spectra and kinetics of erbium photoluminescence and silicon interband photoluminescence in Si:Er/Si structures under conditions of high-intensity pulse optical excitation are studied. It is shown that, in the interband photoluminescence spectra of the Si:Er/Si structures, both the luminescence of free excitons and the emission associated with the electron-hole plasma can be observed, depending on the excitation power and wavelength. It is found that the formation of a peak in the erbium photoluminescence excitation spectra at high pumping powers correlates with the Mott transition from the exciton gas to the electron-hole plasma. It is demonstrated that, in the Si:Er/Si structures, the characteristic rise times of erbium photoluminescence substantially depend on the concentration of charge carriers.

Exciton-exciton annihilation in organic polariton microcavities

We investigate the incoherent diffusion of excitons in thin films ($5.1\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{nm}$ thick) of a highly absorbing J-aggregated cyanine dye material (${10}^{6}\text{ }{\text{cm}}^{\ensuremath{-}1}$ absorption constant) as the excitonic component of a polariton microcavity. Under high-intensity pulsed laser excitation, the J-aggregated molecular films exhibit significant exciton-exciton annihilation, indicating a large exciton diffusion radius of more than 100 nm. When the material is strongly coupled to a cavity, the polaritonic structure also shows exciton-exciton annihilation, which is a competing process against the establishment of a threshold population of polaritons needed for polariton lasing. This study suggests that exciton-exciton annihilation is a loss process which can significantly increase the lasing threshold in polariton microcavities.

On the Nature of the “Dark S*” Excited State of β-Carotene

Femtosecond transient absorption spectroscopy has been applied to the isolated carotenoid β-carotene under a large variety of experimental conditions regarding solvent, temperature, excitation wavelength, and intensity to study the excited state relaxation dynamics in order to elucidate the origin of the so-called "dark S* state", which has been discussed very controversially in the literature. The results are analyzed in terms of lifetime density maps, and various kinetic models are tested on the data. The sample purification was found to be critical. The appearance of a component with a lifetime longer than that of the relaxed S(1) state (i.e., τ > 10 ps), which has been associated previously with the S* (or S(‡)) state is due to the presence of an impurity. For pure samples, four lifetimes are typically observed (all ≤10 ps at room temperature). Consideration of the large body of experimental data leads us to exclude relaxation schemes implying a separate "dark S* state" in β-carotene formed in parallel to the normal S(2) → S(1) relaxation scheme. Vibrational cooling in the S(1) state can explain fully all the features of the transient spectra on the picosecond time scale within a S(2) → S(1v) → S(1v') → S(1) → S(0) relaxation scheme without invoking any additional electronic or distinctly different conformational states. Thus, we exclude assignments of the previously reported "S* state" signals in β-carotene (i) to require the postulate of a separate electronic state, (ii) to require the postulate of a large conformational change and/or a partial cis configuration formed in the relaxation pathway, or (iii) to require a vibrationally excited ground state (GS) species. High intensity excitation leads in part to a two-photon excitation to the S(2N) state which upon relaxation gives rise to a different vibrational excitation pattern in the initially created hot S(1) state(s). The spectral changes in the S(1v) state observed upon both short wave excitation as well as high intensity excitation can be explained well by such a modified vibrational excitation pattern. In contrast, the variations in the difference spectra of the partially (S(1v')) and fully vibrationally relaxed S(1) states (S(1)) are minor. The data do not provide any evidence that would require one to postulate the existence of a separate "S* state".

Measurement of fluorescence emission spectrum of few strongly driven atoms using an optical nanofiber

We show that the fluorescence emission spectrum of few atoms can be measured by using an optical nanofiber combined with the optical heterodyne and photon correlation spectroscopy. The observed fluorescence spectrum of the atoms near the nanofiber shows negligible effects of the atom-surface interaction and agrees well with the Mollow triplet spectrum of free-space atoms at high excitation intensity.

Two-photon spectral hole burning spectroscopy of InAs/GaAs quantum dots

Femtosecond spectral hole burning experiments were performed on InAs/GaAs quantum dots to investigate the influence of high excitation intensities. The observation of additional spectral holes, as well as distinct absorption line broadening are both attributed to two-photon absorption processes. These spectral holes can be unambiguously assigned to degenerate two-photon biexciton creation, whereas the absorption line broadening is a direct result of Coulomb renormalization, initiated by carriers created through two-photon absorption in the surrounding GaAs crystal matrix and the InAs wetting layer.

High-density excitation effect on photoluminescence in ZnO nanoparticles

In this study, photoluminescence (PL) under high excitation intensity as a function of crystalline size was systematically investigated through ZnO nanocrystalline films prepared by spin-coating a colloidal solution of ZnO nanoparticles obtained using the microemulsion method. Annealing of the films at 723, 633, and 593 K allowed us to tune the crystalline radius R. PL studies distinguished different regimes of crystalline size according to the ratio of R to the effective Bohr radius aB(R/aB). For the sample annealed at 723 K (R/aB=7.2), the peak of stimulated emission due to the exciton-exciton collisions appeared on the low-energy side of the exciton emission with an increase in excitation intensity. A further increase in excitation intensity eventually resulted in the occurrence of an electron-hole plasma (EHP) accompanied by consequent band gap renormalization, which indicates that high excitation intensity provokes the dissociation of excitons. For the sample annealed at 633 K (R/aB=4.7), the stimulated emission was observed while the transition to EHP was obscure. For the sample annealed at 593 K (R/aB=2.1), only emissions due to the recombination of the electron-hole pair were observed, and stimulated emission did not appear even when the excitation intensity was increased. The transition from free-exciton emission to donor-bound exciton emission was observed in temperature dependence of PL only for the sample with R/aB=7.2. The origin of annihilation of the stimulated emission with a size reduction is discussed based on nonradiative Auger recombination.

Multicarrier Recombination and Energy Transfer in Mn-Doped CdS Nanocrystals Studied by Femtosecond Pump–Probe Spectroscopy

We studied the photocarrier decay dynamics of Mn-doped CdS nanocrystals (NCs) coated with a ZnS shell layer by femtosecond pump–probe transient absorption spectroscopy. At low excitation intensities, the decay dynamics of photocarriers is determined by energy transfer from electron–hole pairs in CdS NCs to Mn ions. At high excitation intensities, the photocarrier decay curves of Mn-doped CdS NCs are very similar to those of undoped CdS NCs, and they are determined by the fast Auger recombination of photocarriers. These density-dependent photocarrier behaviors in CdS NCs are confirmed by Mn 2+ luminescence spectroscopy.

Amplified Pulse Emissions with Variable Delay Times in Vibronic Transition Bands of Thiophene/Phenylene Co-Oligomer Single Crystals

Time-resolved photoluminescence of thiophene/phenylene co-oligomer (TPCO) single crystals has been measured at 7 K. The amplified pulse emissions with time delay are demonstrated in the 0–2 vibronic transition band of the TPCO single crystal under high-intensity excitation. We identify the delayed pulse emission to the 2A1 vibration line. The pulse peak of the 2A1 line exhibits a maximum delay time of 70 ps at 7 K. We consider that the amplified pulse emissions with time delay in single crystals occur at a vibronic transition band with the largest spontaneous emission intensity in the PL spectrum under weak excitation.

Exciton-exciton annihilation in organic lanthanide complexes

We have investigated the mechanism of exciton-exciton annihilation in organic lanthanide complexes by measuring the excitation density dependence of photoluminescence yield in neat films of a model europium chelate. The observed luminescence efficiency reduction at high excitation intensities has been attributed to the operation of the mutual annihilation of the ligand singlet excitons. The second order interaction rate constant, the exciton diffusivity, and the singlet exciton diffusion length in this material have been determined. These results were used to evaluate the role of exciton annihilation in organic light emitting diodes, lanthanide-based organic lasers, and light up-converters. Indications concerning the optimization of the performances of such devices have also been given.

High-Resolution smFRET in a Microfluidic Gas Exchange Platform

Single-molecule Forster Resonance Energy Transfer (smFRET) is a powerful technique providing new insights in the physics and chemistry of bio-molecules. The smFRET signal quality depends on the photon-flux that can be harvested from the fluorescent dyes. Mere use of high excitation intensities produces counter-productive results, mainly because oxygen-mediated photobleaching of the fluorophores rapidly destroys useful FRET signals. Here, we developed a microfluidic device that ensures a large decrease in the oxygen content of buffers in-situ, allowing us to substantially reduce photobleaching even at high laser powers and obtain dramatic enhancement of signal for several dye-pairs. The principle of the deoxygenation is simple: the microchannels carrying the samples are flanked by large channels ventilated by nitrogen; oxygen is removed by molecular diffusion through porous walls. The device combines this deoxygenation with on-chip mixing and generation of dual-component triplet quenchers (increasing the burst brightness), and also has laminar-flow mixing for kinetic studies.The increased photon flux obtained in the flow device leads to resolution improvements in two key dimensions: (i) it reduces considerably the time (by a factor of ∼ 10 from the typical 500μs) needed to collect high-quality FRET signal, thus providing higher time-resolution, and (ii) it allows the use of higher thresholds, which significantly reduces the width of the smFRET distributions and allows a better resolution of molecular subpopulations. In contrast with a popular enzymatic oxygen scavenger, the method can be used for de-oxygenation in denaturing conditions, hence enabling studies of protein folding. Overall, the platform combines multiple enabling features that can accommodate a range of equilibrium and kinetic and biochemical experiments, along with a simple construction and fabrication, and robust operation. Together, these features have the potential to make it useful for a broad variety of single-molecule fluorescence experiments.

Grating-Assisted Spectrally-Narrowed Emissions from an Organic Slab Crystal Excited with a Mercury Lamp

We report spectrally-narrowed emissions that take place from an organic semiconductor slab crystal of 2,5-bis(4-biphenylyl)thiophene (BP1T) under a low excitation-intensity regime. These emissions are caused with a mercury lamp that operates on a household power supply with an electric current approximately 1 A. The BP1T slab crystal is equipped with a distributed Bragg reflector. To complete this structure the slab crystal is attached to a diffraction grating that is engraved on a surface of a quartz glass substrate. The diffraction gratings have precisely been formed using a focused ion beam with a nanometer-defined precision. The spectral narrowing accompanied by the emission intensity increment is related to the strong mode-coupling between the forward electromagnetic wave and the backward (i.e., reflected) wave within the grating zone. Using a laser we also carried out the emission measurements on the BP1T crystals under a high excitation-intensity regime. The emissions are characterized as the longitudinal multimode laser oscillation, enabling us to determine the group refractive index of 4.56 for the BP1T slab crystal. Under both the low and high excitation-intensity regimes excitons are dominant species of the emission. Their participation in the spectrally-narrowed emissions is briefly discussed.