Low Excitation Intensity Research Articles

Exciton binding and excitonic transition energies in wurtzite Zn1-xCdxO/ZnO quantum wells

Binding energy of the 1s exciton state in the Zn1-xCdxO/ZnO quantum wells, changing between 10 and 50 Å in width, has been calculated for x up to 0.2. The exciton wave function is represented in terms of hydrogenic wave functions and the effects of dielectric mismatch, exciton-phonon interactions and built-in electric field on the binding energy are considered. The calculated built-in electric field is around 19.6x MV cm−1 in a 20-Å Zn1-xCdxO/ZnO quantum well. The numerical results indicate that the exciton binding energy stays below its bulk value virtually for all x because of the built-in electric field dominating the quantum confinement effect. The binding energy is nearly 26 meV at maximum, barely above its bulk value, and about 13 meV at minimum. The contribution of the dielectric mismatch is up to 40% while that of the exciton-phonon interactions is negligible. The exciton binding energy increases up to 66 meV, well above the bulk value, in case the built-in electric field is neglected. A qualitative agreement between the calculated excitonic transition energies and the low temperature and low excitation intensity photoluminescence peak data is obtained although some ZnCdO material properties, such as low temperature band gap, are not available.

Carrier dynamics in hybrid nanostructure with electronic coupling from an InGaAs quantum well to InAs quantum dots

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In0.15Ga0.85As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures.

A seesaw-type approach for enhancing nonlinear energy harvesting

Harvesting sustainable mechanical energy is the ultimate objective of nonlinear energy harvesters. However, overcoming potential barriers, especially without the use of extra excitations, poses a great challenge for the development of nonlinear generators. In contrast to the existing methods, which typically modify the barrier height or utilize additional excitations, this letter proposes a seesaw-type approach to facilitate escape from potential wells by transfer of internal energy, even under low-intensity excitation. This approach is adopted in the design of a seesaw-type nonlinear piezoelectric energy harvester and the energy transfer process is analyzed by deriving expressions for the energy to reveal the working mechanism. Comparison experiments demonstrate that this approach improves energy harvesting in terms of an increase in the working frequency bandwidth by a factor of 60.14 and an increase in the maximum output voltage by a factor of 5.1. Moreover, the output power is increased by a factor of 51.3, which indicates that this approach significantly improves energy collection efficiency. This seesaw-type approach provides a welcome boost to the development of renewable energy collection methods by improving the efficiency of harvesting of low-intensity ambient mechanical energy.

Confocal Microscopy Imaging with an Optical Transition Edge Sensor

Fluorescence color imaging at an extremely low excitation intensity was performed using an optical transition edge sensor (TES) embedded in a confocal microscope for the first time. Optical TES has the ability to resolve incident single photon energy; therefore, the wavelength of each photon can be measured without spectroscopic elements such as diffraction gratings. As target objects, animal cells labeled with two fluorescent dyes were irradiated with an excitation laser at an intensity below $$1\,\upmu \hbox {W}$$ . In our confocal system, an optical fiber-coupled TES device is used to detect photons instead of the pinhole and photomultiplier tube used in typical confocal microscopes. Photons emitted from the dyes were collected by the objective lens, and sent to the optical TES via the fiber. The TES measures the wavelength of each photon arriving in an exposure time of 70 ms, and a fluorescent photon spectrum is constructed. This measurement is repeated by scanning the target sample, and finally a two-dimensional RGB-color image is obtained. The obtained image showed that the photons emitted from the dyes of mitochondria and cytoskeletons were clearly resolved at a detection intensity level of tens of photons. TES exhibits ideal performance as a photon detector with a low dark count rate ( $$<\,1$$ Hz) and wavelength resolving power. In the single-mode fiber-coupled system, the confocal microscope can be operated in the super-resolution mode. These features are very promising to realize high-sensitivity and high-resolution photon spectral imaging, and would help avoid cell damage and photobleaching of fluorescence dyes.

A low-frequency, wideband quad-stable energy harvester using combined nonlinearity and frequency up-conversion by cantilever-surface contact

Abstract This paper proposes models and experiments of a wideband piezoelectric vibration energy harvester with a quadruple-well potential induced by the combined nonlinearity of cantilever-surface contact and magnetoelasticity. The cantilever-surface contact introduces frequency up-conversion mechanism into the present energy harvester and provides favorable mechanical properties such as minimal moving parts, low frictional losses and high shock resistance. The magnetoelasticity, combined with the cantilever-surface contact, can make the present energy harvester exhibit multi-stability, ranging from mono-stability to quad-stability. The formation mechanism of this multi-stability is investigated by a static bifurcation analysis to the harvester’s mathematical model. Dynamic responses of the present energy harvester with a quadruple-well potential are explored by numerical simulations and validated by experiments. The results show that the combined nonlinearity can not only improve the efficiency of electrical power transfer under low intensity excitations, but also extend the wide operating band of harvester to low frequencies.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99.

Photoexcited electron injection dynamics from CsPbI3 quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI3 QDs can be injected into TiO2 with a size-dependent rate ranging from 1.30 × 1010 to 2.10 × 1010 s-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI3/TiO2 electrode is fabricated that delivers a power conversion efficiency of 5%.

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10−5 and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 μs, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime.

Recent advances in the application triplet–triplet annihilation-based photon upconversion systems to solar technologies

Upconversion (UC) of low-energy photons into high-energy light can in principle increase the efficiency of solar devices by converting photons with energies below the energy absorption threshold into radiation that can be utilized. Among UC mechanisms, the sensitized triplet–triplet annihilation-based upconversion (sTTA-UC) is the most recent that would be applied in solar technologies. sTTA-UC was demonstrated using sunlight in 2006, and due to its high efficiency at low excitation intensities with noncoherent light, it is considered a promising strategy for the recovering of subbandgap photons. Here, we describe briefly the working principle of the sTTA-UC, and we review the most recent advances of its use in solar applications.

Direct observation of sequential oxidations of a titania-bound molecular proxy catalyst generated through illumination of molecular sensitizers.

Natural photosynthesis uses the energy in sunlight to oxidize or reduce reaction centres multiple times, therefore preparing each reaction centre for a multiple-electron-transfer reaction that will ultimately generate stable reaction products. This process relies on multiple chromophores per reaction centre to quickly generate the active state of the reaction centre and to outcompete deleterious charge recombination. Using a similar design principle, we report spectroscopic evidence for the generation of a twice-oxidized TiO2-bound molecular proxy catalyst after low-intensity visible-light excitation of co-anchored molecular Ru(II)-polypyridyl dyes. Electron transfer from an excited dye to TiO2 generated a Ru(III) state that subsequently and repeatedly reacted with neighbouring Ru(II) dyes via self-exchange electron transfer to ultimately oxidize a distant co-anchored proxy catalyst before charge recombination. The largest yield for twice-oxidized proxy catalysts occurred when they were present at low coverage, suggesting that large dye/electrocatalyst ratios are also desired in dye-sensitized photoelectrochemical cells.

Sub-picosecond temporal resolution of anomalous Hall currents in GaAs

The anomalous Hall (AH) and spin Hall effects are important tools for the generation, control, and detection of spin and spin-polarized currents in solids and, thus, hold promises for future spintronic applications. Despite tremendous work on these effects, their ultrafast dynamic response is still not well explored. Here, we induce ultrafast AH currents in a magnetically-biased semiconductor by optical femtosecond excitation at room temperature. The currents’ dynamics are studied by detecting the simultaneously emitted THz radiation. We show that the temporal shape of the AH currents can be extracted by comparing its THz radiation to the THz radiation emitted from optically induced currents whose temporal shape is well known. We observe a complex temporal shape of the AH currents suggesting that different microscopic origins contribute to the current dynamics. This is further confirmed by photon energy dependent measurements revealing a current inversion at low optical excitation intensities. Our work is a first step towards full time resolution of AH and spin Hall currents and helps to better understand the underlying microscopic origins, being a prerequisite for ultrafast spintronic applications using such currents.

Dependence of Nonlinear Optical Response of Anatase TiO2 on Shape and Excitation Intensity**Supported by the National Natural Science Foundation of China under Grant Nos 11404410 and 11504105.

Nonlinear optical (NLO) properties of anatase TiO2 with nanostructures of nanoparticle (NP), nanowire (NW) and annealed nanowire (NWA) are studied by open-aperture and closed-aperture Z-scan techniques with a femtosecond pulsed laser at wavelengths of 532 nm and 780 nm simultaneously. At 532 nm, when increasing excitation intensity, NLO absorption of TiO2 NPs transforms from saturable absorption to reverse-saturable absorption. However, NWs and NWAs exhibit the opposite change. At 780 nm, all samples show reverse-saturable absorption, but have different sensitivities to excitation intensity. Due to the larger surface-to-volume ratio of NPs and less defects of NWAs by annealing, nonlinear optical absorption coefficients follow the order NPs NWs NWAs. The results also show that these shape and annealing effects are dominant at low excitation intensity, but do not exhibit at the high excitation intensity. The NLO refractive index of NPs shows a positive linear relationship with the excitation intensity, whereas NW and NWAs exhibit a negative linear relationship. The results could provide some foundational guidance to applications of anatase TiO2 in optoelectronic devices or other aspects.

One, two and three photon absorption of two level system in femto-second laser excitation

Theoretically, we have studied the multi-photon absorption by rate equation approach in the presence of femto-second laser excitation. The time dependent and saturation effects in the nonlinear transmittance are incorporated as they will occur in the absorption process. At low intensity excitation, the transmitted pulse shape has the same shape as incident pulse, but at excitation intensities above the saturation intensity, the shape of the transmitted pulse has got change according to the absorption coefficient variation with intensity. Transmittance in the two and three photon absorption has discussed comparatively in both single beam transmittance and z-scan. In the nonlinear absorption study under femto-second laser excitation, the high peak power of femto-second laser can saturate the absorption in multi-photon absorption even though the process is through virtual levels. We have shown that, at excitation intensities above the saturation intensity, how the absorption coefficient deviates the logarithmic plot of transmittance from its straight line nature.

Superresolution via structured illumination quantum correlation microscopy

We propose to use intensity correlation microscopy in combination with structured illumination to image quantum emitters that exhibit antibunching with a spatial resolution reaching far beyond the Rayleigh limit. Combining intensity measurements and intensity auto correlations up to order $m$ creates an effective PSF with FWHM shrunk by the factor $\sqrt{m}$. Structured Illumination microscopy on the other hand introduces a resolution improvement of factor 2 by use of the principle of moir\'e fringes. Here, we show that for linear low-intensity excitation and linear optical detection the simultaneous use of both techniques leads to an in theory unlimited resolution power with the improvement scaling favorably as $m + \sqrt{m}$ in dependence of the correlation order $m$. Hence, yielding this technique to be of interest in microscopy for imaging a variety of samples including biological ones. We present the underlying theory and simulations demonstrating the highly increased spatial superresolution, and point out requirements for an experimental implementation.

Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission.

Organo-metal halide perovskites are a promising platform for optoelectronic applications in view of their excellent charge-transport and bandgap tunability. However, their low photoluminescence quantum efficiencies, especially in low-excitation regimes, limit their efficiency for light emission. Consequently, perovskite light-emitting devices are operated under high injection, a regime under which the materials have so far been unstable. Here we show that, by concentrating photoexcited states into a small subpopulation of radiative domains, one can achieve a high quantum yield, even at low excitation intensities. We tailor the composition of quasi-2D perovskites to direct the energy transfer into the lowest-bandgap minority phase and to do so faster than it is lost to nonradiative centers. The new material exhibits 60% photoluminescence quantum yield at excitation intensities as low as 1.8 mW/cm2, yielding a ratio of quantum yield to excitation intensity of 0.3 cm2/mW; this represents a decrease of 2 orders of magnitude in the excitation power required to reach high efficiency compared with the best prior reports. Using this strategy, we report light-emitting diodes with external quantum efficiencies of 7.4% and a high luminescence of 8400 cd/m2.

Triplet sensitization by perovskite nanocrystals for photon upconversion.

The potential of three-dimensional (3D) metal-halide perovskites to sensitize organic triplets is unveiled. Nanocrystals of surface-modified inorganic cesium lead halide perovskites (CsPbX3, X = Br/I) are found to work as efficient triplet sensitizers for photon upconversion based on triplet-triplet annihilation (TTA-UC) at low excitation intensity.

The effect of the excitation light intensity during on-line monitoring of biodiesel by fluorescence spectroscopy

In the last years, several methods based on fluorescence spectroscopy have been proposed to investigate different aspects related to biodiesel industry for presenting some advantages such as simplicity, reliability, low-cost and real-time analysis. Nevertheless, the excitation light, depending on its beam intensity, may induce by itself sample degradation during the monitoring due to the low oxidative stability of the biodiesel, providing misleading results. The present study shows for the first time a detailed analysis about how excitation light intensity may distort biodiesel analysis based on fluorescence measurements. The results revealed that fluorescence may be successfully used for on-line and in situ monitoring of biodiesel conditions as long as low excitation intensity interacts with a small sample volume in order to avoid biodiesel photodegradation.

Nature of relaxation processes revealed by the action signals of intensity-modulated light fields

We present a generalized theory and experimental results of the action signals induced by the absorption of two photons from two phase-modulated laser beams. In our experiment, the phases of the laser beams are modulated at the frequencies ${\ensuremath{\phi}}_{1}$ and ${\ensuremath{\phi}}_{2}$, respectively. Their collinear combination leads to the modulation of the total intensity at the frequency $\ensuremath{\phi}=\left|{\ensuremath{\phi}}_{2}\ensuremath{-}{\ensuremath{\phi}}_{1}\right|$. The action signals, such as photoluminescence and photocurrent, which result from the absorption of two photons, are isolated at frequencies $m\ensuremath{\phi}, m\ensuremath{\in}{0,1,2,\ensuremath{\cdots}}$. We demonstrate that the ratio of the amplitudes of the secondary ($m=2$) and the primary ($m=1$) signals ${A}_{2\ensuremath{\phi}}:{A}_{\ensuremath{\phi}}$ is sensitive to the type of relaxation processes in the media. Such sensitivity originates from the cumulative effects of the nonequilibrated state of the matter between the pulses. When the cumulative effects are small, i.e., the relaxation time is much shorter than the laser repetition rate or the laser intensity is high enough to dominate the system behavior, the ratio achieves the reference value of $1:4$. This ratio decreases monotonically as the relaxation time increases. Our experimental results from fluorescent molecules rhodamine 6G and rubrene support these theoretical findings. In the case of a second-order relaxation process, the ratio changes rapidly with the excitation intensity. When the recombination rate in the second-order process is significantly slower than the repetition rate of the laser, we observe nonmonotonic behavior of the ratio as a function of excitation population at low excitation intensity, and when the recombination rate and the excitation intensity are high, the ratio approaches the value of $1:4$. We also use the model to determine the value of the recombination rate of charge carriers in a GaP photodiode.

Acceleration demand of the outer-skin curtain wall system of the Shanghai Tower

The unique complexities of the outer-skin curtain wall (CW) system of the Shanghai Tower increase the difficulty in evaluating its seismic performance. To assess such seismic performance, it is important to understand the floor acceleration demand under expected earthquake actions. Acceleration demand of the CW system consists of floor acceleration amplification (FAA) and floor response spectrum (FRS), which are closely related to the equivalent static seismic design force and the dynamic properties of the CW system. For estimating the FAA and FRS, code spectra compatible ground motions are selected and input to a finite element model of the building structure. The floor responses of key stories are analyzed. Normalized distribution of the horizontal FAA demand is estimated and shows that the obtained values exceed those proposed by current code provisions for low-intensity earthquake excitations. It is noted that the vertical FAA demand has a different distribution profile than the horizontal one. The results indicate that FAA demand under three-dimensional earthquake excitation is larger than that for 1D excitation. Moreover, the prime period range is up to 4.0 s for the horizontal FRS and up to 1.0 s for the vertical FRS. Horizontal and vertical FRS is proposed for dynamic analysis of the outer-skin CW system of the Shanghai Tower.

Image recombination transform algorithm for superresolution structured illumination microscopy.

.Structured illumination microscopy (SIM) is an attractive choice for fast superresolution imaging. The generation of structured illumination patterns made by interference of laser beams is broadly employed to obtain high modulation depth of patterns, while the polarizations of the laser beams must be elaborately controlled to guarantee the high contrast of interference intensity, which brings a more complex configuration for the polarization control. The emerging pattern projection strategy is much more compact, but the modulation depth of patterns is deteriorated by the optical transfer function of the optical system, especially in high spatial frequency near the diffraction limit. Therefore, the traditional superresolution reconstruction algorithm for interference-based SIM will suffer from many artifacts in the case of projection-based SIM that possesses a low modulation depth. Here, we propose an alternative reconstruction algorithm based on image recombination transform, which provides an alternative solution to address this problem even in a weak modulation depth. We demonstrated the effectiveness of this algorithm in the multicolor superresolution imaging of bovine pulmonary arterial endothelial cells in our developed projection-based SIM system, which applies a computer controlled digital micromirror device for fast fringe generation and multicolor light-emitting diodes for illumination. The merit of the system incorporated with the proposed algorithm allows for a low excitation intensity fluorescence imaging even less than , which is beneficial for the long-term, in vivo superresolved imaging of live cells and tissues.

Non-photochemical fluorescence quenching in photosystem II antenna complexes by the reaction center cation radical

In direct experiments, rate constants of photochemical (kP) and non-photochemical (kP(+)) fluorescence quenching were determined in membrane fragments of photosystem II (PSII), in oxygen-evolving PSII core particles, as well as in core particles deprived of the oxygen-evolving complex. For this purpose, a new approach to the pulse fluorometry method was implemented. In the "dark" reaction center (RC) state, antenna fluorescence decay kinetics were measured under low-intensity excitation (532nm, pulse repetition rate1Hz), and the emission was registered by a streak camera. To create a "closed" [P680(+)QA(-)] RC state, a high-intensity pre-excitation pulse (pump pulse, 532nm) of the sample was used. The time advance of the pump pulse against the measuring pulse was 8ns. In this experimental configuration, under the pump pulse, the [P680(+)QA(-)] state was formed in RC, whereupon antenna fluorescence kinetics was measured using a weak testing picosecond pulsed excitation light applied to the sample 8ns after the pump pulse. The data were fitted by a two-exponential approximation. Efficiency of antenna fluorescence quenching by the photoactive RC pigment in its oxidized (P680(+)) state was found to be ~1.5 times higher than that of the neutral (P680) RC state. To verify the data obtained with a streak camera, control measurements of PSII complex fluorescence decay kinetics by the single-photon counting technique were carried out. The results support the conclusions drawn from the measurements registered with the streak camera. In this case, the fitting of fluorescence kinetics was performed in three-exponential approximation, using the value of τ1 obtained by analyzing data registered by the streak camera. An additional third component obtained by modeling the data of single photon counting describes the P680(+)Pheo(-) charge recombination. Thus, for the first time the ratio of kP(+)/kP= 1.5 was determined in a direct experiment. The mechanisms of higher efficiency for non-photochemical antenna fluorescence quenching by RC cation radical in comparison to that of photochemical quenching are discussed.