Low Excitation Intensity Research Articles

Cascade of Multiexciton States Generated by Singlet Fission.

Identifying multiexciton states generated from singlet fission is key to understanding the carrier multiplication process, which presents a strategy for improving the efficiency of photovoltaics and bioimaging. Broadband optically detected magnetic resonance (ODMR) is a sensitive technique to detect multiexciton states. Here, we report a dominant species a weakly exchange coupled triplet pair located on adjacent molecules oriented by nearly 90° (V2) under intense light excitation, contrasting to the π-stacked triplet pair under low excitation intensity. The weakly coupled species model precisely reproduces the intricate spin transitions in the Hilbert space of the triplet pair. Combining the magneto-photoluminescence and high-magnetic-field ODMR, we also identify a strongly exchange-coupled state of three triplet excitons formed by photoexcited V2, which manifests through the magnetic-field-induced level crossings between its quintet and triplet manifolds. Our findings demonstrate the microscopic picture of different multiexciton states and the possible transitions among them during exciton fission, which provide insight for the further molecular designs for fission materials.

Imaging Brain Tissue with Quantum Light at Low Power.

Light-induced tissue damage is a crucial limitation for traditional microscopy of the living brain, underscoring the need for new techniques that minimize exposure of samples to light. Here, we tested the hypothesis that quantum light, i.e., entangled photons, could detect brain structures at a lower excitation energy. In a proof of principle, we show microscopic images of fixed brain tissue in the hippocampus area created by fluorescence selective excitation in the process of entangled two-photon absorption in a scanning microscope. Quantum-enhanced entangled two-photon microscopy (TPM) had brain imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, orders of magnitude lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled TPM is of critical importance in the investigation of neural activity to minimize heating and photobleaching during repetitive imaging. It may have important functional implications in optogenetic technology, removing unintended heating and accumulated photodamage effects. This technology also opens avenues in spatially resolved brain tissue investigations with quantum light, providing new capabilities in local spectroscopy.

Triplet-Triplet Annihilation-Based Photon Upconversion with a Macrocyclic Parallel Dimer.

The integration of multiple chromophore units into a single molecule is expected to improve the performance of photon upconversion based on triplet-triplet annihilation (TTA-UC) that can convert low energy photons to higher energy photons at low excitation intensity. In this study, a macrocyclic parallel dimer of 9,10-diphenylanthracene (DPA) with a precisely parallel orientation, named MPD-2, is synthesized, and its TTA-UC properties are investigated. MPD-2 shows a green-to-blue TTA-UC emission in the presence of a triplet sensitizer, platinum octaethylporphyrin (PtOEP). Compared to monomeric DPA, MPD-2 results in an enhancement of the spin statistical factor of TTA and a decrease in the excitation light intensity due to the intramolecular TTA process. The obtained structure-property relationship provides important information for the further improvement of TTA-UC properties.

Characterizing the free-vibration response of a two-span rocking bridge with free-standing columns

For accelerated bridge construction (ABC), replacing traditional emulative connections of precast elements with simpler discontinuities may lead to more resilient structures with reduced expected damages after strong seismic events. This paper comprises novel free-vibration tests of a 1:10 scale two-span rocking bridge with free-standing columns with proper geometrical and dynamic similarity. The two spans induced in the model the interaction between frames with different tributary masses, as would be expected for a realistic bridge. To excite the bridge uniformly, the experimental protocol consisted of forty-nine incremental sinusoidal ground motions that were interrupted, letting the bridge vibrate freely to rest. The results were compared with six analytical models. The results showed that the structure remained undamaged for the free-vibration tests after drifts up to 6 % and did not wobble unrestricted. The results were used to obtain the dynamic properties of the system and their variation to displacement increments. Construction imperfections induced the model to rock for all excitations, including low-intensity excitations, and to vibrate with periods different from those of theoretical equations at low displacements. This behavior was critical in the comparison between experimental results and the estimates from analytical models. These findings demonstrate that rocking bridges are a suitable system to control structural damage during seismic events despite the apparent variability of the behavior under realistic conditions. However, the design of these structures must account for the effects of construction imperfections at the rocking surface. The experimental results are openly available for download in the DesignSafe Data Depot using this DOI https://doi.org/10.17603/ds2-znfv-8t55.

Donor-acceptor bulk-heterojunction sensitizer for efficient solid-state infrared-to-visible photon up-conversion

Solid-state infrared-to-visible photon up-conversion is important for spectral-tailoring applications. However, existing up-conversion systems not only suffer from low efficiencies and a need for high excitation intensity, but also exhibit a limited selection of materials and complex fabrication processes. Herein, we propose a sensitizer with a bulk-heterojunction structure, comprising both an energy donor and an energy acceptor, for triplet-triplet annihilation up-conversion devices. The up-conversion occurs through charge separation at the donor-acceptor interface, followed by the formation of charge transfer state between the energy donor and annihilator following the spin statistics. The bulk-heterojunction sensitizer ensures efficient charge generation and low charge recombination. Hence, we achieve a highly efficient solid-state up-conversion device with 2.20% efficiency and low excitation intensity (10 mW cm−2) through a one-step solution method. We also demonstrate bright up-conversion devices on highly-flexible large-area substrates. This study introduces a simple and scalable platform strategy for fabricating efficient up-conversion devices.

Best Practices to Directly Assess Heterogeneous Singlet Oxygen Photosensitization by Phosphorescence

AbstractLiterature proves that the direct detection of 1O2(1Δg) at the solid gas interface is systematically performed from its phosphorescence using high intensity excitation sources (i.e., lasers), which lead to quasi‐ubiquitous chemical problems, such as sensitizer degradation, and photophysical counter‐active issues such as ultrafast exciton migration, singlet‐singlet and triplet‐triplet annihilation, and thermally activated delayed fluorescence mediated by 1O2(1Δg). To avoid these inconveniences, low excitation intensity is required but leads to serious analytical challenges. The best practices to reliably detect 1O2(1Δg) phosphorescence at various interfaces using a standard excitation source and near‐IR detector. The two main practices consist in a gas purging test for reliable identification of 1O2(1Δg), and in a particularly fine optimization of the angle made by excitation beam versus substrate plane. These practices are applied to porphyrin sensitizers H2TPP and ZnTPP, either neat or physiosorbed on glass, quartz, paper and hospital bandages, graphene oxide (GO), and embedded inside electrospun polystyrene fibers and spin coated poly(methyl methacrylate) films. Porphyrin‐based metal‐organic framework PCN‐224, freshly activated, is also examined.

Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 μs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

L-Type Calcium Channel Modulates Low-Intensity Pulsed Ultrasound-Induced Excitation in Cultured Hippocampal Neurons.

As a noninvasive technique, ultrasound stimulation is known to modulate neuronal activity both in vitro and in vivo. The latest explanation of this phenomenon is that the acoustic wave can activate the ion channels and further impact the electrophysiological properties of targeted neurons. However, the underlying mechanism of low-intensity pulsed ultrasound (LIPUS)-induced neuro-modulation effects is still unclear. Here, we characterize the excitatory effects of LIPUS on spontaneous activity and the intracellular Ca2+ homeostasis in cultured hippocampal neurons. By whole-cell patch clamp recording, we found that 15min of 1-MHz LIPUS boosts the frequency of both spontaneous action potentials and spontaneous excitatory synaptic currents (sEPSCs) and also increases the amplitude of sEPSCs in hippocampal neurons. This phenomenon lasts for > 10min after LIPUS exposure. Together with Ca2+ imaging, we clarified that LIPUS increases the [Ca2+]cyto level by facilitating L-type Ca2+ channels (LTCCs). In addition, due to the [Ca2+]cyto elevation by LIPUS exposure, the Ca2+-dependent CaMKII-CREB pathway can be activated within 30 min to further regulate the gene transcription and protein expression. Our work suggests that LIPUS regulates neuronal activity in a Ca2+-dependent manner via LTCCs. This may also explain the multi-activation effects of LIPUS beyond neurons. LIPUS stimulation potentiates spontaneous neuronal activity by increasing Ca2+ influx.

Triplet properties and intersystem crossing mechanism of PtAg28 nanocluster sensitizers achieving low threshold and efficient photon upconversion.

Ligand-protected metal nanoclusters have emerged as a promising platform for providing sensitizers for triplet-triplet annihilation upconversion (TTA-UC). Herein, we report [PtAg28(BDT)12]4- (PtAg28; BDT = 1,3-benzenedithiolate) as a sensitizer enabling TTA-UC at low excitation intensities. PtAg28 exhibits a long-lived triplet state (approximately 7 μs) generated with a 100% intersystem crossing (ISC) quantum yield. The mechanism driving this efficient ISC was unveiled with the aid of theoretical calculations. Specifically, the S1-T1 ISC reveals a small spin-orbit coupling (SOC) matrix element, attributed to their similar electron configuration. In contrast, the T2 state, which is energetically close to S1, features a hole distribution derived from the Py superatomic orbital of the icosahedral Pt@Ag12 core. This distribution enables direct SOC based on the orbital angular momentum change from the S1 state with a Pz-derived hole distribution. Consequently, the efficient ISC was rationalized by the S1 → T2 → T1 pathway. The T1 state possesses a metal core-to-surface metal charge transfer character, facilitating triplet energy transfer and conferring superior sensitization ability. Leveraging these characteristics, the combination of PtAg28 sensitizer with a 9,10-diphenylanthracene annihilator/emitter attained an extremely low UC threshold of 0.81 mW cm-2 at 532 nm excitation, along with efficient green-to-blue TTA-UC with an internal quantum yield (ΦUCg) of 12.2% (50% maximum). This results in a pseudo-first-order TTA process with strong UC emission under 1-sun conditions.

Insights into nonlinear absorption transitions in a silver-incorporated reduced graphene oxide-molybdenum disulfide (Ag-rGO-MoS2) hybrid.

Optical nonlinearity in a silver-decorated reduced graphene oxide-molybdenum disulfide (Ag-rGO-MoS2) nanocomposite was experimentally investigated via the Z-scan technique using a Q-switched Nd:YAG nanopulsed green laser. An interesting switching behaviour from saturable to reverse saturable absorption with varying input on-axis intensity of the laser was demonstrated. Under low-intensity laser excitation, Ag-rGO-MoS2 displayed ground-state bleaching, which resulted in saturable absorption (SA) behaviour. Here, the prominent SPR phenomenon of Ag enforced plasmon absorption and is assigned to the SA process. Interestingly at higher intensity, the material switched its nonlinearity to reverse saturable absorption (RSA), attributed to two different mechanisms of two-photon absorption (2PA). At moderate laser pulse energies, the sample underwent sequential 2PA, which is underpinned by calculated varying nonlinear optical parameters and excited-state absorption cross-sections. However, at higher intensities, nonlinear absorption coefficients and excited-state absorption cross-sections remained constant, indicating the occurrence of genuine 2PA. In addition, theoretical estimation of the 2PA cross-section validates the observed transition. Thus, experimental evidence for the validity of nonlinear absorption theory (intensity-dependent transition from linear to nonlinear SA to nonlinear RSA to genuine 2PA via sequential 2PA) is provided for the first time in the literature.

Efficient Lithium-Based Upconversion Nanoparticles for Single-Particle Imaging and Temperature Sensing.

Upconversion Nanoparticles (UCNPs) have attracted exceptional attention due to their great potential in high-contrast, free-background biofluorescence deep tissue imaging and quantum sensing. Most of these interesting studies have been performed using an ensemble of UCNPs as fluorescent probes in bioapplications. Here, we report a synthesis of small and efficient YLiF4:Yb,Er UCNPs for single-particle imaging as well as sensitive optical temperature sensing. The reported particles demonstrated a bright and photostable upconversion emission at a single particle level under a low laser intensity excitation of 20 W/cm2. Furthermore, the synthesized UCNPs were tested and compared to the commonly used two-photon excitation QDs and organic dyes and showed a nine times better performance at a single particle level under the same experimental conditions. In addition, the synthesized UCNPs demonstrated sensitive optical temperature sensing at a single particle level within the biological temperature range. The good optical properties of single YLiF4:Yb,Er UCNPs open an avenue for small and efficient fluorescent markers in imaging and sensing applications.

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor.

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Enabling Self-Induced Back-Action Trapping of Gold Nanoparticles in Metamaterial Plasmonic Tweezers.

The pursuit for efficient nanoparticle trapping with low powers has led to optical tweezers technology moving from the conventional free-space configuration to advanced plasmonic systems. However, trapping nanoparticles smaller than 10 nm still remains a challenge even for plasmonic tweezers. Proper nanocavity design and excitation has given rise to the self-induced back-action (SIBA) effect offering enhanced trap stiffness with decreased laser power. In this work, we investigate the SIBA effect in metamaterial tweezers and its synergy with the exhibited Fano resonance. We demonstrate stable trapping of 20 nm gold particles with trap stiffnesses as high as 4.18 ± 0.2 (fN/nm)/(mW/μm2) and very low excitation intensity. Simulations reveal the existence of two different groups of hotspots on the plasmonic array. The two hotspots exhibit tunable trap stiffnesses, a unique feature that can allow for sorting of particles and biological molecules based on their characteristics.

Steady-state dynamic response analysis of single-degree-of-freedom dual frame-wall resilient system

A mixed dual frame-wall resilient system, composed of a high-strength moment-resisting steel frame (HS-MRF) and self-centering steel plate shear walls (SC-SPSWs), was proposed to address the issue of frame expansion. To study the steady-state dynamic responses of the structural system, an analytical solution of a single-degree-of-freedom nonlinear oscillator under harmonic excitation was solved using the method of averaging. The steady-state responses of HS-MRF, self-centering frame and steel plate shear wall that are represented by the bilinear, self-centering and pinching hysteresis, respectively, were studied individually, followed by a full system study. A stability analysis was also carried out to evaluate the behaviour of the singular points. The results show that the hysteretic damping in the bilinear system can significantly reduce the resonance, and the responses are always stable. Jump phenomenon occurs in the self-centering system, and increasing the post-yield stiffness can reduce the nonlinearity, thus decreasing the resonance and unstable region. Unbounded resonance responses appear in the pinching system under large excitation but can change to jump behaviour under small excitation. A left-unbounded unstable region was also found when the unloading force is small. In the combined-hysteresis system, increasing the portion of bilinear hysteresis has negligible effects on the resonance but can reduce the nonlinearity under lower excitation intensities. However, under higher intensities, increasing bilinear hysteresis can significantly decrease peak responses and reduce both the jump and unbounded phenomena. The unstable regions also strongly depend on the ratio of hysteretic models. These findings shed light on the inherent dynamic properties of the dual frame-wall system and suggest that a well-design can achieve a favourable seismic performance.

Dynamic Characteristic Analysis of Tri-Stable Piezoelectric Energy Harvester with Double Elastic Amplifiers

In order to further improve the vibration energy harvesting efficiency of piezoelectric energy harvester under low frequency environmental excitation, this paper, based on the traditional magnetic tri-stable piezoelectric energy collector model, proposes a tri-stable piezoelectric energy harvester (TPEH+DEM) model with two elastic amplifiers which are installed between the U-shaped frame and the base and between the fixed end of the piezoelectric cantilever beam and the U-shaped frame respectively. Based on Hamilton principle, the motion equation of electromechanical coupling of TPEH+DEM system is established, and the analytical solutions of displacement, output voltage and power of the system are obtained by harmonic balance method. The effects of the mass of elastic amplifier, spring stiffness, magnet spacing and load resistance on the dynamic characteristics of energy harvesting of TPEH+DEM system are analyzed. The result shows that there are two peaks in the response output power of TPEH+DEM system in the operating frequency range. By adjusting the mass and stiffness of the elastic amplifier reasonably, the system can move into the inter-well motion under low external excitation intensity, and produce high output power. Compared with the traditional model which only has an elastic amplifier on the base of piezoelectric energy harvester, TPEH+DEM model has better energy harvesting performance under low frequency and low intensity external excitation.

The effect of the effective electron mass on the hot electron collection

The dominant factor for hot electron collecting in internally photoemitted hot carrier (IPHC) devices is still not clear under steady-state low intensity light. We here use SnO2 as the electron-collecting layer to replace TiO2 to construct IPHC devices. Almost no photoresponse is observed for the pure SnO2-based IPHC device. However, when an insulating MgO layer or TiO2 covered SnO2, relatively large photocurrent generated from hot electrons can be achieved. The effective electron mass (EEM) is figured out to be the dominate factor in hot electron collection in IPHC devices. The very small EEM of SnO2 results in a small emission cone of hot electrons. Also due to the small EEM of SnO2, the leakage of trapped electrons back to the Au is very large. Because of these two reasons, the SnO2-based IPHC device shows almost no photoresponse. MgO can block the backflow of electrons (leakage), while the larger EEM of TiO2 can increase the emission cone of hot electrons. Our finding is significant for understanding hot electrons collection and will give new directions for hot carrier solar cell applications under low-intensity excitation at steady state.

Exploring nonlinear degradation benefit of bio-inspired oscillator for engineering applications

Nonlinear structure design is an indispensable link in the development of vibration isolation, energy harvesting and fault diagnosis technologies. It is also one of the most effective technical means to improve the performance of structures in engineering applications. In this study, a bio-inspired nonlinear oscillator is proposed and the benefits of nonlinear degradation in engineering applications are explored. First, the degradation bistable oscillator and high-order quasi-zero stiffness oscillator can be obtained after the degradation of the bio-inspired nonlinear oscillator. Through the amplitude-frequency analysis, it is found that the nonlinear degradation can reduce the unstable solution in the ultra-low frequency region and reduce the resonant frequency of the bio-inspired nonlinear oscillator. Therefore, the degeneration nonlinear oscillator can realize the large-amplitude inter-well motion at ultra-low frequency or low random excitation intensity. Secondly, the theory and method of vibration isolation and energy harvesting based on the nonlinear degradation mechanism are proposed, and the vibration isolation and energy harvesting performance of the oscillator under harmonic and stochastic excitations are analyzed. Finally, the use of the stochastic resonance of the degeneration bistable oscillator can greatly improve the signal-to-noise ratio, enhance the weak fault signal, and realize early fault diagnosis. The related results help to deepen the understanding of nonlinear degenerate systems and provide new technical approaches for engineering applications.

Design and Experimental Investigation of an Ultra-Low Frequency, Low-Intensity, and Multidirectional Piezoelectric Energy Harvester with Liquid as the Energy-Capture Medium

Traditional piezoelectric vibration energy harvesters (PVEHs) usually adopt a rigid energy-capture structure, which can achieve efficient energy harvesting in single-directional, high-frequency, and high-intensity vibration environments. However, efficient harvesting with the use of low-frequency, low-intensity, and multidirectional vibration energy remains a challenge for existing harvesters. To tackle this problem, we proposed a PVEH with liquid as the energy-capture medium. Our previous research verified that this set up can show a good energy harvesting performance under low-frequency, low-intensity, and horizontal multidirectional vibration excitation. In this paper, we further studied the possibility of vertical multidirectional energy harvesting using this device, as well as the influence of several important parameters (rope margin, liquid level height, and floating block shape) on the output performance. The results showed that the proposed PVEH can realize energy harvesting in three-dimensional space and that the output characteristic is adjustable.

Design of robust particle dampers using inner structures and coated container walls

Classical particle dampers suffer from their non-robust damping behavior, i.e. they can only be efficiently applied to a specific frequency range and amplitude range. The reason for that is that particle motion, also called motion mode, and damper efficiency show a strong correlation. By changing particle or container properties the motion modes are shifted to other excitation conditions but their efficient range is not much affected. To increase the damping performance and robustness of particle dampers, two approaches are presented here by introducing new motion modes. Therefore, the particle dampers are analyzed experimentally using a shaker setup and numerically using the discrete element method. The first design approach uses inner structures inside the particle damper, manufactured by a 3D printer. The inner structures consist of different numbers of beams, placed perpendicular to the container moving direction. They lead to a much more robust damper as the transition between the motion modes gets smoother. For the second approach, the container walls are equipped with different soft polymers. In this way a new motion mode at low excitation intensities is observed, leading to a high efficiency possibly on a large excitation intensity range. For an easy calculation of the necessary wall’s Young’s modulus an analytical formula based on Hertz impact theory is derived.

Evidence for "dark charge" from photoluminescence measurements in wide InGaN quantum wells.

Wide (15-25 nm) InGaN/GaN quantum wells in LED structures were studied by time-resolved photoluminescence (PL) spectroscopy and compared with narrow (2.6 nm) wells in similar LED structures. Using below-barrier pulsed excitation in the microsecond range, we measured increase and decay of PL pulses. These pulses in wide wells at low-intensity excitation show very slow increase and fast decay. Moreover, the shape of the pulses changes when we vary the separation between them. None of these effects occurs for samples with narrow wells. The unusual properties of wide wells are attributed to the presence of "dark charge" i.e., electrons and holes in the ground states. Their wave functions are spatially separated and due to negligible overlap they do not contribute to emission. However, they screen the built-in field in the well very effectively so that excited states appear with significant overlap and give rise to PL. A simple model of recombination kinetics including "dark charge" explains the observations qualitatively.