Low Excitation Power Research Articles

Structural morphing induced by functionalising buckling

The paper presents an overview of a research project at the University of Sydney aimed at developing a general framework for the analysis and design of functional components of buildings and structures, where such components achieve large shape changes (morphing) via buckling. The shape changes are optimised, e.g. to reduce energy consumption by minimising solar radiation loads or maximising natural air ventilation. The underlying driver for the project is to develop innovative building technology solutions to reduce the energy consumption of the next generation of low-, medium- and high-rise buildings.The paper first summarises recent work on plate elements supported along three edges, in which temporary intermediate restraints are used to load the plate into the post-buckling range and subsequently released to generate an abrupt shape change in response to an external signal triggered by shading or ventilation demand. This investigation is backed by an analysis of the placement of intermediate restraints to optimise the plate deflection by maximising the pre-buckling compression of the plate. Next, a study is presented on optimising the topology of plates to maximise their shading or ventilation capacities under applied compression or bending. Considering both buckling and nonlinear post-buckling, the analytical framework optimises the spatial distribution of plate thickness. Experiments on optimised plates are reported as well, in which shape memory alloy (SMA) and piezoelectric (PZT) actuators are used to induce compression and buckling. Subsequently, morphing induced by flexural–torsional buckling is investigated where simple 3-member frame geometries are devised to achieve large lateral buckling displacements and twist rotations under low-power external excitation. Lastly, an application of functionalised buckling for shading of buildings is illustrated which employs a bi-stable mechanism powered by shape memory alloy actuation.

PDT-active upconversion nanoheaters for targeted imaging guided combinatorial cancer phototherapies with low-power single NIR excitation

A versatile nanoformulation is designed by anchoring human transferrin protein (Tf) on fluoromagnetic upconverting nanoheaters, NaGdF4:Yb,Er (UCNP), loaded with Rose Bengal (RB), for multimodal imaging guided synergistic photothermal (PTT) and photodynamic therapy (PDT) at the targeted tumor site. The NIR excitation of the UCNP-RB Forster Resonance Energy Transfer (FRET) pair results in the reactive oxygen species (ROS) generation for PDT, whereas the non-radiative transitions in Er result in the heat required for PTT. The intravenously injected theranostic agent (UCNP@Tf-RB) enabled; (1) combinatorial PTT and PDT of 4T1 tumors with minimal systemic toxicity, (2) dual targeted (passive and active) tumor accumulation, (3) dual-modal imaging (MRI/photothermal), and, (4) excellent stability and biocompatibility. The in vitro therapy data corroborates the MRI findings that Tf conjugation resulted in actively targeted tumor accumulation via over-expressed transferrin receptors (TfR) on 4T1 cells. Real-time photothermal imaging enabled visualization of the tumor while receiving the therapy. The UCNP@Tf-RB, for synergistic PTT-PDT, and UCNP@Tf, for PTT only, caused rapid suppression of tumor with a tumor-growth inhibition index (TGII) of ~0.91, and 0.79, respectively. Histopathological examination demonstrated minimal damage to non-targeted tissues and caused significant damage to the tumor. This theranostic methodology enhances anti-cancer therapeutic efficiency, and announces the potential for pre-clinical cancer therapy.

Lanthanide-doped niobium oxide nanoparticles on nanoarchitectured silica spheres as potential probe for NIR luminescent markers

We have prepared Nb2O5:Eu3+ and Nb2O5:Nd3+ nanostructured materials and SiO2@Nb2O5:Eu3+ core@shell nanoparticles by the sol–gel route to evaluate how the annealing temperature influences their structural and luminescent features, aiming at their potential application as optical markers. An intense red emission under excitation at the blue region was detected from the Nb2O5:Eu3+ solids. Inhomogeneous broadening on the photoluminescence spectra was observed for the sample annealed at 600 °C, while the allocation of the Eu3+ ions on S4 symmetry sites was attributed for higher annealing temperature (1100 °C). A discrete formation of the H-phase (monoclinic) of niobium oxide for the sample annealed at an intermediate temperature of 900 °C was observed, with its photoluminescence spectra revealing the preferential occupation of the Eu3+ ions on sites in this crystalline phase. Photoluminescence decay curves recorded for the 5D0 excited state revealed average lifetime values in the order of milliseconds. The Nb2O5:Nd3+ solids displayed analogous behavior: Nd3+ preferentially occupied the monoclinic phase symmetry sites. In water dispersion and under low-power laser excitation (808 nm), the Nb2O5:Nd3+ systems emitted in biological windows. The sample annealed at 600 °C provided the most promising luminescence intensity and colloidal stability. The surface of the SiO2 nanoparticles was successfully decorated with Nb2O5:Eu3+ nanoparticles in the T-phase. The SiO2@Nb2O5:Eu3+ core@shell nanoparticles presented color tunability due to red light emission from Eu3+ and blue light emission from defect centers in the SiO2 template. The possibility of functionalizing SiO2 nanoparticles with red-emitting Nb2O5:Eu3+ nanoparticles combined with blue excitation and lifetime values in the order of milliseconds renders the prepared SiO2@Nb2O5:Eu3+ core@shell nanoparticles promising biomarkers that can be explored by time-gated luminescence techniques.

Enhancing Triplet-Triplet Annihilation Upconversion: From Molecular Design to Present Applications.

Photon upconversion, the process of converting low-energy photons into high-energy ones, has been widely applied for solar energy conversion, photoredox catalysis, and various biological applications such as background-free bioimaging, cancer therapy, and optogenetics. Upconversion materials that are based on triplet-triplet annihilation (TTA) are of particular interest due to their low excitation power requirements (e.g., ambient sunlight) and easily tunable excitation and emission wavelengths. Despite advances that have been made with respect to TTA upconversion (TTA-UC) in the past decade, several challenges remain for near-infrared light-activatable triplet-triplet annihilation upconversion (NIR TTA-UC). These challenges include low upconversion quantum yield, small anti-Stokes shift, and incompatibility with oxygen, the latter of which seriously limits the practical applications of NIR TTA-UC.This Account will summarize the recent research endeavors to address the above-mentioned challenges and the recent new applications. The first part of this Account highlights recent strategies of molecular design to modulate the excited states of photosensitizers and annihilators, two key factors to determine TTA-UC performance. Novel molecular engineering strategies such as the resonance energy transfer method, dimerization of dye units, and the helix twist molecular structure have been proposed to tune the excited states of photosensitizers. The obtained photosensitizers exhibited enhanced absorption of deep tissue penetrable near-infrared (NIR) light, produced a triplet excited state with elevated energy level and prolonged lifetime, and promoted intersystem crossing, leading to an upgraded TTA-UC system with significantly expanded anti-Stokes shift. With respect to the annihilator, the perylene derivatives were systematically explored, and their attached aromatic groups were found to be the key to adjusting the energy levels of both the triplet and singlet excited states. The resultant optimal TTA-UC system exhibits the highest recorded efficiency among NIR TTA-UC systems.Moreover, to resolve the oxygen-induced TTA-UC quenching, enzymatic reactions were recently introduced. More specifically, the glucose oxidase-catalyzed glucose oxidation reaction showed the ability to rapidly consume oxygen to turn on the TTA-UC luminescence in an aqueous solution. The resultant TTA-UC nanoparticle was able to detect glucose and an enzyme related to glucose metabolism in a highly specific, sensitive, and background-free manner. Further, the upconverted singlet excited state of the annihilator was directly utilized as the catalyst or the excited substrate. For example, the modification of annihilators and drug molecules with photolabile linkages can realize the long wavelength light-induced photolysis. Compared to direct short-wavelength-driven photolysis, this sensitized TTA photolysis (TTAP) exhibits superior reaction yield and lower photodamage, which are important in the release of drugs for tumor treatment in vivo. Moreover, the improved upconversion efficiency can enable the successful coupling of NIR TTA-UC with a visible light absorbing photocatalyst for NIR-driven photoredox catalysis. Compared to direct visible-light photocatalysis, TTA-UC mediated NIR photoredox catalysis showed superior product yield especially in large scale reaction systems owing to the deep penetration power of NIR light. More interestingly, among a few promising technology applications, three-dimensional (3D) printing based on photopolymerization can operate with faster speed and energy-input several orders of magnitude lower when the two-photon polymerization is replaced with TTA-UC mediated polymerization. We believe this Account will spur interest in the further development and application of TTA-UC in the areas of energy, chemistry, material science, and biology.

Driving Third-Order Optical Nonlinearities in Photoluminescent Si Nanoparticles by Nitrogen Co-Implantation in a Silica Matrix.

The photoluminescence and third-order nonlinear optical effects of co-implanted silicon nanoparticles and nitrogen ions in a silica matrix were studied. Experimental evidence shows the potential of nitrogen ions for changing optical properties exhibited by silicon nanoparticles implanted in an integrated system. The modification of the optical bandgap and photoluminescent intensity in the studied nanomaterials by the incorporation of nitrogen was analyzed. Standard two−wave mixing experiments were conducted using nanosecond and picosecond laser pulses at 532 nm wavelength. At this off-resonance condition, only multiphoton excitation can promote electrons at energies above the optical bandgap of the silicon nanoparticles. The picosecond results show that the co-implanted sample with nitrogen exhibits a three-fold enhancement of the nonlinear Kerr response. Femtosecond z-scan measurements were undertaken at 800 nm in order to explore the modification of the ultrafast nonlinear response of the samples that revealed a purely electronic Kerr nonlinearity together to saturable absorption of the SiNPs in the near-infrared. Remarkably, femtosecond results reveal that nitrogen co-implantation in the SiNPs system derives from the quenching of the third-order nonlinear optical behavior. These findings pointed out a simple approach for engineering the optical bandgap of nanocomposites, which can be controlled by a doping process based on ion-implanted nitrogen. It is highlighted that the enhanced light-matter interactions induced by nitrogen implantation can be useful for developing nonlinear integrated silicon photonics nanodevices with low power excitation.

Designing Semiconductor Nanowires for Efficient Photon Upconversion via Heterostructure Engineering.

Energy upconversion via optical processes in semiconductor nanowires (NWs) is attractive for a variety of applications in nano-optoelectronics and nanophotonics. One of the main challenges is to achieve a high upconversion efficiency and, thus, a wide dynamic range of device performance, allowing efficient upconversion even under low excitation power. Here, we demonstrate that the efficiency of energy upconversion via two-photon absorption (TPA) can be drastically enhanced in core/shell NW heterostructures designed to provide a real intermediate TPA step via the band states of the narrow-bandgap region with a long carrier lifetime, fulfilling all the necessary requirements for high-efficiency two-step TPA. We show that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the 1 sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure. This work demonstrates the effectiveness of our approach in providing efficient photon upconversion by exploring core/shell NW heterostructures, yielding an upconversion efficiency being among the highest reported in semiconductor nanostructures. Furthermore, our work provides design guidelines for enhancing efficiency of energy upconversion in NW heterostructures.

Towards the design and operation of a uniformly illuminated NV detector for magnetic field mapping applications

Detectors based on an ensemble of nitrogen-vacancy (NV) centres in diamond are the latest addition to the list of magnetometers capable of detecting weak magnetic fields. Here, we report the design, construction, and testing of an NV magnetometer with a wide spatial area excitation and collection capability. The system was developed for the purpose of magnetic field measurement in an Halbach magnet configuration. The designed system performance was comparable to a conventional NV confocal system in the low power excitation region. From the dip profiles observed in the optically detected magnetic resonance (ODMR) spectrum, each NV centre experienced a static magnetic field in the 2.14–6.07 mT range of the Halbach magnet. The sensitivity of the field mapping system using the continuous wave (CW)-ODMR approach was estimated and found to be approximately 0.2 μT/Hz at 170 MHz. Extensive details of the setup and the experimental procedures are presented, and the system can be easily reproduced by those who are not familiar with NV centre instrumentation. The results obtained pave way for the development of scalable NV sensors using wide area illumination and collection methods, especially for the mapping B0 and B1 field in magnetic resonance imaging (MRI) to facilitate non-FFT image reconstruction.

Emission enhancement and exciton species modulation in monolayer WS2 via decoration of CdTe quantum dots

Transition metal dichalcogenides MX2 (M = Mo/W; X = S/Se) exhibit excellent optical and electrical properties. However, the atomically thin thickness induced low absorption cross sections impede their further applications as efficient absorbers and emitters. Herein, we report the emission enhancement and exciton species modulation in monolayer (ML) WS2 via the decoration of CdTe quantum dots (QDs). The ML WS2 was synthesized by thermal evaporation and showed evident neutral (Ao) and charged (A−) exciton emissions with variable A−-to-Ao ratios under different excitation powers and temperatures. The A− emission played an important role at low temperature (80 K) and high excitation power (5 mW). After the decoration of the CdTe QDs, the photoluminescence (PL) intensity of the ML WS2 enhanced greatly. Moreover, the Ao emission was dominant in WS2 + CdTe even under high excitation power and low temperature. The transfer of numerous holes from CdTe to WS2 induced the nonradiative recombination probability reduction, and p-type doping was critical to the observed PL enhancement and exciton species modulation in WS2 + CdTe. Our results provide a flexible strategy to improve the PL properties of atomically thin WS2 and further deepen the understanding of exciton-physics in ML MX2 for various applications.

Defect-induced distinct exciton-exciton interactions in WS2 monolayers

The optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs) are largely influenced by defect populations (DPs). In this work, we fabricate WS2 monolayers with different DPs by varying the fabrication methods and further reveal their distinct exciton-exciton interactions. Steady-state photoluminescence (PL) experiments show that the monolayer with the lowest DP shows optimal PL intensity at low excitation power; however, it is overtaken and significantly surpassed by monolayers with higher DPs at high excitation powers. Excitation-power-dependent experiments demonstrate that these monolayers exhibit distinct PL saturation behaviors with the threshold power differing by four orders of magnitude. Combined with in situ PL imaging and time-resolved PL experiments, we attribute such PL evolution discrepancies to the different DPs within these monolayers, which largely influence the exciton diffusion behavior and subsequently bring about distinct nonradiative exciton-exciton annihilations (EEAs). Valley polarization experiments are further employed to re-examine the DPs of these monolayers. This work reveals the distinct PL behaviors and underlying exciton dynamics in TMD monolayers with different DPs, which can largely facilitate the engineering of relevant high-performance devices for practical applications.

Optically Coupled PtOEP and DPA Molecules Encapsulated into PLGA-Nanoparticles for Cancer Bioimaging

Triplet-triplet annihilation upconversion (TTA-UC) nanoparticles (NPs) have emerged as imaging probes and therapeutic probes in recent years due to their excellent optical properties. In contrast to lanthanide ion-doped inorganic materials, highly efficient TTA-UC can be generated by low excitation power density, which makes it suitable for clinical applications. In the present study, we used biodegradable poly(lactic-co-glycolic acid) (PLGA)-NPs as a delivery vehicle for TTA-UC based on the heavy metal porphyrin Platinum(II) octaethylporphyrin (PtOEP) and the polycyclic aromatic hydrocarbon 9,10-diphenylanthracene (DPA) as a photosensitizer/emitter pair. TTA-UC-PLGA-NPs were successfully synthesized according to an oil-in-water emulsion and solvent evaporation method. After physicochemical characterization, UC-efficacy of TTA-UC-PLGA-NPs was assessed in vitro and ex vivo. TTA-UC could be detected in the tumour area 96 h after in vivo administration of TTA-UC-PLGA-NPs, confirming the integrity and suitability of PLGA-NPs as a TTA-UC in vivo delivery system. Thus, this study provides proof-of-concept that the advantageous properties of PLGA can be combined with the unique optical properties of TTA-UC for the development of advanced nanocarriers for simultaneous in vivo molecular imaging and drug delivery.

Unifying Perspective of the Ultrafast Photodynamics of Orange Carotenoid Proteins from Synechocystis: Peril of High-Power Excitation, Existence of Different S* States, and Influence of Tagging.

A substantial number of Orange Carotenoid Protein (OCP) studies have aimed to describe the evolution of singlet excited states leading to the formation of a photoactivated form, OCPR. The most recent one suggests that 3 ps-lived excited states are formed after the sub-100 fs decay of the initial S2 state. The S* state, which has the longest reported lifetime of a few to tens of picoseconds, is considered to be the precursor of the first red photoproduct P1. Here, we report the ultrafast photodynamics of the OCP from Synechocystis PCC 6803 carried out using visible–near infrared femtosecond time-resolved absorption spectroscopy as a function of the excitation pulse power and wavelength. We found that a carotenoid radical cation can form even at relatively low excitation power, obscuring the determination of photoactivation yields for P1. Moreover, the comparison of green (540 nm) and blue (470 nm) excitations revealed the existence of an hitherto uncharacterized excited state, denoted as S∼, living a few tens of picoseconds and formed only upon 470 nm excitation. Because neither the P1 quantum yield nor the photoactivation speed over hundreds of seconds vary under green and blue continuous irradiation, this S∼ species is unlikely to be involved in the photoactivation mechanism leading to OCPR. We also addressed the effect of His-tagging at the N- or C-termini on the excited-state photophysical properties. Differences in spectral signatures and lifetimes of the different excited states were observed at a variance with the usual assumption that His-tagging hardly influences protein dynamics and function. Altogether our results advocate for the careful consideration of the excitation power and His-tag position when comparing the photoactivation of different OCP variants and beg to revisit the notion that S* is the precursor of photoactivated OCPR.

Upconversion Nanocrystals with High Lanthanide Content: Luminescence Loss by Energy Migration versus Luminescence Enhancement by Increased NIR Absorption

AbstractLanthanide‐doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background‐free luminescence. The progress in synthesis nowadays enables access to complex core‐shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core‐only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near‐infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF4(20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF4 shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb3+ and 2% Er3+. The impact of size, composition, and core‐shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.

Unveiling spin-flip processes in a neutral quantum dot using an anisotropic photonic structure

We introduce a method to investigate excitonic spin flips in a neutral quantum dot (QD) that is driven nonresonantly. By inserting the QD in an anisotropic photonic structure, one creates an imbalance between the radiative decay rates of the two bright excitons. Direct spin flips between the bright excitons as well as indirect ones (via a dark exciton) mix the level populations and profoundly affect the degree of linear polarization of the excitonic emission. Measuring this quantity under continuous wave optical excitation yields the spin-flip rate over a broad range of excitation powers. Additional time-resolved experiments allow disentangling the contributions of bright-bright and dark-bright spin flips in the low-excitation regime. After providing theoretical background, we demonstrate the method on a self-assembled InAs QD embedded in a GaAs photonic wire featuring an elliptical cross section. For low-excitation power and at $T=5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, bright-bright spin flips are much slower than dark-bright spin flips, which, in turn, remain much slower than the radiative decays. Upon increasing the temperature, we observe a superlinear increase in the bright-bright spin-flip rate which completely reverses the rate hierarchy above $T=50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Moreover, polarization measurements reveal a dramatic increase in the spin-flip rate with the pumping power. Our findings are relevant to spontaneous emission control by anisotropic photonic structures and to the spectral coherence of QD-based quantum light sources.

Upconverted/downshifted NaLnF4 and metal-organic framework heterostructures boosting NIR-II imaging-guided photodynamic immunotherapy toward tumors

Heterostructures of NaLnF4 and porphyrinic metal-organic frameworks (MOF) have been reported as a promising platform for near-infrared (NIR)-driven photodynamic therapy (PDT), while the energy transfer from NaLnF4 to MOF greatly decreased the upconversion emissions. To develop NaLnF4 and MOF heterostructures with significantly improved PDT and imaging simultaneously is important to track their location to implement therapy accurately. Herein, we constructed Er3+-doped NaLnF4@MOF core@shell nanoparticles to integrate NIR-driven PDT and NIR-II imaging. NaLnF4 nanoparticles are designed with upconversion and NIR emission simultaneously under a single 980 nm excitation, where the upconversion luminescence (UCL) overlaps well with the absorption of Zr-based porphyrin MOF, enabling efficient energy transfer from NaLnF4 to the MOF shell to generate 1O2. And the NIR luminescence, which gave out under a lower excitation power than that of UCL, could be used to track the location and distribution of the heterostructures via NIR-II imaging. Moreover, antiprogrammed death-ligand 1 (α-PD-L1) immunotherapy was introduced synergistically to improve antitumor immunity efficacy. The combination of NaLnF4@MOF and α-PD-L1 not only inhibited primary tumors but also suppressed distant one, leading to an effective tumor inhibition rate of 95%. This work provides an integrated nanoplatform for boosting NIR-II imaging-guided PDT/immunotherapy efficacy toward tumor treatment.

Stoichiometric dependence and laser heating effect on the luminescence thermometric performance of Er3+, Yb3+: YuGdwVO4 microparticles in the non-saturation regime

The performance of YuGdwVO4 microcrystals with different atomic concentrations of Y3+ and Gd3+ ions, co-doped Er3+-Yb3+, as thermometers, exploiting two different calculation methods based on the fluorescence intensity ratio is investigated in the non-saturation regime. At a low excitation power density of 20 W cm−2, the thermometric parameters are approximately the same for all the analyzed sensors. Whereas for an excitation power density of 155 W cm−2, they are widely different all along the 303–513 K temperature range of study. Such as an increase on the Gd3+ ion concentration in the microcrystals induces a lattice expansion that can result in the decreasing of the lattice field surrounding the Er3+ ions and then it could enhance greatly the thermometric performances. For both calculation methods, it has been demonstrated that the ∆EFIR (fitted from the FIR curves) and the ∆ETS (determined using the TeSen Calculator program) values are lower than the ∆E` experimental (fitted from the emission spectra measured at room temperature), whenever the sample is heated and then the thermometric parameters are less competitive.

Smartphone-based portable fluorescence sensor with gold nanoparticle mediation for selective detection of nitrite ions

A simple, portable device for the detection of NO2– via a fluorescence method was developed. The proposed device consisted of a dark box containing a blue LED as a low-power excitation light source and a smartphone with a mobile application for RGB analysis as a light detector. Detection was mediated by using synthesized cetyltrimethylammonium bromide-stabilized gold nanoparticles (CTAB-AuNPs). The CTAB-AuNPs were etched with NO2– to yield Au3+, which catalyzes the oxidation of o-phenylenediamine (OPD) in the presence of H2O2 to generate 2,3-diaminophenazine (DAP). Triton X-100 (TX-100) micelles were introduced to improve the DAP fluorescence emission. The fluorescence intensity of DAP was recorded by the smartphone in terms of RGB intensity, which was correlated with the NO2– concentration. This method provided a wide linear working concentration range (0.5–100 μM), a limit of detection of 0.17 μM and excellent selectivity for NO2– over other anions.

The Investigation of Carrier Leakage Mechanism Based on ABC-Models in InGaN/GaN MQW and Its Effect on Internal Quantum Efficiency under Optical Excitation

In this work, a GaN-based multiple quantum well (MQW) sample has a much higher IQE although it has a stronger non-radiative recombination. Through experimental verification, the higher IQE is attributed to the suppressed carrier leakage mechanism, which is normally neglected under optical excitation. To achieve a more reasonable IQE expression in a GaN MQW structure, leakage factor m is introduced into the ABC-models. Meanwhile, by analyzing the Arrhenius fitting of the plot of IQE-temperature and leakage factor m, the key temperature and excitation power turning on the carrier leakage mechanism was roughly determined to be below 220 K and 10 mW, respectively. Such a low turn-on temperature and excitation power indicates a much easier carrier leakage mechanism in GaN-based MQW, which may be caused by the small effective electron mass of InGaN (0.11–0.22 m*) and the narrow thickness of quantum well via the model calculation of energy band structure via simulation software LASTIP. Moreover, higher IQE can be achieved by suppressing the carrier leakage mechanism via structural optimization (such as electron block layer) in GaN-based MQW.

Multiphoton Bleaching of Red Fluorescent Proteins and the Ways to Reduce It.

Red fluorescent proteins and biosensors built upon them are potentially beneficial for two-photon laser microscopy (TPLM) because they can image deeper layers of tissue, compared to green fluorescent proteins. However, some publications report on their very fast photobleaching, especially upon excitation at 750–800 nm. Here we study the multiphoton bleaching properties of mCherry, mPlum, tdTomato, and jREX-GECO1, measuring power dependences of photobleaching rates K at different excitation wavelengths across the whole two-photon absorption spectrum. Although all these proteins contain the chromophore with the same chemical structure, the mechanisms of their multiphoton bleaching are different. The number of photons required to initiate a photochemical reaction varies, depending on wavelength and power, from 2 (all four proteins) to 3 (jREX-GECO1) to 4 (mCherry, mPlum, tdTomato), and even up to 8 (tdTomato). We found that at sufficiently low excitation power P, the rate K often follows a quadratic power dependence, that turns into higher order dependence (K~Pα with α > 2) when the power surpasses a particular threshold P*. An optimum intensity for TPLM is close to the P*, because it provides the highest signal-to-background ratio and any further reduction of laser intensity would not improve the fluorescence/bleaching rate ratio. Additionally, one should avoid using wavelengths shorter than a particular threshold to avoid fast bleaching due to multiphoton ionization.

Compact photothermal self-mixing interferometer for highly sensitive trace detection.

A self-mixing interferometer combined with the photothermal spectroscopy is utilized as a remarkable sensor for highly sensitive trace detection, featuring the beneficial property of a He-Ne laser with back-mounted photodiode, to the best of our knowledge, acting as an excitation laser, also as a probe laser, and even more, as a detector. Utilizing the novel implementation of the photothermal self-mixing (PTSM) interferometer with an external cavity modulation, the concentration of the sample is directly measured by the PTSM parameter extracted from the PTSM signal. The metrological qualities of the PTSM interferometer were investigated by methylene blue trace detection. For a low excitation power of 5 mW, a 7.7 nM of the limit of detection was achieved with a relative standard deviation of ∼3%. The compact and simple structure with high sensitivity has guiding significance to a robust analytical tool for the analysis of photosensitive compounds and in the detection of aquatic product hazards in aquaculture.

Triplet and singlet exciton diffusion in disordered rubrene films: implications for photon upconversion.

Triplet and singlet exciton diffusion plays a decisive role in triplet-triplet annihilation (TTA) and singlet fission (SF) processes of rubrene (Rub) films at low excitation power, and therefore has an important implication for TTA-mediated photon upconversion (UC). Although triplet diffusion in crystalline Rub was studied before, there is no quantitative data on diffusion in disordered Rub films most widely employed for NIR-to-Vis UC. The lack of these data hinders the progress of TTA-UC applications relying on a Rub annihilator (emitter). Herein, a time-resolved PL bulk-quenching technique was employed to estimate the exciton diffusion coefficient (D) and diffusion length (LD) in the neat Rub films as well as Rub-doped PS films at 80 wt% doping concentration, previously reported to be optimal in terms of UC efficiency. The impact of commonly utilized singlet energy collector (sink) DBP on exciton diffusion was also assessed, highlighting its importance exclusively on the dynamics of singlets in Rub films. Our study revealed that triplet diffusion lengths (LTD) of 25-30 nm estimated for the disordered Rub films are sufficient for encountering triplets from the neighboring sensitizer molecules at a low sensitizer PdPc concentration (0.1 wt%), thereby enabling the desired TTA domination regime to be reached. Essentially, the performance of Rub-based UC systems was found to be limited by the modest maximal LTD (up to ∼55 nm) in disordered films resulting from a short maximum triplet lifetime τT (∼100 μs) inherent to this emitter. Thus, to enhance the NIR-to-Vis TTA-UC performance, new emitters with a longer triplet lifetime in the solid state are required.