Excitation Intensity Dependence Research Articles

Distinct Valley Polarization in Vertical Heterobilayers: Difference between Edge- and Center-Nucleated WS2/MoS2.

Structural imperfections can cause both beneficial and detrimental consequences on the excitonic characteristics of transition metal dichalcogenides (TMDs). Regarding valley selection, structural defects typically promote valley depolarization in monolayer TMDs, but defect healing via an additional growth process can restore valley polarization in vertical heterobilayers (VHs). In this study, we analyzed the valley polarization of center-nucleated and edge-nucleated VHs (WS2/MoS2) grown using a controlled growth process and discovered that defect-related photoluminescence (PL) is strongly suppressed in the center-nucleated VHs due to defect healing. Additionally, we demonstrated that the valley polarization of lower-lying intralayer excitons is more sensitive to the defect density of the sample than to higher-lying intralayer excitons. Despite defect healing in the center-nucleated VHs, the temperature-dependent PL study indicated that valley depolarization of the lower-lying intralayer excitons becomes significant below 100 K because of stronger hybridization of defect states. Also, we conducted a comprehensive study on the excitation intensity dependence to investigate the electron-doping-induced Auger recombination mechanism, which also contributes to valley depolarization of intralayer excitons via regeneration of intervalley trions. Our findings provide valuable insight into the development of VH-based valleytronic devices.

Carrier recombination in highly Al doped 4H-SiC: dependence on the injection conditions

We investigate carrier recombination mechanisms in heavily aluminum (Al) doped p-type 4H-SiC, a material crucial for power devices. The recombination mechanisms in Al-doped p-type 4H-SiC have remained unclear, with reports suggesting various possibilities. To gain insights, we employ photoluminescence (PL) measurements, particularly time-resolved PL (TR-PL), as they are well-suited for studying carrier lifetimes in heavily Al-doped p-type 4H-SiC. We examine the temperature and excitation intensity dependencies of TR-PL and PL spectra and discuss the underlying recombination mechanisms. We observe that the dominant recombination mechanism varies with injection conditions for the samples with Al concentration less than 1019 cm−3. Under low injection conditions, recombination via the Al acceptor level appears dominant, exhibiting weak temperature dependence. However, under high injection conditions, Shockley–Read–Hall recombination takes precedence, leading to shorter carrier lifetimes with increasing temperature. This temperature dependence implies that presences of the deep recombination centers with the small capture barrier for holes.

Importance of evaluating the excitation intensity dependence of the temperature-dependent luminescence: Impact on optical thermometry and anti-thermal quenching performance

Afterglow materials exhibit distinct temperature-dependent luminescence compared to general optical materials due to the presence of extra unique energy transfer paths from traps to luminescent centers. These paths include: 1) energy transfer through the conduction band (CB), and 2) energy transfer via quantum tunneling channels to various energy levels. These have a notable impact on both the overall intensity and the relative intensity of emission at different temperatures, which is often unintentionally overlooked. To investigate the influence of these additional paths on temperature-dependent luminescence, we employed a simple yet effective method of varying the excitation light intensity in Y2CaSnGa4O12:Pr3+, an afterglow material. And the influences on optical thermometer and anti-thermal quenching performance were illustrated comprehensively. We ultimately arrived at some interesting and widespread conclusions: 1) The performance of fluorescence intensity ratio (FIR) thermometry, which relies on excitation intensity, can be stabilized by avoiding the presence of charging traps. The high sensitivity of the afterglow intensity ratio (AIR)-based thermometry, which does not require an external light source, may be closely associated with the quantum tunneling process. 2) The performance of trap-assisted anti-thermal quenching is excitation intensity-dependent. We elucidated the specific contribution of traps that had previously been unaccounted for in earlier reports.

Cooperative luminescence of Yb3+ ions in multisite YAM crystal

A series of Yb3+-doped yttrium aluminum monoclinic Y4Al2O9 (Yb:YAM) single crystals with Yb3+ ion concentrations of 0.1, 1, 5 and 10 at.% was grown by means of micro pulling down method. Blue emission was observed in Yb3+ activated YAM crystals under infrared site-selective laser excitation. Through the excitation and emission spectra, decay time measurements and excitation intensity dependence the process responsible for this anti-Stokes emission was assigned to the cooperative deexcitation of two Yb3+ ions. The results show that at 10% Yb3+ dopant concentration investigated system could be considered as single-site system in which cooperative luminescence is dominated by the energetically lowest Yb3+ site. Theoretical cooperative emission spectrum and the cooperative luminescence rate of Yb3+:YAM were calculated.

(Invited) Ternary-Cascaded Triplet Energy Landscape for Solid-State Photon Upconversion

Here, we introduce a new strategy for improving the efficiency of solid-state photon upconversion based on triplet–triplet exciton annihilation (TTA-UC). We designed a ternary blend system consisting of a triplet sensitizer (TS), an exciton-transporting host polymer, and a small amount of an annihilator. The key idea underpinning this concept involves sequential triplet energy transfer from the TS to host and then to the annihilator, driven by the cascaded triplet energy landscape. Because of the small annihilator blend ratio, the local density of triplet excitons in the annihilator domain is higher than the overall density, leading to faster TTA. Based on the excitation-intensity and blend-ratio dependences of TTA-UC efficiency, we found that our concept has a positive impact on accelerating TTA.

Stable and low-threshold photon upconversion in nondegassed water by organic crystals.

Photon upconversion (UC) is a technology that converts lower-energy photons (longer wavelength light) into higher-energy photons (shorter wavelength light), the opposite of fluorescence. Thus, UC is expected to open a vast domain of photonic applications that are not otherwise possible. Recently, UC by triplet-triplet annihilation (TTA) between organic molecules has been studied because of its applicability to low-intensity light, although the majority of such studies have focused on liquid samples in the form of organic solvent solutions. To broaden the range of applications, solid-state UC materials have been an active area of research. We recently developed air-stable, high-performance molecular UC crystals that utilize a stable solid-solution phase of bicomponent organic crystals. This article begins with a brief overview of previous challenges in developing and improving solid-state TTA-UC materials. Then, we briefly review and explain the concept as well as advantages of our molecular solid-solution UC crystals. We applied these organic crystals for the first time to a water environment. We observed blue UC emission upon photoexcitation at 542nm (green-yellow light) and then measured the excitation intensity dependence as well as the temporal stability of the UC emission in air-saturated water. In nondegassed water, these organic crystals were stable, functioned with a low excitation threshold intensity of a few milliwatts per square centimeter, and exhibited high photo-irradiation durability at least over 40h; indicating that the developed organic crystals are also viable for aqueous conditions. Therefore, the organic crystals presented in this report are expected to extend the domain of UC-based photonic applications in practical water systems including in vivo diagnostic, clinical, and therapeutic applications.

Substrate and Excitation Intensity Dependence of Saturable Absorption in Perovskite Quantum Dot Films.

Saturable absorption in perovskite quantum dot (PQD) films, leading to saturation in photoluminescence (PL), is reported. PL of drop-casting films was used to probe how excitation intensity and host-substrate influence the growth of PL intensity. The PQD films were deposited on single-crystal GaAs, InP, Si wafers and glass. Saturable absorption was confirmed through PL saturation in all films, with different excitation intensity thresholds, suggesting strong substrate-dependent optical properties, resulting from absorption nonlinearities in the system. The observations extend our former studies (Appl. Phys. Lett., 2021, 119, 19, 192103), wherein we pointed out that the PL saturation in QDs can be used to create all-optical switches in combination with a bulk semiconductor host.

Optical readout of singlet fission biexcitons in a heteroacene with photoluminescence detected magnetic resonance.

Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in the photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity and revealing the zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically detected magnetic resonance (ODMR) peaks. Fits to these data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the signof the ODMR signal for the triplet features, but not for the quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and highlight anisotropic and multi-scale dynamics of triplet pairs.

Study of the type-I to type-II band alignment transition in InAs(Sb)/GaAs quantum dot nanostructures

InAs(Sb)/GaAs self-assembled quantum dots (QDs) were grown by molecular beam epitaxy though incorporating Sb into InAs with increased beam equivalent pressure ratio, x = Sb2/(Sb2+As2), of x = 0, 0.15, 0.25, and 0.5. From photoluminescence measurements, we observed a transition of the band alignment from type-I for the InAs QDs to type-II for InAsSb QDs. Thus, after adding Sb into the QDs these type II InAsSb QDs have different characteristics from the well-studied type-I InAs QDs regarding to their excitation intensity dependence, temperature dependence, and time dependence of the luminescence. Additionally, the Sb distribution in the QDs and the wetting layer was studied. These results indicate that we are able to fabricate InAsSb QDs with a type II band alignment, which may be good optical materials for the development of intermediate-band photovoltaic cells.

Impact of Excitation Intensity-Dependent Fluorescence Intensity Ratio of Upconversion Nanoparticles on Wide-Field Thermal Imaging

Erbium ion (Er3+)-doped upconversion nanoparticles (UCNPs) are frequently used for nanothermometry because their fluorescence intensity ratio (FIR) between two green emission bands at ∼525 and ∼545 nm is sensitive to temperature variation. One of the prerequisites for nanothermometry is that the FIR be independent of excitation intensity at constant temperature. In this work, the effect of excitation intensity on the FIR of core–double-shell NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+@NaYF4 UCNPs was investigated in two environments. The first environment is in aqueous solution, and the second is a monolayer of UCNPs on top of a silica–silicon substrate in air. The experimental results showed that the FIR decreases with the excitation intensity at constant temperature in each case. We further found that the excitation intensity-dependent FIR indeed deteriorated the thermal images acquired by wide-field upconversion fluorescence microscopy, in which a Gaussian laser beam was used to excite UCNPs uniformly coated on a silica–silicon substrate. The nonuniform excitation intensity of the incident laser beam resulted in thermal images that showed nonuniform temperature distributions in a 100 μm range field of view, even though the whole sample was maintained at constant temperature in air. To tackle this problem, we first measured the excitation intensity and temperature dependence of the FIR and the excitation laser intensity distribution on the sample. We then developed a correction scheme to correct the thermal images. With our correction process, the temperature distribution on the sample can be accurately mapped even with nonuniform illumination.

Direct Detection of Long-Range Interdomain Auger Recombination in Dumbbell-Shaped Quasi-Type-II Nanoparticle.

Multicarrier dynamics in heterostructured ZnS-AgInS2 (ZAIS) dumbbell-like nanoparticle (nanodumbell), which consists of two visible-light absorptive domains (ellipsoidal tip domains) directly linked to each end of a 22 nm length rod domain of the ZAIS nanodumbell with a quasi-type-II heterostructure, was investigated by femtosecond transient absorption spectroscopy under variable excitation intensities. Quantitative analysis together with the numerical simulations for the excitation intensity dependence of the dynamics revealed that only one electron-hole pair survived in the overall dumbbell as a consequence of Auger recombination, even though multiple carriers were formed on both terminal tip domains. This result strongly suggested carrier-carrier interaction between the tip domains, leading to the long-range Auger recombination via tunneling across a rod potential barrier.

Two-Photon Fluorescence in Red and Violet Conjugated Polymer Microspheres

We investigate the two-photon fluorescence (TPF) of conjugated polymer (CP) microspheres with diameters up to tens of micrometers. Two polymers, emitting in either the violet or red, were first synthesized and characterized in terms of their one-photon fluorescence and three-dimensional internal microstructure. Under femtosecond infrared excitation, both types of microspheres showed a strong TPF, which was investigated by the excitation intensity dependence, emission spectroscopy, time-resolved luminescence, and photobleaching dynamics. While the violet-fluorescent microspheres performed similarly compared to dye-doped polystyrene counterparts emitting at a similar wavelength, the red-fluorescent microspheres showed a two-orders-of-magnitude stronger TPF. This excellent performance is attributed to enhanced hyperpolarizability associated with intermolecular interactions in the polymer solid, indicating a route toward designed CP microspheres that could outperform currently-available microparticles for sensing or imaging applications involving two-photon fluorescence.

How Blinking Affects Photon Correlations in Multichromophoric Nanoparticles

A single chromophore can only emit a maximum of one single photon per excitation cycle. This limitation results in a phenomenon commonly referred to as photon antibunching (pAB). When multiple chromophores contribute to the fluorescence measured, the degree of pAB has been used as a metric to "count" the number of chromophores. But the fact that chromophores can switch randomly between bright and dark states also impacts pAB and can lead to incorrect chromophore numbers being determined from pAB measurements. By both simulations and experiment, we demonstrate how pAB is affected by independent and collective chromophore blinking, enabling us to formulate universal guidelines for correct interpretation of pAB measurements. We use DNA-origami nanostructures to design multichromophoric model systems that exhibit either independent or collective chromophore blinking. Two approaches are presented that can distinguish experimentally between these two blinking mechanisms. The first one utilizes the different excitation intensity dependence on the blinking mechanisms. The second approach exploits the fact that collective blinking implies energy transfer to a quenching moiety, which is a time-dependent process. In pulsed-excitation experiments, the degree of collective blinking can therefore be altered by time gating the fluorescence photon stream, enabling us to extract the energy-transfer rate to a quencher. The ability to distinguish between different blinking mechanisms is valuable in materials science, such as for multichromophoric nanoparticles like conjugated-polymer chains as well as in biophysics, for example, for quantitative analysis of protein assemblies by counting chromophores.

Direct determination of molar absorption coefficients of several molecules in the lowest excited singlet states.

Molar absorption coefficient of the lowest excited state is an indispensable information for the quantitative investigation of photochemical reactions by means of transient absorption spectroscopy. In the present work, we quantitatively estimated the molar absorption coefficients of the S1 state of the solute in three solution systems, Rhodamine B in ethanol, ZnTPP in DMF and N,N'-bis(2,6-diisopropylphenyl)terrylene-3,4,11,12-tetracarboxydiimide (TDI) in chloroform, by perfectly bleaching the ground state molecules using the picosecond 532-nm laser pulse with a large number of photons. These solution systems were selected because no obvious photodegradation was detected in the present range of the excitation intensity. The molar absorption coefficient obtained by this method was verified by the numerical analysis of the excitation intensity dependence of the transient absorbance by taking into account the inner filter effect (absorption of the excitation light by the S1 state produced by the leading part of the pump pulse) and the decrease of the ground state molecules by the pump process (depletion). In addition, these molar absorption coefficients were confirmed by the comparison of relations between the excitation intensity and the transient absorbance of the S1 state under the condition where the fraction of the excited solute is ≪ 10% by the femtosecond pulsed laser excitation. From these results, the error of the molar absorption coefficients was estimated to be < 5%. These values can be used as reference ones for the estimation of molar absorption coefficients of other systems leading to the quantitative elucidation of the photochemical reactions detected by the transient absorption spectroscopy.

Broadband sum frequency generation spectroscopy of dark exciton states in hBN-encapsulated monolayer WSe2

We demonstrate that broadband sum frequency generation (SFG) spectroscopy based on a partially incoherent supercontinuum light source can elucidate dark p-series excitons in monolayer WSe2 encapsulated between hexagonal boron nitride (hBN) slabs. The observed 2p exciton peak energy is a few meV higher than that predicted by the Rytova-Keldysh potential model, which is originated from the Berry phase effect. Interestingly, although the radiative relaxation of the 2p exciton is weaker, the 2p exciton peak is broader than the 1s and 2s peaks, which indicates its faster dephasing than the 1s and 2s excitons. Measuring the excitation intensity and temperature dependence, we clarified that this broader linewidth is not caused by excitation- or phonon-induced dephasing, but rather by exciton-electron scattering.

Complex features of the photoluminescence from ZnO nanorods grown by vapor-phase transport method

We present photoluminescence (PL) studies of ZnO nanorods synthesized by high-temperature vapor-phase transport method under pulse laser excitation. The PL spectrum contains a strong near band edge (NBE) emission located at around 387 nm and no emission bands in the visible range were observed. Our studies show that an increase in power density of pulse UV laser results in appearance of a so-called P-band centered at 389 nm and it overlaps the NBE emission. The variations in excitation intensity and detection angles revealed that the band at 389 nm can be deconvoluted into Lorentzian components at ~382 and ~395 nm. Excitation intensity dependence of the band at 389 nm exhibits a clear threshold behavior while the intensity of the band at ~395 nm grows almost linearly with the increase in the excitation intensity. It is possible that the emission band at 395 nm is associated with emission centers caused by zinc vacancy defects or their complexes in ZnO nanorods.

Sensitizer-host-annihilator ternary-cascaded triplet energy landscape for efficient photon upconversion in the solid state.

In this paper, we introduce a new strategy for improving the efficiency of upconversion emissions based on triplet-triplet exciton annihilation (TTA-UC) in the solid state. We designed a ternary blend system consisting of a triplet sensitizer (TS), an exciton-transporting host polymer, and a small amount of an annihilator in which the triplet-state energies of the TS, host, and annihilator decrease in this order. The key idea underpinning this concept involves first transferring the triplet excitons generated by the TS to the host and then to the annihilator, driven by the cascaded triplet energy landscape. Because of the small annihilator blend ratio, the local density of triplet excitons in the annihilator domain is higher than those in conventional binary TS/annihilator systems, which is advantageous for TTA-UC because TTA is a density-dependent bimolecular reaction. We tracked the triplet exciton dynamics in the ternary blend film by transient absorption spectroscopy. Host triplet excitons are generated through triplet energy transfer from the TS following intersystem crossing in the TS. These triplet excitons then diffuse in the host domain and accumulate in the annihilator domain. The accumulated triplet excitons undergo TTA to generate singlet excitons that are higher in energy than the excitation source, resulting in UC emission. Based on the excitation-intensity and blend-ratio dependences of TTA-UC, we found that our concept has a positive impact on accelerating TTA.

Effects of irradiation of ZnO/CdS/Cu2ZnSnSe4/Mo/glass solar cells by 10 MeV electrons on photoluminescence spectra

Solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo/glass were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 1015 cm−2 and then 5.4 × 1015 cm−2 of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant band at 0.94 eV from the CZTSe layer and a lower intensity high energy band (HEB) at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is free-to-bound (FB): the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new band in the examined spectral range (from 0.5 μm to 1.65 μm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure temperature dependencies of the PL spectra. After irradiation the red shift rate of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band.

Niobium Carbide MXenes with Broad-Band Nonlinear Optical Response and Ultrafast Carrier Dynamics.

Exploring the nonlinear photonics of emerging promising two-dimensional (2D) materials like MXenes will boost the development of broad-band optoelectronic and photonic applications. In this paper, the broad-band nonlinear optical response and the excited-carrier dynamics of an emerging MXene, Nb2C, are systematically investigated for the wavelength range of visible to the near-infrared band. The obtained nonlinear optical response shows a wavelength and excitation intensity dependence. The imaginary part of the third-order nonlinear optical susceptibility Imχ(3) and figure of merit were found to be -1.4 × 10-10 esu and 7.5 × 10-12 esu cm, respectively. The interesting nonlinear absorption response inversion properties (e.g., a shift from saturable absorption to two-photon absorption) of Nb2C nanosheets in the near-infrared promise possible important applications in nonlinear photonics, such as an optical switch. We also demonstrate that the wavelength-dependent relaxation times consist of two different relaxation components, that is, time constants in which one is hundreds of femtoseconds and the other is several picoseconds. Our results indicate promising potential in near-infrared nanophotonic applications of 2D Nb2C and offer a promising candidate for 2D-material-based nanophotonic devices and beyond.

Excitation-intensity and temperature dependences of photoluminescence in ZnMgO film

Photoluminescence (PL) properties of Zn0.8Mg0.2O film are investigated under high excitation and low temperature. With increasing excitation intensity, emission from the inelastic exciton-exciton scattering gradually dominates, meanwhile the bound exciton emission decreases. The exciton binding energy is estimated to be ~110–128 meV from the energy of P-band and PL thermal quenching, much higher than that of bulk ZnO. Temperature-dependent PL spectra reveal a room temperature internal quantum efficiency of 9.2% under high excitation, suggesting a high optical quality of the Zn0.8Mg0.2O film.