Two-photon Photoluminescence Research Articles

Two-photon upconversion photoluminescence in CsPbBr3 nanoplates

The utilization of solar energy by light conversion processes in both natural and artificial photosynthesis systems has become a pivotal means of sustaining a consistent power supply for our daily lives and production. In general, these reactions rely on the visible spectra region where the sun produces abundant energy according to the black body's emission. Further expanding the adoption of the near-infrared (NIR) spectral region through upconversion has become an intelligent strategy to improve the entire utilization rate of solar energy. In this work, the two-photon upconversion (TPUC) properties were discovered in CsPbBr3 nanoplates (NPs) emitted at 450 nm, and a sub-gap energy level was observed to serve as the center to facilitate the upconversion process. The excitation threshold for TPUC is drastically lower than that for the two-photon absorption, another nonlinear upconversion path that requires strengthened excitation power. Moreover, a significant negative correlation between the excitation threshold for TPUC in the CsPbBr3 NPs and the exciton binding energy was unveiled, which could be attributed to the enhanced quantum confinement effects. Our work paved the way for a feasible way to expand the utilization of the NIR spectral region in light conversion applications.

Plasmon dephasing time and optical field enhancement in a plasmonic nanobowl substrate studied by scanning near-field optical microscopy.

Plasmonic substrates have been extensively investigated due to their potential applications in fluorescence microscopy, chemical sensing, and photochemical reactions. The optical properties of the substrate depend on the spatial and temporal features of the plasmon excited. Hence, the ability to directly visualize plasmon dynamics is crucial. In this study, we investigated the spatial and temporal properties of plasmon excitation in a plasmonic nanobowl substrate consisting of a periodic hexagonal array of nanoscale bowl-like structures developed with self-assembly. Near-field transmission imaging revealed that multiple plasmon resonance bands are observed from visible to near-infrared spectral region, and the optical contrast of the image is dependent on the observed band. Near-field two-photon photoluminescence microscopy revealed that the probability of excitation inside each nanoscale bowl-like structure is greater than that in the surrounding area. Near-field time-resolved imaging revealed that the nanobowl substrate exhibited a substantially long plasmon dephasing time, exceeding 12fs. Based on the spectral features of the near-field and far-field spectra, we found that optically dark plasmon mode is excited by the near-field illumination and only partly contributes to the long dephasing time observed. This fact indicates that the dephasing time is extended by some other mechanism in the periodic substrate. We revealed from this study that the enhanced optical fields induced in the nanobowl structure originate from the photosynergetic effect of the cavity mode and plasmon mode excited.

Rashba-Type Band Splitting Effect in 2D (PEA)2PbI4 Perovskites and Its Impact on Exciton-Phonon Coupling.

Despite a few recent reports on Rashba effects in two-dimensional (2D) Ruddlesden-Popper (RP) hybrid perovskites, the precise role of organic spacer cations in influencing Rashba band splitting remains unclear. Here, using a combination of temperature-dependent two-photon photoluminescence (2PPL) and time-resolved photoluminescence spectroscopy, alongside density functional theory (DFT) calculations, we contribute to significant insights into the Rashba band splitting found for 2D RP hybrid perovskites. The results demonstrate that the polarity of the organic spacer cation is crucial in inducing structural distortions that lead to Rashba-type band splitting. Our investigations show that the intricate details of the Rashba band splitting occur for organic cations with low polarity but not for more polar ones. Furthermore, we have observed stronger exciton-phonon interactions due to the Rashba-type band splitting effect. These findings clarify the importance of selecting appropriate organic spacer cations to manipulate the electronic properties of 2D perovskites.

Raspberry-like Nanoheterostructures Comprising Glutathione-Capped Gold Nanoclusters Grown on the Lanthanide Nanoparticle Surface.

Bare lanthanide-doped nanoparticles (LnNPs), in particular, NaYF4:Yb3+,Tm3+ NPs (UCTm), have been seeded in situ with gold cations to be used in the subsequent growth of gold nanoclusters (AuNCs) in the presence of glutathione (GSH) to obtain a novel UCTm@AuNC nanoheterostructure (NHS) with a raspberry-like morphology. UCTm@AuNC displays unique optical properties (multiple absorption and emission wavelengths). Specifically, upon 350 nm excitation, it exhibits AuNC photoluminescence (PL) (500-1200 nm, λmax 650 nm) and Yb emission (λmax 980 nm); this is the first example of Yb sensitization in a UCTm@AuNC NHS. Moreover, under 980 nm excitation, it displays (i) upconverting PL of the UCTm (at the blue, red and NIR-I, ca. 800 nm, regions); (ii) two-photon PL of AuNC; and (iii) down-shifting PL of thulium (around 1470 nm). The occurrence of energy transfer from UCTm to AuNCs in the UCTm@AuNC NHS was evidenced by the drastic lengthening of the AuNC PL lifetime (τPL) (from few hundred nanoseconds to more than one hundred microseconds). Initial biological assessment of UCTm@AuNC NHSs in vitro revealed high biocompatibility and bioimaging capabilities upon near-infrared excitation.

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures.

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equationmodel to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time of ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Single- and two-photon excited photoluminescence of 2D perovskite thin films with different organic spacer cations

The length of the organic amine chains of 2D perovskites can significantly affect their low temperature excitonic and two-photon excited luminescent properties.

Near-infrared two-photon excited photoluminescence from Yb3+-doped CsPbClxBr3-x perovskite nanocrystals embedded into amphiphilic silica microspheres.

Nonlinear absorption of metal-halide perovskite nanocrystals (NCs) makes them an ideal candidate for applications which require multiphoton-excited photoluminescence. By doping perovskite NCs with lanthanides, their emission can be extended into the near-infrared (NIR) spectral region. We demonstrate how the combination of Yb3+ doping and bandgap engineering of cesium lead halide perovskite NCs performed by anion exchange (from Cl- to Br-) leads to efficient and tunable emitters that operate under two-photon excitation in the NIR spectral region. By optimizing the anion composition, Yb3+-doped CsPbClxBr3-x NCs exhibited high values of two-photon absorption cross-section reaching 2.3 × 105 GM, and displayed dual-band emission located both in the visible (407-493 nm) and NIR (985 nm). With a view of practical applications of bio-visualisation in the NIR spectral range, these NCs were embedded into silica microspheres which were further wrapped with amphiphilic polymer shells to ensure their water-compatibility. The resulting microspheres with embedded NCs could be easily dispersed in both toluene and water, while still exhibiting a dual-band emission in visible and NIR under both one- and two-photon excitation conditions.

Nonlinear Optical Properties from Engineered 2D Materials.

Two-dimensional (2D) materials with atomic thickness, tunable light-matter interaction, and significant nonlinear susceptibility are emerging as potential candidates for new-generation optoelectronic devices. In this review, we briefly cover the recent research development of typical nonlinear optic (NLO) processes including second harmonic generation (SHG), third harmonic generation (THG), as well as two-photon photoluminescence (2PPL) of 2D materials. Nonlinear light-matter interaction in atomically thin 2D materials is important for both fundamental research and future optoelectronic devices. The NLO performance of 2D materials can be greatly modulated with methods such as carrier injection tuning, strain tuning, artificially stacking, as well as plasmonic resonant enhancement. This review will discuss various nonlinear optical processes and corresponding tuning methods and propose its potential NLO application of 2D materials.

Synthesis, photophysical characterization, and integration of two-photon-responsive fluorophores in mesoporous organosilica nanoparticles for biological imaging use

We report the synthesis of new graftable three-branched octupolar fluorophores (F and F2), based on a triphenylamine core connected to electron-withdrawing sulfone-based peripheral groups through one or two π-conjugated phenylene-vinylene linkers. Their two-photon absorption (TPA), solvatochromism and photoluminescence properties were investigated to derive structure-property relationships. Notably, the chromophores presented TPA bands in the biological window (700–900 nm) and high TPA cross-sections at peak of 810 GM for F and F2. Then, after silylation, fluorophore F was further covalently integrated within the sol-gel synthesis of different periodic mesoporous organosilica nanoparticles (PMO NPs). Transmission electronic microscopy imaging revealed the formation of spherical NPs with good monodispersity in size, as confirmed by dynamic light scattering measurements. The obtained fluorophore-based nanoplatforms were shown to be biocompatible in vitro and demonstrated a good potential for two-photon imaging in cancer cells using two-photon excited fluorescence microscopy.

Gold Nanostars Obviate Limitations to Laser Interstitial Thermal Therapy (LITT) for the Treatment of Intracranial Tumors.

Laser interstitial thermal therapy (LITT) is an effective minimally invasive treatment option for intracranial tumors. Our group produced plasmonics-active gold nanostars (GNS) designed to preferentially accumulate within intracranial tumors and amplify the ablative capacity of LITT. The impact of GNS on LITT coverage capacity was tested in ex vivo models using clinical LITT equipment and agarose gel-based phantoms of control and GNS-infused central "tumors." In vivo accumulation of GNS and amplification of ablation were tested in murine intracranial and extracranial tumor models followed by intravenous GNS injection, PET/CT, two-photon photoluminescence, inductively coupled plasma mass spectrometry (ICP-MS), histopathology, and laser ablation. Monte Carlo simulations demonstrated the potential of GNS to accelerate and specify thermal distributions. In ex vivo cuboid tumor phantoms, the GNS-infused phantom heated 5.5× faster than the control. In a split-cylinder tumor phantom, the GNS-infused border heated 2× faster and the surrounding area was exposed to 30% lower temperatures, with margin conformation observed in a model of irregular GNS distribution. In vivo, GNS preferentially accumulated within intracranial tumors on PET/CT, two-photon photoluminescence, and ICP-MS at 24 and 72 hours and significantly expedited and increased the maximal temperature achieved in laser ablation compared with control. Our results provide evidence for use of GNS to improve the efficiency and potentially safety of LITT. The in vivo data support selective accumulation within intracranial tumors and amplification of laser ablation, and the GNS-infused phantom experiments demonstrate increased rates of heating, heat contouring to tumor borders, and decreased heating of surrounding regions representing normal structures.

Single and two-photo excited violet-blue fluorescence from aza [6]helicenes and the plannar precursors based-on indole, carbazole, and fluorene

Organic π-conjugated materials exhibiting violet-blue emissions are of great interest for diverse applications. Helicenes and stilbenes hold great potential as compounds capable of achieving violet-blue fluorescence. In this study, aza[6]helicenes THNI and DHNC, along with their planar precursors DFVHI and HHIVC, were successfully synthesized using indole, carbazole, and fluorene units as building blocks. The molecular structures were verified through the utilization of 1H and 13C nuclear magnetic resonance (NMR), high resolution mass spectroscopies (HRMS), and the crystal structure of THNI was further elucidated via X-ray analysis. The synthesized material exhibited exceptional thermal stability with a Td value of ≥350 °C. The HOMO and LUMO energy levels were determined through cyclic voltammetry analysis, while density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d) level were employed to investigate ground-state geometries and vertical electronic transitions. Optical properties were characterized using UV–vis absorption spectroscopy as well as single- and two-photon excited photoluminescence. The UV–vis absorption spectra indicate that helicenes possess superior light-harvesting capabilities compared to their planar precursors, owing to the former's higher molar extinction coefficients. These compounds exhibit violet-blue emissions in various solvents and solid-state films. In addition, aza[6]helicenes exhibit higher photoluminescence quantum yields (PLQYs) and shorter maximum emission peaks compared to their planar precursors. However, the maximum two-photon absorption (TPA) cross section (δ) values of aza[6]helicenes are lower than those of their planar counterparts. Therefore, THNI and DHNC may be promising candidates for violet-blue emitters in OLEDs due to their curved confined π-electron systems, high thermal and optical stability, and high PLQYs. Meanwhile, DFVHI and HHIVC exhibit potential as two-photon fluorescent probes for bioimaging.

Photostable organic two-photon dyes with ultrahigh brightness for long-term fluorescence imaging.

Developing photostable two-photon dyes with high brightness and negligible photocytotxicity is of key importance in fluorescence imaging but remains a huge challenge. Here, a two-photon dye with ultrahigh brightness and photostability is demonstrated for high-performance long-term two-photon fluorescence imaging. By terminated donor engineering, the designed DBD shows higher two-photon absorption cross-section (δ, 418 GM vs 329 GM) and photoluminescence quantum yield (ΦPL, 62.74% vs 54.63%) than counterpart DBA. As a consequence, two-photon fluorescence brightness (δ×ΦPL) of DBD exhibits a 10-folded enhancement (262 GM vs 19 GM) in comparison with typical Coumarin 307 dye. More importantly, DBD present ultrahigh photostability and negligible photobleaching under 10 min femtosecond laser irradiation, which stands marked contrast to Coumarin 307. Furthermore, femtosecond transient absorption spectroscopy ascribes this ultrahigh photostability and negligible photobleaching to the inefficient intersystem crossing. With these merits, DBD perform long-term two-photon fluorescence imaging in vitro.

Anisotropic Two-Photon Excited Photoluminescence and Optical Constants of InGaN Nanowires

Gallium nitride (GaN) and its derivatives are important materials for optoelectronic and electronic applications, such as light-emitting diodes, solid-state lasers, optical modulators, artificial photosynthesis, etc. Controlled growth and manipulation of these materials at nanoscale dimensions and understanding their linear and nonlinear optical properties lay the foundation for developing next-generation optoelectronic and electronic devices. In this context, herein, we report on the two-photon absorption and anisotropic two-photon excited photoluminescence of InGaN (IGN) nanowires grown by molecular beam epitaxy. The IGN nanowires were imaged using a home-built two-photon photoluminescence microscope with a tunable femtosecond laser to excite over the near-IR range (690–950 nm). In addition, the polarization angle of the excitation electric field was also rotated, and the emitted photoluminescence was analyzed with a polarizer. Results showed polarized photoluminescence with a stronger signal in the direction perpendicular to the IGN nanowires’ axis. Also, we observed strong two-photon absorption above the bandgap, with several excitation spectrum peaks near 740, 790, and 870 nm. These peaks showed anisotropic response to the excitation polarization angle, which may be due to the anisotropy of the IGN nanowires’ wurtzite phase. The polarized light absorption and emission nature of IGN nanowires, as revealed in this work, may be useful for optimizing materials for future optoelectronic device applications.

Ambient Tip-Enhanced Two Photon Photoluminescence from CdSe/ZnS Quantum Dots

Nonlinear nano-optical measurements that combine ultrafast spectroscopy with tools of scanning probe microscopy are scarce. This is particularly the case when high spatial resolution on the order of a few nanometers is sought after in experiments performed under ambient laboratory conditions. In this work, we demonstrate the latter through measurements that track two-photon photoluminescence from aggregates of CdSe/ZnS quantum dots with sub-5 nm spatial resolution. Our proof-of-principle measurements that only take advantage of a plasmonic probe (as opposed to a gap mode) pave the way for nonlinear photoluminescence-based spectral nanoimaging of realistic/heterogeneous (bio) molecular and (bio) material systems.

Nonlinear Optical Signal Generation Mediated by a Plasmonic Azimuthally Chirped Grating

Plasmonic gratings are simple and effective platforms for nonlinear signal generation since they provide a well-defined momentum for photon-plasmon coupling and local hot spots for frequency conversion. Here, a plasmonic azimuthally chirped grating (ACG), which provides spatially resolved broadband momentum for photon-plasmon coupling, was exploited to investigate the plasmonic enhancement effect in two nonlinear optical processes, namely two-photon photoluminescence (TPPL) and second harmonic generation (SHG). The spatial distributions of the nonlinear signals were determined experimentally by hyperspectral mapping with ultrashort pulsed excitation. The experimental spatial distributions of nonlinear signals agree very well with the analytical prediction based on photon-plasmon coupling with the momentum of the ACG, revealing the "antenna" function of the grating in plasmonic nonlinear signal generation. This work highlights the importance of the antenna effect of the gratings for nonlinear signal generation and provides insight into the enhancement mechanism of plasmonic gratings in addition to local hot spot engineering.

Effects of Single Platinum Atom Doping on Stability and Nonlinear Optical Properties of Ag29 Nanoclusters

The properties of atomically precise noble metal nanoclusters can be modified by the addition of other metals, which may offer augmented characteristics, making them more suitable for real-life applications. In this work, we report the effects of single platinum atom doping of a class of atomically precise silver nanoclusters protected by dithiolated ligands on their optical properties both in linear and nonlinear optical (NLO) regimes and on their stability over time. Pt doping of Ag29 preserves their NLO properties (in particular two-photon excited photoluminescence).

Two-photon excited biexciton photoluminescence in colloidal nanoplatelets CdSe/CdS

The photoluminescence features under resonant two-photon excitation of a light-hole – electron exciton transition in CdSe/CdS nanoplatelets colloidal solution (536 nm, 1lh-1e) were investigated. The discovered power-law dependence of their photoluminescence intensity on the excitation laser femtosecond pulses intensity was revealed to be relating to biexciton radiation. In addition, the conditions for the photoluminescence growth and quenching were determined depending on the concentration and pump intensity. We believe the results presented in our study will provide a clearer comprehension of the electro-optical processes physics in CdSe/CdS nanoplatelets, which will expand the scope of their application, including frequency up-conversion amplification and lasing, also be beneficial for biosensor design.

Probing Excitonic Rydberg States by Plasmon Enhanced Nonlinear Optical Spectroscopy in Monolayer WS2 at Room Temperature.

The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers such as WS2 are largely dominated by excitons due to strong Coulomb interactions in these 2D confined monolayers, which lead to formation of Rydberg-like excitonic states below the free quasiparticle band gap. The precise knowledge of high order Rydberg excitonic states is of great importance for both fundamental understanding such as many-electron effects and device applications such as optical switching and quantum process information. Bright excitonic states could be probed by linear optical spectroscopy, while probing dark excitonic states generally requires nonlinear optical (NLO) spectroscopy. Conventional optical methods for probing high-order Rydberg excitonic states were generally performed at cryogenic temperatures to ensure enough signal-to-noise ratio (SNR) and narrow line width. Here we have designed a hybrid nanostructure of monolayer WS2 integrated with a plasmonic cavity and investigated their NLO properties at the single particle level. Giant enhancement in NLO responses, stronger excitonic resonance effects, and narrowed line widths of NLO excitation spectra were observed when monolayer WS2 was placed in our carefully designed plasmonic cavity. Optimum enhancement of 1000-, 3000-, and 3800-fold were achieved for two-photon photoluminescence (2PPL), second harmonic generation (SHG), and third-harmonic generation (THG), respectively, in the optimized cavity structure. The line width of SHG excitation spectra was reduced from 43 down to 15 meV. Plasmon enhanced NLO responses brought improved SNR and spectral resolution, which allowed us to distinguish discrete excitonic states with small energy differences at room temperature. By using three complementary NLO techniques in combination with linear optical spectroscopy, energies of Rydberg excitonic states of A (1s, 2s, 2p, 3s, 3p, 4s), B (1s), and C and D excitons of monolayer WS2 have been accurately determined, which allow us to determine exciton binding energy and quasiparticle bandgap. It was interesting to find that the 2p lies 30 meV below 2s, which lends strong support to the theoretical prediction of nonlocal dielectric screening effects based on a non-hydrogenic model. Our results show that plasmon enhanced NLO spectroscopy could serve as a general method for probing high order Rydberg excitonic states of 2D materials.

One and Two‐Photon Excited Fluorescence Optimization of Metal–Organic Frameworks with Symmetry‐Reduced AIEgen‐Ligand

By designing a tetraphenylethylene (TPE)-based AIEgen-ligand with reduced symmetry, we obtained two alkaline-earth metal-based MOFs (LIFM-102 and LIFM-103) with dense packing structures and low porosity as proved by single-crystal X-ray diffraction and CO2 sorption data. Excitingly, the desolvated MOFs with rigid environment and reduced lattice free solvent exhibit high quantum yields (QY, 64.9 % and 79.4 %) and excellent two-photon excited photoluminescence (TPA cross-sections, 2946.6 GM and 2899.0 GM), while maintaining the external-stimuli-responsive properties suitable for anticounterfeit fields. The effect of ligand conformation was validated by comparing the structure and fluorescence properties of the samples before and after desolvation and further verified by theoretical calculations. This work expands the study on TPE-cored materials to symmetry-reduced ligand and might bring forward novel structures and excellent photoluminescent properties in the future.

(Invited) Controlling Interlayer Stacking Configuration to Optimize Exciton Extraction Pathways in Van Der Waals Materials

Employing ultrafast microscopy methods, we demonstrate how tuning the interlayer coupling by twisting stacking orientations results in different metastable electronic states like Moiré excitons in twisted bilayer graphene (tBLG) and bound triplet pairs (TT) in molecular crystals. Considering first tBLG, we show how stacking-angle tunable absorption resonances form a strongly-bound exciton state due to the interlayer orbitals' symmetric rehybridization. Using two-photon photoluminescence and intraband-transient absorption (TA) microscopies, we have imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly bound (up to 690 meV), metastable interlayer exciton states.[1] Our observation of resonant PL emission from twisted bilayer graphene materials is best explained by the theoretically predicted coexistence of strongly bound interlayer excitons and metallic graphene continuum states to form Moiré excitons.Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer-directed E-field, inducing electron-hole dissociation as shown in Fig. 1b. Time-resolved photocurrents show that stacked WSe2 devices shown in Fig. 1a can have both IQE >50% and fast (<60 ps) picosecond electron escape times. Our ultrafast photocurrent rates kinetics give the same E-field-dependent electronic escape and dissociation rates seen from ultrafast optical TA microscopy.[2] To rationalize these fast electronic escape rates, we show the ratio of the electronic rates accurately predicts the actual WSe2 device photocurrent generation efficiency. Lastly, we will show how certain intermolecular twist angle packings of athraditiophene molecular crystals make electron-multiplication by singlet fission of TT states favorable. Singlet fission dynamics are indicated in Fig. 1c by the matching singlet (blue) vs. rising triplet dynamics (red) obtained when the probe polarization aligned along the crystal charge-transfer axis. However, other intermolecular packing angles (at 90o) instead localize and trap excitons as excimers, preventing singlet fission.[3] These interlayer stacked systems collectively demonstrate how remarkably different interlayer electronic states evolve from relatively small changes in interlayer twist angles in van der Waals stacked materials and molecular singlet fission materials.