Multiphoton Excited Photoluminescence Research Articles

Plasmon-enhanced multi-photon excited photoluminescence of Au, Ag, and Pt nanoclusters

In this work, we have studied the multi-photon excited photoluminescence from metal nanoclusters (NCs) of Au, Ag and Pt embedded in Al2O3 matrix by ion implantation. The thermal annealing process allows to obtain a system composed of larger plasmonic metal nanoparticles (NPs) surrounded by photoluminescent ultra-small metal NCs. By exciting at 1064 nm, visible emission, ranging from 450 to 800 nm, was detected. The second and fourth-order nature of the multiphoton process was verified in a power-dependent study measured for each sample below the damage threshold. Experiments show that Au and Ag NCs exhibit a four-fold enhanced multiphoton excited photoluminescence with respect to that observed for Pt NCs, which can be explained as a result of a plasmon-mediated near-field process that is of less intensity for Pt NPs. These findings provide new opportunities to combine plasmonic nanoparticles and photoluminescent nanoclusters inside a robust inorganic matrix to improve their optical properties. Plasmon-enhanced multiphoton excited photoluminescence from metal nanoclusters may find potential application as ultrasmall fluorophores in multiphoton sensing, and in the development of solar cells with highly efficient energy conversion modules.

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.

Observation of nanopipes in edge-defined film-fed grown β-Ga2O3 substrate and their effect on homoepitaxial surface hillocks

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (00)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

Combining second harmonic generation and multiphoton excited photo-luminescence to investigate TiO2 nanoparticle powders.

Disentangling Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) signals in microscopy experiments is not an easy task. Two methods have been so far proposed based either on a time domain or a spectral domain analysis of the collected signals. In this report, a new method based on polarization discrimination is proposed to separate these SHG and MEPL contributions. In order to demonstrate this operation, intensity depth profiles are recorded for an anatase titanium dioxide powder consisting of 22 nm diameter nanoparticles using ultrafast femtosecond laser excitation. Polarization analysis of these intensity depth profiles is therefore performed and demonstrates a polarization angle shift for the SHG intensity contribution as compared to the MEPL one, allowing for the discrimination of the two SHG and MEPL contributions. The fundamental beam is set at two different wavelengths in order to provide a SHG photon energy above and below the anatase TiO2 band-gap of 3.2 eV, leading to a change in the relative intensity weight and a spectral shift between the SHG and MEPL contributions. This operation further demonstrates the potential of the method when the spectral domain disentangling cannot be performed. SHG profiles are by far narrower than those of MEPL. This study where both SHG and MEPL contributions are observed offers perspectives in photonics of powder materials as the different origin and properties of the two processes can be separated.

Simultaneous Second Harmonic Generation and multiphoton excited photoluminescence in anatase TiO2 nano powders

Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) are observed in retro-reflection mode from a 22 nm diameter anatase TiO2 nanoparticle powder. Depth intensity profiles are obtained by scanning the fundamental beam focal point through the TiO2 powder surface into its volume for the energy of the incident photons tuned around the anatase TiO2 band-gap energy of 3.2 eV. Evidence for the appearance of both SHG and MEPL is presented, their relative contribution depending on the conditions of photo-excitation, in particular the fundamental wavelength and the focal point depth location. MEPL dominates at fundamental photon energies matching the band-gap energy whereas below and above the band-gap SHG is clearly seen. Depth intensity profiles exhibiting a rising edge determined by the fundamental laser beam focusing conditions and a decreasing edge determined by extinction and multiphoton scattering strongly differ between SHG and MEPL, SHG profiles being by far narrower than that of MEPL. Re-absorption of the emitted SHG and MEPL light occurs due to the spectral proximity with the TiO2 band gap energy but excitation with fundamental light below band-gap further suggests that the intrinsic difference between the two SHG and MEPL processes is responsible for the contrasted depth profiles. This combined study where both SHG and MEPL occur presents perspectives in the study of the photonic properties of powder materials.

Size of dislocation patterns induced by Vickers indentation in hydride vapor-phase epitaxy GaN

3D dislocation structures induced by Vickers indentation depending on the imprint size are precisely investigated using an alternation of cathodoluminescence and chemical mechanical polishing (CMP), multiphoton excitation photoluminescence, and (scanning) transmission electron microscopy under a load range within a constant Vickers hardness. The dislocation structures are composed of a rosette pattern, a flower pattern, and a triangular area. The flower pattern distribution is dispersive along the z direction. The determined penetration depth of the dislocations by practical CMP is almost the same as the length of the imprint diagonal (d1). The dimensions of the individual patterns in the dislocation structure can be described as multiples of d1. That is, the geometric similarity between the imprint size and the dimensions of the dislocation structure is confirmed. This suggests that the dimensions of the dislocation structures induced under scratch can be estimated by measuring the width of the scratch. This shows that a simple method may be developed to determine the maximum thickness of the affected layer over the entire wafer area and the ability to develop low-cost GaN wafers free of affected layers.

Observation of threading dislocations with a c + m type Burgers vector in HVPE GaN substrates using multi-photon excitation photoluminescence and TEM

Dislocations that have a large Burgers vector are potential leakage current pathways in power devices. In this study, we analyzed the dislocations under a large pit (L-pit) produced by chemical etching in a hydride-vapor-phase-epitaxy GaN substrate by using multi-photon excitation photoluminescence (MPPL) and transmission electron microscopy (TEM). Three-dimensional dislocation images reconstructed from MPPL clearly showed that two threading dislocations (TD-1 and TD-2) approached each other laterally while propagating along the growth direction [0001]. They interacted near the sample surface and merged into a single dislocation whose large Burgers vector was responsible for the L-pit formation. A TEM observation operated in large-angle convergent-beam electron diffraction mode unambiguously identified the Burgers vectors of the two dislocations as bTD-1=13[2-1-1-3] (c + a type) and bTD-2 = [10–1–1] (c + m type). The c + m type is extremely rare in GaN. The merged dislocation appeared to constitute Burgers vectors of all three components of c, m, and a.

Highly Efficient Multiphoton Absorption of Zinc-AIEgen Metal-Organic Frameworks.

A series of luminescent frameworks was synthesized from the selective combination of aggregation induced emission (AIE)-linker tetra-(4-carboxylphenyl)ethylene (H4 TCPE) and Zn2+ . Complex 1 was formed by the close packing of Zn-TCPE hinge, and isostructural complexes 2-5 were constructed by the linkage of Zn-TCPE layer and pillar ligands. These complexes exhibit highly efficient multiphoton excited photoluminescence (MEPL) and concomitant third-harmonic generation (THG). The multiphoton absorption (MPA) parameters of 1 are superior to other multiphoton emission materials including the perovskite nanocrystals. The incorporation of pillar linkers slows down the charge transfer between layers of Zn-TCPE, and the aromatic core of pillar linkers has a great influence on the MPA performance of the corresponding frameworks.

Highly Efficient Multiphoton Absorption of Zinc‐AIEgen Metal–Organic Frameworks

AbstractA series of luminescent frameworks was synthesized from the selective combination of aggregation induced emission (AIE)‐linker tetra‐(4‐carboxylphenyl)ethylene (H4TCPE) and Zn2+. Complex 1 was formed by the close packing of Zn‐TCPE hinge, and isostructural complexes 2–5 were constructed by the linkage of Zn‐TCPE layer and pillar ligands. These complexes exhibit highly efficient multiphoton excited photoluminescence (MEPL) and concomitant third‐harmonic generation (THG). The multiphoton absorption (MPA) parameters of 1 are superior to other multiphoton emission materials including the perovskite nanocrystals. The incorporation of pillar linkers slows down the charge transfer between layers of Zn‐TCPE, and the aromatic core of pillar linkers has a great influence on the MPA performance of the corresponding frameworks.

Cover Picture: Highly Efficient Multiphoton Absorption of Zinc‐AIEgen Metal–Organic Frameworks (Angew. Chem. Int. Ed. 12/2022)

Zn-AIEgen crystalline frameworks exhibit highly efficient multiphoton excited photoluminescence and concomitant third-harmonic generation. Exploration of the structural–optical relationship provides a useful methodology for the design of better high-performance multiphoton absorption materials, as reported by Jagadese J. Vittal, Jianzhuang Jiang et al. in their Research Article (e202115205).

Generation of dislocations from scratches on GaN formed during wafer fabrication and dislocation reactions during homoepitaxial growth

Dislocation generation from scratches induced on a bare GaN wafer during the wafer-fabrication process was investigated via multi-photon excitation photoluminescence (MPPL). The scratch was detected as a faint dark contrast by MPPL but had no accompanying dark dislocation loops that expanded on both sides of the scratch in the GaN wafer. The acceptable quality level for the wafer-fabrication process was determined. Dislocations generated from the scratch elongated to the surface with the repetition of the dislocation reaction. The repetition of the dislocation reaction results in the variation of dislocation types ( a , c , and c + a ). The dislocation structures formed after the dislocation reaction appeared to both decrease as well as increase the dislocation energy. The reactions that appeared to increase the dislocation energy could primarily be attributed to the reaction of threading and half-loop dislocations.

In-Plane Anisotropic Nonlinear Optical Properties of Two-Dimensional Organic-Inorganic Hybrid Perovskite.

Two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) with superior nonlinear optical (NLO) properties show great versatility in frequency upconversion applications. Optical anisotropy plays an indispensable role in interpreting the interactions between incoming photons and crystal structure. Recently, the in-plane anisotropic NLO properties of 2D OIHPs have been reported and attracted much attention. However, the structure-related NLO anisotropy of the 2D OIHP framework is not well-established. Here, NLO properties of (C6H5(CH2)2NH3)2PbI4 (PEPI), (C6H11NH3)2PbI4 (C6H11), and (C4H9NH3)2PbI4 (C4PI) were systematically studied to interrogate the correlation between the in-plane anisotropic NLO responses and its lattice structure. In-plane nonparametric NLO responses, e.g., two-photon photoluminescence (2PPL) and three-photon photoluminescence (3PPL), manifest similar anisotropy configurations for PEPI, C6H11, and C4PI regardless of aromatic, cyclic, or linear organic molecules; however, the anisotropies of THG signals are strongly dependent on the specific crystal structures of the individual flakes, and they are much higher than that of the multiphoton excited photoluminescence.

Propagation of threading dislocations and effects of Burgers vectors in HVPE-grown GaN bulk crystals on Na-flux-grown GaN substrates

The propagation behavior of threading dislocations (TDs) and the effects of Burgers vectors in hydride vapor phase epitaxy (HVPE) GaN bulk crystals generated on Na-flux-grown GaN and in a commercially available HVPE-grown GaN bulk crystal were investigated. Analyses based on chemical etching and transmission electron microscopy (TEM) revealed a close correlation between the etch pit sizes and the Burgers vector of these TDs. The existence of TDs with the unique Burgers vector b = 1m + 1c was observed for the first time ever using a large-angle convergent-beam electron diffraction technique and plan-view bright-field scanning TEM. Multi-photon excitation photoluminescence microscopy observations showed that TDs with b = 1c had a meandering morphology in contrast to the linear morphology of TDs with b = 1a, 1a + 1c, or 1m + 1c in both types of HVPE-grown GaN crystals. The inclinations of TDs with b = 1a and 1a + 1c in HVPE-grown GaN on Na-flux-grown GaN were greatly affected by large symmetrical hexagonal hillocks. The TDs with b = 1a were inclined in the slope directions of the hillock planes, while those with b = 1a + 1c were inclined in the a directions parallel or antiparallel to the a component in their Burgers vector. These inclinations were readily explained by the isotropic elastic theory of an individual TD. The hillocks were produced around pairs of parallel mixed TDs for which the a components were opposite to one another. This phenomenon was attributed to inclusions at the interface between the Na-flux-grown GaN substrate and the HVPE-grown layer. The origins of other TDs including unusual ones having b = 1m + 1c are also discussed herein.

Identification of Burgers vectors of threading dislocations in freestanding GaN substrates via multiphoton-excitation photoluminescence mapping

Burgers vectors b of threading dislocations in freestanding GaN substrates were identified through statistical classification using the etch pit and multiphoton-excitation photoluminescence methods. The b = 1a + 1c threading mixed dislocations were propagated with large inclination angles more than four degrees from the c-axis to minimize elastic strain energy. The b = 1c threading edge dislocations had a large contrast difference of more than 0.4 in the photoluminescence image due to their strong nonradiative properties. Based on these characteristics obtained through multiphoton-excitation photoluminescence measurements, major types of dislocations (b = 1a, 1a + 1c, 1c) could statistically be identified.

Local piezoelectric properties in Na-flux GaN bulk single crystals

The local piezoelectricity of a Na-flux GaN crystal grown on a multipoint-seed-GaN template is investigated using piezoresponse force microscopy. The piezoresponse is critically dependent on two types of growth regions that are dominantly formed in the Na-flux GaN crystal: the c-growth sector (cGS), which is grown on top of the point-seed GaN surface with a growth front of (0001) planes, and the facet-growth sector (FGS), which is grown on the side of cGS with {101¯1} facets. Quantitative analyses reveal the GaN surface displacements at cGS that result from the piezoresponses increase with the applied AC voltage: the measured values well reflect the piezoelectric constant of d33 in GaN. The piezoresponses at the FGS and the boundary between the cGS and FGS are less sensitive than that at the cGS. A combination of cathodoluminescence and multiphoton excitation photoluminescence techniques clarifies that a local reduction of the piezoresponse observed in cGS is attributed to microscale FGSs that exist randomly in cGS. The dependence of the piezoresponse on the growth regions is quantitatively discussed from three possible viewpoints that potentially affect the polarization properties of GaN: residual strain, local crystallographic tilting, and inherent carrier distribution. As a result, a carrier screening effect is the most probable candidate to induce reduction of the piezoresponse in the FGSs of GaN crystals.

Anomalous dislocation annihilation behavior observed in a GaN crystal grown on point seeds by the Na-flux method

We recently invented a method called the flux-film-coated technique for purifying a GaN wafer with low dislocation density grown from point-seed crystals. In this study, we investigated the mechanism behind the reduction of dislocation density in the GaN wafer by evaluating the three-dimensional behavior of dislocations using multiphoton-excitation photoluminescence images. We made the surprising discovery that dislocations more than 50 μm away disappeared by annihilating each other as growth proceeded, and this is one of the mechanisms underlying the dislocation density reduction. The moving distance of dislocations before annihilation is uncommon and a unique phenomenon in the Na-flux method.

High quality GaN crystal grown by hydride vapor-phase epitaxy on SCAATTM

GaN crystal with 4.2 mm thickness and size lager than 2 inch without cracks and pits was realized by hydride vapor-phase epitaxy (HVPE) on an acidic ammonothermal (SCAATTM) seed. The average threading dislocation (TD) density was as small as 1.4 × 103 cm−2. Multi-photon excitation photoluminescence revealed that complete one-to-one correspondence of TD at the interface between the seed and epilayer was realized. A comparison between HVPE-GaN film grown on SCAATTM seed and that grown on a conventional self-standing HVPE-GaN seed showed the smaller TDD of SCAATTM than conventional one resulted in a smaller change of residual stress in the crystal.

Nondestructive three-dimensional characterization of GaN using multiphoton-excitation photoluminescence mapping

Nondestructive three-dimensional characterization of GaN using multiphoton-excitation photoluminescence mapping

Correlation between current leakage and structural properties of threading dislocations in GaN bulk single crystals grown using a Na-flux method

This work investigated the electrical and microstructural properties of threading dislocations (TDs) in Na-flux-grown GaN bulk single crystals composed entirely of a c-plane growth sector (cGS). Several microscale Schottky-like contacts fabricated on TD-related etch pits and flat (0001) surfaces were analyzed via a combination of conductive atomic force microscopy, multi-photon excitation photoluminescence and transmission electron microscopy. Significant current leakage was observed in microscale facet growth sectors (μ-FGSs), having high local oxygen impurity concentrations, that had formed in the cGS. Higher leakage currents were found to be associated with bundled a type and a + c type TDs, whereas lower leakage currents were produced by single a type TDs. The leakage current evidently increased as the size of etch pits formed on the μ-FGSs increased. These observations establish a close correlation between current leakage and TD-induced strain fields that affect the distribution of point defects such as oxygen impurities around TDs.

Observation of very high order multi-photon absorption in GeSbS chalcogenide glass

We report high order multi-photon absorption in Ge23Sb7S70 chalcogenide glasses up to the 11th order. Open aperture z-scan measurements with enhanced detection capabilities are performed at short-wave to mid-infrared wavelengths between 1.1 μm and 5.5 μm, enabling very high order multi-photon absorption to be experimentally observed. We report the multi-photon absorption coefficients for Ge23Sb7S70 within the measurement range corresponding to 3–11 photon absorption. The dispersive Kerr nonlinear index of the chalcogenide glass in the short-wave to mid-infrared wavelength range is measured, resulting in a peak n2 value of 4.4 × 10−18 m2/W. Enhancements to the nonlinear refractive index from the presence of defect states are observed. We further report experimentally verified multi-photon absorption excitation of photoluminescence in Ge23Sb7S70 chalcogenide glasses. The results pave the way for previously unexplored, new paradigms of photonic device design leveraging very high order multi-photon absorption effects and multi-photon excited photoluminescence.