Two-photon Photoluminescence Research Articles

A fluorescent probe for the detection of Cu(II) in water and tumor cells

A fluorescent Au nanocluster LRL−AuNCs modified with RGDK-containing peptide was synthesized with a simple one-pot method. Based on the “turn-off” model, LRL−AuNCs displayed high sensitivity and selectivity for sensing Cu2+. It showed a good linear calibration between the fluorescence intensity and Cu2+ concentration in the range of 5 ~ 80 μM (R2 = 0.998), with the detection limit 3.9 μM. which is far lower than the maximum allowable concentration of Cu2+ (20 μM) in drinking water. Therefore, the detection of Cu2+ in water sample was performed in a handy way. As LRL−AuNCs showed low toxicity and good biocompatibility, LRL−AuNCs was used to sense the change of Cu2+ level in tumor cells by two-photon photoluminescence (TPPL) imaging. These results indicated that the LRL−AuNCs as a TPPL probe showed potential in tumor diagnosis and treatment.

Four orders-of-magnitude enhancement in the two-photon excited photoluminescence of homoleptic gold thiolate nanoclusters following zinc ion-induced aggregation

Herein we report unprecedented determination of the molar mass of zinc mediated assemblies of homoleptic gold nanoclusters, based on charge detection mass spectrometry measurements. The accurate determination of the molar mass of zinc coordinated assemblies of gold clusters has further allowed unambiguous determination of the two-photon excited photoluminescence cross section of the same. Furthermore, in line with one-photon excited photoluminescence measurements, four orders-of-magnitude enhancement in two-photon excited photoluminescence of gold nanoclusters has been observed following complexation with zinc ions. The study reported herein is envisioned to not only deepen the fundamental understanding of the multiphoton excitation properties of atomic clusters but also widen their application potential as luminescence markers.

Nature of the different emissive states and strong exciton–phonon couplings in quasi-two-dimensional perovskites derived from phase-modulated two-photon micro-photoluminescence spectroscopy

Quasi two-dimensional perovskites have attracted great attention for applications in light-emitting devices and photovoltaics due to their robustness and tunable highly efficient photoluminescence (PL). However, the mechanism of intrinsic PL in these materials is still not fully understood. In this work, we have analysed the nature of the different emissive states and the impact of temperature on the emissions in quasi two-dimensional methyl ammonium lead bromide perovskite (q-MPB) and cesium lead bromide perovskite (q-CPB). We have used spatially resolved phase-modulated two-photon photoluminescence (2PPL) and temperature-dependent 2PPL to characterize the emissions. Our results show that at room temperature, the PL from q-MPB is due to the recombination of excitons and free carriers while the PL from q-CPB is due to the recombination of excitons only. Temperature-dependent measurements show that in both materials the linewidth broadening is due to the interactions between the excitons and optical phonons at high temperatures. Comparing the characteristics of the emissions in the two systems, we conclude that q-CPB is better suited for light emitting devices. With a further optimization to reduce the impact on the environment, q-CPB-based LEDs could perform as well as OLEDs.

Effect of dipole–dipole interactions on the one-photon and two-photon photoluminescence in an ensemble of quantum dots doped in a polymer matrix

We have developed a theory for the one-photon and two-photon fluorescence of a nanohybrid composite comprised of an ensemble of quantum dots encased in a matrix of polymer. The quantum dots are comprised of four-level quantum emitters encased within a coating of dielectric material. The fluorescence of the quantum dot system is calculated using the master equation method where dipole–dipole interaction (DDI) coupling is present. The application of a probe field to the nanohybrid composite material in conjunction with the DDI between the quantum dots generates an enhancement of both the single-photon and double-photon fluorescence, and in certain conditions causes spectrum splitting. With a strong DDI field, the two-photon emission spectrum experiences slight quenching and shifts from one peak to two peaks. The dressed states are a result of strong coupling effects between the DDI field and the quantum dots. We generated an analytical expression for the fluorescence spectrum and intensity, and quantitatively compared it to experimental data in which ZnSe quantum dots were embedded within a P3HT polymer film, with our theory strongly correlating with the experiment. We also quantitatively compared our theory with experimental data that demonstrated the single-photon emission spectrum of colloidal quantum dots comprised of a CdZnS/ZnS gradient structure in varying probe field strengths. We found that our theory predicted the spectrum peak splitting that the experiment produced.

Giant Quantum Yield Enhancement in CdS/MgF2/Ag Hybrid Nanobelt under Two-Photon Excitation

We demonstrate a giant quantum yield (QY) enhancement (230 times) in a single CdS/MgF2/Ag hybrid nanobelt under the two-photon excitation at 800 nm. The power-dependent two-photon photoluminescence...

Linear and nonlinear chiroptical response from individual 3D printed plasmonic and dielectric micro-helices.

Sub-wavelength chiral resonators formed from artificial structures exhibit exceedingly large chiroptical responses compared to those observed in natural media. Owing to resonant excitation, chiral near fields can be significantly enhanced for these resonators, holding great promise for developing enantioselective photonic components such as biochemical sensors based on circular dichroism (CD) and spin-dependent nonlinear imaging. In the present work, strong linear and nonlinear chiroptical responses (scattering CD > 0.15 and nonlinear differential CDs > 0.4) at visible and near infrared frequencies are reported for the first time for individual micrometer-scale plasmonic and dielectric helical structures. By leveraging dark-field spectroscopy and nonlinear optical microscopy, the circular-polarization-selective scattering behavior and nonlinear optical responses (e.g., second harmonic generation and two-photon photoluminescence) of 3D printed micro-helices with feature sizes comparable to the wavelength (total length is ∼5λ) are demonstrated. These micro-helices provide potential for readily accessible photonic platforms, facilitating an enantiomeric analysis of chiral materials. One such example is the opportunity to explore ultracompact photonic devices based on single, complex meta-atoms enabled by state-of-the-art 3D fabrication techniques.

Second harmonic generation in colloidal CdSe/CdS nanoplatelets

In this Letter, we report the second harmonic generation (SHG) in a colloidal solution of CdSe/CdS nanoplatelets (NPLs) in the case of two-photon excitation in the region of exciton transition from the light holes sub-band to the conduction sub-band (549 nm, 1lh-1e) by femtosecond laser pulses (1064 nm, 100 kHz). Discovery of the SHG in colloidal solutions of NPLs opens up new perspectives for their use as biomarkers or for optical imaging, since in contrast to two-photon photoluminescence the SHG can increase both spectral and temporal resolution by several orders of magnitude. The SHG is also extremely important to consider when designing two-photon pumping lasers.

Au-Coated Ni80Fe20 Submicron Magnetic Nanodisks: Interactions With Tumor Cells

Effective interaction and accumulation of nanoparticles (NPs) within tumour cells is crucial for NP-assisted diagnostic and therapeutic biomedical applications. In this context, the shape and size features of NPs can severely influence the strength of adhesion between NPs and cell and the NPs internalization mechanisms. This study proved the ability of the PT45 and A549 tumor cells to uptake and retain magnetic Au-coated Ni80Fe20 nanodisks (NDs) prepared by means of a bottom-up self-assembling nanolithography technique assisted by polystyrene nanospheres. The chosen geometrical parameters, i.e. diameter (≈ 650 nm) and thickness (≈ 30 nm), give rise to magnetic domain patterns arranged in vortex state at the magnetic remanence. PT45 and A549 cell lines were cultured in presence of different concentrations of Au-coated Ni80Fe20 nanodisks (NDs) and NDs biocompatibility was evaluated by viability and proliferation tests. Electron microscopy techniques and a combined CARS (Coherent Anti-Stokes Raman Scattering) and TPL (two-photon photoluminescence) microscopy allow to localized and distinguish the NDs within or attached to the tumor cells without any labeling. A quantitative measurement of NDs amount retained within tumor cells as a function of NDs concentrations was performed by Instrumental Neutron Activation Analysis (INAA) characterization technique.

Enhanced two-photon photoluminescence assisted by multi-resonant characteristics of a gold nanocylinder

Abstract Multi-resonant plasmonic simple geometries like nanocylinders and nanorods are highly interesting for two-photon photoluminescence and second harmonic generation applications, due to their easy fabrication and reproducibility in comparison with complex multi-resonant systems like dimers or nanoclusters. We demonstrate experimentally that by using a simple gold nanocylinder we can achieve a double resonantly enhanced two-photon photoluminescence of quantum dots, by matching the excitation wavelength of the quantum dots with a dipolar plasmon mode, while the emission is coupled with a radiative quadrupolar mode. We establish a method to separate experimentally the enhancement factor at the excitation and at the emission wavelengths for this double resonant system. The sensitivity of the spectral positions of the dipolar and quadrupolar plasmon resonances to the ellipticity of the nanocylinders and its impact on the two-photon photoluminescence enhancement are discussed.

Multimodal Imaging Iridium(III) Complex for Hypochlorous Acid in Living Systems.

Biomolecule tracing with different imaging methods is of great significance for more accurately unravelling the fundamental processes in living systems. However, considering the different principles of each imaging method for probe design, it is still a great challenge to apply one molecular probe to achieve two or even more imaging analyses for biomarkers. In general, traditional oxime was reported as a recognition group for fluorescence imaging of HOCl. Herein, for the first time, we designed the oxime decorated iridium(III) complex, which can be directly used for chemiluminescence as well as two-photon luminescence and photoluminescence lifetime imaging of HOCl in living systems. Moreover, the novel chemiluminescence mechanism of Ir-CLFLPLIM for HOCl was also proposed and explored by continuously monitoring chemiluminescence peak shapes and mass spectra, inferring the reaction intermediate and calculating the chemical reaction energy range of the reaction process. This strategy could lead us to expand the chemiluminescence application of transition metal complexes and develop more multimodal imaging probes.

(Invited) Ultrafast Photocurrents from Interlayer Excitons in Twisted and Stacked 2D Materials

Using time-space resolved ultrafast microscopy on individual 2D crystal grains, we show how long-range interlayer electronic coupling can be selectively enhanced either by applying an E-field or by twisting the layer stacking orientation. Considering first twisted bilayer graphene (tBLG), we discovered how stacking-angle tunable absorption resonances form a strongly-bound exciton state as a consequence of the symmetrized rehybridization of constrained interlayer 2p orbitals. Using two-photon photoluminescence and intraband-transient absorption microscopies, we have recently 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), quasi-stable interlayer exciton states. 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. Time-resolved photocurrents show that stacked WSe2 devices can have both IQE >50% and fast (<50 ps) picosecond electron escape times. Remarkably our photocurrent response function produces the same E-field-dependent electronic escape and dissociation rates for both the optical and PC addressed ultrafast measurements. To confirm the fast electronic escape rates, we show the ratio of the electronic rates accurately matches our overall TMDC device IQE in the limit of zero Auger recombination. Collectively, we show how optical and photocurrent-based ultrafast microscopies provide an accurate timeline of photocurrent generating dynanamics in both TMDCs and tBLG van der Waals materials.

Determination of absorption coefficients and Urbach tail depth of ZnO below the bandgap with two-photon photoluminescence.

In this article, we demonstrate a novel approach to determine the absorption coefficient of ZnO below the bandgap via measuring the self-absorption (SA) effect on the two-photon luminescence (TPL) spectrum of the ZnO bulk crystal rod at cryogenic temperature. Under a geometric configuration of side-excitation and front-detection, the intensities of several major spectral components of TPL spectra of ZnO can be decisively tuned by precisely varying the transmitting distance of luminescence signal, so that the absorption coefficients at different wavelengths can be determined on the basis of Beer-Lambert law. Furthermore, the peak position of donor bound exciton luminescence exhibits a unique redshift tendency with increasing the transmitting distance. Starting from the product of Lorentzian lineshape function and exponential absorption edge of Urbach tail, an analytical formula is derived to quantitatively interpret the experimental redshift characteristic with the transmitting distance. The energy depth of Urbach tail of the studied ZnO crystal is deduced to be ∼13.3 meV. In principle, this new approach can be used to determine absorption coefficient of any luminescent solids as long as the SA effect happens.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η2σ2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

One- and Two-photon Excited Photoluminescence of Cuprous Iodide Coordination Polymer

One- and Two-photon Excited Photoluminescence of Cuprous Iodide Coordination Polymer

Two-photon photoluminescence of a thin-film hybrid material based on CdSe(core)/ZnS/CdS/ZnS(multishell) semiconductor quantum dots

Semiconductor quantum dots (QDs) are widely used as components of hybrid materials for development of efficient light emitters and convertors. Their unique nonlinear optical properties, such as two-photon absorption and two-photon photoluminescence from biexcitons, make them promising materials for photovoltaic and optoelectronic applications. In this study, thin-film hybrid materials based on the CdSe(core)/ZnS/CdS/ZnS(multishell) QDs have been fabricated, and the two-photon photoluminescence (PL) from the generated biexcitons have been studied. The results show that fabricated thin-film hybrid materials based on the QDs are efficient fluorophores in the one- and two-photon PL regimes for applications in optoelectronics and biosensing.

Synthesis, Structural, Linear, and Nonlinear Optical Studies of Inorganic–Organic Hybrid Semiconductors (R–C6H4CHCH3NH3)2PbI4, (R = CH3, Cl)

Synthesis, crystal structure, and optical properties of two-dimensional (2D) layered structurally slightly different inorganic–organic (IO) hybrid semiconductors (R–C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are presented. They are naturally self-assembled systems where two (RNH3)+ moieties are sandwiched between two infinitely extended 2D layers of the [PbI6]4– octahedral network and treated as natural IO multiple quantum wells. While the former compound crystallizes into an orthorhombic system in the Cmc21 space group, the latter crystallizes into a monoclinic system in the space group P21/c. As a thin film, they are well-oriented along the (l00) direction. Both single crystals and thin films show strong room-temperature Mott type exciton features that are highly sensitive to the self-assembly and crystal packing. Linear (one-photon) and nonlinear (two-photon) optical probing of single crystals for exciton photoluminescence imaging and spectral spatial mapping provide deep insight into the layered re-arrangement and structural crumpling due to organic conformation. The strongly confined excitons, within the lowest band gap of inorganic, show distinctly different one- and two-photon excited photoluminescence peaks: free excitons from perfectly aligned 2D self-assembly and energy down-shifted excitons originated from the locally crumpled layered arrangement. Their structural aspects are successfully presented with proper correlation that emphasize various differences in physical and optical properties associated between these novel IO hybrids.

Gold@Silver@Gold Core Double-Shell Nanoparticles: Synthesis and Aggregation-Enhanced Two-Photon Photoluminescence Evaluation

A facile, straightforward, and low-cost method is proposed to synthesize gold@silver@gold core double-shell nanoparticles. The technique is a seed-mediated growth protocol that contains four steps of (1) gold seed synthesis, (2) gold seed growth, (3) silver layer coating through silver salt reduction, and (4) gold layer deposition via gold precursor reduction. The prepared nanoparticles had a narrow size distribution and the average particle size of 28 ± 1 nm. Cysteine was introduced to the nanoparticles solution as a coupling agent to assemble nanoparticles. Aggregation-induced two-photon photoluminescence enhancement of three types assembled nanoparticles, i.e., gold@silver@gold, gold@silver, and gold nanoparticles, was studied. It was observed that the assembled core double-shell nanoparticles presented huge enhancement in two-photon photoluminescence signal in comparison with two other nanoparticles. Moreover, the gold@silver@gold nanoparticle is a stable and biocompatible plasmonic nanosystem. This paper provides a novel candidate for two-photon photoluminescence excitation sensing and imaging for biomedical applications.

Two-Photon Up-Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi-2D Perovskite Films.

A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA)2 (MA)4 Pb5 Br16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two-photon up-conversion PL signal. This confirms that the gap states are indeed responsible for generating the two-photon up-conversion PL in quasi-2D perovskites. Furthermore, mechanical scratching indicates that the different-n-value nanoplates are essentially uniformly formed in the quasi-2D perovskite films toward generating multi-photon up-conversion light emission. More importantly, the two-photon up-conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi-photon excitation. Polarization-dependent up-conversion PL studies reveal that the gap states experience the orbit-orbit interaction through Coulomb polarization to form spatially extended states toward developing multi-photon up-conversion light emission in quasi-2D perovskites.

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7-Fold to 8-Fold Interpenetrated Metal-Organic Frameworks (MOFs).

Interpenetration in metal-organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7-fold interpenetrated MOF (1) is transformed into an 8-fold interpenetrated MOF by the loss of DMF in a single-crystal-to-single-crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two-photon photoluminescence (ca. 14 times). The strengthened π-π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30-150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single-crystal NLO devices.

Two-photon excited photoluminescence of single perovskite nanocrystals.

Lead-halide perovskite nanocrystals (NCs) have emerged as a novel type of semiconductor nanostructure, attracting great research interests in both fundamental science and practical applications. Here, we compare the optical properties of single CsPbI3 NCs under both one-photon and two-photon excitations, mainly including the photoluminescence (PL) blinking and PL decay dynamics. By means of the PL saturation effect caused by multi-exciton Auger recombination, we have also estimated a two-photon absorption cross section of ∼6.8 × 106 GM for single CsPbI3 NCs. The ability to realize efficient two-photon excitation of single perovskite NCs with strongly suppressed background fluorescence will help not only to promote their bio-imaging and biolabeling applications but also to reveal and manipulate their delicate electronic structures for potential usage in quantum information processing.