Photoluminescence Polarization Research Articles

Giant Modulation of Second-Harmonic Generation in CuInP2S6 by Interfacing with MoS2 Atomic Layers.

Probing and manipulating the intriguing nonlinear optical responses in van der Waals (vdW) ferroelectrics offer opportunities for their applications in nanophotonics. Here, we report the observation of giant and tunable second-harmonic generation (SHG) in ferroelectric CuInP2S6 (CIPS) and CIPS/MoS2 heterostructures. The results show that CIPS, ranging from multilayer to bulk-like samples, all exhibit strong SHG with giant anisotropy. The SHG anisotropy is attributed to the local strain along the a-axis that naturally exists in CIPS, as evidenced by piezoresponse force microscopy measurement. We further realized the strong modulation of SHG in CIPS by interfacing with monolayer MoS2. A combination of polarization, temperature, and thickness-dependent SHG and photoluminescence analyses shows that the nonlinear optical signal control in CIPS/MoS2 heterostructures is unrelated to the polar symmetry of CIPS and MoS2 but is driven by light absorption-mediated interfacial coupling. Our study provides a material platform based on vdW ferroelectric heterostructures for achieving dynamic control of nonlinear optical responses, which shows great potential applications in modern nanophotonics.

Direct Linearly Polarized Emission in van der Waals LEDs via Flexoelectric Effect

AbstractFollowing the rapid development of information technology, modern polarized light, which is a critical component for display and data transmission, has been in demand for miniaturization and high efficiency, rendering two‐dimensional (2D) semiconductors potential candidates. The traditional polarized light is usually generated by external optical structures or polarizers that influence the scaling and bring up losses. Previous works have reported polarized light emission from inversion‐asymmetric 2D semiconductors such as black phosphorus (BP), black arsenide phosphorus (AsP), and rhenium disulfide (ReS2), however, their emission wavelengths are not in the visible range. Here, a direct emission of linearly polarized light is demonstrated from van der Waals light–emitting diodes (vdWLEDs) via the flexoelectric effects by inducing the non‐uniform strain in monolayer (ML) transition metal dichalcogenides (TMDCs). In this work, the effects of strain including excitonic binding energy and exciton dipole moment distribution is analyzed by the density functional theory (DFT) then we show that linearly polarized photoluminescence (PL) with a degree of linear polarization (DOLP) of ≈17% can be realized at room temperature (RT), and the polarization angle is perpendicular to the direction of the strain‐gradient. By incorporating the strained ML TMDCs into vdWLEDs, electroluminescence (EL) with DOLP of ≈19% can be observed at RT. This work puts forward a direct and universal strategy for fabricating polarized LEDs based on inversion‐symmetric semiconductors.

(Invited) Chiral Metamaterials for Polarized Photoluminescence

Metasurfaces offer compact routes to spatial and polarization control of luminescence from nearby emitters, enabling applications ranging from document security to stereoscopic displays to sensing.Most of the time, light-emitters are distributed evenly across the surface. This talk will discuss an alternate approach, where we use direct-write electron beam lithography to pattern semiconductor nanocrystals into targeted shapes and locations, creating a highly tailored metasurface. We use a combination of nanoscale fabrication, Fourier-space photoluminescence measurements, and computational methods to study the circularly polarized photoluminescence from these designer structures.We will discuss the influence of patterning on the resulting optical properties, including the effects of spatial averaging and dipole orientation. We will then show an optimization strategy to produce highly polarized light at chosen angles of outcoupling. The final part of the talk will discuss adding layers of complexity, including the chirality of the underlying pattern, and creating light-emitting solids from intrinsically chiral semiconductor nanocrystals.

Seeing is believing: Correlating optoelectronic functionality with atomic scale imaging of single semiconductor nanocrystals.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical-structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam-scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor-metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical-structural correlation method for numerous scenarios and timely questions in the field of QD research.

Inverse exciton spin orientation due to trion formation in modulation doped quantum wells

Time-resolved and time-integrated circularly polarized photoluminescence of excitons and trions in external magnetic fields up to 10 T has been studied in undoped and n-type doped quantum well structures based on ZnSe. In an undoped structure, a circular polarization of photoluminescence induced by magnetic fields corresponded to the Boltzmann distribution of excitons on Zeeman sublevels. The inverse spin orientation of excitons is observed in doped samples with a carrier density of 3×1010 cm−2 and higher. Model calculations show that the reason for the inverse spin orientation is the effective depletion of the lowest-exciton Zeeman sublevel as a result of the spin-dependent formation of trions. The trion formation time as a result of exciton-electron binding was determined as 2 ps. This is noticeably shorter than the characteristic time of the exciton-photon interaction. The observed effect can be considered as a way of spin manipulation by electric fields.

Polarized photoluminescence and g-factor in an inhomogeneous ensemble of quantum dots in magnetic fields

In this work, polarized photoluminescence caused by quasi-equilibrium spin orientation of excitons at Zeeman sublevels in an ensemble of quantum dots of different sizes is studied theoretically. The following unexpected results are found: (i) the splitting of photoluminescence bands in a magnetic field in an ensemble of quantum dots is much larger than the Zeeman splitting of exciton levels in a single dot, (ii) contrary to the Boltzmann distribution the low-energy luminescence band has a lower intensity than the high-energy band, (iii) the sign of the circular polarization of photoluminescence changes along the contour of the bands, (iv) a universal formula for the dependence of the exciton g-factor on the quantum dot size is obtained. All of the above features of the polarized luminescence spectra are explained by variations of the exciton g-factor when its quantization energy changes. The results obtained are applicable not only to ensembles of quantum dots, but also to any ensemble of localized states: excitons in quantum wells under conditions of well-width fluctuations, impurity states in quantum wells, etc.

Self‐Assembled 2D Sheets of an Amphiphilic Sexiphenyl Exhibit Intense Polarized Blue Emission in the Solid State

AbstractDistinct from inorganic 2D nanomaterials (e.g., graphene, MoS2, etc,), self‐assembled organic 2D materials are significant due to the unique advantages of fine structure tunability and functionality. Electronically and photofunctionally‐active compounds (e.g., pentacene, p‐sexiphenyl) are highly attractive for applications but are limited by difficult handling and the existence of only a few native structures of the compounds. Here, it is shown that a p‐sexiphenyl derivative incorporated into 2D multilamellar sheets with chromophores oriented in the layers exhibits a significant polarization of photoluminescence. Highly emissive, amphiphilic sexiphenyl with hydrophilic/hydrophobic groups self‐assembles at air–water interface with chromophore orientation in the resulting films being controlled by monolayer compression. Large‐area multilamellar structures with p‐sexiphenyl chromophores highly oriented against the 2D plane are prepared by consecutive film transfers. The 2D sheets exhibit significant polarization of photoluminescence with polarization ratio 0.8 between orthogonal in‐plane axes. Notably, multilamellar structures cannot be established for the same p‐sexiphenyl using thermotropic processing thus emphasizing the importance of the 2D sheet multilayer synthesis process. Amphiphile design can be applied for the tailored synthesis of large‐area multilayered 2D sheets and used to construct highly efficient light‐emitting devices.

Room-Temperature Magnetic-Induced Circularly Polarized Photoluminescence in Two-Dimensional Er2O2S.

Two-dimensional (2D) materials with spin polarization have great potential for achieving next-generation spintronic applications. However, spin polarization of 2D materials is usually produced at a cryogenic temperature because of thermal fluctuations, which severely hinder their further applications. Here, we report room-temperature intrinsic magnetic-induced circularly polarized photoluminescence (PL) in 2D Er2O2S flakes. The geff factor of 2D Er2O2S stays at around -6.3 from the liquid He temperature limit to room temperature, which is independent of temperature. This anomalous phenomenon in Er2O2S is totally different from previous materials, which all have a decreasing Zeeman splitting with increasing temperature resulting from thermal fluctuations. The anomalous temperature-dependent magnetic-induced circularly polarized PL originates from the weak electron-phonon coupling in 2D Er2O2S, which has been proven by both the temperature-dependent Raman and theoretical calculations. This work sheds light on the understanding and manipulation of 2D materials for practical spintronic applications.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S−3AMP)x(R−3AMP)1−xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 − 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Localized Temperature Engineering Enables Writing of Heterostructures in Glass for Polarized Photoluminescence of Perovskites.

Metal halide perovskite (MHP) structures that exhibit polarized photoluminescence (PL) have attracted significant interest in fabricating light field regulation elements for display, imaging, and information storage applications. We report a three-dimensional direct lithography of heterostructures for controllable polarized PL inside glass by laser-induced localized temperature engineering. The heterostructures consisted of oriented periodic structures (OPSs) and MHP nanocrystals, and the mechanism for hierarchical distribution of heterostructures was illustrated. The patterning of heterostructures for manipulable polarized PL can be used for information encryption, wave-plate, and polarized micro-LEDs.

Polarized photoluminescence from Sn, Fe, and unintentionally doped β-Ga2O3

In this work, we demonstrate that β-Ga2O3 shows orientation-dependent polarized photoluminescence (PL) emission and give a comprehensive insight into gallium oxide's PL spectral properties. We characterized the polarization and spectral dependencies of both the incident and emitted light for (−201) unintentionally doped (UID) as well as (−201) and (010) Sn-doped and Fe-doped crystals. We observed for UID and Sn-doped samples that the electron to self-trapped hole and native defect-related emission bands are linearly polarized with polarized emission intensities ordered as E || c (and c*) > E || a (and a*) > E || b. Furthermore, the spectral shape of emission does not change between the UID and Sn-doped samples; instead, the Sn-doping quenches the total PL spectral intensity. For Fe-doped samples, polarized red emission caused by unintentional Cr3+ doping generates emission intensities ordered E || b > E || c (and c*) > E || a (and a*). It is also observed that in some circumstances, for some doped crystals, the PL spectra can show variations not only in intensity but also in spectral shape along different polarization directions. As an example, the PL emission band for emission along c is blueshifted relative to that along a in Sn-doped β-Ga2O3.

Crystalline architectures of C84 with tunable morphology and linearly polarized red emission.

Fullerene-based micro/nano-architectures (FMNAs) with remarkable photoluminescence (PL) emissions have attracted considerable interest as potential building blocks for optical and biolabeling applications, by virtue of their low toxicity and environmentally friendly nature. Nevertheless, the PL polarization properties of FMNAs have rarely been explored. Herein, we demonstrate the preparation of highly crystalline architectures of C84, which exhibit polymorphism depending on the preparation conditions but possess similar hexagonal close-packed (hcp) structures. The PL data demonstrate that the as-prepared carambola-like hexagonal microprisms (c-HPs) show enhanced red emission compared to regular hexagonal microprisms (r-HPs). More importantly, the linear polarization of the PL emission is verified and estimated through single-prism spectroscopy, which changes from 0.42 (r-HP) to 0.58 (c-HP), comparable to those of traditional rod-like semiconductors. Thus, we demonstrate a significant correlation between the morphology and emission characteristics of C84-based microprisms, highlighting the possibility of controlling the photophysical properties of FMNAs by finely tailoring their external morphologies. This study expands the range of carbon materials with linearly polarized emissions and offers potential for use in polarization-based micro-scale sensors or detectors.

Rapid Spin Depolarization in the Layered 2D Ruddlesden-Popper Perovskite (BA)(MA)PbI.

We report temperature-dependent spectroscopy on the layered (n = 4) two-dimensional (2D) Ruddlesden-Popper perovskite (BA)(MA)PbI. Helicity-resolved steady-state photoluminescence (PL) reveals no optical degree of polarization. Time-resolved PL shows a photocarrier lifetime on the order of nanoseconds. From simultaneously recorded time-resolved differential reflectivity (TRΔR) and time-resolved Kerr ellipticity (TRKE), a photocarrier lifetime of a few nanoseconds and a spin relaxation time on the order of picoseconds was found. This stark contrast in lifetimes clearly explains the lack of spin polarization in steady-state PL. While we observe clear temperature-dependent effects on the PL dynamics that can be related to structural dynamics, spin relaxation is nearly T-independent. Our results highlight that spin relaxation in 2D (BA)(MA)PbI occurs at time scales faster than the exciton recombination time, which poses a bottleneck for applications aiming to utilize this degree of freedom.

Scanning the time and space evolution of anisotropy in single molecular crystals based on 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile

The 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile (LAXYUI)), acting as a model, is a type of outstanding organic fluorophore. Using a transient absorption approach, the excitation relaxation behavior of LAXYUI in the monomolecular system and ordered crystals was identified. The polarization optical properties of LAXYUI crystals, including anisotropy of absorption and photoluminescence, polarization PL decay at different temperatures, and the evolution of optical anisotropy as a function of propagation length, are thoroughly investigated using a series of home-built polarization optical techniques. Our results can help people better understand the time- and space-dependent optical anisotropy that occurs in organic crystals, which will be important in the future to improve or broaden the performance of polarization optical systems based on organic crystals.

Dramatically Enhanced Valley-Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe2 x S2(1- x ) Alloys at Room Temperature with 1T'-WTe2 -Contact.

Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two-dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe2 x S2(1- x ) monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto-valleytronic applications. Here, new strategies to efficiently tailor the valley-polarized PL from semiconducting monolayer WTe2 x S2(1- x ) at room temperature (RT) through alloying and back-gating are presented. The DVP at RT is found to increase drastically from < 5% in WS2 to 40% in WTe0.12 S1.88 by Te-alloying to enhance the spin-orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe0.12 S1.88 via metallic 1T'-WTe2 electrodes, where the use of 1T'-WTe2 substantially lowers the Schottky barrier height (SBH) and weakens the Fermi-level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley-polarized emission from 1T'-WTe2 /WTe0.12 S1.88 heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications.

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Eco-Friendly Methanolic Myrrh Extract Corrosion Inhibitor for Aluminum in 1 M HCl.

Aluminum corrosion was inhibited by myrrh extract when it was placed in a solution of 1 M HCl. Several procedures were used for these tests, including weight loss WL, potential dynamic polarization PL, and electrochemical impedance EIS in addition to theoretical calculations like density functional theory (DFT), Fukui functions, and Monte Carlo simulation. Fourier transform infrared spectroscopy was used to analyze the compositional surface of Al. Scanning electron microscopy was used to determine the shape of the Al surface. The inhibition rate of Al corrosion in HCl with varying myrrh extract contents at 25-45 °C was studied. An analysis of the PL curves indicates that myrrh extract is an inhibitor of mixed type. Upon increasing the concentration of myrrh, the inhibition efficiency increased. Moreover, rising temperatures decrease inhibition efficiency. It was discovered that the inhibition process follows the Langmuir isotherm, demonstrating that a monolayer has formed on the surface of aluminum. Theoretical and practical studies proved the validity of the conclusions.

Polarization-sensitive visible emission of ordered CdSe/CdS nanorod films

The polarization-sensitive optical properties of semiconductor nanorods (NRs) are attractive characteristics for optical devices. Although oriented NR films can provide convenient integration and new application opportunities, the realization of large-area NR films with polarized photoluminescence (PL) remains a challenge. Herein, we fabricated ordered CdSe/CdS NR films on an arbitrary substrate using the Langmuir-Blodgett technology. Polarization-sensitive PL was observed for the NR films. The band-edge exciton fine structure owing to the rod-like morphology and the dielectric effect of the orderly arranged NRs were discussed with a focus on the contribution to polarized photons. The degree of linear polarization of the CdSe/CdS NR films excited a circularly polarized light was approximately 0.30. When linearly polarized ultraviolet (UV) light was used as the excitation source, the PL intensity of the NR films periodically varied with the polarization direction of the UV light. The corresponding PL polarization (i.e., “excitation polarization”) was approximately 0.28. The polarization-sensitive fluorescence (approximately 526 nm) excited by the UV-polarized light (325 nm) demonstrates the potential for realizing low-cost polarization-sensitive devices using only a visible light charge-coupled device system.

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate fully valley-coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

Dark Exciton in 2D Hybrid Halide Perovskite Films Revealed by Magneto‐Photoluminescence at High Magnetic Field

AbstractA comprehensive study of the exciton fine structure (EFS) is presented in 2D‐phenethylammonium lead iodide films using magnetic field‐induced polarization of photoluminescence (PL) in both Faraday and Voigt configurations at fields up to 25 Tesla. Three exciton bands are identified in the PL spectrum associated with bound, dark, and bright excitons, respectively. Under a high magnetic field in Faraday/Voigt configuration, large field‐induced circular/linear polarization is observed in the PL band related to the dark exciton, which is magnetically activated. Furthermore, it is found that the dark exciton has an anomalous field‐induced circular polarization, which cannot be explained by the classical Boltzmann distribution of spin‐polarized species. These findings are well explained by an effective mass model that includes exchange terms unique to the monoclinic symmetry as a perturbation of the EFS in the approximate tetragonal symmetry. It is also confirmed that the field‐induced linear polarization is sensitive to the monoclinic exchange term, whereas the field‐induced circular polarization is immune to such term.