Optical Polarization Anisotropy Research Articles

Plasmon-enhanced upconversion luminescence on horizontally aligned gold nanorod arrays with self-contained spacer

The growing interest in converting infrared light to visible light in many photovoltaic fields requires the development of upconversion systems with high luminescence quantum yields and stability without blinking. However, the weak and narrow absorption of rare earth ions has greatly restricted the luminescent efficiency of rare earth dopants and hampered their translation from experimental research to real world applications. In this work, horizontally aligned gold nanorod arrays with self-contained spacer and obvious polarization dependent plasmonic properties are prepared on the substrate of silicon wafers by a self-assembly method of droplet evaporation, and polarization-dependent enhancement on the upconversion luminescence of rare earth ions doped nanoparticles (NaYF4: Yb3+, Er3+) are investigated. The results show that the luminescence intensity of the nanoparticles is greatly enhanced by a factor of about 140, when the horizontally aligned gold nanorod arrays are available as a substrate. The effect of self-contained spacer of the arrays to prevent quenching of the luminescence is investigated with the introduction of an Al2O3 layer as an isolate and the adjustment of its thickness. It is found that the self-contained spacer can effectively block the non-radiative energy transfer without the need of additional spacer. In addition, the horizontally aligned gold nanorod arrays also exhibit excellent optical polarization anisotropy. As the polarization angle of the incident light gradually increases, the intensity of the fluorescent radiation decreases, which is interpreted and analyzed by simulating the near field distribution. This strategy not only provides a powerful solution to build high-efficiency surface enhanced fluorescence systems, but also suggests a new pathway to realize the polarization-selective excitation of upconversion luminescence.

Polarized light emission from graphene induced by terahertz pulses

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was investigated experimentally and explained theoretically. We found that emitted photons are polarized predominantly perpendicular to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. Comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate a momentum isotropization time for electrons in graphene to be ~25 fs and roughly reconstruct the distribution function evolution in k-space.

Calculations of the Structural, Electronic, Optical, and Elastic Parameters of CdSiX2 (X = P, As) Compounds Based on First‐Principles Theory

Density functional theory is used to investigate the structural, electronic, optical, and mechanical aspects of the chalcopyrites CdSiP2 and CdSiAs2 using both the generalized gradient approximation and the local density approximation. The substitution of As for P alters the lattice constants, elastic constants, and dielectric characteristics considerably. The theoretical calculations correspond reasonably well with experimental data and other theoretical computations. The results of the electronic band structure analysis indicate that all of the compounds investigated are semiconductors with direct bands located at the Γ point. CdSiP2 possesses the highest elastic constants and bulk modulus, whereas CdSiAs2 has the highest values of shear modulus and Young's modulus. Moreover, specific focus is also given to obtaining the orientation‐dependent linear compressibility and Young's modulus. In bulk, CdSiP2 and CdSiAs2 are isotropic, although their Young's modulus is anisotropic. In addition, the optical response of the compounds is examined in the energy range of 0–14 eV in terms of dielectric functions, optical reflectivity, refractive index, extinction coefficient, optical conductivity, and electron energy loss. The computed optical results indicate that all compounds exhibit optical polarization anisotropy, making them suitable for optoelectronic devices.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus.

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Strong and robust polarization anisotropy of site- and size-controlled single InGaN/GaN quantum wires

Optical polarization is an indispensable component in photonic applications, the orthogonality of which extends the degree of freedom of information, and strongly polarized and highly efficient small-size emitters are essential for compact polarization-based devices. We propose a group III-nitride quantum wire for a highly-efficient, strongly-polarized emitter, the polarization anisotropy of which stems solely from its one-dimensionality. We fabricated a site-selective and size-controlled single quantum wire using the geometrical shape of a three-dimensional structure under a self-limited growth mechanism. We present a strong and robust optical polarization anisotropy at room temperature emerging from a group III-nitride single quantum wire. Based on polarization-resolved spectroscopy and strain-included 6-band k·p calculations, the strong anisotropy is mainly attributed to the anisotropic strain distribution caused by the one-dimensionality, and its robustness to temperature is associated with an asymmetric quantum confinement effect.

Control of the 3-Fold Symmetric Shape of Group III-Nitride Quantum Dots: Suppression of Fine-Structure Splitting.

Controlling the in-plane symmetry of wide-bandgap semiconductor quantum dots (QDs) is essential for room temperature quantum photonic applications using polarization entangled photon pairs. Herein, we report the formation of 3-fold symmetric group III-nitride QDs at the apex of a triangular pyramid via a self-limited growth mechanism. We employed the in-plane rotational symmetry of the c-plane of a Wurtzite crystal and the large built-in piezoelectric field to reduce fine-structure splitting. The QDs exhibit emission that is distinguishable from that of sidewall quantum wells, and the biexciton-exciton cascade possesses a single-photon nature. We observed the relatively low optical polarization anisotropy and small fine structure splitting under the measurement limit (270 μeV) with the 3-fold symmetric QD. In contrast with current strategies that consider group III-nitride QDs as strongly polarized single-photon emitters, our approach for controlling the QD symmetry provides a new perspective on such QDs, as polarization-entangled photon pairs.

The pressure effect on optoelectronic and mechanical properties of chalcopyrite BeSiN2

The most significant physical properties under pressure are investigated for BeSiN2 compound through the density functional theory (DFT). It is found that all parameters computed within the generalized gradient theory (GGA-PBE) are compatible with the other related theoretical research outcomes. The equilibrium structural parameters are calculated using the full-optimization results and the obtained elastic data prove that BeSiN2 is mechanically stable. Furthermore, the pressure effects on mechanical properties of this chalcopyrite structure are presented. The value of bulk to shear modulus ratio for BeSiN2 reveals that this compound should be considered as a brittle material. The elastic anisotropy of Young modulus and Poisson ratio in 2D and 3D under pressure are also studied. Then, the density of states (DOS) and charge density map in direction (110) are visualized to examine the electronic characteristics in addition to the band structure analysis. It is predicted that BeSiN2 is a semiconductor, which has the direct band gap of 3.52 eV. The plotted phonon dispersion curves of BeSiN2 have no significant imaginary branches throughout the phonon spectrum. The optical properties (i.e. electron energy loss, reflectivity, extinction coefficient, and refractive index) are discussed besides the dielectric functions for the 0–30 eV energy range. It is concluded that BeSiN2 is suitable for optoelectronic applications since having an optical polarization anisotropy.

Optical polarization anisotropy induced by molecular alignment in pentacene films confined in micron sized grooves etched in SiO2 substrate

Pentacene thin films grown on SiO2 substrates with few micron-spaced patterns were found to show anisotropy in their photoluminescence (PL) responses. PL emission was maximum for excitation polarization parallel to the groove direction. Detailed analysis of polarized Raman spectra gave insight on the intermolecular coupling. The anisotropy in PL was found to arise from preferential in-plane molecular alignment of pentacene, induced by the walls of the grooves. This was verified by spatial scan of the polarized micro-PL emission. Though earlier reports have shown anisotropy for nanoscale patterned substrates, to our knowledge, controlled orientation of molecules induced by micron spaced patterning has not been reported so far. This work reports observation of six fold increase in PL response for polarization parallel to the groove of widths 5 μm.

Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton.

Controlled lateral quantum confinement in single-layer transition-metal chalcogenides (TMCs) can potentially combine the unique properties of two-dimensional (2D) exciton with the size-tunability of exciton energy, creating the single-layer quantum dots (SQDs) of 2D TMC materials. However, exploring such opportunities has been challenging due to the limited ability to produce well-defined SQDs with sufficiently high quality and size control, in conjunction with the commonly observed inconsistency in the optical properties. Here, we report an effective method to synthesize high-quality and size-controlled SQDs of WSe2 via multilayer quantum dots (MQDs) precursors, which enables grasping a clear picture of the role of lateral confinement on the optical properties of the 2D exciton. From the single-particle optical spectra and polarization anisotropy of WSe2 SQDs of varying sizes in addition to their ensemble data, we reveal how the properties of 2D exciton in single-layer TMCs evolve with increasing lateral quantum confinement.

Structural and Thermoelectronic Properties of Chalcopyrite MgSiX2 (X = P, As, Sb)

We have explored the structural, electronic, optical, and mechanical properties of the magnesium-based chalcopyrites MgSiP2, MgSiAs2, and MgSiSb2 using density functional theory with five different generalized gradient approximation (GGA) functionals: Perdew–Wang (1991), Perdew–Burke–Ernzerhof, revised Perdew–Burke–Ernzerhof, modified Perdew–Burke–Ernzerhof for solids, and Armiento–Mattson (2005) as well as the local density approximation. Change of the constituent element from P to Sb significantly affected the lattice constants, elastic constants, and thermal and dielectric properties. Our theoretically computed results are in reasonable agreement with experiments and other theoretical calculations. The electronic band structure results imply that all three considered compounds are semiconductors. MgSiP2 has the highest value of elastic constants, and bulk and shear moduli compared with the other two binary chalcopyrites. Furthermore, the optical response in terms of the dielectric functions, optical reflectivity, refractive index, extinction coefficient, and electron energy loss of the compounds were also investigated in the energy range from 0 eV to 15 eV. The calculated optical results reveal optical polarization anisotropy for all three compounds, making them useful for optoelectronic device applications. Moreover, specific focus is also given to quantify the dependence of various thermal properties on finite pressure/temperature within the quasiharmonic approximation.

Ab-initio calculations of semiconductor MgGeP2 and MgGeAs2

In this study, we focus on structural, electronic, elastic, lattice dynamic and optic properties of MgGeP2 and MgGeAs2 using ab-initio density-functional theory (DFT) within Armiento-Mattson 2005 (AM05) scheme of the generalized gradient approximation (GGA) for the exchange-correlation potential. Our computed structural results are in reasonable agreement with the literature. The band gap of these compounds is predicted to be direct. Our elastic results prove that these compounds are mechanically stable. The obtained phonon spectra of MgGeP2 and MgGeAs2 do not exhibit any significant imaginary branches using GGA-AM05 for the exchange-correlation approximation. Further analysis of the optical response of the dielectric functions, optical reflectivity, refractive index, extinction coefficient and electron energy loss delves into for the energy range of 0–22.5eV. It motivated that there exists an optical polarization anisotropy of these compounds for optoelectronic device applications.

Electronic, magnetic, optical, and edge-reactivity properties of semiconducting and metallic WS2 nanoribbons

First-principles density functional theory calculations are performed in one-dimensional single-layer WS2 nanoribbons with zigzag- and armchair-edges. Magnetic ordering, optical response, and chemical reactivity are investigated. Our results demonstrated that WS2 zigzag nanoribbons exhibit a ferromagnetic-metallic behavior that is attributed to the edges; the resulting magnetic moments are mainly localized in S and W edge atoms. Furthermore, the magnetic ordering along the edges depends on the zigzag nanoribbon’s width. Armchair nanoribbons exhibit semiconducting behavior. Optical response results demonstrated that there exists a strong optical polarization anisotropy enhancing a well defined absorption intensity peak, with polarization along the nanoribbons axis. Regarding chemical reactivity, ribbons are exposed to water (H2O), thiophene (C4H4S), and carbon monoxide (CO) molecules. Results reveal that H2O can be covalently joined to the edges via the W-atoms in the ribbons with zigzag-edges, whereas in ribbons with armchair edges, H2O is dissociated in OH and H, and these species are joined to W and S atoms respectively. Results for thiophene on zigzag nanoribbons demonstrated that C4H4S molecules are absorbed by W-terminated edges, whereas in armchair ribbons, the C4H4S is linked to the edges by binding to the sulfur. Interestingly, CO molecules give rise to half-metallicity and surprising ferromagnetism in zigzag and armchair nanoribbons, respectively. The results discussed here could help to understand the physical and chemical properties of edges in transition metal dichalcogenides materials.

Engineering of optical polarization based on electronic band structures of A-plane ZnO layers under biaxial strains

In-plane anisotropic strains in A-plane layers on the electronic band structure of ZnO were investigated from the viewpoint of optical polarization anisotropy. Investigations utilizing k·p perturbation theory revealed that energy transitions and associated oscillation strengths were dependent on in-plane strains. The theoretical correlation between optical polarizations and in-plane strains was experimentally demonstrated using A-plane ZnO layers with different in-plane strains. Finally, optical polarization anisotropy and its implications for in-plane optical properties are discussed in relation to the energy shift between two orthogonal directions. Higher polarization rotations were obtained in an A-plane ZnO layer with in-plane biaxially compressive strains as compared to strain-free ZnO. This study provides detailed information concerning the role played by in-plane strains in optically polarized applications based on nonpolar ZnO in the ultra-violet region.

Giant growth-plane optical anisotropy in wurtzite InN/GaN disk-in-wire structures

Using a combination of valence force-field molecular mechanics, 20-band sp3d5s∗ atomistic tight-binding approach, and appropriate post-processing tools, we have studied the origin and nature of optical polarization anisotropy in semiconducting GaN/InN/GaN disk-in-wire structures having wurtzite crystal symmetry and varying InN disk thicknesses. True atomistic symmetry due to the presence of strong internal fields, coupled with quantum mechanical size quantization effects, results in unconventional characteristics in the electronic structure related to non-degeneracy in the excited P states and rotation (symmetry lowering) in the wavefunctions. The optical polarization ratio projected on the XY (growth) plane and, in particular, the transition rates have been shown to be strongly dependent on the crystal internal fields and the thickness of the InN disk.

Large optical polarization anisotropy due to anisotropic in-plane strain in m-plane GaInN quantum well structures grown on m-plane 6H-SiC

We investigated the optical polarization anisotropy of m-plane GaInN/GaN quantum well structures on m-plane SiC and bulk GaN substrates. On bulk GaN, the degree of polarization increases with increasing indium content according to the larger strain-induced separation of the topmost valence bands. On m-plane SiC, however, we observe constantly large polarization ratios of around 90% and more. From an x-ray strain state analysis and calculations of the valence band energies, we find that an anisotropic strain of the GaN buffer layer leads to a very strong separation of the topmost valence bands resulting in a large degree of polarization.

Polarization dependences of electroluminescence and absorption of vertically correlated InAs/GaAs QDs

The results of experimental studies concerning the optical polarization anisotropy of electroluminescence and absorption spectra of systems with a varied number of tunnel-coupled vertically correlated In(Ga)As/GaAs quantum dots (QDs), built into a double-section laser with equal-length sections, are presented. One such system is a QD superlattice exhibiting the Wannier-Stark effect. The involvement of heavyhole ground states in optical transitions for light polarized both in the plane perpendicular to the growth axis (X-Y) and along the growth direction Z of the structure was observed. The degree of polarization anisotropy depends on the height of vertically correlated QDs and the QD superlattice: the total thickness of all In(Ga)As QD layers and GaAs spacers between the QDs, which is related to the Z component of the wave function of heavy-hole ground states for vertically correlated QDs and for the QD superlattice.

Comprehensive control of optical polarization anisotropy in semiconducting nanowires

The demonstration of strong photoluminescence polarization anisotropy in semiconducting nanowires embodies both technological promise and scientific challenge. Here, we present progress on both fronts through the study of the photoluminescence polarization anisotropy of randomly oriented nanowire ensembles in materials without/with crystalline anisotropy, small/wide bandgap, and both III-V/II-VI chemistry (InP/ZnO nanowires, respectively). Comprehensive control of the polarization anisotropy is realized by dielectric matching with conformally deposited Ta2O5 (dielectric ratios of 9.6:4.41 and 4.0:4.41 for InP and ZnO, respectively). After dielectric matching, the polarization anisotropy of the nanowire ensembles is reduced by 86% for InP:Ta2O5 and 84% for ZnO:Ta2O5.

Optical polarization anisotropy of tensile strained InGaN/AlInN quantum wells for TM mode lasers

In this paper, we discuss the optical characteristics and polarization anisotropy of a tensile strained polar c-plane InGaN/AlInN quantum well. We found that if the quantum well is under the tensile strain, the |Z⟩-like state will be lifted up so that the emitted light will be TM mode. In addition, with a particular aluminum composition of the AlInN alloy as the barrier for the tensile strained InGaN quantum well, it is possible to reduce quantum-confined Stark effect. The self-consistent Poisson and 6×6 k⋅p Schrödinger solver has been used for studying light emitting characteristics. Our results show that the tensile strained InGaN quantum well on AlInN barrier has much larger optical gain and lower threshold carrier density compared to the conventional InGaN/GaN system, and it has a potential to be TM light source for edge emitting laser diodes with the photonic crystal cavity made by nanorod arrays.

Polarization sensitive lateral photoconductivity in GaAs/AlGaAs quantum well based structures on low-temperature grown GaAs(001)

Polarization-resolved lateral-photoconductivity measurements are reported on device structures made of GaAs/Al0.3Ga0.7As quantum wells sandwiched between low-temperature grown GaAs(001) layers. The mesa device structures have long length (3 mm∥y) and narrow width (10 and 20 μm∥x) in the (001) plane. For light incident along [001], the ground state light-hole exciton transition is much stronger for light polarization E∥x, compared to E∥y. The heavy-hole exciton transition shows a weaker polarization anisotropy of opposite sign, being stronger for E∥y. Through calculations based on the Bir–Pikus Hamiltonian, the observed in-plane optical polarization anisotropy is shown to arise from valence band mixing induced by anisotropic strain in the plane of quantum wells.

Semipolar III Nitride Semiconductors: Crystal Growth, Device Fabrication, and Optical Anisotropy

AbstractSemipolar InGaN/GaN quantum wells (QWs) are quite attractive as visible light emitters. One of the reasons is that a better optical transition probability is expected because of weaker internal electric fields, compared to conventional polar QWs. In addition, in-plane optical polarization anisotropy, which is absent in conventional QWs, is another relevant property because it affects device design and also may provide a means for novel applications. We revealed that the in-plane optical anisotropy in semipolar QWs switched from one direction perpendicular to the [0001] crystal axis to the perpendicular direction as the In composition increases. This is a property unique to semipolar QWs and enables, for example, to make cavity mirrors of laser diodes by cleavage. In this article, we describe the concept of semipolar planes and fabrication of high-quality epitaxial films for semipolar QWs. Furthermore, we discuss device fabrication and optical polarization anisotropy.