Y. L. Xie

Photoluminescence from 8-hydroxy quinoline aluminum embedded in porous anodic alumina membrane

By immersing porous alumina membranes in 8-hydroxy quinoline aluminum (Alq3) solutions, we embed Alq3 molecules into the nanovoids of the membranes. Photoluminescence (PL) spectral examinations show that the Alq3 molecules embedded in the alumina membrane show a blueshifted luminescent spectrum compared to that of Alq3 solid film. Since the blueshift of ∼18nm does not change with nanopore diameter, we consider it to be due to the confinement of nanovoids in the nanopore walls, which prevents Alq3 molecules from aggregation and thus weakens the interaction between them. Further spectral analyses suggest that the photogeneration of carriers takes place in oxygen vacancies at the surfaces of nanopore walls, whereas the radiative recombination occurs in the embedded Alq3 molecules. Forster energy transfer from oxygen vacancies to the embedded Alq3 molecules is proposed to be responsible for the enhancement of the PL intensity.

Light emission from silicon-based porous anodic alumina formed in 0.5 M oxalic acid

We have examined the photoluminescence (PL) spectra of silicon-based porous anodic alumina formed in 0.5 M oxalic acid. A broad PL band centered at 565 nm was observed. Spectral analyses reveal that the broad PL arises from optical transitions between the energy levels of F2+ center (a combined oxygen vacancy with one positive charge). In addition, two ultraviolet and one violet PL band, together with a blue band, were also obtained from this kind of Si-based porous anodic alumina. Based on annealing behavior of these PL bands, we suggest that the ultraviolet and violet PL bands are closely related to excess aluminum ions in the alumina membrane rather than oxygen vacancy defects reported previously. The blue emission originates in oxygen vacancies (F and F+ centers). This work provides a good understanding of the light-emitting properties of silicon-based porous anodic alumina templates.

Amplified spontaneous emission via the coupling between Fabry-Perot cavity and surface plasmon polariton modes.

We demonstrated amplified spontaneous emission by embedding dye molecules within a dielectric layer of a metal-dielectric-metal subwavelength structure. It was reinforced when a strong coupling occurred between the Fabry-Perot mode supported by the dielectric layer and the surface plasmon polariton mode supported by the adjacent metallic grating. Here, we adjust the two mode interaction via tuning the depth of the metallic grating grooves. The stronger the interaction, the smaller the full width at half-maximum of the emission spectra and the lower the threshold of the amplified spontaneous emission.

Miniature polarization analyzer based on surface plasmon polaritons

We investigated a miniature plasmonic polarization analyzer measuring Stokes parameters of a light. The optical component consists of a 2 × 2 polarizer array, three linear polarizers, and one right-handed circular polarizer. These polarizers are formed with bulls eye structures on a metal surface. The measurements of Stokes parameters in a unit radius Poincare sphere were demonstrated. Compact polarization-dependent optical sensing and imaging can be envisioned based on the miniature polarization analyzer.

Ultra-low coercive field of improper ferroelectric Ca3Ti2O7 epitaxial thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions, such as the rotation and tilting of Ti-O octahedral in the Ca3Ti2O7 (CTO) family. In this work, we prepare the high quality (010)-oriented CTO thin films on (110) SrTiO3 (STO) substrates by pulsed laser deposition. The good epitaxial growth of the CTO thin films on the substrates with the interfacial epitaxial relationship of [001]CTO//[001]STO and [100]CTO//[-110]STO is revealed. The in-plane ferroelectric hysteresis unveils an ultralow coercive field of ∼5 kV/cm even at low temperature, nearly two orders of magnitude lower than that of bulk CTO single crystals. The huge difference between the epitaxial thin films and bulk crystals is most likely due to the lattice imperfections in the thin films rather than substrate induced lattice strains, suggesting high sensitivity of the ferroelectric properties to lattice defects.

Controlling the state of polarization via optical nanoantenna feeding with surface plasmon polaritons

Optical nanoantennas, usually referring to metal structures with localized surface plasmon resonance, could efficiently convert confined optical energy to free-space light, and vice versa. But it is difficult to manipulate the confined visible light energy for its nanoscale spatial extent. Here, a simple method is proposed to solve this problem by controlling surface plasmonpolaritons to indirectly manipulate the localized plasmons. As a proof of principle, we demonstrate an optical rotation device which is a grating with central circular polarization optical nanoantenna. It realized the arbitrary optical rotation of linear polarized light by controlling the retard of dual surface plasmonpolaritons sources from both side grating structures. Furthermore, we use a two-parameter theoretical model to explain the experimental results.

Magnetic phase transitions and monopole excitations in spin ice under uniaxial pressure: A Monte Carlo simulation

In this work, we explore the spin ice model under uniaxial pressure using the Monte Carlo simulation method. For the known spin ices, the interaction correction (δ) introduced by the uniaxial pressure varies in quite a wide range from positive to negative. When δ is positive, the ground state characterized by the ferromagnetic spin chains is quite unstable, and in real materials it serves as intermediate state connecting the ice state and the long range ordered dipolar spin ice ground state. In the case of negative δ, the system relaxes from highly degenerate ice state to ordered ferromagnetic state via a first order phase transition. Furthermore, the domain walls in such ferromagnetic state are the hotbed of the excitations of magnetic monopoles, thus indicating that the uniaxial pressure can greatly increase the monopole density.

Phase transitions in classical biquadratic Heisenberg model for strained iron pnictides

Based on the notion of magnetically driven nematicity, we study the phase transitions in a frustrated classical Heisenberg model with biquadratic and anisotropic exchange interactions on square lattice by means of Monte Carlo simulations, in order to understand the uniaxial strain effect in BaFe2As2. The variation of both structural and antiferromagnetic transition temperatures with uniaxial stress reported in experiments can be qualitatively reproduced by including the exchange anisotropy, suggesting that the anisotropy may be responsible for the strain effect on the phase transitions.

Manipulation of magnetic state in nanostructures by perpendicular anisotropy and magnetic field

We investigate the transitions of spin configurations in ultrathin nanostructures by tuning the perpendicular anisotropy (Kz) and out-of-plane magnetic field (H), using the Monte Carlo simulation. It is revealed that enhancing the anisotropy Kz can drive the evolution of in-plane vortex state into intriguing saturated magnetization states under various H, such as the bubble domain state and quadruple-block-domain state etc. The spin configurations of these states exhibit remarkable H-dependence. In addition, the strong effects of geometry and size on the spin configurations of nanostructures are observed. In particular, a series of edged states occur in the circular disk-shaped lattices, and rich intricate saturated magnetization patterns appear in big lattices. It is suggested that the magnetic states can be manipulated by varying the perpendicular anisotropy, magnetic field, and geometry/size of the nanostructures. Furthermore, the stability (retention capacity) of the saturated magnetization states upo...

Domain splitting and enhanced piezoelectricity in ferroelectric thin films with square grid structure

The pattern evolution of the stripe-like domain structure in tetragonal ferroelectric thin films upon introduction of square void arrays is investigated using Monte Carlo simulation based on the Landau-Devonshire phenomenological theory. The regular splitting of the stripe-like domains near the voids and substantial release of both the dipole-dipole interaction and the electromechanical energy are revealed. Consequently, a significant enhancement of piezoelectric coefficient is predicted. It is suggested that the delicate balance between the two energy terms modulated by the void array periodicity and void size is responsible for the domain splitting. The present work proposes an alternative promising approach to tune the domain structure via simple micro-fabrication techniques.