Direction Of Linear Polarization Research Articles

Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics.

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Focused ion beam based nano-kirigami/origami for three-dimensional micro/nanomanufacturing and photonic applications

<sec>High-resolution three-dimensional (3D) micro-/nano-fabrication techniques form an important basis for developing the modern micro-/nano-photonics. In this paper, we systematically introduce a new 3D micro-/nano-fabrication technique named as nano-kirigami, which is based on the focused ion beam (FIB) induced in-situ “cutting” and “folding” of a suspended gold film. The high-dose FIB milling fulfils the “cutting” process, and the low-dose FIB irradiation of the effective sample area acts as a mean of “folding”, well representing the fabrication principle of macro-kirigami. By utilizing the topography-guided stress equilibrium during FIB irradiation, nano-kirigami can reliably achieve rich 3D shape transformations, such as multi-directional folding, bending, buckling and twisting, with resolution down to below 50 nm. Taking the FIB-induced stress as a stimulus, the FIB-based nano-kirigami can be divided into “tree-type” and “closed-loop” fabrication from the perspective of topological classifications in multibody dynamics. The characteristics, advantages and disadvantages between the two different types of processes are analyzed and summarized in the paper.</sec><sec>The rich 3D shape deformation characteristics bring about unimaginable possibilities for the fabrication of various novel 3D structures, and the generation of new geometries facilitates the exploration of new physics. For example, by taking advantage of the 3D plasmonic conductive couplings, the composite symmetry-broken SRR-based configuration is constructed for the efficient generation of significant chiral Fano resonances. Moreover, with the close-loop nano-kirigami induced by FIB global irradiation, the giant intrinsic optical chirality is achieved in a 3D pinwheel-like structure with exotic twisting and rotation geometric features. Especially, the rotation of the linear polarization direction, caused by the circular birefringence, reaches an efficiency of 310000°/mm which exceeds the reported value of chiral metamaterial and congeneric two-dimensional planar nanostructure. Another example is the metasurface formed by alternately arranging the left-handed pinwheel structure and right-handed pinwheel structure in the horizontal plane, which can diffract cross-polarized transmitted light efficiently while clear photon spin Hall effect is observed.</sec><sec>The introduced nano-kirigami technique, as well as the generated exotic 3D nanostructures and their photonic applications, can build up a novel 3D miro-/nano-fabrication platform for versatile structural geometries with promising functionalities, which may find great potential applications in areas such as micro-/nano-photonics, electronic and magnetic devices, plasmonics, optomechanics, micro-/nano-electromechanical systems, etc.</sec>

The tests of CP and CPT symmetry using the J-PET detector

Symmetries under the parity transformation (P), charge-conjugation (C) and time reversal (T) are of fundamental importance in nuclear and elementary particle physics. Studies of the observables violating the combined CP symmetry constitute precise tests of the Standard Model. However, CP violation was observed to date only for systems involving quarks, raising the importance of searches its manifestations e.g. in purely leptonic systems. The 3γ decay of spin-aligned ortho-positronium atoms (o-Ps) can be used to test CP invariance in such a purely leptonic system. The Jagiellonian Positron Emission Tomograph (J-PET) detection system enables experimental tests of CP and CPT through measurement of the expectation values of angular correlation operators odd under these transformations and constructed from (i) spin vector of the ortho-positronium atom, (ii) co-planar momentum vectors of photons originating from the decay of the positronium atom, and (iii) linear polarization direction of annihilation photons. Precise experimental symmetry tests with J-PET are possible thanks to a dedicated reconstruction technique of 3γ ortho-positronium decays and a positronium production chamber including a highly porous aerogel target, whose setup allows for determining the orthopositronium spin polarization without the use of an external magnetic field.

Polarization-selective spin wave driven domain-wall motion in antiferromagnets

The control of magnetic domain walls is essential for the magnetic-based memory and logic applications. As an elementary excitation of magnetic order, spin wave is capable of moving magnetic domain walls just as the conducting electric current. Ferromagnetic spin waves can only be right-circularly polarized. In contrast, antiferromagnetic spin waves have full polarization degree of freedom, including both left- and right-circular polarizations, as well as all possible linear or elliptical ones. Here we demonstrate that, due to the Dzyaloshinskii-Moriya interaction, the spin wave driven domain wall motion in antiferromagnets strongly depends on the linear polarization direction of the injected spin waves. Steering domain wall motion by simply tuning the polarization of spin waves offers new designing principles for domain-wall based information processing devices.

All-fiber cylindrical vector beams laser based on an acoustically-induced fiber grating

We presented a method to construct an all-fiber laser by using an acoustically-induced fiber grating (AIFG) in two-mode fiber (TMF) within the laser cavity to generate cylindrical vector beams (CVBs) in communication band. Based on the polarization dependent vector mode coupling characteristic of an AIFG, the CVBs were generated by respectively coupling and modes to TM01 and TE01 modes, which have radial and azimuthal polarizations characteristics, respectively. The output of the all-fiber laser could be adjusted from a linearly-polarized Gaussian mode to a radially or an azimuthally polarized vector mode through a polarization controller to adjust the linear polarization direction of the input mode of and an acoustic flexural wave applied on TMF. This work provides a method for generating CVBs with convenient mode conversion characteristics.

High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films.

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

Measurement of linear polarization in the HeI 21P–31D emission line in an electron cyclotron resonance discharge plasma

The magnitude and direction of linear polarization in the HeI 21P–31D emission line are simultaneously measured using a beam-splitting-type polarization spectroscopy system. The polarization measured in the steady-state phase of an electron cyclotron resonance (ECR) discharge produced in a simple cusp field is directed closely perpendicular to the magnetic field at the ECR surface, and the degree of polarization increases up to 8% when the neutral pressure is reduced to less than 23 mPa. These results suggest that the cause of the polarization is attributable to impact excitation by high-energy electrons produced at the ECR surface.

Zeeman structure of red lines of lanthanum observed by laser spectroscopy methods

Laser Induced Fluorescence (LIF) Spectroscopy and Optogalvanic (OG) Spectroscopy were used for the investigation of the Zeeman hyperfine (hf) structures of 27 spectral lines of La I in the wavelength range between 633.86 and 667.54 nm. As a source of free La atoms a hollow cathode discharge lamp was used. Spectra were recorded in the presence of a relatively weak magnetic field (about 800G) produced by a permanent magnet, for two linear polarization directions of the exciting laser beam. As a result of the measurements, we determined for the first time the Landé gJ- factors of 18 levels of La I. The Landé gJ- factors of 12 other levels were re-investigated and determined with higher accuracy.

Handedness-switchable chiral field in the 1D metal grooves for plasmonic circular dichroism spectroscopy

The electromagnetic fields possessing optical chirality are very important to circular dichroism (CD) analysis for chiral molecules. Plasmonic nanostructures are often adopted to generate the chiral near-fields for enhancing the weak CD signal of the chiral molecules, which is called plasmonic-enhanced CD spectroscopy. However, due to the conservation of optical chirality the general nanostructures cannot provide sign-uniform chiral near-fields close to the structures, which dramatically reduce the performance of plasmonic-enhanced CD spectroscopy. The prerequisite of plasmonic-based CD analysis is to generate two enantiomeric chiral fields with opposite optical chirality close to the nanostructures in two independent measurements. Therefore, the design that can provide a handedness-switchable and equivalent optical chirality of near-field of plasmonic structure is desired. In this paper, we show that simple 1D periodic metal grooves can provide handedness-switchable, sign-uniform chiral fields inside the grooves with enhanced optical chirality through adjusting the linear polarization direction of incident light from +45° to −45° off the periodic direction. A reflection-type plasmonic CD spectroscopy scheme was proposed based on metal grooves, which works under illumination of linearly polarized light. Moreover, the numerical simulation including chiral analytes was implemented to verify the feasibility. The proposed CD spectroscopy scheme has the potential for rapid and low-cost chiroptical sensing for a small quantity of chiral molecules.

Design of a High Directive Gain Reconfigurable Circular Patch Antenna

A new probe-fed patch antenna polarization reconfiguration is presented in this proposed work. The antenna is composed of a circular radiating patch and a switchable feed network. By controlling the operating states of four pairs of pin diode in the feed network, the feed point of the circular patch can be switched. Therefore, a reconfiguration between four linear polarization directions at a 45 degree interval (22.5 degree, 67.5 degree, 112.5 degree and 157.5 degree) can be realized. All the different polarization states own similar matching and radiation characteristics. Simulated and measured results indicate that antenna can achieve reconfigurable quadri-polarization diversity features with an invariable operating frequency and excellent radiation performance, which are very attractive for wireless communications. In addition, the proposed design can offer more reconfigurable linear polarization directions by adding more switchable paths in the feed network.

Polarization-Rotated Waveguide Antennas for Base-Station Applications

A novel base-station antenna element is proposed. It consists of an artificial surface composed of parallel strips rotating in the polarization direction and a segment of a rectangular waveguide. The surface is designed on a single-sided substrate, which has the same area as the aperture of the waveguide. To achieve the polarization rotation, the parallel strips on the surface are rotated by 45° with respect to the orientation of the waveguide antenna. By adding the surface, the linear polarization direction of the rectangular waveguide antenna rotates by 45° to comply with the requirements of the cellular industry. To verify the simulation results, the proposed antenna was fabricated and measured. Results show that the antenna has an operating bandwidth from 698-960 MHz, where a stable radiation pattern is achieved.

Circular Dichroism of Electric-Field-Oriented CdSe/CdS Quantum Dots-in-Rods.

Here we report anisotropy of intrinsic chiroptical response in CdSe/CdS quantum dot-in-rod systems. These nanostructures being oriented in an external electric field demonstrate dependence of circular dichroism signal on the orientation of the nanocrystals. The type of circular polarization in these nanostructures correlates with preferential direction of linear polarization, and the degree of circular polarization is the maximal for the first circular dichroism band corresponding to the absorption band edge. We also support our experimental data with a theoretical model. Using this model, we show a direct connection between theoretically derived morphological parameters of twisting in nanocrystals lattices and calculated from experimental data parameters of circular dichroism anisotropy.

Vector cavity solitons in broad area Vertical-Cavity Surface-Emitting Lasers.

We report the experimental observation of two-dimensional vector cavity solitons in a Vertical-Cavity Surface-Emitting Laser (VCSEL) under linearly polarized optical injection when varying optical injection linear polarization direction. The polarization of the cavity soliton is not the one of the optical injection as it acquires a distinct ellipticity. These experimental results are qualitatively reproduced by the spin-flip VCSEL model. Our findings open the road to polarization multiplexing when using cavity solitons in broad-area lasers as pixels in information technology.

Test of target independence for free-free scattering in a Nd:YAG laser field

We report measurements of one-, two-, and three-photon processes during the elastic scattering of electrons through ${90}^{\ensuremath{\circ}}$ by helium, argon, and molecular-nitrogen targets, in the presence of 1.17-eV photons from a Nd:YAG laser. The incident energy of the electrons was 200 and 350 eV, and the linear polarization direction of the laser was parallel to the momentum transfer direction. Our measured free-free count rates for the three processes are target independent within the experimental uncertainties, perfectly consistent with the Kroll-Watson approximation, which assumes no interaction of the laser radiation with the target.

A New Quadri-Polarization Reconfigurable Circular Patch Antenna

A new probe-fed patch antenna with polarization reconfiguration is presented in this paper. The antenna is composed of a circular radiating patch and a switchable feed network. By controlling the operating states of four pairs of PIN diodes in the feed network, the feed point of the circular patch can be switched. Therefore, a reconfiguration between four linear polarization directions at a 45° interval (22.5°, 67.5°, 112.5°, and 157.5°) can be realized. All the different polarization states own similar matching and radiation characteristics. Simulated and measured results indicate that the antenna can achieve reconfigurable quadri-polarization diversity features with an invariable operating frequency and excellent radiation performance, which are very attractive for wireless communications. In addition, the proposed design can offer more reconfigurable linear polarization directions by adding more switchable paths in the feed network.

Studies of discrete symmetries in a purely leptonic system using the Jagiellonian Positron Emission Tomograph

Discrete symmetries such as parity (P), charge-conjugation (C) and time reversal (T) are of fundamental importance in physics and cosmology. Breaking of charge conjugation symmetry (C) and its combination with parity (CP) constitute necessary conditions for the existence of the asymmetry between matter and antimatter in the observed Universe. The presently known sources of discrete symmetries violations can account for only a tiny fraction of the excess of matter over antimatter. So far CP and T symmetries violations were observed only for systems involving quarks and they were never reported for the purely leptonic objects. In this article we describe briefly an experimental proposal for the test of discrete symmetries in the decays of positronium atom which is made exclusively of leptons. The experiments are conducted by means of the Jagiellonian Positron Emission Tomograph (J-PET) which is constructed from strips of plastic scintillators enabling registration of photons from the positronium annihilation. J-PET tomograph together with the positronium target system enable to measure expectation values for the discrete symmetries odd operators constructed from (i) spin vector of the ortho-positronium atom, (ii) momentum vectors of photons originating from the decay of positronium, and (iii) linear polarization direction of annihilation photons. Linearly polarized positronium will be produced in the highly porous aerogel or polymer targets, exploiting longitudinally polarized positrons emitted by the sodium 22 Na isotope. Information about the polarization vector of orthopositronium will be available on the event by event basis and will be reconstructed from the known position of the positron source and the reconstructed position of the orthopositronium annihilation. In 2016 the first tests and calibration runs are planned, and the data collection with high statistics will commence in the year 2017.

Self-Assembly and Visualization of Poly(3-hexyl-thiophene) Chain Alignment along Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs) were noncovalently functionalized with poly(3-hexyl-thiophene) (P3HT) to produce stable suspensions. The conjugated polymer and BNNTs interact via π–π stacking, and the resulting self-assembly of regiorandom P3HT on the BNNT surface is immediately evident through a color change and the appearance of a partially resolved absorption band. By employing polarized excitation fluorescence microscopy, we directly visualized the polymer-functionalized BNNTs and determined the conformation of the polymer on BNNT surfaces. Variation of fluorescence intensity with linear polarization direction showed conclusively that the P3HT chains were always aligned parallel to the BNNT long axis.

Compact linearly polarized ceramic laser made with anisotropic nanostructured thin films.

A novel method is proposed for the fabrication of polarizing laser mirrors for compact solid-state lasers using glancing angle deposition. Changing the inclination angle and the azimuthal orientation of the substrate during deposition allows one to create and control in-plane birefringence of a deposited thin film by changing its nanostructure. Principal refractive indices of tungsten trioxide films were determined for various deposition angles using transmission and reflection ellipsometry. High-reflectance contrast between orthogonal linear polarization directions was obtained using a single material without any additional processing steps. These birefringent films were the building blocks of a Bragg mirror that was tested as an output coupler of a (Yb0.1Y0.9)3Al5O12 ceramic laser in a laser-diode end-pumped configuration. Continuous-wave, linearly polarized, transverse single-mode laser emission was obtained at a wavelength of 1030 nm with a polarization extinction ratio higher than 973 (30 dB).

Human perception of visual stimuli modulated by direction of linear polarization

This study explores both theoretically and experimentally the human perception of polarized light beyond that currently established. The radial analyser theory of Haidinger’s phenomenon (HP) is used to predict the effect of observing visual stimuli comprising patterned zones characterized by orthogonal planes of linear polarization (linear polarization direction fields, LPD-fields). Any pattern can be represented as an LPD-field including optotypes and geometric forms. Simulated percepts differ from the original patterns although edges are mostly preserved. In edge-rich images a cross of attenuating contrast spanning the field of view is predicted.The mathematical model is verified experimentally using a liquid crystal display (LCD)-based polarization modulator imaged through a tangential (azimuthal) analyser with properties complementary to a radial analyser. The LCD device is then used in vivo to elicit perceptual responses in human subjects. Normal humans are found to readily detect spatially and temporally modulated isoluminant spatially-isochromatic, highly polarized LPD stimuli. Most subjects match the stimuli to corresponding images of theoretically predicted percepts. In particular edge perception and the presence of the contrast cross was confirmed. Unlike HP, static patterned LPD stimuli are perceived without difficulty.The simplest manifestation of human polarization perception is HP which is the fundamental element of an open set of stimulus-dependent percepts. This study demonstrates that humans have the ability to perceive and identify visual pattern stimuli defined solely by polarization state modulation.

Phase differences in the photoemission from krypton in the fine-structure-split ionization channelsP3/22andP1/22

Using the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique, a nonzero phase difference is observed in the two-photon ionization transition of krypton atoms into the ionization channels $^{2}P_{1/2}$ and $^{2}P_{3/2}$ when an electron is removed from the $4p$ shell. The phase difference is studied for excitation energies ranging from $\ensuremath{\approx}18\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ up to $\ensuremath{\approx}28\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The phase differences are found to depend on the relative directions of linear polarization of the photoionization laser pulses and the pulses inducing the subsequent continuum-continuum transition. The experimental results are compared to phase differences calculated on the basis of dipole matrix elements for photoionization and scattering phases known for krypton from earlier experiments. The results indicate that the RABBITT phase differences do not give an obvious direct access to time-delay differences in the primary photoionization step of the atom.