Tunable Angle Research Articles

Manipulating topological states by imprinting non-collinear spin textures.

Topological magnetic states, such as chiral skyrmions, are of great scientific interest and show huge potential for novel spintronics applications, provided their topological charges can be fully controlled. So far skyrmionic textures have been observed in noncentrosymmetric crystalline materials with low symmetry and at low temperatures. We propose theoretically and demonstrate experimentally the design of spin textures with topological charge densities that can be tailored at ambient temperatures. Tuning the interlayer coupling in vertically stacked nanopatterned magnetic heterostructures, such as a model system of a Co/Pd multilayer coupled to Permalloy, the in-plane non-collinear spin texture of one layer can be imprinted into the out-of-plane magnetised material. We observe distinct spin textures, e.g. vortices, magnetic swirls with tunable opening angle, donut states and skyrmion core configurations. We show that applying a small magnetic field, a reliable switching between topologically distinct textures can be achieved at remanence.

Photoacoustic transformation of Bessel light beams in magnetoactive superlattices

Photoacoustic transformation of the TE mode of a Bessel light beam (BLB) has been studied for piezoelectric detection in short-period superlattices formed by magnetoactive crystals of bismuth germanate (Bi12GeO20) and bismuth silicate (Bi12SiO20) types. It is shown that the resulting signal amplitude can be controlled using optical schemes of BLB formation with a tunable cone angle. A resonant increase in the signal amplitude has been found in the megahertz range of modulation frequencies and its dependences on the BLB modulation frequency, geometric sizes of the two-layer structure and piezoelectric transducer, radial coordinate of the polarization BLB mode, and dissipative superlattice parameters are analyzed.

Generating and moving Dirac points in a two-dimensional deformed honeycomb lattice arrayed by coupled semiconductor quantum dots

Analysis of the electronic properties of a two-dimensional (2D) deformed honeycomb structure arrayed by semiconductor quantum dots (QDs) is conducted theoretically by using tight-binding method in the present paper. Through the compressive or tensile deformation of the honeycomb lattice, the variation of energy spectrum has been explored. We show that, the massless Dirac fermions are generated in this adjustable system and the positions of the Dirac cones as well as slope of the linear dispersions could be manipulated. Furthermore, a clear linear correspondence between the distance of movement d (the distance from the Dirac points to the Brillouin zone corners) and the tunable bond angle α of the lattice are found in this artificial planar QD structure. These results provide the theoretical basis for manipulating Dirac fermions and should be very helpful for the fabrication and application of high-mobility semiconductor QD devices.

Deposition temperature mediated tunable tilt angle magnetization in Co–Pt/Ni81Fe19 exchange springs

Abstract In this study, we investigate the effect of deposition temperature of Co–Pt fixed layer, T d,CoPt (150, 250 and 350 °C) on the tilt angle magnetization ( θ M ) of Ni 81 Fe 19 -layer grown at room temperature (RT) and at different thicknesses ( t NiFe =0, 1.0, 2.5 and 4.0 nm) in Co–Pt( T d,CoPt )/NiFe( t NiFe ) exchange springs. The magnetic studies demonstrated a strong perpendicular magnetic anisotropy (PMA) for the equi-compositional ordered Co–Pt layer grown on glass substrate using the film sequence: Ta(20 nm)/Pt(20 nm)/CoPt(5 nm), regardless of T d,CoPt . The PMA can be retained with the addition of a 4-nm NiFe layer on the top when T d,CoPt ≥250 °C. In contrast, relatively a thin layer of Ni–Fe (2.5 nm) can destroy the perpendicular exchange-spring behavior if the Co–Pt layer is deposited at RT. Using 3-D micromagnetic simulation, the interfacial exchange coupling strength ( A ij ) between the Co–Pt and NiFe-layers was estimated and the A ij value is found to increase rapidly when T d,CoPt is increased from RT to 300 °C. Besides, the magnetization tilted angle ( θ M ) of NiFe can be easily tuned from completely out-of-plane to almost 60° when t NiFe =4.0 nm. Through this study, it is demonstrated that the θ M of NiFe-layer can be tuned by not only altering the t NiFe ; but also by varying the T d,CoPt .

Leaky‐wave plasma antenna with tunable radiation angle

ABSTRACTA beam scanning leaky‐wave antenna working at a fixed operating frequency and based on plasma discharge is proposed. The investigation of leaky‐poles for the proposed geometry shows that, at a fixed frequency, the radiation angle scans from broadside toward endfire by varying the plasma density in a relevant range. Theoretical results demonstrate that, at 10 GHz, this antenna may produce a beam scanning from 1.6 to 74.7° by varying the plasma density from to . Measurement results of the plasma characteristics performed on a radiofrequency plasma source in our laboratory are also discussed. Their integration in full‐wave simulations confirms the excitation of leaky‐waves. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2601–2604, 2014

Optical modulation of continuous terahertz waves towards cost-effective reconfigurable quasi-optical terahertz components

We report optical modulation of continuous terahertz (THz) wave in the frequency range of 570-600 GHz using photo-induced reconfigurable patterns on a silicon wafer. The patterns were implemented using programmable illumination from a commercially-available digital light processing (DLP) projector. A modulation depth of 20 dB at 585 GHz has been demonstrated. Modulation speed measurement shows a 3-dB bandwidth of ~1.3 kHz which is primarily limited by the DLP system. A photo-induced polarizer with tunable polarization angle has been demonstrated, showing a 3-dB extinction ratio. Reconfigurable aperture-arrays (4 x 4 pixels) have been attempted for room-temperature coded-aperture imaging using a single Schottky diode detector at 585 GHz. We envision that this technique will provide a simple but powerful means to realize a variety of cost-effective reconfigurable quasi-optical THz circuits and components.

Transformation optofluidics for large-angle light bending and tuning

Transformation optics is a new art of light bending by designing materials with spatially variable parameters for developing wave-manipulation devices. Here, we introduce a transformation optofluidic Y-branch splitter with large-angle bending and tuning based on the design of a spatially variable index. Differing from traditional splitters, the optofluidic splitter is achieved in an inhomogeneous medium by coordinate transformation. The designed bidirectional gradient index (GRIN) distribution can be achieved practically by the convection-diffusion process of liquid flowing streams. The transformation optofluidic splitter can achieve a much larger split angle with little bend loss than the traditional ones. In the experiments, a large tunable split angle up to 30° is achieved by tuning the flow rates, allowing optical signals to be freely transferred to different channels. Besides the symmetrical branch splitting, asymmetrical Y-branch splitting with approximately equal power splitting is also demonstrated by changing the composition of the liquids. The optofluidic splitter has high potential applications in biological, chemical and biomedical solution measurement and detection.

Tunable gradient refractive index optics using graded plasmonic crystals with semiconductor rods

Using numerical simulations, we demonstrate the feasibility of tunable gradient refractive index optics at terahertz frequencies based on two-dimensional graded plasmonic crystals (GPCs). They consist of semiconductor rods with spatially dependent radii. In the effective medium approximation, the GPCs can be considered as effective media with a graded effective dielectric permittivity. The semiconductor rods have the Drude-type dispersion. By varying free charge carrier concentration in the rods, it is possible to tune their permittivity. In accordance to effective medium theory, the effective permittivity of the whole GPC is changed at the same time. This property is used for the demonstration of a GPC-based lens with a tunable focus, beam deflector with tunable angle of the beam deflection, and the half Maxwell-fisheye and the Luneburg lens as antennas with tunable radiation patterns. In particular, these GPCs can be made invisible to the incoming radiation by equaling the real part of the rods permittivity to the permittivity of air background.

Tunable contact angle hysteresis by micropatterning surfaces

Lithography is used to form circular micropatterns which govern the evaporation of a water droplet. The surfaces are composed of concentric circular defects having a smooth indentation profile. When a droplet encounters a micropattern, evaporation occurs with distinct discontinuities in the droplet wetting contact angle and base radius. The addition of gaps into the patterns enables modification of the contact angle hysteresis; the receding angle of fluorocarbon coated SU-8 can be tuned between 34.6° and 89.1° and that of SU-8 from 5.6° to 43.3° depending on the gap length. A model is developed which accurately predicts the observed behavior.

Polarization modulation by tunable electromagnetic metamaterial reflector/absorber

We propose a polarization modulation scheme of electromagnetic (EM) waves through reflection of a tunable metamaterial reflector/absorber. By constructing the metamaterial with resonant unit cells coupled by diodes, we demonstrate that the EM reflections for orthogonal polarized incident waves can be tuned independently by adjusting the bias voltages on the corresponding diodes. Owing to this feature, the reflected EM waves can be electrically controlled to a linear polarization with continuously tunable azimuth angle from 0° to 90° at the resonant frequency, or an elliptical polarization with tunable azimuth angle of the major axis when off the resonant frequency. The proposed property has been verified through both numerical simulations and experimental measurements at microwave band, which enables us to electrically modulate the polarization state of EM waves flexibly.

Leaky wave antenna with amplitude controlled beam steering based on composite right/left-handed transmission lines

Abstract. An antenna comprising two different composite right/left-handed transmission line structures is proposed which enables easy beam steering at an operation frequency of 10 GHz. The composite right/left-handed transmission lines are based on planar, periodically arranged via free unit cells, implemented in microstrip technology. Both transmission lines exhibit the infinite wavelength phenomenon which occurs at 9.72 GHz and 9.89 GHz, respectively. Thus, operating the different leaky wave structures at 10 GHz, radiation with azimuth angles of ±8° and ±17° can be achieved depending on the selected input port. In order to obtain a tunable main beam direction, the radiation patterns of both structures are superimposed by feeding them simultaneously. The influence of each guiding structure, and hence the direction of the main beam, can be controlled via the feeding amplitude. As a result of this, the beam can be steered between ±17° with a gain of up to 10 dBi. The guiding structures are arranged in parallel with a clearance of a=12.2 mm which is less than half of the wavelength in free space. This allows in a further step the attachment of additional guiding structures in order to increase the tunable angle range or creating an antenna array with a small beamwidth in the elevation plane without the occurrence of grating lobes. An antenna prototype was fabricated and validated by measurements.

The nonlinear effect from the interplay between the nonlinearity and the supercollimation of photonic crystal

The authors theoretically and numerically investigate the beam propagation near the supercollimation frequency ωs0 in a photonic crystal made of nonlinear material. Since the value and sign of the equal-frequency-contour curvature which dominates the beam behaviors can be nonlinearly tuned near ωs0, a kind of nonlinear effect is generated. The envelope equation with unique form is also obtained. Beam-control mechanisms are theoretically predicted and observed in numerical experiments, such as tunable collimation, tunable beam-divergence angle, and self-lock of collimation. These mechanisms can be utilized to function as fiber, lens and coupler, or to design photonic devices.

Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth

A metamaterial-based electronically controlled transmission-line structure is presented and demonstrated as a novel leaky-wave (LW) antenna with tunable radiation angle and beamwidth functionalities. This structure is, in essence, a composite right/left-handed (CRLH) microstrip structure incorporating varactor diodes for fixed-frequency voltage-controlled operation. Angle scanning at a fixed frequency is achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to the varactors. Beamwidth tuning is obtained by making the structure nonuniform by application of a nonuniform bias voltage distribution of the varactors. A rigorous analysis based on an extension of the CRLH concept is proposed and the corresponding dispersion curves, obtained by equivalent-circuit formulas with LC parameters extracted from full-wave simulation, are shown. A 30-cell LW antenna structure, incorporating both series and shunt varactors for optimal impedance matching and maximal tuning range, is designed. This prototype exhibits continuous scanning capability from 50/spl deg/ to -49/spl deg/ by tuning the bias voltages from 0 to 21 V at 3.33 GHz. A maximum gain of 18 dBi at broadside is also achieved. In addition, it provides half-power beamwidth variation of up to 200% with comparison to the case of uniform biasing. The effect of intermodulation due to the nonlinearity of the varactors is shown to be negligible for antenna applications. The antenna is tested in a 10-Mb/s binary phase-shift keying transmission link and successful recovery of the baseband data is demonstrated.

Metamaterial-Based Electronically Controlled Transmission-Line Structure as a Novel Leaky-Wave Antenna With Tunable Radiation Angle and Beamwidth

A metamaterial-based electronically controlled transmission-line structure is presented and demonstrated as a novel leaky-wave (LW) antenna with tunable radiation angle and beamwidth functionalities. This structure is, in essence, a composite right/left-handed (CRLH) microstrip structure incorporating varactor diodes for fixed-frequency voltage-controlled operation. Angle scanning at a fixed frequency is achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to the varactors. Beamwidth tuning is obtained by making the structure nonuniform by application of a nonuniform bias voltage distribution of the varactors. A rigorous analysis based on an extension of the CRLH concept is proposed and the corresponding dispersion curves, obtained by equivalent-circuit formulas with LC parameters extracted from full-wave simulation, are shown. A 30-cell LW antenna structure, incorporating both series and shunt varactors for optimal impedance matching and maximal tuning range, is designed. This prototype exhibits continuous scanning capability from 50/spl deg/ to -49/spl deg/ by tuning the bias voltages from 0 to 21 V at 3.33 GHz. A maximum gain of 18 dBi at broadside is also achieved. In addition, it provides half-power beamwidth variation of up to 200% with comparison to the case of uniform biasing. The effect of intermodulation due to the nonlinearity of the varactors is shown to be negligible for antenna applications. The antenna is tested in a 10-Mb/s binary phase-shift keying transmission link and successful recovery of the baseband data is demonstrated.

Thermally controlled lateral beam shift and beam steering in semiconductor lasers

We demonstrate a thermally controlled, offset-gain and index-guiding (OGIG) structure, which permits continuous control of output beam position and direction from a semiconductor laser. The gain and index guiding axes in this structure are parallel but transversely offset. For parabolic gain and index profiles the resulting modes will be Hermite-Gaussian modes with complex-valued spot sizes and tilted wavefronts such that the beam emerges from the end face at a tunable angle with respect to the facet normal. An experimental demonstration of thermally controlled mode displacement and beam steering by as much as 15/spl deg/ is obtained by applying asymmetric heating to a semiconductor laser using a microstripe heater. >

Detection of thin a-Si:H antireflective coatings on oxidized c-Si by resonant-detected spectroscopic ellipsometry

The cleaning procedures applied in the wafer processing of thin a-Si:H overlayers have been monitored by spectroscopic ellipsometry. By selecting a suitable sample configuration and exploiting a tunable angle of incidence, we show that spectroscopic ellipsometry is extremely sensitive to small modifications at the vacuum-a-Si:H interface induced by the cleaning procedures. Experimental results are presented on the characterization of thin (3-12-nm) a-Si:H films on top of thermally oxidized crystalline silicon. Submonolayer sensitivity of the ellipsometric measurement to changes in a-Si:H film thickness is shown.