Tunable spin-wave delay line based on ferrite and vanadium dioxide

The temperature dependence of electrical capacitance of planar microstructures with vanadium dioxide (VO2) film near the insulator-metal phase transition has been investigated at the frequency of 1 MHz. Electrical capacitance measurements of the microstructures were performed by the technique based on the using of a two-terminal resistor-capacitor module simulating the VO2 layer behavior at the insulator-metal phase transition. At temperatures 325–342 K, the anomalous increase in microstructures capacitance was observed. Calculation of electric field in the microstructure showed that VO2 relative permittivity (ε) reaches ∼108 at the percolation threshold. The high value of ε can be explained by the fractal nature of the interface between metal and insulator clusters formed near the insulator-metal phase transition.

Modern optical communication systems require tunable optical delay lines for synchronization of data packages. Chirped fiber grating are typically used as a dispersive element for this application. Fiber-based schemes, however, suffer from bulkiness, slow response and/or low time resolution. Integrated photonics on silicon is one alternative approach. Several more advanced device architectures have been subsequently demonstrated on silicon, including ringresonator and photonic crystal (PhC) line-defect waveguide types. The coauthors of this paper have recently proposed a novel class of tunable optical delay lines based on apodized grating waveguides. It was shown that high bit-rate operation is achievable by cascading two complementary (inward and outward) apodized gratings. In this paper, we have experimentally demonstrated our proposed approach and studied performance of the fabricated delay lines for optical telecommunication applications. The bit-rate was extracted from broadening of transform-limited input pulses of the measured dispersion of the delay lines. Characterization of the compact delay lines shows that the single grating devices offer true-time delays as long as 164 ps, tuning range of ~ 81 ps, and a minimum bit rate of ~ 15 Gb/s. The cascaded scheme offer true-time delays of 82 ps and tuning range of 32 ps and can potentially operate at bit rates as high as 107 Gb/s.

Heterophase textured films of vanadium dioxide on single-crystal silicon substrates have been synthesized by the sol-gel method from solutions of triethoxyvanadyl VO(OEt)3 in methyl cellosolve CH3OCH2CH2OH. The effect of ozone and ethanol vapor chemisorption on the parameters of the semiconductor-metal phase transition in vanadium dioxide VO2(B) has been studied. It has been found that the transition temperature increases in a reducing atmosphere and decreases in an oxidizing atmosphere.

We have analyzed the dielectric spectra of thin (1200 A) films of vanadium dioxide, a material with strong electron–electron correlations. We have tested both undoped and germanium-doped VO2 : Ge films. The latter exhibits an additional peak in the frequency spectrum of the tangent of the dielectric loss angle. We have analyzed the fine structure of the spectra and provided physical interpretation of two peaks on the frequency dependence of the tangent of dielectric loss angle and two semi-circles on the Cole–Cole diagram. Analysis of results has been performed based on equivalent electric circuit diagrams of samples, viz., one-loop RC circuit for the undoped VO2 film and two-loop circuit for VO2 : Ge film. The numerical values of parameters of model systems have been determined.

A large change in optical constants of phase-change vanadium dioxide enables active control over the transmission and reflection properties of structures incorporating VO2. In this paper, we demonstrate electrically tunable mid-infrared strip array antennas based on metal–insulator transition of vanadium dioxide. The antennas consist of an interdigitated metal strip array separated from a metallic ground plane by a film of vanadium dioxide. The interdigitated metal strips serve as both antennas and electrodes. As the insulator-to-metal phase transition of vanadium dioxide is induced with applied voltages, resonance of the strip antenna array redshifts with reduced absorbance before it is eventually switched off. A tuning magnitude of 30% in reflectivity is measured at 25.5 THz. Our measurements of sample temperature reveal that tuning mechanism of the antennas is primarily thermal in nature. The demonstrated electrical tuning of mid-infrared antennas could be used for reconfigurable bolometric sensing, camouflaging and modulation of infrared radiations.

A phase noise of the microwave oscillators based on spin-wave delay lines is theoretically investigated. Theoretical results show that spin-wave damping in the delay line limits a phase reduction with increasing time delay. It is possible to find an optimum value of the time delay for each particular value of damping decrement in order to provide minimum phase noise of the generated microwave signal.

Single phase monoclinic vanadium dioxide (VO2) films were successfully grown on sapphire (001) substrate by means of an inductively coupled plasma (ICP)-assisted sputtering with an internal coil. In x-ray diffraction patterns, we observed reflections originating from monoclinic VO2 with a main peak at 2θ = 39.77° corresponding to VO2 (020) for films on sapphire (001) substrates. The VO2 films exhibited metal-insulater transition (MIT) at temperatures around 65°C with three orders of change in resistivity. A simple device of VO2 film on sapphire (001) substrate having aluminum electrodes as ohmic contacts with gap separation of 1.0 mm showed current jump at an applied voltage of 48 V. The current jumped from 7 mA to 200 mA which was a limited value of voltage source.

The theory describing a tunability of the spin-wave spectrum for the ferrite/vanadium dioxide layered structures through changing the VO2 conductivity is suggested. An influence of the various parameters on the spin-wave wavenumber variations and damping decrement is studied. We show that an effective wavenumber variation with minimal losses is achieved by matching the thicknesses of the ferrite and vanadium dioxide films, as well as the value of external magnetic field. We show also that owing to the electrodynamic interaction, a quality of contact between the ferrite and vanadium dioxide layers is not critical.

We report on the effects of energetic ions incident to a substrate on the growth of vanadium dioxide (VO2) films on conductive layers by inductively coupled plasma (ICP)-assisted sputtering (ICPS). Ion energy distributions (IEDs) of Ar+ were measured using an electrostatic energy analyzer consisting of three meshed plates. Ions with kinetic energies up to 150 eV with peak positions corresponding to the plasma space potential were observed in ICP-assisted sputtering, in contrast with lower energies in conventional sputtering. Crystalline VO2 films with an insulator-to-metal transition (IMT) exhibiting resistance change over 2 orders of magnitudes at around 70 °C were successfully grown on conductive layers of titanium (Ti) and indium tin oxide (ITO) even at a low substrate temperature of 250 °C. Characteristic IMTs with multistep abrupt changes and hysteresis width of around 30 °C were first realized for polycrystalline VO2 film with the M2 phase grown on ITO layers on glass substrates. Through the analyses of surface morphology and stress states, we elucidated that high-energy ion irradiation strongly assists the low-temperature (<250 °C) crystalline growth of VO2, while it is, at the same time, accompanied by high growth stress.

Vanadium dioxide (VO2) films were prepared by the sol–gel method on high-purity silicon substrates, in which cetyltrimethyl ammonium bromide (CTAB) was used as a functional additive to form nano-porous structure in the VO2 films. The morphology, crystalline structure and stoichiometry of the films were investigated by field emission scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. Furthermore, the effects of nano-scaled grain size and pores on the THz transmission properties across the phase transition in the VO2 films were studied. The results indicated that the film modified with CTAB can form a nano-porous VO2 structure with a uniform grain size of about 30 nm. The nano-porous VO2 film exhibited significantly broader hysteresis loops and a slight decrease in THz transmission reduction across the phase transition, compared with the common film without a nano-porous structure. A tentative interpretation is given for these phenomena.

We demonstrate the generation of a persistent conductivity increase in vanadium dioxide thin films grown on single crystal silicon by irradiation with 1 GeV 238U swift heavy ions at room temperature. VO2 undergoes a temperature driven metal-insulator-transition (MIT) at 67 °C. After room temperature ion irradiation with high electronic energy loss of 50 keV/nm the conductivity of the films below the transition temperature is strongly increased proportional to the ion fluence of 5·109 U/cm2 and 1·1010 U/cm2. At high temperatures the conductivity decreases slightly. The ion irradiation slightly reduces the MIT temperature. This observed conductivity change is persistent and remains after heating the samples above the transition temperature and subsequent cooling. Low temperature measurements down to 15 K show no further MIT below room temperature. Although the conductivity increase after irradiation at such low fluences is due to single ion track effects, atomic force microscopy (AFM) measurements do not show surface hillocks, which are characteristic for ion tracks in other materials. Conductive AFM gives no evidence for conducting ion tracks but rather suggests the existence of conducting regions around poorly conducting ion tracks, possible due to stress generation. Another explanation of the persistent conductivity change could be the ion-induced modification of a high resistivity interface layer formed during film growth between the vanadium dioxide film and the n-Silicon substrate. The swift heavy ions may generate conducting filaments through this layer, thus increasing the effective contact area. Swift heavy ion irradiation can thus be used to tune the conductivity of VO2 films on silicon substrates.

In this paper, we propose integrated one-dimensional (1D) grating waveguide-based true-time delay (TTD) lines on a silicon-on-insulator (SOI) platform. Through optimizing the structure of the proposed waveguide, a time delay of 77.23ps/mm can be readily achieved in a wavelength tuning range from 1540.20nm to 1558.97nm. Compared to conventional photonic crystal waveguide-based TTDs, the proposed waveguide occupies 70% less chip surface and has much lower propagation loss when compared with 2D photonic crystal devices. Therefore, a larger time delay can be achieved on-chip. To facilitate low loss coupling from strip waveguides to 1D grating waveguides and vice versa, a novel step taper is designed and shows a coupling efficiency of over 78%. Based on the 1D grating waveguide, a 1×4 beam steering module is designed and simulated. A wide beam steering angle from -67.84° to 67.84° for the X-band four-element phased array antenna with an array pitch size of 1.25cm is obtained.

Tunable optical delay devices have numerous applications in optical communications [1] and have been successfully implemented using slow light elements and fiber or waveguide gratings. There has been considerable interest in siliconon- insulator (SOI) as a technology platform for compact integration of optical signal processing systems. SOI-based delay lines have been realized using coupled ring resonators [2], photonic crystals [3], and various Bragg grating-based configurations including single or coupled chirped sidewall gratings [4,5] as well as tapered rib waveguide gratings [6]. By linearly chirping the period in sidewall gratings, relatively small delays (a few ps) over a bandwidth of tens of nm were demonstrated [4]; with tapered waveguides, significantly larger delays (300-500 ps) were obtained, albeit over a narrower bandwidth (&lt; 2 nm) [6]. On the other hand, some signal processing applications may require large delays (e.g., tens to hundreds of ps) over large bandwidths (several to tens of nm). Several designs have been proposed to meet these requirements, e.g., a step-chirped rib waveguide grating providing 50 ps delay over 15 nm [7] or complementary apodized sidewall gratings providing up to 275 ps over 3 nm [8], however, they have not been realized experimentally. In this paper, we demonstrate discretely tunable optical delay lines that provide tens of ps delay (up to 65 ps) in steps of 15-32 ps over bandwidths of several tens of nm (35-70 nm). The devices are fabricated on SOI using electron beam lithography and implemented through two different approaches: serial sidewall Bragg grating arrays and the step-chirped sidewall Bragg gratings.

We review our last work on dispersion-engineered heterogeneous multicore fiber links designed to act as tunable true time delay lines for radiofrequency signals. This approach allows the realization of fiber distributed signal processing in the context of fiber-wireless communications, providing both radiofrequency access distribution and signal processing in the same fiber medium. We show how to design trench-assisted heterogeneous multicore fibers to fulfil the requirements for sampled true time delay line operation while assuring a low level of crosstalk, bend sensitivity and tolerance to possible fabrication errors. The performance of the designed radiofrequency photonic delay lines is evaluated in the context of tunable microwave signal filtering and optical beamforming for phased array antennas.

Recent interest in employing single-crystal yttrium iron garnet (YIG) films for miniaturized microwave integrated circuit applications has prompted us to study detaching liquid phase epitaxy (LPE) grown YIG films from their gadolinium gallium garnet (GGG) substrates via crystal ion slicing (CIS). We report studies of magnetization and ferromagnetic resonance (FMR) of narrow linewidth gallium-doped YIG (GaYIG) films in various stages of separation from their 〈111〉 oriented GGG substrates. All samples were diced from the same three-inch wafer of Y3Fe4.6Ga0.4O12 with 4πMeff of 1070 G and 9.45 GHz FMR linewidth of 0.4±0.1 Oe. The CIS separation process involved (1) implantation with helium ions, (2) flash annealing to remove surface damage, and (3) chemical etching to detach the YIG at the implantation damaged layer. The starting films were 10.8-μm thick. Separation occurred 7 to 8 μm from the front surface in the YIG. The implantation (5×1016 cm−2 dosage, 3.8 MeV bias) increased the uniaxial anisotropy of these films. A small cubic anisotropy (∼5 G 60° in-plane periodicity) persisted in both implanted and unimplanted samples. Upon implantation the FMR linewidth increased from 0.4 to 3±0.5 Oe. The detached samples have linewidths ranging from 1.7 to 2.5 Oe. The temperature dependence of the linewidths is roughly exponential decreasing as the temperature is lowered for the unprocessed sample, increasing with reduced temperature for all processed samples. Post-detachment annealing restores the FMR linewidth to 0.55±0.5 Oe.