In this topical review, we will discuss recent advances in the field of skyrmionics (fundamental and applied aspects) mainly focusing on skyrmions that can be realized in thin film structures where ...an ultrathin ferromagnetic layer (<1 nm) is coupled to materials with large spin-orbit coupling. We review the basic topological nature of the skyrmion spin structure that can entail a stabilization due to the chiral exchange interaction present in many multilayer systems with structural inversion asymmetry. The static spin structures and the dynamics of the skyrmions are also discussed. In particular, we show that skyrmions can be displaced with high reliability and efficiency as needed for the use in devices. We discuss major possible applications, such as memory, microwave oscillators and logic, and combinations of these, making skyrmions very promising candidates for future low power IT devices.
Previously a method was reported to determine the mathematical representation of the microwave oscillator admittance by using numerical calculation. When analyzing the load characteristics and ...synchronization phenomena by using this formula, the analysis results meet with the experimental results. This paper describes a method to determine the mathematical representation manually.
The metallic mesh is a typical transparent conductive film commonly used in the field of transparent electromagnetic shielding. The nondestructive evaluation (NDE) of such films has attracted ...considerable interest from researchers due to its applicability to the quality assessment and maintenance of the films. In this study, we developed a microwave feedback-type oscillator with an integrated complementary spiral resonator (CSR) for the NDE of metallic meshes. The optimized CSR structure with bandpass characteristics can transform two typical defects of metallic meshes-cracks and increase in sheet resistance into a downshift in resonant frequency and decrease in quality factor. Next, the CSR was integrated into a feedback oscillator with an additional loop filter to achieve self-sustained measurement and self-switching effect for cracks. Both small-signal and steady-state analyses were conducted to predict the variation in the output power and the static drain current under different defect conditions. The experimental results show that the developed sensor can detect cracks with 0.4 mm width with self-switching effect and can detect a 3.6% increase in sheet resistance with a variable measurement range. The sensor has a low profile and power consumption and requires only a microwave power detector or dc ammeter as the readout device. The proposed sensor can not only be applied to detect defects in metallic meshes but also a good candidate for the NDE of other conductive films.
An narrow linewidth all-optical optomechanically microwave oscillator (OM-MO) based on forward stimulated Brillouin scattering (FSBS) for microwave photonics (MWPs) generation is proposed and ...demonstrated. The all-optical OM-MO consists of a 100 m SMF ring cavity which provide FSBS gain and a Sagnac loop with 10 m un-pumped Erbium-doped fiber (EDF) that used as saturable absorber. At 280 mw 980 nm pump threshold power, the 128∼368 MHz tunable range with about 50 MHz frequency interval of the MWPs are achieved by adjusting PCs to tune the offset frequency between the repetition rate of the driving pulse and the R 03 -R 08 FSBS acoustic mode resonance frequency of the all-optical OM-MO. At the same time, the acoustic and side-mode suppression rates of R 03 -R 08 acoustic mode based R 03 -R 08 MWPs are 21, 28, 22, 28, 31, 27 dB and 38, 28, 20, 30, 30, 32 dB respectively. Compared with the FSBS based OEO and passively mode locked fiber laser, the all-optical OM-MO in longer length fiber without any electrical microwave devices can generate narrower linewidth MWPs by narrowing the passive resonator intrinsic linewidth, which is different from the backward Brillouin fiber laser of narrowing pump linewidth. The linewidths of R 03 -R 08 MWPs are 11.9, 11.1, 10.3, 10.8, 10.4, 10.5 Hz. Within 20 minutes of the stability experiment, the power and frequency stability fluctuation of the R 04 MWP are ±0.5 dB and ±5 kHz respectively. The best phase noise value at 10 kHz frequency offset is -136.1 dBc/Hz of R 04 MWP which is superior to FSBS based OEO. This narrow linewidth MWP generation technology can be promising for radar monitoring and wireless communication applications.
An all-optical microwave oscillator based on parity-time symmetry was proposed and experimentally demonstrated. It mainly consists of a big feedback loop, an active small loop and a ...polarization-dependent Sagnac loop. The microwave seed signal originates from the beating between the continuous wave and the Stokes wave. The function of microwave envelope detection and feedback modulation are implemented through cross gain modulation of two semiconductor optical amplifiers. By employing the parity time symmetry loops and the Vernier effect, the single mode selectivity capability of the oscillator has been enhanced, and a single mode microwave signal with high quality can be generated. In the experiment, a highly quality microwave signal with a frequency of 10.71 GHz was generated. The single side band phase noise can reach −94.1 dBc/Hz@10 kHz and the side mode suppression ratio of the generated microwave signal reaches 41 dB. Compared to the previously reported all-photonic microwave oscillator, the single side band phase noise is improved by 7 dB at a frequency offset of 10 kHz, and by 20 dB at a frequency offset of 100 Hz. The results indicate that our proposed scheme significantly improves the quality of the generated microwave signals.
•This manuscript provides a feasible pure optical scheme to generate microwave photonic signal source.•The parity time symmetry loops can significantly enhance the single mode selectivity capability of the oscillator.•The SSB is improved by 7 dB at a frequency offset of 10 kHz, and by 20 dB at a frequency offset of 100 Hz.
A microwave planar negative resistance oscillator with higher frequency stability and lower phase noise (PN) is introduced for sensing applications. Split-ring resonator is used as sensing element at ...2.4 GHz and is used for microfluidic concentration sensing of high lossy solution of water in ethanol. Stabilizing the frequency response with the use of active filter leads to 14-times enhancement in the limit of detection, from 7% (passive filter) to 0.5% (active filter), while the linearity of the sensor is preserved; the sensing range is then increased. Nearly 13-dBc/Hz improvement in the measured PN, in great agreement with theory, also verifies the frequency domain stability of the oscillator.
A novel displacement sensor was proposed based on a frequency delta-sigma modulator (FDSM) employing a microwave oscillator. To demonstrate basic operation, we fabricated a stylus surface profiler ...using a cylindrical cavity resonator, where one end of the cavity is replaced by a thin metal diaphragm with a stylus probe tip. Good surface profile was successfully obtained with this device. A 10 nm depth trench was clearly observed together with a 10 µm trench in a single scan without gain control. This result clearly demonstrates an extremely wide dynamic range of the FDSM displacement sensors.
A semianalytical method for the global prediction and understanding of the transient dynamics of oscillator circuits is presented. It covers both the linear and nonlinear transient stages, which are ...related with the circuit generalized eigenvalues, here introduced for the first time. The transient model relies on the application of the implicit function theorem to the harmonic-balance (HB) system, in order to derive a reduced-order nonlinear differential equation from a given observation node. This requires the extraction of a nonlinear admittance function, depending on the voltage excitation and oscillation frequency, which is done with a forcing auxiliary generator (AG). The linearization of this admittance function for each excitation amplitude provides a sequence of linear ordinary differential equations (ODEs), describing the system dynamics in the vicinity of each point of the transient trajectory, which can be reconstructed from the expression of the solution increment at each time step. The sequence of differential equations provides a set of generalized eigenvalues, responsible for the acceleration or deceleration of the oscillation growth and capable to detect spurious transient frequencies. The concept of escape time, or time required by the transient trajectory to go through a certain interval of amplitude values, is also introduced, for the first time to our knowledge. The method has been successfully applied to analyze the transient dynamics of several FET oscillators, including dual-frequency oscillators and switched oscillators.
The single-sideband phase noise power spectral density and Allan deviation of a portable microwave oscillator system based on an optical frequency comb stabilized to an ultra-low expansion ...Fabry-Perot reference cavity is characterized at carrier frequencies of 4.0, 8.0, and 16.0 GHz. Compared to industry standard sapphire loaded cavity oscillators, the photonic microwave oscillator shows superior phase noise at offset frequencies < 100 Hz, and similar performance for offset frequencies from 100 Hz to 100 kHz. At 1-kHz offset, the phase noise power spectral density is better than −140 dBc/Hz at each carrier frequency. Relative frequency stability of <inline-formula> <tex-math notation="LaTeX">3.0 \times 10^{-14} </tex-math></inline-formula> obtains at an averaging time of 400 ms.
A coupled all-optical microwave oscillator with large tuning range is proposed. Without optical–electrical–optical conversion components or any electrical microwave devices, microwave signals can be ...generated through a pure optical oscillation. In this system, a fiber ring laser provides a set of frequency references for frequency selection. Meanwhile, it also serves as a high-Q active resonator to optimize phase noise performance. By transferring the cavity modes to an optical carrier and selectively amplifying some of the modes via stimulated Brillouin scattering, the oscillation frequency is preliminarily selected. A semiconductor optical amplifier in the fiber ring laser is employed to accomplish the optical envelope detection and feedback modulation. Once the oscillation system is closed, good quality and high stability single-mode microwave signals are generated. By varying the wavelength of Brillouin pump light, a tuning range from 2 GHz to 26 GHz is demonstrated. Furthermore, the single side-band phase noise of these microwave signals are around −90 dBc/Hz@10 kHz.
•A good-quality microwave signal is generated by a coupled all-optical oscillator.•The system has no any electrical microwave device as well as electro-optic (E/O) and opto-electronic (O/E) conversion.•The oscillation frequency of the signal is chosen by Brillouin-selective sideband amplification.•The optical envelope detection and feedback modulation are implemented by a semiconductor optical amplifier.
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