Continuous Tuning Research Articles

High Power RF Discharge Detection Technique Based on the In-Phase and Quadrature Signals

High power radio frequency (RF) breakdown testing is a subject of great relevance in the space industry, due to the increasing need of higher transmission power and smaller devices. This work presents a novel RF breakdown detection system, which monitors the same parameters as the microwave nulling system but with several advantages. Where microwave nulling--a de facto standard in RF breakdown testing--is narrowband and requires continuous tuning to keep its sensitivity, the proposed technique is broadband and maintains its performance for any RF signal. On top of that, defining the detection threshold is cumbersome due to the lack of an international standardized criterion. Small responses may appear in the detection system during the test and, sometimes, it is not possible to determine if these are an actual RF breakdown or random noise. This new detection system uses a larger analysis bandwidth, thus reducing the cases in which a small response is difficult to be classified. The proposed detection method represents a major step forward in high power testing as it runs without human intervention, warning the operator or decreasing the RF power automatically much faster than any human operator.

Improved Hopfield Network Optimization Using Manufacturable Three-Terminal Electronic Synapses

We illustrate novel optimization techniques via simulations for Hopfield networks constructed from manufacturable three-terminal Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) synaptic circuit elements. We first present a computationally-light, memristor-based, highly accurate static compact model for the SONOS synapses used in our simulations. We then show how to exploit analog errors in programming resistances and current leakage, and the continuous tunability of the SONOS synapses to enable transient chaotic group dynamics, to accelerate the convergence of a Hopfield network. We project improvements in energy consumption and time to solution relative to existing CPUs and GPUs by at least 4 orders of magnitude, and also exceed the projected performance of two-terminal memristor-based crossbars in addition to a 100-fold increase in error-resilient array size (i.e. problem size).

Floquet engineering of low-energy dispersions and dynamical localization in a periodically kicked three-band system

Much having learned about Floquet dynamics of pseudospin-$1/2$ system namely, graphene, we here address the stroboscopic properties of a periodically kicked {three-band fermionic system such as $\alpha$-T$_3$ lattice. This particular model provides an interpolation between graphene and dice lattice via the continuous tuning of the parameter $\alpha$ from 0 to 1.} In the case of dice lattice ($\alpha=1$), we reveal that one can, in principle, engineer various types of low energy dispersions around some specific points in the Brillouin zone by tuning the kicking parameter in the Hamiltonian along a particular direction. Our analytical analysis shows that one can experience different quasienergy dispersions for example, Dirac type, semi-Dirac type, gapless line, absolute flat quasienergy bands, depending on the specific values of the kicking parameter. Moreover, we numerically study the dynamics of a wave packet in dice lattice. The quasienergy dispersion allows us to understand the instantaneous structure of wave packet at stroboscopic times. We find a situation where absolute flat quasienergy bands lead to a complete dynamical localization of the wave packet. {Aditionally, we calculate the quasienergy spectrum numerically for $\alpha$-T$_3$ lattice. A periodic kick in a perpendicular (planar) direction breaks (preserves) the particle-hole symmetry for $0<\alpha<1$. Furthermore, it is also revealed that the dynamical localization of wave packet does not occur at any intermediate $\alpha \ne 0,\,1$.}

Midinfrared Tunable Laser with Noncritical Frequency Matching in Box Resonator Geometry.

Monolithic optical parametric oscillators extend laser frequencies in compact architectures, but normally guide and circulate all pump, signal, and idler beams. Critical frequency matching is raised among these resonances, limiting operation stability and continuous tuning. Here, we develop a box resonator geometry that guides all beams but only resonates for signal. Such noncritical frequency matching enables 227GHz continuous tuning, with sub-10kHz linewidth and 0.43W power at 3310nm. Our results confirm that monolithic resonator can be effectively used as a tunable laser including midinfrared wavelength, as further harnessed with methane fine spectral measurement at MHz accuracy.

Nearly Room-Temperature Ferromagnetism in a Pressure-Induced Correlated Metallic State of the van der Waals Insulator CrGeTe_{3}.

A complex interplay of different energy scales involving Coulomb repulsion, spin-orbit coupling, and Hund's coupling energy in 2D van der Waals (vdW) material produces a novel emerging physical state. For instance, ferromagnetism in vdW charge transfer insulator CrGeTe_{3} provides a promising platform to simultaneously manipulate the magnetic and electrical properties for potential device implementation using few nanometers thick materials. Here, we show a continuous tuning of magnetic and electrical properties of a CrGeTe_{3} single crystal using pressure. With application of pressure, CrGeTe_{3} transforms from a ferromagnetic insulator with Curie temperature T_{C}∼66 K at ambient condition to a correlated 2D Fermi metal with T_{C} exceeding ∼250 K. Notably, absence of an accompanying structural distortion across the insulator-metal transition (IMT) suggests that the pressure induced modification of electronic ground states is driven by electronic correlation furnishing a rare example of bandwidth-controlled IMT in a vdW material.

Phase‐Only Tuning of Extreme Huygens Metasurfaces Enabled by Optical Anisotropy

AbstractPure phase modulation of light is vital for a number of optical devices such as spatial light modulators and beam steering in light detection and ranging (LIDAR) technologies. Tunable metasurfaces have recently provided a feasible alternative to existing technologies, allowing for ultra‐high miniaturisation while enabling high transmission efficiency and tunability under small external stimuli. However, despite the recent advances in the field, no pure phase tuning has been demonstrated in transmissive devices, for example, any implementation of continuous phase tuning is accompanied by sizable amplitude modulation or low efficiency. Here, it is shown that the optical anisotropy of the surrounding material can enable phase‐only tuning of optical metasurfaces in the full 2π range with unitary efficiency over a sizable bandwidth. A practical implementation of this concept based on a liquid‐crystal infiltrated metasurface operating in the regime of extreme Huygens condition is further proposed. In this way, the full 2π phase‐only tunability in transmission can be enabled by controlling bias voltage and temperature variation of the surrounding liquid crystal.

Continuous Tuning of the Fermi Level in Disorder-Engineered Amorphous Films of Li-Doped ZnO for Thermoelectric Applications.

Amorphous metal-oxide semiconductors can be readily prepared by a solution process at low temperatures, and their energy band structures and carrier concentrations can be controlled based on the oxide composition or the addition of dopants in the design of thermoelectric (TE) materials. However, research on the correlation between the charge transport and TE performance of amorphous metal-oxide semiconductors is still in its infancy. Herein, we present the energy-dependent TE performance characteristics of Li-doped ZnO thin films with different doping levels and charge carrier concentrations. Thin films were prepared by the solution process, and the Li doping level was controlled by the Li precursor concentration added to a Zn precursor solution. Subsequently, a field-effect-modulated Seebeck coefficient measurement device was built to study the energy-dependent TE performance. Notably, the higher ratio of interstitial Li (Liinter) and oxygen vacancies (Ova) in the Li-ZnO device indicates an improved n-type TE performance. To investigate more thoroughly the charge transport phenomena, the localized density of states (DOS) was derived from the temperature-dependent transfer curve; the higher ratio of interstitial Li (Liinter) and oxygen vacancy (Ova) induces a reduction in the localized DOS and lowers the degree of disorder in their DOS. The determined energy-dependent TE characteristics can be used as guidance for the design of efficient TE devices with amorphous metal-oxide semiconductors.

A Modular Design of Continuously Tunable Full Color Plasmonic Pixels with Broken Rotational Symmetry

AbstractColor tuning is a fascinating and indispensable property in various applications. Thus far, a variety of reconfigurable approaches have been implemented to achieve color change. However, it is still a challenge to enable continuous color tuning over the entire hue range in a simple, stable, and rapid manner without changes in configuration and material properties. Here, an all‐optical continuously tunable plasmonic pixel scheme is demonstrated via a modular design approach to realize polarization‐controlled full color tuning by breaking the intrinsic symmetry of the unit cell layout. The polarization‐controlled full color tunable plasmonic pixels consist of three different types of color modules corresponding to three subtractive primary colors. Without changing the structural properties or surrounding environment, the structural colors can be continuously and precisely tuned across all hues by illuminating linearly polarized light with different polarization directions. Meanwhile, the plasmonic pixels can be flexibly customized for various color tuning processes through the appropriate choice of component modules and the elaborate design of module layouts. Furthermore, the color tuning is extended to achromatic colors with the utilization of a single module or the introduction of a black module. The proposed plasmonic pixels hold considerable potential to function as next‐generation color pixels integrated with liquid‐crystal polarizers.

Mechanical Regulation of the Magnetic Properties of Uniaxial Anisotropic Hexaferrite Thin Films

Flexible oxide thin films with uniaxial magnetocrystalline anisotropy $({K}_{u})$ not only have many applications in flexible electronics, but also provide an ideal low-crystal-symmetry material for a fundamental understanding of the competitive interplay among magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy. However, an understanding of the mechanical tuning of magnetic parameters in flexible oxide films with ${K}_{u}$ has not been realized up to now. In this work, epitaxial flexible hexaferrite ${\mathrm{Ba}\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ thin films with a room temperature out-of-plane ${K}_{u}$ of 1.1 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ J/m${}^{3}$ are fabricated. The continuous and controllable tuning of magnetic parameters is found to be realizable by different bending radii of the film. The changes in magnetic parameters are symmetrical under tensile and compressive bending due to the dominance of film geometry and bending-stress directions, which are perpendicular to the easy axis of ${K}_{u}$. The micromagnetic simulations further show that the magnitude of ${K}_{u}$ plays a critical role in the mechanical tuning performance of the film, where high ${K}_{u}$ drastically reduces the role of stress anisotropy and increases the magnetic bending repeatability of the film by suppressing bending-defect-related anisotropy degradation. This work not only provides an understanding of the role of the direction and magnitude of ${K}_{u}$ in the flexible thin films, but also demonstrates that the flexible hexaferrite film is one of the best materials for mechanical tunable devices up to the millimeter-wave region.

Matching of generators and gas-discharge loads with radio frequency pumping

The article considers the problem of matching power high-frequency generators and gas discharge loads. Variants of various types of tunable matching circuits are proposed. A method of adaptive continuous tuning is described and a variant of implementing tunable elements on varicaps is proposed.

Multifunctional Optical Device with a Continuous Tunability over 500 nm Spectral Range Using Polymerized Cholesteric Liquid Crystals.

We report that polymerization makes a robust, practically applicable multifunctional optical device with a continuous wavelength tunable over 500 nm spectral range using UV-polymerizable cholesteric liquid crystals (CLCs). It can be used as a circular polarizer generating an extremely high degree of circularly polarized light with |g| = 1.85~2.00. It can also be used for optical notch filters, bandwidth-variable (from ~28 nm to ~93 nm) bandpass filters, mirrors, and intensity-variable beam splitters. Furthermore, this CLC device shows excellent stability owing to the polymerization of CLC cells. Its performance remains constant for a long time (~2 years) after a high-temperature exposure (170 °C for 1 h) and an extremely high laser beam intensity exposure (~143 W/cm2 of CW 532 nm diode laser and ~2.98 MW/cm2 of Nd: YAG pulse laser operation for two hours, respectively). The optical properties of polymerized CLC were theoretically analyzed by Berreman’s 4 × 4 matrix method. The characteristics of this device were significantly improved by introducing an anti-reflection layer on the device. This wavelength-tunable and multifunctional device could dramatically increase optical research efficiency in various spectroscopic works. It could be applied to many instruments using visible and near-infrared wavelengths.

51-W average power, 169-fs pulses from an ultrafast non-collinear optical parametric oscillator.

We present a high power optical parametric oscillator (OPO) synchronously pumped by the second-harmonic of a modelocked 1030-nm thin-disk laser (TDL) oscillator. The OPO delivers an average power of 51.1 W around degeneracy (1030 nm) with a 10.2-MHz repetition-rate. After extra-cavity dispersion compensation using dispersive mirrors, we obtain a pulse duration of 169 fs, which is 4.6× shorter than the TDL pulse duration of 770 fs. The TDL has 250W average power, which is converted to 215W at the second-harmonic. Hence, the OPO exhibits a high photon conversion efficiency of 47% (ratio of signal photons to 515-nm pump photons). Moreover, the OPO generates a peak power of 26.2 MW, which is very similar to the 28.0-MW peak power of the TDL. To facilitate continuous tuning around degeneracy and convenient extraction of the pump and idler beams, the OPO is operated in a noncollinear configuration. A linear cavity configuration was chosen since it offers easy alignment and straightforward cavity length tuning. To the best of our knowledge, this source has the highest average power generated by any ultrafast OPO, and the shortest pulse duration for any >5-W OPO. This result is an important step to adding wavelength tunability to high power Yb-based laser sources without the complexity of either laser or parametric amplifier systems.

A highly selective tri‐band bandpass‐to‐bandstop filter with the third band independently tunable

AbstractA highly selective tri‐band bandpass (BP)‐to‐bandstop (BS) filter with the third tunable band and the other two fixed ones is proposed in this article. This filter is based on a novel crossed resonator that is composed of a two‐end‐open UIR with an open stub and a short stub connected in parallel on both sides of the resonator center. A PIN diode is utilized to achieve BP‐to‐BS switchable characteristics. Then, varactor diodes loaded at the open stub are employed to obtain the independently tunable third band. In addition, due to the symmetry of the resonator structure, the even‐ and odd‐ mode analysis method is used to derive the resonant frequencies. For validation, a tri‐band BP‐to‐BS filter operating at 2.4 GHz (WLAN), 3.3 GHz (WiMAX), and 4.9 GHz (LAA/LTE‐U) is designed, manufactured, and measured that achieves the continuous frequency tuning range (FTR) of the third band from 4.52 to 4.90 GHz with return loss (RL) better than 10.05 dB under the BP characteristic, from 4.55 to 4.90 GHz with the rejection level (RL) higher than 15.53 dB under the BS characteristic. The measured results agree well with the simulated ones.

Continuously Tunable and Energy-Enhanced Injection Pulse-Seeded Terahertz Parametric Generator Based on KTP Crystal

A continuously tunable, energy-enhanced injection, pulse-seeded THz parametric generator based on a KTP crystal (KTP ips-TPG) has been demonstrated in this paper. Theoretical analysis denotes that improving the initial Stokes intensity is favorable to achieve a continuously tunable, high-energy THz generation in a KTP crystal, which is clarified to originate from the enhancement of stimulated polariton scattering (SPS) process in the photon-like region of polariton and parametric amplification process in the phonon-like region of polariton. In the experiment, the THz continuously tunable range was from 2.93 to 6.21 THz, and the THz energies were 6.27, 2.62, 5.43, 4.59, and 5.88 μJ at the output peaks of 3.33, 3.83, 4.41, 4.85, and 5.69 THz, respectively. Compared with the traditional KTP THz parametric oscillator (KTP TPO) with discontinuous frequency tuning, the KTP ips-TPG has great improvement in terms of continuous frequency tuning range and THz output energy, where the enhancement ratios of THz output energy were 6.88× at 3.36 THz, 5.25× at 4.35 THz, 18.21× at 4.79 THz, and 1.82× at 5.69 THz. This injection pulse-seeded THz parametric source based on a KTP crystal is not only suitable to the practical applications in THz high frequency range, but also provides a technical solution for the KTP-family crystals to achieve a continuously tunable and high energy THz generation in a wide THz region.

Pressure-induced ferromagnetism in the topological semimetal EuCd2As2

The antiferromagnet and semimetal $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$ axis in $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$. In the search for such phases, we investigate here the effects of hydrostatic pressure in $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$. For that, we present specific heat, transport, and $\ensuremath{\mu}\mathrm{SR}$ measurements under hydrostatic pressure up to $\ensuremath{\sim}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, combined with ab initio density functional theory (DFT) calculations. Experimentally, we establish that the ground state of $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ changes from in-plane antiferromagnetic $({\mathrm{AFM}}_{ab})$ to ferromagnetic at a critical pressure of $\ensuremath{\approx}2$ GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane $({\mathrm{FM}}_{ab})$. The ${\mathrm{AFM}}_{ab}\text{\ensuremath{-}}{\mathrm{FM}}_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our theoretical results indicate that $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ moves closer to the sought-for ${\mathrm{FM}}_{c}$ state (moments $\ensuremath{\parallel}c$) with increasing pressure further. We predict that a pressure of $\ensuremath{\approx}23$ GPa will stabilize the ${\mathrm{FM}}_{c}$ state if Eu remains in a $2+$ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{As}}_{2}$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

Electro-superlubric springs for continuously tunable resonators and oscillators

Resonators and resonator-based oscillators are used in most electronics systems and they are classified as either mechanical or electrical, with fixed or difficult-to-tune resonant frequencies. Here, we propose an electro-superlubric spring, whose restoring force between two contacting sliding solid surfaces in the structural superlubric state is linearly dependent on the sliding displacement from the balanced position. We use theoretical analysis and finite element methods to study the restoring force and stability. The stiffness of this electro-superlubric spring is proportional to the square of the applied electric bias, facilitating continuous tuning from zero to several megahertz or gigahertz for the microscale or nanoscale resonators, respectively. Furthermore, we propose an electro-superlubric oscillator that is easily operated by varying a pair of harmonic voltages. The resonant frequency, resonant amplitude, quality factor, and maximum resonant speed can be continuously tuned via the applied voltage and bias. These results indicate significant potential in the applications of electro-superlubric resonators and oscillators.

SESAM Mode-Locked Yb:Ca3Gd2(BO3)4 Femtosecond Laser

We report on the first passively mode-locked femtosecond-laser operation of a disordered Yb:Ca3Gd2(BO3)4 crystal using a SEmiconductor Saturable Absorber Mirror (SESAM). Pumping with a single-transverse mode fiber-coupled laser diode at 976 nm, nearly Fourier-transform-limited pulses as short as 96 fs are generated at 1045 nm with an average output power of 205 mW and a pulse repetition rate of ~67.3 MHz. In the continuous-wave regime, high slope efficiency up to 59.2% and low laser thresholds down to 25 mW are obtained. Continuous wavelength tuning between 1006–1074 nm (a tuning range of 68 nm) is demonstrated. Yb:Ca3Gd2(BO3)4 crystals are promising for the development of ultrafast lasers at ~1 μm.

Tunable quantum-cascade VECSEL operating at 1.9 THz.

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) emitting around 1.9 THz with up to 10% continuous fractional frequency tuning of a single laser mode. The device shows lasing operation in pulsed mode up to 102 K in a high-quality beam, with the maximum output power of 37 mW and slope efficiency of 295 mW/A at 77 K. Challenges for up-scaling the operating wavelength in QC metasurface VECSELs are identified.

Continuous tuning of unidirectional emission wavelength by bending a notched-elliptical polymer microdisk

An approach of continuously tunable unidirectional emission through bending a notched-elliptical polymer microdisk is proposed. The characteristics of the bending-dependent action are carefully analyzed, and the resonance wavelength for unidirectional emission can be tuned continuously through bending the device. Such a whispering-gallery-mode microresonator enables unidirectional emission with ultra-low divergence, of which the emission efficiency and Q factor are stabilized, demonstrating the whole structure is robust and relatively insensitive within a certain bending angle range. A maximum resonance wavelength shift of ∼100 nm and Q factor of 1500 can be achieved with the total size of the microdisk less than 10 μm. This kind of microresonator is promising for applications in multilevel integrated photonics circuits and may open the door to new functionalities of resonator devices, from sensing to optical amplification.

Tunable injection-seeded fan-out-PPLN optical parametric oscillator for high-sensitivity gas detection

We demonstrate the possibility to enhance spectroscopic and lidar capabilities of mid-infrared pulsed optical parametric oscillators (OPOs) based on fan-out periodically-poled lithium niobate (fan-out PPLN) by combining their own wavelength tunability with tunable continuous-wave injection seeding. To that effect, we have developed an original test-bed singly-resonant OPO configuration in which continuous wavelength tuning via intracavity translation of the fan-out PPLN is assisted by simultaneous tuning of a narrow-linewidth fiber-coupled seed diode laser. It is shown that such an approach provides strong (at least by one order of magnitude) spectral narrowing, wavelength stabilization and beam-quality improvement for the signal and idler radiations (tunable around 1.54 μm and 3.33 μm, respectively). The achieved spectral narrowing does not noticeably affect the OPO output pulse energy, but allows fine wavelength tuning and significantly increases the power spectral density which can be available at an analyzed gas absorption line. Thus, it leads to a noticeable increase in sensitivity of the photoacoustic gas sensing carried out with the developed OPO. We have achieved an almost double (∼80%) increase in the photoacoustic response of a methane detector to the reference gas mixture with low concentration methane. The application of the method can be simply extended to many gas absorptions lines within the range 3.1–3.4 μm, due to the potentially wide tunability of the developed OPO. The demonstrated OPO features simplicity, reliability and energy efficiency. It can advance various LIDAR techniques due to the unique combination of spectral and energy characteristics.