Continuous Tuning Research Articles

All-Optical Tunability of Metalenses Permeated with Liquid Crystals.

Metasurfaces have been extensively engineered to produce a wide range of optical phenomena, allowing exceptional control over the propagation of light. However, they are generally designed as single-purpose devices without a modifiable postfabrication optical response, which can be a limitation to real-world applications. In this work, we report a nanostructured planar-fused silica metalens permeated with a nematic liquid crystal (NLC) and gold nanoparticle solution. The physical properties of embedded NLCs can be manipulated with the application of external stimuli, enabling reconfigurable optical metasurfaces. We report the all-optical, dynamic control of the metalens optical response resulting from thermoplasmonic-induced changes of the NLC solution associated with the nematic-isotropic phase transition. A continuous and reversible tuning of the metalens focal length is experimentally demonstrated, with a variation of 80 μm (0.16% of the 5 cm nominal focal length) along the optical axis. This is achieved without direct mechanical or electrical manipulation of the device. The reconfigurable properties are compared with corroborating numerical simulations of the focal length shift and exhibit close correspondence.

Red-diode-clad-pumped Er3+/Dy3+ codoped ZrF4 fiber: A promising mid-infrared laser platform.

We report, for the first time, to the best of our knowledge, mid-infrared (mid-IR) laser generation, from a red-diode-clad-pumped Er3+/Dy3+ codoped ZrF4 fiber laser. A free-running laser at ∼3.4 µm, mainly from the 4F9/2→4I9/2 transition of Er3+, directly excited by a 659-nm laser diodehas been achieved at room temperature with a maximum power of 0.8 W and 8.8% slope efficiency. In this system, the long-lived 4I11/2 and 4I13/2 states are rapidly depopulated by energy transfer to the codoped Dy3+ ions and energy transfer upconversion between the Er3+ ions, resulting in the accelerated recycling of ions. Additionally, the free-running dual-wavelength operation state at ∼3.3 and ∼3.5 µm is also observed, producing a total maximum power of 0.95 W with 10.7% slope efficiency, representing the first watt-class output from a diode-pumped rare-earth-doped fiber laser far beyond 3 µm. By employing a diffraction grating, continuous spectral tuning across the 642-nm range from 3053.9 to 3695.9 nm has been demonstrated. The proposed scheme provides, to the best of our knowledge, a promising new platform for laser generation in the mid-IR region of 3-4 µm.

Recent progress in continuously tunable low-noise all-solid-state single-frequency continuous-wave laser based on intracavity locked etalon

All-solid-state single-frequency continuous-wave (CW) lasers have been applied in many fields of scientific research owing to their intrinsic advantages of high beam quality, low noise, narrow linewidth, and high coherence. In atom-based applications, single-frequency lasers should also be continuously tuned to precisely match their wavelengths with the transition lines of the corresponding atoms. Continuous frequency tuning of the laser is mainly achieved by continuously scanning the laser cavity length after the intracavity tuning element etalon is locked to an oscillating laser mode. However, the modulation signals necessary in current etalon locking systems increase the noise of the continuously tunable lasers and in some respects limit their applications in Frontier scientific research. Moreover, the obtained continuous frequency tuning range with the etalon locking technique is restricted by the free spectrum range of the adopted etalon. In this paper, we systematically summarize recent progress of the continuously tunable single-frequency CW lasers based on intracavity locked etalon, including the advanced etalon locking techniques and the tuning range expansion approach. As a result, the low noise and high stable all-solid-state single-frequency CW tunable lasers are successfully developed.

Large Thermal Conductivity Switching in Ferroelectrics by Electric Field-Triggered Crystal Symmetry Engineering

A convenient, reversible, fast, and wide-range switching of thermal conductivity is desired for efficient heat energy management. However, traditional methods, such as temperature-induced phase transition and chemical doping, have many limitations, e.g., the lack of continuous tunability over a wide temperature range and low switching speed. In this work, a strategy of electric field-driven crystal symmetry engineering to efficiently modulate thermal conductivity is reported with first-principles calculations. By simply changing the direction of an external electric field loaded in ferroelectric PbZr0.5Ti0.5O3, near the morphotropic phase boundary composition, we obtain the largest switching of thermal conductivity for ferroelectric materials at room temperature based on the dual-phonon theory, i.e., normal and diffuson-like phonons, with three different criteria. The calculation results indicate that with decreasing crystal symmetry, the degeneracy of phonon modes reduces and the avoid-crossing behavior of phonon branches enhances, leading to the increase of diffuson-like phonons and weighted phonon-phonon scattering phase space. A thermal switch prototype based on PbZr0.5Ti0.5O3 is further shown that can protect the Li-ion battery by modulating its temperature up to 17.5 °C. Our studies would pave the way for designing next-generation thermal switch with high speed, a wide temperature range, and a large switching ratio.

Design and simulation of continuous tuning band stop filter by2-D periodic defected ground structure

In this paper, the design and simulation of a continuous tuning bandstopfilter (BSF) is done by using two-dimensional (2-D) periodic DGS for the coplanar waveguide (CPW) transmission line. The proposed BSF filter structure has DGS cells on the ground plane of the CPW transmission line. This structure can easily be designed for another frequency band by changing the number of periodic DGS unit cells. This BSF has a fixed bandwidth of 8.2 GHz over a wide range of frequencies (16–26) GHz.The tunability of the BSF is achieved by moving plates, which are kept above the substrate of the CPW. The size of these plates is the same as the size of the DGS cell. The tunability of the BSF is observed while changing the distance between the metal plates and substrate in the up position. In the down position the metal plates are covered by the DGS cells, then the filter act as a normal transmission line. The extraction of the equivalent circuit model parameters has also been derived. The electromagnetic (EM) and equivalent model simulations are done with the HFSS and ADS tools, both these simulations have a good agreement with each other. From this, the proposed BSF can be used for satellite and radar communication in K band frequency range.

Wavelength-Tuning of Nanosecond Pulses in Er-Doped Fluoride Fibre Amplifier

We investigate the gain bandwidth and wavelength-tuning range of an erbium-doped fluoride fibre amplifier. The presented experimental setup consisted of a widely wavelength-tunable optical parametric oscillator (OPO), which was amplified in a single-stage Er-doped fluoride fibre amplifier. The OPO laser provided seed pulses with a pulse width of 5.2 ns and a repetition rate of 10 kHz. The fibre section consisted of 2.2 m of double-clad, single-mode fibre with a doping concentration of 7 mol%. Wavelength-tuning was analysed at gain values of up to 26 dB and amplified pulse energies of up to 37.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \mathrm{J}$</tex-math></inline-formula> . Using this setup, we demonstrate continuous wavelength tuning of more than 100 nm, covering the wavelength range from 2712 nm to 2818 nm.

Wideband and Tunable Reflective Cross-Polarization Conversion Metasurface for TeraHertz Applications

An ultrathin and wideband reflective cross-polarization converter is presented in this paper. The design consists of a pair of modified dumbbell shaped resonators which are connected together with a conductive strip. The proposed single-layer THz metasurface achieved a high polarization conversion ratio (PCR) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\ge$</tex-math></inline-formula> 95% in the operating frequency ranging from 1.76–4.26 THz. This wide-bandwidth cross-polarization conversion is obtained by exciting multiple closely spaced resonant frequencies at 1.98 THz, 2.61 THz, 2.95 THz, and 3.93 THz. The working mechanism of the proposed structure is analyzed and the polarizer exhibits an excellent angular stability upto 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{0}$</tex-math></inline-formula> of incidence. Further, the proposed cross-polarizer exhibits continuous tunability of PCR over the operating frequencies when photoconductive silicon (Si) is filled in the gap between pair of dumbbell-resonators. The wideband characteristics and the switchable property show that the proposed reflective cross polarizer is an excellent candidate for manipulation and control of polarization in THz communication systems.

Tunable continuous-wave laser operation of Tm3+ ion doped tellurite glass near 2 μm

We provide an experimental investigation of the absorption saturation and 1.9-μm lasing characteristics of a bulk Tm3+:TZLN glass with the host composition of 74TeO2–12ZnO–4La2O3–10Na2O. By using a z-scan saturation measurement, the absorption cross section at 794 nm was determined to be 5×10−21cm2, in reasonable agreement with the value of 6×10−21cm2 obtained from small-signal transmission measurement. In lasing experiments, double pumping was employed at 794 nm and the bulk Tm3+:TZLN glass laser operated at the free running wavelength of 1926 nm. As high as 15.4 mW of continuous-wave output power was obtained with a 2.15% transmitting output coupler at the absorbed pump power of 284 mW. The slope efficiency with respect to the absorbed pump power was 7.1%. By using the threshold and power efficiency data, the stimulated emission cross section was further determined to be 6.8×10−21cm2at 1926 nm. Finally, continuous tuning of the laser output was demonstrated between 1845 and 1973 nm by using a quartz birefringent plate.

Superconducting Sub-Terahertz Oscillator with Continuous Frequency Tuning

The development and approbation of a superconducting local oscillator based on a long Josephson junction made it possible to create a fully superconducting integrated receiver in sub-terahertz frequency range, which was successfully tested both on board a high-altitude balloon and in the laboratory. In order to expand the frequency range of a superconducting integrated local oscillator, it is necessary to ensure the continuous tuning of its frequency at an arbitrary bias current, including a so-called resonant mode regime. The resonant mode regime takes place for high-quality tunnel junctions with low leakage; in this regime, stable generation is possible only at Fiske steps, the distance in frequency between which is tens of GHz. A method for suppressing resonances has been proposed and implemented; this method is based on the introduction of normal metal layers into the region near the long Josephson junction. Modeling of the propagation of electromagnetic waves in the proposed integrated structure was carried out; experimental samples were fabricated, and their comprehensive study was performed. The complete suppression of resonances and the possibility of the continuous tuning of the frequency of a superconducting local oscillator in the range of 200–700 GHz have been demonstrated. The linewidth of the FFO radiation does not exceed 15 MHz over the entire frequency range, which makes it possible to implement the phase locked loop mode in an integrated receiver intended for spectral studies.

Microwave Vortex Transceiver System with Continuous Tunability Using Identical Plasmonic Resonators

AbstractElectromagnetic vortices have attracted vast interest for their unique physics and promising applications. Tremendous efforts have been devoted to vortex generations, but receiving vortex modes remains challenging and commonly requires spatial phase gradient measurements. Here, a compact microwave vortex transceiver system based on reversible superposition and decomposition of degenerate and orthogonal dipole modes is reported. Identical plasmonic resonators are designed as both transmitting and receiving antennas. The received vortex modes are conveniently characterized by amplitude and phase signals measured at the receiving resonator ports. The transmitted orbital angular momentum (OAM) can be continuously tunable while the topological charge maintains discrete values of ±1. A transceiver system for degradation and reconstruction of OAMs is experimentally constructed, which can dynamically tune the vortex modes via simple voltage modulations. This research provides a convenient solution to transmit and receive microwave vortices, and paves novel routes for sensing and wireless communications based on continuously tunable vortex modes.

Phase Control of Terahertz Waves Using Injection-Locked Resonant Tunneling Diode Oscillator

Controlling the phase of terahertz (THz) waves is necessary for various applications, particularly beam steering with phased arrays. Resonant tunneling diodes (RTDs) are promising candidates for compact THz sources, and injection-locking phenomenon can be used to control the phase of RTD oscillators. In this study, we experimentally demonstrate the phase control of an injection-locked RTD oscillator. We fabricated an RTD oscillator with a slot antenna and developed a device package that reduces external THz feedback to obtain continuous frequency tuning. The free-running frequency varied from 380.5 to 386.7 GHz over a bias range of 0.61 to 0.65 V. To measure the phase of RTD oscillator, we constructed a heterodyne detection system in which all signals were well synchronized to achieve good phase measurement accuracy. The frequency of the RTD oscillator was locked to an injection signal of 383.2 GHz from an external source, and the phase was controlled by the bias voltage under the injection locking. The measured phase changed from −81° to +73° within the locking range. The bias-voltage dependence of the phase change reasonably agreed with the theoretical expectation.

Phase-Matching in Nonlinear Crystal-Based Monochromatic Terahertz-Wave Generation

Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources and nonlinear crystals and determines the direction of energy flow (as well as the output power). In past decades, a variety of peculiar PM configurations in the THz region have been invented and are different from the traditional ones in the optical region. We summarize the configurations that have been applied in nonlinear THz-wave generation, which mainly fall in two categories: scalar (collinear) PM and vector PM (including macroscopic noncollinear PM and microscopic vector PM). The development of this technique could relax the matching conditions in a wide range of nonlinear crystals and pump wavelengths and could finally promote the improvement of coherent THz sources.

Reconfigurable UWB Circularly Polarized Slot Antenna With Three Modes of Operation and Continuous Tuning Rang

In this communication, a tunable circularly polarized (CP) antenna with a reconfigurable integrated feed network is proposed for cognitive radio (CR) applications. The integrated feed network is composed of an ultrawideband (UWB) bandpass filter (BPF), a continuously tunable T-shaped resonator, and an L-shaped microstrip line. The feed network employs three p-i-n diodes to switch between different RF paths, resulting in three modes of operation: an UWB mode (3.5–8.5 GHz), a narrowband bandpass (BP) mode and a narrowband bandstop (BS) mode. The continuous tuning range of the BP/BS narrowband mode is realized by two dc-controlled varactor diodes. The stand-alone antenna attains a −10 dB impedance bandwidth of 92.3% (3.5–9.5 GHz), while it attains an axial ratio (AR) bandwidth of 88.3% (3.1–8 GHz) in its wideband mode. The overlapped bandwidth (matching and AR) of the reconfigurable antenna is 78.2% (3.5–8 GHz). The communicating state covers 80% of the sensing spectrum while maintaining CP radiation. The measured peak realized gain values for the wideband mode range from 2 to 3.2 dBic while the maximum gain for the narrowband mode ranged from 1.95 to 3.1 dBic. The measured results show that the proposed filtenna is suitable for UWB CR systems.

Coherent terahertz radiation with 2.8-octave tunability through chip-scale photomixed microresonator optical parametric oscillation

High-spectral-purity frequency-agile room-temperature sources in the terahertz spectrum are foundational elements for imaging, sensing, metrology, and communications. Here we present a chip-scale optical parametric oscillator based on an integrated nonlinear microresonator that provides broadly tunable single-frequency and multi-frequency oscillators in the terahertz regime. Through optical-to-terahertz down-conversion using a plasmonic nanoantenna array, coherent terahertz radiation spanning 2.8-octaves is achieved from 330 GHz to 2.3 THz, with ≈20 GHz cavity-mode-limited frequency tuning step and ≈10 MHz intracavity-mode continuous frequency tuning range at each step. By controlling the microresonator intracavity power and pump-resonance detuning, tunable multi-frequency terahertz oscillators are also realized. Furthermore, by stabilizing the microresonator pump power and wavelength, sub-100 Hz linewidth of the terahertz radiation with 10−15 residual frequency instability is demonstrated. The room-temperature generation of both single-frequency, frequency-agile terahertz radiation and multi-frequency terahertz oscillators in the chip-scale platform offers unique capabilities in metrology, sensing, imaging and communications.

Active modulation of a metasurface emitter based on phase-change material GST arrays

Tunable thermal emitters have attracted much attention due to their applications in communications, sensing, and control system. To date, the emission performance of many reported thermal emitters require continuous external energy for the purpose of continuous modulation. Here, a Ge2Sb2Te5 (GST) array-based metasurface thermal emitter is proposed and measured. The maximum emissivity (crystalline) is 95.3%, while the minimum emission (amorphous) amplitude is 15.1%. Based on the phase transition states (amorphous and crystalline) of the GST array, the emission amplitude and resonance wavelength of this metasurface thermal emitter are clearly switchable. Moreover, the thickness of the GST arrays is set to be 1.5μm, 2.0μm, and 2.5μm, which results in the emissivity peak shifting to longer resonant wavelengths. The thermal emitter reveals continuous tunability, broad wavelength selectivity, and switchability.

Topological waves guided by a glide-reflection symmetric crystal interface

A domain wall separating two different topological phases of the same crystal can support the propagation of backscattering-immune guided waves. In valley-Hall and quantum-Hall crystal waveguides, this property stems from symmetry protection and results from a topological transition at a Dirac point. Since an initially closed band gap has to open, the guidance bandwidth remains limited compared to that of wide band gap crystals. When a glide-symmetric dislocation is introduced in a 2D crystal, we show that a pair of wide-bandwidth, single-mode, and symmetry-protected guided waves appear in the bulk band gap. The 2D Zak phase changes by $\pi$ on either side of the interface, providing a topological invariant protected by glide-reflection symmetry at the X point of the Brillouin zone. A demonstration experiment is performed with acoustic waves in water, at ultrasonic frequencies, and shows the continuous tuning of transmission as a function of the glide parameter. The concept further extends to other types of waves, including the case of elastic waves in solids, but also of optical and electromagnetic waves.

Physical Activity Dynamics During a Digital Messaging Intervention Changed After the Pandemic Declaration.

BackgroundThe COVID-19 pandemic adversely impacted physical activity, but little is known about how contextual changes following the pandemic declaration impacted either the dynamics of people’s physical activity or their responses to micro-interventions for promoting physical activity.PurposeThis paper explored the effect of the COVID-19 pandemic on the dynamics of physical activity responses to digital message interventions.MethodsInsufficiently-active young adults (18–29 years; N = 22) were recruited from November 2019 to January 2020 and wore a Fitbit smartwatch for 6 months. They received 0–6 messages/day via smartphone app notifications, timed and selected at random from three content libraries (Move More, Sit Less, and Inspirational Quotes). System identification techniques from control systems engineering were used to identify person-specific dynamical models of physical activity in response to messages before and after the pandemic declaration on March 13, 2020.ResultsDaily step counts decreased significantly following the pandemic declaration on weekdays (Cohen’s d = ‐1.40) but not on weekends (d = ‐0.26). The mean overall speed of the response describing physical activity (dominant pole magnitude) did not change significantly on either weekdays (d = ‐0.18) or weekends (d = ‐0.21). In contrast, there was limited rank-order consistency in specific features of intervention responses from before to after the pandemic declaration.ConclusionsGeneralizing models of behavioral dynamics across dramatically different environmental contexts (and participants) may lead to flawed decision rules for just-in-time physical activity interventions. Periodic model-based adaptations to person-specific decision rules (i.e., continuous tuning interventions) for digital messages are recommended when contexts change.

GaSb surface grating distributed feedback interband cascade laser emitting at 3.25 µm.

A second-order distributed feedback interband cascade laser emitting at 3.25 µm was designed, grown, and fabricated. By coherent epitaxy of a GaSb cap layer instead of the conventional thin InAs cap on top of the laser structure, a high-quality surface grating was made of GaSb and gold. Enough coupling strength and a significant inter-modal loss difference were predicted according to the simulation within the framework of couple-wave theory. Lasers having 2-mm-long cavities and 4.5-µm-wide ridges with high-/anti-reflection coatings were fabricated. The continuous-wave threshold current and maximum single-mode output power were 60 mA and 24 mW at 20°C, respectively. The output power of 5 mW was still kept at 55°C. Continuous tuning free from mode hopping and high single-mode suppression ratios (>20 dB) were realized at all injection currents and heat-sink temperatures, covering a spectral range of over 20 cm-1.

Continuously wavelength‐tunable passband‐flattened fibre multiwavelength filter based on heterogeneous combination of wave retarders

By incorporating the heterogeneous combination of wave retarders, the authors demonstrated a continuously wavelength-tunable passband-flattened fibre multiwavelength filter comprised of a four-port polarization beam splitter (PBS), two equal-length polarization-maintaining fibre (PMF) segments, one wave retarder combination (WRC) of dual quarter-wave retarders (QWRs), and another WRC of a half-wave retarder (HWR) and a QWR. The former WRC lies before the first PMF segment, and the latter one is located between the two PMF segments. From the theoretically derived filter transmittance, the authors found the orientation angle (OA) sets of the four wave retarders where the additional phase ϕ of the transmittance function could be continuously modified from 0° to 360°. For eight OA sets allowing ϕ to increase from 0° to 315° by 45°, the authors calculated the passband-flattened transmission spectrum of the filter and confirmed its redshift by 0.1 nm per set. Then, this theoretical prediction was verified by the experimental implementation of the filter, which clearly indicates the continuous wavelength tuning capability of the proposed filter.

A Multimode Extended Interaction Oscillator With Broad Continuous Electric Tuning Range

A novel extended interaction oscillator (EIO) capable of continuous multimode operation is proposed to make the leap from the single-mode operation and continuous dual-mode operation to the continuous multimode operation and overcome the continuous electric tuning range limitation of the conventional EIO. The “cold cavity” simulation and theory demonstrate the possibility of a stable continuous multimode operation. A −3 dB continuous electric tuning range of 4.15 GHz (4.3% relative bandwidth) with a center frequency of 95.74 GHz is predicted by the “hot cavity” simulation. Meanwhile, a comparative study on the uniform-gap and stagger-gap circuit explains the working mechanism of the conventional stagger-gap circuit with a single-mode ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> mode) operation.