Tunable Bandwidth Research Articles

Dual-band electromagnetically induced transparent metamaterial with slow light effect and energy storage

The realization of electromagnetically induced transparency (EIT) on metamaterials has special properties, such as strong slow-light, frequency-selection and so on, which have allowed EIT to be widely used in the fields of slow-light, optical storages and filters. In this paper, a metamaterial with two pairs of split ring resonators and one cut-wire is designed to achieve dual-band EIT effect at 0.5–2.14 GHz and 0.4–2.10 GHz with independently tunable bandwidths of 1.64 GHz and 2.7 GHz, respectively. The coupled Lorentz model is adopted to principally study the coupling characteristics between dark and bright modes. It is shown that the coupling strength between the dark and bright modes could be modulated by the coupling distance, which make the dual-band transparent window could be independently modulated by only changing the coupling distance between the bright and dark mode. The group delay and energy storage are also simulated by setting the Gaussian pulse signal passing through the EIT structure. The results show that the group delay of the designed EIT structure is 16.9 times that of the same thickness of dielectric material. The manufactured metamaterial is tested in a microwave anechoic chamber. The experimental and theoretical results are well consistent. These results could be beneficial for the development of EIT research toward some up-and-coming novel slow-light, optical storage, sensor and optical filter applications.

The Research on Backward Wave Oscillator with Wide Tunable Bandwidth and High Power

A continuous wave backward wave oscillator is studied in this paper. The design of the slowing wave system is completed through theoretical calculation and numerical simulation, and the slowing wave system is manufactured through precision machining. In order to improve the output power of the electron tube, a high resistance diode electron gun is used to produce a sheet electron beam with high emission current density. The test shows that the developed BWO can work in the frequency range of 250 ~ 310 GHz, and the output power is 20 mW to 42 mW.

A balanced dual-band BPF with quasi-independently tunable center frequency and bandwidth

Abstract By introducing an open stub loaded resonator (OSLR) and a uniform impedance resonator (UIR) into balanced stepped-impedance microstrip-slotline transition structures, a balanced dual-band bandpass filter (BPF) with high selectivity and low insertion loss performance is presented in this paper. The balanced microstrip-slotline transition structures can achieve a wideband common-mode (CM) suppression. Meanwhile, the differential-mode (DM) passbands are independent from the CM responses, which greatly simplifies the design procedure. In addition, four varactors are loaded into OSLR and UIR to achieve the electrical reconfiguration. The proposed balanced dual-band BPF can realize quasi-independently tunable center frequencies and bandwidths. In order to verify the feasibility of the proposed design method, a prototype circuit of the proposed reconfigurable balanced dual-band BPF is simulated, fabricated and measured. A good agreement between the simulation and measurement results is observed.

Integrated dual-laser photonic chip for high-purity carrier generation enabling ultrafast terahertz wireless communications

Photonic generation of Terahertz (THz) carriers displays high potential for THz communications with a large tunable range and high modulation bandwidth. While many photonics-based THz generations have recently been demonstrated with discrete bulky components, their practical applications are significantly hindered by the large footprint and high energy consumption. Herein, we present an injection-locked heterodyne source based on generic foundry-fabricated photonic integrated circuits (PIC) attached to a uni-traveling carrier photodiode generating high-purity THz carriers. The generated THz carrier is tunable within the range of 0–1.4 THz, determined by the wavelength spacing between the two monolithically integrated distributed feedback (DFB) lasers. This scheme generates and transmits a 131 Gbits−1 net rate signal over a 10.7-m distance with −24 dBm emitted power at 0.4 THz. This monolithic dual-DFB PIC-based THz generation approach is a significant step towards fully integrated, cost-effective, and energy-efficient THz transmitters.

Metamaterial microbolometers for multi-spectral infrared polarization imaging

Vanadium oxide (VOx) microbolometers enable the construction of high-performance yet low-cost and uncooled imaging detectors in the mid-infrared spectrum. Typical micro-bolometers are broadband sensors with no polarization selectivity. Thus, imaging detectors based on microbolometers have to use separate spectral and polarization filters to select the target spectral bands and polarization states, and the resulting systems are complicated and bulky. Here we demonstrate that by using metamaterial absorbers (MAs), which are arrays of optical resonators with sub-wavelength dimensions and spacing, we simultaneously tailor the VOx microbolometers' spectral and polarization responses, the need for separate spectral filters and polarizers can be mitigated. The MAs selectively absorb the TM polarization component of the incident light in a spectral band with tunable central wavelength and bandwidth while rejecting the TE polarization component. Two MAs with average TM absorption of 0.8322 in the 5.150 µm - 6.422 µm band and 0.7720 in the 5.867 µm - 7.467 µm band are fabricated, and the polarization extinction ratio (PER) are 42.24 and 42.65, respectively. The MAs are applied to VOx micro-bolometers, and the measured detector responses agree well with the absorption spectra of the MAs. The achieved peak responsivities of two fabricated detectors are 1.0 V/W at 6.0 µm and 1.46 V/W at 6.8 µm, respectively. And the two detectors achieve a D* of 6.94×105 cm·Hz1/2W-1 at 11Hz and 9.95×105 cm·Hz1/2W-1 at 36Hz, respectively. Our work paved the way towards large format room temperature multi-spectral infrared polarization imaging detector.

Constant Absolute Bandwidth Tunable Symmetric and Asymmetric Bandpass Responses Based on Reconfigurable Transmission Zeros and Bandwidth

A two-pole reconfigurable and tunable bandpass filter (BPF) with constant absolute bandwidth (CABW) and tunable transmission zeros (TZs) is presented in this paper. The BPF consists of asymmetrical spiral resonators having open ends on one side and capacitor loaded on the other side. The varactor loaded coupled-feed scheme is used at input and output. The two coupling feed-lines are also coupled to each other. It is shown that the control over TZs using the coupling-capacitor loaded between the two resonators leads to two different symmetric frequency responses – first with one TZ on each side of the passband, second with no TZ on either side of the passband. Further, asymmetrical frequency responses having single TZ either on lower or upper side of the passband are achieved by varying the capacitors loaded to the end of coupling-line. CABW while tuning as well as tunable bandwidth (BW) are achieved in all type of responses by using the coupling-capacitor. While tuning, quality factor is maintained by varying the capacitor loaded at the end of the coupled-feed. For the demonstration of the concept, a two-pole tunable BPF is designed and fabricated. The fabricated BPF having size of 13.1 mm × 9.4 mm, shows four different responses based on the position of TZs. All four CABW responses of the BPF have a tuning range of 300 MHz. Also, a tunable bandwidth is achieved in every response of the BPF.

High-power narrow-linewidth fiber lasers using optical spectrum broadening based on high-order phase modulation of inversion probability-tuning sequence.

Continuous fiber laser with ultra-high power and narrow linewidth is one of the key devices in the field of high-precision industrial processing, beam combining, and nonlinear frequency conversion. Under the premise of ensuring the signal quality, continuously increasing the output power is the focus of high-power narrow-linewidth fiber lasers. Driven by the white noise or pseudo-random binary sequence (PRBS), using cascaded phase modulations to broaden the spectrum of the seed source to suppress the stimulated Brillouin scattering (SBS) effect in the master oscillator power amplifier (MOPA) structure is an effective solution to increase the output power. However, this type of optical spectrum needs to be optimized, and the randomness of the driving signal causes a self-pulsing effect, which limits the further increase of the output power. In this paper, the influence of the frequency interval and randomness of the driving signal on the SBS effect in the laser system is analyzed. The modulated spectral type can be simply adjusted through changing the bit rate and inversion probability. Combining with high-order phase modulation, an approximate rectangular spectral broadening of the seed source with a tunable bandwidth up to 30 GHz is achieved. Compared with the cascaded white noise case, the output power of this scheme is increased by 600 W under the extended bandwidth of 27 GHz. It is fully verified that the seed source spectrum with high in-band flatness and low randomness can effectively suppress the SBS effect in the fiber laser and increase the output power.

Passive repetition-rate stabilization for a mode-locked fiber laser by electro-optic modulation.

We report a passive stabilization of the repetition rate for a mode-locked fiber laser by using an electro-optic modulator in a phase-biased nonlinear amplifying loop mirror. The underlying mechanism, in contrast to active feedback operations, lies in the cross-phase modulation between electrical and optical pulses within an electro-optic crystal. The resulting spectral shift can automatically compensate for the cavity-length drift via the group velocity dispersion. Consequently, the artificial actuator enables a capture range up to 2.3 mm, much longer than that achieved by index changes of the modulator. A robust and tight locking for the repetition rate is then realized with a standard deviation as low as 9 μHz with a 1-s sample time over 11 hours, corresponding to a fractional instability of 4.3 × 10-13. Furthermore, a dynamic optical sampling by repetition-rate tuning has been manifested with a fast refresh rate at 100 kHz and a broad scanning range over 305 ps. The demonstrated passive servo action may provide a simple yet effective way to stabilize the repetition rate with high precision, large bandwidth, and wide tunability.

Extremely Narrow and Actively Tunable Mie Surface Lattice Resonances in GeSbTe Metasurfaces: Study.

Mie surface lattice resonances (SLRs) supported by periodic all-dielectric nanoparticles emerge from the radiative coupling of localized Mie resonances in individual nanoparticles through Rayleigh anomaly diffraction. To date, it remains challenging to achieve narrow bandwidth and active tuning simultaneously. In this work, we report extremely narrow and actively tunable electric dipole SLRs (ED-SLRs) in Ge2Se2Te5 (GST) metasurfaces. Simulation results show that, under oblique incidence with TE polarization, ED-SLRs with extremely narrow linewidth down to 12 nm and high quality factor up to 409 can be excited in the mid-infrared regime. By varying the incidence angle, the ED-SLR can be tuned over an extremely large spectral region covering almost the entire mid-infrared regime. We further numerically show that, by changing the GST crystalline fraction, the ED-SLR can be actively tuned, leading to nonvolatile, reconfigurable, and narrowband filtering, all-optical multilevel modulation, or all-optical switching with high performance. We expect that this work will advance the engineering of Mie SLRs and will find intriguing applications in optical telecommunication, networks, and microsystems.

SnSe-Coated Microfiber Resonator for All-Optical Modulation.

In this study, a tin monoselenide (SnSe)-based all-optical modulator is firstly demonstrated with high tuning efficiency, broad bandwidth, and fast response time. The SnSe nanoplates are deposited in the microfiber knot resonator (MKR) on MgF2 substrate and change its transmission spectra by the external laser irradiation. The SnSe nanoplates and the microfiber are fabricated using the liquid-phase exfoliation method and the heat-flame taper-drawing method, respectively. Due to the strong absorption and enhanced light–matter interaction of the SnSe nanoplates, the largest transmitted power tunability is approximately 0.29 dB/mW with the response time of less than 2 ms. The broad tuning bandwidth is confirmed by four external pump lights ranging from ultraviolet to near-infrared. The proposed SnSe-coated microfiber resonator holds promising potential for wide application in the fields of all-optical tuning and fiber sensors.

Tunable Perfect Absorber of Graphene Metamaterial in the Terahertz Band and Its Sensing Properties

Herein, a dual‐band tunable ideal absorber based on graphene array, and the dynamic tuning performance of the perfect absorber in the terahertz band and its sensing properties based on graphene metamaterials are studied. The absorber shows two perfect absorption peaks at 1.01 and 1.86 THz, with an absorbance of 99.5% and 99.9%, respectively. With an elevation angle in the range of 0°–30° and a frequency in the range of 0.971–1.100 THz or 1.794–1.897 THz, the absorber designed herein can achieve an absorbance greater than 90%. Owing to the superior properties of graphene, dynamic tuning of the absorption frequency band, absorption bandwidth, and relative bandwidth of the absorber can be achieved by adjusting the Fermi level of graphene. In addition, adjusting the relaxation time realizes dynamic tuning of the absorption bandwidth and the absorbance with constant resonant frequency and fixed geometric parameters. Moreover, the sensitivity of the absorber reaches 492.5 GHz/RIU, which can achieve the sensing resolution required for the resonance frequency. This research provides a new concept for the development of high‐performance terahertz devices.

Design and fabrication of multiband reconfigurable copper and liquid multiple complementary split-ring resonator based patch antenna

The paper presents a multiband frequency reconfigurable microstrip patch antenna structure loaded by copper-based and water liquid-based superstrate resonators. The frequency tunability is achieved by three PIN diodes with the highest tunability of 320 MHz. The enhancement of gain is attained by the concept of superstrates in which the periodic structure implemented by copper-based and liquid-based split-ring resonators is considered for performance analysis. By loading the patch with the three copper-based and liquid-based split-ring resonators (SRR), the presented design achieves the maximum broadside directivities of 7.70 dB and 5.31 dB, and provides the maximum tunable frequency bandwidths of 250 MHz and 140 MHz, respectively. Out of all the configurations the copper-loaded three SRRs provide the peak gain of 9.11 dB. Comparison between simulated (by HFSS tool) and fabricated structures (using low profile material FR-4) is carried out in the frequency range from 6 MHz to 14 GHz for verification. The presented design opens a new way for achieving the different wireless communication applications such as satellite, aeronautical radio navigation, X-band synthetic aperture radar (SAR), etc.

Wide color gamut white light-emitting diodes based on two-dimensional semiconductor nanoplatelets.

II-VI colloidal semiconductor nanoplatelets (NPLs) are a kind of two-dimensional nanomaterial with uniform thickness at the atomic scale, thus leading to the characteristics of tunable emission wavelength and narrow bandwidth. Here, we report wide color gamut white light-emitting diodes (WLEDs) based on high-performance CdSe-based heterostructure NPLs. The narrow-band CdSe/CdS core/crown and CdSe/ZnCdS core/shell NPLs are chosen as green (∼521 nm) and red (∼653 nm) luminescent materials, respectively. They represent excellent PL properties, such as narrow linewidth, high quantum yields, and high photostability. Importantly, the further fabricated NPL-WLEDs exhibits an ultrawide color gamut covering up ∼141.7% of the NTSC standard in the CIE 1931 color space and excellent stability towards driving currents. These outstanding device performances indicate that the colloidal semiconductor NPLs possess huge potentiality to achieve higher color saturation and wide color gamut for applications in new-generation lightings and displays.

Tuneable Microwave Photonics Filter Based on Stimulated Brillouin Scattering With Enhanced Gain and Bandwidth Control

A Tuneable Microwave Photonics Gain Filter (TMWPF) is proposed based on Stimulated Brillouin Scattering (SBS). The TMWPF configuration exhibits a gain of 70dBm and tuneable bandwidth control which makes it attractive for applications requiring high gain and bandwidth variations with high precision. The proposed TMWPF configuration employs a tuneable laser, an Erbium-Doped Fibre Amplifiers (EDFA), Radio Frequency (RF) mixers, an RF Amplifier (RFA), Single Mode Fibre (SMF) and Gallium Arsenide (GaAs) I-Q Dual parallel Mach-Zehnder Modulator (DPMZM). The modulation scheme performed by I-Q DPMZM is controlled by a voltage bias controller. The performance of the filter is compared whilst using LiNbO3 Phase Modulators (PMs) and Intensity Modulators (IMs) instead of GaAs I-Q DPMZM. The TMWPF delivers 44dBm, 57dBm & 44dBm of gain for I-Q DPMZM, PM, and IM, respectively. The use of RFA resulted in a total gain of 62dBm, 69dBm & 52dBm for I-Q DPMZM, PM and Amplitude Modulators (AM), respectively. This configuration exhibits high Q, tuneable bandwidth by employing stokes SBS gain, RF mixer and RFA. The bandwidth of the filter is varied from 20MHz to 40MHz. The operation principle of the TMWPF is based on SBS stokes gain. By controlling the bias point of the I-Q DPMZM, the operating point can be varied between PM and IM allowing 6dBm gain control and the choice of PM or IM for specific applications. This gives improved filter performance. The full control of TMWFP's gain is obtained by using EDFA. The EDFA controls the power of the tuneable laser pump. The control challenges of the proposed TMWPF along with improvement enhancements are reported. The proposed TMWPF exhibits tuneability of 32GHz in RF domain, and 40GHz in optical domain. To the best of our knowledge, this TMWPF exhibits the highest reported gain.

An adaptive composite disturbance rejection for attitude control of the agricultural quadrotor UAV

Aiming at the problem that agricultural quadrotor UAV is easily disturbed in ultra-low altitude phenotype remote sensing and precision hovering of spraying, an adaptive composite anti-disturbance attitude controller is proposed for ground effect and propeller failure disturbances rejection. The adaptive composite disturbance rejection control (ACDRC) is composed of active disturbance rejection control (ADRC) and disturbance observer (DO) based on nominal inverse model, which is used to estimate wind disturbance, payload disturbance and propeller failure disturbance in real time. For the bandwidth tuning of the extended state observer (ESO), an online tuning method based on iterative learning control (ILC) is proposed to realize the adaptive extended state observer (ESO). And the stability of the composite anti-disturbance controller is analyzed. In the experiments, the wind disturbance experiments under the side-down flow and the horizontal flow, the failure experiments under the single propeller failure and twin propeller failure, and the composite disturbances experiments under the simultaneous action of the wind disturbance, propeller failure and payload disturbance are carried out. The experimental results show that under wind disturbance, the anti-disturbance performance of ACDRC is increased by 82.5%; under the disturbance of propeller fault, the anti-disturbance performance of ACDRC is increased by 60%; under the composite disturbance, the anti-disturbance performance of ACDRC is increased by 50%. Finally, the effectiveness of ACDRC is further verified in vegetable and cotton fields.

Bandwidth Tunable Optical Bandpass Filter Based on Parity-Time Symmetry.

A chip-scale tunable optical filter is indispensable to meeting the demand for reconfigurability in wavelength division multiplexing systems, channel routing, and switching, etc. Here, we propose a new scheme of bandwidth tunable band-pass filters based on a parity-time (PT) symmetric coupled microresonator system. Large bandwidth tunability is realized on the basis of the tuning of the relative resonant frequency between coupled rings and by making use of the concept of the exception point (EP) in the PT symmetric systems. Theoretical investigations show that the bandwidth tuning range depends on the intrinsic loss of the microresonators, as well as on the loss contrast between the two cavities. Our proof-of-concept device confirms the tunability and shows a bandwidth tuning range from 21 GHz to 49 GHz, with an extinction ratio larger than 15 dB. The discrepancy between theory and experiment is due to the non-optimized design of the coupling coefficients, as well as to fabrication errors. Our design based on PT symmetry shows a distinct route towards the realization of tunable band-pass filters, providing new ways to explore non-Hermitian light manipulation in conventional integrated devices.

Dual-Band Bandpass Filter With Controllable Stopband Bandwidth

A novel dual-band bandpass filter (BPF) is proposed with independently controllable transmission zeros (TZs) which can realize widely tunable stopband bandwidth (BW). The planar microstrip filter consists of a three-degree L-C ladder lowpass filter loaded with two unsymmetrical shorted stubs which are used to produce different TZs. By tuning the parameters of the two unsymmetrical shorted stubs, the TZs can be independently controlled. Therefore, the BPF has independent controllable center frequencies (CFs), passband bandwidths, and stopband bandwidths between adjacent passbands. All the L-C values in the equivalent circuit of the proposed filter are optimized to fulfill the design specifications. For demonstration, a dual-band BPF is designed. The measured results show good agreement with the simulated ones.

Plasmon induced transparency effect based on graphene nanoribbon waveguide side-coupled with rectangle cavities system

In order to reduce the size of the device and realize the ultrafast response time and dynamic tunableness, the single-band and dual-band plasmon induced transparency (PIT) effect are investigated based on graphene nanoribbon waveguide side-coupled rectangle cavity. The slow light properties of the model are analyzed numerically and theoretically by coupled mode theory and finite difference time domain method. With controlling the chemical potential of the graphene rectangle cavity, the tunability of the resonant wavelength and the transmission peak can be achieved simultaneously in single-band and dual-band PIT model. As the chemical potential of graphene increases, the resonant wavelength of each transmission window of PIT effect decreases gradually and presents the blue shift. In addition, through dynamically tuning the resonant wavelength of the graphene rectangle cavity, when the chemical potential of the graphene rectangle cavity increases from 0.41 to 0.44 eV, the group index of single PIT system is controlled to be between 79.2 and 28.3, and the tunable bandwidth is 477 nm. Moreover, the group index of dual PIT system is controlled to be between 143.2 and 108.6 when the chemical potentials of graphene rectangle cavities 1, 2, and 3 are 0.39–0.42 eV, 0.40–0.43 eV, and 0.41–0.44 eV, respectively. The size of the entire PIT structure is <0.5 μm<sup>2</sup>. The research results here in this work are of reference significance in designing and fabricating the optical sensors, optical filters, slow light and light storage devices with ultrafast, ultracompact and dynamic tunableness.

Plasmonic random laser enabled artefact-free wide-field fluorescence bioimaging: uncovering finer cellular features.

Narrow bandwidth, high brightness, and spectral tunability are the unique properties of lasers that make them extremely desirable for fluorescence imaging applications. However, due to the high spatial coherence, conventional lasers are often incompatible for wide-field fluorescence imaging. The presence of parasitic artefacts under coherent illumination causes uneven excitation of fluorophores, which has a critical impact on the reliability, resolution, and efficiency of fluorescence imaging. Here, we demonstrate artefact-free wide-field fluorescence imaging with a bright and low threshold silver nanorod based plasmonic random laser, offering the capability to image finer cellular features with sub-micrometer resolution even in highly diffusive biological samples. A spatial resolution of 454 nm and up to 23% enhancement in the image contrast in comparison to conventional laser illumination are attained. Based on the results presented in this paper, random lasers, with their laser-like properties and spatial incoherence are envisioned to be the next-generation sources for developing highly efficient wide-field fluorescence imaging systems having high spatial and temporal resolution for real-time, in vivo bioimaging.

Ultra-Narrow Bandwidth and Large Tuning Range Single-Passband Microwave Photonic Filter Based on Brillouin Fiber Laser

Ultra-Narrow Bandwidth and Large Tuning Range Single-Passband Microwave Photonic Filter Based on Brillouin Fiber Laser