Frequency Modulation Depth Research Articles

3D Dirac Semimetal Supported Tunable Multi‐Frequency Terahertz Metamaterial Absorbers

AbstractIn recent years, 3D Dirac semimetals (DSM) with linear energy‐momentum dispersion near the Fermi points have emerged as promising material candidates for novel tunable metamaterial devices due to their prominent electromagnetic performance and excellent tunability. In this work, the propagation characteristics of 3D DSM‐supported double and triple stripe patterned metamaterial absorbers (MMAs) are investigated in the terahertz (THz) regime by facile modulation of the Fermi level. The results manifest that for the double stripe patterned devices, two strong absorption peaks with near unity are observed at 0.97 THz and 1.63 THz, respectively. When the Fermi level varies in the range of 0.01–0.15 eV, the amplitude (frequency) modulation depth of resonance is up to 77% (27.3%). Additionally, triple stripe MMAs supported by 3D DSM are devised, which can achieve resonant absorption peaks at three specific frequencies of 0.23, 0.345, and 0.46 THz, while the amplitudes of the peaks are as high as 0.997, 0.940, and 0.779, respectively. Furthermore, the resonances can also be regulated, and the amplitude MD reaches ≈20%. These results are very helpful in understanding the tuning mechanisms of metamaterial devices and aiding the design of THz functional devices, such as receivers, detectors, and modulators.

Soliton wave parameter estimation with the help of artificial neural network by using the experimental data carried out on the nonlinear transmission line

In this study, an artificial neural network (ANN) model is generated, which is used to estimate the output parameters of soliton waves produced as a result of nonlinear transmission lines (NLTLs). Three different output parameters are acquired as a consequence of the experiments carried out utilizing the five various input parameters that are set in the ANN-based study. Input parameters for NLTL designs with 116 different experiments; inductor (L), input voltage (Vi) value, number of nodes (n), capacitance (C(V)) and load resistance (RLoad) values. Output parameters values, which are maximum voltage (Vmax), center frequency (fcenter), and voltage modulation depth (VMD). Input and output data; 70 % is set aside for training, 15 % for validation and the remaining 15 % for testing. Training, validation, and testing steps are repeated for the output parameters, in which case more than 99 % correlation is found as a result of each operation. An absolute percentage error value is found for each output parameter. Moreover, Mean absolute percentage error (MAPE) is calculated for these output datasets. The data set is tested for the experimental studies carried out in the literature, and it is observed that there is a compliance of over 99 % for this situation.

Tunable dual‐spectral plasmon‐induced transparency in terahertz Dirac semimetal metamaterials

AbstractWe have numerically studied a tunable dual‐spectral plasmon‐induced transparency metamaterial structure based on Dirac semimetal films in the terahertz region. The structure was composed of three length‐variant parallel coplanar strips. The dual‐spectral plasmon‐induced transparency is mainly induced by the bright‐bright modes coupling between neighboring Dirac semimetal strips. By adjusting the Fermi energies of Dirac semimetal strips, the individual and synchronous modulation of the dual‐transparency window in resonance frequency, bandwidth and strength can be obtained, respectively. Simultaneous change of the Fermi energies of all the strips can achieve an overall frequency shift of the dual‐transparency window, with the frequency modulation depth of 21.7% and 20.0%, respectively. The maximum group delays of 5.26 and 3.57 ps for each window were calculated in our proposed structure. The simulation results demonstrated the potential of the proposed structure to expand the applications for slow‐light systems, filters, and switchers in the terahertz region.

La réalité virtuelle, un outil pertinent pour la sensibilisation au métier de masseur-kinésithérapeute

The motor cortex plays a critical role in accurate visually guided movements such as reaching and target stepping. However, the manner in which vision influences the movement-related activity of neurons in the motor cortex is not well understood. In this study we have investigated how the locomotion-related activity of neurons in the motor cortex is modified when subjects switch between walking in the darkness and in light.Three adult cats were trained to walk through corridors of an experimental chamber for a food reward. On randomly selected trials, lights were extinguished for approximately 4 s when the cat was in a straight portion of the chamber's corridor. Discharges of 146 neurons from layer V of the motor cortex, including 51 pyramidal tract cells (PTNs), were recorded and compared between light and dark conditions. It was found that while cats’ movements during locomotion in light and darkness were similar (as judged from the analysis of three-dimensional limb kinematics and the activity of limb muscles), the firing behavior of 49% (71/146) of neurons was different between the two walking conditions. This included differences in the mean discharge rate (19%, 28/146 of neurons), depth of stride-related frequency modulation (24%, 32/131), duration of the period of elevated firing ([PEF], 19%, 25/131), and number of PEFs among stride-related neurons (26%, 34/131). 20% of responding neurons exhibited more than one type of change.We conclude that visual input plays a very significant role in determining neuronal activity in the motor cortex during locomotion by altering one, or occasionally multiple, parameters of locomotion-related discharges of its neurons.

3D Dirac semimetals supported tunable terahertz BIC metamaterials

Abstract Based on the 3D Dirac semimetals (DSM) supported tilted double elliptical resonators, the tunable propagation properties of quasi-bound in continuum (BIC) resonance have been investigated in the THz regime, including the effects of rotation angles, DSM Fermi level, and the configuration of resonators. The results manifest that by altering the rotation angle of elliptical resonator, an obvious sharp BIC transmission dip is observed with the Q-factor of more than 60. The DSM Fermi level affects the BIC resonance significantly, a sharp resonant dip is observed if Fermi level is larger than 0.05 eV, resulting from the contributions of reflection and absorption. If Fermi level changes in the range of 0.01–0.15 eV, the amplitude and frequency modulation depths are 92.75 and 44.99%, respectively. Additionally, with the modified configurations of elliptical resonators, e.g. inserting a dielectric hole into the elliptical resonator, another transmission dip resonance is excited and indicates a red shift with the increase of the permittivity of the dielectric filling material. The results are very helpful to understand the mechanisms of DSM plasmonic structures and develop novel tunable THz devices, such as modulators, filters, and sensors in the future.

Chaotic dynamics of an atomic Bose–Einstein condensate in a frequency-modulated cavity QED

Nonlinear systems, including atom–field interaction, are investigated due to their fundamental applications in quantum mechanics and rapidly growing fields of quantum communication, especially secure communication with chaotic dynamics. In this paper, we study the chaotic dynamics of a system consisting of an atomic Bose–Einstein condensate interacting with a quantized radiation field in a high-quality cavity with a periodically modulated length. The frequency modulation is adapted by a periodic time-dependent atom–field coupling strength. We use a semiclassical approach to decouple the atoms and field variables and then numerically solve the corresponding nonlinear dynamical equations of the system. Generally, the dynamics of the system sensitively depends on its initial conditions, thereby long-term prediction is impossible. We show that the system demonstrates the emergence of classical dynamical chaos from quantum electrodynamics. The chaotic behavior of energy transfer in the system can be enhanced by increasing the depth of frequency modulation. The strange attractor clearly illustrates that the system presents extremely exotic dynamics over a wide range of parameters. This implies that the dynamical quantities oscillate irregularly, never exactly repeating but always remaining in a bounded region of the phase space.

Investigation of terahertz high Q-factor of all-dielectric metamaterials

We investigated the propagation properties of all-dielectric metamaterials (ADMs) based on a SiO2-Si asymmetric hybrid block, including the effects of structural parameters, asymmetrical degrees, carrier doping concentrations, and graphene Fermi levels. The Q-factor of Fano resonance reaches more than 270, and the amplitude modulation depth (MD) is about 75% if the asymmetric degree changes in the range of 2–10 μm. The carrier concentration of silicon significantly affects the intensity of excited Fano resonance. When the carrier concentration of Si is 1 × 1014 cm−3, the excited Fano resonance is the strongest, and the transmission peak is about 0.92. With the help of a uniform graphene layer, the Fano resonance can be effectively modulated, the frequency MD is about 20% if the Fermi level changes in the scope of 0.01–0.3 eV. These results can help us to design THz high Q-factor devices, such as sensors, filters, and modulators.

Ultrasound from underground: cryptic communication in subterranean wild-living and captive northern mole voles (Ellobius talpinus)

ABSTRACT This study provides the first evidence of ultrasonic vocalisations (USVs) in a truly subterranean rodent, the northern mole vole Ellobius talpinus. Calls were recorded by attracting callers with a bait to burrow entrances, where they were mostly visible to researchers. USVs recorded from 14 different burrows in southern Russia were verified as belonging to Ellobius talpinus by comparison with USVs of two wild-captured young males and by comparison with USVs of four adults from a captive colony. As a first attempt at exploring the function of USV diversity, we defined upward-intense USVs, with a maximum fundamental frequency (f0) of 35.32 ± 5.11 kHz, and variable-faint USVs, with a maximum f0 of 31.40 ± 7.78 kHz. Compared to variable-faint USVs, the upward-intense USVs were longer, had a larger depth of frequency modulation and were produced at high intensity in regular series. The upward-intense USVs were lower in the maximum and peak frequencies in the wild than in captivity, whereas the variable-faint USVs did not differ between recordings from the wild and from captivity. We discuss that similar ranges of acoustic variables found in USVs of Ellobius talpinus and surface-dwelling Arvicolinae species do not support the hypothesis that subterranean life has drastically reduced ultrasonic vocalisation in rodents.

Graphene-supported tunable bidirectional terahertz metamaterials absorbers

Based on asymmetric graphene ellipses, the tunable propagation characteristics of metamaterial absorber (MMA) have been investigated in the THz region. Two distinct absorption peaks of 84% and 90% are observed at 1.06 THz and 1.67 THz, respectively. Besides a high Q factor exceeding 20, the Fano resonance can also be modulated in a wide range (e.g., the frequency modulation depth reaches more than 43.8% if the Fermi energy level changes in the range of 0.2-1.0 eV). Additionally, a bidirectional THz MMA is achieved by replacing the metal substrate with a uniform graphene layer. If the terahertz wave is incident in the forward direction, the proposed graphene double stripe microstructure shows a typical MMA with its absorption reaching 88%. On the other hand, if the terahertz wave is incident in the reverse direction, the graphene double stripe microstructure behaves as a reflective modulator, and its amplitude and frequency MD will reach 60% and 85%. These results contribute to the design of tunable THz devices, such as filters, absorbers, and modulators.

Tunable terahertz Dirac semimetal metamaterials

The tunable propagation properties of 3D Dirac semimetal (DSM) patterned metamaterial (MM) structures have been symmetrically investigated in the terahertz (THz) regime. The results demonstrate that the resonant properties are very sensitive to the thicknesses of DSM MMs, and hundreds of nanometers are required to excite strong resonant curves. The DSM MMs support both strong LC and dipolar resonances, quite different from graphene MM patterns which mainly depend on dipolar resonance. As the Fermi level increases, the resonant strength becomes stronger, and significant modulation can be achieved, e.g. the amplitude and frequency modulation depths of transmission curves are more than 99% and 80%, respectively. In addition, by utilizing asymmetrical resonators, a very sharp Fano resonant peak is achieved with a large Q-factor of more than 25, for which the figure of merit is about 20. The results are very helpful to understand the tunable mechanisms of DSM devices and design novel THz plasmonic components, such as modulators, filters, and sensors.

Tunable 3D Dirac-semimetals supported mid-IR hybrid plasmonic waveguides.

Based on 3D Dirac-semimetal (DSM) modified hybrid waveguides, tunable propagation properties have been investigated, including the effects of Fermi levels, structural parameters, and operation frequency. The results show that if the operation frequency is smaller (larger) than the transition frequency (ℏω≈2|μc|), the proposed hybrid waveguides indicate strong (weak) confinement because the DSM layer manifests a high plasmonic (dielectric low) loss property. The dielectric fiber shape affects the propagation property obviously, as the elliptical parameter decreases, the confinement and figure of merit increase, and the loss reduces. With the increase in Fermi level, the propagation constant increases, and the frequency (amplitude) modulation depth is 32.31% (12.93%) if the Fermi level changes in the range of 0.01-0.15 eV. The results are very helpful in understanding the tunable mechanisms of hybrid waveguides and designing novel plasmonic devices in the future, e.g., modulators, filters, lasers, and resonators.

Noise-Induced Limits of Detection in Frequency Locked Optical Microcavities

Ultra-high quality (Q) whispering gallery mode (WGM) optical microcavities have been shown to be sensitive biomolecular sensors due to their long photon confinement times. We have previously experimentally demonstrated that a system known as FLOWER (frequency locked optical whispering evanescent resonator) can detect single macromolecules. FLOWER uses frequency locking in combination with balanced detection and data processing to greatly improve the sensitivity, stabilization, signal-to-noise ratio (SNR), and the detection limit of ultra-high-Q microcavities. Here we present the analytical basis for FLOWER and explore its limits of detection via numerical simulation. We examine the effects of key parameters such as Q-factor and frequency modulation depth on the SNR of FLOWER. We demonstrate that the frequency locked optical microcavity system is limited by the shot noise from the receiver, as well as the laser intensity noise. Using median filtering in combination with step-fitting algorithms, frequency locked ultra-high-Q microcavities can detect resonance shifts as small as 0.05 attometers at one millisecond time intervals. Our results can guide the choice of experimental parameters to achieve better sensing performance in a variety of target applications, including fundamental studies of protein-protein interactions and medical diagnostics and prognostics.

Investigation of graphene supported terahertz elliptical metamaterials

Abstract Based on a graphene asymmetric sdouble ellipse (ADE) structure, the tunable Fano resonance characteristics in the terahertz range have been symmetrically studied, including the effects of graphene Fermi levels, asymmetrical degrees, operation frequencies and structural parameters. The results show that due to the strong coupling between the incident THz waves and graphene ADE structures, a strong Fano peak can be observed by arranging double ellipse asymmetrically. The maximum peak of Fano resonance reaches 0.918, and its Q-factor is about 20. As the asymmetrical degree of the ADE structure increases, the Fano peak value increases, and the amplitude modulation depth reaches about 75%. As graphene level increases, the amplitude and frequency modulation depths reaches more than 31% and 11%, respectively. These results are very helpful to design novel tunable graphene devices with high Q-factor devices in the future, such as sensors, modulators, and filters.

Triple mode coupling effect and dynamic tuning based on the zipper-type graphene terahertz metamaterial

We construct a laminated metamaterial structure of zipper-type monolayer graphene with silica, which is equipped with a distinctively dual plasmon-induced transparency (PIT) phenomenon. The graphene pattern in this structure craftily connects with the electrode so that we can dynamically control the PIT response (via regulating the bias voltage that applied between the electrode and the substrate to control the graphene Fermi energy), in which the range of voltage-regulate is calculated theoretically, proving to be a feasible measure for PIT response-tune. Then, the tuning discipline for this structure is summarized from the calculated result of the coupled mode theory deduced theoretical model and the simulation result. It is found that the structure possesses a great tuning performance within the tuning range we studied; also, as the structure ameliorates the monolayer graphene absorbance from 2.3% to about 50% within a broad dynamic frequency tuning range, the absorption peak frequency modulation depth is up to 45.46%. Besides, the group refractive index is discussed for reflecting the system capability of slow light, and the maximum of the coefficient can catch up to 595. Even if the impact of dielectric light- absorbance to slow light is taken into account, the slow light index at the transparent window is still as high as 200.

Performance Optimization of Fiber Optic Interferometric Accelerometer Based on Phase Noise Analysis

The phase noise of a Michelson interferometer-based fiber optic accelerometer is analyzed to determine the optimal modulation depth of the laser light frequency and the length difference between the interference arms. The output intensity fluctuation of the interferometer, originating from the phase noise of the modulated laser source, is examined theoretically, revealing that both the interference intensity noise power and the phase modulation depth of the interferometer are proportional to the frequency modulation depth of the laser and the interference arm length difference. In addition, it is determined experimentally that the laser linewidth, or equivalently, the laser phase noise, varies approximate linearly with the light frequency. As the optimum phase modulation depth is a constant, the interference intensity noise power has a minimum value, when the light frequency modulation depth is at its maximum. The proposed theory is demonstrated experimentally, with results showing that a high-performance broadband fiber optic interferometric accelerometer achieves the lowest noise level when the light frequency modulation depth of the source laser is at its maximum value.

P‐48: A Novel “Always‐on” Spread Spectrum Clock Technique for Reducing EMI in Display Electronics

Spread Spectrum Clock Generation (SSCG) is an important technique for EMI reduction. It is especially applicable for display electronics since modern society is increasingly equipped with growing number of myriad types of semiconductor display devices. To achieve the goal of lowering peak power, clock signal is intentionally “jittered”. This fact negatively impacts the clock signal integrity. For this reason, the common practice nowadays in industry is to turn off the SSCG function in normal work mode. It is only turned on when the device is actively tested for EMI influence in the lab. In this paper, a novel SSCG technique is presented. It is based on the emerging frequency synthesis technique of Time‐Average‐Frequency Direct Period Synthesis. Its aim is to maximize frequency modulation depth while precisely controls clock’s impact on system operation so that the SSCG function can be “always‐on”.

A Novel Spread Spectrum Clock Generation Technique: Always-On Boundary Spread SSCG

Spread spectrum clock generation (SSCG) is an important technique for electromagnetic interference reduction. To achieve the goal of lowering peak power, clock signal is intentionally “jittered.” In circuit implementation, there are mainly three ways to carry out the task of jittering the clock: voltage-controlled oscillator modulation, phase-locked loop divider dithering, and clock-period direct manipulation. From the perspective of spread result, there are three types including down spread, central spread, and up spread. Being an effective solution for lowering the harmful electromagnetic peak power, SSCG is also faced with a challenge of preserving the integrity of the clock signal. In this paper, a novel approach of always-on boundary spread is presented. It is based on time-average-frequency concept. Its aim is to maximize frequency modulation depth while precisely controls clock's impact on system operation. Theoretical analysis is performed, experimental validation is provided through an implementation on field programmable gate array (FPGA). Furthermore, a commercial product is used to demonstrate the feature of “always on” enabled by this SSCG technology.

Tunable terahertz hybrid graphene-metal patterns metamaterials

Based on the hybrid metal-graphene structures, we investigated the tunable Fano resonances in the terahertz region, including the effects of graphene Fermi levels, structural parameters, and operation frequencies. The results reveal that an obvious Fano resonance can be observed, the maximum peak value of Fano resonance can reach 0.9711, and its Q-quality factor is more about 20. With the help of the graphene layer, the resonant curves of the proposed structures can be effectively modulated, the frequency modulation depth can reach 60% as the Fermi level changes in the range of 0.1–1.0 eV. In addition, by varying the length of graphene bar in the scope of 10–60 μm, the amplitude modulation depths are about 21.0%. The results are helpful for designing novel graphene-based tunable terahertz devices with high Q factor, e.g. modulators, sensors and antenna.

Ppbv-Level Ethane Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a Continuous-Wave, Room Temperature Interband Cascade Laser.

A ppbv-level quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane (C2H6) sensor was demonstrated by using a 3.3 μm continuous-wave (CW), distributed feedback (DFB) interband cascade laser (ICL). The ICL was employed for targeting a strong C2H6 absorption line located at 2996.88 cm−1 in its fundamental absorption band. Wavelength modulation spectroscopy (WMS) combined with the second harmonic (2f) detection technique was utilized to increase the signal-to-noise ratio (SNR) and simplify data acquisition and processing. Gas pressure and laser frequency modulation depth were optimized to be 100 Torr and 0.106 cm−1, respectively, for maximizing the 2f signal amplitude. Performance of the QEPAS sensor was evaluated using specially prepared C2H6 samples. A detection limit of 11 parts per billion in volume (ppbv) was obtained with a 1-s integration time based on an Allan-Werle variance analysis, and the detection precision can be further improved to ~1.5 ppbv by increasing the integration time up to 230 s.

Fiber-amplifier-enhanced resonant photoacoustic sensor for sub-ppb level acetylene detection

A near-infrared fiber-amplifier-enhanced resonant photoacoustic spectroscopy (PAS) sensor is developed for sub-ppb level acetylene (C2H2) detection. The photoacoustic excitation source is composed of a telecommunication distributed feedback (DFB) laser and an erbium-doped fiber amplifier (EDFA). The weak photoacoustic second-harmonic signal is demodulated by a field-programmable gate array (FPGA) based lock-in amplifier. For sensitivity improvement, wavelength modulation spectrum and wavelet denoising methods are employed. The PAS sensor is optimized in terms of resonant frequency and current modulation depth for C2H2 detection at the wavelength of 1532.83 nm. The linearity of the sensor response to the laser power and C2H2 concentration confirms that saturation does not occur with the excitation power less than 1 W. With 1 W excitation power and 60 s averaging time, the detection limit (1δ) is achieved to be 0.37 ppb, which is the best value compared with other photoacoustic C2H2 sensors ever reported so far.