Modulation Frequency Research Articles

Inducing exceptional points, enhancing plasmon quality and creating correlated plasmon states with modulated Floquet parametric driving.

The control of low frequency collective modes in solids by light presents important challenges and opportunities for condensed matter physics. We propose a method to parametricallydrive low THz range collective modes in an interacting many body system using Floquet driving at optical frequencies with a modulated amplitude. We demonstrate that it can be used to design plasmonic time-varying media with singular dispersions. Plasmons near resonance with half the modulation frequency exhibit two lines of exceptional points connected by dispersionless states. Above a critical driving strength, resonant plasmon modes become unstable and undergo a continuous transition towards a crystal-like structure stabilized by interactions and nonlinearities. This new state breaks the discrete time translational symmetry of the drive as well as the translational and rotational spatial symmetries of the system and exhibits soft, Goldstone-like phononic excitations. Below the instability threshold, our method can be used to enhance the quality of plasmon resonances.

Mechanisms of Increasing Weld Depth during Temporal Power Modulation in High‐Power Laser Beam Welding

Understanding the fundamental mechanisms of action for increasing weld depth during temporal power modulation in laser beam welding could allow dissimilar rotational welding without the introduction of concomitant turbulence, but with enhanced intermixing. The investigations are conducted on 30 mm‐diameter round bars of stainless steel alloy 1.4301 and nickel base alloy 2.4856 utilizing a 16 kW disk laser beam source. Modulation frequencies are 0/50/100/200 Hz at low, medium, and high amplitudes of laser beam power. The influence on the process and weld characteristics is investigated through high‐speed imaging with grayscale analysis, keyhole depth measurements, metallographic sections, and energy‐dispersive X‐ray spectroscopy analysis. The objectives are successfully achieved, and the underlying mechanism is maintaining the keyhole depth at a higher level for modulation frequencies of 200 Hz and a high amplitude of laser beam power, which is related to the keyhole inertia. Based on this, a novel welding mode with a constant keyhole depth is proposed. Furthermore, up to 20% increase in weld depth is achieved, a saturation limit for the modulation frequency is identified, intermixing within the weld is enhanced, and a model for predicting the weld depth based solely on measurements of the surface width is developed.

A magnetic pendulum for demonstrating Mathieu-type parametric resonance

Abstract We describe a magnetic pendulum apparatus designed for studying and demonstrating Mathieu-type parametric resonance. The apparatus features a T-shaped rigid arm with a neodymium (NdFeB) magnet bob, suspended by double V-shaped thread suspensions. It is parametrically driven by an alternating uniform magnetic field produced by Helmholtz coils. The pendulum is structurally simple and easy to implement, and its parameter-driven mechanism is transparent in terms of physics. The use of electromagnetic driving allows for precise control over both the frequency and amplitude of parameter modulation, enabling quantitative studying on parametric resonance. We have observed the first and second instability region of parametric resonance. We also theoretically analyze the impact of non-uniformity in the Helmholtz coils' magnetic field on the pendulum motion, revealing that the non-uniformity can significantly alter the pendulum's nonlinearity, influencing both the magnitude and the sign of the coefficients of the nonlinear terms. However, under the parameters of our experimental setup, the nonlinearity effect is mainly manifested in large swing angle motions. For small swing angle within 5\textsuperscript{$\circ$}, the impact of magnetic field non-uniformity on parametric resonance is negligible. The experiment reported here is appropriate for undergraduates to carry out experimental research, helping them develop a more intuitive and quantitative understanding of parametric resonance.

Signal Processing for Novel Noise Radar Based on de-chirp and Delay Matching

Modern radar technology requires high-quality signals and detection performance. However, traditional frequency-modulated continuous wave (FMCW) radar often has poor anti-jamming capabilities, and the high sampling rates associated with large time-bandwidth product signals can lead to increased system hardware costs and reduced data processing efficiency. This paper constructed a composite radar waveform based on noise frequency modulation (NFM) and linear frequency modulation (LFM) signals, enhancing the signal’s complexity and anti-jamming capability. Furthermore, a method for optimizing the processing of echo signals based on de-chirp and delay matching is proposed. The locally generated LFM signal is used to de-chirp the received echoes, resulting in a narrowband difference frequency noise signal. Subsequently, delay matching is performed in the fast time domain using the locally generated NFM signal according to the number of sampling points in the traversal processing period, allowing for the acquisition of target delay information. While reducing the analog-to-digital (A/D) sampling rate, the detection performance for wideband echo signals under high sampling rates is still maintained, with sidelobe levels and range resolution preserved. Accumulating this information in the slow time domain enables accurate target detection. The effectiveness of the proposed method is validated through simulation experiments.

Fundamental effects of array density and modulation frequency on image quality of diffuse optical tomography.

Diffuse optical tomography (DOT) provides three-dimensional image reconstruction of chromophore perturbations within a turbid volume. Two leading strategies to optimize DOT image quality include, (i) arrays of regular, interlacing, high-density (HD) grids of sources and detectors with closest spacing less than 15mm, or (ii) source modulated light of order ∼100MHz. However, the general principles for how these crucial design parameters of array density and modulation frequency may interact to provide an optimal system design have yet to be elucidated. Herein, we systematically evaluated how these design parameters effect image quality via multiple key metrics. Specifically, we simulated 32 system designs with realistic measurement noise and quantified localization error, spatial resolution, signal-to-noise, and localization depth of field for each of ∼85000 point spread functions in each model. We found that array density had a far stronger effect on image quality metrics than modulation frequency. Additionally, model fits for image quality metrics revealed that potential improvements diminish with regular arrays denser than 9mm closest spacing. Further, for a given array density, 300MHz source modulation provided the deepest reliable imaging compared to other frequencies. Our results indicate that both array density and modulation frequency affect the spatial sampling of tissue, which asymptotically saturates due to photon diffusivity within a turbid volume. In summary, our results provide comprehensive perspectives for optimizing future DOT system designs in applications from wearable functional brain imaging to breast tumor detection.

Laser power modulation between disturbed and undisturbed material removal regime in laser chemical processing

Laser chemical machining (LCM) is a method for removing material by thermally induced chemical dissolution of self-passivating metals. However, the process window is limited by disrupted material removal due to gas bubble formation and metallic salts and oxides deposition at higher energy input. Since the temperature increases, and therefore gas bubble growth takes time, it is hypothesized that the temporal modulation of laser power can remove the metal homogeneously, i.e., without disrupted material removal, while achieving a higher removal rate. Based on this, the dynamic process behavior of material removal is investigated for the LCM of titanium in phosphoric acid, using a rectangular modulation of the laser power with varying irradiation durations. As a result, however, high-speed videos show that gas bubbles are consistently generated, regardless of the applied laser power and power modulation, although the quantity of bubbles varies with different parameters. Even with short power durations (10 ms), the material deposition occurs after multiple irradiations. When the duration is longer, the material deposition increases in height along the laser scan direction. For the studied process parameters, a Fourier analysis in the spatial domain further indicates the correlation between the material removal frequencies and the modulation frequencies. In conclusion, the laser power modulation cannot prevent the disturbed material removal at high laser powers. Nevertheless, the material deposition can be utilized to generate periodic surface structures with a depth below and above the initial surface.

Frequency-domain instrument with custom ASIC for dual-slope near-infrared spectroscopy.

Real-time and non-invasive measurements of tissue concentrations of oxyhemoglobin (HbO2) and deoxyhemoglobin (HbR) are invaluable for research and clinical use. Frequency-domain near-infrared spectroscopy (FD-NIRS) enables non-invasive measurement of these chromophore concentrations in human tissue. We present a small form factor, dual-wavelength, miniaturized FD-NIRS instrument for absolute optical measurements, built around a custom application-specific integrated circuit and a dual-slope/self-calibrating (DS/SC) probe. The modulation frequency is 55MHz, and the heterodyning technique was used for intensity and phase readout, with an acquisition rate of 0.7Hz. The instrument consists of a 14 × 17cm2 printed circuit board (PCB), a Raspberry Pi 4, an STM32G491 microcontroller, and the DS/SC probe. The DS/SC approach enables this instrument to be selective to deeper tissue and conduct absolute measurements without calibration. The instrument was initially validated using a tissue-mimicking solid phantom, and upon confirming its suitability for in vivo, a vascular occlusion experiment on a human subject was conducted. For the phantom experiments, an average of 0.08° phase noise and 0.10% standard deviation over the mean for the intensities was measured at a source-detector distance of 35mm. The absorption and reduced scattering coefficients had average precisions (variation of measurement over time) of 0.5% and 0.9%, respectively, on a window of ten frames. Results from the in vivo experiment yielded the expected increase in HbO2 and HbR concentration for all measurement types tested, namely SC, DS intensity, and DS phase.

Signal growth in a pure time-modulated transmission line and the loss effect.

We present the first comprehensive study for signal growth in transmission lines (TL) with purely time-modulated characteristic impedance (infinite superluminality). This study pioneers the investigation into the effects of varying the cell's electrical length and the impact of loss on momentum bandgaps and amplification levels. It also thoroughly examines how time-modulated transmission line truncation by a static load influences the sensitivity of amplification gain to the relative phase between the incoming signal and modulation, comparing these findings with the case of parametric amplification. Varying is accomplished by loading TLs with a sinusoidally time-modulated capacitor (TMC). The study starts with a simple lumped model cell to facilitate understanding of the phenomena. Following this, transmission lines are introduced, and the effects of incorporating loss are examined. To accomplish this, three models are investigated: a lossless L-C TL lumped model loaded with a shunt lossless TMC and a TL loaded with a shunt lossless and lossy TMC. Dispersion diagrams are plotted and momentum bandgaps are identified at a modulation frequency double the signal frequency. Within the momentum bandgap, only imaginary frequencies are found and correlated to momentum bandgap width and signal growth level. Signal growth is confirmed using harmonic balance and transient simulations, and the results are consistent with the dispersion diagram outcomes.

Lineshape of Amplitude-Modulated Stimulated Raman Spectra.

The amplitude modulation of a pump field and the phase-sensitive detection of a pump-induced intensity change of a probe field encompass a common practice in nonlinear spectroscopies to enhance the detection sensitivity. A drawback of this approach arises when the modulation frequency is comparable to the width of the spectral feature of interest, since the presence of sidebands in the amplitude-modulated pump field provides distortion to the observed spectral lineshape. This represents a problem when accurate measurements of spectral lineshapes and line positions are pursued, as recently happened in our group with the metrology of the Q(1) line in the 1-0 band of molecular hydrogen. The measurement was performed with a Stimulated Raman Scattering spectrometer that was calibrated, for the first time, against an optical frequency comb. In this work, we develop an analytical tool for nonlinear Stimulated Raman spectroscopies that allows us to precisely quantify spectral distortions arising from high-frequency amplitude modulation in one of the interacting fields. Once they are known, spectral distortions can be deconvolved from the measured spectra to retrieve unbiased data. The application of this tool to the measured spectra is discussed.

Melting and solidification in periodically modulated thermal convection

Melting and solidification in periodically time-modulated thermal convection are relevant for numerous natural and engineering systems, for example, glacial melting under periodic sun radiation and latent thermal energy storage under periodically pulsating heating. It is highly relevant for the estimation of melt rate and melt efficiency management. However, even the dynamics of a solid–liquid interface shape subjected to a simple sinusoidal heating has not yet been investigated in detail. In this paper, we offer a better understanding of the modulation frequency dependence of the melting and solidification front. We numerically investigate periodic melting and solidification in turbulent convective flow with the solid above and the melted liquid below, and sinusoidal heating at the bottom plate with the mean temperature equal to the melting temperature. We investigate how the periodic heating can prevent the full solidification, and the resulting flow structures and the quasi-equilibrium interface height. We further study the dependence on the heating modulation frequency. As the frequency decreases, we found two distinct regimes, which are ‘partially solid’ and ‘fully solid’. In the fully solid regime, the liquid freezes completely, and the effect of the modulation is limited. In the partially solid regime, the solid partially melts, and a steady or unsteady solid–liquid interface forms depending on the frequency. The interface height can be derived based on the energy balance through the interface. In the partially solid regime, the interface height oscillates periodically, following the frequency of modulation. Here, we propose a perturbation approach that can predict the dependency of the oscillation amplitude on the modulation frequency.

Stability of thermal convection in two superimposed miscible viscous fluids under gravitational modulation

Abstract The thermal stability of a system of two superimposed miscible viscous fluid layers, under gravitational modulation, is investigated using linear stability analysis. It is well known in the literature that in the absence of gravitational modulation, there exist two convection regimes depending on the buoyancy number: the single-cell regime, where convection is oscillating and occupies the entire volume of the two fluid layers, and the stratified regime where stationary convection occurs in each fluid layer. Using Chebyshev's spectral collocation method and Floquet's theory, the numerical results show that under gravitational modulation, single-cell convection oscillates at half the frequency of the external oscillation (subharmonic threshold), while the stratified regime oscillates at a frequency equal to that of the forcing (harmonic threshold). The critical buoyancy number corresponding to the transition between the two convection regimes depends, in addition to the viscosity ratio and the depth of the two layers, on certain dimensionless frequencies permeating this transition and also on the amplitude of oscillation via the Froude number. In the plane of the critical Rayleigh number versus dimensionless modulation frequency, the branch associated with stratified convection does not depend on the buoyancy number for equal viscosities whereas this is the case for the whole-convection regime. For different viscosities, both convection regimes depend on the buoyancy number. At last, in the stratified regime (harmonic threshold), the critical value of the viscosity contrast, corresponding to the passage from an active layer to a passive layer and vice versa, increases with the modulation frequency.

Single pulse shaping for higher harmonic demodulation in terahertz time-domain spectroscopy

We present a simple, highly stable, low noise, and rapid detection scheme using higher harmonic demodulation applied to terahertz time domain spectroscopy (THz-TDS). The presence of higher harmonics in the detected periodic signal is because of the non-sinusoidal shape of a single pulse, which is controlled by an ultrafast current pre-amplifier. Instead of using external signal modulators, the use of the inherent repetition rate (frep) of the femtosecond laser and its harmonics as reference for the lock-in amplifier simplifies the setup, while allows rapid and low noise detection owing to the megahertz modulation frequencies. Unlike the signal detected at the fundamental frep, signals detected at higher harmonics have much lower offset and are unaffected by perturbations in the environment present during measurements, which is an essential characteristic for an analytical tool. Our proposed technique can be readily integrated to existing THz-TDS systems and is applicable to scans with rapid acquisition times and to scans that require long periods of time (e.g., hyperspectral imaging).

Investigating the Role of Exercise Pattern in Acute Cardiovagal Recovery.

Research on intermittent training has mainly focused on the effects of exercise intensity while overlooking the specific impact of the modulations associated with alternating exercise and recovery. This study investigated how the frequency of modulations during moderate-intensity exercise affects post-exercise vagal reactivation. Healthy, active females and males aged 18-39 years were recruited for the study. Participants completed three treadmill running sessions on separate days. Each moderate-intensity session accumulated 30 min at 90% of the intensity associated with the second ventilatory threshold and were performed as either high-frequency intermittent (HiFi; 15 x [2 min + 2 min recovery]), low-frequency intermittent (LoFi; 5 x [6 min + 2 min recovery]), or continuous training (MICT; 1 x 30 min). Heart rate recovery (HRrec) at 1 min and heart rate variability recovery (HRVrec; lnRMSSD) were assessed in response to submaximal constant-speed tests performed prior to (CST1) and following (CST2) each of the exercise sessions. HRrec, HRVrec, blood lactate (BLa), and blood pressure were also collected during the exercise sessions. Twenty-one individuals (8 females, 13 males) participated in the study. HRrec from CST2 was faster in HiFi vs. MICT (p < 0.001), while HRVrec post-CST2 was higher following HiFi vs. both LoFi (p = 0.024) and MICT (p < 0.001). BLa increased in all conditions (p = 0.007) but remained lower during HiFi compared to LoFi and MICT (both p < 0.001). Diastolic blood pressure did not change during exercise with HiFi (p = 0.939) but decreased during LoFi (p = 0.006) and MICT (p = 0.008). Exercise pattern influences the physiologic response to exercise. Higher frequencies of modulations can preserve vagal activity and expedite post-exercise recovery, suggesting moderate-intensity intermittent exercise as a potential strategy to mitigate autonomic impact and acute physiological stress while maintaining total work performed.

Scalable In2S3 based optical memristor devices as artificial synapse for logic realization and neuromorphic computing

This paper presents the scalable production of indium sulfide (In2S3) based optical memristors as an artificial synapse for logic realization and neuromorphic computing applications. The optical memristor device's performance was tested using the electrical and optical stimulation (ultraviolet (UV) to near infrared (NIR)) at room temperature. The memristor has shown the improved performance metrics for optical stimulation as compared to electrical stimulation. The In2S3 based optical memristor offers a resistance switching ratio of ∼103 at room temperature. In addition, the memristor has ultra-fast program and reprogram capability (38.8/39 μs) at 880 nm. Moreover, stability/scalability of the device was tested using the cycle-to-cycle variability test at different modulated frequencies. The electrical and optical signals were used as an input for logic gate realization. The neuromorphic computing model was implemented to predict the digits and alphabets with an accuracy of 98.19 % and 99.18 % respectively. These demonstrated optical memristors can be the key technology for future memory devices that surpass the limitations of von-Neumann bottleneck.

Manipulation of High-Fidelity Sidebands under Large Detuning by Floquet Technology: Application to Multi-Mode Cooling

Abstract The Floquet technology, a powerful way to manipulate quantum states, is employed to drive sidebands transition under large detuning. Our results demonstrate that high fidelities over 99% can be achieved through optimizing suitable modulation frequencies under large detuning. We observe high-fidelity transitions within a high bandwidth by utilizing a single modulation frequency and reveal that this capability is due to the emergence of a flat-band structure in the bandwidth range. The key finding of high-fidelity sideband manipulation under large detuning is experimentally confirmed in nuclear magnetic resonance platform. Finally, we propose a new parallel sideband cooling scheme that enables simultaneous cooling of multiple motional modes. This approach improves the cooling rate compared to conventional schemes with fixed laser frequency and power, and eliminates the need for mode-specific addressing. Our Floquet parallel scheme is applicable to any harmonic oscillator system and is not limited by bandwidth in theory.

Comparison of space vector and switching frequency optimal pulse width modulation for diode free F‐type multi level inverter

AbstractThis study presents a comprehensive examination of space vector pulse width modulation (SVPWM) and switching frequency optimal PWM (SFOPWM) for an F‐type multilevel inverter (FTMLI). SVPWM offers a simple, digital implementation for three‐phase, three‐level inverter structures, and carrier‐based SFOPWM allows for maximum use of switching frequency with a simple construction. This approach reveals the underlying relationship between the above two PWM techniques with various amplitude and frequency modulation indices to ensure the performance of FTMLI. A suitable comparative study was carried out to verify the PWM techniques based on the inverter output voltage, total harmonic distortion, switching stress, and filter size. The analytical conclusions are supported by the simulation results and their experimental validation.

Mechanisms of multi-layered Rayleigh noise in Brillouin optical correlation-domain reflectometry

In Brillouin optical correlation-domain reflectometry (BOCDR), sinusoidal modulation is applied to the output frequency of a light source, with spatial resolution inversely related to the modulation amplitude. We have developed an effective method to estimate the modulation amplitude using the width of the noise spectrum caused by Rayleigh scattering, eliminating the need for an optical spectrum analyzer or modifications to existing equipment. However, the Rayleigh noise spectrum often displays a three-layered structure, complicating the identification of the appropriate spectral components for estimating the modulation amplitude. In this work, we investigate the origins of this three-layered Rayleigh noise spectrum and identify the directivity of an optical circulator as the source of the third noise component. As replacing the circulator with alternative optical components is not easy, it remains an essential part of the system. Our analysis shows that the third noise component exhibits significantly small variation in spectral width with changes in modulation frequency compared to the first and second components. This characteristic allows for the effective separation and identification of the third noise component, thereby enhancing the accuracy and convenience of modulation amplitude estimation in BOCDR.

Quantum tunneling high-speed nano-excitonic modulator

High-speed electrical control of nano-optoelectronic properties in two-dimensional semiconductors is a building block for the development of excitonic devices, allowing the seamless integration of nano-electronics and -photonics. Here, we demonstrate a high-speed electrical modulation of nanoscale exciton behaviors in a MoS2 monolayer at room temperature through a quantum tunneling nanoplasmonic cavity. Electrical control of tunneling electrons between Au tip and MoS2 monolayer facilitates the dynamic switching of neutral exciton- and trion-dominant states at the nanoscale. Through tip-induced spectroscopic analysis, we locally characterize the modified recombination dynamics, resulting in a significant change in the photoluminescence quantum yield. Furthermore, by obtaining a time-resolved second-order correlation function, we demonstrate that this electrically-driven nanoscale exciton-trion interconversion achieves a modulation frequency of up to 8 MHz. Our approach provides a versatile platform for dynamically manipulating nano-optoelectronic properties in the form of transformable excitonic quasiparticles, including valley polarization, recombination, and transport dynamics.

Experimental verification of the variability of amplitude modulation (AM) from wind turbines in relation to its assessment for annoyance

The analysis of the influence of amplitude modulation (AM) on the annoyance of noise generated by wind turbines (WT) is still the subject of numerous studies. However, there is still a lack of widely accepted quantitative methods that would determine the degree of influence of modulation phenomena on noise perception. The study analyzed the variability of the depth of AM over time within different frequency-ranges. The research was based on long-term measurements at selected measurement locations. Long-term measurement ensured consideration of diverse propagation conditions. The collected data were segmented into time intervals, which included basic AM parameters - depth, modulation frequency, and frequency band in which the AM was observed, as well as wind speed and direction. Statistical analysis of the gathered dataset was used to assess the regularity of distinguished AM parameters in correlation with WT noise energy values and recorded non-acoustic parameters. A significant variability of AM depth was observed depending on the frequency band, masking level, and position relative to the wind direction. By adopting a criterion for assessing the annoyance of WT noise (according to AMWG) with a threshold depth of AM not less than 2 dB, it was noticed in over 30% of the analyzed intervals.

Whistle or squeal? Distinguish and evaluate two typical abnormal sounds of collaborative robots

The whistle and squeal are two typical abnormal sounds among the radiated noise of running collaborative robots, indicating potential faults and causing discomfort through auditory perception. This study investigated whether psychoacoustic parameters for sound quality could distinguish the whistle and squeal and evaluate their discomfort. A total of 20 abnormal sound stimuli, including 6 whistles and 14 squeals, were obtained from various collaborative robots. The cepstrum characteristics and sound quality metrics (i.e., roughness, sharpness, fluctuation strength and tonality) of all stimuli were analyzed by controlling the equal loudness. Twenty-four participants rated the 'discomfort of abnormal sound in the stimuli' on an 11-point scale. The results show that the modulation frequencies of the fundamental frequency of the whistles were 2.9 Hz to 5.5 Hz, whereas the squeals had no obvious modulation characteristics. The modulation frequencies of the fundamental frequency were highly correlated (with a correlation coefficient of 0.89) to the discomfort responses for whistles, whereas tonality was significantly correlated (with a correlation coefficient of 0.82) to the discomfort responses for squeals. The linear models using single dependent variables of modulation frequency in cepstrum and tonality had determination coefficients of 0.77 and 0.66, respectively, in evaluating the discomfort of whistles and squeals.