Spectral Oscillations Research Articles

Polarization‐Induced Buildup and Switching Mechanisms for Soliton Molecules Composed of Noise‐Like‐Pulse Transition States

AbstractBuildup and switching mechanisms of solitons in complex nonlinear systems are fundamentally important dynamical regimes. Using a novel strongly nonlinear optical system, including saturable absorber metal‐organic framework (MOF)‐253@Au and a polarization controller (PC), the work reveals a new buildup scenario for “soliton molecules (SMs)”, which includes a long‐duration stage dominated by the emergence of transient noise‐like pulses (NLPs) modes to withstand strong disturbances arising from “turbulence” and extreme nonlinearity in the optical cavity. The switching between SMs and NLPs is controlled by the cavity polarization state. The switching involves the spectral collapse, following spectral oscillations with a variable period, and self‐organization of NLPs, following energy overshoot. This switching mechanism applies to various patterns with single, paired, and clustered pulses. In the multi‐pulses stage, XPM (cross‐phase‐modulation)‐induced interactions between solitons facilitate a specific mode of energy exchange between them, proportional to interaction duration, ensuring pulse stability during and after state transitions. Systematic simulations reveal the effects of the PC's rotation angle and intra‐cavity nonlinearity on the periodic phase transitions between the different soliton states and accurately reproduce the experimentally observed buildup and switching mechanisms. These findings can enhance the fundamental study and points to potential uses in designing information encoding systems.

Additional Lighting Effects for Photovoltaic Improvements in the Performance of Solar Cells

The basic function of the water flow lens (WFL) system is to cool, decrease, and increase light intensity with inevitable spectral oscillations, but in reality, that manipulation helps to better understand the possible additional optical and light effects and, thus, the nature of light itself, in the hopes of making significant progress toward the use of solar energy. According to the published research on a variety of solar devices, including commercial monocrystalline and amorphous Si‐solar cells, differently designed Sb2S3‐based solar cells made of synthesized undoped and doped semiconductors, and dye‐sensitized solar cells (Dyesol/Greatcell Solar DSL 30 NRD‐T) with varying sensitizers and cosensitizers, photovoltaic performance using the WFL system can show significant improvements in all tested conditions. Based on all previous results on different solar devices, many potential explanations for demonstrating common extra‐light effects for an increase in the performance of solar cells are experimentally compared and discussed. The theoretical history of the nature of light is reviewed, and our findings are commented on along with new disclosure.

Investigating Diurnal and Seasonal Turbulence Variations of the Martian Atmosphere Using a Spectral Approach

We use a spectral approach to analyze the pressure and wind data from the InSight mission and investigate the diurnal and seasonal trends. Our analyses show that the daytime pressure and wind spectra have slopes of approximately −1.7 and −1.3 and, therefore, do not follow the Kolmogorov scaling (as was also previously reported for a reduced data set in Banfield et al.). We find that the nighttime pressure spectral slope is close to −1 (as reported in Temel et al.), and that the wind speed spectral slope is close to −0.5, flatter than the theoretical slope expected for the shear-dominated regime. We observe strong nocturnal (likely shear-generated) turbulent behavior starting around L s = 150° (InSight sol 440) that shifts to progressively earlier local times before reaching the “5th season” (InSight sols 530–710) identified by Chatain et al.. The diurnal spectral slope analyses indicate an asymmetry in the diurnal behavior of the Martian boundary layer, with a slow growth and fast collapse mechanism. Finally, the low-frequency (5–30 mHz) pressure data exhibit large spectral slope oscillations. These occur particularly during the periods with a highly stable atmosphere and, therefore, may be linked to gravity wave activity.

Asymmetric manipulations on the coherent Raman sideband generation under molecular modulations

We study the light evolution through a Raman-active medium, where a pair of fields pump molecules near-resonantly and excite the molecular coherence. A synthetic lattice including multiple frequency sidebands via the coherent Raman process is constructed. By varying the density of molecules in the light propagation direction, the system can present a lattice model undergoing dynamic modulations. Further considering the frequency-dependent couplings between synthetic lattice sites, we explore intriguing phenomena, including spectral Bloch oscillation and super Bloch oscillation with asymmetric patterns, which leads to interesting frequency-conversion phenomena with the Raman sideband generations. Our work could be useful for future experiments in coherent Raman sideband processes towards ultrashort pulse generations and higher-frequency conversion.

Influence of ac Stark shift on below-threshold harmonic generation of H molecular ions in intense laser fields

The effects of laser-induced ac Stark shift on the high-order harmonic generation below the ionization threshold of H molecular ions are investigated. The results show that several sub-peak harmonics generated in the vicinity of odd harmonics are clearly observed. Combined with the synchrosqueezing transform of the quantum time-frequency spectrum and a two-state model, we confirm that the transition between the first excited state related to ac Stark-shift and the ground state contributes to the sub-peak harmonics generation. In addition, we find some fine oscillatory structures of the sub-peak harmonic due to spectral interference. The sub-peak harmonics generation and spectral oscillations can be controlled by the laser intensity and the pulse duration. Our study provides a new alternative way to measure the energy difference of ac Stark shift between the excited state and the ground state of atoms and molecules in intense laser fields.

Theory of x-ray absorption spectroscopy: A microscopic Bloch equation approach for two-dimensional solid states

We develop a self-consistent Maxwell-Bloch formalism for the interaction of x-rays with two-dimensional crystalline materials by incorporating the Bloch theorem and Coulomb many-body interaction. This formalism is illustrated for graphene, by calculating the polarization-dependent XANES, formulating expressions for the radiative and Meinter-Auger recombination of core holes, and the discussion of microscopic insights into the spectral oscillations of EXAFS beyond point scattering theory. In particular, the correct inclusion of lattice periodicity in our evaluation allows us to assign so far uninterpreted spectral features in the Fourier transformed EXAFS spectrum.

Propagation characteristics of radial phase-locked discrete vector beam generated from a Gaussian Schell-Model laser array in atmospheric turbulence

In this paper, combining the advantages of partial coherence, vector beam, and laser array in better resistance to atmospheric turbulence, high capacity, and high output power, we proposed a radial phase-locked discrete vector beam generated from a Gaussian Schell-Model (GSM) laser array (i.e., phase-locked GSM discrete vector beam array) and theoretically studied its propagation characteristics in atmospheric turbulence. The analytical expressions for spectral intensity, spectral degree of coherence (DOC), and spectral degree of polarization (DOP) of a phase-locked GSM discrete vector beam array were derived based on the extended Huygens-Fresnel principle. And also, based on numerical simulation, we investigated the influence of atmospheric and laser parameters on the propagation properties in detail. The results show that the beam profile will gradually evolve from hollow structure distribution, then form a flat-topped beam and finally present the Gaussian-like distribution; the spectral DOC exhibits decaying oscillation analogous to under-damping vibration; a dip appears in the distribution of the spectral DOP. The evolution rate of spectral intensity, oscillation amplitude, and period of spectral DOC and dip width of spectral DOP are determined by propagation distance, refraction structure constant, beam waist, spatial correlation length, and the number of beamlets. More importantly, by analyzing Stokes parameters, different initial polarization distributions correspond to different crosslines of S1/S0, and even with the increase of the propagation distance, the crosslines of S1/S0 still show similar performance. This work may provide novel thoughts and methods to improve propagation performance and benefit potential applications in free-space optical communication.

Nanometer-scale depth-resolved hard x-ray absorption spectroscopy based on the detection of energy-loss Auger electrons with low energies

X-ray absorption spectroscopy (XAS) provides information on the chemical state and atomic structure of a target element even without long-range periodicity. A depth-resolved surface-sensitive XAS method for K-edge using hard x rays is proposed herein. The principle of this method is based on the selective detection of low-energy electrons using an electron analyzer. The detected electrons originate from the LMM Auger electrons that cascade from the KLL Auger process caused by K-shell absorption and lose the energy corresponding to their travel distance in a solid. The analysis depth was confirmed to be in the nanometer range using a GaN substrate with a thin oxide film of the defined thickness. In addition, depth-resolved information regarding the local atomic structure, including the interatomic distance and crystallinity, was obtained via the Fourier transformation of the spectral oscillations. Because the proposed technique does not require a change in the detection angle, it is also expected to be used for particles and samples with uneven surfaces.

X-ray absorption spectroscopy without the self-absorption effect by detecting l-line fluorescence at the K-edge

A novel measurement method that addresses the self-absorption effect, which is a significant disadvantage in X-ray absorption spectroscopy in fluorescence yield mode for surface analysis, is proposed. The validity and features were experimentally verified using copper. This method uses the l-line fluorescence that cascades from the KLL Auger process caused by K-shell absorption. The obtained spectrum is unaffected by the self-absorption effect and is not distorted. The confirmed features are that the spectral oscillation with a wide energy range is obtained, and the analysis depth is comparable to that of the l-edge measurement. In addition, the proposed method theoretically applies to samples with uneven surfaces, that is, various actual materials.

Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser.

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.

Weak Langmuir turbulence in disordered multimode optical fibers

We consider the propagation of temporally incoherent waves in multimode optical fibers (MMFs) in the framework of the multimode nonlinear Schr\"odinger (NLS) equation accounting for the impact of the natural structural disorder that affects light propagation in standard MMFs (random mode coupling and polarization fluctuations). By averaging the dynamics over the fast disordered fluctuations, we derive a Manakov equation from the multimode NLS equation, which reveals that the Raman effect introduces a previously unrecognized nonlinear coupling among the modes. Applying the wave turbulence theory on the Manakov equation, we derive a very simple scalar kinetic equation describing the evolution of the multimode incoherent waves. The structure of the kinetic equation is analogous to that developed in plasma physics to describe weak Langmuir turbulence. The extreme simplicity of the derived kinetic equation provides physical insight into the multimode incoherent wave dynamics. It reveals the existence of different collective behaviors where all modes self-consistently form a multimode spectral incoherent soliton state. Such an incoherent soliton can exhibit a discrete behavior characterized by collective synchronized spectral oscillations in frequency space. The theory is validated by accurate numerical simulations: The simulations of the generalized multimode NLS equation are found in quantitative agreement with those of the derived scalar kinetic equation without using adjustable parameters.

Mechanism of Spectral Distortion for Real-Time Crack Propagation on Aluminum Alloy Structure

This study investigates four characteristic damage mechanisms of fiber Bragg grating (FBG) sensors, with fatigue crack propagation in aluminum alloy. The multipeak wavelength distinguish algorithm was developed for FBG spectrum quantitative analysis. The results distinguish a subordinate peak skewing significantly, associated with strain patterns along the FBG, corresponding to various crack lengths. For parallel bonded direction grating, the subordinate peak skewing appears at the strain pattern transition region. This is located at the ratio 32%-34% of crack length lying in the crack tip. Meanwhile, the four damage characteristics correspond to subordinate peak skewing. When the strain is distributed along the grating, spectral distortion occurs. In this region, the cubic strain pattern determines the shorter wavelength location of subordinate peaks. This corresponds to the 15%-17% ratio of crack length lying in the grating, causing spectral oscillations.

Precise spectrophotometric method for semitransparent metallic thin-film index determination using interference enhancement

A precise spectrophotometric method to determine the refractive index of a semitransparent metallic thin film is presented. This method relies on interference enhancement of the measured spectra, employing an opaque substrate with a dielectric spacer layer beneath the absorbing layer of interest to create interference fringes.The resulting spectral oscillations of the stack are highly sensitive to the properties of the top absorbing layer, allowing precise determination of the refractive index via fitting. The performance of this method is verified using simulations in comparison to the typical method of depositing the absorbing thin film directly onto a transparent substrate. An experimental demonstration is made for titanium thin films over the visible range (370-835 nm). The refractive index of these films is extracted from experimental data using a combination of the Modified Drude and Forouhi-Bloomer models. This method showed high repeatability and precision, and is verified for Ti films between 6-70 nm thickness.

Temporal loss boundary engineered photonic cavity

Losses are ubiquitous and unavoidable in nature inhibiting the performance of most optical processes. Manipulating losses to adjust the dissipation of photons is analogous to braking a running car that is as important as populating photons via a gain medium. Here, we introduce the transient loss boundary into a photon populated cavity that functions as a ‘photon brake’ and probe photon dynamics by engineering the ‘brake timing’ and ‘brake strength’. Coupled cavity photons can be distinguished by stripping one photonic mode through controlling the loss boundary, which enables the transition from a coupled to an uncoupled state. We interpret the transient boundary as a perturbation by considering both real and imaginary parts of permittivity, and the dynamic process is modeled with a temporal two-dipole oscillator: one with the natural resonant polarization and the other with a frequency-shift polarization. The model unravels the underlying mechanism of concomitant coherent spectral oscillations and generation of tone-tuning cavity photons in the braking process. By synthesizing the temporal loss boundary into a photon populated cavity, a plethora of interesting phenomena and applications are envisioned such as the observation of quantum squeezed states, low-loss nonreciprocal waveguides and ultrafast beam scanning devices.

Label-Free, Real-Time Measurement of Metabolism of Adherent and Suspended Single Cells by In-Cell Fourier Transform Infrared Microspectroscopy.

We used infrared (IR) microscopy to monitor in real-time the metabolic turnover of individual mammalian cells in morphologically different states. By relying on the intrinsic absorption of mid-IR light by molecular components, we could discriminate the metabolism of adherent cells as compared to suspended cells. We identified major biochemical differences between the two cellular states, whereby only adherent cells appeared to rely heavily on glycolytic turnover and lactic fermentation. We also report spectroscopic variations that appear as spectral oscillations in the IR domain, observed only when using synchrotron infrared radiation. We propose that this effect could be used as a reporter of the cellular conditions. Our results are instrumental in establishing IR microscopy as a label-free method for real-time metabolic studies of individual cells in different morphological states, and in more complex cellular ensembles.

Revisiting the analysis of axion-like particles with the Fermi-LAT gamma-ray observation of NGC1275

In this work, we re-analyze the Fermi-LAT observation of NGC 1275 to search for axion-like particle (ALP) effects and constrain ALP parameters. Instead of fitting the observed spectrum with ALP models, we adopt an alternative method for the analysis of this source which calculates the irregularity of the spectrum. With the newly used method, we find no spectral oscillation for the NGC 1275 and rule out couplings gaγ>3×10−12GeV−1 around ALP mass of ma∼1 neV at 95% confidence level, which is more stringent than the previous results. We also show that the constraints can be further improved by combining the observation of PKS 2155-304. We suggest that with more sources taken into account, we could obtain a much wider exclusion region.

3D simulation of hybrid Fabry-Pérot for single mode fibre

3D modal simulation of Hybrid Fabry Pérot (H-FP) consisting of a Bragg grating (Bg) and a multilayer mirror between single-mode optical fibres shows that diffraction in their multilayer is negligible, but that multimode 3D S-matrix calculation is essential in the multilayer to determine the adequate thickness of the interface layer with the Bg. The single-mode approximation made in 1D calculation is justified for their sinusoidal profile Bg, but is not at the top of the transmission peak for their high step profile Bg. The study of manufacturing constraints shows that feasible H-FPs, with rejection broad of several hundred nanometers, provide access to bandpass widths as low as 15 fm, but that the control of the bandwidth shift remains delicate for the realization of large series. The principle of H-FP with a high refraction index step reflector and a low step one is also valid for integrated optics. All weak spectral oscillations in the rejection band of H-FPs have been interpreted. Finally, the simulation of H-FPs with Interleaved Fibre (IF) between their 2 mirrors makes it possible to deduce also for H-FPs without IF and for some of their properties a quantitative model of equivalent cavity filled with homogeneous medium and with localized mirrors equidistant by a fraction of the length of the Bg depending only on its reflection coefficient. This equivalent cavity is used to evaluate the line width. This model also applies to 2 Bg-FPs without IF with a double length of the equivalent cavity for the same grating.

Local field effects in ultrafast light–matter interaction measured by pump-probe spectroscopy of monolayer MoSe2

Abstract Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.

Temporal and spectral hybrid interference from phase transition of Eu3+/Pr3+: YPO4 and evolution of amplifier and multiplexer

We study the temporal and spectral oscillation originated from the interference between the fluorescence (FL) and spontaneous parametric four-wave mixing (SP-FWM) signals. The FL and SP-FWM are generated from different phases of Eu3+ or Pr3+ doped YPO4 under the nonlinear SP-FWM process. We discuss and compare the spectral interfering oscillations and self-Rabi oscillations controlled via multiple phase transitions (hexagonal-phase and mixed-phase) of Eu3+: YPO4 and Pr3+: YPO4. The coexistence mechanism of interference and self-oscillations in time domain is also investigated. Further, we discuss the cross-Rabi oscillation in correlated light beams, which evolves from FL signal (having Sinc profile) to SP-FWM (having Cosine profile) in the hybrid signal regime. Based on these results, we have realized wavelength division multiplexing, and temporal amplifier with the temporal interference being controlled by the gate position, and frequency detuning. Our experimental results provide a technique to achieve higher channel equalization ratio about 85%.

Spatial distribution of spectral SST oscillations over the equatorial pacific in the period 1888–2014

AbstractThis study identified and analysed the spatial and temporal patterns of spectral oscillation ranges of the monthly sea surface temperature, SST, over the tropical Pacific region, from 1888 to 2014, through wavelet analysis. The strategic steps considered the variance analysis, obtained from the wavelet computation, for each single grid point instead of the analysis of a spatial mean variance, as is commonly done, allowing us to visualize the spatial distribution of SST variance during different time periods. The mean results indicate that lower‐frequency oscillations (2–4 and 4–8 years) over the considered area are becoming more intense while higher‐frequency oscillations (1–12 months) are becoming less intense through time, which can lead to more persistent climates around the globe. While the 1–2 year oscillations feature the highest positive trend along the equatorial belt over central and eastern locations, the 4–8 year oscillations feature the strongest signal closer to the western South American coast, and the 2–4 year oscillations present positive signals over areas closer to the continental coast and over more distant western locations intercalated by negative signals. Throughout time, the position of the highest EN signals occurs either over central or eastern areas of the equatorial Pacific. The SST variance in the Niño areas mainly oscillates within the 2–4 and 4–8 year spectral ranges. Niño1 + 2 and Niño3 regions, in opposition to Niño3 and Niño3 + 4 regions, present the highest SST variance values for the four considered spectral ranges. The strongest El Niño events present their main oscillations in the 2–4 and 4–8 year spectral ranges, especially in the Niño1 + 2 and Niño3 regions, while weaker El Niño events show more association with the 1–2 year spectral range. Although there are some patterns identified for distinct El Niño/Southern Oscillation, ENSO, intensities and the associated frequency oscillations, the distinct spatial spectral energy distribution over the equatorial areas suggests the importance of the spatial distribution of the spectral energy through time in order to better characterize the ENSO events.