Coherent Backscattering Research Articles

Coherent instabilities in thulium-based fiber amplifiers induced by laser frequency modulation

Frequency modulation of narrow-linewidth lasers can cause coherent backscattering in cladding-pumped fiber amplifiers. This detrimental effect can be observed in Tm-based fiber amplifiers and can be an additional limitation for power scaling applications. We investigate such instabilities in Tm- and Tm/Ho-doped fiber amplifiers for a wide range of design parameters (active fiber length, pumping scheme, dopant type) and operation regimes (laser frequency tuning rate, amplifier gain). For each amplifier configuration, the backward-propagating (BP) signal is found to peak at a specific laser frequency tuning rate, with an amplitude and a frequency that increase with increasing amplifier gain and fiber length.

Enhancement of time-dependent coherent backscattering: Leveraging phase information

Enhancement of time-dependent coherent backscattering: Leveraging phase information

Coherent and low-coherent enhanced backscattering in tissue models

We perform simulations of coherent backscattering enhancement, explicitly accounting for the finiteness of spatial coherence of radiation. Three plausible models are considered, which produce the broadening of the coherent backscattering angular profile with the loss of spatial coherence. These models show fair agreement with data obtained earlier for biological media. Additionally, the exact solution for the double scattering term is obtained; its numerical magnitude is shown to be non-dominant for evaluations in the biomedical area due to high scattering anisotropy.

Generation of attosecond gigawatt soft x-ray pulses through coherent Thomson backscattering.

Collision between relativistic electron sheets and counterpropagating laser pulses is recognized as a promising way to produce intense attosecond x rays through coherent Thomson backscattering (TBS). In a double-layer scheme, the electrons in an ultrathin solid foil are first pushed out by an intense laser driver and then interact with the laser reflected off a second foil to form a high-density relativistic electron sheet with vanishing transverse momentum. However, the repulsion between these concentrated electrons can increase the thickness of the layer, reducing both its density and subsequently the coherent TBS. Here, we present a systematic study on the evolution of the flying electron layer and find that its resulting thickness is determined by the interplay between the intrinsic space-charge expansion and the velocity compression induced by the drive laser. How the laser driver, the target areal density, the reflector, and the collision laser intensity affect the properties of the produced x rays is explored. Multidimensional particle-in-cell simulations indicate that employing this scheme in the nonlinear regime has the potential to stably produce soft x rays with several gigawatt peak power in hundreds of terawatt ultrafast laser facilities. The pulse duration can be tuned to tens of attoseconds. This compact and intense attosecond x-ray source may have broad applications in attosecond science.

Crop Height Estimation Based on a Novel Semi-Empirical Model Considering Double-Bounce Scattering Using RADARSAT-2 PolSAR Data

Abstract. Obtaining precise and rapid crop height is essential to facilitate agricultural production services, field management, disaster monitoring, and yield assessment. With the capability to penetrate vegetation and record vertical structure information, Polarimetric Synthetic Aperture Radar (PolSAR) holds significant potential for application in vegetation height inversion. The Water Cloud Model (WCM) and its enhanced versions are extensively utilized for estimating crop heights from PolSAR data owing to their physical significance and simplicity. However, the method is not practical for stalk crops due to the neglect of double-bounce scattering considerations. Therefore, according to the growth characteristics of stalk crops, a three-component polarimetric coherent backscattering model considering crop target double-bounce scattering is established by simplifying the Random Volume over Ground (RVoG) coherent scattering model. The empirical coefficient is introduced to simplify the model into a semi-empirical for crop height inversion. The suitability of applying the RVoG-B three-component model for crop height inversion at the early stage in corn fields was assessed using Multi-temporal C-band PolSAR RADARSAT-2 data in three polarimetric channels. The results show that the HV channel exhibits superior potential in inverting the height of corn compared with the HH and the VV channels. The results of corn height inversion demonstrate that the RVoG-B three-component semi-empirical model performs effectively in estimating corn height, with its inversion accuracy having an RMSE ranging from 11.66cm to 24.51cm. This study demonstrates the potential application of the RVoG-B three-component semi-empirical model for inverting crop height at the early stage dominated by double-bounce scattering.

Polar Decomposition of Jones Matrix and Mueller Matrix of Coherent Rayleigh Backscattering in Single-Mode Fibers.

The Jones matrix and the Mueller matrix of the coherent Rayleigh backscattering (RB) in single-mode fibers (SMFs) have been derived recently. It has been shown that both matrices depict two polarization effects-birefringence and polarization-dependent loss (PDL)-although the SMF under investigation is purely birefringent, having no PDL. In this paper, we aim to perform a theoretical analysis of both matrices using polar decomposition. The derived sub-Jones/Mueller matrices, representing birefringence and PDL, respectively, can be used to investigate the polarization properties of the coherent RB. As an application of the theoretical results, we use the derived formulas to investigate the polarization properties of the optical signals in phase-sensitive optical time-domain reflectometry (φ-OTDR). For the first time, to our knowledge, by using the derived birefringence-Jones matrix, the common optical phase of the optical signal in φ-OTDR is obtained as the function of the forward phase and birefringence distributions. By using the derived PDL-Mueller matrix, the optical intensity of the optical signal in φ-OTDR is obtained as the function of the forward phase and birefringence distributions as well as the input state of polarization (SOP). Further theoretical predictions show that, in φ-OTDR, the common optical phase depends on only the local birefringence in the first half of the fiber section, which is occupied by the sensing pulse, irrelevant of the input SOP. However, the intensity of the φ-OTDR signal is not a local parameter, which depends on the input SOP and the birefringence distribution along the entire fiber section before the optical pulse. Moreover, the PDL measured in φ-OTDR is theoretically proven to be a local parameter, which is determined by the local birefringence and local optical phase distributions.

New Polarimetric Data for the Galilean Satellites: Io and Ganymede Observations and Modeling

New high-precision disk-integrated measurements of the polarization of Io and Ganymede in the UBVRI bands are presented. The observations were obtained using polarimeters mounted on the Crimean Astrophysical Observatory and the Peak Terskol Observatory in 2019–2023. For Io, the negative polarization branch (NPB) reaches a minimum of P min ≈ −0.25 ± 0.02% in the V band at a phase angle of α min = 2.°1 ± 0.°5. The inversion angle is α inv = 26° ± 6° in the V and R bands. The NPB for Ganymede is an asymmetric curve, with P min = −0.34 ± 0.01% at α min = 0.°52 ± 0.°06 and α inv = 8.°5 ± 0.°2 in the V band. Although Io and Europa have similar geometric albedos (0.63 and 0.67, respectively), their NPB shapes differ. The NPB of Ganymede (albedo of 0.43) is morphologically similar to that of Europa, although it is described by different parameter values (P min, α min, and α inv). This discrepancy is likely due to the compositions of their surfaces: Europa’s with H2O ice, Io’s with sulfuric/silicate composition, and Ganymede’s with H2O ice and silicates. Numerical computations using the radiative transfer coherent backscattering method demonstrated a match to the polarimetric observations and to the geometric albedos for Ganymede with the single-scattering albedo ≈ 0.943 and mean free path length kl = 2πl/λ eff ≈ 150, where λ eff is the wavelength. For Io’s regolith, the single-scattering albedo was found to be ≈ 0.979 and kl ≈ 40.

Possible zero-magnetic field fractional quantization in In0.75Ga0.25As heterostructures

In this Letter, we report a systematic study of a structure found in zero magnetic field at or near 0.2 ×(e2/h) in In0.75Ga0.25As heterostructures, where e is the fundamental unit of charge and h is Planck's constant. This structure has been observed in many samples and stays at near constant conductance despite a large range of external potential changes, the stability indicating a quantum state. We have also studied the structure in the presence of high in-plane magnetic fields and find an anisotropy which can be related to the Rashba spin–orbit interaction and agrees with a recent theory based on the formation of coherent back-scattering. A possible state with conductance at 0.25 ×(e2/h) has also been found. The quantum states described here will help with the fundamental understanding of low-dimensional electronic systems with strong spin–orbit coupling and may offer new perspectives for future applications in quantum information schemes.

Degree of linear polarization of light at backscattering by a single large particle of irregular shape.

Light backscattering by large randomly oriented particles of irregular shape is calculated using the physical optics approximation (PhOA). It is shown that the degree of linear polarization reveals the polarization surge because of the coherent backscattering. The sign of the polarization surge for pairs of conjugate beams can be either negative or positive depending on the shape of the photon trajectory that predominantly contributes to backscattering. Summing up the surges, we conclude that the conventional negative polarization phenomena are not obligatory in the case of a single backscattering particle.

Coherent Backscattering Peak for Radiation with Low Spatial Coherence

Coherent Backscattering Peak for Radiation with Low Spatial Coherence

Multifractal Conductance Fluctuations of Helical Edge States.

Two-dimensional topological insulators are characterized by the bulk gap and one-dimensional helical states running along the edges. The theory predicts the topological protection of the helical transport from coherent backscattering. However, the unexpected deviations of the conductance from the quantized value and localization of the helical modes are generally observed in long samples. Moreover, at millikelvin temperatures significant mesoscopic fluctuations are developed as a function of the electron energy. Here we report the results of an experimental study of the transport in a HgTe quantum well with an inverted energy spectrum that reveal a multifractality of the conductance fluctuations in the helical edge state dominated transport regime. We attribute observed multifractality to mesoscopic fluctuations of the electron wave function or local density of states at the spin quantum Hall transition. We have shown that the mesoscopic two-dimensional topological insulator provides a highly tunable experimental system in which to explore the physics of the Anderson transition between topological states.

Extension of Radiative Transfer Coherent Backscattering RT-CB code to dense discrete random media

The Radiative transfer coherent backscattering (RT-CB) code is extended to apply to dense discrete random media of optically soft spherical particles. This is achieved by utilizing the well-known static-structure-factor (SSF) correction derived from the Percus–Yevick approximation for sticky-hard-sphere (SHS) pair correlation function. The code is verified against the numerically exact electromagnetic method for small spherical media consisting of submicrometer-sized icy particles at optical wavelengths. The SSF-corrected RT-CB method significantly improves solution accuracy, and the solution agrees well with the numerically exact solution when the packing density is less than 20% and particles are optically soft, i.e., the refractive index of particles is close to that of the background medium.

Polarization Characteristics of the Coherent Backscattering Peak for Large Nonspherical Particles with Random Orientation in Space

Polarization Characteristics of the Coherent Backscattering Peak for Large Nonspherical Particles with Random Orientation in Space

The Phase Response of a Rough Rectangular Facet for Radar Sounder Simulations of Both Coherent and Incoherent Scattering

AbstractWith radar sounders, coherent backscattering simulations from global planetary digital elevation models (DEMs) typically display a deficit in diffuse clutter, which is mainly due to the implicit assumption that roughness at scales below the resolution of the DEM is absent. Indeed, while polynomial approximations of the phase evolution across the facet allow for fast and mathematically rigorous simulators, the coarse resolution of these planetary DEMs leads to a potentially significant portion of the backscattering response being neglected. In this paper, we derive the analytical phase response of a rough rectangular facet characterized by Gaussian roughness and a Gaussian isotropic correlation function under the linear phase approximation. Formulae for the coherent and incoherent power scattered by such an object are obtained for arbitrary bistatic scattering angles. Validation is done both in isolation and after inclusion in different Stratton‐Chu simulators. In order to illustrate the different uses of such a formulation, we reproduce two lunar radargrams acquired by the Lunar Radar Sounder instrument with a Stratton‐Chu simulator incorporating the proposed rough facet phase integral, and we show that the original radargrams are significantly better‐reproduced than with state‐of‐the‐art methods, at a similar computational cost. We also show how the rough facet integral formulation can be used in isolation to better characterize subglacial water bodies on Earth.

Main Mechanisms of Celestial Bodies Negative Polarization Formation: A Review

The scattered light by the vast majority of celestial bodies without atmospheres has a characteristic feature: A negative branch of linear polarization degree at little phase angles. Researchers have proposed many theoretical mechanisms to explain this feature. This review describes the main mechanisms that form a negative branch of linear polarization degree. The results of ground-based observations of the negative branch of the degree of linear polarization of various objects of the solar system are described. Scattering by single particles, shadow effect, coherent backscattering enhancement, and effects of the near field are considered. The review will be useful for all researchers studying the scattering of light by celestial bodies.

Coherent backscattering in the topological Hall effect

The mutual interplay between electron transport and magnetism has attracted considerable attention in recent years, primarily motivated by strategies to manipulate magnetic degrees of freedom electrically, such as spin–orbit torques and domain wall motion. Within this field the topological Hall effect, which originates from scalar spin chirality, is an example of inter-band quantum coherence induced by real-space inhomogeneous magnetic textures, and its magnitude depends on the winding number and chiral spin features that establish the total topological charge of the system. Remarkably, in the two decades since its discovery, there has been no research on the quantum correction to the topological Hall effect. Here we will show that, unlike the ordinary Hall effect, the inhomogeneous magnetization arising from the spin texture will give additional scattering terms in the kinetic equation, which result in a quantum correction to the topological Hall resistivity. We focus on two-dimensional systems, where weak localization is strongest, and determine the complicated gradient corrections to the Cooperon and kinetic equation. Whereas the weak localization correction to the topological Hall effect is not large in currently known materials, we show that it is experimentally observable in dilute magnetic semiconductors. Our theoretical results will stimulate experiments on the topological Hall effect and fill the theoretical knowledge gap on weak localization corrections to transverse transport.

Highly-twisted states of light from a high quality factor photonic crystal ring

Twisted light with orbital angular momentum (OAM) has been extensively studied for applications in quantum and classical communications, microscopy, and optical micromanipulation. Ejecting high angular momentum states of a whispering gallery mode (WGM) microresonator through a grating-assisted mechanism provides a scalable, chip-integrated solution for OAM generation. However, demonstrated OAM microresonators have exhibited a much lower quality factor (Q) than conventional WGM resonators (by >100×), and an understanding of the limits on Q has been lacking. This is crucial given the importance of Q in enhancing light-matter interactions. Moreover, though high-OAM states are often desirable, the limits on what is achievable in a microresonator are not well understood. Here, we provide insight on these two questions, through understanding OAM from the perspective of mode coupling in a photonic crystal ring and linking it to coherent backscattering between counter-propagating WGMs. In addition to demonstrating high-Q (105 to 106), a high estimated upper bound on OAM ejection efficiency (up to 90%), and high-OAM number (up to l = 60), our empirical model is supported by experiments and provides a quantitative explanation for the behavior of Q and the upper bound of OAM ejection efficiency with l. The state-of-the-art performance and understanding of microresonator OAM generation opens opportunities for OAM applications using chip-integrated technologies.

Coherent backscattering of entangled photon pairs

Correlations between entangled photons are a key ingredient for testing fundamental aspects of quantum mechanics and an invaluable resource for quantum technologies. However, scattering from a dynamic medium typically scrambles and averages out such correlations. Here we show that multiply scattered entangled photons reflected from a dynamic complex medium remain partially correlated. In experiments and full-wave simulations we observe enhanced correlations, within an angular range determined by the transport mean free path, which prevail over disorder averaging. Theoretical analysis reveals that this enhancement arises from the interference between scattering trajectories, in which the photons leave the sample and are then virtually reinjected back into it. These paths are the quantum counterpart of the paths that lead to the coherent backscattering of classical light. This work points to opportunities for entanglement transport despite dynamic multiple scattering in complex systems. Dynamic and disordered media destroy the correlations that underlie many quantum measurement protocols and applications. However, coherently backscattered photons can remain partially correlated due to interference between scattering trajectories.

Polarization properties of the coherent backscattering enhancement for large nonspherical particles with random orientation

Polarization properties of the coherent backscattering enhancement for large nonspherical particles with random orientation

Emergence of coherent backscattering from sparse and finite disordered media

Coherent backscattering (CBS) arises from complex interactions of a coherent beam with randomly positioned particles, which has been typically studied in media with large numbers of scatterers and high opacity. We develop a first-principles scattering model for scalar waves to study the CBS cone formation in finite-sized and sparse random media with specific geometries. The current study provides insights into the effects of density, volume size, and other relevant parameters on the angular characteristics of the CBS cone emerging from sparse and bounded random media for various types of illumination, with results consistent with well-known CBS studies which are typically based on samples with much larger number of scatterers and higher opacity. The enhancements are observed in scattering medium with dimensions between 10× and 40× wavelength and the number of particles as few as 370. This work also highlights some of the potentials and limitations of employing the CBS phenomenon to characterize disordered configurations. The method developed here provides a foundation for studies of complex electromagnetic fields beyond simple incident classical beams in randomized geometries, including structured wavefronts in illumination and quantized fields for investigating the effects of the quantum nature of light in multiple scattering, with no further numerical complications.