arXiv subjects

Ziqiang Cai

Publications and source records attributed to Ziqiang Cai.

17 recordsLinked to original sources

arXiv2026

Images of shadow and thin accretion disk around Bardeen black hole surrounded by perfect fluid dark matter

We investigate the shadow and optical appearance of Bardeen black hole (BH) immersed in perfect fluid dark matter (PFDM). Using EHT observations of M87* and Sgr A*, we constrain the DM parameter to a narrow range $b/M \sim \mathcal{O}(10^{-1}-10^{-2})$ for M87* and to $\mathcal{O}(10^{-2}-10^{-3})$ for Sgr A*. From these constraints we derive a rough prediction for the PFDM density near the shadow scale ($R_{\mathrm{sh}}\sim5M$): $ρ_{\mathrm{PFDM}} \sim 0.27$-$2.67\,\mathrm{g/cm^3}$ for Sgr A*, dropping to $\sim10^{-24}$-$10^{-25}\,\mathrm{g/cm^3}$ at 100 pc. Moreover, increasing $b$ substantially enlarges the photon sphere, impact parameter, shadow radius, and suppresses the observed disk brightness, while the magnetic charge $g$ produces only negligible corrections completely masked by PFDM on macroscopic scales. Subsequently, we investigate the primary/secondary images, flux, and redshift profiles for the PFDM-Bardeen BH using the Novikov-Thorne disk model, and compare these quantities with those of NFW, Dehnen-type and Moore DM BHs. The four BH types exhibit distinct densities at the shadow radius and at 100 pc, offering a potential distinguishing signature. Furthermore, for all DM BH models, blueshift appears in the primary image as inclination increases, while the secondary image remains redshift dominated even at high inclinations. Hence, if significant blueshifted emission were detected at low inclination, the predictions of these four DM models would be seriously challenged.

astro-ph.HE↗

arXiv2026

Shadow of rotating black hole surrounded by dark matter

Dark matter (DM), a fundamental cosmic component, motivates the study of its influence on black hole (BH) shadows, especially for spinning BHs confirmed by EHT observations. This work generalizes the Schwarzschild BH surrounded by DM to an axisymmetric Kerr BH using the Newman-Janis Algorithm (NJA), investigating the resulting event horizon and ergosphere structures. Employing null geodesics, we examine the effects of DM mass ($Δ$M) on BH shadow, including its radius, distortion, and the associated energy emission rate. Our analysis reveals that DM has a negligible effect below a critical mass, once this threshold is surpassed, all BH structures expand significantly. Furthermore, DM robustly contributes to the shadow maintaining a near circular shape, even for highly spinning BHs. This pronounced structural expansion under high DM mass may potentially exceed current observational constraints, suggesting that DM must either be absent in the immediate vicinity of the BH or its localized mass must remain below this critical value to be consistent with astrophysical observations.

gr-qc↗

arXiv2026

Periodic orbits and gravitational waveforms around a Schwarzschild black hole with a cloud of strings embedded in perfect fluid dark matter

In this study, we explore the dynamics of particle orbits and their corresponding gravitational wave signatures in the vicinity of a Schwarzschild black hole (BH) surrounded by a cloud of strings and embedded in a perfect fluid dark matter medium. The model is characterized by two parameters: $a$, associated with the string cloud, and $α$, representing the dark matter distribution. We systematically analyze how the marginally bound orbit (MBO) and the innermost stable circular orbit (ISCO) depend on these parameters. Our findings reveal that while both the orbital radius and angular momentum increase with increasing $a$, they decrease as $α$ increases; notably, the energy exhibits the opposite trend, decreasing with $a$ and increasing with $α$. Furthermore, we examine periodic orbits indexed by rational numbers $q$ and the gravitational waveforms they generate. The results demonstrate that an increase in the string cloud parameter $a$ induces a significant phase delay in the waveform. Specifically, waveforms with lower values of $a$ oscillate over shorter time intervals, whereas those with higher values extend to longer time scales. These distinct features, including noticeable differences in amplitude, allow the waveforms to be clearly distinguished from those in a pure Schwarzschild spacetime.

gr-qc↗

arXiv2026

Connection Between the Shadow Radius and Quasinormal Frequencies for Black Holes in STVG with Perfect Fluid Dark Matter

We investigate the connection between black hole shadow and quasinormal mode (QNM) spectra in the context of scalar-tensor-vector gravity (STVG) coupled to perfect fluid dark matter (PFDM), characterized by the MOG parameter $α$ and the dark matter intensity $λ$. Employing complementary methods -- namely the sixth-order WKB approximation, Padé resummation, and time-domain numerical integration -- we compute QNM frequencies for scalar ($s=0$), electromagnetic ($s=1$), and axial gravitational ($s=2$) perturbations. Both the real part of the QNM frequencies and the peak height of the effective potential display a consistent parametric dependence: they increase with $λ$ yet decrease with growing $α$. In the eikonal limit ($l \gg 1$), we derive an exact analytical link between the shadow radius $R_{\mathrm{sh}}$ and the QNM frequency $ω_R$. Noting that $R_{\mathrm{sh}}$ is determined by the critical impact parameter $b_c = r_{\mathrm{ph}}/\sqrt{f(r_{\mathrm{ph}})}$, while $ω_R = Ωl$ with photon angular velocity $Ω= \sqrt{f(r_{\mathrm{ph}})}/r_{\mathrm{ph}}$, we obtain the precise relation $ω_R = l / b_c$, identifying $R_{\mathrm{sh}} \equiv b_c$ for an asymptotically flat observer. This prediction is robustly validated by numerical results across all three computational approaches at large multipole numbers. Our findings reveal that the black hole shadow and gravitational ringdown are not independent phenomena, but dual observational signatures of the same underlying structure -- the unstable photon orbit -- thereby offering a unified multi-messenger framework to simultaneously constrain modified gravity and dark matter in the strong-field regime.

gr-qc↗

arXiv2026

The perturbation solutions to the Blandford-Znajek mechanism in the Kerr-Sen black hole

We investigate the steady, axisymmetric, force-free magnetosphere of Kerr-Sen black hole (BH) within the framework of the Einstein-Maxwell-dilaton-axion (EMDA) theory. By perturbatively solving the nonlinear Grad-Shafranov (GS) equation, we determine the magnetic field configuration and quantify the influence of the dilaton parameter $r_2$ on the energy extraction rate and radiative efficiency. Our results show that both the energy extraction power and the radiative efficiency increase with $r_2$, exceeding those of the standard Kerr BH, whereas the extraction efficiency remain consistent with the Kerr case. In addition, we perform $χ^2$ statistical analysis using observational data from six binary BH systems, which indicates that the Kerr BH currently provides a better fit for bulk Lorentz factors $Γ= 2$ and $5$.

gr-qc↗

arXiv2025

Shadow and thin accretion disk around Ayón-Beato-García black hole coupled with cloud of strings

In this paper, we investigate the shadow and thin accretion disk around Ayón-Beato-García (ABG) black hole (BH) coupled with a cloud of strings (CS), characterized by the nonlinear electrodynamics (NLED) parameter $g$, and the CS parameter $a$. By comparing shadow diameters with Event Horizon Telescope (EHT) observations of M87$^{*}$ and Sgr A$^*$, we have established constraints on the BH parameters $g$ and $a$. Additionally, we analyze the BH shadow, lensing ring, and photon ring features for the ABG BH coupled with CS. Our results indicate that the shadow radius increases monotonically with the CS parameter $a$, while it decreases with increasing $g$. Finally, the study explores the physical properties and observational signatures of thin accretion disks around ABG BH with CS. The results show that an increase in parameter $g$ leads to a hotter and more luminous disk, while an increase in parameter $a$ results in a cooler and less luminous disk.

gr-qc↗

arXiv2025

Polarization-sensitive GeSn Mid-Infrared Membrane Photodetectors with Integrated Plasmonic Metasurface

Germanium-Tin (GeSn) semiconductors are promising for mid-infrared optoelectronics owing to their silicon compatibility, tunable bandgap, and potential for room-temperature operation. Released GeSn membranes provide an additional degree of freedom to extend the operation wavelength through epitaxial strain relaxation, while their transferability expands design flexibility. On the other hand, metasurfaces have become an effective strategy to engineer light--matter interaction, and their integration with photodetectors can enhance performance and introduce new functionalities. Here, we demonstrate a mid-infrared photodetector consisting of a transfer-printed Ge$_{0.89}$Sn$_{0.11}$ membrane integrated with an Au plasmonic metasurface. The photodetector exhibits a wavelength cutoff exceeding 3.0~$μ$m with nearly fourfold increase in responsivity at 2.5~$μ$m as compared to unreleased films, attributed to Fabry--Pérot resonance. Furthermore, the integration with an anisotropic metasurface yields detectors with strong polarization sensitivity, achieving a measured contrast ratio of $\sim$4:1 between orthogonal polarizations. Moreover, the operation wavelength of the photodetector can be selectively tuned by varying the geometric scale of the metasurface. The experimental results show excellent agreement with simulations, confirming the effectiveness and versatility of this integrated metasurface--membrane design.

physics.app-ph↗

arXiv2025

Observational properties and quasinormal Modes of the Hayward black Hole surrounded by a cloud of strings

In this work, we explored the Hayward black hole surrounded by a cloud of strings, with a focus on the effects of the regularization parameter $l$ and the string cloud parameter $a$ on its observational properties and quasinormal modes (QNMs). Utilizing the spacetime metric and geodesic equations, we calculated several geometric quantities characterizing the black hole. To visualize the observational appearance of the accretion disk, we employed the Novikov--Thorne model to simulate both its primary and secondary images. Furthermore, we analyzed the QNMs of the black hole under scalar and electromagnetic perturbations for different parameter values. The results indicate that as the regularization parameter $l$ increases, the outer horizon radius $r_{+}$, photon-sphere radius $r_{\text{ph}}$, critical impact parameter $b_{c}$, and innermost stable circular orbit $r_{\text{isco}}$ exhibit a gradual decrease, while the inner horizon radius $r_{-}$ and the real part of the QNMs frequency $ω_{r}$ increase. In contrast, as the string cloud parameter $a$ increases, $r_{+}$, $r_{\text{ph}}$, $b_{c}$, and $r_{\text{ isco}}$ demonstrate a rapid increase, whereas $r_{-}$ and $ω_{r}$ decrease. In both cases, the absolute value of the imaginary part of the QNMs frequency decreases with the increase $l$ or $a$. This work offers a theoretical foundation for understanding the coupling between regular black holes and surrounding string clouds.

gr-qc↗

arXiv2025

Rotating Charged Black Holes with Scalar Hair Constructed via the Newman-Janis Algorithm: Accretion Disk Structure and Shadow Characteristics

In this paper, we generate a rotating charged black hole (BH) with scalar hair via the Newman--Janis algorithm (NJA) and study its thin accretion disk and shadow. The structure of the event horizon and ergosurface is analyzed in detail, revealing how the charge parameter $Q$ and scalar hair parameter $s$ influence the spacetime geometry. We analyze the energy flux and temperature distribution of the accretion disk, finding that increasing either $Q$ or $s$ leads to higher energy flux and peak temperature. The BH shadow is also examined, showing that its apparent size decreases monotonically with increasing $Q$ or $s$. Notably, in the near-extremal regime, the shadow develops a distinctive cuspy edge, indicative of strong light bending in the scalarized and charged spacetime. By comparing the theoretically predicted shadow diameter with Event Horizon Telescope (EHT) observations of Sgr A$^*$, we derive observational constraints on the model parameters. For inclination angles of $17^\circ$ and $90^\circ$, a joint analysis constrains the charge parameter to $0<Q<0.522745$ (at fixed $s=0.1$) and the scalar hair parameter to $0<s<0.283373$ (at fixed $Q=0.3$). Our results demonstrate how scalar hair and electric charge leave imprints on accretion disk emissions and black hole shadows, offering new observational signatures for testing gravity theories beyond general relativity.

gr-qc↗

arXiv2025

Thin accretion disk around Schwarzschild-like black hole in bumblebee gravity

The physical properties and optical appearance of a thin accretion disk surrounding a Schwarzschild-like black hole (BH) are investigated within the framework of bumblebee gravity. To understand how the Lorentz symmetry breaking (LSB) parameter $l$ affects the disk's behavior, we analyze main characteristics such as energy flux, temperature distribution, and emission spectrum. In addition, direct and secondary images of the accretion disk are generated and examined to explore how both the observational inclination angle and the LSB parameter $l$ shape the visual profile. Furthermore, we compute the redshift and observed flux distributions of the disk from the perspective of distant observers at various inclination angles. Our results indicate that the redshift factor grows as $l$ decreases. When the parameter $l$ assumes negative values, the BH exhibits enhanced luminosity with decreasing $l$. These findings highlight the crucial influence of the LSB parameter $l$ on the observable features of BHs.

gr-qc↗

arXiv2025

Probing Einstein-Maxwell-Scalar Black hole via Thin Accretion Disks and Shadows with EHT Observations of M87* and Sgr A*

We investigated the shadows and thin accretion disks of Einstein-Maxwell-Scalar (EMS) black hole. Firstly, we investigated the influence of EMS parameters on the black hole shadow using the null geodesic method and constrained these parameters based on EHT observations of M87* and Sgr A*. Furthermore, we analyzed the direct emission, lensing ring, and photon ring structures in EMS black hole. Comparing our results with the Schwarzschild and Reissner-Nordstr$\ddot{\mathrm{o}}$m (RN) black holes, we found that the Schwarzschild black hole exhibits the largest shadow radius and the highest observed intensity.

gr-qc↗

arXiv2025

Optical Propulsion and Levitation of Metajets

The quintessential hallmark distinguishing metasurfaces from traditional optical components is the engineering of subwavelength meta-atoms to manipulate light at will. Enabling this freedom, in a reverse manner, to control objects constituted by metasurfaces could expand our capability of optical manipulation to go beyond the predominant microscopic and sub-microscopic scales. Here, we introduce a driving metaphotonic force fully controllable by meta-atoms to manipulate structured objects named metajets. Upon Newton's law of motion that can apply to classical and relativistic mechanics, we develop a first-principles theory to analyze optical forces generated by refraction and reflection at an interface. We find that three-dimensional motions of metajets would be possible if one could introduce an extra wavevector component. We achieve that by creating a spatially distributed phase gradient with deliberately arranged silicon nanopillars. Our experiments and simulations reveal an in-plane propulsion and, very importantly, out-of-plane levitation of the metajets, aligning well with the theory. We also find that the metaphotonic force augments with increased light power but is not limited by the size of metajets, which could unleash new opportunities for metaphotonic control in large settings, such as interstellar light sails.

physics.optics↗

arXiv2024

Electrically Tuning Quasi-Bound States in the Continuum with Hybrid Graphene-Silicon Metasurfaces

Metasurfaces have become one of the most prominent research topics in the field of optics owing to their unprecedented properties and novel applications on an ultrathin platform. By combining graphene with metasurfaces, electrical tunable functions can be achieved with fast tuning speed, large modulation depth and broad tuning range. However, the tuning efficiency of hybrid graphene metasurfaces within the short-wavelength infrared (SWIR) spectrum is typically low because of the small resonance wavelength shift in this wavelength range. In this work, through the integration of graphene and silicon metasurfaces that support quasi-bound states in the continuum (quasi-BIC), we experimentally demonstrate significant transmittance tuning even with less than 30 nm resonance wavelength shift thanks to the high quality-factor of quasi-BIC metasurfaces. The tunable transmittance spectrum was measured using Fourier Transform Infrared Spectroscopy (FTIR) with a modified reflective lens to improve the accuracy, and the electrical tuning was realized utilizing the cut-and-stick method of ion gel. At the wavelength of 3.0 um, the measured change of transmittance T_max-T_min and modulation depth (T_max-T_min)/T_max can reach 22.2% and 28.9%, respectively, under a small bias voltage ranging from -2 V to +2 V. To the best of our knowledge, this work is the first experimental demonstration of tunable graphene/quasi-BIC metasurfaces, which have potential applications in optical modulation, reconfigurable photonic devices, and optical communications.

physics.optics↗

arXiv2024

Polarization-Encoded Lenticular Nano-Printing with Single-Layer Metasurfaces

Metasurface-based nano-printing has enabled ultrahigh-resolution grayscale or color image display. However, the maximum number of independent nano-printing images allowed by one single-layer metasurface is still limited despite many multiplexing methods that have been proposed to increase the design degree of freedom. In this work, we substantially push the multiplexing limit of nano-printing by transforming images at different observation angles into mapping the corresponding images to different positions in the Fourier space, and simultaneously controlling the complex electric field across multiple polarization channels. Our proposed Polarization-Encoded Lenticular Nano-Printing (Pollen), aided by a modified evolutionary algorithm, allows the display of several images based on the viewing angle, similar to traditional lenticular printing but without requiring a lenticular layer. In addition, it extends the display capability to encompass multiple polarization states. Empowered by the ability to control the complex amplitude of three polarization channels, we numerically and experimentally demonstrate the generation of 13 distinguished gray-scale Chinese ink wash painting images, 49 binary patterns, and three sets of 3D nano-printing images, totaling 25 unique visuals. These results present the largest number of recorded images with ultra-high resolution to date. Our innovative Pollen technique is expected to benefit the development of modern optical applications, including but not limited to optical encryption, optical data storage, lightweight display, and augmented reality and virtual reality.

physics.optics↗

arXiv2023

Types of the geodesic motions in Kerr-Sen-AdS$_{4}$ spacetime

We consider the geodesic motions in the Kerr-Sen-AdS$_4$ spacetime. We obtain the equations of motion for light rays and test particles. Using the parametric diagrams, we shown some regions where the radial and latitudinal geodesic motions are allowed. We analyse the impact of parameter related to dilatonic scalar on the orbit and find that it will result in more rich and complex orbital types.

gr-qc↗

arXiv2022

Thin accretion disks around a black hole in Einstein-Aether-scalar theory

We consider the accretion process in the thin disk around a supermassive black hole in Einstein-aether-scalar theory. We probe the effects of the model parameter on the physical properties of the disk. The results show that with increasing value of the parameter, the energy flux, the radiation temperature, the spectra cut-off frequency, the spectra luminosity, and the conversion efficiency of the disk decrease. The disk is hotter and more luminous than that in general relativity for negative parameter, while it is cooler and less luminous for positive parameter. We also find some values of the parameter allowed by the theory are excluded by the physical properties of the disk.

gr-qc↗

arXiv2022

Accretion of the Vlasov gas onto a Schwarzschild-like black hole

We consider spherical steady accretion of the relativistic Vlasov gas onto a Schwarzschild-like black hole. We determine the expressions for the particle current density and accretion rate and present the limiting expressions for the mass accretion rate at high and low temperature. The results show that the parameter characterizing the breaking of Lorentz symmetry can affects the radial component of the particle current density and the mass accretion rate.

gr-qc↗