arXiv subjects

Mustafa Sarisaman

Publications and source records attributed to Mustafa Sarisaman.

At least 19 recordsLinked to original sources

arXiv2026

Etendue and Radiance Conservation in Transformation Optics: Strict Analytical Bounds on Field Enhancement

We establish an intuitive connection between transformation optics (TO) and the classical invariants of etendue and radiance that hasn't been made before. Through explicit application of the optical metric formulation of TO, we demonstrate that any smooth, passive, impedance-matched transformation performs as a canonical (symplectic) mapping on optical phase space. In combination with Hamiltonian ray dynamics, this indicates that Liouville theorem is as well applicable to TO media: the phase-space volume element is preserved, radiance remains constant along rays, and etendue won't decrease under any passive TO mapping. Based on this novel notion, which has never been mentioned in TO literature before, we develop a radiance-invariant phase-space measure specific to TO media. We then utilize it to find severe analytical limitations on field enhancement. We demonstrate that the maximum attainable average intensity in any passive TO concentrator is exclusively constrained by the area-compression ratio of the coordinate mapping, irrespective of material implementation. Using the same paradigm on zero-index media, optical-null media, and illusion devices reveals the same rules. Consequently, our findings demonstrate that transformation optics (TO) can redistribute intensity but cannot increase radiance, and they give a Liouville-type theorem within this field. This provides a consistent, metric-based elucidation for the intrinsic limitations of concentration in passive metamaterials and extreme-index platforms.

physics.optics↗

arXiv2026

Exploring Spectral Singularities in Dirac Semimetals: The Role of Non-Hermitian Physics and Dichroism

In this study, motivated by recent advancements in non-Hermitian physics, we explore new characteristics of Dirac semimetals (DSMs) using the spectral singularities by means of scattering techniques, with the goal of uncovering additional unique properties. To achieve this, we investigate how the axion texture of a DSM affects its topological properties by analyzing its interaction with electromagnetic waves. We examine the transverse electric (TE) mode configuration, where the magneto-electric effect induces a dichroic property in these materials. This behavior is particularly interesting and commonly seen in potential DSM candidates. Consequently, we report for the first time that a dichroic DSM generates 12 unique topological laser types. We discover that surface currents are generated by topological terms on the surface of the DSM slab. Furthermore, we examine how the θ term associated with axions in topological materials contributes to these topological properties. Our study reveals distinct topological role of the θ term more clearly than ever before. Our results confirm that the topological properties of DSMs with a single Dirac cone remain stable under external influences and that a topologically robust DSM laser can be developed accordingly

cond-mat.mes-hall↗

arXiv2025

Exploring Spectral Singularities and Topological Lasers in PT-Symmetric Weyl Semimetals

This paper investigates the unique properties of PT-symmetric Topological Weyl Semimetals (TWS) within the framework of non-Hermitian physics, focusing on their potential for generating topological lasers. By exploring the role of spectral singularities and their relationship to exceptional points, we examine how these materials, characterized by Weyl nodes and topologically protected surface states, can support novel optical phenomena such as unidirectional propagation and enhanced lasing. Through a theoretical model based on the transfer matrix approach, we reveal how the interplay between the PT symmetry and the axion term introduces new dynamics, leading to 12 distinct topological laser configurations. The study also investigates the impact of the $θ$-term on spectral singularities, showing how it quantizes the system's gain values and influences the topological properties of the lasers. By applying our model to the TaAs material, a known Weyl semimetal, we uncover previously unreported effects, demonstrating the potential of PT-symmetric TWS materials for advanced optoelectronic applications. We show that the axion-induced cyclotron-like Hall current in a PT-symmetric TWS medium, revealing its topological characteristics and distinct flow patterns in the gain and loss regions, which serve as indicators of the system's topological symmetry. Our findings open new avenues for the development of robust, tunable, and efficient topological lasers with applications in quantum information processing and beyond.

physics.optics↗

arXiv2023

Tilt induced vortical response and mixed anomaly in inhomogeneous Weyl matter

We propose a non-dissipative transport effect and vortical response in Weyl semimetals in the presence of spatial inhomogeneities, namely a spatially varying tilt of the Weyl cones. We show that when the spectrum is anisotropic and tilted due to spatial lattice variations, one is confronted with generalized quantum anomalies due to the effective fields stemming from the tilt structure. In particular, we demonstrate that the position-dependent tilt parameter induces local vorticity, thus generating a chiral vortical effect even in the absence of rotation or magnetic fields. As a consequence, it couples to the electric field and thus contributes to the anomalous Hall effect.

cond-mat.mes-hall↗

arXiv2022

Charge-pseudospin coupled diffusion in semi-Dirac graphene: pseudospin assisted valley transport

Modifying the hexagonal lattices of graphene enables the repositioning and merging of the Dirac cones which proves to be a key element in the use of these materials for alternative electronic applications such as valleytronics. Here we study the nonequilibrium transport of carriers within a system containing two Dirac cones in both standard graphene and semi-Dirac graphene. In the latter, the lattice modifications cause the relativistic and parabolic dispersion bands to coexist, furnishing the Fermi surface with a rich pseudospin texture and a versatile Dirac cones separation. We construct a kinetic theory to investigate the carrier diffusion and uncover that the pseudospin index contributes to the particle current and, like the real spin, can induce a magnetoelectric effect, and argue that the pseudospin-charge coupling can be utilized to design a pseudospin filter. We explore the charge dynamics inside a quasi-one-dimensional conductor using the drift-diffusion model and detect the pseudospin accumulation at the sample boundaries. We find that, while, for graphene, the accumulation contributes to an extra voltage drop between the sample interfaces, the semi-Dirac system presents a similar accumulation that is strikingly equipped with valley polarization, signifying an essential tool for the control of valley manipulation and chirality transport using the pseudospin.

cond-mat.mes-hall↗

arXiv2020

Spectral singularities and tunable slab lasers with 2D material coating

We investigate linear and nonlinear spectral singularities in the transverse electric and transverse magnetic modes of a slab laser consisting of an active planar slab sandwiched between a pair of Graphene or Weyl semimetal thin sheets. The requirement of the presence of linear spectral singularities gives the laser threshold condition while the existence of nonlinear spectral singularities due to an induced weak Kerr nonlinearity allows for computing the laser output intensity in the vicinity of the threshold. The presence of the Graphene and Weyl semimetal sheets introduces additional physical parameters that we can use to tune the output intensity of the laser. We provide a comprehensive study of this phenomenon and report peculiarities of lasing in the TM modes of the slab with Weyl semimetal coatings. In particular, we reveal the existence of a critical angle such that no lasing seems possible for TM modes of the slab with smaller emission angle. Our results suggest that for TM modes with emission angle slightly exceeding the critical angle, the laser output intensity becomes highly sensitive to the physical parameters of the coating.

physics.optics↗

arXiv2020

Lasing with Topological Weyl Semimetal

Lasing behavior of optically active planar topological Weyl semimetal (TWS) is investigated in view of the Kerr and Faraday rotations. Robust topological character of TWS is revealed by the presence of Weyl nodes and relevant surface conductivities. We focus our attention on the surfaces where no Fermi arcs are formed, and thus Maxwell equations contain topological terms. We explicitly demonstrate that two distinct lasing modes arise because of the presence of effective refractive indices which lead to the birefringence phenomena. Transfer matrix is constructed in such a way that reflection and transmission amplitudes involve $2\times2$ matrix-valued components describing the bimodal character of the TWS laser. We provide associated parameters of the topological laser system yielding the optimal impacts. We reveal that gain values corresponding to the lasing threshold display a quantized behavior, which occurs due to topological character of the system. Our proposal is supported by the corresponding graphical demonstrations. Our observations and predictions suggest a concrete way of forming TWS laser and coherent perfect absorber; and are awaited to be confirmed by an experimental realization based on our computations.

physics.optics↗

arXiv2019

Broadband and Wide-Angle Invisibility with PT-Symmetric 2D-Weyl Semimetal

Inspired by the magnificent features of two-dimensional (2D) materials which aroused much of the interest in recent materials science research, we study PT-symmetric 2D Weyl semimetal (WSM) to reveal the broadband and wide-angle invisible configurations in a PT-symmetric optical slab system. Desired unidirectional reflectionlessness and invisibility phenomena is obtained by the optimal control of system parameters. We unravel the mystery of broadband and wide-angle invisibility in regular slab materials with finite refractive indices by means of the plenary expressions. We show that materials whose refractive indices relatively small (usually around $η=1$) give rise to quite a lot broadband and wide-angle (almost all incidence angles) invisible configurations. This is not observed with any 2D material other than 2D WSMs. Our findings suggest a concrete expedience to experimental realizations in this direction.

physics.optics↗

arXiv2019

Broadband Coherent Perfect Absorber with PT-Symmetric 2D-Materials

We suggest graphene and a two-dimensional (2D) Weyl semimetal (WSM) as 2D materials for the realization of a broadband coherent perfect absorber (CPA) respecting overall PT-symmetry. We also demonstrate the conditions for mutually equal amplitudes and phases of the left and right incoming waves to realize a CPA. 2D materials in our system play the role to enhance the absorption rate of a CPA once the appropriate parameters are inserted in the system. We show that a 2D WSM is more effective than graphene in obtaining the optimal conditions. We display the behavior of each parameter governing the optical system and show that optimal conditions of these parameters give rise to enhancement and possible experimental realization of a broadband CPA-laser.

physics.optics↗

arXiv2018

$\mathcal{P}\mathcal{T}$-Symmetric Coherent Perfect Absorber with Graphene

We investigate $\mathcal{PT}$-symmetric coherent perfect absorbers (CPAs) in the TE mode solution of a linear homogeneous optical system surrounded by graphene sheets. It is revealed that presence of graphene sheets contributes the enhancement of absorption in a coherent perfect absorber. We derive exact analytic expressions, and work through their possible impacts on lasing threshold and CPA conditions. We point out roles of each parameter governing optical system with graphene and show that optimal conditions of these parameters give rise to enhancement and possible experimental realization of a CPA laser. Presence of graphene leads the required gain amount to reduce considerably based on its chemical potential and temperature. We obtain that relation between system parameters decides the measure of CPA condition. We find out that graphene features contributing to resonance effect in graphene sheets are rather preferable to build a better coherent perfect absorber.

physics.optics↗

arXiv2018

Unidirectional Invisibility and PT-Symmetry with Graphene

We investigate the reflectionlessness and invisibility properties in the transverse electric (TE) mode solution of a linear homogeneous optical system which comprises the $\mathcal{PT}$-symmetric structures covered by graphene sheets. We derive analytic expressions, indicate roles of each parameter governing optical system with graphene and justify that optimal conditions of these parameters give rise to broadband and wide angle invisibility. Presence of graphene turns out to shift the invisible wavelength range and to reduce the required gain amount considerably, based on its chemical potential and temperature. We substantiate that our results yield broadband reflectionless and invisible configurations for realistic materials of small refractive indices, usually around $η= 1$, and of small thickness sizes with graphene sheets of rather small temperatures and chemical potentials. Finally, we demonstrate that pure $\mathcal{PT}$-symmetric graphene yields invisibility at small temperatures and chemical potentials.

physics.optics↗

arXiv2017

Nonlinear Spectral Singularities and Laser Output Intensity

The mathematical notion of spectral singularity admits a description in terms of purely outgoing solutions of a corresponding linear wave equation. This leads to a nonlinear generalization of this notion for nonlinearities that are confined in space. We examine the nonlinear spectral singularities in arbitrary TE and TM modes of a mirrorless slab laser that involves a weak Kerr nonlinearity. This provides a computational scheme for the determination of the laser output intensity $I$ for these modes. In particular, we offer an essentially mathematical derivation of the linear-dependence of $I$ on the gain coefficient $g$ and obtain an explicit analytic expression for its slope. This shows that if the real part $η$ of the refractive index of the slab does not exceed 3, there is a lower bound on $θ$ below which lasing in both its TE and TM modes requires $η$ to be shifted by a small amount as $g$ surpasses the threshold gain. Our results suggest that lasing in the oblique TM modes of the slab is forbidden if the incidence (emission) angle of the TM mode exceeds the Brewster's angle.

physics.optics↗

arXiv2016

Unidirectional reflectionlessness and invisibility in the TE and TM modes of a PT-symmetric slab system

Unidirectional invisibility of a PT-symmetric optical system is of great interest, but challenging as well since it is infeasible to fulfill it through wide optical frequency ranges in all angular directions. Accordingly we study reflectionless and invisible patterns in the TE and TM modes of an optical slab system consisting of adjacent or separated pair of balanced gain and loss layers with a gap. We provide a comprehensive study of one of the simplest experimentally accessible examples of a unidirectionally reflectionless and invisible PT-symmetric optical slab system. We obtain the physically optimal conditions for the realization of these phenomena. We derive analytic expressions, and show that only certain gain amounts restricted to take values between certain minimum and maximum values give rise to uni/bi-directionally invisible configurations. The size of gap decides the measure of reflectionlessness and invisibility parameters, especially on gain value and incident angle.

physics.optics↗

arXiv2016

Spectral Singularities in the TE and TM modes of a PT-Symmetric Slab System: Optimal conditions for realizing a CPA-Laser

Among the interesting outcomes of the study of the physical applications of spectral singularities in PT-symmetric optical systems is the discovery of CPA-lasers. These are devices that act both as a threshold laser and a coherent perfect absorber (CPA) for the same values of their physical parameters. Unlike a homogeneous slab that is made to act as a CPA, a slab CPA-laser would absorb the incident waves coming from the left and right of the device provided that they have appropriate intensity and phase contrasts. We provide a comprehensive study of one of the simplest experimentally accessible examples of a CPA-laser, namely a PT-symmetric optical slab system consisting of a balanced pair of adjacent or separated gain and loss components. In particular, we give a closed form expression describing the spectral singularities of the system which correspond to its CPA-laser configurations. We determine the intensity and phase contrasts for the TE and TM waves that are emitted (absorbed) whenever the slab acts as a laser (CPA). We also investigate the behavior of the time-averaged energy density and Poynting vector for these waves. This is necessary for determining the optimal values of the physical parameters of the system that make it act as a CPA-laser. These turn out to correspond to situations where the separation distance $s$ between the gain and loss layers is an odd multiple of a characteristic length scale $s_0$. A curious by-product of our study is that, except for the cases where $s$ is an even integer multiple of $s_0$, there is a critical angle of polarization beyond which the energy of the waves emitted from the lossy layer can be larger than the energy of those emitted from the gain layer.

physics.optics↗

arXiv2015

Lasing Threshold Condition for Oblique TE and TM Modes, Spectral Singularities, and Coherent Perfect Absorption

We study spectral singularities and their application in determining the threshold gain coefficient $g^{(E/M)}$ for oblique transverse electric/magnetic (TE/TM) modes of an infinite planar slab of homogenous optically active material. We show that $g^{(E)}$ is a monotonically decreasing function of the incidence angle $θ$ (measured with respect to the normal direction to the slab), while $g^{(M)}$ has a single maximum, $θ_c$, where it takes an extremely large value. We identify $θ_c$ with the Brewster's angle and show that $g^{(E)}$ and $g^{(M)}$ coincide for $θ=0$ (normal incidence), tend to zero as $θ\to 90^\circ$, and satisfy $g^{(E)} θ_c$ is smaller inside the gain region than outside it. The converse is true for the TM waves with $θ<θ_c$ and all TE waves.

quant-ph↗

arXiv2013

Spectral Singularities in the Surface Modes of a Spherical Gain Medium

We study the surface modes of a homogeneous spherical gain medium and provide a comprehensive analytic treatment of a special class of these modes that support spectral singularities. Because the latter have a divergent quality factor, we call them the singular gallery modes. We show that they can be excited using arbitrarily small amounts of gain, and as a result, the system lacks a lasing threshold, effectively. This shows that we can realize spectral singularities in the surface modes of extremely small spherical samples with modest amounts of gain. We also examine the possibility of exciting singular gallery modes with different wavelengths using the same amount of gain. This corresponds to the situation where the system undergoes simultaneous lasing at different wavelengths.

quant-ph↗

arXiv2013

Spectral Singularities and Whispering Gallery Modes of a Cylindrical Gain Medium

Complex scattering potentials can admit scattering states that behave exactly like a zero-width resonance. Their energy is what mathematicians call a spectral singularity. This phenomenon admits optical realizations in the form of lasing at the threshold gain, and its time-reversal is responsible for antilasing. We study spectral singularities and whispering gallery modes (WGMs) of a cylindrical gain medium. In particular, we introduce a new class of WGMs that support a spectral singularity and, as a result, have a divergent quality factor. These singular gallery modes (SGMs) are excited only if the system has a positive gain coefficient, but typically the required gain is extremely small. More importantly given any amount of gain, there are SGMs requiring smaller gain than this amount. This means that, in principle, the system lacks a lasing threshold. Furthermore, the abundance of these modes allows for configurations where a particular value of the gain coefficient yields an effective excitation of two distant SGMs. This induces lasing at two different wavelengths.

quant-ph↗

arXiv2012

Pseudoduality and Complex Geometry in Sigma Models

We study the pseudoduality transformations in two dimensional N = (2, 2) sigma models on Kähler manifolds. We show that structures on the target space can be transformed into the pseudodual manifolds by means of (anti)holomorphic preserving mapping. This map requires that torsions related to individual spaces and riemann connection on pseudodual manifold must vanish. We also consider holomorphic isometries which puts additional constraints on the pseudoduality.

hep-th↗