Polarization Symmetry Breaking Research Articles

Strong spinning thermal radiation enabled by germanium-based chiral dielectric metasurface

Spinning thermal radiation refers to the phenomenon of selective emission of circularly polarized waves from chiral structures with polarization dependence or symmetry breaking. This phenomenon finds diverse applications in fields such as radiation detection and chiral sensing. In this study, we introduce a dielectric metasurface composed of a periodic arrangement of germanium (Ge) elliptical disks, which can exhibit circular dichroism (CD) with a maximum value of approximately 0.93 at the optimal structural parameters. The physical mechanism of the strong CD is analyzed through the polarization conversion and distributions of the electric field. Moreover, the influence of structural parameters on the spinning thermal radiation is also analyzed. It is found that the CD is closely related to the height and period of the Ge-based chiral dielectric metasurface rather than the rotation angle. This work not only provides valuable insights into the design and optimization of spinning thermal radiation using metasurfaces, but also holds promise for its engineering applications in the field of thermal detection.

Surface alignment of ferroelectric nematic liquid crystals.

The success of nematic liquid crystals in displays and optical applications is due to the combination of their optical uniaxiality, fluidity, elasticity, responsiveness to electric fields and controllable coupling of the molecular orientation at the interface with solid surfaces. The discovery of a polar nematic phase opens new possibilities for liquid crystal-based applications, but also requires a new study of how this phase couples with surfaces. Here we explore the surface alignment of the ferroelectric nematic phase by testing different rubbed and unrubbed substrates that differ in coupling strength and anchoring orientation and find a variety of behaviors - in terms of nematic orientation, topological defects and electric field response - that are specific to the ferroelectric nematic phase and can be understood as a consequence of the polar symmetry breaking. In particular, we show that by using rubbed polymer surfaces it is easy to produce cells with a planar polar preferential alignment and that cell electrostatics (e.g. grounding the electrodes) has a remarkable effect on the overall homogeneity of the ferroelectric ordering.

Asymmetric balance in symmetry breaking

Spontaneous symmetry breaking is central to our understanding of physics and explains many natural phenomena, from cosmic scales to subatomic particles. Its use for applications requires devices with a high level of symmetry, but engineered systems are always imperfect. Surprisingly, the impact of such imperfections has barely been studied, and restricted to a single asymmetry. Here, we experimentally study spontaneous symmetry breaking with two controllable asymmetries. We remarkably find that features typical of spontaneous symmetry breaking, while destroyed by one asymmetry, can be restored by introducing a second asymmetry. In essence, asymmetries are found to balance each other. Our study illustrates aspects of the universal unfolding of the pitchfork bifurcation, and provides new insights into a key fundamental process. It also has practical implications, showing that asymmetry can be exploited as an additional degree of freedom. In particular, it would enable sensors based on symmetry breaking or exceptional points to reach divergent sensitivity even in presence of imperfections. Our experimental implementation built around an optical fiber ring additionally constitutes the first observation of the polarization symmetry breaking of passive driven nonlinear resonators.

Interplay of Polarization and Time-Reversal Symmetry Breaking in Synchronously Pumped Ring Resonators.

Optically induced breaking of symmetries plays an important role in nonlinear photonics, with applications ranging from optical switching in integrated photonic circuits to soliton generation in ring lasers. In this work we study for the first time the interplay of two types of spontaneous symmetry breaking that can occur simultaneously in optical ring resonators. Specifically we investigate a ring resonator that is synchronously pumped with short pulses of light. In this system we numerically study the interplay and transition between regimes of temporal symmetry breaking (in which pulses in the resonator either run ahead or behind the seed pulses) and polarization symmetry breaking (in which the resonator spontaneously generates elliptically polarized light out of linearly polarized seed pulses). We find ranges of pump parameters for which each symmetry breaking can be independently observed, but also a regime in which a dynamical interplay takes place. Besides the fundamentally interesting physics of the interplay of different types of symmetry breaking, our work contributes to a better understanding of the nonlinear dynamics of optical ring cavities which are of interest for future applications including all-optical logic gates, synchronously pumped optical frequency comb generation, and resonator-based sensor technologies.

Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics.

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Topological properties of bound states in the continuum in geometries with broken anisotropy symmetry

Waveguiding structures made of anisotropic media support bound states in the continuum (BICs) that arise when the radiation channel of otherwise semi-leaky modes is suppressed. Hitherto, only structures with optical axes aligned in symmetric orientations inside the waveguide plane, where BICs appear as lines in the momentum-frequency dispersion diagram, have been considered. Here we address settings where such symmetry is broken and unveil a number of fundamental new features. Weak and strong symmetry-breaking regimes are identified, corresponding to azimuthal and polar optical axes orientation asymmetries, respectively. The azimuthal symmetry-breaking is found to still preserve the existence loci of BICs in the momentum-frequency dispersion diagram as lines. However, all possible BICs become interferometric, while the polarization separable states that occur in symmetric settings cease to exist. The polar symmetry-breaking has stronger effects and transforms the BICs' existence loci from lines to points, which correspond to full-vector states that exist at discrete values of the optical axis orientation for a given wavelength. Such transformation results in fundamental changes in the topological properties of the radiated field around the BICs.

Magnetoelectric multipoles in metals

In this paper, we demonstrate computationally the existence of magnetoelectric multipoles, arising from the second-order term in the multipole expansion of a magnetization density in a magnetic field, in non-centrosymmetric magnetic metals. While magnetoelectric multipoles have long been discussed in the context of the magnetoelectric effect in non-centrosymmetric magnetic insulators, they have not previously been identified in metallic systems, in which the mobile carriers screen any electrical polarization. Using first-principles density functional calculations, we explore three specific systems: first, a conventional centrosymmetric magnetic metal, Fe, in which we break inversion symmetry by introducing a surface, which both generates magnetoelectric monopoles and allows a perpendicular magnetoelectric response. Next, the hypothetical cation-ordered perovskite, SrCaRu2O6, in which we study the interplay between the magnitude of the polar symmetry breaking and that of the magnetic dipoles and multipoles, finding that both scale proportionally to the structural distortion. Finally, we identify a hidden antiferromultipolar order in the non-centrosymmetric, antiferromagnetic metal Ca3Ru2O7, and show that, while its competing magnetic phases have similar magnetic dipolar structures, their magnetoelectric multipolar structures are distinctly different, reflecting the strong differences in transport properties.This article is part of the theme issue 'Celebrating 125 years of Oliver Heaviside's 'Electromagnetic Theory''.

Substantial bulk photovoltaic effect enhancement via nanolayering.

Spontaneous polarization and inversion symmetry breaking in ferroelectric materials lead to their use as photovoltaic devices. However, further advancement of their applications are hindered by the paucity of ways of reducing bandgaps and enhancing photocurrent. By unravelling the correlation between ferroelectric materials' responses to solar irradiation and their local structure and electric polarization landscapes, here we show from first principles that substantial bulk photovoltaic effect enhancement can be achieved by nanolayering PbTiO3 with nickel ions and oxygen vacancies ((PbNiO2)x(PbTiO3)1−x). The enhancement of the total photocurrent for different spacings between the Ni-containing layers can be as high as 43 times due to a smaller bandgap and photocurrent direction alignment for all absorption energies. This is due to the electrostatic effect that arises from nanolayering. This opens up the possibility for control of the bulk photovoltaic effect in ferroelectric materials by nanoscale engineering of their structure and composition.

Vacuum polarization and dynamical chiral symmetry breaking: Phase diagram of QED with four-fermion contact interaction

We study chiral symmetry breaking for fundamental charged fermions coupled electromagnetically to photons with the inclusion of a four-fermion contact self-interaction term, characterized by coupling strengths $\ensuremath{\alpha}$ and $\ensuremath{\lambda}$, respectively. We employ multiplicatively renormalizable models for the photon dressing function and the electron-photon vertex that minimally ensures mass anomalous dimension ${\ensuremath{\gamma}}_{m}=1$. Vacuum polarization screens the interaction strength. Consequently, the pattern of dynamical mass generation for fermions is characterized by a critical number of massless fermion flavors ${N}_{f}={N}_{f}^{c}$ above which chiral symmetry is restored. This effect is in diametrical opposition to the existence of criticality for the minimum interaction strengths, ${\ensuremath{\alpha}}_{c}$ and ${\ensuremath{\lambda}}_{c}$, necessary to break chiral symmetry dynamically. The presence of virtual fermions dictates the nature of phase transition. Miransky scaling laws for the electromagnetic interaction strength $\ensuremath{\alpha}$ and the four-fermion coupling $\ensuremath{\lambda}$, observed for quenched QED, are replaced by a mean field power law behavior corresponding to a second-order phase transition. These results are derived analytically by employing the bifurcation analysis and are later confirmed numerically by solving the original nonlinearized gap equation. A three-dimensional critical surface is drawn in the phase space of $(\ensuremath{\alpha},\ensuremath{\lambda},{N}_{f})$ to clearly depict the interplay of their relative strengths to separate the two phases. We also compute the $\ensuremath{\beta}$ functions (${\ensuremath{\beta}}_{\ensuremath{\alpha}}$ and ${\ensuremath{\beta}}_{\ensuremath{\lambda}}$) and observe that ${\ensuremath{\alpha}}_{c}$ and ${\ensuremath{\lambda}}_{c}$ are their respective ultraviolet fixed points. The power law part of the momentum dependence, describing the mass function, implies ${\ensuremath{\gamma}}_{m}=1+s$, which reproduces the quenched limit trivially. We also comment on the continuum limit and the triviality of QED.

Symmetry breaking phenomena in vector-field lasers

Polarization and phase symmetry breaking phenomena have been studied on the basis of elaborated and experimentally tested mathematical models of gas class A lasers. It is shown that, polarization symmetry breaking appears in a single-mode linear laser: for stationary regime--at the transition from the isotropic to an anisotropic cavity and destruction of the laser modes degeneracy; for periodical regime--in the presence of a sinusoidal longitudinal magnetic field on the active medium. Spontaneous phase symmetry breaking occurs in a single-mode ring laser at the transition from the running-wave to a standing-wave operation. For periodical regime this phenomenon can be accompanied by the appearance of deterministic and noise-induced chaos.

Noncritical quadrature squeezing through spontaneous polarization symmetry breaking

We discuss the possibility of generating noncritical quadrature squeezing by spontaneous polarization symmetry breaking. We first consider Type II frequency-degenerate optical parametric oscillators but discard them for a number of reasons. Then we propose a four-wave-mixing cavity, in which the polarization of the output mode is always linear but has an arbitrary orientation. We show that in such a cavity, complete noise suppression in a quadrature of the output field occurs, irrespective of the parameter values.

Phase behavior of bent-core molecules.

Recently, a new class of smectic liquid crystal phases characterized by the spontaneous formation of macroscopic chiral domains from achiral bent-core molecules has been discovered. We have carried out Monte Carlo simulations of a minimal hard spherocylinder dimer model to investigate the role of excluded volume interactions in determining the phase behavior of bent-core materials and to probe the molecular origins of polar and chiral symmetry breaking. We present the phase diagram of hard spherocylinder dimers of length-diameter ratio of 5 as a function of pressure or density and dimer opening angle psi. With decreasing psi, a transition from a nonpolar to a polar smectic A phase is observed near psi=167 degrees, and the nematic phase becomes thermodynamically unstable for psi<135 degrees. Free energy calculations indicate that the antipolar smectic A (SmAP(A)) phase is more stable than the polar smectic A phase (SmAP(F)). No chiral smectic or biaxial nematic phases were found.

Two-Photon Excitation Study of Peridinin in Benzene and in the Peridinin Chlorophyll a-Protein (PCP)

Peridinin chlorophyll a-protein (PCP) is a unique light harvesting protein found in dinoflagellates, that contains a large amount of the carotenoid peridinin. Carotenoids have unusual spectroscopic properties due to theirapproximate C 2 h symmetry, which makes transitions from their ground states to their S 1 (S 2 ) states one-photon forbidden (allowed). To gain information about one-photon forbidden electronic states in peridinin, fluorescence excitation spectra were measured after two-photon excitation for peridinin in benzene and in the PCP. The samples were excited using 920-1320 nm light. Fluorescence of the isolated peridinin S, state was then measured at 750 nm. In PCP, the excited peridinin transfers energy to chlorophyll whose fluorescence was monitored at 670 nm. Surprisingly, two-photon absorption was observed in both the peridinin S 1 and S 2 regions, with the spectrum slightly red-shifted in the protein sample. The peridinin S, energy was found to be higher than that of typical light harvesting carotenoids, making its S 1 state very close in energy to its S 2 state. We suggest that peridinin's polar groups, symmetry breaking, and possible mixing of electronic states lead to two-photon character of the normally one-photon allowed S 0 -S 2 transition.

Polarization symmetry breaking and restoration in vector-field lasers

Polarization symmetry breaking and restoration phenomena in vector-field lasers are described by singular (symmetric) bifurcations of stationary and periodic solutions.

Symmetric bifurcations and phenomena of polarization symmetry breaking in a single-mode gas laser

Invariance properties of equations of lasing of a single-mode gas laser with linear phase anisotropy of the resonator operating on the j b = 1 → j a = 2 transition are analyzed. The phenomena of polarization symmetry breaking and restoration caused by symmetric bifurcations of the stationary and periodic solutions are described. In passing from an isotropic resonator to an anisotropic resonator, a spontaneous breaking of the polarization symmetry is revealed, which is caused by the lifting of degeneracy of laser modes.

A ferroelectric liquid crystal conglomerate composed of racemic molecules

We describe the design and synthesis of a ferroelectric liquid crystal composed of racemic molecules. The ferroelectric polarization results from spontaneous polar symmetry breaking in a fluid smectic. The ferroelectric phase is also chiral, resulting in the formation of a mixture of macroscopic domains of either handedness at the isotropic-to-liquid crystal phase transition. This smectic liquid crystal is thus a fluid conglomerate. Detailed investigation of the electrooptic and polarization current behavior within individual domains in liquid crystal cells shows the thermodynamically stable structure to be a uniformly tilted smectic bow-phase (banana phase), with all layer pairs homochiral and ferroelectric (SmC(S)P(F)).

Supermolecular stereochemistry in ferroelectric liquid crystals

Until recently, it was an empirical fact that chiral liquid crystal phases were produced only from enantiomerically enriched liquid crystal materials. Ferroelectric smectic liquid crystals have always been composed of enantiomerically enriched molecules as well. Here we describe the first example of the formation of chiral supermolecular liquid crystalline structures from achiral bent-core molecules (bow phases or banana phases). In one case, a metastable antiferroelectric bow-phase structure is macroscopically chiral, with bulk samples composed of chiral macroscopic domains of either handedness. This system represents the first known liquid conglomerate. In addition, based upon a directed design approach exploiting control of interlayer clinicity, a ferroelectric liquid crystal conglomerate has been created from a racemic mixture. In this case bulk ferroelectricity is obtained by spontaneous polar symmetry breaking. Copyright © 2000 John Wiley & Sons, Ltd.

Erratum: Modulational instability of bright solitary waves in incoherently coupled nonlinear Schrödinger equations [Phys. Rev. E60, 1019 (1999)

We present a detailed analysis of the modulational instability (MI) of ground-state bright solitary solutions of two incoherently coupled nonlinear Schr\"odinger equations. Varying the relative strength of cross-phase and self-phase effects we show existence and origin of four branches of MI of the two-wave solitary solutions. We give a physical interpretation of our results in terms of the group velocity dispersion (GVD) induced polarization dynamics of spatial solitary waves. In particular, we show that in media with normal GVD spatial symmetry breaking changes to polarization symmetry breaking when the relative strength of the cross-phase modulation exceeds a certain threshold value. The analytical and numerical stability analyses are fully supported by an extensive series of numerical simulations of the full model.

Modulational instability of bright solitary waves in incoherently coupled nonlinear Schrödinger equations.

We present a detailed analysis of the modulational instability (MI) of ground-state bright solitary solutions of two incoherently coupled nonlinear Schrödinger equations. Varying the relative strength of cross-phase and self-phase effects we show the existence and origin of four branches of MI of the two-wave solitary solutions. We give a physical interpretation of our results in terms of the group-velocity-dispersion- (GVD-) induced polarization dynamics of spatial solitary waves. In particular, we show that in media with normal GVD spatial symmetry breaking changes to polarization symmetry breaking when the relative strength of the cross-phase modulation exceeds a certain threshold value. The analytical and numerical stability analyses are fully supported by an extensive series of numerical simulations of the full model.

Polarization chaos in a vector nonautonomous class-A laser

We investigate the dynamics of a vector (weakly anisotropic) class-A laser subjected to the modulation of the resonator anisotropy directions. We report on polarization symmetry breaking and, to our knowledge, this is the first explicit example of chaos affecting amplitude and polarization parameters of the field emitted by a laser in which material dynamics is not essential. The physical mechanism of the chaotic behaviour is discussed.