Lattice vibrations carrying angular momentum give rise to fundamental phenomena such as the phonon Hall effect and the Einstein-de Haas effect, offering new opportunities for manipulating angular momentum in condensed-matter systems. Generating such circularly polarized phonons requires either the use of an external magnetic field, or of polarized light pulses. The latter approach does not require the use of any magnetic response of the material, but it has limitations, in particular regarding the duration of the pulse. Consequently, any effect stemming from phonons generated by light pulses is transient. In this paper, we propose the use of a driven electromagnetic cavity as a route to generate a steady-state population of chiral phonons. We derive a general description of cavity-chiral phonon interaction, with a particular focus on modes of chiral cavities. We show that, under optimized conditions, a significant effective phonon-induced magnetic field can be generated by means of an external drive with realistic power. Our approach is specially tailored for Gamma point phonons with THz frequencies, and opens a new route for investigating chiral phonons with electromagnetic cavities.
Lattice vibrations carrying angular momentum give rise to fundamental phenomena such as the phonon Hall effect and the Einstein-de Haas effect, offering new opportunities for manipulating angular momentum in condensed-matter systems. Generating such circularly polarized phonons requires either the use of an external magnetic field, or of polarized light pulses. The latter approach does not require the use of any magnetic response of the material, but it has limitations, in particular regarding the duration of the pulse. Consequently, any effect stemming from phonons generated by light pulses is transient. In this paper, we propose the use of a driven electromagnetic cavity as a route to generate a steady-state population of chiral phonons. We derive a general description of cavity-chiral phonon interaction, with a particular focus on modes of chiral cavities. We show that, under optimized conditions, a significant effective phonon-induced magnetic field can be generated by means of an external drive with realistic power. Our approach is specially tailored for Gamma point phonons with THz frequencies, and opens a new route for investigating chiral phonons with electromagnetic cavities.
Chiral phonons are quantized vibrations where the atomic motion in a solid breaks improper rotation symmetries. In many cases, chiral phonons possess angular momenta and are therefore selective to circularly polarized light. Both fundamental and applied research efforts on chiral phonons have been gaining increasing attention owing to their importance in a variety of fields including spintronics, spin-selective chemical reactions, thermal transport, quantum information processing and biosensing, where the bi-directional spin-lattice coupling enabled by chiral phonons can be harnessed in new ways, and potentially lead to new functionalities. Thus far, the studies of chiral phonons across diverse materials platforms have evolved largely independently within these fields, but the experimental techniques are often interrelated. In this perspective, we present a detailed description, as well as advantages and disadvantages of the current approaches for experimentally measuring chiral phonons in chiral and achiral materials. We conclude with a discussion of new methods for measuring chiral phonons. Ultimately, this work seeks to offer an experimental guide for systematically investigating the properties of chiral phonons in various materials systems and applications.
The advent of high-intensity ultrafast laser pulses has opened new opportunities for controlling and designing quantum materials. In particular, terahertz (THz) pulses can resonantly drive optical phonon modes, enabling dynamic manipulation of lattice degrees of freedom. In this work, we investigate the ultrafast quantum thermodynamics of optical phonon mode driven by a THz pulse by treating the phonon as an open quantum system coupled to a thermal environment within a Caldeira-Leggett-type framework. We derive the quantum heat current between the phonon and the bath and analyze its behavior under realistic pulse protocols. Our results demonstrate that ultrafast laser driving can reveal and even induce significant deviations from the commonly adopted Markovian approximation, thereby providing a pathway to probe and control non-Markovian dissipation in driven solid-state systems.
Chiral quantum optics is a growing field of research in which light-matter interactions become asymmetrically dependent on momentum and spin, offering novel control over photonic and electronic degrees of freedom. Recently, the platforms for investigating chiral light-matter interactions have expanded from laser-cooled atoms and quantum dots to various solid-state systems, such as microcavity polaritons and two-dimensional layered materials, integrated into photonic structures such as waveguides, cavities, and ring resonators. In this Perspective, we begin by establishing the foundation for understanding and engineering these chiral light-matter regimes. We review the cutting-edge platforms that have enabled their successful realization in recent years, focusing on solid-state platforms, and discuss the most relevant experimental challenges to fully harness their potential. Finally, we explore the vast opportunities that these chiral light-matter interfaces present, particularly their ability to reveal exotic quantum many-body phenomena, such as chiral many-body superradiance and fractional quantum Hall physics.
We reveal that a chiral cavity distinguishes the resonant and antiresonant interactions between a two-dimensional electron gas and the cavity vacuum. For co-rotating helicities, the modes anticross, exhibiting vacuum Rabi splitting, and retain a bare vacuum ground state. For counter-rotating helicities, the modes cross, acquiring vacuum-mediated energy shifts and a two-mode squeezed ground state. We trace the origin of these observations to the SU(2) and SU(1,1) symmetries of the respective Hamiltonians and present their spectroscopic signatures in chiral photonic crystal cavities. We further predict that cavity chirality produces a handedness-dependent correction to Hall resistivity at finite polariton broadening, while such a correction is not present in an achiral cavity. For the longitudinal resistivity, we find an asymmetric response of the Shubnikov--de Haas oscillations for the different helicities. Finally, we present an experimentally accessible route for detecting the squeezed ground state through current fluctuations. Parameters based on a recently demonstrated terahertz chiral cavity place these effects within experimental reach.
In this thesis, we theoretically investigate exotic quantum phenomena underlying periodically-driven systems.We discuss three distinct systems, noninteracting Dirac systems, lattice systems with a strong repulsive interaction, and lattice systems with a strong attractive interaction.We reveal various novel phenomena underlying these systems, and provide clear understanding of them.Periodically-driven systems, i.e., systems with a discrete time-translational symmetry, are described by the Floquet formalism, which maps the time-dependent Hamiltonian to a static "photon-dressed Hamiltonian".While a perturbative expansion of this Hamiltonian from the limit of high driving frequency has clarified various novel phenomena peculiar to periodically-driven systems, such an approach suffers from serious heating or damage when it is applied to realistic condensed matters.In this thesis, we focus on a lower-frequency regime, especially when the heating should be suppressed due to the small driving amplitude or an energy gap larger than the driving frequency.There, the high-frequency expansion cannot capture the relevant physics, so that we first formulate various perturbative expansions for three distinct regimes, i.e., high-frequency, small-amplitude, and strong-coupling regimes.We apply the small-amplitude expansion to two-and three-dimensional Dirac electrons driven by a circularly-polarized light to explore their behavior in the lowfrequency regime.We find an emergence of infinite numbers of topological gaps for the former and Weyl nodes for the latter.We also discuss how a regularization of the model affects this topological structure.We then introduce a strong repulsive interaction, and discuss behaviors of spins using the strong-coupling expansion.We predict an emergence of scalar spin chirality when a circularly-polarized light is illuminated, and discuss possible applications to photo-induced chiral spin liquids, and optical probing of spin chirality.Finally we turn to a strong attractive interaction to discuss the fate of the ordinary s-wave superconductivity under the periodic driving.We demonstrate an emergence of long sought-after η-pairing superconductivity, first as a ground state of the photon-dressed Hamiltonian, and then as a consequence of a dynamical phase transition in an isolated system such as an optical lattice system.
Ultrastrong coupling between matter and cavity vacuum fields can turn the electromagnetic vacuum into a structured quantum environment, thereby opening passive routes for modifying and manipulating material properties. Recent work has identified light--matter entanglement as an important ingredient in these property changes, which raises the question of where the relevant vacuum correlations actually reside. Landau polaritons provide chiral ultrastrong coupling systems in which one circular cavity polarization forms the bright polariton branches. Here, using a quantum information approach, we show that an exact chiral charge in a multimode Hopfield model routes the dominant anomalous correlations, squeezing, and cavity--matter entanglement into the opposite polarization. We find that, using parameters extracted from a multimode Landau polariton system, this hidden sector correlates the cyclotron resonance with finite momentum magnetoplasmons through Gaussian discord, while pairwise matter--matter entanglement remains absent. We further predict a polarization anisotropy of dressed vacuum electric field fluctuations as a signature of this chiral routing. These results identify chirality as a symmetry principle for organizing ultrastrong coupling vacua and show that quantum information tools provide a powerful framework for revealing the salient properties of Landau polaritons.