Microcavity phonoritons -- a coherent optical-to-microwave interface

Optomechanical systems provide a pathway for the bidirectional optical-to-microwave interconversion in (quantum) networks. We demonstrate the implementation of this functionality and non-adiabatic optomechanical control in a single, $\mu$m-sized potential trap for phonons and exciton-polariton condensates in a structured semiconductor microcavity. The exciton-enhanced optomechanical coupling leads to self-oscillations (phonon lasing) -- thus proving reversible photon-to-phonon conversion. We show that these oscillations are a signature of the optomechanical strong coupling signalizing the emergence of elusive phonon-exciton-photon quasiparticles -- the phonoritons. We then demonstrate full control of the phonoriton spectrum as well as coherent microwave-to-photon interconversion using electrically generated GHz-vibrations and a resonant optical laser beam. These findings establish the zero-dimensional polariton condensates as a scalable coherent interface between microwave and optical domains with enhanced microwave-to-mechanical and mechanical-to-optical coupling rates

Effect of fermion indistinguishability on optical absorption of doped two-dimensional semiconductors

Funding: AT and FMM acknowledge financial support from the Ministerio de Ciencia e Innovación (MICINN), projects No. AEI/10.13039/501100011033 (2DEnLight) and No. MAT2017-83772-R (QLMC-2D). FMM acknowledges financial support from the Proyecto Sinérgico CAM 2020 Y2020/TCS-6545 (NanoQuCo-CM). JL and MMP acknowledge support from the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (CE170100039). JL and MMP are also supported through the Australian Research Council Future Fellowships FT160100244 and FT200100619, respectively. JK acknowledges financial support from EPSRC program “Hybrid Polaritonics” (EP/M025330/1).We study the optical absorption spectrum of a doped two-dimensional semiconductor in the spin-valley polarized limit. In this configuration, the carriers in the Fermi sea are indistinguishable from one of the two carriers forming the exciton. Most notably, this indistinguishability requires the three-body trion state to have p-wave symmetry. To explore the consequences of this, we evaluate the system's optical properties within a polaron description, which can interpolate from the low-density limit, where the relevant excitations are few-body bound states, to higher-density many-body states. In the parameter regime where the trion is bound, we demonstrate that the spectrum is characterized by an attractive quasiparticle branch, a repulsive branch, and a many-body continuum, and we evaluate the doping dependence of the corresponding energies and spectral weights. In particular, at low doping we find that the oscillator strength of the attractive branch scales with the square of the Fermi energy as a result of the trion's p-wave symmetry. Upon increasing density, we find that the orbital character of the states associated with these branches interchanges. We compare our results with previous investigations of the scenario where the Fermi sea involves carriers distinguishable from those in the exciton, for which the trion ground state is s wave.PreprintPostprintPeer reviewe

Light-induced switching between singlet and triplet superconducting states

While the search for topological triplet-pairing superconductivity has remained a challenge, recent developments in optically stabilizing metastable superconducting states suggest a new route to realizing this elusive phase. Here, we devise a testable theory of competing superconducting orders that permits ultrafast switching to an opposite-parity superconducting phase in centrosymmetric crystals with strong spin-orbit coupling. Using both microscopic and phenomenological models, we show that dynamical inversion symmetry breaking with a tailored light pulse can induce odd-parity (spin triplet) order parameter oscillations in a conventional even-parity (spin singlet) superconductor, which when driven strongly can send the system to a competing minimum in its free energy landscape. Our results provide new guiding principles for engineering unconventional electronic phases using light, suggesting a fundamentally non-equilibrium route toward realizing topological superconductivity

Crossover from exciton polarons to trions in doped two-dimensional semiconductors at finite temperature

We study systematically the role of temperature in the optical response of doped two-dimensional semiconductors. By making use of a finite-temperature Fermi-polaron theory, we reveal a crossover from a quantum-degenerate regime with well-defined polaron quasiparticles to an incoherent regime at high temperature or low doping where the lowest energy "attractive" polaron quasiparticle is destroyed, becoming subsumed into a broad trion-hole continuum. We demonstrate that the crossover is accompanied by significant qualitative changes in both absorption and photoluminescence. In particular, with increasing temperature (or decreasing doping), the emission profile of the attractive branch evolves from a symmetric Lorentzian to an asymmetric peak with an exponential tail involving trions and recoil electrons at finite momentum. We discuss the effect of temperature on the coupling to light for structures embedded into a microcavity, and we show that there can exist well-defined polariton quasiparticles even when the exciton-polaron quasiparticle has been destroyed, where the transition from weak to strong light-matter coupling can be explained in terms of the polaron linewidths and spectral weights.Comment: 19 pages, 9 figure

Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy

By using intense coherent electromagnetic radiation, it may be possible to manipulate the properties of quantum materials very quickly, or even induce new and potentially useful phases that are absent in equilibrium. For instance, ultrafast control of magnetic dynamics is crucial for a number of proposed spintronic devices and can also shed light on the possible dynamics of correlated phases out of equilibrium. Inspired by recent experiments on spin-orbital ferromagnet YTiO$_3$ we consider the nonequilibrium dynamics of Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model the dynamics of the magnon excitations in this system following an optical pulse which resonantly excites infrared-active phonon modes. As the phonons ring down they can dynamically couple the orbitals with the low-lying magnons, leading to a dramatically modified effective bath for the magnons. We show this transient coupling can lead to a dynamical acceleration of the magnetization dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the parameter space more we find that the magnon dynamics can also even completely reverse, leading to a negative relaxation rate when the pump is blue-detuned with respect to the orbital bath resonance. We therefore show that by using specially targeted optical pulses, one can exert a much greater degree of control over the magnetization dynamics, allowing one to optically steer magnetic order in this system. We conclude by discussing interesting parallels between the magnetization dynamics we find here and recent experiments on photo-induced superconductivity, where it is similarly observed that depending on the initial pump frequency, an apparent metastable superconducting phase emerges.Comment: 16 pages, 11 figures + 5 pages, no figure

Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy

By using intense coherent electromagnetic radiation, it may be possible to manipulate the properties of quantum materials very quickly, or even induce new and potentially useful phases that are absent in equilibrium. For instance, ultrafast control of magnetic dynamics is crucial for a number of proposed spintronic devices and can also shed light on the possible dynamics of correlated phases out of equilibrium. Inspired by recent experiments on spin-orbital ferromagnet YTiO3 we consider the nonequilibrium dynamics of Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model the dynamics of the magnon excitations in this system following an optical pulse which resonantly excites infrared-active phonon modes. As the phonons ring down they can dynamically couple the orbitals with the low-lying magnons, leading to a dramatically modified effective bath for the magnons. We show this transient coupling can lead to a dynamical acceleration of the magnetization dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the parameter space more we find that the magnon dynamics can also even completely reverse, leading to a negative relaxation rate when the pump is blue-detuned with respect to the orbital bath resonance. We therefore show that by using specially targeted optical pulses, one can exert a much greater degree of control over the magnetization dynamics, allowing one to optically steer magnetic order in this system. We conclude by discussing interesting parallels between the magnetization dynamics we find here and recent experiments on photo-induced superconductivity, where it is similarly observed that depending on the initial pump frequency, an apparent metastable superconducting phase emerges

Topology and symmetry-breaking in the strong light-matter coupling regime

The physics of light-matter interactions is a rapidly developing research area at the junction between condensed matter physics and quantum optics. Depending on the strength of the light-matter interaction the systems behave very differently. In the current thesis, we mainly (but not exclusively), focus on the regime of (ultra)strong light-matter interaction. Typically, the strong coupling regime implies the employment of various low-dimensional semiconductor structures embedded into a microcavity, or irradiation by a strong laser field, or considering cold atoms, trapped in the vicinity of a waveguide. In the current thesis, we investigate various quantum systems and phenomena in the (ultra)strong coupling regime, including: 1)topological insulator based on a two-dimensional array of dressed quantum rings; 2)the new type of a polariton Z topological insulator; 3)Bose-Einstein condensate in a tilted polariton ring; 4)chiral waveguide quantum optomechanics; 5)the new type of a Hall effect for composite particles (excitons), that we refer to as the anomalous exciton Hall effect; 6)transition metal dichalcogenide polaritons in the presence of free carriers; and other related phenomena.Eðlisfræði víxlverkunar ljóss og efnis þróast hratt sem rannsóknarsvið á mörkum þéttefnisfræði og skammtaljósfræði. Hegðun eðlisfræðikerfa þar sem þessi víxlverkun á við er mjög mismunandi og fer eftir styrkleika þess. Í þessari ritgerð, skoðum við aðallega svið ofursterkrar víxlverkunar ljóss og efnis án þess að einskorða okkur við það. Svið ofursterkrar víxlverkunar ljóss og efnis gefur almennt til kynna notkun á ýmsum lágvídda hálfleiðarakerfum, ágeislun með sterku leysissviði, eða kaldar frumeindir í gildru nálægt bylgjuleiðurum. Ritgerð þessi fjallar um rannsóknir okkar á mismunandi skammtakerfum og fyrirbærum í sviði ofursterkrar kúplunar, svo sem: 1) grenndareinangrara byggða á tvívíðri grind af ágeisluðum skammtahringjum; 2) nýja tegund Z grenndareinangrara byggða á ljósskauteindum; 3) Bose-Einstein þéttingu í hallandi hring ljósskauteindahring; 4) skammtaljósaflfræði með hendnum bylgjuleiðara; 5) nýja tegund Hallhrifa fyrir samsettar eindir (örveindir), sem við köllum afbrigðileg Hallhrif örveinda; 6) hliðarmálms díkalkogeníða (TMD) ljósskauteindir í nærveru frjálsra rafbera; og fleiri tengd fyrirbæri

Strong electron phonon correlation in quasi one-dimensional crystals and the excitonic insulator candidate Ta₂NiSe₅

Understanding and controlling the interaction between quasiparticles in quantum materials is still an ongoing endeavour in condensed matter physics. In this thesis the interaction between phononic, electronic and excitonic degrees of freedom is investigated using first principles and model calculations for prototype candidates of a family of correlated materials.In the first part of this thesis I am going to present two different cases, where the strong coupling between electrons and phonons can be used to control the material properties in a solid. The first example, which I am going to discuss, is SiP2. I will show that its quasi one-dimensional structure gives rise to peculiar hybrid dimensional excitons. These are shown to be detectable through their strong coupling to the ionic degrees of freedom, which leads to the emergence of exciton phonon sidebands. These have been detected by our experimental collaborators, which marks the first measurement of such low dimensional exciton phonon sidebands in a bulk system and show a prime example of symmetry engineering of the electronic degrees of freedom. For a second example I will show, how the ionic system can be dynamically controlled via an electronic excitation which allows to manipulate the reflectivity in the THz regime. I will identify the microscopical coupling mechanism of this phenomenon, which arises through the strong coupling between the involved electronic and phononic states, and explain how this manifests in an enhancement of the reflectivity of the system.In the second part of the thesis I will present how one can understand the nature of competing phase transitions using a combined ab-initio and model calculation approach. I will discuss Ta2NiSe5, which is currently the most discussed candidate to host a phase transition to an excitonic insulating state. The difficulty in understanding this transition is that it is intrinsically coupled to a structural phase transition which makes the unique signature of the conjectured excitonic insulating groundstate elusive. Therefore, it is necessary to understand the nature of both transitions separately and disentangle the contribution of the two. I will discuss how an excitonic instability in this system could arise and identify its order parameter, but then show that the actual material does not realize it. Instead Ta2NiSe5 displays a structural instability, which leads to changes in the electronic system that is in agreement with the experiments for this material. Thus, we conclude that the phase transition is stemming from a structural instability rather than an excitonic instability.Das Verständnis und die Kontrolle von Quasiteilchen in Quantenmaterialien ist bis heute eine Herausforderung für die aktuelle Forschung. In dieser Dissertation werde ich die Wechselwirkung zwischen eletronischen, exzitonischen und phononischen Freiheitsgraden mittels ab-initio und Modellrechnungen untersuchen.In dem ersten Teil dieser Arbeit werde ich zwei verschiedene Beispiele von Systemen präsentieren, deren Eigenschaften sich durch ihre starke Wechselwirkung zwischen Ionen und Elektronen manipulieren lassen. In dem ersten Projekt diskutiere ich SiP2. Dies ist ein Kristall mit quasi-eindimensionaler Struktur, welcher Exzitonen mit einer besonderen hybrid-dimensionalen Struktur beheimatet. Es wird gezeigt, dass diese Struktur erlaubt die starke Wechselwirkung dieser Teilchen mit Phononen zu messen, da dieser stark korrelierte Exziton-Phonon Zustand als Nebenpeak zu den exzitonischen Hauptpeaks sichtbar wird. Diese wurden von unseren experimentellen Kollegen gemessen, was die erste Messung dieser niedrigdimensionalen Exziton-Phonon Nebenmaxima in einem dreidimensionalen Kristall darstellt. In einem zweiten Projekt werde ich zeigen wie das ionische System durch seine starke Kopplung zu dem elektronischen System gezielt durch die Anregung des letzteren manipuliert werden kann. So ist es möglich die Reflektivität des Systems mittels eines gezielten Laserpulses signifikant zu erhöhen. Ich werde in dieser Arbeit den mikroskopische Mechanismus dieses Phänomens identifizieren, zeigen welche elektronischen und ionischen Zustände dazu stark miteinander koppeln und erklären wie dies zu der Vergrößerung der Reflektivität des Kristalls führt.In dem zweiten Teil dieser Arbeit werde ich zeigen wie mittels eines kombinierten Ansatzes aus ab-initio-Rechnungen, Modellrechnungen und experimentellen Daten ein kompetitiver Phasenübergang verstanden werden kann. Ich werde den Ta2NiSe5 Kristall diskutieren, welcher gegenwärtig der meistdiskutierte Kandidat ist um den exzitonischen Isolator Zustand zu realisieren. Die Schwierigkeit bei der Suche nach diesem Zustand ist, dass dieser in Ta2NiSe5 intrinsisch mit einem strukturellen Phasenübergang gekoppelt ist. Dies macht es schwer den vermuteten exzitonisch isolierenden Zustand eindeutig zu identifizieren. Deshalb ist es nötig beide Phasenübergänge einzelnd genau zu verstehen, um ihre Signatur in dem kompetitiven Phasenübergang unterscheiden zu können. Ich werde dies anhand dieses exzitonischen Isolator Kandidaten aufzeigen und diskutieren wie solch eine exzitonische Instabilität aussehen könnte und ihren Ordnungsparameter identifizieren. Anschließend werde ich jedoch zeigen, dass diese exzitonische Instabilität im realen Material nicht realisiert ist und stattdessen eine strukturelle Instabilität den Phasenübergang dominiert. Diese führt zu einer Signatur, welche im Einklang mit den experimentellen Messungen ist. Deshalb ist das Fazit, dass der exzitonische Isolator in dem Phasenübergang in Ta2NiSe5 keine Rolle spielt

Strong electron phonon correlation in quasi one-dimensional crystals and the excitonic insulator candidate Ta₂NiSe₅

Understanding and controlling the interaction between quasiparticles in quantum materials is still an ongoing endeavour in condensed matter physics. In this thesis the interaction between phononic, electronic and excitonic degrees of freedom is investigated using first principles and model calculations for prototype candidates of a family of correlated materials.In the first part of this thesis I am going to present two different cases, where the strong coupling between electrons and phonons can be used to control the material properties in a solid. The first example, which I am going to discuss, is SiP2. I will show that its quasi one-dimensional structure gives rise to peculiar hybrid dimensional excitons. These are shown to be detectable through their strong coupling to the ionic degrees of freedom, which leads to the emergence of exciton phonon sidebands. These have been detected by our experimental collaborators, which marks the first measurement of such low dimensional exciton phonon sidebands in a bulk system and show a prime example of symmetry engineering of the electronic degrees of freedom. For a second example I will show, how the ionic system can be dynamically controlled via an electronic excitation which allows to manipulate the reflectivity in the THz regime. I will identify the microscopical coupling mechanism of this phenomenon, which arises through the strong coupling between the involved electronic and phononic states, and explain how this manifests in an enhancement of the reflectivity of the system.In the second part of the thesis I will present how one can understand the nature of competing phase transitions using a combined ab-initio and model calculation approach. I will discuss Ta2NiSe5, which is currently the most discussed candidate to host a phase transition to an excitonic insulating state. The difficulty in understanding this transition is that it is intrinsically coupled to a structural phase transition which makes the unique signature of the conjectured excitonic insulating groundstate elusive. Therefore, it is necessary to understand the nature of both transitions separately and disentangle the contribution of the two. I will discuss how an excitonic instability in this system could arise and identify its order parameter, but then show that the actual material does not realize it. Instead Ta2NiSe5 displays a structural instability, which leads to changes in the electronic system that is in agreement with the experiments for this material. Thus, we conclude that the phase transition is stemming from a structural instability rather than an excitonic instability.Das Verständnis und die Kontrolle von Quasiteilchen in Quantenmaterialien ist bis heute eine Herausforderung für die aktuelle Forschung. In dieser Dissertation werde ich die Wechselwirkung zwischen eletronischen, exzitonischen und phononischen Freiheitsgraden mittels ab-initio und Modellrechnungen untersuchen.In dem ersten Teil dieser Arbeit werde ich zwei verschiedene Beispiele von Systemen präsentieren, deren Eigenschaften sich durch ihre starke Wechselwirkung zwischen Ionen und Elektronen manipulieren lassen. In dem ersten Projekt diskutiere ich SiP2. Dies ist ein Kristall mit quasi-eindimensionaler Struktur, welcher Exzitonen mit einer besonderen hybrid-dimensionalen Struktur beheimatet. Es wird gezeigt, dass diese Struktur erlaubt die starke Wechselwirkung dieser Teilchen mit Phononen zu messen, da dieser stark korrelierte Exziton-Phonon Zustand als Nebenpeak zu den exzitonischen Hauptpeaks sichtbar wird. Diese wurden von unseren experimentellen Kollegen gemessen, was die erste Messung dieser niedrigdimensionalen Exziton-Phonon Nebenmaxima in einem dreidimensionalen Kristall darstellt. In einem zweiten Projekt werde ich zeigen wie das ionische System durch seine starke Kopplung zu dem elektronischen System gezielt durch die Anregung des letzteren manipuliert werden kann. So ist es möglich die Reflektivität des Systems mittels eines gezielten Laserpulses signifikant zu erhöhen. Ich werde in dieser Arbeit den mikroskopische Mechanismus dieses Phänomens identifizieren, zeigen welche elektronischen und ionischen Zustände dazu stark miteinander koppeln und erklären wie dies zu der Vergrößerung der Reflektivität des Kristalls führt.In dem zweiten Teil dieser Arbeit werde ich zeigen wie mittels eines kombinierten Ansatzes aus ab-initio-Rechnungen, Modellrechnungen und experimentellen Daten ein kompetitiver Phasenübergang verstanden werden kann. Ich werde den Ta2NiSe5 Kristall diskutieren, welcher gegenwärtig der meistdiskutierte Kandidat ist um den exzitonischen Isolator Zustand zu realisieren. Die Schwierigkeit bei der Suche nach diesem Zustand ist, dass dieser in Ta2NiSe5 intrinsisch mit einem strukturellen Phasenübergang gekoppelt ist. Dies macht es schwer den vermuteten exzitonisch isolierenden Zustand eindeutig zu identifizieren. Deshalb ist es nötig beide Phasenübergänge einzelnd genau zu verstehen, um ihre Signatur in dem kompetitiven Phasenübergang unterscheiden zu können. Ich werde dies anhand dieses exzitonischen Isolator Kandidaten aufzeigen und diskutieren wie solch eine exzitonische Instabilität aussehen könnte und ihren Ordnungsparameter identifizieren. Anschließend werde ich jedoch zeigen, dass diese exzitonische Instabilität im realen Material nicht realisiert ist und stattdessen eine strukturelle Instabilität den Phasenübergang dominiert. Diese führt zu einer Signatur, welche im Einklang mit den experimentellen Messungen ist. Deshalb ist das Fazit, dass der exzitonische Isolator in dem Phasenübergang in Ta2NiSe5 keine Rolle spielt

Electronic Structure of Novel Two-dimensional Materials and Graphene Heterostructures

Today a well-equipped library of two-dimensional materials can be synthesized or exfoliated, ranging from insulating hexagonal boron nitride, to semi-metallic graphene, and metallic as well as superconducting transition metal dichalcogenides and many others. Due to strong intra-layer covalent bondings, but weak inter-layer Van-der-Waals interactions, these layered materials can be stacked in a Lego-like fashion to artificial heterostructures which do not occur in nature. Thereby, these novel systems offer the possibility to combine specific properties of each of its constituents to tailor the heterostructure's properties on demand which might allow for completely new device classes. In fact, these kind of systems are already constructed and studied in labs around the world. In order to guide these efforts, we need an in-depth understanding of these complex heterostructures starting with its smallest components, namely the different two-dimensional materials and their mutual interactions. To this end, we study electronic and optical properties of novel two-dimensional materials in this thesis. In more detail, we here aim to investigate functionalized graphene, graphene heterostructures and doped or optically excited molybdenum disulfide (MoS$_2$) monolayers for which we combine \abinitio based models with many-body or multi-scale approaches. The first part is devoted to functionalized graphene and is subdivided into the investigation of disorder-induced optical effects of fluorographene and into a detailed study of the Coulomb interaction in graphene heterostructures in form of multilayer graphene, intercalated graphite and few-layer graphene within a dielectric environment. In the case of fluorographene we use a multi-scale approach to study the effects of realistic disorder patterns to the optical conductivity. Thereby, we provide important insights into the role of non-perfect fluorination of graphene. Regarding the graphene heterostructures we present a novel approach to easily and reliably derive Coulomb-interaction matrix elements in these structures. This method is used to study the robustness of bilayer graphene's ground state to changes in its dielectric surrounding. In the second part of the thesis we study a variety of many-body effects that arise in doped and optically excited MoS$_2$ monolayers. Once again, by deriving simplified yet accurate models from first-principles we are able to investigate many-body excitations like plasmons or excitons as well as many-body instabilities like superconductivity or charge-density wave phases. Regarding the latter, we are able to extend the electron-doping phase diagram of MoS$_2$ by the formation of a charge-density-wave phase and reveal its potential coexistence with the superconducting state. In the field of many-body excitations we study in detail excitonic line shifts upon optical excitations and we precisely describe different types of plasmonic excitations under electron or hole doping in MoS$_2$. Finally, we make use of the fundamental properties of the many-body interactions in layered materials in order to externally induce heterojunctions within homogeneous semiconducting monolayers by non-local manipulations of the Coulomb interaction