Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session D66: Topological States in AMO Systems IFocus

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Sponsoring Units: DAMOP Chair: Ying Su, University of Texas at Dallas Room: Room 413 
Monday, March 6, 2023 3:00PM  3:36PM 
D66.00001: Recent advanced in topological photonics Invited Speaker: Mohammad Hafezi We discuss our recent advances in topological photonics. First, we report our progress in investigating the nonlinear optical properties of topological ring resonators. Then, we show various topological phenomena in fiber loops by temporal multiplexing. In the end, we discuss the topological interplay between photons and electrons. 
Monday, March 6, 2023 3:36PM  3:48PM 
D66.00002: NonAbelian Floquet braiding and anomalous Dirac string phase in periodically driven systems RobertJan Slager, Adrien Bouhon, Nur Unal Topological phases of matter span a wide area of research shaping fundamental pursuits and offering promise for future applications. While a significant fraction of topological materials has been characterized using symmetry requirements of wave functions, the past two years have witnessed the rise of novel multigap dependent topological states, the properties of which go beyond these approaches and are yet to be fully explored. Thriving upon these insights, we report on uncharted anomalous phases and properties that can only arise in outofequilibrium Floquet settings. In particular, we identify Floquetinduced nonAbelian braiding mechanisms, which in turn lead to a phase characterized by an anomalous Euler class, the prime example of a multigap topological invariant. Most strikingly, we also retrieve the first example of an `anomalous Dirac string phase'. This gapped outofequilibrium phase features an unconventional Dirac string configuration that physically manifests itself via anomalous edge states on the boundary. Our results therefore not only provide a stepping stone for the exploration of intrinsically dynamical and experimentally viable multigap topological phases, but also demonstrate a powerful way to observe these nonAbelian processes notably in quantum simulators. 
Monday, March 6, 2023 3:48PM  4:00PM 
D66.00003: Identifying topological markers in gapless photonic systems with topological phase transitions at an interface Kahlil Y Dixon Photonic topological insulators exhibit bulkboundary correspondence, which requires that boundarylocalized states appear at the interface between topologically distinct insulating materials. However, in many topological photonic systems, this raises a subtle problem, as freespace is gapless for photons above the lightline. Although experiments have observed localized photonic edge states in this regime, their topology has been defined using theories which effectively approximate freespace to be insulating. Here, we show that even at the interface between a gapless material and a topological insulator these systems still exhibit both a topological phase transition and bulkboundary correspondence. To do so, we employ the system's spectral localizer, which uses a system’s realspace description to calculate local topological markers and local topological protection independent of the material’s bulk band gap (or lack thereof). Ultimately, we show that topological photonic crystals can still demonstrate topological behavior and associated edge localizer resonances while in contact with freespace or any other gapless media. 
Monday, March 6, 2023 4:00PM  4:12PM 
D66.00004: Polaritonic Chern insulators in monolayer semiconductors Li He, Jingda Wu, Jicheng Jin, Eugene J Mele, Bo Zhen Systems with strong lightmatter interaction opens up new avenues for studying topological phases of matter. Examples include excitonpolaritons, mixed lightmatter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from lightmatter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the wellknown optical selection rules, which were derived under the plane wave approximation. Here we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both timereversal and inversion symmetries. Lifting exciton valley degeneracy by breaking timereversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. 
Monday, March 6, 2023 4:12PM  4:24PM 
D66.00005: Disordered topological graphs enhancing nonlinear phenomena Zhetao Jia, Matteo Seclì, Alexander Avdoshkin, Walid Redjem, Elizabeth Dresselhaus, Joel E Moore, Boubacar Kante Since their discovery in crystalline materials, topological insulators have also been realized in amorphous solids, where nontrivial topology is captured by the real space version of the Chern number. Unlike the periodic lattice, disorder in amorphous structure induces Anderson localization of the bulk modes. We propose and demonstrate topological structurally disordered systems with a modal structure that enhances nonlinear phenomena by inhibiting the leakage of energy from topological edge modes to bulk modes in the presence of nonlinearities. We present the construction of the graph and show that its dynamics enhances the photon pair generation rate in an optical realization. [1] 
Monday, March 6, 2023 4:24PM  4:36PM 
D66.00006: Topological control of light with graphene devices Coskun Kocabas The topological structure associated with the branchpoint singularity around an exceptional point (EP) can provide tools for controlling the propagation of light. Using graphenebased devices, we demonstrate the emergence of EPs in the electrically controlled interaction of light with a collection of organic molecules in the terahertz regime at room temperature. We show that the intensity and phase of terahertz pulses can be controlled by a gate voltage which drives the device across the EP. Our electrically tuneable system allows reconstructing the Riemann surface associated with the complex energy landscape and provides a topological control of light by tuning the lossimbalance and frequency detuning of interacting modes. Our approach provides a platform for developing topological optoelectronics and studying the manifestations of EP physics in lightmatter interactions. 
Monday, March 6, 2023 4:36PM  4:48PM 
D66.00007: Second Euler number in fourdimensional synthetic matter Giandomenico Palumbo, RobertJan Slager, Adrien Bouhon, YanQing Zhu Twodimensional Euler insulators are novel kind of spinless fermionic systems that support topological phases, which exhibit a quantised first Euler number in their bulk. This topological invariant is protected by the spacetime inversion symmetry. Recently, these phases have been experimentally realised in suitable twodimensional synthetic matter setups. Artificial engineered systems, ranging from ultracold atoms to photonics and electric circuits, offer the unique way to implement higherdimensional phases that cannot exist in real quantum materials. Although the second Euler invariant is a familiar invariant in both differential topology (GaussBonnet theorem) and in fourdimensional Euclidean gravity, its existence in synthetic matter has never been explored so far. In this talk, we firstly introduce and describe two specific novel models in four dimensions that support a nonzero second Euler number in the bulk together with peculiar gapless boundary states. Secondly, we discuss its robustness in general spacetimeinversion invariant phases and its role in the classification of topological degenerate real bands through real Grassmannians. Finally, we show how to engineer these new topological phases in a fourdimensional ultracold atom setup. Our results naturally generalize the second Chern and spin Chern numbers to the case of fourdimensional phases that are characterised by real Hamiltonians. 
Monday, March 6, 2023 4:48PM  5:00PM 
D66.00008: Topological phases of drivendissipative lattices: nonHermitian physics and experimental implementations Diego Porras, Tomas Ramos, Alejandro GonzalezTudela, Álvaro GómezLeón

Monday, March 6, 2023 5:00PM  5:12PM 
D66.00009: Lossdriven topological transitions in lasing from quasiBIC in plasmonic lattices Grazia Salerno, Paivi E Torma Plasmonic lattices of metal nanoparticles have emerged as an effective platform for strong lightmatter coupling, lasing, and BoseEinstein condensation. However, the full potential of complex unit cell structures has not been exploited. On the other hand, bound states in continuum (BICs) have attracted attention, as they provide topologically protected optical modes with diverging quality factors. We show that nanoparticle lattices with complex unit cells enable lasing in quasiBIC modes with a exceedingly high quality Q factor. By combining theory with polarizationresolved measurements of the emission, we show that the lasing mode has a nontrivial polarization winding, which changes when tuning the scale of the unit cell. By a theoretical analysis we identify the lasing modes as quasi bound states in continuum (quasiBICs) of topological charges of zero, one or two. A Tmatrix simulation of the structure reveals that the mode Q factors depend on the scale of the unit cell, with highestQ modes favored by lasing. The system thus shows a lossdriven transition between lasing in modes of trivial and highorder topological charge.

Monday, March 6, 2023 5:12PM  5:24PM 
D66.00010: Singlemode robust defectbased laser Fargol Seifollahi, Abouzar Gharajeh, Qing Gu, Hamidreza Ramezani NonHermitian systems can exhibit unique topological effects that can be realized in photonic crystals, metamaterials, and coupled resonators. In our theoretical and experimental work, by utilizing nonHermitian degeneracies known as the exceptional points (EP) and a locally embedded defect in a trivial array of microring resonators, we form a singlemode robust defectbased laser emission that is immune from disorder in the couplings in the array. Our proposed design facilitates going beyond edge mode lasing and bypasses the requirements sought to be evaded in previous approaches towards achieving topological lasing modes. 
Monday, March 6, 2023 5:24PM  5:36PM 
D66.00011: Dynamical Degeneracy Splitting and Directional Invisibility in NonHermitian Systems Kai Zhang The nonHermitian skin effect (NHSE) refers to extensive bulk modes localized at the open boundaries, which makes the energy spectra and wave functions sensitive to the change in boundary conditions. In higher dimensions, the geometrydependent skin effect has recently been proposed, i.e., the NHSE disappears in some specified openboundary geometries but reappears in generic openboundary geometries. Although theoretically, the geometrydependent skin effect can universally exist in higherdimensional nonHermitian systems, a detectable signature experimentally is still lacking. 
Monday, March 6, 2023 5:36PM  5:48PM 
D66.00012: Topological multimode waveguide QED Alejandro GonzalezTudela, Diego Porras, Carlos Vega, Alberto Muñoz de las Heras Twodimensional topological insulators with quantized large Chern numbers feature a number of protected, chiral edge modes linked to the value of the invariant. When implemented in photonic setups, these systems then naturally display a topologicallyprotected multimode waveguide at their edges. Here, we show how to take advantage of these setups by interfacing them with quantum emitters. Using a HarperHofstadter lattice as a particular model, we find situations in which the emitters feature quasiquantized decay rates due to the increasing number of edge modes in the different emergent bandgaps, and where their spontaneous emission spatially separates in different modes, enabling the generation of nonlocal timebin entangled states already with a single πpulse. Besides, we show how the emitters can selectively interact with the different channels using nonlocal lightmatter couplings as the ones that can be obtained with giant atoms. Such capabilities pave the way for generating quantum gates among topologicallyprotected photons as well as generating more complex entangled states of light in topological channels. 
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