Localized optical field enhancement enables strong light-matter interactions necessary for efficient manipulation and sensing of light. Specifically, tunable broadband energy localization in nanoscale hotspots offers many applications in nanophotonics and quantum optics. We experimentally demonstrate a mechanism for the local enhancement of electromagnetic fields based on strong suppression of backscattering. This is achieved at a designed termination of a topologically nontrivial waveguide that nearly preserves the valley degree of freedom. The symmetry origin of the valley degree of freedom prevents edge states to undergo intervalley scattering at waveguide discontinuities that obey the symmetry of the crystal. Using near-field microscopy, we reveal that this leads to strong confinement of light at the termination of a topological photonic waveguide, even without breaking reciprocity. We emphasize the importance of symmetry conservation by comparing different waveguide termination geometries, confirming that the origin of suppressed backscattering lies with the near conservation of the valley degree of freedom, and show the broad bandwidth of the effect.

Control over light propagation and localization in photonic crystals offers wide applications ranging from sensing and on-chip routing to lasing and quantum light–matter interfaces. Although in electronic crystals, magnetic fields can be used to induce a multitude of unique phenomena, the uncharged nature of photons necessitates alternative approaches to bring about similar control over photons at the nanoscale. Here we experimentally realize pseudomagnetic fields in two-dimensional photonic crystals through engineered strain of the lattice. Analogous to strained graphene, this induces flat-band Landau levels at discrete energies. We study the spatial and spectral properties of these states in silicon photonic crystals at telecom wavelengths with far-field spectroscopy. Moreover, taking advantage of the photonic crystal’s design freedom, we realize domains of opposite pseudomagnetic field and observe chiral edge states at their interface. We reveal that the strain-induced states can achieve remarkably high quality factors despite being phase matched to the radiation continuum. Together with the high density of states and high degeneracy associated with flat bands, this provides powerful prospects for enhancing light–matter interactions, and illustrates the broad potential of psdeudomagnetic fields in the nanophotonic domain. This work, thus, establishes a new design principle to govern both on-chip and radiating light fields.

Distributing quantum entanglement on a chip is a crucial step toward realizing scalable quantum processors. Using traveling phonons-quantized guided mechanical wave packets-as a medium to transmit quantum states is now gaining substantial attention due to their small size and low propagation speed compared to other carriers, such as electrons or photons. Moreover, phonons are highly promising candidates to connect heterogeneous quantum systems on a chip, such as microwave and optical photons for long-distance transmission of quantum states via optical fibers. Here, we experimentally demonstrate the feasibility of distributing quantum information using phonons by realizing quantum entanglement between two traveling phonons and creating a time-bin-encoded traveling phononic qubit. The mechanical quantum state is generated in an optomechanical cavity and then launched into a phononic waveguide in which it propagates for around 200 micrometers. We further show how the phononic, together with a photonic qubit, can be used to violate a Bell-type inequality.

Experimental characterization of the electromagnetic vector field in topological photonic crystals featuring the photonic quantum valley Hall effect, using phase-resolving near-field optical microscopy, reveals two orders of magnitude higher robustness compared to a conventional waveguide.

Topological on-chip photonics based on tailored photonic crystals (PhCs) that emulate quantum valley-Hall effects has recently gained widespread interest owing to its promise of robust unidirectional transport of classical and quantum information. We present a direct quantitative evaluation of topological photonic edge eigenstates and their transport properties in the telecom wavelength range using phase-resolved near-field optical microscopy. Experimentally visualizing the detailed sub-wavelength structure of these modes propagating along the interface between two topologically non-trivial mirror-symmetric lattices allows us to map their dispersion relation and differentiate between the contributions of several higher-order Bloch harmonics. Selective probing of forward- and backward-propagating modes as defined by their phase velocities enables direct quantification of topological robustness. Studying near-field propagation in controlled defects allows us to extract upper limits of topological protection in on-chip photonic systems in comparison with conventional PhC waveguides. We find that protected edge states are two orders of magnitude more robust than modes of conventional PhC waveguides. This direct experimental quantification of topological robustness comprises a crucial step toward the application of topologically protected guiding in integrated photonics, allowing for unprecedented error-free photonic quantum networks.

With phase-and polarization-resolving near-field optical microscopy we directly visualize the electromagnetic vector field in topological photonic crystals featuring the optical quantum spin Hall effect. We reveal that the local optical spin of spin-protected edge states is highly structured.

We employ near-and far-field optical microscopy to characterize the propagation of edge states in topological photonic crystal waveguides and cavities. We test fundamental and practical limits to topological protection, quantifying dispersion, loss, and scattering.

Topological protection in photonics offers new prospects for guiding and manipulating classical and quantum information. The mechanism of spin-orbit coupling promises the emergence of edge states that are helical, exhibiting unidirectional propagation that is topologically protected against back scattering. We directly observe the topological states of a photonic analog of electronic materials exhibiting the quantum spin Hall effect, living at the interface between two silicon photonic crystals with different topological order. Through the far-field radiation that is inherent to the states' existence, we characterize their properties, including linear dispersion and low loss. We find that the edge state pseudospin is encoded in unique circular far-field polarization and linked to unidirectional propagation, thus revealing a signature of the underlying photonic spin-orbit coupling. We use this connection to selectively excite different edge states with polarized light and directly visualize their routing along sharp chiral waveguide junctions.

Two-dimensional photonic crystals allow for various types of photonic topological insulators. In this paper, we present our efforts to directly image on-chip light propagation in topological edge states. We quantify the robustness of such states to scattering at sharp corners and defects.

We directly observe the states of topological photonic crystals at telecom wavelengths. Using the states’ intrinsic radiation, we measure dispersion, loss, pseudospin, and spin-spin scattering. We image spin-selective unidirectional propagation around sharp corners and junctions.

The concept of topology has proven immensely powerful in physics, describing new phases of matter with unique properties. There has been a recent surge in attempts to implement topological protection in the photonic domain, owing to the application potential of robust transport immune to scattering at disorder. A famous class of electronic topological insulators relies on the quantum spin-Hall effect (QSHE). Photonic analogues of QSHE were recently predicted to occur in photonic crystals with special symmetries [1,2]. Interestingly, topological photonic crystals employing QSHE offer the possibility to access their properties via far-field radiation [3].

The scattering matrix is a fundamental tool to quantitatively describe the properties of resonant systems. In particular, it enables the understanding of many photonic devices of current interest, such as photonic metasurfaces and nanostructured optical scatterers. In this contribution, we show that the scattering matrix of a photonic system is completely determined by its quasinormal modes, i.e., the self-sustaining electromagnetic excitations at a complex frequency. On the basis of temporal coupled-mode theory, we derive an expression for the expansion of the scattering matrix on quasinormal modes, which is directly applicable to an arbitrary number of modes and input/output channels. Our theory does not require any ad-hoc assumptions, such as the fitting of an additional nonresonant background. We validate and discuss the theoretical formalism with some illustrative examples. This demonstrates that the theory represents a powerful and predictive tool for calculating the highly structured spectra of resonant nanophotonic systems, and, at the same time, a key for unravelling the physical mechanisms at the heart of such intricate spectral structures.

We observe that the asymmetric transmission (AT) through photonic systems with a resonant chiral response is strongly related to the far-field properties of eigenmodes of the system. This understanding can be used to predict the AT for any resonant system from its complex eigenmodes. We find that the resonant chiral phenomenon of AT is related to, and is bounded by, the nonresonant scattering properties of the system. Using the principle of reciprocity, we determine a fundamental limit to the maximum AT possible for a single mode in any chiral resonator. We propose and follow a design route for a highly chiral dielectric photonic crystal structure that reaches this fundamental limit for AT.

We develop a theoretical formalism which explains asymmetric transmission (AT) in chiral resonators from their eigenmodes. We derive a fundamental limit for AT and propose the design of a chiral photonic crystal offering 84% AT.