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.
The control of light fields on subwavelength scales in nanophotonic structures has become ubiquitous, driven by both curiosity and a multitude of applications in fields ranging from biosensing to ...quantum optics. Mapping these fields in detail is crucial, as theoretical modelling is far from trivial and highly dependent on nanoscale geometry. Recent developments of nanoscale field mapping, particularly with near-field microscopy, have not only led to a vastly increased resolution, but have also resulted in increased functionality. The phase and amplitude of different vector components of both the electric and magnetic fields are now accessible, as is the ultrafast temporal or spectral evolution of propagating pulses in nanostructures. In this Review we assess the current state-of-the-art of subwavelength light mapping, highlighting the new science and nanostructures that have subsequently become accessible.
The emergence of two-dimensional transition metal dichalcogenide materials has sparked intense activity in valleytronics, as their valley information can be encoded and detected with the spin angular ...momentum of light. We demonstrate the valley-dependent directional coupling of light using a plasmonic nanowire-tungsten disulfide (WS
) layers system. We show that the valley pseudospin in WS
couples to transverse optical spin of the same handedness with a directional coupling efficiency of 90 ± 1%. Our results provide a platform for controlling, detecting, and processing valley and spin information with precise optical control at the nanoscale.
Controlling photon emission by single emitters with nanostructures is crucial for scalable on-chip information processing. Nowadays, nanoresonators can affect the lifetime of linear dipole emitters, ...while nanoantennas can steer the emission direction. Expanding this control to the emission of orbital angular momentum-changing transitions would enable a future coupling between solid state and photonic qubits. As these transitions are associated with circular dipoles, such control requires knowledge of the interaction of a complex dipole with optical eigenstates containing local helicity. We experimentally map the coupling of classical, circular dipoles to photonic modes in a photonic crystal waveguide. We show that, depending on the combination of the local helicity of the mode and the dipole helicity, circular dipoles can couple to left- or rightwards propagating modes with a near-unity directionality. The experimental maps are in excellent agreement with calculations. Our measurements, therefore, demonstrate the possibility of coupling the spin to photonic pathway.
Solid-state nanopores are single-molecule sensors that hold great potential for rapid protein and nucleic-acid analysis. Despite their many opportunities, the conventional ionic current detection ...scheme that is at the heart of the sensor suffers inherent limitations. This scheme intrinsically couples signal strength to the driving voltage, requires the use of high-concentration electrolytes, suffers from capacitive noise, and impairs high-density sensor integration. Here, we propose a fundamentally different detection scheme based on the enhanced light transmission through a plasmonic nanopore. We demonstrate that translocations of single DNA molecules can be optically detected, without the need of any labeling, in the transmitted light intensity through an inverted-bowtie plasmonic nanopore. Characterization and the cross-correlation of the optical signals with their electrical counterparts verify the plasmonic basis of the optical signal. We demonstrate DNA translocation event detection in a regime of driving voltages and buffer conditions where traditional ionic current sensing fails. This label-free optical detection scheme offers opportunities to probe native DNA–protein interactions at physiological conditions.
We investigate the focusing of surface plasmon polaritons (SPPs) excited with 1.5 microm light in a tapered Au waveguide on a planar dielectric substrate by experiments and simulations. We find that ...nanofocusing can be obtained when the asymmetric bound mode at the substrate side of the metal film is excited. The propagation and concentration of this mode to the tip is demonstrated. No sign of a cutoff waveguide width is observed as the SPPs propagate along the tapered waveguide. Simulations show that such concentrating behavior is not possible for excitation of the mode at the low-index side of the film. The mode that enables the focusing exhibits a strong resemblance to the asymmetric mode responsible for focusing in conical waveguides. This work demonstrates a practical implementation of plasmonic nanofocusing on a planar substrate.
We show with both experiment and calculation that highly confined surface plasmon polaritons can be efficiently excited on metallic nanowires through the process of mode transformation. One specific ...mode in a metallic waveguide is identified that adiabatically transforms to the confined nanowire mode as the waveguide width is reduced. Phase- and polarization-sensitive near-field investigation reveals the characteristic antisymmetric polarization nature of the mode and explains the coupling mechanism.
Valley pseudospin has emerged as a good quantum number to encode information, analogous to spin in spintronics. Two-dimensional transition metal dichalcogenides (2D TMDCs) recently attracted enormous ...attention for their easy access to the valley pseudospin through valley-dependent optical transitions. Different ways have been reported to read out the valley pseudospin state. For practical applications on-chip access to and manipulation of valley pseudospins is paramount, not only to read out, but especially to initiate the valley pseudospin state. Here, we experimentally demonstrate the selective on-chip, optical near-field initiation of valley pseudospins at room-temperature. We exploit a nanowire optical waveguide, such that the local transverse optical spin of its guided modes selectively excites a specific valley pseudospin. Furthermore, spin-momentum locking of the transverse optical spin enables us to flip valley pseudospins with the opposite propagation direction. Thus, we open up ways to realize integrated hybrid opto-valleytronic devices.
It is well known that the quasinormal modes (or resonant states) of photonic structures can be associated with the poles of the scattering matrix of the system in the complex-frequency plane. In this ...work, the inverse problem, i.e., the reconstruction of the scattering matrix from the knowledge of the quasinormal modes, is addressed. We develop a general and scalable quasinormal-mode expansion of the scattering matrix, requiring only the complex eigenfrequencies and the far-field properties of the eigenmodes. The theory is validated by applying it to illustrative nanophotonic systems with multiple overlapping electromagnetic modes. The examples demonstrate that our theory provides an accurate first-principles prediction of the scattering properties, without the need for postulating ad hoc nonresonant channels.
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