Valley topological materials, in which electrons possess valley pseudospin, have attracted a growing interest recently. The additional valley degree of freedom offers a great potential for its use in information encoding and processing. The valley pseudospin and valley edge transport have been investigated in photonic and phononic crystals for electromagnetic and acoustic waves, respectively. In this work, by using a micromanufacturing technology, valley topological materials are fabricated on silicon chips, which allows the observation of gyral valley states and valley edge transport for elastic waves. The edge states protected by the valley topology are robust against the bending and weak randomness of the channel between distinct valley Hall phases. At the channel intersection, a counterintuitive partition of the valley edge states manifests for elastic waves, in which the partition ratio can be freely adjusted. These results may enable the creation of on-chip high-performance micro-ultrasonic materials and devices.

On-chip valley topological materials for elastic wave manipulation.

Manipulating the properties of the isofrequency contours (IFCs) of materials provides a powerful means of controlling the interaction between light and matter. Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic materials with hyperbolic IFCs, have been intensively investigated. Because of their open dispersion curves, HMMs support propagating high-k modes and possess an enhanced photonic density of states. As a result, HMMs can be utilized to realize hyperlenses breaking the diffraction limit, metacavity lasers with subwavelength scale, high-sensitivity sensors, long-range energy transfer, and so on. Aimed at those who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs but also discusses dispersion manipulation in HMMs and HMM-based structures such as hypercrystals. Both theoretical methods and experimental platforms are detailed. Finally, some potential applications associated with hyperbolic dispersion are introduced.

Hyperbolic metamaterials: From dispersion manipulation to applications

The valley degree of freedom in crystals offers great potential for manipulating classical waves, however, few studies have investigated valley states with complex wavenumbers, valley states in graded systems, or dispersion tuning for valley states. Here, we present tunable valley phononic crystals (PCs) composed of hybrid channel-cavity cells with three tunable parameters. Our PCs support valley states and Dirac cones with complex wavenumbers. They can be configured to form chirped valley PCs in which edge modes are slowed to zero group velocity states, where the energy at different frequencies accumulates at different designated locations. They enable multiple functionalities, including tuning of dispersion relations for valley states, robust routing of surface acoustic waves, and spatial modulation of group velocities. This work may spark future investigations of topological states with complex wavenumbers in other classical systems, further study of topological states in graded materials, and the development of acoustic devices. The valley degree of freedom gives additional flexibility to tunable phononic and photonic crystals. Here, the authors realise a honeycomb phononic structure where both the size of the cavities and of the air channel can be actively tuned, allowing several functionalities in a broad frequency range.

https://rdw.rowan.edu/cgi/viewcontent.cgi?article=1109&context=engineering_facpub

Dispersion tuning and route reconfiguration of acoustic waves in valley topological phononic crystals.

Manipulating the property of iso-frequency contour (IFC) will provide a powerful control for the interaction between light and matter. Importantly, hyperbolic metamaterials (HMMs), a class of artificial anisotropic materials with hyperbolic IFC have been intensively investigated. Because of the open dispersion curves, HMMs support propagating high-k modes and possess enhanced photonic density of states. As a result, HMMs can be utilized to realize hyper-lens breaking the diffraction limit, meta-cavity laser with subwavelength scale, high sensitivity sensor, long-range energy transfer and so on. In order to make it easier for people who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs, but also discusses the dispersion manipulation of HMMs and HMM-based structures such as hypercrystals. The theoretical methods and experimental platforms are given in this tutorial. Finally, some potential applications associated with hyperbolic dispersion are also introduced

Hyperbolic Metamaterials: From Dispersion Manipulation to Application

Recent advances in condensed matter physics have shown that the spin degree of freedom of electrons can be efficiently exploited in the emergent field of spintronics, offering unique opportunities for efficient data transfer, computing, and storage ( 1 – 3 ). These concepts have been inspiring analogous approaches in photonics, where the manipulation of an artificially engineered pseudospin degree of freedom can be enabled by synthetic gauge fields acting on light ( 4 – 6 ). The ability to control these degrees of freedom significantly expands the landscape of available optical responses, which may revolutionize optical computing and the basic means of controlling light in photonic devices across the entire electromagnetic spectrum. We demonstrate a new class of photonic systems, described by effective Hamiltonians in which competing synthetic gauge fields, engineered in pseudospin, chirality/sublattice, and valley subspaces, result in bandgap opening at one of the valleys, whereas the other valley exhibits Dirac-like conical dispersion. We show that this effective response has marked implications on photon transport, among which are as follows: (i) a robust pseudospin- and valley-polarized one-way Klein tunneling and (ii) topological edge states that coexist within the Dirac continuum for opposite valley and pseudospin polarizations. These phenomena offer new ways to control light in photonics, in particular, for on-chip optical isolation, filtering, and wave-division multiplexing by selective action on their pseudospin and valley degrees of freedom.

https://advances.sciencemag.org/content/advances/4/5/eaap8802.full.pdf

Spin- and valley-polarized one-way Klein tunneling in photonic topological insulators

Valley-dependent propagation of light in an artificial photonic hexagonal lattice, akin to electrons in graphene, is investigated in microwave regime. Both numerical and experimental results show that the valley degeneracy in the photonic graphene is broken when the frequency is away from the Dirac point. The peculiar anisotropic wave transport property due to distinct valleys is analyzed using the equifrequency contours. More interestingly, the valley-dependent self-collimation and beam splitting phenomena are experimentally demonstrated with the armchair and zigzag interfaces, respectively. Our results confirm that there are two inequivalent Dirac points that lead to two distinct valleys in photonic graphene, which could be used to control the flow of light and might be used to carry information in valley polarized beam splitter, collimator or guiding device.

Observation of valley-dependent beams in photonic graphene

The generation and control of valley pseudospin currents are the core of valleytronics Here, the photonic analogy for generation and control of valley pseudospin currents using the pseudomagnetic fields induced in strained graphene is investigated in a microwave regime In photonic graphene with uniaxial distortion, photons in two different valleys experience pseudomagnetic fields with opposite signs, and valley-dependent propagations in bended paths are observed The external-field-free photonic transportation behavior may be very useful in controlling the flow of light in future valley-polarized devices

Valley-dependent beams controlled by pseudomagnetic field in distorted photonic graphene

At the Dirac-like point at the Brillouin zone center, the photonic crystals (PhCs) can mimic a zero-index medium In the band structure, an additional flat band of longitudinal mode will intersect the Dirac cone This longitudinal mode can be excited in PhCs with finite sizes at the Dirac-like point By introducing positional shift in the PhCs, we study the dependence of the longitudinal mode on the disorder At the Dirac-like point, the transmission peak induced by the longitudinal mode decreases as the random degree increases However, at a frequency slightly above the Dirac-like point, in which the longitudinal mode is absent, the transmission is insensitive to the disorder because the effective index is still near zero and the effective wavelength in the PhC is very large

Xiao Wang

Papers

Observation of valley-dependent beams in photonic graphene

Valley-dependent beams controlled by pseudomagnetic field in distorted photonic graphene

Transport properties of disordered photonic crystals around a Dirac-like point