Qian Lin

Hyperbolic Weyl point in reciprocal chiral metamaterial

We report the existence of Weyl points in a class of noncentral symmetric metamaterials, which has time reversal symmetry, but does not have inversion symmetry due to chiral coupling between electric and magnetic fields. This class of metamaterial exhibits either type-I or type-II Weyl points depending on its nonlocal response. We also provide a physical realization of such metamaterial consisting of an array of metal wires in the shape of elliptical helices which exhibits type-II Weyl points.

Photonic Weyl point in a two-dimensional resonator lattice with a synthetic frequency dimension.

Weyl points, as a signature of 3D topological states, have been extensively studied in condensed matter systems. Recently, the physics of Weyl points has also been explored in electromagnetic structures such as photonic crystals and metamaterials. These structures typically have complex three-dimensional geometries, which limits the potential for exploring Weyl point physics in on-chip integrated systems. Here we show that Weyl point physics emerges in a system of two-dimensional arrays of resonators undergoing dynamic modulation of refractive index. In addition, the phase of modulation can be controlled to explore Weyl points under different symmetries. Furthermore, unlike static structures, in this system the non-trivial topology of the Weyl point manifests in terms of surface state arcs in the synthetic space that exhibit one-way frequency conversion. Our system therefore provides a versatile platform to explore and exploit Weyl point physics on chip.

Pulse shortening in an actively mode-locked laser with parity-time symmetry

We consider an actively mode-locked laser system with parity-time symmetry. The system consists of two ring cavities, each incorporating an amplitude modulator operating at the same modulation frequency, but with the modulation phases differing by π. We show that spontaneous parity-time symmetry breaking can be used to shorten the temporal width of the pulse generated through active mode locking in this system. Our work highlights the importance of applying the concept of parity-time symmetry in pulsed laser systems.We consider an actively mode-locked laser system with parity-time symmetry. The system consists of two ring cavities, each incorporating an amplitude modulator operating at the same modulation frequency, but with the modulation phases differing by π. We show that spontaneous parity-time symmetry breaking can be used to shorten the temporal width of the pulse generated through active mode locking in this system. Our work highlights the importance of applying the concept of parity-time symmetry in pulsed laser systems.

Resonator-free realization of effective magnetic field for photons

We propose to create an effective magnetic field for photons in a two-dimensional waveguide network with strong scattering at waveguide junctions. The effective magnetic field is realized by imposing a direction-dependent phase along each waveguide link. Such a direction-dependent phase can be produced by dynamic modulation or by the magneto-optical effect. Compared to previous proposals for creating an effective magnetic field for photons, this scheme is resonator-free, thus potentially reduces the experimental complexity. We also show that such a waveguide network can be used to explore photonic analogue of integer quantum Hall effect for massless particles.

A three-dimensional photonic topological insulator using a two-dimensional ring resonator lattice with a synthetic frequency dimension

Topology traps photons in a one-way channel in the frequency space with the help of ring resonators. In the development of topological photonics, achieving three-dimensional topological insulators is of notable interest since it enables the exploration of new topological physics with photons and promises novel photonic devices that are robust against disorders in three dimensions. Previous theoretical proposals toward three-dimensional topological insulators use complex geometries that are challenging to implement. On the basis of the concept of synthetic dimension, we show that a two-dimensional array of ring resonators, which was previously demonstrated to exhibit a two-dimensional topological insulator phase, automatically becomes a three-dimensional topological insulator when the frequency dimension is taken into account. Moreover, by modulating a few of the resonators, a screw dislocation along the frequency axis can be created, which provides robust one-way transport of photons along the frequency axis. Demonstrating the physics of screw dislocation in a topological system has been a substantial challenge in solid-state systems. Our work indicates that the physics of three-dimensional topological insulators can be explored in standard integrated photonic platforms, leading to opportunities for novel devices that control the frequency of light.

Topological and complex birefringent metamaterial (Conference Presentation)

We discuss novel electromagnetic effects in topological metamaterial and in complex birefringent meta material. In particular, we discuss the creation of novel topology using meta-material geometry., We also discuss three-dimensional meta-materials with balanced gain and loss for the purpose of achieving arbitrary control of a pair of polarization states.

Dynamic modulation approach to topological photonics(Conference Presentation)

We show that dynamic refractive index modulation provides a route towards non-reciprocal topological photonics. In particular, the phase of the modulation provides an effective gauge field for photons that breaks time-reversal symmetry, and can be used to create a wide range of topological effects, in both real space as well as in spaces that involve a synthetic frequency dimension. These topological effects are pointing to new capabilities for controlling the properties of light.

Light guiding by gauge field for photons

We propose a waveguiding mechanism based on the effective gauge potential for photons, where the core and cladding regions have the same dispersion relation but are subject to different gauge potentials. This can be realized in a dynamically modulated resonator lattice, and provides a dynamically reconfigurable mechanism for generating a one-way waveguide.