Surface potential-adjusted surface states in 3D topological photonic crystals.

Surface potential in a topological matter could unprecedentedly localize the waves. However, this surface potential is yet to be exploited in topological photonic systems. Here, we demonstrate that photonic surface states can be induced and controlled by the surface potential in a dielectric double gyroid (DG) photonic crystal. The basis translation in a unit cell enables tuning of the surface potential, which in turn regulates the degree of wave localization. The gradual modulation of DG photonic crystals enables the generation of a pseudomagnetic field. Overall, this study shows the interplay between surface potential and pseudomagnetic field regarding the surface states. The physical consequences outlined herein not only widen the scope of surface states in 3D photonic crystals but also highlight the importance of surface treatments in a photonic system.

Curved InGaAs nanowire array lasers grown directly on silicon-on-insulator.

Catalyst-free, selective nano-epitaxy of III-V nanowires provides an excellent materials platform for designing and fabricating ultra-compact, bottom-up photonic crystal lasers. In this work, we propose a new type of photonic crystal laser with a curved cavity formed by InGaAs nanowires grown directly on silicon-on-insulator. This paper investigates the effect of the radius of the curved cavity on the emission wavelength, quality factor as well as laser beam emission angle. We find that the introduction of curvature does not degrade the quality factor of the cavity, thereby offering another degree of freedom when designing low-footprint multiwavelength photonic crystal nanowire lasers. The experimentally demonstrated device shows a lasing threshold of 157 µJ/cm2 at room temperature at telecom O-band wavelengths.

Tailoring topological edge states with photonic crystal nanobeam cavities.

The realization of topological edge states (TESs) in photonic systems has provided unprecedented opportunities for manipulating light in novel manners. The Su-Schrieffer-Heeger (SSH) model has recently gained significant attention and has been exploited in a wide range of photonic platforms to create TESs. We develop a photonic topological insulator strategy based on SSH photonic crystal nanobeam cavities. In contrast to the conventional photonic SSH schemes which are based on alternately tuned coupling strength in one-dimensional lattice, our proposal provides higher flexibility and allows tailoring TESs by manipulating mode coupling in a two-dimensional manner. We reveal that the proposed hole-array based nanobeams in a dielectric membrane can selectively tailor single or double TESs in the telecommunication region by controlling the coupling strength of the adjacent SSH nanobeams in both transverse and axial directions. Our finding provides an additional degree of freedom in exploiting the SSH model for integrated topological photonic devices and functionalities based on the well-established photonic crystal nanobeam cavity platforms.

Suppressing spatiotemporal lasing instabilities with wave-chaotic microcavities.

Spatiotemporal instabilities are widespread phenomena resulting from complexity and nonlinearity. In broad-area edge-emitting semiconductor lasers, the nonlinear interactions of multiple spatial modes with the active medium can result in filamentation and spatiotemporal chaos. These instabilities degrade the laser performance and are extremely challenging to control. We demonstrate a powerful approach to suppress spatiotemporal instabilities using wave-chaotic or disordered cavities. The interference of many propagating waves with random phases in such cavities disrupts the formation of self-organized structures such as filaments, resulting in stable lasing dynamics. Our method provides a general and robust scheme to prevent the formation and growth of nonlinear instabilities for a large variety of high-power lasers.

Frequency-domain modelling of gain in pump-probe experiment by an inhomogeneous medium.

Introduction of a gain medium in lossy plasmonic metamaterials reduces and compensates losses or even amplifies an incident light often with nonlinear optical effect. Here, optical gain in a pump-probe experimental setup is effectively calculated in the frequency-domain by approximating a gain material as an inhomogeneous medium. Spatially varying local field amplitudes of the pump and probe beams are included in the model to reproduce the inhomogeneous gain effect, in which population inversion occurs most strongly near the surface and decays along the propagation direction. We demonstrate that transmission spectra calculated by this method agree well with finite-difference time-domain simulation results. This simplified approach of gain modelling offers an easy and reliable way to analyze wave propagation in a gain medium without nonlinear time-domain calculation.

Electrical access to critical coupling of circularly polarized waves in graphene chiral metamaterials.

Active control of polarization states of electromagnetic waves is highly desirable because of its diverse applications in information processing, telecommunications, and spectroscopy. However, despite the recent advances using artificial materials, most active polarization control schemes require optical stimuli necessitating complex optical setups. We experimentally demonstrate an alternative-direct electrical tuning of the polarization state of terahertz waves. Combining a chiral metamaterial with a gated single-layer sheet of graphene, we show that transmission of a terahertz wave with one circular polarization can be electrically controlled without affecting that of the other circular polarization, leading to large-intensity modulation depths (>99%) with a low gate voltage. This effective control of polarization is made possible by the full accessibility of three coupling regimes, that is, underdamped, critically damped, and overdamped regimes by electrical control of the graphene properties.

Control of terahertz nonlinear transmission with electrically gated graphene metadevices.

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

Chiral metamaterials: enhancement and control of optical activity and circular dichroism.

The control of the optical activity and ellipticity of a medium has drawn considerable attention due to the recent developments in metamaterial design techniques and a deeper understanding of the light matter interaction in composite metallic structures. Indeed, recently proposed designs of metaatoms have enabled the realisation of materials with unprecedented chiral optical properties e.g. strong optical activity, broadband optical activity, and nondispersive zero ellipticity. Combining chiral metamaterials with nonlinear materials has opened up new possibilities in the field of nonlinear chirality as well as provided the foundation for switchable chiral devices. Furthermore, chirality together with hyperbolicity can be used to realise new exciting materials such as photonic topological insulators. In this review, we will outline the fundamental principles of chiral metamaterials and report on recent progress in providing the foundations for promising applications of switchable chiral metamaterials.

Optical activity enhanced by strong inter-molecular coupling in planar chiral metamaterials.

The polarization of light can be rotated in materials with an absence of molecular or structural mirror symmetry. While this rotating ability is normally rather weak in naturally occurring chiral materials, artificial chiral metamaterials have demonstrated extraordinary rotational ability by engineering intra-molecular couplings. However, while in general, chiral metamaterials can exhibit strong rotatory power at or around resonances, they convert linearly polarized waves into elliptically polarized ones. Here, we demonstrate that strong inter-molecular coupling through a small gap between adjacent chiral metamolecules can lead to a broadband enhanced rotating ability with pure rotation of linearly polarized electromagnetic waves. Strong inter-molecular coupling leads to nearly identical behaviour in magnitude, but engenders substantial difference in phase between transmitted left and right-handed waves.

Tunable 3D extended self-assembled gold metamaterials with enhanced light transmission.

The optical properties of metamaterials made by block copolymer self-assembly are tuned by structural and environmental variations. The plasma frequency red-shifts with increasing lattice constant and blue-shifts as the network filling fraction increases. Infiltration with dielectric liquids leads also to a red-shift of the plasma edge. A 300 nm-thick slab of gyroid-structured gold has a remarkable transmission of 20%.

On the origin of chirality in nanoplasmonic gyroid metamaterials.

Metallic single gyroids, a new class of self-assembled nanoplasmonic metamaterials, are analyzed on the basis of a tri-helical metamaterial model. The physical mechanisms underlying the chiral optical behavior of the nanoplasmonic single gyroid are identified and it is shown that the optical chirality in this metallic structure is primarily determined by structural chirality and the connectivity of helices along the main cubic axes.

Self-collimation phenomena of surface waves in structured perfect electric conductors and metal surfaces.

We demonstrate that surface waves in structured perfect electric conductor surfaces can be self-collimated using the finite-difference time-domain method. The self-collimation frequency is obtained from the equi-frequency contours of a perfect electric conductor patterned with an array of square holes. The field patterns of the self-collimated surface wave, obtained using the periodic boundary conditions, show that the surface waves propagate with almost no spreading. We also show that self-collimation phenomena can be observed for the hybrid surface plasmon waves in structured metal surfaces using the finite-difference time-domain method with the Drude model. It is shown that for a structured silver surface the self-collimation can be achieved at the frequencies in the infrared region.