Teun-Teun Kim

Switching terahertz waves with gate-controlled active graphene metamaterials

The extraordinary electronic properties of graphene provided the main thrusts for the rapid advance of graphene electronics. In photonics, the gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of photons with graphene, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications owing to its non-resonant Drude-like behaviour. Here, we demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light-matter interaction in the graphene layer can be greatly enhanced by the strong resonances of the metamaterial. Although the thickness of the embedded single-layer graphene is more than six orders of magnitude smaller than the wavelength (<λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 47% and its phase by 32.2° at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, which is indicative of persistent photonic memory effects.

Reversibly Stretchable and Tunable Terahertz Metamaterials with Wrinkled Layouts

The use of wrinkling provides a generic route to stretchable metamaterials, with unprecedented terahertz tunability. The wrinkled metamaterial can be stretched reversibly up to 52.5%; the structural integrity can be maintained during at least 100 stretching/relaxing cycles. Importantly, the wrinkling of meta-atoms offers a deterministic way to achieve controlled broadening of electrical resonance.

Asymmetric Mach-Zehnder filter based on self-collimation phenomenon in two-dimensional photonic crystals

A two-dimensional photonic crystal asymmetric Mach-Zehnder filter (AMZF) based on the self-collimation effect is studied by numerical simulations and experimental measurements in microwave region. A self-collimated beam is effectively controlled by employing line-defect beam splitters and mirrors. The measured transmission spectra at the two output ports of the AMZF sinusoidally oscillate with the phase difference of pi in the self-collimation frequency range. Position of the transmission peaks and dips can be controlled by varying the size of the defect rod of perfect mirrors, and therefore this AMZF can be used as a tunable power filter.

Nondispersive optical activity of meshed helical metamaterials

Extreme optical properties can be realized by the strong resonant response of metamaterials consisting of subwavelength-scale metallic resonators. However, highly dispersive optical properties resulting from strong resonances have impeded the broadband operation required for frequency-independent optical components or devices. Here we demonstrate that strong, flat broadband optical activity with high transparency can be obtained with meshed helical metamaterials in which metallic helical structures are networked and arranged to have fourfold rotational symmetry around the propagation axis. This nondispersive optical activity originates from the Drude-like response as well as the fourfold rotational symmetry of the meshed helical metamaterials. The theoretical concept is validated in a microwave experiment in which flat broadband optical activity with a designed magnitude of 45° per layer of metamaterial is measured. The broadband capabilities of chiral metamaterials may provide opportunities in the design of various broadband optical systems and applications.

Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

Experimental demonstration of reflection minimization at two-dimensional photonic crystal interfaces via antireflection structures

We experimentally confirm that the antireflection structures effectively minimize unnecessary reflections of self-collimated microwave beams at the interfaces of a two-dimensional photonic crystal, which is composed of cylindrical alumina rods. Optimized design parameters for the antireflection structures are obtained from the one-dimensional antireflection coating theory and the finite-difference time-domain simulations. Measured transmittance through the photonic crystal samples with and without the antireflection structures agree well with the simulation results. The measured results show that the photonic crystal with an antireflection structure yields about 90% transmission of incident power on the average in the frequency range of 12.0 to 13.0 GHz.

Surface plasmon polariton resonance and transmission enhancement of light through subwavelength slit arrays in metallic films

In this study, we present experimentally measured transmission enhancement of microwaves through periodic slit arrays in metallic films. Enhanced transmission peaks and sharp transmission dips are clearly observed around the theoretically expected surface plasmon polariton(SPP) resonance frequencies. Dependence of the transmittance spectra on the geometrical properties of slits is also demonstrated by varying the slit width, slit periodicity and the thickness of metallic films. Transmission peaks and dips are originated from the coupling between the incident light and SPPs which are caused by the slit array that acts like a grating coupler. The obtained results are theoretically explained by solving the Maxwells equations and by the diffraction theory with appropriate boundary conditions, and they are in good agreement with those calculated by the finite-difference time-domain method.

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.

Ring-type Fabry-Pérot filter based on the self-collimation effect in a 2D photonic crystal

We propose a ring-type Fabry-Pérot filter (RFPF) based on the self-collimation effect in photonic crystals. The transmission characteristics of self-collimated beams are experimentally measured in this structure and compared with the results obtained with the simulations. Bending and splitting mechanisms of light beams by the line defects introduced into the RFPF are used to control the self-collimated beam. Antireflection structures are also employed at the input and output photonic crystal interfaces in order to minimize the coupling loss. Reflectance of the line-defect beam splitters can be controlled by adjusting the radius of defect rods. As the reflectance of the line-defect beam splitters increases, the transmission peaks become sharper and the filter provides a Q-factor as high as 1037. Proposed RFPF can be used as a sharply tuned optical filter or as a spectrum analyzer based on the self-collimation phenomena of photonic crystals. Furthermore, it is suitable for a building block of photonic integrated circuits, as it does not back reflect any of the incoming self-collimated beams owing to the antireflection structure applied.

Experimental demonstration of bending and splitting of self-collimated beams in two-dimensional photonic crystals

The authors have experimentally demonstrated the bending and splitting phenomena of self-collimated microwave beams in a two-dimensional square lattice photonic crystal composed of alumina rods. The bending and splitting were achieved by introducing a line defect in the photonic crystal. The power ratio of two split beams can be controlled by varying the radii of rods in the line defect.