Koray Aydin

Frequency tunable near-infrared metamaterials based on VO_2 phase transition

Engineering metamaterials with tunable resonances from mid-infrared to near-infrared wavelengths could have far-reaching consequences for chip based optical devices, active filters, modulators, and sensors. Utilizing the metal-insulator phase transition in vanadium oxide (VO(2)), we demonstrate frequency-tunable metamaterials in the near-IR range, from 1.5 - 5 microns. Arrays of Ag split ring resonators (SRRs) are patterned with e-beam lithography onto planar VO(2) and etched via reactive ion etching to yield Ag/VO(2) hybrid SRRs. FTIR reflection data and FDTD simulation results show the resonant peak position red shifts upon heating above the phase transition temperature. We also show that, by including coupling elements in the design of these hybrid Ag/VO(2) bi-layer structures, we can achieve resonant peak position tuning of up to 110 nm.

Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability

Metamaterial designs are typically limited to operation over a narrow bandwidth dictated by the resonant line width. Here we report a compliant metamaterial with tunability of Δλ ∼ 400 nm, greater than the resonant line width at optical frequencies, using high-strain mechanical deformation of an elastomeric substrate to controllably modify the distance between the resonant elements. Using this compliant platform, we demonstrate dynamic surface-enhanced infrared absorption by tuning the metamaterial resonant frequency through a CH stretch vibrational mode, enhancing the reflection signal by a factor of 180. Manipulation of resonator components is also used to tune and modulate the Fano resonance of a coupled system.

Investigation of magnetic resonances for different split-ring resonator parameters and designs

We investigate the magnetic resonance of split-ring resonators (SRR) experimentally and numerically. The dependence of the geometrical parameters on the magnetic resonance frequency of SRR is studied. We further investigate the effect of lumped capacitors integrated to the SRR on the magnetic resonance frequency for tunable SRR designs. Different resonator structures are shown to exhibit magnetic resonances at various frequencies depending on the number of rings and splits used in the resonators.

Transmission properties of composite metamaterials in free space

We propose and demonstrate a type of composite metamaterial which is constructed by combining thin copper wires and split ring resonators (SRRs) on the same board. The transmission measurements performed in free space exhibit a passband within the stop bands of SRRs and thin wire structures. The experimental results are in good agreement with the predictions of the transfer matrix method simulations.

Equivalent-Circuit Models for the Design of Metamaterials Based on Artificial Magnetic Inclusions

In this paper, we derive quasi-static equivalent-circuit models for the analysis and design of different types of artificial magnetic resonators-i.e., the multiple split-ring resonator, spiral resonator, and labyrinth resonator-which represent popular inclusions to synthesize artificial materials and metamaterials with anomalous values of the permeability in the microwave and millimeter-wave frequency ranges. The proposed models, derived in terms of equivalent circuits, represent an extension of the models presented in a recent publication. In particular, the extended models take into account the presence of a dielectric substrate hosting the metallic inclusions and the losses due to the finite conductivity of the conductors and the finite resistivity of the dielectrics. Exploiting these circuit models, it is possible to accurately predict not only the resonant frequency of the individual inclusions, but also their quality factor and the relative permeability of metamaterial samples made by given arrangements of such inclusions. Finally, the three models have been tested against full-wave simulations and measurements, showing a good accuracy. This result opens the door to a quick and accurate design of the artificial magnetic inclusions to fabricate real-life metamaterial samples with anomalous values of the permeability.

Subwavelength resolution with a negative-index metamaterial superlens

Negative-index metamaterials are candidates for imaging objects with sizes smaller than a half-wavelength. The authors report an impedance-matched, low loss negative-index metamaterial superlens that is capable of resolving subwavelength features of a point source with a 0.13λ resolution, which is the highest resolution achieved by a negative-index metamaterial. By separating two point sources with a distance of λ∕8, they were able to detect two distinct peaks on the image plane. They also showed that the metamaterial based structure has a flat lens behavior.

Experimental observation of true left-handed transmission peaks in metamaterials

We report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of -1.2 dB (-0.3 dB/cm) was obtained. The experimental results agree extremely well with the theoretical calculations.

Enhanced Light Emission from Large-Area Monolayer MoS2 Using Plasmonic Nanodisc Arrays

Single-layer direct band gap semiconductors such as transition metal dichalcogenides are quite attractive for a wide range of electronics, photonics, and optoelectronics applications. Their monolayer thickness provides significant advantages in many applications such as field-effect transistors for high-performance electronics, sensor/detector applications, and flexible electronics. However, for optoelectronics and photonics applications, inherent monolayer thickness poses a significant challenge for the interaction of light with the material, which therefore results in poor light emission and absorption behavior. Here, we demonstrate enhanced light emission from large-area monolayer MoS2 using plasmonic silver nanodisc arrays, where enhanced photoluminescence up to 12-times has been measured. Observed phenomena stem from the fact that plasmonic resonance couples to both excitation and emission fields and thus boosts the light-matter interaction at the nanoscale. Reported results allow us to engineer light-matter interactions in two-dimensional materials and could enable highly efficient photodetectors, sensors, and photovoltaic devices, where photon absorption and emission efficiency highly dictate the device performance.

Capacitor-loaded split ring resonators as tunable metamaterial components

Transmission through split ring resonator (SRR) structures loaded with capacitors is investigated both experimentally and numerically. Magnetic resonance frequency (ωm) is observed to shift to lower frequencies when capacitors are mounted to the various capacitive regions of the SRR structure. The amount of change in ωm depends strongly on the place where the capacitors are loaded. The magnetic resonance is obtained at 0.99GHz with subwavelength SRR size of λ∕42 when the capacitor (C=2.2pF) is integrated at the split region of the outer ring.

Symmetry breaking and strong coupling in planar optical metamaterials

We demonstrate narrow transmission resonances at near-infrared wavelengths utilizing coupled asymmetric split-ring resonators (SRRs). By breaking the symmetry of the coupled SRR system, one can excite dark (subradiant) resonant modes that are not readily accessible to symmetric SRR structures. We also show that the quality factor of metamaterial resonant elements can be controlled by tailoring the degree of asymmetry. Changing the distance between asymmetric resonators changes the coupling strength and results in resonant frequency tuning due to resonance hybridization.