Sara Arezoomandan

Graphene-based tunable metamaterial terahertz filters

We propose and describe a micro-machined tunable metamaterial terahertz filter based on graphene. The device structure consists of periodic metallic rings with several gaps where tunable graphene stripes are located. We demonstrate that the filter resonance frequency can be adjusted easily by varying the conductivity of graphene and implement this by changing the number of stacked graphene layers. Moreover, the proposed design is scalable, in the sense that the resonance frequency tuning can be controlled by scaling the inner and outer radius of the metal rings. Using numerical simulations and terahertz time-domain spectroscopy measurements of the fabricated samples, we show that the resonance frequency of the structure can be altered by 40% (i.e., from ∼0.2 THz to ∼0.12 THz) by simply tuning the conductivity of graphene. Importantly, the active area of the device is ≪0.1% of the total unit cell area, which can boost the device speed upon electrostatic actuation.

Geometrical tradeoffs in graphene-based deeply-scaled electrically reconfigurable metasurfaces

In this work we study the terahertz light propagation through deeply-scaled graphene-based reconfigurable metasurfaces, i.e. metasurfaces with unit-cell dimensions much smaller than the terahertz wavelength. These metasurfaces are analyzed as phase modulators for constructing reconfigurable phase gradients along an optical interface for the purpose of beam shaping. Two types of deeply-scaled metacell geometries are analyzed and compared, which consist of: (i) multi split ring resonators, and (ii) multi spiral resonators. Two figures of merit, related to: (a) the loss and (b) the degree of reconfigurability achievable by such metamaterials -when applied in beam shaping applications-, are introduced and discussed. Simulations of these two types of deep-subwavelength geometries, when changing the metal coverage-fraction, show that there is an optimal coverage-fraction that gives the best tradeoff in terms of loss versus degree of reconfigurability. For both types of geometries the best tradeoff occurs when the area covered by the metallic region is around 40% of the metacell total area. From this point of view, reconfigurable deeply-scaled metamaterials can indeed provide a superior performance for beam shaping applications when compared to not deeply-scaled ones; however, counterintuitively, employing very highly-packed structures might not be beneficial for such applications.

Tunable Terahertz Metamaterials Employing Layered 2-D Materials Beyond Graphene

In this study, we extend recent investigations on graphene/metal hybrid tunable terahertz metamaterials to other two-dimensional (2-D) materials beyond graphene. For the first time, use of a nongraphitic 2-D material, molybdenum disulfide (MoS2), is reported as the active medium on a terahertz metamaterial device. For this purpose, high-quality few atomic layer MoS2 films with controlled numbers of layers were deposited on host substrates by means of pulsed laser deposition methods. The terahertz conductivity swing in those films is studied under optical excitation. Although no-appreciable conductivity modulation is observed in single-layer MoS2 samples, a substantial conductivity swing, i.e., 0 to ~0.6 mS, is seen in samples with ~60 atomic layers. Therefore, although exhibiting much smaller maximum terahertz conductivity than that in graphene, which is a consequence of much smaller carrier mobility, MoS2 can still be employed for terahertz applications by means of utilizing multilayer films. With this in mind, we design and demonstrate optically actuated terahertz metamaterials that simultaneously exhibit a large modulation depth (i.e., >2× larger than the intrinsic modulation depth by a bare MoS2 film) and low insertion loss (i.e., <;3 dB). The advantages of using a 2-D material with a bandgap, such as MoS2, rather than a gapless material, such as graphene, are: 1) a reduced insertion loss, which is owed to the possibility of achieving zero minimum conductivity, and 2) an enhanced modulation depth for a given maximum conductivity level, which is due to the possibility of placing the active material in a much closer proximity to the metallic frequency selective surface, thus allowing us to take full advantage of the near-field enhancement. These results indicate the promise of layered 2D materials beyond graphene for terahertz applications.

Large nanoscale electronic conductivity in complex oxide heterostructures with ultra high electron density

We study the two-dimensional electron gas at the interface of NdTiO3 and SrTiO3 to reveal its nanoscale transport properties. At electron densities approaching 1015 cm−2, our terahertz spectroscopy data show conductivity levels that are up to six times larger than those extracted from DC electrical measurements. Moreover, the largest conductivity enhancements are observed in samples intentionally grown with larger defect densities. This is a signature of electron transport over the characteristic length-scales typically probed by electrical measurements being significantly affected by scattering by structural defects introduced during growth, and, a trait of a much larger electron mobility at the nanoscale.

THz characterization and demonstration of visible-transparent/terahertz-functional electromagnetic structures in ultra-conductive La-doped BaSnO3 Films

We report on terahertz characterization of La-doped BaSnO3 (BSO) thin-films. BSO is a transparent complex oxide material, which has attracted substantial interest due to its large electrical conductivity and wide bandgap. The complex refractive index of these films is extracted in the 0.3 to 1.5 THz frequency range, which shows a metal-like response across this broad frequency window. The large optical conductivity found in these films at terahertz wavelengths makes this material an interesting platform for developing electromagnetic structures having a strong response at terahertz wavelengths, i.e. terahertz-functional, while being transparent at visible and near-IR wavelengths. As an example of such application, we demonstrate a visible-transparent terahertz polarizer.

Terahertz spectroscopy and demonstration of visible-transparent/terahertz-functional electromagnetic structures in La-doped BaSnO3 films (Conference Presentation)

BaSnO_3 (BSO) is a transparent conductive oxide. This category of materials is interesting for applications such as optically transparent electrodes in solar cells and displays. This perovskite-material possesses many interesting properties including a wide bandgap, 3 eV, and a high electrical conductivity (exceeding 10^4 S/cm at room-temperature), which make it very interesting for visible-transparent applications. The DC conductivity in BSO can be superior to that in ITO, which is a commonly used transparent conductive oxide. Thin films used in our study were grown by molecular beam epitaxy (MBE) on LSAT substrates. The epitaxial structure of the samples consist of 45 nm of La-doped BSO on top of a 45 nm thick undoped BSO film grown on LSAT. The BSO films were characterized by means of terahertz spectroscopy. The terahertz-extracted optical conductivity was ~0.8x10^3 S/cm in the 0.1 to 2 THz frequency range. Using these films, upon patterning into stripes, we demonstrate a terahertz polarizer. The polarizer is transparent at visible wavelengths, and functional at terahertz wavelengths; it achieves 96% transmission for terahertz polarization parallel to the stripes and 16% transmission for the perpendicular polarization. Furthermore, we also show that resonant structures, such as cross resonators, are also realizable in this material. The large optical conductivity in BSO films at terahertz frequencies, together with being transparent at visible wavelengths, makes it a very good candidate for developing visible-transparent electromagnetic structures responding in the terahertz frequency range.

High-Q terahertz reconfigurable metamaterials using graphene

We propose and discuss high-Q reconfigurable metamaterials based on graphene. The key components of the device are periodic concentric metallic ring resonators with interdigitated fingers, which are placed in-between the rings and provide for the large Q in the metamaterial, as well as several strategically located gaps where active graphene sheets are placed. We can easily adjust the frequency response of the metamaterial by means of varying a couple of parameters, such as the ring dimensions, number of fingers, etc., but also dynamically by means of varying conductivity in graphene.

Terahertz conductivity of ultra high electron concentration 2DEGs in NTO/STO heterostructures

We analyze the terahertz properties of complex oxide hetero-structures with record-high carrier concentration approaching 1015 cm-2. Our results evidence a large room temperature terahertz conductivity, which corresponds to 3X to 6X larger mobility than what is extracted from electrical measurements. That is, in spite of a relatively lower mobility, when taking into account its ultra-large carrier concentration, the 2DEG in complex oxide hetero-structures can still attain a large terahertz conductivity, which is comparable with that in traditional high-mobility semiconductors or large-area CVVD graphene films. Moreover, we also discuss the perspectives off these hetero-structures for terahertz and high frequency electronic applications.

Near-field Enhancement and Optimal Performance in Metamaterial Terahertz Modulators Based on 2D-materials

In this work we analyze metamaterial terahertz modulators consisting of 2D-material/metal hybrid-structures. Influence of the near-field enhancement in the modulator performance is discussed. Devices are fabricated and tested using graphene and MoS2 as active materials.

Design, simulation, and fabrication of a novel reconfigurable graphene terahertz filter (presentation video)

We propose and discuss a novel micro-machined reconfigurable terahertz (THz) filter based on graphene. The key components are the periodic metal ring with several gaps where active graphene strips are placed. We can easily adjust the resonance frequency of the terahertz filter by varying a couple of parameters such as the inner and outer radius of the metal ring, the gap, the number of graphene layers, and by dynamically varying the conductivity of each graphene layer. This work explains the principle of operation of the device, its design based on numerical simulations, its fabrication process, and also presents experimental results.