Ashish Chanana

Hiding multi-level multi-color images in terahertz metasurfaces

Our work presents a novel technique to encode information onto terahertz metasurfaces comprised of geometrically identical unit cell arrays. Previous demonstrations on metasurfaces or frequency-selective surfaces have shown interesting concepts to engineer electromagnetic radiation, but such designs often require a spatial arrangement of geometrically varying unit cells, either by shape, size, orientation, etc. In some cases, the output response can be mapped by examining the arrangement of atoms. Here, we show that by fabricating an array of resonant structures that are nominally identical visually, but where individual structures can have different conductivities, we can hide image information that is revealed when imaged using the appropriate terahertz frequency and polarization. This is achieved because changes in the structure’s conductivity correspond to changes in the depth of the resonant absorption observed in transmission. Using the simplest unit cell consisting of a single dipole, we create images that have up to 9 different discernible gray levels when interrogated at a single frequency. When a slightly more complex cross structure is used in the unit cell, 36 discernible levels are encoded in the image using two different polarizations. Finally, when the unit cell consists of multiple dipoles designed for multiple frequencies, we observe 64 unique colors in an encoded image. We believe our results present a unique approach for hiding information that could be applied to security-related 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.

Non-Drude like behaviour of metals in the terahertz spectral range

Abstract We review measurements of the dielectric properties of metals, which have resurfaced as a timely topic given the ongoing interest in plasmonics across a broad range of the electromagnetic spectrum. It is generally accepted that the Drude model fully describes the optical response of metals. This is certainly true at optical frequencies. This also appears to be the case when THz time-domain spectroscopy is used to measure the properties of thin films. However, for a variety of plasmonics-based implementations in the terahertz (THz) spectral range, there appear to be significant discrepancies. We discuss these observations, as well as a new family of measurement techniques based on the excitation and detection of surface plasmon-polaritons. Finally, we conclude with a brief discussion regarding the implications of these new measurements for the field of THz plasmonics.

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.

Ultrafast frequency-agile terahertz devices using methylammonium lead halide perovskites

We demonstrate new capabilities for frequency-agile terahertz devices using perovskites. The ability to control the response of metamaterial structures can facilitate the development of new terahertz devices, with applications in spectroscopy and communications. We demonstrate ultrafast frequency-agile terahertz metamaterial devices that enable such a capability, in which multiple perovskites can be patterned in each unit cell with micrometer-scale precision. To accomplish this, we developed a fabrication technique that shields already deposited perovskites from organic solvents, allowing for multiple perovskites to be patterned in close proximity. By doing so, we demonstrate tuning of the terahertz resonant response that is based not only on the optical pump fluence but also on the optical wavelength. Because polycrystalline perovskites have subnanosecond photocarrier recombination lifetimes, switching between resonances can occur on an ultrafast time scale. The use of multiple perovskites allows for new functionalities that are not possible using a single semiconducting material. For example, by patterning one perovskite in the gaps of split-ring resonators and bringing a uniform thin film of a second perovskite in close proximity, we demonstrate tuning of the resonant response using one optical wavelength and suppression of the resonance using a different optical wavelength. This general approach offers new capabilities for creating tunable terahertz devices.

Colour selective control of terahertz radiation using two-dimensional hybrid organic inorganic lead-trihalide perovskites

Controlling and modulating terahertz signals is of fundamental importance to allow systems level applications. We demonstrate an innovative approach for controlling the propagation properties of terahertz (THz) radiation, through use of both the excitation optical wavelength (colour) and intensity. We accomplish this using two-dimensional (2D) layered hybrid trihalide perovskites that are deposited onto silicon substrates. The absorption properties of these materials in the visible range can be tuned by changing the number of inorganic atomic layers in between the organic cation layers. Optical absorption in 2D perovskites occurs over a broad spectral range above the bandgap, resulting in free carrier generation, as well as over a narrow spectral range near the bandedge due to exciton formation. We find that only the latter contribution gives rise to photo-induced THz absorption. By patterning multiple 2D perovskites with different optical absorption properties onto a single device, we demonstrate both colour selective modulation and focusing of THz radiation. These findings open new directions for creating active THz devices.All-optical approaches to modulate signals are of wide interest. Here the authors demonstrate the use of two-dimensional perovskites on silicon for optically controlling the propagation and attenuation of terahertz radiation in the visible by changing the number of atomic layers.

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.

Comparison of unit cell coupling for grating‐gate and high electron mobility transistor array THz resonant absorbers

We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the...

Experimental demonstration of enhanced terahertz coupling to plasmon in ultra-thin membrane AlGaN/GaN HEMT arrays

THz technology offers multiple applications in areas such as remote sensing, spectroscopy, biomedical imaging, and ultra-wide bandwidth communications [1]. However, obtaining high-frequency performance at THz frequencies has proven challenging in conventional electronic devices. This difficulty motivated the exploration of unconventional transport mechanisms such as electron plasma waves. Two dimensional electron gases (2DEGs) in semiconductor heterostructures can allow for collective motion of electrons, i.e. plasma waves, whose group velocity is >10X larger than typical electron drift velocities (i.e. vg >108 cm/s) [2-3]. Devices based on electron plasma waves have attracted significant attention during recent years for THz generation, detection and amplification [4]. In this context, efficient coupling of external THz radiation into and out of plasmons in semiconductor heterostructures is essential for the operation of these devices. A conventional approach to excite plasmons in a 2DEG is via a grating gate coupler as illustrated in Fig. 1(a). In a grating gate configuration, adjacent unit-cells interact with each other making this a coupled resonant system. In contrast, via addition of source (S) and drain (D) electrodes, in a HEMT array configuration as depicted in Fig. 1(b), every unit cell becomes effectively independent. In this configuration, the THz to plasmon coupling is enhanced due to a cooperative effect by synchronizing the electron plasma waves in each unit-cell of the array as theoretically discussed by Popov et al [5]. Here we present the first experimental demonstration of enhanced THz coupling to electron plasma wave or plasmon in ultra-thin membrane HEMT arrays via plasmon synchronization. A thin-membrane configuration enables us to remove substrate effects and further enhance the coupling. The proposed approach allows: (i) more efficient excitation of high order plasmonic modes, and (ii) superior overall coupling-even in configurations having less number of devices per unit area-. Our results reveal a simple way to enhance the THz to plasmon coupling and thus improve the performance of electron plasma wave based devices; this effect can be exploited, for example, to improve the response of HEMT THz detectors.

Applications of spatially varying conductivity in plasmonics and metamaterials (Conference Presentation)

Conventional plasmonic materials are typically fabricated using a single homogenous metal and structured to obtain useful functionality. Alternatively, structures are occasionally made in which several homogenous materials are deposited using a layer-by-layer process, such as metal-dielectric-metal structures [1]. However additional control over the propagation properties of surface plasmon-polaritons should be possible if the metal conductivity could also be varied spatially. This is not straightforward using conventional microfabrication techniques. We demonstrate the ability to vary the conductivity spatially using a conventional inkjet printer, yielding either step-wise changes or continuous changes in the conductivity. We accomplish this using a commercially available inkjet printer, where one inkjet cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink. By varying the fractional amounts of the two inks in each printed dot, we can spatially vary the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than the bulk silver, while the carbon ink acts as a lossy dielectric at terahertz frequencies. Both inks sinter immediately after being printed on a treated PET transparency. We demonstrate the utility of this approach with both plasmonics and metamaterial applications, demonstrating the ability to control beam profiles, create new filter capabilities and hide images in THz metasurfaces.