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Micro-/nanocavities that combine high quality factor (Q) and small mode volume (V) have been used to enhance light–matter interactions for cavity quantum electrodynamics (cQED). Whispering gallery mode (WGM) geometries such as microdisks and microrings support high-Q and are design- and fabrication-friendly, but V is often limited to tens of cubic wavelengths to avoid WGM radiation. The stronger modal confinement provided by either one-dimensional or two-dimensional photonic crystal defect geometries can yield sub-cubic-wavelength V, yet the requirements on precise design and dimensional control are typically much more stringent to ensure high-Q. Given their complementary features, there has been sustained interest in geometries that combine the advantages of WGM and photonic crystal cavities. Recently, a “microgear” photonic crystal ring (MPhCR) has shown promise in enabling additional defect localization ( > $ > $ 10× reduction of V) of a WGM, while maintaining high-Q ( ≈ 1 0 6 ) $(\approx 1{0}^{6})$ and other WGM characteristics in ease of coupling and design. However, the unit cell geometry used is unlike traditional PhC cavities, and etched surfaces may be too close to embedded quantum nodes (quantum dots, atomic defect spins, etc.) for cQED applications. Here, we report two novel PhCR designs with “rod” and “slit” unit cells, whose geometries are more traditional and suitable for solid-state cQED. Both rod and slit PhCRs have high-Q ( > 1 0 6 ) $( > 1{0}^{6})$ with WGM coupling properties preserved. A further ≈10× reduction of V by defect localization is observed in rod PhCRs. Moreover, both fundamental and 2nd-order PhC modes co-exist in slit PhCRs with high Qs and good coupling. Our work showcases that high-Q/V PhCRs are in general straightforward to design and fabricate and are a promising platform to explore for cQED.

Continuous wave optical parametric oscillation (OPO) provides a flexible approach for accessing mid-infrared wavelengths between 2 $\mu$m to 5 $\mu$m, but has not yet been integrated into silicon nanophotonics. Typically, Kerr OPO uses a single transverse mode family for pump, signal, and idler modes, and relies on a delicate balance to achieve normal (but close-to-zero) dispersion near the pump and the requisite higher-order dispersion needed for phase- and frequency-matching. Within integrated photonics platforms, this approach results in two major problems. First, the dispersion is very sensitive to geometry, so that small fabrication errors can have a large impact. Second, the device is susceptible to competing nonlinear processes near the pump. In this letter, we propose a flexible solution to infrared OPO that addresses these two problems, by using a silicon nitride photonic crystal microring (PhCR). The frequency shifts created by the PhCR bandgap enable OPO that would otherwise be forbidden. We report an intrinsic optical quality factor up to (1.2 $\pm$ 0.1)$\times$10$^6$ in the 2 $\mu$m band, and use a PhCR ring to demonstrate an OPO with threshold power of (90 $\pm$ 20) mW dropped into the cavity, with the pump wavelength at 1998~nm, and the signal and idler wavelengths at 1937 nm and 2063 nm, respectively. We further discuss how to extend OPO spectral coverage in the mid-infrared. These results establish the PhCR OPO as a promising route for integrated laser sources in the infrared., Comment: 4 pages, 3 figures

We use inverse design to develop an optimized nanophotonic geometry for efficient, waveguide-coupled, path-entangled photon pair sources based on single embedded quan-tum dots.

Magneto-electroluminescence (MEL) represents the electroluminescence intensity change upon application of an external magnetic field. We show that the MEL field response in “magnetic” organic light-emitting diodes, where one electrode is ferromagnetic (FM), is a powerful technique for measuring the induced fringe field, BF, from the FM electrode in the organic layer. We found that the in-plane fringe field, BF∥, from 3 nm Co and Ni80Fe20 FM electrodes is proportional to the applied field, B∥. The fringe field of the 3 nm Ni80Fe20 film was also investigated for an applied out-of-plane magnetic field, B⊥. We found that the out-of-plane fringe field has two components: a component that is parallel or antiparallel to B⊥ and remains unchanged with the distance, d, from the FM electrode and the other component that is highly inhomogeneous, parallel to the surface, and steeply decreases with d. We show that the obtained BF is independent of the underlying mechanism for the MEL(B) response and thus may be c...

Three-dimensional (3D) semimetals have been predicted and demonstrated to have a wide variety of interesting properties associated with their linear energy dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as graphene, Cd3As2 has shown ultrahigh mobility and large Fermi velocity and has been hypothesized to support plasmons at terahertz frequencies. In this work, we experimentally demonstrate synthesis of high-quality large-area Cd3As2 thin films through thermal evaporation as well as the experimental realization of plasmonic structures consisting of periodic arrays of Cd3As2 stripes. These arrays exhibit sharp resonances at terahertz frequencies with associated quality factors ( Q) as high as ∼3.7 (at 0.82 THz). Such spectrally narrow resonances can be understood on the basis of a long momentum scattering time, which in our films can approach ∼1 ps at room temperature. Moreover, we demonstrate an ultrafast tunable response through excitation of photoinduced carriers in optical pump/terahertz probe experiments. Our results evidence that the intrinsic 3D nature of Cd3As2 might provide for a very robust platform for terahertz plasmonic applications. Moreover, the long momentum scattering time as well as large kinetic inductance in Cd3As2 also holds enormous potential for the redesign of passive elements such as inductors and hence can have a profound impact in the field of RF integrated circuits.

Utilizing the spin degree of freedom of photoexcitations in hybrid organic inorganic perovskites for quantum information science applications has been recently proposed and explored. However, it is still unclear whether the stable photoexcitations in these compounds correspond to excitons, free/trapped electron-hole pairs, or charged exciton complexes such as trions. Here we investigate quantum beating oscillations in the picosecond time-resolved circularly polarized photoinduced reflection of single crystal methyl-ammonium tri-iodine perovskite (MAPbI

We demonstrate hybrid quantum photonic circuits comprising Si3N4 waveguides featuring losses in the dB/m range, with directly integrated quantum dot based single-photon sources.

We report on the anisotropy of the terahertz permittivity of β-Ga 2 O 3 through terahertz transmission measurements in single crystals of (010) and (-201) orientation. The electromagnetic wave propagation in this anisotropic crystal is best described with an orthogonal set of eigenmodes, i.e., polarization eigenvectors, wherein each eigenmode possess a distinct refractive index. Polarization-dependent transmission measurements were carried out to ascertain the nature of the eigenmodes and their directions with respect to the crystal axes.

Hybrid organic-inorganic perovskites (HOIPs) are prime candidates for studying Rashba effects due to the heavy metal and halogen atoms in their crystal structure coupled with predicted inversion symmetry breaking. Nevertheless, observation of the Rashba effect in cubic CH3NH3PbBr3 single crystals that possess bulk inversion symmetry is the subject of extensive debate due to the lack of conclusive experiments and theoretical explanations. Here, we provide experimental evidence that Rashba state in cubic CH3NH3PbBr3 single crystals at room temperature occurs exclusively on the crystal surface and depends on specific surface termination that results in local symmetry breaking. We demonstrate this using a suite of spatially resolved and depth-sensitive techniques, including circular photogalvanic effect, inverse spin Hall effect, and multiphoton microscopy, that are supported by first principle calculations. Our work suggests using surface Rashba states in these materials for spintronic applications.

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.

Three-dimensional (3D) semimetals have been predicted and demonstrated to have a wide variety of interesting properties associated with their linear energy dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as graphene, Cd

We correct three labeling errors, one in Fig. 4(a), and the other two in Section 3.2 of the manuscript text [Opt. Mater. Express 9, 826 (2019)10.1364/OME.9.000826].

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.

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.

The quasi-static anisotropic permittivity parameters of electrically insulating beta gallium oxide (β-Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200 GHz to 1 THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c and b and reciprocal lattice direction a *. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high-frequency contributions and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of β-Ga2O3 in high-frequency electronic devices.

The referenced paper, Optica 3, 1466 (2016)OPTIC82334-253610.1364/OPTICA.3.001466, has been retracted.

In this work we report on synchronized plasma wave resonances in ultrathin-membrane GaN heterostructures. In contrast to commonly employed grating-gate configurations, the analyzed structure contains periodically-patterned ohmic contacts to the two-dimensional electron gas (2DEG), which are laid-out parallel to the gate fingers. Our work demonstrates that the proposed approach allows: more efficient excitation of high order plasmon modes, and superior overall coupling, even in configurations having less number of devices per unit area.

Semiconducting TMDs are promising candidates for THz optoelectronic devices with their ultrafast carrier dynamics and large modulation depths stemming from high photo-induced carrier concentration. In this work, we exploit the ultrafast carrier recombination in photoexcited WSe2 thin films for active THz modulation. The THz absorption was modelled as a sum of a free carrier Drude term and a Lorentzian oscillator for excitonic bound carriers. Based on this, we explore a THz modulator based on a metamaterial structure capacitively coupled to a CVD grown WSe2 film. While the THz absorption upon photoexcitation is strong close to the exciton resonance, the modulation response is limited by free carriers.

Computational diffractive optics is a powerful tool for designing THz optical elements. Furthermore, an optimization based modified search algorithm can enable us to find optimal solutions much faster than what is possible by employing search algorithms. In view of this, the proposed gradient descent optimization based modified binary search algorithm cannot just enable a faster convergence (up to l0-l00X), but also allows us to increase the dimension of the search space, thus the number of degrees of freedom on the problem, which can lead to much better design performance. Using this approach, we demonstrate a series of ultra-thin (1.5-3λ 0 ), error-tolerant and efficient THz optical elements including (i) large N.A. 1D and 2D lenses for aberration-rectified narrow and broadband focusing and (ii) the THz equivalent of an optical spectrometer, i.e. a structure focusing different THz frequencies at different points in space. We present experimental results performed using 3D printed structures, which are backed by full wave simulations.

Linear energy band dispersion in Dirac semimetals has been associated with strong plasmon coupling in terahertz regime. Graphene, in this regard, has been projected to play crucial role in terahertz optoelectronics due to high carrier mobility and conductivity. However, in the realm of quantum materials, a three-dimension analogue of graphene, Cd 3 As 2 , so called 3D semimetal, has been shown to have carrier mobility and fermi-velocity well exceeding that in graphene. We experimentally demonstrate synthesis of high-quality large-area Cd 3 As 2 thin-films and realization of THz plasmonic structures. The structures exhibit quality factor ~5, at frequency of ~0.7 THz, the highest reported to-date at room temperature in any semiconductor or semimetal. Our results evidence that the 3D nature of Cd 3 As 2 provides for a more robust platform for terahertz applications than what is otherwise possible in graphene.

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.

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.

We study the role of complex scattering effects in transferred bi-layer and tri-layer CVD graphene, wherein the dynamics of intrinsic and photo-excited carriers is governed by interactions between adjacent layers. We observed non-Drude conductivity stemming from scattering in electrically connected systems; these effects being more pronounced at low temperatures. Dynamics of photo-excited carriers are studied in the context of intense terahertz field and optical fluence. We observe enhanced scattering and attribute it to coulombic interactions in the stacked systems investigated under various non-equilibrium conditions.

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.

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.

The fullerene C60, C70, and C84 molecules, that are composed of ∼99% naturally abundant 12C having spinless nuclei, are considered to have miniature hyperfine interaction and also weak intrinsic spin-orbit coupling (SOC) due to the light carbon atoms. However, it has been theoretically predicted that the curvature of the fullerene molecules may increase the SOC due to the induced hybridization of the π and σ electrons on the carbon atoms that reside on the fullerene molecule surface. In this work, we have measured the spin diffusion length in films of C60, C70, and C84 in NiFe/fullerene/Pt trilayer devices, where pure spin current is injected into the fullerene film at the NiFe/fullerene interface via spin pumping induced by microwave absorption at ferromagnet resonance conditions, and subsequently detected at the fullerene/Pt interface as electrical current via the inverse spin-Hall effect. The obtained spin diffusion lengths in the fullerene films are of the order of 10 nm and increase from C60 to C84 in which the fullerene molecule’s curvature decreases; this finding validates the existence of curvature-induced SOC in the fullerenes. Our results deepen the understanding of spin transport in fullerene films and may benefit the design of molecular spintronic devices.

We demonstrate 100% modulation of selective Terahertz resonances using series of two-dimensional hybrid lead halide perovskites. The device operation in perovskite/silicon devices was achieved using a simple halogen lamp and a set of color filters.

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.

We demonstrate a novel approach for encoding images using onto a metasurface design composed of resonant dipole based unit cells. This is accomplished by varying the conductivity of individual dipole, resulting in amplitude modulation of the transmitted THz beam. However, since only a narrow regime of conductivity variation was used, the dipole array appears to be uniform visually. Thus, this offers an interesting approach for hiding information. Using one to three dipoles per unit cell, we experimentally demonstrate 9 level encoding of information using simple dipole and 64 (43) color encoding per bit with RGB color metasurface as demonstrated via THz imaging.

We introduce a novel approach for photo-induced modulation of THz resonances using organometal trihalide perovskites. In contrast to equivalent devices based on conventional semiconductors or graphene, a wide variety of different perovskites, with tunable chemical compositions, can be either solution processed or thermally deposited on to desired substrates. We demonstrate 100% depth of modulation for THz plasmonic resonances using methylammonium lead iodide (MAPbb) perovskite, where the medium is photo-excited using a standard halogen lamp.

Boiling, a dynamic and multiscale process, has been studied for several decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle, which occurs over millisecond time-span, makes it extremely challenging to study near-surface interfacial characteristics of a single bubble. Here, we create a steady-state vapor bubble that can remain stable for hours in a pool of sub-cooled water using a femtosecond laser source. The stability of the bubble allows us to measure the contact-angle and perform in-situ imaging of the contact-line region and the microlayer, on hydrophilic and hydrophobic surfaces and in both degassed and regular (with dissolved air) water. The early growth stage of vapor bubble in degassed water shows a completely wetted bubble base with the microlayer and the bubble does not depart from the surface due to reduced liquid pressure in the microlayer. Using experimental data and numerical simulations, we obtain permissible range of maximum heat transfer coefficient possible in nucleate boiling and the width of the evaporating layer in the contact-line region. This technique of creating and measuring fundamental characteristics of a stable vapor bubble will facilitate rational design of nanostructures for boiling enhancement and advance thermal management in electronics.

Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2 thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (

In this paper we discuss graphene–dielectric integrated terahertz metasurfaces as an alternative approach to graphene–metal metamaterials for providing effective control of electromagnetic beam propagation at terahertz wavelengths. Our structures consist of a passive lossless dielectric pattern and a reconfigurable graphene sheet whose terahertz optical conductivity could be actively tuned chemically, electrically, or optically. In particular, we investigate dielectric patterns consisting of pillar- and circular hole arrays and show that via optimizing the geometric dimensions in these patterns it is possible to attain almost complete terahertz absorption at an arbitrary frequency of interest. Furthermore, via either (i) controlling the thickness of a low-index dielectric spacer located between the dielectric pattern and the graphene sheet, or (ii) choosing a material with an appropriate index of refraction when constructing the dielectric pattern, it is possible to control the sensitivity of the overall structure to variations in the graphene sheet conductivity. Full-wave electromagnetic simulations are supported by proof-of-principle experiments on structures fabricated via patterning of a silicon-on-insulator wafer followed by graphene transfer and chemical doping. Overall, the proposed approach can lead to the construction of efficient tunable terahertz absorbers; however, other tailored electromagnetic responses might be also possible via the selection of appropriate dielectric patterns.

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 describe studies on the nanoscale transport dynamics of carriers in strained AlN/GaN/AlN quantum wells: an electron-hole bilayer charge system with large difference in transport properties between the two charge layers. From electronic band diagram analysis, the presence of spatially separated two-dimensional electron and hole charge layers is predicted at opposite interfaces. Since these charge layers exhibit distinct spectral signatures at terahertz frequencies, a combination of terahertz and far-infrared spectroscopy enables us to extract (a) individual contributions to the total conductivity, as well as (b) effective scattering rates for charge-carriers in each layer. Furthermore, by comparing direct-current and terahertz extracted conductivity levels, we are able to determine the extent to which structural defects affect charge transport. Our results evidence that (i) a non-unity Hall-factor and (ii) the considerable contribution of holes to the overall conductivity, lead to a lower apparent mobility in Hall-effect measurements. Overall, our work demonstrates that terahertz spectroscopy is a suitable technique for the study of bilayer charge systems with large differences in transport properties between layers, such as quantum wells in III-Nitride semiconductors., Applied Physics Letters

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.

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.

We experimentally demonstrate thin, error tolerant, and aberration corrected 3D printed diffractive 1D and 2D THz lenses for focusing THz beams with NA > 0.25 and an average efficiency greater than 75% (for all cases). Our work has relevance to THz lens design, and in general, THz imaging systems.

We demonstrate high-efficiency (>70%) all-dielectric Terahertz metasurfaces operating in the transmission mode. Example devices include high numerical-aperture (NA-0.9) meta-lenses as well as cylindrical vector beam generator.