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While terahertz spectroscopy can provide valuable information regarding the charge transport properties in semiconductors, its application for the characterization of low-conductive two-dimensional layers, i.e., σs

This paper discusses the terahertz electromagnetic response of metallic gratings on anisotropic dielectric substrates. The metallic gratings consist of parallel gold stripes. Utilizing numerical simulations, we observe that it is possible to excite a series of resonant modes in these structures. These modes are affected differently by the different indices on the anisotropic substrate. An analytical model is discussed to show that modes associated with transmission peaks are due to the excitation of (a) Fabry–Pérot modes with polarization along the grating and/or (b) waveguide modes with polarization perpendicular to the grating. It is observed that the resonance associated with the TM1,1 mode is a narrow linewidth resonance which, in some particular circumstances, becomes nearly independent of substrate thickness. Therefore, from the spectral position of this resonance, it is possible to extract the out-of-plane component of the substrate refractive index with very small uncertainty. Based on this observation, we demonstrate the refractive index characterization of several lossless semiconductor substrates through frequency-domain polarized terahertz transmission measurements in the frequency range of 0.2–0.6 THz at normal incidence. The reliability of the technique is demonstrated on well-known materials, such as high-resistivity silicon and sapphire substrates. This technique is also applied for the characterization of a Fe-doped β-Ga2O3 single-crystal substrate.

We study the application of metallic gratings with subwavelength apertures to characterize sheet charges of very low conductivity (~μSie) in a transmission geometry. The gratings provide increased E-field enhancement and interaction length in the vicinity of the aperture which allows for enhanced absorption by the two-dimensional sheet charges, which would otherwise be undetectable in a transmission measurement. We explore the design space to obtain both broadband and resonant behavior by altering the pitch of the gratings (w.r.t. λ) in order to be applicable for both characterization and device applications.

We program the optical properties of colloidal Au nanocrystal (NC) assemblies via an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design ma...

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.

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.

In this work, we study transient THz conductivity of tungsten diselenide (WSe2) thin films to assess its potential for high-speed THz modulators, i.e., with pulsed optical excitation one could potentially realize all-optical, terahertz devices. Time-resolved THz spectroscopy, across different samples, reveal carrier recombination lifetimes of ~10-100 ps and a transient conductivity that exhibits a non-Drude behavior. We observe a strong dependence of the photoconductivity on the grain size. Based on this study, we can provide a general framework to understand the interplay between grain size and transient conductivity and leverage its role for ultrafast modulator design in terahertz metamaterials.

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.

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 book chapter provides a brief overview of some of the prominent features of non-carbon 2D materials that are currently being investigated and predicted to play significant role in the development of ultrathin electronic and optoelectronic devices in the coming years that could push the boundaries of current CMOS technology.

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.

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 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.

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 a low-cost, large-area fabrication process using solution-based nanoimprinting and compact ligand exchange of colloidal Au nanocrystals to define anisotropic, subwavelength, plasmonic nanoinclusions for optical metasurfaces. Rod-shaped, Au nanocrystal-based nanoantennas possess strong, localized, plasmonic resonances able to control polarization. We fabricate metasurfaces from rod-shaped nanoantennas tailored in size and spacing to demonstrate Au nanocrystal-based quarter-wave plates that operate with extreme bandwidths and provide high polarization conversion efficiencies in the near-to-mid infrared.