Terahertz time-domain spectroscopy and near-field microscopy of transparent silver nanowire networks

\u3cp\u3eTransparent conductive layers are key components of optoelectronic devices. Here, a polyol method is used to synthesize large quantities of monodisperse silver nanowires (AgNWs) and these are used to fabricate transparent conducting networks over large areas. The optical extinction and terahertz (THz) conductance of these networks are simultaneously investigated, using optical and THz spectroscopy, and THz near-field microscopy. The combination of optical and THz measurements allows the identification of transparent regions with high conductance. The THz near-field measurements reveal local variations in the THz transmission and conductance that are averaged in far-field measurements. These results demonstrate that THz near-field microscopy is a powerful tool for the quantitative investigation of new conductive transparent electrodes.\u3c/p\u3

Diffraction enhanced transparency (DET) using frequency detuned and displaced resonant rods

\u3cp\u3eA periodic lattice of frequency-detuned and displaced resonant metallic rods is used to generate diffraction enhanced transparency (DET) at terahertz frequencies. Using far-field spectroscopy, we demonstrate the appearance of a sharp transmission line in the otherwise broad extinction spectrum of the rod array, accompanied by strong group delay at the frequency of the transparency. Probing further with near-field microscopy we unravel the underlying mode profiles of this structure. We recover a quadrupolar field distribution in the near-field of each unit cell of the array, that arises due to the interference between lattice modes corresponding to the individual rods, resulting in a transparency.\u3c/p\u3

Diffraction enhanced transparency (DET) using frequency detuned and displaced resonant rods

In this contribution, we demonstrate that a periodic lattice of frequency-detuned and displaced resonant rods can suppress the THz extinction at the central resonant frequency, leading to an enhanced spectral window of near perfect transparency. The system consists of a periodic lattice of metallic rods of two different sizes. Each of the rods supports a strong half-wavelength (λ/2) resonance which are detuned with respect to each other. Furthermore, both the rods are spatially displaced within each unit cell of the lattice. The group-index obtained from far-field measurements shows that the THz field is strongly delayed by more than four orders of magnitude at the spectral transparency window. Using micro-spectroscopic measurements of the electric near fields, we show that this transparency window has its origin in the interference between two surface lattice resonances, arising from the diffractively enhanced radiative coupling of the two λ/2 resonances in the lattice. Thus, we term this phenomenon as Diffraction Enhanced Transparency (DET). Since DET does not involve near-field coupling between resonators, the fabrication tolerance to imperfections is expected to be very high. This remarkable response and ease of fabrication renders these systems as very interesting components for THz communicatio

Effect of optical damage resistant dopants on the dielectric properties of LiNbO\u3csub\u3e3\u3c/sub\u3e:Insight from broadband impedance spectroscopy and Raman scattering

\u3cp\u3eOptical damage limits the application range of congruent LiNbO\u3csub\u3e3\u3c/sub\u3e. This problem is commonly overcome by adding optical-damage-resistant cations. Here, the influence of doping with optical-damage-resistant Mg and Zn on the ionic and piezoelectric contributions to the dielectric permittivity is investigated in a broad frequency range (1 mHz-2 THz). It is shown that the two dopants have radically different influences on the variation of ionic permittivity with doping, in spite of their similarities with respect to the crystallographic structure. Raman spectroscopy reveals that the difference in permittivity can be traced to the effect of Mg and Zn doping on the susceptibility of the phonon modes. Both observations point to differences in the defect incorporation mechanisms.\u3c/p\u3

Terahertz diffraction enhanced transparency probed in the near field

\u3cp\u3eElectromagnetically induced transparency in metamaterials allows to engineer structures which transmit narrow spectral ranges of radiation while exhibiting a large group index. Implementation of this phenomenon frequently calls for strong near-field coupling of bright (dipolar) resonances to dark (multipolar) resonances in the metamolecules comprising the metamaterials. The sharpness and contrast of the resulting transparency windows thus depends strongly on how closely these metamolecules can be placed to one another, placing constraints on fabrication capabilities. In this manuscript, we demonstrate that the reliance on near-field interaction strength can be relaxed, and the magnitude of the electromagnetic-induced transparency enhanced, by exploiting the long-range coupling between metamolecules in periodic lattices. By placing dolmen structures resonant at THz frequencies in a periodic lattice, we show a significant increase of the transparency window when the in-plane diffraction is tuned to the resonant frequency of the metamolecules, as confirmed by direct mapping of the THz near-field amplitude across a lattice of dolmens. Through the direct interrogation of the dark resonance in the near field, we show the interplay of near- and far-field couplings in optimizing the response of planar dolmen arrays via diffraction-enhanced transparency.\u3c/p\u3

Near-field microscopy of electromagnetically induced transparency in terahertz dolmens

Summary form only given. Terahertz (THz) resonant structures present an attractive canvas for the design of novel optoelectronic devices in the far-infrared. By exploiting the strong scattering properties of metallic resonators, effects such as beam steering [1], spectral filtering [2], and enhanced group delays [3] have been demonstrated over very thin optical path lengths with high figures of merit. In many implementations, such effects arise due to interferences occurring in local fields of the resonators via evanescent coupling. We use scanning probe near-field THz time-domain microscopy [4] to present spectral maps of a periodic array of gold dolmen structures. The system consists of a first resonator that supports a bright mode, which couples efficiently to the radiation field, and a second resonator that supports a dark mode which cannot be driven by incident plane wave radiation. The interaction of these two modes in the near-field gives rise to electromagnetically induced transparency, observed in far-field extinction spectra. The coupling between bright and dark modes supported by the structure is visualized through the hybridization of these modes, and by the enhanced field recorded at position of the dark resonator which cannot be directly excited from the far-field. These measurements present a platform for disentangling the local field distribution near more complex optoelectronic devices in the THz range, for applications in the emerging field of THz photonics

Non-invasive local (photo)conductivity measurements of metallic and semiconductor nanowires in the near-field

\u3cp\u3eThe capability of THz-time domain spectroscopy (TDS) for the non-invasive extraction of the conductive properties of metal and semiconductor surfaces is essential for advancements in material science and device analysis. Here, we demonstrate that this technique can be successfully applied to image and analyze sub-diffraction inhomogeneities using THz near-field microscopy. Additionally, a novel total internal reflection geometry enables time-resolved THz time-domain near-field microscopy using ultrashort optical pulses on photoexcited semiconducting materials with a resolution of < 50~ mu { mathrm{ m}}.\u3c/p\u3

Spectral and temporal evidence of robust photonic bound states in the continuum on terahertz metasurfaces

\u3cp\u3ePhotonic bound states in the continuum (BICs) are protected eigenstates in optical systems with infinite lifetimes. This unique property, which translates in infinite Q-factor resonances, makes BICs extremely interesting not only from a fundamental perspective but also for various applications such as lasing and sensing. General means to achieve robust BICs are, however, elusive. Here we demonstrate analytically that BICs emerge in metasurfaces formed by arrays of detuned resonant dipolar dimers as a universal behavior occurring regardless of both dipole position within the unit cell and lattice constant in the nondiffracting regime. These resonances evolve continuously from a Fano resonance into a symmetry-protected BIC as the dipole detuning vanishes. We have experimentally verified this very robust response at terahertz frequencies through dimer rod arrays with different rod sizes by simultaneously measuring the reduction of linewidth and the increase of lifetime before the BIC is formed, as it is impossible to couple to it from the continuum. Similar configurations can be straightforwardly envisioned throughout the electromagnetic spectrum, enabling a simple geometry that is easy to fabricate with resonances of arbitrarily high Q factors.\u3c/p\u3

THz resonances with infinite lifetime in array of gold resonators

\u3cp\u3eMetasurfaces consisting of two metallic rods per unit-cell in a lattice can support resonances at THz frequencies with infinitely long lifetime. These resonances are known as bound states in the continuum (BICs). We investigate theoretically and experimentally the conditions leading to the formation of these resonances in gold particle arrays.\u3c/p\u3