Planar Terahertz Research Articles

Orthogonally twisted planar concentric split ring resonators towards strong near field coupled terahertz metamaterials

We present strongly coupled planar terahertz metamaterials in which the metamolecule design comprised two concentric split ring resonators (SRRs) with their capacitive gaps oriented orthogonally in order to establish strong near field coupling. Experimental results clearly demonstrate huge splitting in the fundamental inductive-capacitive resonance when the incident terahertz polarization couples to the metamolecule system through the outer SRR. However, the strengths of split resonances are too weak to detect experimentally when the meta-molecule system is excited through the inner SRR. Such strongly coupled metamolecules can enable additional dispersion tuning and polarization control in metamaterials.

Low-index-metamaterial for gain enhancement of planar terahertz antenna

We theoretically present a high gain planar antenna at terahertz (THz) frequencies by combing a conventional log-periodic antenna (LPA) with a low-index-metamaterial (LIM, |n| < 1). The LIM is realized by properly designing a fishnet metamaterial using full-wave finite-element simulation. Owing to the impedance matching, the LIM can be placed seamlessly on the substrate of the LPA without noticeable reflection. The effectiveness of using LIM for antenna gain enhancement is confirmed by comparing the antenna performance with and without LIM, where significantly improved half-power beam-width (3-dB beam-width) and more than 4 dB gain enhancement are seen within a certain frequency range. The presented LIM-enhanced planar THz antenna is compact, flat, low profile, and high gain, which has extensive applications in THz systems, including communications, radar, and spectroscopy.

Dispersion characteristic of ultrathin terahertz planar lenses based on metasurface

Terahertz (THz) radiation has attached a lot of attention due to its potential applications. The smart optical components will greatly boost the application of the THz technology. Ultrathin planar lenses which can greatly reduce the thickness of the components are designed and fabricated in the THz region based on the metasurface. The dispersion characteristic of this kind of ultrathin planar lenses is systematically investigated. It is demonstrated that the focal lengths of the proposed lenses become shorter with the wavelength of the illuminating light increasing. Moreover, the ultrathin planar lenses perform a good focusing characteristic over a broad wavelength range. The results help us to further understand the optical properties of the lenses based on the metasurface and provide a considerable important reference for its future applications in miniaturization and integration of the THz system.

Ultrastrong coupling of intersubband plasmons and terahertz metamaterials

We report on the ultrastrong-coupling between localized plasmons of a planar terahertz metamaterial and intersubband plasmons in a modulation doped quantum well sample. Such a system exhibits the formation of a lower and an upper polariton branch when the metamaterial eigenfrequency is tuned close to resonance with the intersubband transition. We achieve a normalized polariton splitting of 22% and a polaritonic gap of 2.4% of the intersubband transition frequency. In addition to the usual geometrical scaling, we demonstrate the effective tuning of the metamaterial resonance by dry etching with a tuning range of more than 1 THz.

Electromagnetic Response of Finite Terahertz Metafilm Arrays Excited on Total Internal Reflection Boundaries

Resonant excitation of planar terahertz metamaterials using attenuated total reflection is demonstrated. Experimental results reveal an anomalous increase in the resonance strength while the sample is illuminated near the edge of the metamaterial array with a finite-size terahertz beam. A re-radiation signal at the fundamental metamaterial resonance is observed on the transmission side of the total internal reflection interface where no signal was expected. Multiple theoretical approaches address the physical origins of this re-radiation signal and rich behavior has been simulated with numeric simulations. Although models indicate that surface waves could exist, radiation coupled across the total internal reflection surface appears predominately mediated by finite currents oscillating in resonators at the edge of the metafilm array. The observations could lead to a better understanding of boundary effects in finite, planar metamaterials and more accurate modeling of MM-mediated total reflection spectroscopy.

Nano-antenna in a photoconductive photomixer for highly efficient continuous wave terahertz emission

We report highly efficient continuous-wave terahertz (THz) photoconductive antenna based photomixer employing nano-gap electrodes in the active region. The tip-to-tip nano-gap electrode structure provides strong THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it provides good impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation especially at the higher part of the THz spectrum. As a result, the output intensity of the photomixer with the new nano-gap electrode structure in the active region is two orders of magnitude higher than that of a photomixer with typical interdigitated electrodes. Significant improvement in the THz emission bandwidth was also observed. An efficient continuous wave THz source will greatly benefit compact THz system development for high resolution THz spectroscopy and imaging applications.

Femtosecond direct laser writing of gold nanostructures by ionic liquid assisted multiphoton photoreduction

Amino-terminated ionic liquid assisted multiphoton photoreduction (IL-MPR) was developed for the direct writing of subwavelength gold nanostructures in AuCl4- ions aqueous solution by femtosecond laser. It was revealed that the carbon chain length was crucial for morphology and size control of gold nanostructures. A 228 nm width of gold nanostructure, which was beyond the optical diffraction limit, was fabricated by the matching between IL and the power and scanning speed of the laser beam. The measured conductivity is of the same order as that of bulk gold. Furthermore, we successfully fabricated a U-shaped terahertz planar metamaterial whose spectral response is consistent with the theoretical expectation. The IL-MPR nanofabrication protocol is expected to play an important role in the fabrication of fine metallic micro/nanostructures for applications in microelectromechanical systems, nanoelectronics and nanophotonics.

Influence of film thickness in THz active metamaterial devices: A comparison between superconductor and metal split-ring resonators

We experimentally demonstrate thickness-dependent resonance tuning in planar terahertz superconducting metamaterials. Inductive-capacitive resonance of arrays of split-ring resonators fabricated from 50, 100, and 200 nm thick YBa2Cu3O7−δ (YBCO) and gold films were characterized and compared as a function of temperature. In the YBCO metamaterials the resonance frequency strongly depends on the thickness, and they show high thermal tunability in both resonance strength and frequency below the superconducting transition temperature, where the imaginary conductivity varies by three orders of magnitude. In contrast, the resonance in the gold metamaterials exhibits little thickness-dependence and very small tunability.

Planar terahertz metamaterial with three-resonant frequencies

In this paper, we study a three-resonant metamaterial with the combination of dual-resonant and single-resonant metamaterials. We present a new method to design multi-resonant metamaterial, which has a smaller dimension than general symmetric and asymmetric multi-resonant metamaterials. Theoretical and experimental results show that the structure has three distinct absorption frequencies centering around 0.29 THz, 0.46 THz, and 0.92 THz, and that each of them corresponds to a different resonant mode. Due to the good separation of the different resonances, this design provides a unique and effective method to construct multiband terahertz devices.

Resonance mode-switching in terahertz metamaterials based on varying gallium arsenide conductivity

An asymmetric planar terahertz (THz) metamaterial (MM) is designed to be composed of two different single split-ring resonators (SRR) and its character is simulated in the THz region. Gallium arsenide (GaAs) is inserted between the gap of two separated single SRR, and through modulation of its conductivity (σ GaAs ) we achieved the switching of three different resonance modes and researched the influence of σ GaAs on transmission of MM. The resonant structure of MM can be switched from the two fundamental LC-modes to the new LC-mode and dipole resonance mode through the coupling LC-mode with the increasing σ GaAs . Such dynamical control of MM resonances provides an efficient way to manipulate electromagnetic wave to push mode-switching of resonance and could be implemented in terahertz devices to achieve additional functionalities.

Low-loss ultra-high-Q dark mode plasmonic Fano metamaterials

We experimentally demonstrate a planar terahertz Fano metamaterial with an ultrahigh quality (Q) factor of 227. This is achieved by the excitation of the nonradiative dark modes by introducing a tiny asymmetry in the metamaterial structure. The extremely sharp quadrupole and Fano resonances are excited at normal incidence for orthogonal polarizations of the electric field. In order to capture the narrow linewidth of the dark resonance modes, we perform high resolution terahertz time-domain measurements with a scan length of 200 picoseconds and frequency resolution of 5 GHz. These high-Q metamaterials can be used in ultrasensitive label-free terahertz sensing, dense photonic integration, and narrowband filtering.

Electromagnetically induced transparency in terahertz plasmonic metamaterials via dual excitation pathways of the dark mode

We observe the excitation and tuning of electromagnetically induced transparency (EIT) by the interference between different excitation pathways of the dark mode in a planar terahertz metamaterial. The EIT unit cell consists of a cut wire as the bright resonator and a pair of split ring resonators (SRRs) as the dark element. The dark mode resonance is excited by both the electric and magnetic fields when the SRR pair translates along the wire without altering the lateral distance between the resonators. The electric and magnetic pathways of exciting the dark mode allows for a giant amplitude modulation of the EIT resonance.

Superconducting terahertz metamaterials mimicking electromagnetically induced transparency

We designed and fabricated planar terahertz (THz) metamaterials made from superconducting NbN films to mimic electromagnetically induced transparency (EIT) system. They are characterized using THz time domain spectroscopy over a temperature range from 8 to 300 K. High transmittance and large delay-bandwidth product at transparency window are demonstrated, which mainly arise from the enhanced coupling and decreased damping in superconducting state. The EIT-like spectral response could be tuned in a wide frequency range. By applying two dark resonators with different resonance frequencies coupled with a radiative resonator, we experimentally demonstrated the planar metamaterials mimicking four-level EIT system.

Impact of Eddy Currents and Crowding Effects on High-Frequency Losses in Planar Schottky Diodes

In this paper, we present the influence of eddy currents, skin and proximity effects on high-frequency losses in planar terahertz Schottky diodes. The high-frequency losses, particularly losses due to the spreading resistance, are analyzed as a function of the ohmic-contact mesa geometry for frequencies up to 600 GHz. A combination of 3-D electromagnetic (EM) simulations and parameter extraction based on lumped equivalent circuit is used for the analysis. The extracted low-frequency spreading resistance shows a good agreement with the results from electrostatic simulations and experimental data. By taking into consideration the EM field couplings, the analysis shows that the optimum ohmic-contact mesa thickness is approximately one-skin depth at the operating frequency. It is also shown that, for a typical diode, the onset of eddy current loss starts at <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\sim$</tex></formula> 200 GHz, and the onset of a mixture of skin and proximity effects occurs around <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 400 GHz.

A broadband planar terahertz metamaterial with nested structure

We demonstrate the broadening of fundamental resonance in terahertz metamaterial by successive insertion of metal rings in the original unit cell of a split ring resonator (SRR) forming an inter connected nested structure. With the subsequent addition of each inner ring, the fundamental resonance mode shows gradual broadening and blue shift. For a total of four rings in the structure the resonance linewidth is enhanced by a factor of four and the blue shift is as large as 316 GHz. The dramatic increase in fundamental resonance broadening and its blue shifting is attributed to the decrease in the effective inductance of the entire SRR structure with addition of each smaller ring. We also observe that while the fundamental resonance is well preserved, the dipolar mode resonance undergoes multiple splittings with the addition of each ring in the nest. Such planar metamaterials, possessing broadband resonant response in the fundamental mode of operation, could have potential applications for extending the properties of metamaterials over a broader frequency range of operations.

Tuning of superconducting niobium nitride terahertz metamaterials

Superconducting planar terahertz (THz) metamaterials (MMs), with unit cells of different sizes, are fabricated on 200 nm-thick niobium nitride (NbN) films deposited on MgO substrates. They are characterized using THz time domain spectroscopy over a temperature range from 8.1 K to 300 K, crossing the critical temperature of NbN films. As the gap frequency (f(g) = 2Δ0/h, where Δ0 is the energy gap at 0 K and h is the Plank constant) of NbN is 1.18 THz, the experimentally observed THz spectra span a frequency range from below f(g) to above it. We have found that, as the resonance frequency approaches f(g), the relative tuning range of MMs is quite wide (30%). We attribute this observation to the large change of kinetic inductance of superconducting film.

Manipulating the plasmon-induced transparency in terahertz metamaterials

Coupling between superradiant and subradiant mode resonators in a metamaterial unit cell plays an important role in observing the sharp transparency peak due to destructive interference between the resonators. This effect is enhanced as the resonators are brought closer to each other in a conventional planar arrangement. We present a novel coupling scheme of planar terahertz metamaterial to tune the plasmon-induced transparency peak by physically varying the distance between the superradiant and the subradiant resonators in such a way that the transparency peak begins to disappear as the coupled resonators are brought closer than a critical separation distance. The effect is attributed to the disappearance of the resonant behavior of the subradiant resonator in a closely coupled regime. The simple planar design presented here demonstrates a scheme to manipulate the electromagnetically induced transparency-like behavior in terahertz metamaterials and this could lead to the development of unique slow light devices for terahertz applications.

Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates

We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO₃) substrate. Cooling the metamaterial starting from 409 K down to 150 K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials.

Sharp Fano resonances in THz metamaterials

We report on the occurrence of sharp Fano resonances in planar terahertz metamaterials by introducing a weak asymmetry in a two gap split ring resonator. As the structural symmetry of the metamaterial is broken a Fano resonance evolves in the low-frequency flank of the symmetric fundamental dipole mode resonance. This Fano resonance can have much higher Q factors than that known from single gap split ring resonators. Supporting simulations indicate a Q factor of 50 for lowest degree of asymmetry. The Q factor decreases exponentially with increasing asymmetry. Hence, minute structural variations allow for a tuning of the Fano resonance. Such sharp resonances could be exploited for biochemical sensing. Besides, the strong current oscillations excited at the Fano resonance frequency could lead to the design of novel terahertz narrow band emitters.

Planar plasmonic terahertz waveguides based on periodically corrugated metal films

We demonstrate that a one-dimensional periodically corrugated metal film can be used to create planar terahertz (THz) waveguides. The periodic corrugation is in the form of rectangular blind holes (i.e. holes that do not completely perforate the metal film) that are fabricated using a multilayer construction. The approach allows for the creation of structures in which the hole depth can be more than four times the hole width. This is necessary to achieve tightly confined THz guided-wave modes. We find that the modes can be modeled using an effective cavity resonance model and that the mode properties depend sensitively on the depth of corrugation. We use numerical simulations to validate the experimental results. We also highlight the differences between simulations that incorporate idealized input parameters and our experimental measurements. Using these data, we fabricate and characterize a Y-splitter to demonstrate the utility of this approach.