Research into metasurfaces is developing rapidly and is topical due to their importance and applications in various fields such as communications, cryptography, and sensing, to name a few. These materials are artificially engineered to manipulate/control electromagnetic (EM) waves, in order to present a particular functionality. In this regard, nonlinear metasurfaces may present particular functionalities that remain to be discovered. In this paper, we numerically investigate the dynamic behaviors caused by the motion of charge carriers under the intense EM field at the gap of a single nonlinear split-ring resonator (NSRR) in the terahertz (THz) frequency range. We derive the mathematical model that is used to examine the excitation properties of the NSRR and then demonstrate various tuning regions. Analysis of the two-dimensional parameter space reveals that the NSRR exhibits periodic, chaotic patterns as the amplitude of the excitation field and the loss parameter vary. However, this chaotic behavior disappears when the loss parameter is very large. The period doubling that confirms the transition between the periodic and chaotic modes is explored using the bifurcation diagram. The sensitivity of the initial conditions is examined on three dynamic region plots. Our results correctly demonstrate that the NSRR exhibits the attractive phenomenon of multistability. The coexistence of two stable states is studied and confirmed on the basin of attractions for a fixed set of amplitude or loss parameters. The energy balance of the proposed model is well analyzed on the dynamic states and parameters to characterize the different oscillation regimes. The study of the multistability in the work represents an important first step toward the development of photonic memory devices in the THz frequency range.

In this Letter, we present a significant improvement to time and space resolutions in electro-optic sampling (EO) for two-dimensional terahertz (THz) near-field imaging. Using a THz microscope, we readapt a recent EO sampling scheme based on optical probe spectrum filtering. Combined with an ultra-thin EO crystal, we achieve record broadband video-rate THz near-field imaging. Particularly, this new scheme improves the THz bandwidth, the imaging contrast, and the spatial resolution. To validate our method, we show THz near-field images ranging from 100 GHz to 4 THz with a spatial resolution up to $\lambda /600$ at 100 GHz. This demonstration positively affects the detection of intense THz pulses derived from the tilted-pulse-front excitation of lithium niobate and will accelerate our understanding of the interaction processes between electromagnetic waves and the conducting electrons of metallic interfaces.

We report on terahertz (THz) generation via optical rectification in a room temperature gallium arsenide (GaAs) crystal pumped at a wavelength of 1.8 μm using a modified tilted-pulse-front scheme, leading to a 0.05% energy conversion efficiency. The spectral content of the measured THz pulses, ranging from 0.1 to 3 THz, confirmed a good broadband phase matching between the pump and the THz pulses over several millimeters (>20 mm) in a semi-insulating 〈110〉 cut bulk GaAs crystal. Our findings also suggest that the pump-to-THz conversion efficiency can be further increased by using a pump source with a narrower bandwidth.

Metasurfaces, artificial materials with unique electromagnetic (EM) properties arising from electron oscillations in highly conductive metals, often utilize split-ring resonators (SRRs) as key components for nonlinear effects. In this paper, we investigate the nonlinear phenomena caused by charge carrier motion at the gap of a single SRR operating in the terahertz (THz) frequency range. Unlike previous approaches, our model incorporates an additional controlled field to adjust the loss parameter, providing a more generalized framework for understanding nonlinear behavior in SRRs. The nonlinear dynamics are revealed by the two-parameter space diagrams, which allow us to identify periodic and chaotic oscillations for different parameter sets of the nonlinear SRR material. In order to explore the multifunctionality of the material, we construct two-dimensional diagrams by sweeping each parameter in two directions for the same initial values and we find a plethora of hysteresis regions, demonstrating the interesting phenomenon of multistability. Our results show that this behavior occurs in extremely large dynamic regions for all cases studied, favoring the coexistence of up to three different signals/attractors. The different types of multistable patterns are studied in the cross-section of the demarcation region and presented as the coexistence of chaotic with periodic states, and the coexistence of only periodic states. We analyze the parallel branches in hysteresis regions, and we identify a parallel branch in some range where the bifurcation presents a jump. According to our results, the parallel branch has a key impact on multistability, since it increases the number of coexisting states. However, it appears in a very tiny range of parameters and annihilates after a collision crisis. Finally, we analyze the influence of the loss parameter on nonlinear material dynamics using two-parameter spatial diagrams, demonstrating that control of the loss parameter can gradually eliminate the presence of chaotic behavior, thus serving as a pivotal control mechanism for future applications.

Printable electronics is a promising manufacturing technology for the potential production of low-cost flexible electronic devices, ranging from displays to active wear. It is known that rapid printing of conductive ink on a flexible substrate is vulnerable to several sources of variation during the manufacturing process. However, this process is still not being subjected to a quality control method that is both non-invasive and in situ. To address this issue, we propose controlling the printing accuracy by monitoring the spatial distribution of the deposited ink using terahertz (THz) waves. The parameters studied are the printing speed of an industrial roll-to-roll press with flexography printing units and the pre-calibration compression, or expansion factor, for a pattern printed on a flexible plastic substrate. The pattern, which is carefully selected, has Babinet’s electromagnetic transmission properties in the THz frequency range. To validate our choice, we quantified the geometric variations of the printed pattern by visible microscopy and compared its accuracy using one-dimensional THz spectroscopy. Our study shows a remarkable agreement between visible microscopic observation of the printing performance and the signature of the THz transmission. Notably, under specific conditions, one-dimensional (1D) THz information from a resonant pattern can be more accurate than two-dimensional (2D) microscopy information. This result paves the way for a simple strategy for non-invasive and contactless in situ monitoring of printable electronics production.

Printed electronic devices are attracting significant interest due to their versatility and low cost; however, quality control during manufacturing is a significant challenge, preventing the widespread adoption of this promising technology. We show that terahertz (THz) radiation can be used for the in situ inspection of printed electronic devices, as confirmed through a comparison with conventional electrical conductivity methods. Our in situ method consists of printing a simple test pattern exhibiting a distinct signature in the THz range that enables the precise characterization of the static electrical conductivities of the printed ink. We demonstrate that contactless dual-wavelength THz spectroscopy analysis, which requires only a single THz measurement, is more precise and repeatable than the conventional four-point probe conductivity measurement method. Our results open the door to a simple strategy for performing contactless quality control in real time of printed electronic devices at any stage of its production line.