No abstract

Graphene has recently been shown to exhibit ultrafast conductivity modulation due to periodic carrier heating by either terahertz (THz) waves, leading to self-induced harmonic generation, or the intensity beat-note of two-color optical radiation. We exploit the latter to realize an optoelectronic photomixer for coherent, continuous-wave THz detection, based on a photoconductive antenna with multilayer graphene in the gap. While for biased THz emitters, the dark current would pose a serious detriment for performance, we show that this is not the case for bias-free THz detection, and demonstrate a detection bandwidth of at least 700 GHz at room temperature, even without optimized tuning of the doping. We account for the photocurrent and photomixing response using detailed simulations of the time-dependent carrier distribution, which also indicate significant potential for enhancement of the sensitivity, to become competitive with well-established semiconductor photomixers.

Guided electromagnetic wave propagation using ultra-wideband signals is a barely new approach for damage detection. However, still many challenges are present, including the way to deal with the GHz domain signals and the physics behind the interaction phenomena enabled by any type of flaw. The present work proposes a feasibility analysis for a structural health monitoring system employing permanently integrated microwave sensors. This setup allows to interrogate the structure continuously using multiple transmitters and multiple receivers when the electromagnetic waveguide is established. To this end, a metallic plate is equipped with a dielectric waveguide patch attached to the structures’ surface. To validate the detectability of damage, a reversible defect is modeled through removable bolts accessible from the other surface of the plate. The experiments are carried out considering different bottom holes at different spatial locations of the plate. In addition, concurrent measurements are adopted to characterize the noise level within the signal. The characteristic changes of electromagnetic wave signals are caught using a damage index approach returning whether the defect can be detected sensitively or not. Different coupling conditions are used to let the guided electromagnetic waves propagate and interact with underlaying structure. The results show that this approach can be adopted for damage detection with a reasonable signal to noise ratio, especially when the waveguide is well coupled. In addition, both transmission and reflection loss can be monitored reliably.