Terahertz transmissive half-wave metasurface with enhanced bandwidth.

Polarization conversion is useful for studies of chiral structures in biology and chemistry, and for polarization diversity in communications. It is conventionally realized with wave plates, which, however, present challenges due to limited material availability, as well as narrow bandwidth and low efficiency at terahertz frequencies. To enhance bandwidth and efficiency, the concept of the Huygens' metasurface is adopted here for a transmissive half-wave plate. The half-wave metasurface is designed following the optimal frequency-independent circuit parameters provided by a broadband semi-analytical approach. Simulation results of an optimal design suggest that a 15-dB extinction ratio can be sustained from 219 GHz to 334 GHz, corresponding to a fractional bandwidth of 41.6%. The measured results indicate that the fabricated structure enables a 15-dB extinction ratio from 220 GHz to 303 GHz, with a cross-polarization transmission efficiency above 76.7% for both linear and circular polarizations. This half-wave metasurface design can be readily integrated into compact terahertz systems for diverse applications.

Terahertz transmissive half-wave metasurface with enhanced bandwidth: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 4164 (2021)OPLEDP0146-959210.1364/OL.431285.

Assessing frost damage in barley using terahertz imaging.

Frost is estimated to cost Australian grain growers $ 360 million in direct and indirect losses every year. Assessing frost damage manually in barley is labor intensive and involves destructive sampling. To mitigate against significant economic loss, it is crucial that assessment decisions on whether to cut for hay or continue to harvest are made soon after frost damage has occurred. In this paper, we propose a non-destructive technique by using raster-scan terahertz imaging. Terahertz waves can penetrate the spike to determine differences between frosted and unfrosted grains. With terahertz raster-scan imaging, conducted in both transmission and reflection at 275 GHz, frosted and unfrosted barley spikes show significant differences. In addition, terahertz imaging allows to determine individual grain positions. The emergence of compact terahertz sources and cameras would enable field deployment of terahertz non-destructive inspection for early frost damage.

Dielectric-resonator metasurfaces for broadband terahertz quarter- and half-wave mirrors.

Polarization conversion of terahertz waves is important for applications in imaging and communications. Conventional wave plates used for polarization conversion are inherently bulky and operate at discrete wavelengths. As a substitute, we employ reflective metasurfaces composed of subwavelength resonators to obtain similar functionality but with enhanced performance. More specifically, we demonstrate low-order dielectric resonators in place of commonly used planar metallic resonators to achieve high radiation efficiencies. As a demonstration of the concept, we present firstly, a quarter-wave mirror that converts 45° incident linearly polarized waves into circularly polarized waves. Next, we present a half-wave mirror that preserves the handedness of circularly polarized waves upon reflection, and in addition, rotates linearly polarized waves by 90° upon reflection. Both metasurfaces operate with high efficiency over a measurable relative bandwidth of 49% for the quarter-wave mirror and 53% for the half-wave mirror. This broadband and high efficiency capabilities of our metasurfaces will allow to leverage maximum benefits from a vast terahertz bandwidth.

Terahertz near-field imaging of dielectric resonators.

As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

Thin film notch filters as platforms for biological image processing.

Many image processing operations involve the modification of the spatial frequency content of images. Here we demonstrate object-plane spatial frequency filtering utilizing the angular sensitivity of a commercial spectral bandstop filter. This approach to all-optical image processing is shown to generate real-time pseudo-3D images of transparent biological and other samples, such as human cervical cancer cells. This work demonstrates the potential of non-local, non-interferometric approaches to image processing for uses in label-free biological cell imaging and dynamical monitoring.