For measurements designed to accurately determine layer thickness, there is a natural trade-off between sensitivity to optical thickness and lateral resolution due to the angular ray distribution required for a focused beam. We demonstrate a near-field imaging approach that enables both sub-wavelength lateral resolution and optical thickness sensitivity. We illuminate a sample in a total internal reflection geometry, with a photo-activated spatial modulator in the near-field, which allows optical thickness images to be computationally reconstructed in a few seconds. We demonstrate our approach at 140 GHz (wavelength 2.15 mm), where images are normally severely limited in spatial resolution, and demonstrate mapping of optical thickness variation in inhomogeneous biological tissues.
Adoption of terahertz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a fundamentally new way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer's emissivity in the THz part of the electromagnetic spectrum. We show how this unconventional source of controllable THz radiation enables a new form of incoherent computational THz imaging. We use it to image various concealed objects, demonstrating this scheme has the penetrating capability of state-of-the-art THz imaging approaches, without the requirement of femto-second pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control paves the way towards a new family of long-wavelength structured illumination, imaging and spectroscopy systems.
Optical systems often largely consist of empty space as diffraction effects that occur through free-space propagation can be crucial for their function. Contracting these voids offers a path to the miniaturization of a wide range of optical devices. Recently, a new optical element—coined “spaceplate”—has been proposed, which is capable of emulating the effects of diffraction over a specified propagation distance using a thinner non-local metamaterial [Reshef et al., Nat. Commun. 12, 3512 (2021)]. The compression factor of such an element is given by the ratio of the length of free-space that is replaced to the thickness of the spaceplate itself. In this work, we test a prototype spaceplate in the microwave spectral region (20–23 GHz)—the first such demonstration designed to operate in ambient air. Our device consists of a Fabry–Pérot cavity formed from two reflective metasurfaces with a compression factor that can be tuned by varying the size of perforations within each layer. Using a pair of directive horn antennas, we measure a space compression factor of up to ∼6 over a numerical aperture (NA) of 0.34 and a fractional bandwidth of 6%. We also investigate the fundamental trade-offs that exist between the compression factor, transmission efficiency, NA, and bandwidth of this single resonator spaceplate design and highlight that it can reach arbitrarily high compression factors by restricting its NA and bandwidth.
The paper describes a corrugated substrate integrated horn antenna designed for surface wave excitation in the UWB band groups 3, 4, 5 and 6 (6-10 GHz). The antenna is optimized for integration into 3.4 mm thick textile of relative permittivity of 1.2. The corrugated substrate integrated waveguide is fed by a microstrip line and for connectivity a transition from microstrip line to coaxial line is designed. In order to enhance the impedance matching, we utilize thin strip lines placed in front of the aperture. The directivity of the antenna is 12.5 dBi at the 8 GHz and the antenna efficiency is about 70 %.
Optical systems often consist largely of empty space, as diffraction effects that occur through free-space propagation can be crucial to their function. Contracting these voids offers a path to the miniaturisation of a wide range of optical devices. Recently, a new optical element - coined a 'spaceplate' - has been proposed, that is capable of emulating the effects of diffraction over a specified propagation distance using a thinner non-local metamaterial [Nat. Commun. 12, 3512 (2021)]. The compression factor of such an element is given by the ratio of the length of free-space that is replaced to the thickness of the spaceplate itself. In this work we test a prototype spaceplate in the microwave spectral region (17-18 GHz) - the first such demonstration designed to operate in ambient air. Our device consists of a Fabry-P\'erot cavity formed from two perforated conductive sheets, with a compression factor that can be directly tuned by varying the size of the perforations. Using a pair of directive horn antennas, we show evidence for a compression factor of up to ~6.6. We also observe some distortion to the transmitted field, and we discuss future improvements to minimise aberrations. Finally, we investigate the fundamental trade-offs that exist between the compression factor, transmission efficiency, numerical aperture (NA) and bandwidth of this single resonator spaceplate design, and highlight that it can reach arbitrarily high compression factors by restricting its NA and bandwidth.
Between the design and final realization of a working metasurface lies the potential for a myriad of complications: fabrication tolerances, material permittivity uncertainties, alignment issues and localized defects to name just a few. Global methods of characterizing an entire surface are often incapable of separating these candidates and typically one must resort to the simulation of a wide parameter space to begin to understand experimental discrepancies. In this work we introduce a new imaging technique that is able to locate and discern the resonant frequencies of individual antennas in a complex microwave metasurface. This is achieved with a microwave single-pixel camera using patterned optical excitation of a silicon layer adjacent to the metamaterial to achieve super-resolution. This approach allows us to locate and diagnose fabrication defects, spectrally characterize individual meta-atoms, and visualize inhomogeneous broadening across our samples with below λ/20 resolution, over large areas and in near real-time.
In this article, an original design of a range illuminator based on a stepped-septum polarizer and a dual-mode horn is discussed. However, the designed polarizer can also be used with other waveguide antennas, such as pyramidal horns and corrugated horns. A simple procedure for determining the initial septum geometry for the subsequent optimization is described, and based on this, a two-step septum polarizer is developed. A carefully designed dual-mode horn antenna with a rectangular aperture is then utilized to increase the illuminator's gain and suppress sidelobe levels. Results of full-wave electromagnetic simulations are presented and compared to experimentally measured data with good agreement.
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