45 research outputs found
Two-dimensional imaging in hyperbolic media-the role of field components and ordinary waves
We study full vector imaging of two dimensional source fields through finite slabs of media with extreme anisotropy, such as hyperbolic media. For this, we adapt the exact transfer matrix method for uniaxial media to calculate the two dimensional transfer functions and point spread functions for arbitrary vector fields described in Cartesian coordinates. This is more convenient for imaging simulations than the use of the natural, propagation direction-dependent TE/TM basis and clarifies which field components contribute to sub-diffraction imaging. We study the effect of ordinary waves on image quality, which previous one-dimensional approaches could not consider. Perfect sub-diffraction imaging can be achieved if longitudinal fields are measured, but in the more common case where field intensities or transverse fields are measured, ordinary waves cause artefacts. These become more prevalent when attempting to image large objects with high resolution. We discuss implications for curved hyperbolic imaging geometries such as hyperlenses
Subwavelength terahertz imaging via virtual superlensing in the radiating near field
Paradoxically, imaging with resolution much below the wavelength -
now common place in the visible spectrum - remains challenging at lower
frequencies, where arguably it is needed most due to the large wavelengths
used. Techniques to break the diffraction limit in microscopy have led to many
breakthroughs across sciences, but remain largely confined to the optical
spectrum, where near-field coupled fluorophores operate. At lower frequencies,
exponentially decaying evanescent waves must be measured directly, requiring a
tip or antenna to be brought into very close vicinity to the object. This is
often difficult, and can be problematic as the probe can perturb the near-field
distribution itself. Here we show the information encoded in evanescent waves
can be probed further than previously thought possible, and a truthful image of
the near-field reconstructed through selective amplification of evanescent
waves - akin to a virtual superlens reversing the evanescent decay. We quantify
the trade-off between noise and measurement distance, and experimentally
demonstrate reconstruction of complex images with subwavelength features, down
to a resolution of and amplitude signal-to-noise ratios below 25dB
between 0.18-1.5THz. Our procedure can be implemented with any near field probe
far from the reactive near field region, greatly relaxes experimental
requirements for subwavelength imaging in particular at sub-optical
frequencies, and opens the door to non-perturbing near-field scanning
Terahertz orbital angular momentum modes with flexible twisted hollow core antiresonant fiber
THz radiation is more and more commonplace in research laboratories as well
as in everyday life, with applications ranging from body scanners at airport
security to short range wireless communications. In the optical domain,
waveguides and other devices to manipulate radiation are well established. This
is not yet the case in the THz regime because of the strong interaction of THz
radiation with matter, leading to absorption, and the millimeter size of the
wavelength and therefore of the required waveguides. We propose the use of a
new material, polyurethane, for waveguides that allows high flexibility,
overcoming the problem that large sizes otherwise result in rigid structures.
With this material we realize antiresonant hollow-core waveguides and we use
the flexibility of the material to mechanically twist the waveguide in a
tunable and reversible manner, with twist periods as short as tens of
wavelengths. Twisting the waveguide, we demonstrate the generation of modes
carrying orbital angular momentum. We use THz time domain spectroscopy to
measure and clearly visualize the vortex nature of the mode, which is difficult
in the optical domain. The proposed waveguide is a new platform offering new
perspectives for THz guidance and particularly mode manipulation. The
demonstrated ability to generate modes with orbital angular momentum within a
waveguide, in a controllable manner, will be beneficial to both fundamental,
e.g. matter-radiation interaction, and applied, e.g. THz telecommunications,
advances of THz research and technology. Moreover, this platform is not limited
to the THz domain and could be scaled for other electromagnetic wavelengths.Comment: Figure 6 and 10 are video and are available at
https://doi.org/10.1063/1.5016283.1 and https://doi.org/10.1063/1.5016283.2
respectively. Supplementary material available at
ftp://ftp.aip.org/epaps/apl_photonics/E-APPHD2-3-011891 and data are
available at https://doi.org/10.5281/zenodo.1164309. Note: the title has
changed in the latest version compared to the original on
A prism based magnifying hyperlens with broad-band imaging
Magnification in metamaterial hyperlenses has been demonstrated using curved geometries or tapered devices, at frequencies ranging from the microwave to the ultraviolet spectrum. One of the main issues of such hyperlenses is the difficulty in manufacturing. In this letter, we numerically and experimentally study a wire medium prism as an imaging device at THz frequencies. We characterize the transmission of the image of two sub-wavelength apertures, observing that our device is capable of resolving the apertures and producing a two-fold magnified image at the output. The hyperlens shows strong frequency dependent artefacts, a priori limiting the use of the device for broad-band imaging. We identify the main source of image aberration as the reflections supported by the wire medium and also show that even the weaker reflections severely affect the imaging quality. In order to correct for the reflections, we devise a filtering technique equivalent to spatially variable time gating so that ultra-broad band imaging is achieved
Fiber-Drawn Metamaterial for THz Waveguiding and Imaging
In this paper, we review the work of our group in fabricating metamaterials for terahertz (THz) applications by fiber drawing. We discuss the fabrication technique and the structures that can be obtained before focusing on two particular applications of terahertz metamaterials, i.e., waveguiding and sub-diffraction imaging. We show the experimental demonstration of THz radiation guidance through hollow core waveguides with metamaterial cladding, where substantial improvements were realized compared to conventional hollow core waveguides, such as reduction of size, greater flexibility, increased single-mode operating regime, and guiding due to magnetic and electric resonances. We also report recent and new experimental work on near- and far-field THz imaging using wire array metamaterials that are capable of resolving features as small as λ/28
Applications of long period gratings in solid core photonic bandgap fibers
Solid core photonic bandgap fibres are photonic crystal fibres with a solid core surrounded by high index inclusions. The guidance properties of these fibers are very sensitive to the refractive index of the inclusions, making them widely tunable and making them very promising for sensing applications. Combining these fibers with long period gratings unleashes their full potential, enabling narrow band notch filters tunable over hundreds of nm, refractive index sensors with sensitivity comparable to that of surface plasmon resonance sensors, but also the extraction of the full band diagrams of these bandgap fibres
Towards subdiffraction imaging with wire array metamaterial hyperlenses at MIR frequencies
We describe the fabrication of metamaterial magnifying hyperlenses with subwavelength wire array structures for operation in the mid-infrared (around 3 µm). The metadevices are composed of approximately 500 tin wires embedded in soda-lime glass, where the metallic wires vary in diameter from 500 nm to 1.2 µm along the tapered structure. The modeling of the hyperlenses indicates that the expected overall losses for the high spatial frequency modes in such metadevices are between 20 dB to 45 dB, depending on the structural parameters selected, being promising candidates for far-field subdiffraction imaging in the mid-infrared. Initial far-field subdiffraction imaging attempts are described, and the problems encountered discussed