Loïc Markley

Spatially shifted beam approach to subwavelength focusing.

Although negative-refractive-index metamaterials have successfully achieved subwavelength focusing, image resolution is limited by the presence of losses. In this Letter, a metal transmission screen with subwavelength spaced slots is proposed that focuses the near-field beyond the diffraction limit and, furthermore, is easily scaled from microwave frequencies to the optical regime. An analytical model based on the superposition of shifted-beam patterns is developed that agrees very well with full-wave simulations and is corroborated by experimental results at microwave frequencies.

Two-Dimensional Subwavelength Focusing Using a Slotted Meta-Screen

A transmission screen with subwavelength slots is described which can focus electromagnetic radiation to a 2-D subwavelength spot. Unlike negative-refractive-index focusing implementations, this ldquometa-screenrdquo does not suffer from image degradation when losses are introduced and is easily scalable from microwave to Terahertz frequencies and beyond. The slotted geometry is designed using a theory of shifted beams to determine the necessary weighting factors for each slot element, which are then converted to appropriate slot dimensions. The screen was designed using a pair of rotated orthogonal focusing axes to produce a spot on a focal plane located 0.25 lambda away. Full-wave simulations are corroborated by experimental measurements at 10 GHz reporting a full-width half-maximum beam width of 0.27 lambda along the x-axis and 0.38 lambda along the z-axis.

A Negative-Refractive-Index Metamaterial for Incident Plane Waves of Arbitrary Polarization

In this letter, a left-handed artificial dielectric is proposed. Using resonating broadside coupled patches to generate a negative permeability and thin strips to generate negative permittivity, a negative index of refraction that is matched to free space can be designed. The structure is fabricated from a single dielectric sheet printed on both sides by metal patterns that couple to externally incident plane waves of any linear polarization. Unlike many previously published structures, the waves interact through perpendicular incidence to the surface. An experiment was carried out at 10 GHz to verify the left-handed behavior with an extracted index of refraction of -1.04 matched to free-space with 26.0 dB return loss and 0.77 dB insertion loss. A multilayer version was also designed that achieved fractional dispersion bandwidths comparable to those of transmission-line implementations

Two-dimensional subwavelength-focused imaging using a near-field probe at a λ/4 working distance

Two-dimensional (2D) subwavelength imaging using a near-field antenna array probe is demonstrated experimentally at a probe-to-object separation distance of λ/4. Field perturbations caused by the presence of small objects are detected by monitoring the input reflection coefficient as the probe is scanned. The probe is designed to produce a subwavelength focal spot with a 0.217λ full-width half-maximum beam width. Propagating components are suppressed through destructive interference, enhancing the evanescent field enough to resolve objects spaced by 0.259λ. The poor resolving capability of a conventional single-element probe at this working distance is also addressed; it is shown that even objects spaced over a wavelength apart are inaccurately detected. A full 2D imaging experiment is carried out using eight scattered objects. The single element probe is unable to resolve any of the objects, while all but the two most closely spaced pairs are resolved by the array probe.

A Near-Field Probe for Subwavelength-Focused Imaging

A near-field antenna array probe is presented that can produce a subwavelength focal spot at a distance of 0.25λ. The array is composed of a central fed dipole with loaded secondary dipoles placed less than half a wavelength away on each side. When performing a surface raster scan, the probe return loss can be monitored to detect the presence of small wire loops with subwavelength resolution. An experiment was simulated to scan for two loops placed 0.25λ away from the probe and separated by 0.4λ. The array probe was able to clearly resolve both loops while a single dipole probe could not. The minimum resolvable loop separation was 0.3λ for the array probe and 0.65λ for the single dipole probe.

A near-field probe for subwavelength-focused imaging

A near-field antenna array probe is presented that can perform subwavelength-focused imaging at a distance of 0.25¿ . The array is composed of a centrally fed dipole with open-circuited secondary dipoles placed less than half a wavelength away on each side. Together, they produce a subwavelength focal spot that can be used to detect the presence of small wire loops with subwavelength resolution. This is done by raster scanning the array over the focal plane and monitoring the probe return loss for perturbations. An experiment was conducted that verified the 1-D subwavelength imaging of two loops placed in the focal plane. The minimum resolvable loop separation was 0.31¿ for the array probe compared to 0.72¿ for the single-dipole probe. The array probe had a 13% resolution bandwidth for two loops spaced at 0.4¿.

Two-Dimensional Subwavelength-Focused Imaging Using a Near-Field End-Fire Antenna-Array Probe

A two-dimensional imaging technique is proposed based on an array of monopole antennas that produces a near-field subwavelength focal spot. The antennas are designed to produce an end-fire, rather than broadside, near-field beam in order to increase the degree of rotational symmetry in the structure. Since two-dimensional focusing does not automatically translate to two-dimensional imaging, symmetry was the key to extending subwavelength imaging to two dimensions. Simulations were conducted to demonstrate the improved resolution of the antenna array probe with a final minimum resolvable spacing of lambda at a distance of lambda/4.

Experimental verification of subwavelength acoustic focusing using a near-field array of closely spaced elements

A linear array of closely spaced sound transducers is presented that can produce a subwavelength-focused intensity profile at a distance of a quarter wavelength. This work is related to research on super-resolution using metamaterials in both the acoustic and optical domains. It is designed using the principle of shifted beams, a near-field antenna array theory developed for the subwavelength focusing of electromagnetic waves. Once the spatial sound pattern is characterized for each source, the optimal weights for a minimum beam width can be calculated. An experiment operating at 4 kHz was able to successfully construct a super-focused beam.

A polarization independent negative-refractive-index metamaterial for incident plane waves

In this paper a new left-handed artificial dielectric is proposed. Using resonating broadside coupled patches to generate a negative permeability and thin strips to generate negative permittivity, a negative index of refraction that is matched to free space can be designed. This structure is fabricated from a single dielectric sheet printed on both sides by metal patterns that couple to externally incident plane waves of any polarization

Detecting buried objects with subwavelength resolution using a near-field antenna-array probe

A near-field antenna-array probe is used to perform subwavelength imaging on objects buried a quarter-wavelength beneath the surface of a dielectric slab. Short passive dipole objects are detected by monitoring the input reflection coefficient of the probe as it is scanned over the dielectric. Experimental measurements are presented that verify the subwavelength imaging behavior. A minimum resolvable separation between two objects in terms of dielectric wavelengths was measured to be 0:295λ at a distance of λ/4, compared to 0:639λ for a single monopole probe.