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Abstract— It is demonstrated that the electromagnetic (EM) transmission through a subwavelength or non-resonant aperture in a conductive screen can be dramatically enhanced by loading it with folded metallic strips exhibiting resonant properties. When illuminated by an EM plane wave these loaded apertures enable very tight, subwavelength, collimation of the EM power in the near field zone. We propose planar and quasi-planar resonant insertion geometries that should allow, for the first time, two-dimensional dual-polarization subwavelength field confinement along with ability to focus both electric and magnetic fields.

The proposed technique for resonance transmission enhancement and near field confinement forms a basis for a new class of microwave near field imaging probe with subwavelength resolution capable of operating over a wide range of imaging distances (0.05-0.25λ). Measurement results demonstrate the possibility of high contrast (more than 3dB in amplitude and 40 degrees in phase) near field subwavelength imaging of 2D and 3D resonant and non-resonant metallic and dielectric targets in free space and in moderately lossy layered media.

Index Terms— Enhanced transmission, focusing, near field enhancement, resonance, sub-diffraction imaging, subwavelength aperture.

I. INTRODUCTION

IGH RESOLUTION near field electromagnetic (EM) imaging is important in a number of application

including materials characterization [1], biomolecular and medical non-invasive probing [2], non-destructive testing [3], etc.

HAt present, there are a number of different near-field

imaging techniques applicable across different spectral ranges. These techniques can be broadly divided into two categories: aperture-based and apertureless near field probes. In the first case the probe consists of a small, typically subwavelength, aperture in a metallic screen [4]. Alternatively it can be an open waveguide loaded with a dielectric insert [5]. The object to be imaged is positioned in close vicinity of the aperture and illuminated by an EM wave. The radiation scattered off the object is transmitted through the aperture and is collected by an EM radiation detector where the properties of the imaged object are estimated by measuring the detector output current. In this scenario, the spatial resolution of an aperture-based probe is determined by the size of the aperture [6], [7].

In the apertureless imaging scheme EM radiation is scattered by a sharp tip of a probe located in the vicinity of an

Date submitted. This work was supported in part by the Leverhulme Trust, UK .

The authors are with The Institute of Electronics, Communications and Information Technology, Queen's University Belfast, Queen's Rd, Belfast, BT3 9DT, UK. e-mails:[email protected]; [email protected].

imaged object and collected in the far or near field [8]. Knowing the scattering characteristics of a probe it is possible to reconstruct the object properties [8]. In this scenario image spatial resolution depends on the probe tip dimensions, geometry and the probe-to-object separation distance.

These techniques can also be used for the active source characterization where the radiation from the source can be collected through the aperture or by a small coaxial probe [9].

The above mentioned techniques have key limitations. The main limiting factor that restricts the use of small apertures is a dramatic reduction of the transmitted power as aperture size decreases, proportional to the third power of the aperture size [6], [7]. This leads to the necessity of using sources of considerable power or large signal amplification. The availability of these imparts practical difficulties especially at millimeter and sub-millimeter wavelengths. In the apertureless scenario the operating distance is extremely small, typically λ/50- λ/100 [8], λ is a wavelength of radiation, leading to the fast deterioration of the image as the probe tip is located farther away from the sample. This compounds the interpretation of the images obtained, which in general, is not a trivial task [10] requiring some prior knowledge of the imaged scene and de-embedding of the probe tip intrinsic scattering properties.

In this paper we propose a simple method which overcomes the main drawbacks of previous aperture-based near field probe solutions by using resonantly loaded apertures. It was first noted in [11] that the electrically small apertures can exhibit resonance transmission properties in the presence of a conductive object. Several structures that can enhance the transmission through a small hole based on this principle have been proposed in recent years, particularly metal gratings [12] or metamaterials [13], [14]. In [15] a very simple arrangement consisting of a rectangular slot aperture loaded with strip has been proposed. It was shown that such a loaded aperture enables up to 25dB transmitted field enhancement with respect to the unloaded slot transmission in the L or S band. Additionally superior subwavelength near field confinement was observed with transmitted beam full width at half maximum (FWHM) measured approximately one tenth of a wavelength at 0.1λ distance from the slot. This probe demonstrated very high-contrast (with more than 10dB discrimination) near field subwavelength resolution of metallic scatterers separated by less than quarter-wavelength at 0.1λ to 0.2λ imaging distance.

In the work to be presented here we significantly extend the results of our previous studies [15] in two ways.

First, we propose several new insert and aperture geometries that result in significant further improvement of

Near Field Enhancement and Subwavelength Imaging Using Resonantly Loaded Apertures

Oleksandr Malyuskin, Member, IEEE, Vincent Fusco, Fellow, IEEE

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loaded aperture probe performance in terms of the near field enhancement, spatial squeezing and enhanced imaging contrast. In particular we propose planar and quasi-planar resonant insertion geometries that allow, for the first time, two-dimensional dual-polarization subwavelength field confinement along with the ability to focus both electric and magnetic fields. Secondly, the experimental results we present demonstrate significant sensitivity of the resonantly loaded aperture probe to the dielectric properties of the imaged samples. This makes the approach a promising candidate for dielectric material characterization.

It should be noted that while papers such as [16]-[18] deal with the problem of slot–strip interaction and the transmission properties of such arrangements, these works only consider the case of orthogonal complementary structures. The geometries considered below lead to fundamentally new scattering effects not studied before.

II.PRINCIPLE OF OPERATION OF RESONANTLY LOADED APERTURE

A resonantly loaded slot aperture in a metallic screen illuminated by a plane wave is shown in Fig.1(a). In this arrangement, the lengths of the slot and the strip are 0.6λ and 0.5λ and respective widths are 0.1λ and 0.05λ. In the absence of the strip insert, the transmitted EM field, when co-polarized to the slot, is significantly attenuated, Fig. 1(b), dotted line. In this graph the transmitted electric field amplitude is plotted as a function of the transversal range x, at z = -0.1λ, i.e. in the near field zone. This transmitted field attenuation occurs due to the unloaded slot radiation mechanism [17] which in this case is cut-off for the radiating magnetic currents. Insertion of a half-wavelength strip results in electric currents being supported that can re-radiate, generating resonantly enhanced EM field transmission whose electric field component is polarized along the slot, Fig.1(b), solid line. Additional near field spatial squeezing of the EM field transmitted through the loaded aperture, solid line, as compared to the scattered by a standalone strip in free space, dashed line, Fig.1(b) is also observed. This can be explained in simple terms by the fact that at the metal boundaries of the slot the tangential field is zero which confines the spatial signature of the transmitted near field in transversal directions and leads to its amplitude enhancement in the “hot spot”. In Fig.1(b), the FWHM of the transmitted by the resonantly loaded aperture near field is 0.18λ at z = -0.1λ, while the FWHM of the near field due to the unloaded slot aperture and strip in free space are both 0.26λ at the same distance z = -0.1λ.

The scattering properties of the slot can be qualitatively understood on the basis of the system of coupled integral electric field equations which can be represented in terms of the equivalent circuit network [19]. The detailed analysis of this problem is out of the scope of the present paper however some important points have to be mentioned.

(a)

(b)

Fig.1 Resonant near field enhancement and spatial confinement due to loaded aperture. (a) Loaded aperture geometry; (b) FEKO simulated near field amplitude across the transversal range at the z = -0.1λ offset distance.

First, it should be noted that both the Ex and Ey components of the electric field inside a gap between the loaded narrow slot and a strip are significantly enhanced (compared to the incident field of unit amplitude), due to strong near field coupling, Fig.2.

Fig.2. Simulated (FEKO) electric field magnitude in the gap between the strip of width 0.02λ and slot (0.1λ wide). The field is plotted near the strip end at y=0.2λ, z=0 where the electric field intensity is large. Red lines show strip and slot edges.

However, in the narrow slots (width<0.2λ) the transversal Ex

component is dominating over the longitudinal component Ey

everywhere in the gap area. This can be explained by the fact

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that the last term in the mixed potential integral equation [19]

E⃗ ( r⃗ )=− jω∫S

μ0 exp (− jkR )4πR

J⃗ ( r⃗ ' ) d r⃗ '

−∇∫S

exp (− jkR )4 πε0 R

ρ ( r⃗ ' ) d r⃗ '

(1)

contains spatial derivatives resulting in the terms with ~ 1/ R2 , ~ 1/ R3

spatial dependence as opposed to the ~ 1/ R

dependence of the first term. In (1) J⃗ ( r⃗ ' ) and ρ ( r⃗ ' )are the current and electric charge density on the strip, the first term defines the equivalent inductance while the second term contributes to the equivalent capacitance [19] of the loaded aperture which can be viewed as a capacitance of the strip insert in free space and additional series quasi-static capacitance due to near field coupling. When the gap width is reduced the quasi-static capacitance is also reduced

proportionally to the ratioK (√1−ν2) /K ( ν ) , where ν is the

ratio of strip width to the gap width and K (ν ) is the complete elliptic integral of the first kind [20], [21]. The considered loaded aperture EM transmission properties will be studied experimentally in the next section.

III. NEAR FIELD PROBE BASED ON THE LOADED APERTURE AND ITS CHARACTERISTIC RESOLUTION

A. Near Field Imaging Probe DescriptionA microwave probe can be composed using a resonantly

loaded aperture backed by a cavity or horn antenna, Fig.3(a). In our experiment the measurement setup consists of a pyramidal horn antenna excited with a vertical E-field polarized signal and a conductive plate with resonantly loaded aperture, covering a pyramidal horn with aperture dimensions 140mm x 105mm. Microwave imaging is based on the measurements of the reflection coefficient S11 at the coaxial excitation port of the horn antenna. Resonant EM transmissiondue to the loaded aperture leads to significant near field enhancement in a tight focal spot in the vicinity of the probe insert. This, in its turn, leads to very strong near field coupling of the probe to objects positioned within the imaging scene, Fig. 3(b). The geometry and EM properties of these objects can be characterized by the variation of the reflection coefficient S11 as the probe scans the scene.

B. Microwave Probe Transmission Enhancement and Near Field Collimation

In the previous section it has been shown that the EM field is dramatically enhanced both outside the loaded slot and inside the gap between the strip insert and slot. The latter effect leads to the quasi-static capacitive coupling between the strip insert and slot aperture.

(a)

(b)

Fig.3. (a) Microwave near field probe based on the horn antenna terminated with metal plate with resonantly loaded aperture; (b) imaging scene and probe set-up geometry.

Fig.4.Measured EM reflection and transmission of the straight strip loaded aperture with slot width as a parameter. Strip with is 2mm, strip length is 52mm, slot length is 62.5mm. Green hollow dotted lines show the transmission and reflection parameters for the case of unloaded aperture.

The measured resonance transmission properties of the strip-loaded aperture geometry are shown in Fig. 4, where the slot width varies as a parameter. The transmitted field is measured

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by the wideband standard horn (positioned 300mm away from the probe) with a 140mm x 240mm aperture. It can be seen that for narrow slots the resonance frequency increases due to abovementioned equivalent capacitance reduction. It is also interesting to note that wider slots ensure higher level of transmission and increased frequency bandwidth of operation. It is expected that wider slots should result, however, to poorer E-field collimation in the near field zone. To verify this, the near electric field transversal distribution FWHM has been calculated (at y = 0.2λ, i.e. close to the strip ends) as a function of slot width and distance from the loaded aperture using FEKO solver, Fig.5.

Fig.5.Calculated FWHM for the straight strip loaded aperture vs. distance with slot width as a parameter. Wavelength λ=0.125m (@2.4GHz). Other geometry parameters are the same as in Fig.4. Parameter w sl stands for the slot width.

From Fig.5 it can be concluded that for very narrow slots (~0.05λ wide) the E-field is tightly collimated in the vicinity of the strip, -z ranges from 0.05 to 0.10λ, and then rapidly diverges as the imaging distance increases (-z>0.15λ). For wide slots (~0.2λ) the divergence of the E-field at larger imaging distances is slower, however, as expected in the vicinity of the aperture (0.05÷0.10λ) the FWHM is ~0.05λ wider than for the narrower slot. Apparently there is some optimal slot width, viz. 10mm (~0.1λ) that can be chosen in microwave probe design. It also can be noted that based on the FWHM properties the probes with different slot widths should ensure very similar imaging resolution (with ~λ/20 difference) in the imaging distances range 0.05÷0.15λ. Fig.5 also gives a characteristic resolution of the loaded aperture probe based on the FWHM as a function of imaging distance. Another important aspect affecting the ability of the probe to resolve multiple targets is discussed below.

C.Characteristic Transversal Resolution of Two Electric Hertzian Dipoles

Imaging multiple targets in the cross range, Fig.3(b) is associated with the near field cross-coupling between these targets which affects the microwave probe imaging resolution. Since EM coupling is caused by the induced currents on the

targets, the characteristic resolution limits can be estimated using a canonical model of two parallel y-oriented Hertzian dipoles located in the imaging plane and separated by the subwavelength distance d across the range x.

(a)

(b)

Fig.6. (a) Calculated normalized near field distribution of two Hertzian dipoles in free space separated by quarter wavelength at various distances z; (b) resolution contrast as a function of dipoles cross-range separation and imaging distance.

Fig.6(a) shows the near field distribution of two y-oriented Hertzian point-like dipoles separated by λ/4 in cross range at various stand-off distances z. It can be seen that the resolution between the dipole sources decreases very rapidly with the distance – it is noteworthy that two λ/4 separated dipoles cannot be resolved at 0.15λ stand-off distance. Next, Fig. 6(b) shows the resolution contrast of two Hertzian dipoles, measured as 20log of the ratio of a peak-to-dip field magnitude for a range of dipoles separation and stand-off distances. It can be seen that the resolution contrast for the considered dipole sources decreases exponentially with the stand-off distance. The results provided in Figs. 5, 6 can be used to estimate the characteristic resolution of the considered near field probe.

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IV. LOADED APERTURE NEAR FIELD PROBE PERFORMANCE ENHANCEMENT

For subwavelength resolution imaging in two dimensions (in the x-y plane) it is essential to reduce the dimensions of the insert, especially its length. The major drawback of the straight strip insert loaded aperture considered in the previous Sections is its wide near E-field pattern in the longitudinal y range [22] which allows subwavelength imaging only in one direction. Additionally, in many applications dual polarization probes are required. Insert configurations fulfilling these requirements will now be discussed.

A. Insertion of Folded Strip in Rectangular Slot The geometry of the folded strip insert (whose total unfolded length is ~ λ/2) is shown in Fig.7(a) and its measured reflection S11 parameter in Fig.7(b).

(a)

(b)

Fig.7. (a) Slot aperture loaded with folded strip. The aperture in dual slotted screen is shown. Small red arrows show the current density flow path (FEKO simulated) at the resonance frequency. (b) S11 parameters of the folded strip loaded aperture. Vertical separation between the screens dscr = 3mm.

A single or dual screen configuration as shown in Fig. 7(b) can be used. Experimental results and numerical simulations not discussed here in detail for the brevity’s sake show that the dual slotted screen loaded aperture leads to better near field confinement as compared to the single screen aperture. Also, enhanced transmission, Fig.7(b), through the slotted dual

screen aperture should lead to better imaging contrast as compared to a single screen configuration(~10dB difference in the S11 levels for these two structures).

Fig.7(b) shows the measured S11 parameter and Fig.8 displays calculated (with FEKO) near field spatial distribution for the folded strip loaded aperture with geometric parameters: total unfolded length of the strip is λ/2; top horizontal segment is λ/4; vertical segments are λ/20 each; bottom horizontal segments are λ/13.3 each; strip and slot widths are λ/30 and λ/10 correspondingly. The dual layers are separated by 3mm, the wavelength λ=13.15cm corresponds to the frequency 2.28GHz.

The simulated field intensity distribution in the vicinity of the dual-screen loaded aperture at z = -0.1λ demonstrates that the folded strip insertion results in a dominant dipole mode in the near electric field, Fig. 8(a), with a subwavelength average FWHM approximately equal to the length of folded strip in longitudinal direction (0.25λ in this case). The FWHM in the transversal directions depends on the strip and slot widths and intrinsically is very narrow (Table I).

(a)

(b)

Fig. 8. (a)Simulated electric field intensity distribution in the near field vicinity of the probe at z = -0.1λ; (b) Simulated magnetic field intensity distribution at the plane z = -0.1λ.

It should also be noted that the magnetic field intensity (with dominant Hx component) forms a tightly localized circular spot with FWHM ~ 0.1λ or less at offset distance z = -0.1λ, Fig. 8(b).

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B. Folded Cross Dipole in Circular Subwavelength Aperture

The folded strip insertion considered in the previous section offers considerable near electric and magnetic field collimation as compared to the straight metallic insert. However, again, it can operate only for single EM field polarization. A compacted cross dipole insertion in a circular subwavelength aperture, Fig. 9(a) is now proposed as planar probe geometry with dual-polarization capability.

(a)

(b)

Fig.9. (a) Geometry of the compacted dipole cross insertion in circular hole and (b) its measured S11 parameter. Red arrows in Fig.9(a) show the current density flow path at the resonance frequency.

Here resonant transmission occurs due to the electric currents along the crosses dipole arms whose unfolded length is approximately λ/2, λ is the wavelength corresponding to resonance. Fig. 10 shows the simulated electric and magnetic field intensity distribution in the xy plane at the offset distance z = -0.1λ. The geometric parameters of the structure are: circular aperture diameter Dh is λ/5; strip width is λ/50; gap g is λ/35 and the incident field is polarized along the y axis.

It can be seen that the normally incident Ey polarized plane wave induces a dominant dipole mode in the electric response of the compacted cross inclusion; both the electric and magnetic components of the near field intensity distribution are tightly collimated with FWHM data detailed in Table I.

(a)

(b)

Fig. 10. (a) Simulated electric field intensity distribution in the near field vicinity of the folded cross loaded aperture at z = -0.1λ; (b) simulated magnetic field intensity distribution at the plane z = -0.1λ.

A comparison between the near field patterns of folded strip and compacted cross inserts loaded apertures is summarized in Table I.

TABLE IFWHM (in λ) OF THE LOADED APERTURE NEAR FIELD

-z/λ Folded strip, |E|

Folded strip, |H|

Cross insert,|E|

Cross insert,|H|

0.05 0.11 0.14 0.23 0.280.1 0.18 0.19 0.26 0.37

0.15 0.23 0.26 0.34 0.46

0.2 0.30 0.4 0.46 0.54

0.25 0.42 0.53 0.58 0.70

In this table the FWHM data are provided for the slotted dual screen and compacted cross loaded apertures discussed above. It can be seen from Table I that the folded strip loaded aperture enables much tighter (~0.1λ smaller FWHM) near field collimation than the compacted cross geometry. Also the near field pattern of the circular compacted cross loaded aperture diverges fast at larger stand-off distances (FWHM>0.5λ at |z|>0.25λ) which makes sub-diffraction

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resolution impossible for more than quarter-wavelength imaging distance.

V. NEAR FIELD SUBWAVELENGTH RESOLUTION IMAGING OF METALLIC AND DIELECTRIC OBJECTS

In this section we study the imaging properties of dual screen aperture arrangements of the folded strip and compacted cross resonant insert probe. Characteristic targets are represented by resonant and non-resonant metallic 1D and 2D shapes in free space and in moderately lossy layered media. Also, imaging of non-resonant dielectric 2D and 3D targets is experimentally studied in free space and in the presence of a lossy absorbing screen. In all experiments the targets have subwavelength dimensions or are separated by the subwavelength distance in cross-range.

A. Planar Metallic Targets in Free SpaceThe first target to be imaged consists of a pair of resonant

half-wavelength strips, separated along the x-axis by a quarter wavelength (at 2.28GHz), Fig. 11(a). The imaging distance is 0.1λ for all experiments. Fig. 11 shows the measured S11

amplitude and phase obtained using the folded strip insert probe. It can be seen that the probe reconstructs the EM properties (near field distribution) of the strips with very high contrast (~10dB in amplitude and ~200o in phase).

(a)

(b)

(c)Fig. 11. Microwave image of two sub-wavelength spaced resonant strips on a 20mm thick layer of foam εr=1.1 in free space. (a)Imaged target; (b) measured S11 magnitude; (c) S11 phase. The results are obtained with folded strip insert probe. The scale in Fig.11(a) is in millimeters.

(a)

(b)

Fig. 12. Microwave image of two resonant strips in free space. (a) measured S11 magnitude; (c) S11 phase. The results are obtained with folded cross insert probe.

Fig.12 demonstrates the microwave image of the same two-strip structure obtained with a probe using folded cross insert in circular aperture described in Section IV B. From Figs.11, 12 it can be seen that the coupling mechanism of these two probes is substantially different; the loaded circular hole probe tends to average the EM coupling effect across its aperture.

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Even in this case the obtained image has a very high amplitude and phase contrast, however the shape of the wire scatterers represented not as clearly as in Fig.11.

It is important to note at this stage that in this paper we show “raw” imaging data with probe coupling effect embedded into images. The images could be potentially improved using numerical post-processing de-embedding the probe coupling effect, this will be reported in future work.

(a)

(b)

(c)

Fig. 13. Microwave image of two different metal shapes in free space. (a) Imaged target; (b) measured S11 magnitude; (c) S11 phase. The results are

obtained with folded strip insert in rectangular slot probe. The scale in Fig.13(a) is in millimeters.

Figs.13, 14 demonstrate the near field microwave image of two non-resonant planar metallic shapes in free space as obtained using the folded strip and compacted cross loaded aperture probes correspondingly.

(a)

(b)

Fig. 14. Microwave image of two planar metal shapes in free space. (a) Measured S11 magnitude; (c) S11 phase. The results are obtained with compacted cross insert probe.

The shapes are separated by ~λ/3 in the cross-range and have ~λ/4 characteristic size. It can be seen that the folded strip loaded slot probe recovers (3dB amplitude and ~50 degrees phase contrast) the EM near field distribution of these targets. The compacted crossed probe results in much higher amplitude contrast (~6dB) image. The features of these shapes are less obvious due to the abovementioned averaging of the coupling effects by the loaded circular aperture, however in both cases – folded strip probe, Fig.13 and compacted cross loaded aperture probe, the recovered difference in the near field properties between the shapes is quite significant (up to 5dB in amplitude and about 30 degrees in phase) even though the characteristic dimensions of the shapes differ by less than 0.1λ.

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B. Planar Metallic Targets in Lossy Medium

In the next experiment the planar metal shapes depicted in Fig. 13(a) were positioned behind a 2mm thick sheet of microwave absorbing magnetodielectric material Eccosorb FGM-U40 [23] which has ~10dB/cm attenuation at around 2.3GHz.

Fig. 15. Microwave image of two different metal shapes behind the 2mm layer of FGM-U-40 material [23]. Microwave probe based on the folded probe insert in rectangular slot.

Fig. 15 demonstrates the amplitude near field image of the structure obtained with the folded strip insert probe. It can be seen that the absorbing material in front of the shapes leads to much higher contrast imaging (~6dB) than when the shapes were positioned in free space. This is believed to be principally due to two reasons. Firstly, the absorbing material increases the resonance frequency of the shapes which are non-resonant in free space because it behaves as a superstrate with higher than unity relative permittivity and permeability. Secondly, the absorber leads to higher attenuation of the Fresnel’s components of the near field (responsible for image smearing) due to the better collimation of the near field by the loaded aperture in the presence of lossy layer. It was noticed that the probe based on the folded cross in circular aperture cannot resolve these targets at z=-0.1λ or even closer distance z=-0.05λ.

Fig. 16. Electric field of the horizontal and vertical magnetic dipoles of 1µAm2 dipole moment amplitude as a function of vertical offset distance in the near field.

This can be explained by the fact that the folded strip in rectangular slot has its dominant horizontal magnetic dipole component parallel to the sample surface. The compacted cross in circular aperture has dominant vertical magnetic dipole component, perpendicular to the sample surface. The electric field (with dominant horizontal Ey component) of the vertical magnetic dipole decays very much faster than the electric field of the horizontal magnetic dipole in the near field zone, Fig. 16. This electric field behavior leads to the strong EM coupling of the compacted cross probe to the sheet of absorbing material and inability to resolve targets behind the lossy material.

C.Dielectric Target ImagingIn this section microwave imaging of dielectric targets with moderate value of permittivity is studied.

(a)

(b)

Fig. 17. Microwave image of two identical rubber balls, permittivity ~3, of λ/4 diameter whose centers separated by λ/3 in the x range. (a) Folded strip loaded aperture; (b) folded cross loaded aperture probe.

Fig. 17 demonstrates the amplitude near field image of two solid rubber balls (permittivity ~3) of 25mm diameter (λ/4 at 2.28GHz) immersed in foam (permittivity is ~1.1). The centers of the balls are separated by 44mm (λ/3) in the x range. Fig. 17(a) shows the near field image obtained with folded strip insert probe, while Fig.17(b) demonstrates the image

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collected by compacted cross loaded aperture. It can be seen that the use of the folded strip loaded aperture results in a clear image with ~3dB maximum contrast. Near field coupling leads to a dipole-like response of the dielectric targets. The compacted cross loaded aperture results in a slightly better image contrast, ~5dB. Asymmetry observed in the image between left and right targets is due to 2 mm difference in the offset positions z of the balls (the right ball is 2mm farther away from the probe than the left one).

(a)

(b)

(c)Fig. 18. Near field amplitude image of a dielectric tile (εr~10) on a lossy 1mm sheet of absorber Eccosorb FGM-U40, (a). Near field image obtained with folded strip (b) and compacted cross (c) loaded aperture

In the next experiment a 20mm x 20mm dielectric tile (with εr ~ 10 and thickness 2mm) was positioned on 1mm thick

Eccosorb FGM-U40 layer, Fig. 18(a). This scenario is relevant to a number of applications, e.g. microwave skin cancer detection, non-destructive surface probing and quality control, etc. It can be seen that both probes, based on folded strip and compacted cross loaded apertures, collect a reasonably high-contrast (~3dB) image of a tile. S11 phase images demonstrate very high contrast in phase (~35-40 degrees).

Fig. 19. Near field amplitude image of a water-filled plastic ball behind a lossy sheet of absorbing material. The imaging offset distance is ~ λ/4.

Finally, Fig. 19 demonstrates a near field amplitude image of a plastic ball of 25mm diameter (λ/4) wall thickness 1mm filled with water (εr ~ 80) and obtained with folded strip loaded aperture probe. The ball is positioned behind a 1mm layer of Eccosorb FGM-U40 material at 30mm offset distance (~λ/4). It can be seen that very high contrast image of a subwavelength dielectric target is possible at larger imaging distances when high permittivity dielectric targets are being imaged. The compacted cross probe has not been able to resolve this target due to mentioned behavior of the electric field component of the vertical magnetic dipole, Fig.16.

From the experiments in Section V it is possible to conclude that the folded insert in rectangular slot and compacted cross in circular aperture probes behave in similar fashion when non-resonant metallic objects are being imaged. In such cases the increased level of reflection amplitude S11 corresponds to the presence of metal surface, cf. Figs. 11(a)-12(a) and 13(a)-14(a).

In the case of dielectric objects in free space, the coupling mechanisms of these two probes are opposite. In general, the folded strip couples to the inhomogeneity in such a way that the EM transmission is enhanced by the presence of dielectric object, Figs.17(a), 18(b) while the compacted cross probe detects the dielectric object through the increased level of reflection as compared to the background, Figs.17(b)-18(c). Therefore using data obtained simultaneously by both probe types should result in better object characterization.

It should be noted that in many practical imaging scenarios, especially involving dielectric media, frequency tuning of the probe resonance is required [15]. In the microwave range this can be accomplished using above considered inserts loaded with electrically or mechanically tunable varactors [15].

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VI. CONCLUSIONS

It has been shown that the EM transmission through the subwavelength or non-resonant apertures in conductive screens can be dramatically enhanced by loading them with folded metallic strips exhibiting resonant properties. We propose planar and quasi-planar resonant insertion geometries that enable two-dimensional dual-polarization subwavelength field confinement along with the ability to focus both electric and magnetic fields. When illuminated by an EM beam these loaded apertures enable very tight, subwavelength, collimation of the EM power in the near field zone. This effect forms a basis for a class of microwave near field imaging probes with subwavelength resolution operating in a wide range of imaging distances (0.05-0.25λ). Measurement results demonstrate a possibility of high-contrast (more than 3dB in amplitude and 40 degrees in phase) near field subwavelength imaging of two-and three-dimensional resonant and non-resonant metallic and dielectric targets in free space and moderately lossy layered media. The results of the paper should be useful for applications of microwave imaging in medical applications, non-destructive testing and non-invasive surface probing.

ACKNOWLEDGMENT

The Authors are most thankful to the Reviewers of this paper whose valuable and insightful comments allowed to improve the original manuscript.

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Journ. Selected Topics Quant. Electron., vol. 19, no.1, 8400416, Jan/Feb. 2013.

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[20]R.E. Collin, Foundations for microwave engineering, Wiley 2007. [21]K.J. Binns and P.J. Lawrenson, Analysis and Computation of Electric and

Magnetic Field Problems, Pergamon Press, 1973.[22]T. Pochiraju, O. Malyuskin, V. Fusco, “Sub-wavelength near field

imaging using a cross polarised slot with wire insert”, 2010 URSI Int. Symp. Electromagnetic Theory (EMTS), pp. 559 – 561, 2010.

[23]http://www.eccosorb.eu/products/eccosorb/eccosorb-fgm-u-40

Oleksandr Malyuskin (M’04) received the MSc degree in Radiophysics and Electronics and the PhD degree in Radiophysics from Kharkiv National University, Ukraine, in 1997 and 2001 respectively. He joined the Institute of Electronics, Communications and Information Technology, Queens University Belfast, in March 2004 as a Post Doctoral Research Fellow involved in the development of novel composite materials for advanced EM applications. His research

interests include analytic and numerical methods in electromagnetic wave theory, characterization and application of complex and nonlinear materials, antenna arrays and time reversal techniques. Dr Malyuskin has published over 50 scientific papers in major journals and in refereed international conferences, and has acted as a reviewer for the IEEE publications.

Vincent Fusco (S’82–M82–SM’96–F’04) received the Bachelors degree (1st class honors) in electrical and electronic engineering, the Ph.D. degree in microwave electronics, and the D.Sc. degree, for his work on advanced front end architectures with enhanced functionality,

from The Queens University of Belfast (QUB), Belfast, Northern Ireland, in 1979, 1982, and 2000, respectively.His research interests include nonlinear microwave circuit design, and active and passive antenna techniques. He is the Director of the High Frequency Laboratories, Queens University of Belfast, and is also Director of the International Centre for Research for System on Chip and Advanced MicroWireless Integration, SoCaM. He has published over 420 scientific papers in major journals and international conferences, and is the author of two textbooks. He holds several patents on active and retrodirective antennas and has contributed invited chapters to books in the fields of active antenna design and EM field computation.

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http://www.eccosorb.eu/products/eccosorb/eccosorb-fgm-u-40


Prof. Fusco is a Fellow of the Royal Academy of Engineering and a Member of the Royal Irish Academy. In 1986, he was awarded a British Telecommunications Fellowship and 1997 he was awarded the NI Engineering Federation Trophy for outstanding industrially relevant research.

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