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Deliverable 5 – Page D 5: 1 Report on the properties of basic test structures During the first year of the project we fabricated and characterized many 1- and 2D test structures. The analytical reports on their fabrication and characterization are described in WP 2, M 2.1 and WP 1, M 1.2 respectively. Here, we present results on the most important 1-D structures: 1) 10 GHz 1-D composite metamaterials (CMMs). [FORTH, Bilkent] The in-plane CMM at ~10 GHz was fabricated and tested at FORTH in order to understand better the interactions of SRRs and wires that lead to the evolution of the LH peak (see progress report M 1.2). The lattice parameters of the CMM are αx= 5 mm and αy= 3.63 mm, while the geometrical parameters of the SRR are w= d= t= 0.33 mm and l= 3 mm (see Figure 1 of WP 1, M 1.2). The SRRs-only structure shows a dip in the transmission data at ~8.5-10.5 GHz corresponding to the magnetic resonance of the SRR, while the wires-only structure shows a cutoff frequency at ~10.5 GHz that corresponds to the plasma frequency of the “thin wires” structure. The CMM shows a transmission peak between 8.5 and 10 GHz, at almost the same frequency region where the SRR dip appears. This was originally interpreted as the signature of the appearance of LH behavior. We then varied the separation between the boards in order to study how the “LH peak” evolves. We firstly observed that the “SRR dip” remains almost unaffected by the variation of the separation between the boards. However, by decreasing the separation between the boards the dip broadens and a second drop in the transmission appears at higher frequencies. This second drop could be attributed (see also WP1, Task 1.1: Modeling) to the electric response of the SRR structure. The wires-only structures show large dependence on the board separation. Increasing the board separation leads to a decrease of the characteristic cutoff frequency of the wires array. Finally, the in-plane CMMs behave remarkably. By increasing the board separation, the so-called “LH peak” broadens and shifts to lower frequencies. This behavior could be explained by the fact that the corresponding decrease of the cutoff frequency of the wires-only array suggests that εeff remains positive down to lower frequencies. Since both εeff and μeff are then positive, a transmission peak appears. However, this peak should then be right-handed and not left-handed, as it was initially thought. We can conclude that the electric response of the CMM should be treated as the result of the interactions between the SRRs and the wires. The conclusion that a peak in the transmission data is due to LH could be misleading if these interactions are not taken into account. It came out from our theoretical simulations that the best model to really investigate the electric response of the CMM is to close the gaps of the SRR in the CMM structure. By this way, we expect the magnetic resonance of the SRRs to be switched off and thus to really observe the electric response of the CMM. The results of this study shed light on the appearance of different peaks in the transmission data of the in-plane CMMs (see also WP1, Task 1.1: Modeling). Deliverable 5 – Page 2 The SRRs contribute to the electric response of the CMM, thus shifting the cutoff frequency of the wires-only array to lower frequencies. It is remarkable that the new cutoff frequency that corresponds to the CMM (ωp΄ as annotated in FORTH’s theoretical model) perfectly coincides with the onset of the “LH peak”. In the case of the “closed SRRs” CMM there is no dip in the transmission data since the magnetic response of the SRRs is switched off. We strongly believe that the observed peak in the transmission data of the in-plane CMM is right-handed since εeff and μeff are both positive between 8.5 and 9.5 GHz. The dip at ~9.5-10.5 GHz is due to the magnetic resonance of the SRRs that renders μeff negative. As εeff remains positive above ωp΄ no transmission of the electromagnetic waves (EM) is then allowed between 9.5 and 10.5 GHz. Beyond 10.5 GHz μeff switches again to positive values and this results again in the transmission of the EM waves. CMMs with circular shaped SRRs alternating with cut-wire arrays yielded higher transmission values than the ones with square shaped SRRs (see progress report M 1.2). In contrary to the continuous wire structures that exhibit a stop band with no lower edge, the cut-wire configuration exhibits a stop band with a well-defined lower edge due to the discontinuous nature of the wires. The stop band of the discontinuous thin wire structure extends from 6 GHz to 18 GHz. The reflection measurements indicate that all of the incident EM waves are reflected back from the structures within the stop band. So, the structure behaves like a good mirror throughout the stop-band. For the pass-band region, the measured reflection is near –15 dB. As the transmitted power is also low at these frequencies, we can conclude that the EM waves cannot efficiently couple into propagating modes and strongly scatter within the structure. The CMM structure is constructed by stacking the SRR and wire mediums periodically. There appears a broad pass band extending from 9.6 to 14.3 GHz. The average transmission within the pass band is around -4.5 dB, corresponding to a transmission -0.3 dB for each unit cell. This transmission is significantly higher than the previously reported composite metamaterial transmission properties. In this frequency range, both effective permeability and permittivity are negative. Since if only one of the constitutive parameters is negative and the other is positive we would have evanescent waves rather than propagating waves in the medium. So, the structure can be named as a double negative metamaterial. However, we are still working on getting more understanding on the effect of the cut-wires array on the CMM transmission properties. 2) 10 GHz 2-D composite metamaterials (CMMs). [Bilkent] For an efficient 2D CMM design, the split-ring resonators and wires are printed on the same board with off-plane configuration, i.e. SRRs are on one plane of the board and wires are on the other plane of the board. Spacing between similar elements (SRRSRR or wire-wire) is 9.3 mm. The distance between overlapping SRR and wire is ~1.2 mm (thickness of the board). The boards are then assembled in interlocking slits to form the 2-D CMM structure. The constructed structure has 15 layers along the horizontal directions. Transmission analysis presented here is based on the normal propagation, where one side of the 2-D structure is parallel to the propagation direction. Deliverable 5 – Page 3 We first verify that each of the 1-D CMMs building up the 2-D CMM structure has indeed a transmission band observed in previous 1-D CMM structures. Each side of the 2-D CMM (denoted here as side “A” and side “B”) has a similar 1-D CMM transmission spectrum with a definite transmission band between 10.5-12.5 GHz. From the 2-D CMM transmission spectrum it can be seen that the transmission level is reduced by about 10 dB in amplitude, and the bandwidth is reduced to ~1 GHz when compared with the 1-D CMM. We note that in the normal propagation through the 2D CMM structure, one of the 1-D CMMs is facing perpendicular to the propagation direction. Therefore the propagating field is incident on a large metallic surface, which might decrease the transmission amplitude considerably. We are currently working on structures to maximize the transmission amplitude. The next step will be to construct a 2D structure with sufficiently large aspect ratio to perform refraction experiments for various incident angles of the incoming electromagnetic field. 3) 35 GHz GaAs-based composite metamaterials. [Forth, Bilkent] Originally, CMMs at 35 GHz were fabricated consisting of 600 μm-thick GaAs substrates patterned with square SRRs and thin wires on the same plane. Figure 1 shows a single SRR of the type used for these experiments. The thickness of the deposited metal (silver or copper) is 1 μm. The parameters of the SRR are t= 20 μm, w= 30 μm, d= 40 μm and l= 260 μm. After constructing the SRRs and the wires on each GaAs substrate following conventional fabrication sequence steps applied in lithographic processes, the substrates were aligned and stacked in a periodical arrangement. From the transmission spectra of the GaAs-based CMMs it can be seen that the CMM allows propagation of EM waves in the frequency range 31.8-33.8 GHz. The peak transmission amplitude of the passband is ~-10 dB. We tried to exploit the speculation that this transmission peak is due to LH behavior of the GaAs-based CMMs. Therefore, numerical calculations were performed for the constructed CMMs. The GaAs-based CMM exhibits a passband between 29-34 GHz, which is in good agreement with the experimental results. The SRRs-only structure shows a forbidden band between 3134 GHz, which points to negative values of the effective magnetic permeability, μeff, of the SRR structure. It is, however, still necessary to study the electric response of both the wires-only structure and the “closed SRR plus wires” structure in order to determine both ωp and ωp΄ (see also the theoretical model developed at FORTH for the behavior of the “LH” CMMs, Task 1.1: Modeling). In this way, we shall conclude if the effective permittivity, εeff, of the CMM is negative in the frequency range < 34 GHz where a peak in the transmission data for the CMM and a dip in the transmission data for the SRR-only structure appears. Consequently, it will be clarified if real LH behavior is observed for the GaAs-based CMM. Deliverable 5 – Page 4) 4 Negative refraction and subwavelength resolution in a two-dimensional photonic crystal. [Bilkent] One important application of left-handed materials is their possible use for constructing a perfect lens. In this part of the report, we explain our experimental and theoretical work on the negative refraction and subwavelength focusing of electromagnetic waves in a 2-D PC. Our structure consists of a square array of alumina rods in air. The frequency range that gives negative refraction extends from 13.10 GHz to 15.44 GHz. Transmission measurements were performed to verify the predicted negative refraction behaviour in our structure. The centre of the outgoing Gaussian beam is shifted towards the left hand side of the center of the incident Gaussian beam, which clearly corresponds to negative refraction. Negative index of refraction determined from the experiment is -1.94, which is very close to the theoretical value of -2.06 calculated by the FDTD method. For comparison purposes, the measurements and the simulations are repeated with a slab that contains only polystyrene pellets, which has a refractive index of 1.46. The refracted beam is now on the right hand side of the incident beam corresponding to a positive index of 1.52. Since we know the optimum frequency for a broad angle negative refraction, we can use our crystal to test the superlensing effect that was predicted for negative refractive materials. For this purpose, a PC having 15 layers in the propagation direction and 21 layers in the lateral direction is used. To show the focusing on the image plane in the vicinity of the PC, the time averaged intensity distributions along the image plane with and without the PC are calculated. The full width half maximum (FWHM) of the measured focused beam is found to be 0.21 , which is in good agreement with the calculated FWHM. The calculated FWHM of the beam at this plane without the PC is found to be 5.94 . So, our structure exhibits 25 focusing at this plane with respect to free space. Subwavelength resolution using negative refractive materials has been theoretically suggested by other researchers. We performed the experiments and simulations with two incoherent point sources 0.7 mm away from the PC, which are separated by a distance of 6.78 mm and having frequencies 13.698 GHz and 13.608 GHz respectively. The peaks corresponding to each point source are clearly resolved in both measurement and simulation. To our knowledge, this is the first experimental observation of subwavelength resolution of two incoherent sources in negative refractive materials. The negative refraction effect we have observed depends only on the refractive index of the dielectric material and the geometrical parameters used in 2D PCs. So, this effect can also be observed at optical wavelengths where it is possible to obtain similar refractive indices using transparent semiconductors. This is in contrast to the previously reported metal-based LHMs. In such structures, increased absorption in metals prohibits the scaling of these structures to the optical wavelengths. In terms of fabrication, a slab shaped lens structure is easier to fabricate than a conventional curved shaped lens structure. So, our slab shaped lens structure can also be used for nanophotonics and nanooptics applications. Deliverable 5 – Page 5) 5 Photonic crystal based high-gain antenna. [Bilkent] We have experimentally and theoretically studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. In our experiments and FDTD calculations we have calculated and measured the angular distribution of power emitted from a monopole source embedded inside a 2D square array of cylindrical alumina rods whose radius is 1.55 mm and dielectric constant is 9.61. The separation between the centers of the rods along the lattice vectors is 1.1 cm. The monopole source used in the experiments is obtained by removing 0.5 cm of the cladding from a coaxial cable and leaving the metal part. An HP-8510C network analyzer is used to excite the monopole source and to measure the power emitted from the monopole source. We have measured the angular distribution of power at the upper band edge frequency for various crystal lengths. There is an optimum crystal length, which is found to be 24 layers in our case. The minimum half power beam width is obtained at 24 layers and it is found to be 6 degrees. We observed that as the number of layers is increased from the optimum layer number the change in the far field radiation pattern is not as strong as the change observed when the layer number is decreased from the optimum layer number. This can be explained by fact that as we move away from the center radiator the amplitude of the radiators decrease very rapidly. Hence, as we increase the number of layers from the optimum layer number the effect of the increasing the number of radiators will be small compared to the effect of the decreasing the number of radiators. To our knowledge this is the minimum half power beam width value obtained by using PCs. 6) Swiss-Roll structures. [ICSTM2] The emphasis of the first year’s effort in this workpackage has been to develop, characterize and use magnetic metamaterials (MM) in MRI. In particular, we have built an RF “lens” (or, more accurately, a “faceplate”). This has been fully characterized, and its impact on the RF field distribution determined for the full range of permeability available from the material. We have used the “lens” to demonstrate geometry preserving flux ducting in a 0.5 Tesla MRI machine operating at 21.3 MHz. To complement the work on bulk MMs, we have also started the development of RF components based on individual MM elements. For example, we are developing a flux compressor based on loop resonators, the RF equivalent of the SRR used at microwave frequency. This will be a key element in the development of a MM yoke, which will assist in the delivery or detection of RF signals. Transmission measurements The behaviour of the prism was characterized by measuring the transmitted field distribution from a dipole source as a function of frequency. A small (3 mm diameter) loop was used as the source, and was placed at the back of the box containing the prism, some 5 mm away from the end of the rolls, oriented so that its axis was parallel to the axis of the Swiss Rolls. Similar loops, oriented to receive the three orthogonal components of the transmitted field were scanned in the XY and XZ planes of the output Deliverable 5 – Page 6 space. The loops were connected to a network analyser that recorded the signal as a function of frequency in the range 15 – 35 MHz, thus spanning the magnetically active frequency range. The data were processed to provide maps of field intensity in the output planes at each frequency, and hence as a function of the permeability. At both low and high frequency, where μ ~ 1, a dipole field distribution is observed. On resonance, at 21.3 MHz, an intense central peak is found. Above the resonant frequency, where μ < 0, a complicated variety of field patterns is observed, that cover the entire surface of the prism, constrained only by the boundary conditions on its edges. For example, at 24.75 MHz, where μ ~ -1, there is a central peak, surrounded by a ring of intensity of radius ~ 80 mm. At 29.7 MHz, where μ ~ 0, the field distribution is quite uniform. It is clear that a wide variety of field behaviour is obtained, and detailed analysis is being performed. Meanwhile, a simple examination of the propagation of radiation through an anisotropic effective medium has been undertaken to understand certain of the features of these data, and predict aspects of the MM performance so they can be demonstrated in the MR environment. Preliminary measurements of flux ducting Preliminary characterization was carried out using 3 mm diameter loops as both source and receiver. The source was placed centrally on the outside of the base of the box, about 5 mm from the base of the Swiss Roll array. The receiver loop was scanned across the surface of the array, 68 mm above the source, and the signal was measured using a network analyzer. The intensities at 15 and 21.3 MHz are shown as the red lines in Fig. 30, as a function of distance from the center of the slab measured along a diagonal. The material was then removed, and the scan repeated (at a height of 68 mm. Finally, the receiver was set at a height of 8 mm, the equivalent height had the 60 mm long Swiss Rolls not been present, and the scan repeated. At low frequency (15 MHz), the permeability is slightly elevated (μ = 1.4). The signal through the material slab lies between the two background scans, and the peak intensity is 20 dB below the 8 mm reference level. At 21.3 MHz, however, the signal matches the reference level closely, across the whole extent of the scan, except at the nearest neighbor positions, where the reference signal passes through a minimum. Scanning Measurements To test the two-dimensional imaging performance of the material, we constructed an antenna from a pair of anti-parallel wires, bent into the shape of the letter M. This generated a line of magnetic flux, so providing a characteristic field pattern for imaging. It was placed horizontally, and the material was positioned on top of it. The transmitted field was measured by scanning a 3 mm diameter loop probe in a horizontal plane, about 2 mm above the surface of the material. Measurements were made on a grid, 2 mm square, using a network analyzer. The pattern thus observed at 21.3 MHz is shown in Fig. 33 of the progress report of WP 1-M 1.2, in which the Swiss Roll structure is overlaid on the field pattern. Deliverable 5 – Page 7 It is clearly shown that the material does indeed act as an image transfer device for the magnetic field. The shape of the antenna is faithfully reproduced in the output plane, both in the distribution of the peak intensity, and in the “valleys” that bound the M. These mimic the minima in the input field pattern either side of the central line of flux. The upper right arm of the M itself was twisted, so that the flux pattern was launched with a reduced vertical component. This is reproduced in the weaker intensity observed in this region.