Using micromanufactured S-shaped gold strings suspended in free space by means of window-frames, we experimentally demonstrate an electromagnetic meta-material (EM3) in which the metallic structures are no longer embedded in matrices or deposited on substrates such that the response is solely determined by the geometrical parameters and the properties of the metal. Two carefully aligned and assembled window-frames form a bi-layer chip that exhibits 2D left-handed pass-bands corresponding to two different magnetic resonant loops in the range of 1.4 to 2.2 THz as characterized by Fourier transform interferometry and numerical simulation. Chips have a comparably large useful area of 56 mm2. Our results are a step towards providing EM3 that fulfill the common notions of a material.
©2008 Optical Society of America
Seminal work by Veselago  and Pendry and co-workers  has spurred, over the past decade, a rapid development of electromagnetic metamaterials (EM3) fueled by curiosity in the unusual properties of EM3 like negative refraction or rekindling of evanescent waves within resonance pass-bands. First experiments in the gigahertz range [3–11] were followed by a wave of micro- and nanomanufactured terahertz devices [12–23]. Radically new potential applications have caught the attention of researchers such as sub-wavelength resolution imaging , invisibility cloaking , and advanced antennae , relevant to fields including microscopy, lithography, electromagnetic shielding, and telecommunication.
The electromagnetic response of EM3 is commonly due to tiny artificial metallic elements that are densely distributed in space, either embedded in a plastic matrix or deposited on a dielectric substrate. Isolated elements such as split-rings, U shapes, rectangles and crosses have been used at THz frequencies [12–23] while string-like elements like S and Ω have been more common in the microwave domain [7–11].
However, matrices and substrates may constrain applications due to their electromagnetic, mechanical, thermal, and radiative properties. Interaction of the electromagnetic waves with the dielectric of either matrix or substrate may reduce resonance frequency and increase loss. In particular, polymer matrices would show strong absorption bands in the “fingerprint region” (400–4000 cm-1) preventing working frequencies from being freely selectable. EM3 on substrates may work only in reflection if the substrate is not transparent over the left-handed pass-band. Besides spectral features, commonly used polymer matrices may suffer from ageing and radiation degradation. They may also be limited in operation temperature and may be sensitive to humidity. Finally, the rigidity of substrates may constrain applications further.
In the following, we show how to overcome such restrictions and demonstrate the successful fabrication and characterization of free-space THz EM3 that are suspended by a window-frame over areas of 56 mm2.
We use string-like resonators that are fixed at both ends in a window-frame such that the EM3 structure is in free space. Here, we chose S resonators as thoroughly investigated, in the microwave range, by J.A. Kong and co-workers [7–9]. Two window-frames are assembled to form a bi-layer of aligned strings separated by a gap (Fig. 1(a)). Although window-frames are presently made of polymers like SU-8 resist, for experimental convenience, numerical simulations show they could as well be metallic.
A second key aspect of our work is the capability of producing large quantities of good quality devices by means of UV and/or X-ray lithography, thereby restricting to the initial mask-making the time-consuming primary pattern generation by electron beam or laser beam writing. Finally, our micromanufacturing approach offers control of the device geometry and, thus, resonance frequency over a wide range, opening up a way towards tunable and two-dimensionally isotropic devices at THz frequencies.
We manufactured gold S-strings (Fig. 1(b)) with a length a=104.2 µm, width b=34.9 µm, top width h=15.9 µm, bottom width h’=13.4 µm, and thickness t=11.4 µm. The gap d takes three values 0.6 µm, 1.1 µm, and 6.1 µm. The SU-8 window-frame that holds S-strings by their ends has four thin stabilizing ribs. The total size of chip and window is 14.7×12.3 mm2 and 8.1×6.9 mm2, respectively, resulting in a useful area as large as 56 mm2. A sub-window is 3.2×1.8 mm2 large and the window-frame is 260 µm thick.
For the initial photomask, a 5” soda lime blank (Nanofilm, Wetlake Village, California) with 100 nm-thick chrome and 0.5 µm-thick layer of AZ1518 photoresist is used. A Heidelberg Instruments direct-write laser system DWL 66 serves for patterning the resist. Upon resist development, chromium is etched away where not protected by the resist. Finally, the resist is dissolved leaving the thin chromium pattern that serves to absorb UV light.
Two distinct photomasks are needed, one containing the S-string array pattern, the other the window-frame pattern. 4×3 fields with either string or window-frame pattern are inscribed in a circle of 80 mm diameter such as to fit onto a standard 100 mm diameter Si wafer that serves as a substrate during processing. Both masks have alignment marks for accurate relative positioning prior to the second exposure. They are used either in a UV lithography process to expose structures in photoresist or to make X-ray masks that feature thick (≈20 µm) gold structures capable to absorb hard X-rays. S-string arrays can be made from both types of masks. The difference is that UV lithography leads to sidewalls slanted by about 6° with AZ9260 resist while deep X-ray lithography delivers nearly vertical sidewalls.
In the UV lithography process, S-string arrays are preferably patterned into a ≈20 µm thick AZ9260 resist layer which is spin coated on the silicon wafer that was covered with a plating base of Cr/Au (100/50 nm) before. After a soft baking at 95 °C for 3 min, the resist is exposed to UV light through the photomask in a Karl Suss Mask & Bond Aligner (MA8/BA6). Finally, the exposed resist is removed with AZ 400k developer, the remaining patterned resist serving as a mold for gold plating of the S-string arrays.
To produce X-ray masks cost-effectively, a 100 mm diameter graphite wafer of 200 µm thickness is used as a membrane onto which a 35 µm thick SU-8 resist layer is spin coated. Photomask pattern are transferred to the SU-8 resist layer by UV exposure in the MA8/BA6 mask aligner. Post-exposure bake and development are followed by electroplating of 20 µm thick gold into the SU-8 mould. The graphite membrane with the patterned gold absorber is finally glued on an aluminium NIST-standard mask holder.
For X-ray lithography, the usual Si wafer with its plating base is covered with a ≈20 µm thick PMMA layer by multiple spin coating and soft baking. As each spin process yields ≈4 µm thickness, five layers are required. The baking temperature is 180 °C, applied for about 2 min after each spin and 5 min after the final spin. The substrate is then exposed to synchrotron X-rays through an X-ray mask at SSLS’ LiMiNT facility. Finally, exposed PMMA is removed using the standard GG developer . The short wavelength (≈0.2 nm) and small local divergence (≤1 mrad) of the synchrotron X-rays used result in smooth and nearly vertical sidewalls. Thus formed PMMA structures serve for gold plating of the S-string arrays.
Whether AZ or PMMA mold, a standard Enthone electrolyte is used for Au plating at a temperature of 50 °C. After electroplating, AZ resist is removed using a proprietary AZ remover whereas PMMA is dissolved by acetone.
For holding and stabilizing the freely-suspended matrix-free S-string array and for easily handling the final device, a window-frame is built that must be precisely aligned with the finished gold S-string array. Again, UV or X-ray lithography is used to expose this window-frame. First, SU-8 resist is spin coated to a thickness of 260 µm, followed by soft baking at 95 °C for 4 hours. Then, the sample is exposed either to UV light or X-rays as described above. Removing the unexposed SU-8 resist by SU-8 developer, we finally obtain ≈15 µm thick gold S-string arrays held by 260 µm thick SU-8 window-frames on a Si wafer.
To finally release these single-layer chips from the Si substrate, the Cr layer of the plating base is etched away as a sacrificial layer. For easy etching, holes are designed into the window-frame and the stabilizing ribs (Fig. 2(a)). First, the gold of the plating base is removed by submerging it for about 30 s into a KI solution (4g KI (potassium iodide) +1 g I2 (iodine) +40 ml DI water). The then exposed Cr layer (100 nm) is removed by chrome etchant CEP-200 (Microchrome Technology), thus releasing the single-layer chip from the substrate.
Although using two pieces of single-layer window-frames to make one bi-layer chip, we need to manufacture only one type of single-layer chip for symmetry reasons. In a bi-layer chip, two single-layer chips are facing each other with their bottom sides. In this way, the alternating loop-forming S-structure is generated. Critical issues for the assembly are the relative alignment of the S-string arrays of both single-layer chips and the control of the gap. A customized optical microscope serves for alignment.
The gap is set by sandwiching a spacer made from SU-8 using similar processes as above. The shape of the spacer matches the window-frame. Different thicknesses ranging from 4–20 µm are fabricated to control the pass-band frequency. Finally, the sandwich of two single-layer chips and spacer is fixed by glueing. Figure 2 shows various views of the bi-layer chip taken with a digital camera, an optical microscope (Leica), and a scanning electron microscope (FEI Sirion). It can be seen that the regular structure and the alignment of the two layers of S-strings are excellent over a number of S periods and strings between frame edges as large as about 80 and 200, respectively.
3. Spectral characterization
We measured the spectral response of our materials by Fourier transform interferometry (FTIR) within 1–4 THz using synchrotron infrared radiation from the edge-effect source at SSLS . The bi-layer chip is put on a rotational stage in the sample chamber of a Bruker IFS 66v/S Fourier transform interferometer. The rotation axis is parallel to the string direction as is the electric field vector of the incident radiation. The beam spot on sample is about 1 mm wide horizontally and 0.4 mm vertically. A far infrared (FIR) polarizer (gold grid on polyethylene substrate) is introduced before the sample. Transmission spectra are acquired by means of a DTGS detector (deuterated triglycine sulfate) comparing signals with and without samples. Spectral resolution is 0.12 THz (4 cm-1). The sample chamber is kept at 4 mbar of dry nitrogen gas in order to avoid signal contributions from water vapour and other gases.
The incidence angle α is varied from normal incidence 0° up to 60°. In this way, the magnitude of the magnetic field component perpendicular to the inductance loop is varied. Using the transmission of SU-8 in the THz range as measured earlier , namely, 0.93 for 25 µm thickness at 1 THz, we estimate that one leg of the window-frame would have a transmission of 3.9·10-4, thus ruling out measurements under 90° incidence angle.
4. Results and discussion
Figure 3 compares measured (left column) and simulated (right column) transmission spectra of our samples. Numerical simulations were made using MWS . Rows correspond to gap values of 0.6 µm, 1.1 µm, and 6.1 µm from top to bottom, respectively. The simulation reproduces the general features of the measured spectra fairly well. We find three major resonance pass-bands in the ranges 1.2–1.8 THz, around 2.2 THz, and above 2.6–2.8 THz. At normal incidence (α=0°), we measure pass-bands at 2.2 THz and 2.6–2.8 THz which correspond to peaks at 2.1–2.4 THz and 2.7–3.6 THz in the simulation. Frequency deviations will be discussed below. At tangential incidence (α=90°), simulation shows a strong peak at 1.2–1.8 THz that cannot be observed experimentally because of the window-frame. However, at intermediate incidence angles, experimental spectra indicate a faint peak at these frequencies. Note that, for future experiments, the window-frame can be modified to enable 90° incidence.
The strong peak at 1.2–1.8 THz belongs to the loop formed by strings (m,1) and (m,2) (Fig. 1(b)) whereas the peak at 2.2 THz is due to the loop formed by the bottom leg a/2 of string (m-1,2) and the top leg a/2 of the adjacent string (m,1). The peak at 2.8 THz is due to an electrical resonance as shown by the parameter retrieval below. The two latter peaks went unnoticed hitherto because there had been no simulations or measurements at 0°–60° incidence.
The effective permittivity ε and permeability µ of the devices are derived from the reflection and transmission coefficients of a wave incident onto a slab of the metamaterial, analysed by a robust retrieval algorithm . Numerically simulated values of ε and µ for 0o and 90o incidence are plotted in Figs. 4(a) and 4(b), respectively. Figure 4(a) shows that, for 0o incidence, the lower-frequency pass-band around 2.2 THz is left-handed involving a magnetic resonance with the real parts of both µ and ε negative while imaginary parts are small indicating low loss. In this case, the magnetic field points along the x direction, and the magnetic resonance loop is formed by the rod pair (m-1,2), (m,1) as sketched in the inset of Fig. 4(a). The left-handedness of this peak was further confirmed by a numerical phase tracking method that showed backward wave propagation inside the bi-layer. In contrast, the higher-frequency pass-band between 2.7–3.6 THz is an electric resonance with both ε and µ positive and thus right-handed. We can clearly see from the results that the first pass-band is double-negative while the second pass-band is double-positive.
Figure 4(b) shows that, for 90o incidence, the pass-band around 1.9 THz is left-handed involving a magnetic resonance. In this case, the magnetic field points along the y direction, and the magnetic resonance is produced by the two half areas of the figure-‘8’-pattern with the current as indicated in the inset of Fig. 4(b). A detailed interpretation of this kind of resonance was given in . The left-handedness of this pass-band is further confirmed by a numerical phase tracking method showing backward wave propagation inside the bi-layer.
In Figs. 4(a) and 4(b), frequency bands masked in pink indicate that retrieved effective parameters are always accompanied by a negative imaginary part of permittivity or permeability, and are very sensitive to the simulation environment. The reason is that, in these frequency regions, the wavelength inside the structure is smaller than the structure periodicity which makes it inadequate to characterize the structure by using effective parameters. A detailed study of such phenomena that are related to negative imaginary parameters can be found in . In the other regions, the retrieved effective parameters are very robust and the imaginary parts of both permittivity and permeability are close to zero.
The variation of peak positions with incidence angle is reasonably reproduced for gap values 0.6 µm and 1.1 µm, whereas it deviates visibly for the gap of 6.1 µm. This angular dependency is related to the fat aspect ratio of the conductors that causes the location and size of a resonance loop as defined by the actual current flow to vary with incidence angle, thus changing inductance, capacitance, and frequency.
The apparent frequency deviations between measured and simulated peaks are most likely due to the measurement error of the gap which is about ±25%, and to mesh discretisation errors in the simulation, particularly for small gaps. We note that frequency deviations of the 2.2 THz peak are small as the capacitance and inductance of the corresponding loop vary oppositely upon gap changes, so compensating influences on the resonance frequency. In contrast, the >2.6 THz pass-band shows larger deviations and a stronger dependency on the gap which is caused by a decrease of the metallic string density, and thus, an increase of the string self-inductance. The fading of measured signals with increasing incidence angle is ascribed to the diminishing transparency of the first S-string layer which looks closed from about 66° onwards.
Interestingly, the shift of the main peak from 1.2 THz to 1.8 THz upon extending the gap from 0.6 µm to 6.1 µm indicates that the bi-layer chip could be made continuously tunable by providing microactuators such as piezoelectric or spring-loaded capacitive positioning elements instead of fixed-thickness spacers to vary the gap width and, thus, the resonance frequency.
Finally, it is very interesting to see that our THz bi-layer S-string structure exhibits two different magnetic resonant loops with different orientations. The related left-handed behavior found for both, x and y directions, as shown in Fig. 4, implies a potential realization of a two-dimensionally isotropic metamaterial by optimising geometrical parameters of the S-string bi-layer. This feature seems inaccessible to other THz devices known from literature so far.
In summary, we have demonstrated that it is possible to solve the issues of the accuracy of the structure definition within an individual S-string layer as well as of the alignment between the two layers in a bi-layer chip over areas as large as 56 mm2 such as to achieve devices that are working in free space. As a next step, the microfabrication of self-supporting materials in which strings are held together by interconnecting rods is within reach. Such materials may find immediate applications as narrow-band filters in THz detector systems for chemicals  and for extremely wide-band data transfer in THz telecommunication .
We demonstrate the first EM3 that are freely suspended in space without using any matrix or substrate. They feature precisely aligned bi-layers of S-strings that are micro-manufactured via lithography-based processes. Spectral characterization by FTIR and numerical simulation reveal left-handed pass-bands at 1.2 to 1.8 THz and around 2.2 THz, solely determined by the geometrical parameters and the properties of the metal. Useful areas as large as 56 mm2 have been achieved. Larger device areas are possible via process extensions. As micromanufacturing allows controlling the geometrical dimensions of strings and gap within a broad range, resonance frequencies can be matched to requirements. In future work, 2D isotropic materials may be built and optical devices made tunable by including microactuators to vary the gap. Finally, the state-of-the-art presented here also enables to build self-supporting EM3 as a next step. Such EM3 would come close to being materials in the common sense.
In memoriam: Professor Jin Au Kong (1942-2008)
Work partly performed at SSLS and supported by DARPA HR0011-06-1-0030, NUS Core Support C-380-003-003-001, A*STAR/MOE RP 3979908M and A*STAR 12 105 0038 grants. JAK and HSC would also like to acknowledge the Chinese National Science Foundation under contract 60531020.
References and links
1. V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of ε and µ,” Sov. Phys. Usp. 10, 509–514 (1968). [CrossRef]
2. J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. Microwave Theory Tech. 47, 2075–2084 (1999). [CrossRef]
3. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett. 84, 4184–4187 (2000). [CrossRef] [PubMed]
4. C. G. Parazzoli, R. B. Greegor, K. Li, B. E. C. Koltenbah, and M. Tanielian, “Experimental verification and simulation of negative index of refraction using Snell’s law,” Phys. Rev. Lett. 90, 107401 (2003). [CrossRef] [PubMed]
5. K. Aydin, K. Guven, M. Kafesaki, L. Zhang, C. M. Soukoulis, and E. Ozbay, “Experimental observation of true left-handed transmission peaks in metamaterials,” Opt. Lett. 29, 2623–2625 (2004). [CrossRef] [PubMed]
6. J. D. Baena, R. Marqués, and F. Medina, “Artificial magnetic metamaterial design by using spiral resonators,” Phys. Rev. B 69, 014402 (2004).
7. H. S. Chen, L. Ran, J. Huangfu, X. Zhang, K. Chen, T. M. Grzegorczyk, and J. A. Kong, “Left- handed materials composed of only S- shaped resonators,” Phys. Rev. E 70, 057605 (2004).
8. H. S. Chen, L. Ran, J. Huangfu, X. Zhang, K. Chen, T. M. Grzegorczyk, and J. A. Kong, “Metamaterial exhibiting left-handed properties over multiple frequency,” J. Appl. Phys. 96, 5338–5340 (2004). [CrossRef]
9. J. Huangfu, L. Ran, H. Chen, X. Zhang, K. Chen, T. M. Grzegorczyk, and J. A. Kong, “Experimental confirmation of negative refractive index of a meta-material composed of Ω-like metallic patterns, Appl. Phys. Lett. 84, 1537–1539 (2004). [CrossRef]
10. M. Gokkavas, K. Guven, I. Bulu, K. Aydin, R. S. Penciu, M. Kafesaki, C. M. Soukoulis, and E. Ozbay, “Experimental demonstration of a left-handed metamaterial operating at 100 GHz,” Phys. Rev. B 73, 193103 (2006).
11. E. Lheurette, O. Vanbeisen, and D. Lippens, “Double negative media using interconnected Omega -type metallic particles,” Microwave Opt. Technol. Lett. 49, 84–90 (2007). [CrossRef]
13. T. J. Yen, W. J. Padilla, N. Fang, D. C. Vier, D. R. Smith, J. B. Pendry, D. N. Basov, and X. Zhang, “Terahertz Magnetic Response from Artificial Materials,” Science 303, 1494–1496 (2004). [CrossRef] [PubMed]
14. H. O. Moser, B. D. F. Casse, O. Wilhelmi, and B. T. Saw, “Terahertz response of a microfabricated rod-split-ring-resonator electromagnetic metamaterial,” Phys. Rev. Lett. 94, 063901 (2005). [CrossRef] [PubMed]
15. G. Dolling, C. Enkrich, M. Wegener, J. Zhou, C. M. Soukoulis, and S. Linden, “Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials,” Opt. Lett. 30, 3198–3200 (2005). [CrossRef] [PubMed]
16. W. J. Padilla, D. R. Smith, and D. N. Basov, “Spectroscopy of metamaterials from infrared to optical frequencies,” J. Opt. Soc. Am. B 23, 404–414 (2006).
17. T. F. Gundogdu, I. Tsiapa, A. Kostopoulos, G. Konstantinidis, N. Katsarakis, R. S. Penciu, M. Kafesaki, E. N. Economou, Th. Koschny, and C. M. Soukoulis, “Experimental demonstration of negative magnetic permeability in the far-infrared frequency regime,” Appl. Phys. Lett. 89, 084103 (2006). [CrossRef]
18. B. D. F. Casse, H. O. Moser, J. W. Lee, M. Bahou, S. Inglis, and L. K. Jian, “Towards three-dimensional and multilayer rod-split-ring metamaterial structures by means of deep x-ray lithography,” Appl. Phys. Lett. 90, 254106 (2007). [CrossRef]
19. N. Liu, H. Guo, L. Fu, S. Kaiser, H. Schweizer, and H. Giessen, “Three-dimensional photonic metamaterials at optical frequencies,” Nat. Mater. 7, 31–37 (2008). [CrossRef]
20. O. Paul, C. Imhof, B. Reinhard, R. Zengerle, and R. Beigang, “Negative index bulk metamaterial at terahertz frequencies,” Opt. Express 16, 6737 (2008). [CrossRef]
21. S. Zhang, W. Fan, N. C. Panoiu, K. J. Malloy, R. M. Osgood, and S. R. J. Brueck, “Experimental demonstration of Near-Infrared Negative-Index Metamaterials,” Phys. Rev. Lett. 95, 137404 (2005). [CrossRef] [PubMed]
22. G. Shvets, “Photonic approach to making a material with a negative index of refraction,” Phys. Rev. B 67, 035109 (2003).
26. A. Lai, C. Caloz, and T. Itoh, “Composite right/left-handed transmission line metamaterials,” IEEE Microw. Mag. 5, 34–50 (2004). [CrossRef]
27. V. Ghica and W. Glashauser, “Verfahren fuer die spannungsrissfreie Entwicklung von bestrahlten Polymethylmethacrylat-Schichten,” Deutsche Offenlegungsschrift DE 3039110 (1982).
28. M. Bahou, L. Wen, X. Ding, B. D. F. Casse, S. P. Heussler, P. Gu, C. Diao, H. O. Moser, W. S. Sim, J. Gu, and Y. L. Mathis , “Infrared Spectro/Microscopy at SSLS — Edge Effect Source in a Compact Superconducting Storage Ring,” in Synchrotron Radiation Instrumentation, Jae-Young Choi and Seungyu Rah, eds., AIP Conf. Proc. 879, 603–606 (2007). [CrossRef]
29. Microwave Studio (MWS) is a registered trademark of CST GmbH, Darmstadt, Germany.
30. X. Chen, T. M. Grzegorczyk, B. I. Wu, J. Pacheco, and J. A. Kong, “Robust method to retrieve the constitutive effective parameters of metamaterials,” Phys. Rev. E 70, 016608 (2004).
31. Th. Koschny, P. Markos, E. N. Economou, D. R. Smith, D. C. Vier, and C. M. Soukoulis, “Impact of inherent periodic structure on effective medium description of left-handed and related metamaterials,” Phys. Rev. B 71, 245105 (2005).
32. H. Zhong, A. Redo-Sanchez, and X.-C. Zhang, “Identification and classification of chemicals using terahertz reflective spectroscopic focal-plane imaging system,” Opt. Express 14, 9130–9141 (2006). [CrossRef] [PubMed]
33. C. Jastrow, K. Münter, R. Piesiewicz, T. Kürner, M. Koch, and T. Kleine-Ostmann, “300 GHz Transmission System,” Electron. Lett. 44, 213–214 (2008). [CrossRef]