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Microwave Emission from MetalPhotonic Crystals Fabricated by usingStereolithographyDaisuke Sano a & Soshu Kirihara ba Graduate school of engineering , Osaka University , Osaka, Japanb Joining and Welding Research Institute Osaka University , Osaka,JapanPublished online: 20 Sep 2010.

To cite this article: Daisuke Sano & Soshu Kirihara (2009) Microwave Emission from MetalPhotonic Crystals Fabricated by using Stereolithography, Ferroelectrics, 388:1, 23-30, DOI:10.1080/00150190902963708

To link to this article: http://dx.doi.org/10.1080/00150190902963708

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Ferroelectrics, 388:23–30, 2009Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190902963708

Microwave Emission from Metal Photonic CrystalsFabricated by using Stereolithography

DAISUKE SANO1,∗ AND SOSHU KIRIHARA2

1Graduate school of engineering Osaka University, Osaka, Japan2Joining and Welding Research Institute Osaka University, Osaka, Japan

Micro-scale photonic crystals composed of pure copper for microwave emission controlwere fabricated by using micro-stereolithography system and sintering process. As acrystal structure, we designed diamond lattice structure with or without stretched latticespacing. Diamond lattices exhibited common bandgap where electromagnetic wavecan not propagate at any direction. Stretched lattices showed directional transmissionproperty. Simulated and measured transmission properties of microwave propagatingthrough the metal photonic crystals showed good agreements.

Keywords Metal photonic crystal; electromagnetic band gap; micro stereolithogra-phy; microwave; terahertz frequency

Introduction

Photonic crystals with periodic arrangements of dielectric or metal lattices can reflectelectromagnetic waves perfectly and form photonic bandgap in the specific frequency rangeby Bragg diffraction [1]. Microwave with micrometer or millimeter order wavelength canbe applied for clean and easily-controlled power sources used for such as heating/sinteringprocesses, non-contact energy transmission system, medical treatments and disposals ofhazardous organic substances [2]. There is a great deal of growing interest to apply artificialmaterials with photonic bandgap to microwave devices such as antennas, waveguides andfrequency-selective filters [3, 4]. For effective control of microwave, structural modificationof photonic crystal is necessary. In the past research, we fabricated dielectric rods withdiamond structure to control microwave [6, 7]. Diamond structure exhibit complete bandgapin which propagation of electromagnetic wave is forbidden in all direction at a certainfrequency range by three-dimensional Bragg diffraction. In addition, modified diamondstructure in which lattice spacing was stretched in one direction was designed and fabricatedfor directional control of emission [8]. The bandgap range of the stretched lattices shiftedtoward lower frequency range depending on the stretching ratio, and the modified crystalexhibited directional transmission property. By emitting microwave from monopole antennasource embedded inside the modified crystal, we succeeded to confine the emitted powerto narrow angular region.

In this research, micro-scale diamond lattice structures with or without stretched lat-tice spacing composed of pure copper were fabricated by using micro-stereolithography

Received November 1, 2008; in final from January 26, 2009.∗Corresponding author. E-mail: sanwou@jwri.osaka-u.ac.jp

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Figure 1. CAD models of diamond lattice structure: (A) Lattices consisting of 5 × 5 × 5 units, (B)<100> direction, (C) <110> direction and (D) <111> direction.

to control high-energy electromagnetic waves with micrometer order wavelength. Copperis thermal-shock resistant material and nearly perfect reflector without absorption prob-lem. Copper photonic crystal with stretched lattice spacing expected to exhibit directionaltransmission property, which can be used to improve the directivity of microwave emission.

Experimental Procedure

Photonic crystals with diamond lattice structure were designed by using Computer AidedDesign (CAD) software (Thinkdesign Ver. 2007.1, Toyota Caelum Co. Ltd., Japan). Figure 1shows CAD models of diamond lattice structure through <100>, <110>, and <111>

Figure 2. Designed models of diamond lattices stretched in a <100> direction: (A) <100> directionand (B) <010> direction.

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Figure 3. A schematic illustration of micro-stereolithgraphy process.

direction. The lattice constant was 1 mm in length, and the rod diameter was 290 µm. Thewhole structure was 6 × 6 × 2 mm, consisting of 6 × 6 × 2 unit cells. The stretchedcrystal was designed by stretching lattice spacing in <100> direction. Adequate stretchingratio was determined by using electromagnetic simulation of transmission line modeling(TLM) method so that stretched lattices exhibit directional transmission. In a TLM method,analyzed model is divided into tiny cells of 50 µm in edge length, and transmission propertythrough a structure was simulated as sequential wave propagation between linked cells.After the simulation, stretching ratio was determined to be 70% and the lattice constantalong <100> and <010> direction was designed to be 3.4 mm and 2 mm, respectively.The CAD model of the Stretched diamond structures are shown in Fig. 2. The whole sizewas 12 × 12 × 6.8 mm and 12 × 12 × 4 mm. The designed CAD models were slicedinto a series of 2D layers. The structures were fabricated by using micro-stereolithographysystem (SI-C1000, D-MEC Co. Ltd., Japan). This machine consists of laser unit, an elevatorstage, moving blade and Digital Micromirror Device (DMD) chips. DMD is the reflectingoptical system which has 768 × 1024 mirrors of 14 µm in edge length. Each mirror isautomatically controlled by piezo elements and the mask patterns are dynamically generatedaccording to the sliced data. A schematic illustration of micro-stereolithography processis shown in Fig. 3. In this system, the photo sensitive resin paste including pure copperparticles of 5 µm in diameter at 54 vol. % was fed on the fabrication stage by air pressure.The highly viscosity paste was uniformly coated by moving mechanical-controlled blade.The layer thickness was controlled to 10 µm. The laser light of two-dimensional imageaccording to sliced data was illuminated on the coated resin surface by using DMD,and illuminated area became solidified simultaneously. When a layer was formed, theelevator stage moved downward 10 µm and then new layer was stacked. Three dimensionalstructures were fabricated by stacking these layers under computer control. Unsolidifiedresin was removed by washing in ethanol solvent. We also used stereolithography machine(SCS-300P, D-MEC Co. Ltd.) to fabricate the stretched structures. This machine useshigh-speed laser scanning system to solidify 2-D patterns, and large structure can befabricated rapidly. The fabricated precursors were heated to dewax resin and sinter copper

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Figure 4. SEM images of copper diamond lattice structure: (A) <100> direction, (B) <110>

direction, (C) <111> direction.

particles. The dewaxing and sintering temperature was 600◦C and 1000◦C. Heating processwas handled in argon atmosphere in order to block the oxidation process. The densityof sintered samples was measured by the Archimedes’ method. The apparent conditionand microstructure of samples were observed by digital optical microscope (VH-Z100,KEYENCE Co., Osaka, Japan) and scanning electron microscope (SEM), respectively. Thetransmission properties of microwave propagating through the diamond lattice structureswere measured by using terahertz time-domain spectroscopy (AISPEC Co. Ltd., Japan,J-spec2001 spc/ou). Measured microwave properties were compared with simulation byTLM method.

Figure 5. SEM images of copper diamond structure with stretched lattice spacing: (A) <100>

direction and (B) <010> direction.

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Figure 6. Transmission properties of microwave propagating through the normal diamond latticestructure for (A) the <100> direction, (B) the <110> direction and (C) the <111> direction.

Result and Discussion

Figure 4 and Fig. 5 shows SEM images of the fabricated diamond lattice structures with orwithout stretched lattice spacing composed of sintered pure copper. The achieved spatialresolution was approximately 0.5%. The peaks of copper oxides were not detected by XRD

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Figure 7. Electric field intensity around the diamond lattices for (A) the <100> direction, (B) the<110> direction and (C) the <111> direction.

analysis. The linear shrinkage was 10% and the relative density reached 87%. Dense purecopper structures were obtained without cracking and deformation by the proper dewaxingand sintering condition.

Figure 6 shows simulated and measured transmission spectra of the metal photoniccrystals for the <100>, <110> and <111> directions. In the simulated spectra, suddendecreases of transmission meaning photonic bandgap were observed in every direction,and a common bandgap appeared at the frequency from 400 to 450 GHz. Diamond latticeshave isotropic periodicity, so wavelength of diffracted wave is almost the same lengthin all directions and photonic bandgap appeared in common region. Dot lines meaningmeasured transmission spectra indicate a common bandgap in the frequency range from415 to 430 GHz, which was in good agreement with simulation. Figure 7 illustrates simu-lated electric field intensity around the diamond lattices at 430 GHz of the common bandfrequency. Electromagnetic waves propagating from left side attenuated gradually in thestructure and did not propagate across the metal lattices. When electromagnetic wavesare reflected by Bragg diffraction, attenuation wave is formed in periodic space due tothe mutual interference between traveling and diffracted waves. Copper diamond latticesforbidden propagation of electromagnetic wave in all directions.

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Figure 8. The bandgap variation against stretching ratios of the lattice spacing for the <100>

direction simulated by TLM method. Solid squares show simulated bandgap edges.

Figure 9. Transmission spectra of microwave propagating through the stretched lattice structuresfor (A) the <100> direction and (B) the <010> direction.

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Figure 8 shows the bandgap variation against stretching ratios of the lattice spacing forthe <100> direction calculated by TLM method. The bandgap shifts to lower frequencyrange with an increase in stretching ratio. In the stretched structure, waves with longerwavelength can be reflected by stretched lattices and bandgap is considered to appear atlower frequency region.

Transmission spectra of microwave propagating thorough the stretched lattices along<100> and <010> direction are shown in Fig. 9. The bandgaps of fabricated crystalsalong <100> and <010> directions existed in the frequency range below 200 GHz and185 to 210 GHz, respectively. Fabricated photonic crystals exhibit transmission weakerthan simulated value in the pass band region. The reason for that are copper particleson the surface of fabricated structures leading to wave scattering. At the frequency from200 to 210 GHz, wave can propagate along <100> direction, while wave propagationis forbidden along <010> direction. These results indicate the copper diamond latticestructure stretched for <100> direction can confine direction and power distribution ofmicrowave emission to the <100> direction.

Conclusion

We fabricated diamond lattice structure with or without stretched lattice spacing composedof acrylic resin/copper composite by using micro-stereolithography. Pure copper structureswere precisely formed without deformation and cracking by proper dewaxing/sinteringprocess. The normal diamond structure showed common bandgap at the frequency from415 GHz to 430 GHz. The stretched crystal exhibited directional transmission propertyat the frequency from 200 to 210 GHz. The simulated result by TLM method showed agood agreement with the measured result. Our investigation indicates microwave emissionfrom metal photonic crystal can be controlled by modification of lattice structure. Stretchedcrystals composed of copper are possible devices controlling emission of high-energymicrowave.

References

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2. H. Matsumoto, Research on solar power satellites and microwave power transmission in Japan.IEEE Microwave Mag. 3, 36–45 (2002).

3. M. M. Sigalas, R. Biswas, K. M. Ho, and C. M. Soukoulis, Waveguide in three-dimensinalmetallic photonic band-gap materials. Phys. Rev. B. 60, 4426–4429 (1999).

4. H. Caglayan, I. Bulu, and E. Ozbay, Highly directional enhanced radiation from sources embed-ded inside three-dimensional photonic crystals. Opt. Exp. 13, 7645–7652 (2005).

5. N. Guerin, C. Hafner, and R. Vahldieck: Compact directive antennas using metallic photoniccrystals. Proc. Antennas Propagation Society Int. Symp. IEEE 3A, 6–9 (2005).

6. S. Kirihara, M. Takeda, K. Sakoda, and Y. Miyamoto, Electromagnetic wave control of ce-ramic/resin photonic crystals with diamond structure. Sci. Tech. Adv. Mater. 5, 225–230 (2004).

7. H. Kanaoka, S. Kirihara, and Y. Miyamoto, Terahertz wave properties of alumina micro photoniccrystals with a diamond structure. J. Mater. Res. 23, 1036–1041 (2008).

8. S. Kirihara, M. W. Takeda, K. Sakoda, and Y. Miyamoto, Control of microwave emission fromelectromagnetic crystals by lattice modifications. Sol. Stat. Comm. 124, 135–139 (2002).

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