Infrared Antenna Designs at 30 THz

Mohammed Ahmed Nassor, Erdal Korkmaz

Electrical and Electronics Engineering Department

Fatih University

Istanbul, TURKEY

ekorkmaz@fatih.edu.tr

Korkut Yegin

Electrical and Electronics Engineering Department

Yeditepe University

Istanbul, TURKEY

Abstract This paper presents different types of planar infrared antennas resonating at 30 THz. The designed antennas are

bowtie, trapezoidal, modified crossed dipole and balanced spiral.

The sensitivity of the antennas is analyzed at the aperture of the

antennas by varying geometrical parameter. The balanced spiral

and the modified crossed dipole antennas have higher sensitivity

than the other two antennas.

I. INTRODUCTION

Infrared antennas are essential to couple incident radiation to sensors such as bolometers and tunnel diodes. These antennas are usually coupled to metal-insulator-metal (MIM) tunnel junction (ACTJ) devices which can be used as terahertz and IR radiation detectors [1]. For better coupling efficiency of the antenna, these devices are usually illuminated from the substrate side. Planar antennas radiate part of their energy into the substrate in the form of surface waves. Depending on substrate thickness, radiation from the substrate side of silicon

(si =11.7 for 30 THz) can be 40 times greater than the

radiation in air [2].

Antenna can be designed separate from the sensor itself, which makes antenna tuning and optimization easier. Different planar antennas such as dipole antennas [3], spiral antennas [4]-[5], bowtie antennas and log-periodic antennas [5]-[6], trapezoidal [7], microstrip patch [8] antennas have been made studied to couple radiation into the sensor. These devices are light weight, fast, CMOS compatible and use low power which makes them suitable in various applications such as thermal imaging, target detection, tracking, navigation in autonomous vehicles and energy harvesting [8].

In this study, we designed our antennas at 30 THz simply because at a normal daily temperature human body and other mammals emit radiation at this frequency according to Wiens displacement law. In addition, between 8 to 14 m there is a gap at atmospheric opacity (atmospheric window) where absorption is low [9].

We simulated four different types of antennas on a grounded and ungrounded substrate. We designed bowtie, balanced spiral, modified crossed dipole, and trapezoidal antenna tuned to 30 THz and compared their results to each other for performance metrics.

II. ANTENNA GEOMETRY

Designs of four types of antennas are shown from their front views in Fig. 1. The antennas are designed on top of a

2SiO substrate ( s = 4.84 at 30 THz) with three different

thicknesses of 150 nm, 300 nm and 600 nm in a grounded

situation. The antenna material is chosen as gold. All the designed antennas are optimized to resonate at 30 THz. The bowtie antenna has a length of 4.89 m from end to end and a gap of 300 nm. For the balanced spiral antenna the outer end to the other outer end distance is 2.988 m, its strip width is 272 nm and the gap at the antenna aperture between arms is 314 nm. The trapezoidal antenna has a length of 4.25 m from end to end, the gap is 150 nm and the ratio of two consecutive radii of its arms was 0.7. The modified crossed dipole composed of four parts with square shape of total length 3.44 m, width of the strips is 200 nm and a gap between them is approximately 200 nm.

III. RESULTS

The design and simulation of antennas are performed by means of commercial electromagnetic softwares CST Microwave Studio. In order to observe the sensitivity of the antennas in receiving mode the antennas are illuminated by a plane wave. At the aperture of the antennas electric field probes are placed to monitor the coupled fields sensitivity with respect to its geometry. In all simulations results the ungrounded antenna substrate is kept constant since it has no significant effect on the sensitivity whereas for grounded

Figure 1. The front view of the antennas: bowtie (a),

balanced spiral (b), trapezoidal (c) and modified crossed

dipole (d).

Figure 1. The top view of bowtie (A)

antennas a parameter sweep is performed for the dielectric thickness. The electric fields measured at the antenna aperture probes are depicted for bowtie in Fig. 2, balanced spiral in Fig. 3, trapezoidal in Fig. 4 and modified crossed dipole in Fig. 5. For the bowtie antenna the results are best for an ungrounded configuration and the result for a grounded configuration can be optimized with choosing a thicker substrate. However the trapezoidal antenna has a better sensitivity with grounded configuration and thinner substrate whereas the sensitivity of both antennas is comparable. The balanced spiral shows the best performance in a grounded configuration with a 300 nm substrate thickness. The modified crossed dipole show a better performance than bowtie and trapezoidal but is worse than the spiral antenna whereas the ground does not improve the results.

IV. CONCLUSION

Four different planar 30 THz antennas are designed and

their sensitivity is compared in a grounded and ungrounded

substrate with different thicknesses. The balanced spiral

antenna has the best sensitivity followed by the modified

crossed dipole antenna which performs better than the

trapezoidal and the bowtie whereas there is no significant

difference observed between the latter two antennas.

REFERENCES

[1] P. C. D. Hobbs, R. B. Laibowitz, and F. R. Libsch, Ni-NiO-Ni tunnel

junctions for terahertz and infrared detection, Appl. Opt., vol. 44,no. 32, pp. 68136822, 2005.

[2] J. Alda, C. Fumeaux, M.A. Gritz, D. Spencer, G.D. Boreman, "Responsivity of infrared antenna-coupled microbolometers for air-side and substrate-side illumination", Infrared Physics & Technology 41(1), 1-9 (2000).

[3] C. Fumeaux, M. A. Gritz, I. Codreanu, W. L. Schaich, F. J. Gonzlez, and G. D. Boreman, "Measurement of the Resonant Lengths of Infrared Dipole Antennas," Infrared Physics & Technology, vol. 41, p. 271, 2000.

[4] G.D. Boreman, C. Fumeaux, W. Herrmann, F.K. Kneubhl, H. Rothuizen, "Tunable polarization response of a planar asymmetric-spiral infrared antenna", Optics Letters 23(24), 1912-1914 (1998).

[5] Isa Kocakarin and Korkut Yegin, Glass Superstrate Nano Antennas For Infrared Energy Harvesting Applications, International Journal of Antennas and Propagation, in press.

[6] F. Gonzalez and G. Boreman, Comparison of dipole, bowtie, spiral and log-periodic IR antennas, Infrared Phys. Technol., vol. 46, no. 5,pp. 418428, Jun. 2005.

[7] M. Navarro, S.A. Maier Broad-Band Near-Infrared Plasmonic Nanoantennas for Higher Harmonic Generation ACS Nano, 2012, 6 (4), pp 35373544

[8] I. Codreanu, C. Fumeaux, D. F. Spencer, and G. D. Boreman, "Microstrip Antenna-Coupled Infrared Detector," Electronics Letters, vol. 35, p. 2166,1999.

[9] M. Midrio, S. Boscolo, A. Locatelli et al., Flared monopole antennas for 10 m energy harvesting, Proc. of the 40th European Microwave Conference, Paris (France), October 2010.

[10] J.A. Bean, B. Tiwari, G. Szakmany, G.H. Bernstein, P. Fay, W. Porod Long wave infrared detection using dipole antenna-coupled metal oxide-metal diodes, In proceeding of: Infrared, Millimeter and Terahertz Waves, 2008. IRMMW-THz 2008. 33rd International Conference on 10/2008.

Figure 2. Electric field values of the probe at the bowtie

antenna aperture.

Figure 3. Electric field values of the probe at the balanced

spiral antenna aperture.

Figure 4. Electric field values of the probe at the

trapezoidal antenna aperture.

Figure 5. Electric field values of the probe at the modified

crossed dipole antenna aperture.