March 15, 2009 / Vol. 34, No. 6 / OPTICS LETTERS
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Fresnel zone antenna for dual-band detection at millimeter and infrared wavelengths Javier Alda1,* and Francisco Javier González2 1
Applied Optics Complutense Group, University Complutense of Madrid, School of Optics, Avenida Arcos de Jalon, 118, 28037 Madrid, Spain 2 Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí, Álvaro Obregón 64, 78000, San Luis Potosí, SLP, México *Corresponding author:
[email protected] Received October 29, 2008; revised February 9, 2009; accepted February 9, 2009; posted February 18, 2009 (Doc. ID 103413); published March 11, 2009 In this work the concept of a Fresnel zone antenna for dual-band detection in the IR and millimeter wave region is presented. The design is based on a Fresnel zone plate lens in the IR that is transformed to serve as a millimeter-wave antenna. Two different designs are presented, a circular-zone design that gives a high diffractive efficiency in the IR and a square-zone design that gives a higher response in the millimeter band but a lower focusing efficiency in the IR. Both designs have an operation bandwidth with the same low frequency limit of 400 GHz 共750 m兲, which can be tailored by changing the number of Fresnel zones, and a high frequency limit of 4.5 THz 共65 m兲 for the circular-zone design and 5 THz 共59 m兲 for the square-zone design. © 2009 Optical Society of America OCIS codes: 040.6808, 050.1965, 040.2235.
For certain applications the simultaneous detection of IR and millimeter waves can be useful. Millimeter wavelengths can detect signals in low visibility conditions such as fog, haze, clouds, and smoke and can even penetrate clothing [1], while shorter wavelengths in the IR provide better resolution [2]. Uncooled antenna-coupled microbolometers have been used successfully to detect both IR radiation [3] and millimeter waves [4] by using different designs tuned for each wavelength. To increase the collection area of antenna-coupled microbolometers Fresnel zone plate lenses (FZPLs) have been used to focus IR radiation onto spiralcoupled microbolometers [5]. Since the size and shape of FZPLs in the IR is similar to a spiral antenna tuned at millimeter waves, a design based on a modified FZPL that serves the double purpose of focusing IR radiation and detecting millimeter waves is presented. In this work the concept of a Fresnel zone antenna (FZA) used for dual-band detection purposes in the IR and millimeter wave region is presented and characterized using full-wave finite-element simulations and scalar diffraction calculations. Figure 1 shows the proposed design for the FZA for dual-band detection. It consists of two antennacoupled detectors in which the millimeter-wave detector also serves as a diffractive lens for the IR detector. The center of the millimeter-wave detector is occupied by a bolometric transducer. They could be fabricated on a silicon substrate and aligned to each other using a backside alignment lithographic procedure. The proposed IR detector is a square-spiralcoupled microbolometer, which has been successfully coupled to a diffraction lens and characterized at these wavelengths [5]. The design procedure begins with the calculation of an FZPL that focuses IR radiation at a given wavelength on a detector placed at the opposite side of the substrate; for this work an IR wavelength of 10.6 m 0146-9592/09/060809-3/$15.00
and a 350-m-thick silicon wafer was used. The antenna for millimeter-wave detection is a transformation of the FZPL into an antenna layout. Both detectors are aligned to each other. To evaluate the efficiency in the IR of the proposed FZA a diffractive calculation was made using a scalar approach. This treatment has been successfully applied to the design and analysis of FZPLs as focusing elements [5].
Fig. 1. (Color online) Dual-band FZA consisting of two antenna-coupled microbolometers tuned for IR and millimeter-wave detection. The bolometer for the millimeter-wave detection is at the center of the FZA, and it is shown in gray. The millimeter-wave antenna also serves the purpose of a diffractive focusing element in the IR. © 2009 Optical Society of America
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Fig. 2. (Color online) Spiral antenna response at millimeter wavelengths for linear and circular polarizations. The shape of the modified circular FZPL is shown in the inset. LHCP, left-handed circular polarization; RHCP, righthanded circular polarization.
Fig. 3. (Color online) Square-spiral antenna response at millimeter wavelengths for linear and circular polarizations. The shape of the modified square FZPL is shown in the inset. LHCP, left-handed circular polarization; RHCP, right-handed circular polarization.
Numerical simulations of the millimeter-wave antenna-coupled microbolometer were performed using the finite-element method implemented in COMSOL Multiphysics. The simulated antenna and microbolometer were made out of 150 nm films of gold and nickel, respectively. The electromagnetic simulation was performed by launching a 1 V / m plane wave with linear or circular polarization and measuring the induced current in the microbolometer as a function of the plane wave’s frequency. Matched boundary conditions were used in the simulations, and tetrahedral elements were used to discretize the computational domain. Figure 2 shows the millimeter-wave response of a spiral antenna at linear and circular polarizations; this antenna was designed from an FZPL at 10.6 m having a focal length of 350 m. The outer diameter of the FZPL is 351 m. Even though spiral antennas are frequency independent [6], the antenna showed a working frequency bandwidth from 400 GHz 共750 m兲 given by the outer diameter of the spiral and a higher frequency limit given by the smallest resonant structure, in which this case was the bolometer, which would give a higher frequency limit around 4.5 THz 共65 m兲. Figure 3 shows the millimeter-wave response of a square-spiral antenna at linear and circular polarization. This antenna was designed from a variant of the FZPL, where the zones are built from squares instead of circles. The outer square’s side is 276 m long. In this design the open zones add in phase [7]. Owing to the similar size of the zone plate with the one presented in Fig. 2, the antenna also presents a lower working frequency around 400 GHz, and the higher frequency limit would be around 5 THz 共59 m兲 owing to the smaller bolometer. It is worth noting that this square antenna has a higher response than the circular spiral owing to its larger metallic surface; however, the frequency response is not as smooth as the one shown in Fig. 2, because this design resembles more a folded-arm dipole than a spiral antenna. Table 1 shows the gain of the two FZA designs at 10.6 m; the gain is defined as the ratio of the irra-
diance at a given point using the FZPL and the irradiance at the same point without the FZPL. Also an integrated gain has been obtained as the integration of the calculated irradiance distribution over the measured 12.5 m ⫻ 12.5 m collection area of a square-spiral antenna at 10.6 m [3]. Using computer simulations the concept of an FZA used for dual-band detection in the IR and millimeter wave region is demonstrated. This device is based on a diffraction lens in the IR that is transformed to serve as a millimeter-wave antenna. The response of the two millimeter-wave antenna designs, obtained using full-wave finite-element simulations, have an operation bandwidth with the same low frequency limit of 400 GHz 共750 m兲 and a high frequency limit of 4.5 THz 共65 m兲 for the circular-zone design and 5 THz 共59 m兲 for the square-zone design. Its focusing efficiency in the IR obtained using a scalar diffraction calculation gives an irradiance enhancement of 263 for the circular design and 4.6 for the square design. Even though the square-zone antenna gives a higher response in the millimeter range than the circular design, the 50⫻ relative increase in the IR irradiance of the circular FZA with respect to the square FZA might be an advantage for certain applications. It is also worth noting that the performance of the millimeter-wave spiral antenna can be tailored to shorter or longer wavelengths by changing the number of Fresnel zones and therefore changing the size of the spiral. Table 1. Values of the Gain Obtained at the Plane of the IR Detector When Including the FZAa
Gain at the focus Integrated gain a
Circular FZPL
Square FZPL
263.4 194.1
4.6 3.9
The integrated gain uses the responsivity map of the proposed infrared detector. This responsivity has a total area of about 12.5 m2.
March 15, 2009 / Vol. 34, No. 6 / OPTICS LETTERS
This paper has been developed during a stay of F. J. González at the University Complutense of Madrid. This stay has been partially funded by the University Complutense de Madrid and the Ministry of Science of Spain (TEC2006-1882). F. J. González would also like to acknowledge support from Programa de Mejoramiento del Profesorado, Consejo Nacional de Ciencia y Tecnología, and Fondo Mixto del Estado de San Luis Postosí and FOMIX-SLP through grants FMSLP-2005-C01-28, FMSLP-C01-87127 and CB-2006-60349. References 1. L. Yujiri, M. Shoucri, and P. Moffa, IEEE Microw. Mag. 4, 39 (2003).
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2. H. Chen and P. K. Varshney, Proc. SPIE 3719, 152 (1999). 3. F. J. González and G. D. Boreman, Infrared Phys. Technol. 46, 418 (2005). 4. E. N. Grossman, A. K. Bhupathiraju, A. J. Miller, and C. D. Reintsema, Proc. SPIE 4719, 364 (2002). 5. F. J. González, J. Alda, B. Ilic, and G. D. Boreman, Appl. Opt. 43, 6067 (2004). 6. P. E. Mayes, Proc. IEEE 80, 103 (1992). 7. J. Alda and G. Boreman, Microwave Opt. Technol. Lett. 50, 536 (2008).
