Frequency Selective Surface for High-Sensitivity ...

70 downloads 0 Views 64KB Size Report
Christian Debus and Peter Haring Bolivar. Institute of High Frequency and Quantum Electronics, Siegen University,. Hölderlinstr.3, 57076 Siegen, Germany,.
a2365_1.pdf CTuCC7.pdf

Frequency Selective Surfaces for High-Sensitivity Terahertz Sensors Christian Debus and Peter Haring Bolivar Institute of High Frequency and Quantum Electronics, Siegen University, Hölderlinstr.3, 57076 Siegen, Germany, Email: [email protected]

Abstract: We present a frequency selective surface (FSS) of asymmetric split-ring resonators for terahertz (THz) sensor applications. Multiple resonances of the rings combine to sharp edges in the FSS’s frequency response to achieve high sensitivity.

©2007 Optical Society of America

OCIS codes: (260.3090) far Infrared; (120.5700) Reflection; (999.9999) Biosensing

In the radio and microwave technology frequency selective surfaces (FSSs) are widely used as filters for freely propagating waves [1]. A FSS is a two-dimensional periodic array of identical resonating metallo-dielectric structures. Depending on the shape of the resonators and on the direction of the incident wave, some frequencies will be transmitted through the surface while others will be reflected. With an appropriate design, high-Q bandpass or bandstop filters can be realized even for terahertz (THz) frequencies [2]. However, until now FSS have barely been used for other submillimeter applications. In this paper we propose the use of FSS as a flexible technology for the development of high sensitivity sensor concepts. The “open” geometry of FSSs enables the simple integration of such sensing surfaces into various THz sensing systems. For those applications, the sharp edge in the frequency response of such filters is suitable for high sensitivity THz sensing. By loading the individual resonators of the FSS with the analyte material, the resonant frequency shifts in proportion to the dielectric properties of the analyte. By using a sharp, steep frequency edge at the operation point this frequency shift is associated with a large change of the transmission or reflection of the signal. This enables detecting small amounts of material or small variations in the dielectric constant of a material. Many applications for such sensors can be foreseen, e.g. for biomolecular sensing, chemical sensing or security applications [3-5]. Higher sensitivity is attained with spectrally sharper properties. Here we present a FSS made of split-ring resonators. In a typical single-split structure only a single resonant frequency with a moderate Q-factor is observed. Symmetric double split-rings being rings with two splits at opposite positions still have only one resonance with a similar Q-factor slightly higher than 10, as shown in Fig. 1 at 1.04 and 1.07 THz. At these frequencies the half circumference is about half a wavelength so that the behavior corresponds to that of classical dipole antennas. The edges of this moderate Q bandstop filter are not sufficient to achieve high sensitivity when utilized as a sensor. Harmonics can also be excited, but these show an even worse performance. To gain a more dynamic frequency response the structure must have multiple resonances and exploit interference effects between them. 0

Reflection (dB) .

-6 -12 -18 -24 -30 -36 0.8

0.9

1

1.1

1.2

Frequency (THz)

Fig. 1. Numerically simulated absolute value of the reflection coefficient of split-ring FSSs. Dashed curve: single split-ring; Dotted curve: symmetric double split-ring; Solid curve: asymmetric double split-ring.

a2365_1.pdf CTuCC7.pdf

We propose an asymmetric double split-ring resonator (a-DSRR). By breaking the symmetry, different parts of the structure become resonant at different frequencies. An a-DSRR, e.g. with splits shifted by 4° from the x-axis as depicted in Fig. 2, does still show the broad resonance at 1.08 THz. But in addition it reveals a dual-resonance interference feature with a sharp local maximum and a zero close to 0.9 THz. Numerical simulation using HFSS shows a steep edge of 34 dB over 12 GHz in the frequency response of this FSS for a perfect metal configuration. Realistic losses reduce the steepness of the spectral response by a factor of approximatedly 2, only. Therefore a-DSSR structures enable attractive sensing surfaces. One should note that alternative designs, which have been investigated in the past (e.g. [6], [7]), show concentric combinations of split-rings of different sizes. Such concepts show much broader spectral features, caused by a higher coupling between the concentric resonators and by higher losses resulting from a higher coverage of the surface with metal.

Fig. 2. Drawing of one asymmetric double split-ring in a FSS cell.

Fig. 3. Magnitude of the simulated E-field of the FSS, excited with a plane wave of 1 V/m. Picture shows one cell of the periodic array.

Regarding biosensing applications, one advantage of the double split-ring approach is the appearance of high E-field concentrations at the end of the arcs, as shown in Fig. 3. By positioning probe material at these locations only, e.g. via chemically selective functionalization, the effect of the analyte on the reflection can be maximized so that the amount of required material is drastically reduced. a-DSSR FSSs constitute therefore an attractive concept for sensing applications, e.g. for the development of easily manufacturable and disposable THz biochip technologies [3, 4]. One should highlight that FSS sensor concepts are advantageous in terms of flexibility, as the direct interaction of the resonant sensors with free-space THz radiation greatly simplifies system integration. [1] B. A. Munk, “Frequency Selective Surfaces: Theory and Design”, A Wiley-Interscience Publication (2000). [2] D. Qu, D. Grischkowsky, W. Zhang, “Terahertz transmission properties of thin, subwavelength metallic hole arrays”, Opt. Let. 29, pp. 896898 (2004). [3] P. H. Siegel, “Terahertz Technology in Biology and Medicine”, IEEE transactions on microwave theory and techniques, Vol. 52 No. 10 (2004). [4] P. Haring Bolivar et al., “Label-free THz sensing of genetic sequences: towards ‘THz biochips’”, Phil. Trans. R. Soc. Lond. A 362, pp. 323335 (2003). [5] S. P. Mickan, A. Menikhu, H. Liu, C. A. Mannella, R. MacColl, D. Abbott, J. Munch, and X.-C. Zhang, “Label-free bioaffinity detection using terahertz technology,” Phys. Med. Biol., vol. 47, no. 21, pp. 3789–3795, (2002). [6] R. Marques et al., “Ab initio analysis of frequency selective surfaces based on conventional and complementary split ring resonators”, J. Opt. A: Pure Apll. Opt. 7, S38-S43 (2004). [7] A. K. Azad, J. Dai, W. Zhang, “Transmission properties of terahertz pulses through subwavelength double split-ring resonators”, Opt. Let. 31, pp. 634-636 (2006).