Enhanced Coupling of Subterahertz Radiation with Semiconductor Periodic Slot Arrays Ramakrishnan Parthasarathy, 1 Alexei Bykhovski, 1 Boris Gelmont, 1, * Tatiana Globus, 1 Nathan Swami, 1 and Dwight Woolard 2 1 Department of Electrical & Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 2 U.S Army Research Laboratory, Army Research Office, Research Triangle Park, North Carolina 27709, USA (Received 3 November 2006; published 12 April 2007) In this work, a theoretical study of the coupling of TM polarized subterahertz (THz) radiation with periodic semiconductor rectangular slot arrays was conducted, using InSb as an example. Simulation results showed that the structure with 4–12 m thickness provides over a 20 –30-fold increase in the electric field at slot edges in a nanosize region (  500 nm). The enhancement of the THz electromagnetic field extends across the slots and reaches peak values at the edges due to discontinuity effects. Because of the strong local electromagnetic field enhancement, the structure can potentially be used for the development of novel biophotonic sensors, leading to improved detection sensitivity. DOI: 10.1103/PhysRevLett.98.153906 PACS numbers: 42.25.Bs, 42.25.Fx, 42.25.Gy, 42.79.Dj Vibrational resonance spectroscopy in the terahertz (THz) gap or in the sub-THz range is a fast emerging technique for fingerprinting biological molecules and spe- cies with broad potential applications such as biomedicine or detection and identification of biological molecules. The technique is based on the specificity of spectroscopic sig- natures of molecules, which reflect absorption of THz radiation by the weakest hydrogen type bonds and non- bonded interactions within biological matter at character- istic resonance frequencies. Experimental and computa- tional studies [1– 3] have demonstrated the capability of THz spectroscopy as a very promising technique to dis- criminate between species. However, the problem of im- proving detection sensitivity of molecules with low absorp- tion characteristics at THz remains an important issue. In order to increase the sensitivity and reliability of THz fingerprinting techniques, coupling of incident THz radia- tion to biological molecules has to be enhanced. One possible solution is to use periodic slot arrays. Such arrays were previously used for THz bandpass filters fabricated from lossy metal films deposited on dielectric membranes [4]. Experimental work on enhanced transmission is mostly available at optical and near-infrared frequencies for metallic periodic structures (gratings [5 – 8] and hole arrays [9 –11]). Recently, it has been shown that waveguide resonance and diffraction are the main factors [8] contrib- uting to enhanced transmission of narrow slot subwave- length metallic gratings. The phenomenon of extraordinary optical transmission (transmission efficiency exceeding unity when normalized to the surface of the holes) through hole arrays, first experimentally observed in Ag [9,10], has been attributed to the resonant tunneling of surface plas- mons [10 –15] through thin films. Recently, similar studies were done in the THz range with hole arrays in films made of metals (Ag-coated stainless steel [16], Al-coated Si wafers [17]) and doped semiconductors (Si [18] and InSb [19]). However, the observed transmission enhancement at THz in lossy metal foils with hole arrays is not well under- stood and a rigorous treatment is not provided [16,17] since the qualitative analysis relied on the dispersion of surface plasmons in a uniform film [20] that is inappropriate in the case of hole arrays. In the THz region, interaction between radiation and metals is quite different from higher frequency regions due to the change in material dielectric properties. In the visible and near-IR regions, where frequencies are only slightly less than plasma frequency, the permittivities are predominantly real and negative (for example, at wave- length 1 m, " Au 51:4  j1:6), and metals are reflec- tive. On the contrary, as we go down in frequency to the THz, the real part continues to be negative and large, but the dissipative imaginary part becomes larger, and hence metals are very conducting and absorbing (at wavelength 500 m, " Au 5:5  10 4  j8:5  10 5 ). Therefore, to reduce radiation losses, it is preferable to substitute metals with doped semiconductors with plasma frequencies in the low THz range. InSb with high electron mobility and low effective mass is most suited for this purpose, but still has a substantial absorbing imaginary part compared to the real component. The absorbing component requires the as- sumption of a small film thickness, which makes the InSb skin depth at both InSb-air interfaces larger than half the film thickness throughout our frequency range of interest. This renders the surface impedance boundary con- ditions for perfect conductors [21,22] to be unsuitable for our case. On the other hand, in contrast with the behavior of metals in short wavelength ranges, the Fourier expan- sion method for field diffracted from gratings [5] can be applied in the THz region for InSb films, since the imagi- nary permittivity component damps the Gibbs oscillations [23]. In this Letter, we study the mechanism of coupling of TM polarized THz radiation to the periodic semiconductor structure, consisting of a doped semiconductor film with rectangular slot arrays, using InSb as an example. Trans- mission properties of subwavelength slot arrays are funda- PRL 98, 153906 (2007) PHYSICAL REVIEW LETTERS week ending 13 APRIL 2007 0031-9007= 07=98(15)=153906(4) 153906-1  2007 The American Physical Society