Surface plasmon mediated transmission of subwavelength slits at THz frequencies

Abstract

We present measurements and a numerical modeling that elucidate the role of surface plasmons in the resonant transmission of a subwavelength slit in a conducting material. By using THz time domain spectroscopy, we determine the Fabry-Pérot transmission resonances for a single slit formed from a wafer of a semiconductor with a surface plasma frequency in the THz frequency range. We measure large redshifts in the resonant frequencies close to the surface plasma frequency, which are 50% lower than the resonance frequencies expected well below the surface plasma frequency. This is an effect attributed to the coupling of plasmons on the adjacent surfaces of the slit. DOI: 10.1103/PhysRevB.77.113411 PACS number͑s͒: 73.20.Mf, 42.79.Ag, 71.45.Gm, 78.20.Ci The resonant transmission of radiation through a subwavelength slit in a screen of a conducting material has recently been the subject of considerable theoretical 1-5 and experimental 6-8 work. For some time, it has been known that narrow slits in metals at low frequencies support FabryPérot ͑FP͒ resonances, where an approximately half-integer number of wavelengths are quantized along the length of the slit. These quantized modes give rise to transmission resonances 9 of frequency FP given bywhere l is the length of the slit and n is an integer. For a slit in a slab of a perfect metal ͑a material with infinite conductivity at all frequencies͒, it has been shown that Eq. ͑1͒ provides an accurate estimate of the resonant transmission frequencies, provided that the slits are very narrow compared to the wavelength of the transmitted radiation. 3,10However, for the case of narrow slits in real conductors with finite conductivity, the modes within the slits are coupled surface plasmons ͑SPs͒, and one obtains very large redshifts 1,2 from the resonant transmission frequencies predicted by Eq. ͑1͒. Resonant slit cavities are featured in many photonic structures, many of which have potential applications in sensing or spectroscopy. 5,11,12 It is, therefore, important to understand the role of SPs in influencing the resonant behavior of this fundamental system.Here, we present measurements and a numerical modeling that elucidate the role of SPs in the transmission of subwavelength slits in conducting materials. By using THz time domain spectroscopy ͑THz-TDS͒, we determine the behavior of FP transmission resonances for slits formed in wafers of indium antimonide ͑InSb͒, which has a surface plasma frequency ͑ SP ͒ of approximately 1.7 THz ͑10.7ϫ 10 12 rad/ s͒. For narrow slits in the InSb, we find that near the surface plasma frequency, resonances are redshifted by more than 50% from the values predicted by Eq. ͑1͒. This frequency shift is more than 2 orders of magnitude larger than the shifts reported in metals at low frequencies.9 By measuring the phase delay of the transmitted radiation, we demonstrate that the coupling of SPs on the surfaces within the slit modifies the effective index of the mode within the cavity and, hence, the frequency of resonant transmission. This e...

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…An interesting aspect of THz radiation interacting with photoexcited semiconductors that has not been discussed in this review is the possibility to optically tune the plasma frequency in highly ordered nanostructured semiconductors, which has opened a new field of THz plasmonics (Rivas et al, 2004;Isaac, Rivas et al, 2008;) allows for the (ultra)fast switching of THz transmission at specific frequencies with a high degree of control Azad et al, 2009;Berrier et al, 2010).…”

“…Similarly, in inhomogeneous conducting materials one can couple to local plasmon oscillations. Semiconductors, on the other hand, exhibit significantly lower plasma frequencies than metals Isaac, Rivas et al, 2008), so that one can expect coupling to THz plasmon oscillations in structured semiconductors. Such plasmon oscillations produce resonances in the effective dielectric function, similar to those expected from carrier localization effects (see preceding section).…”

Light Trapping in Plasmonic Solar Cells

“…Historically, optical excitation of SPPs has been performed by prism coupling [35,81] or by gratings, [82] which can be highly efficient but limited in both wavelength and angular range. More recently, phenomena such as extraordinary transmission through hole arrays and slits have been intensively studied [76,77,83,84,85,86,87]. Gratings with small numbers of elements are interesting because they are less frequency selective, and we show here that the limiting case of a single scattering object, such as a ridge or a groove in a metallic film, can also act as a relatively efficient broadband coupler from freely propagating modes to guided modes [75,88,89].…”

Determining the terahertz optical properties of subwavelength films using semiconductor surface plasmons

“…13 In particular, narrow gap semiconductors such as InSb have an intrinsic electron density appropriate for supporting low loss, highly confined terahertz SPs at room temperature. 14,15 Indeed, the dielectric function of InSb in the terahertz frequency range 14 is remarkably similar to that of plasmon supporting metals such as gold and silver in the UV/visible frequency range. 16 In this paper we present phase-resolved measurements which demonstrate that it is possible to determine the optical properties of a submicron sized dielectric layer above an InSb surface using a propagating SP.…”