By employing a combination of time-domain measurements and numerical calculations, we demonstrate that the semiconductor InSb supports a strongly confined surface plasmon ͑SP͒ in the terahertz frequency range. We show that these SPs can be used to enhance the light-matter interaction with dielectric layers above the semiconductor surface, thereby allowing us to detect the presence of polystyrene layers around 1000 times thinner than the free space wavelength of the terahertz light. Finally we discuss the viability of using semiconductor SPs for the purposes of terahertz sensing and spectroscopy. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3049350͔ Due to its nonionizing nature and the distinctive optical response of many molecules in the terahertz frequency range, there has been much recent interest in using terahertz radiation for chemical and biological analysis.1-3 However, with terahertz wavelengths ranging from 100 to 1000 m, the diffraction limit for this long wavelength light means that it is difficult to measure small volume samples. Recent work [4][5][6] has demonstrated that by propagating terahertz radiation along a metallic surface or waveguide it is possible to detect the presence of thin dielectric layers on the metal surface by allowing the terahertz surface wave to traverse a greater length of analyte. The effectiveness of this type of measurement geometry has recently been demonstrated by the development of a terahertz biosensor for DNA binding. 7Near the surface plasma frequency of a conductor ͑typi-cally in the UV/visible spectral region for most metals͒ a propagating surface plasmon ͑SP͒ can exhibit subwavelength confinement of electric field in the direction normal to a conductor-dielectric interface.8 This subwavelength field distribution enhances light-matter interactions with material in the region above the conductor surface-a property which has been utilized in the development of SP biosensors. 9,10 In contrast, at frequencies well below the metal plasma frequency, SPs exhibit very weak field confinement.11 Semiconductors, on the other hand, exhibit a plasma frequency that depends on the conduction band electron density, so that the properties of semiconductor SPs can be tailored within the terahertz frequency range through doping 12 and photoexcitation. 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. 16In 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. We show that the SP on InSb is significantly more sensitive to the dielectric layer than surface modes supported on a gold substrate. Moreover, ...

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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...

We measure the enhanced transmission of Terahertz radiation through a metal film perforated with arrays of subwavelength holes of varying hole size. By measuring transmission spectra in the time domain and comparing our experimental results to a rigorous modal-matching model, we are able to assess the relative contributions of resonant and nonresonant transmission channels. We see that the contribution of the resonant transmission becomes more important with decreasing hole size because the lifetime of the surface mode mediating the transmission is increased with reducing hole size. Using low-temperature measurements to control the nonradiative loss levels in our system, we show that losses limit the lifetime of the surface mode, thereby limiting the resonant transmission intensity for the smallest holes.

We modulate the transmission of terahertz ͑THz͒ radiation through periodic arrays of subwavelength holes in a metallic film by using pulses of visible-wavelength light to photoexcite the semiconducting substrate of the hole arrays. By varying the photodoping level of the semiconductor we are able to switch off the resonant transmission of THz radiation through the array. By varying the size of the holes, we demonstrate the crucial role that surface modes play in the resonant transmission and ultimately in the photomodulation behavior of these structures. We demonstrate that the surface-wave transmission mechanism can allow for very efficient optical modulation of radiation transmission. DOI: 10.1103/PhysRevB.80.193412 PACS number͑s͒: 71.45.Gm, 41.20.Jb, 84.40.Ϫx Photonic structures which incorporate some degree of dynamic control over their electromagnetic properties 1,2 are interesting for numerous reasons. Some structures have direct applications in proposed photonic devices; in others the dynamic control can provide direct evidence for transmission pathways and for the role of material properties in determining the behavior of a structure. 3 Manipulating material properties optically 4-10 is of particular interest, as changes to the structure can be made on the same time scale as the transit of light pulses through the system.One very fundamental photonic structure is an array of subwavelength holes perforated in a conducting screen. Such arrays can exhibit narrow transmission resonances 11 for wavelengths determined by the periodicity of the array-this is known as extraordinary optical transmission or EOT. The mechanisms underlying EOT in these arrays have been the subject of considerable debate, 12-14 however consensus has gradually emerged that for many structures the transmission is mediated at least in part by electromagnetic surface modes at the interface between the perforated conducting screen and the dielectric layers by which it is bound. 15,16 In this contribution we use pulses of visible light to modulate the transmission of terahertz ͑THz͒ radiation through periodic arrays of subwavelength holes in a metallic film fabricated at the interface of a substrate of crystalline silicon. By varying the photodoping level of the silicon we are able to switch off EOT of THz radiation through the array. By varying the size of the holes we are able to explain the photomodulation effects in terms of the properties of the surface mode which mediates the enhanced transmission; in particular, we can make a direct link between the lifetime of the surface mode and the magnitude of the photomodulation. We show that if we extend the surface-mode lifetime by minimizing losses and reducing the hole size it is possible to attain photomodulation levels which are orders of magnitude greater than those found for a plain silicon surface.The hole-array structure we shall consider in this work ͑Fig. 1͒ is formed from a 150-nm-thick film of gold on a silicon substrate. The gold is perforated with a square lattice ͑pitch 10...