Observation of a New Type of THz Resonance of Surface Plasmons Propagating on Metal-Film Hole Arrays

Qu, Dongxia; Grischkowsky, D.

doi:10.1103/physrevlett.93.196804

Cited by 67 publications

(31 citation statements)

References 38 publications

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“…The Ag film had much higher conductivity than Al and Au films of the same thickness, which may be technologically important. These results are of particular importance to research studies with thin metal film THz surface plasmons [13][14][15] and metamaterials. 16 This work was partially supported by the National Science Foundation.…”

Section: ͑2͒mentioning

confidence: 99%

Terahertz conductivity of thin metal films

Laman¹,

Grischkowsky²

2008

Applied Physics Letters

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183

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The conductivities of thin Al, Au, and Ag films were measured via their transmission at terahertz frequencies. The conductivities of all the films, particularly the thinner films and Al films, were much smaller than their bulk dc values. This reduced conductivity can be quantitatively understood in terms of an increased scattering rate from defects. The transmission is consistent with a frequency independent conductivity, implying a very fast electron scattering time. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2968308͔The conductivity of metals at terahertz ͑THz͒ frequencies is becoming increasingly important as both microcircuit technology and microwave resonators are being driven at ever-higher frequencies. Our earlier work with terahertz metal parallel-plate waveguides 1 has shown that the conductivity of Al and Cu at THz frequencies is considerably less than the expected handbook value, particularly at cryogenic temperatures, due to defects within the 100 nm skin-depth layer. We have now extended and confirmed these results by measuring the terahertz transmission of a number of thin films comprised of different metals and different thicknesses. Unlike our previous work, this allows us to measure the conductivity of the thin film as a function of the thickness, since we are probing the entire film and not just the skin-depth layer.Thin films of Al, Au, and Ag were deposited on highresistivity, 2 in. diameter, 0.43 mm thick Si wafers via thermal evaporation. The typical evaporation rates were 2 nm/s onto an unheated wafer in order to ensure a relatively high conductivity film. The thickness of the film was measured via a quartz thickness monitor. Six films were made in total: Al films of 36, 88, and 152 nm thickness, Au films of 85 and 150 nm thickness, and an Ag film of 86 nm thickness. The films were not annealed after evaporation. The metal coated wafers were placed in a standard terahertz time-domain spectroscopy ͑THz-TDS͒ apparatus.2,3 Their transmission was measured at both 295 and 77 K. The terahertz pulses and their amplitude spectra are shown in Fig. 1.The amplitude transmission at a frequency f of the films and wafers can be modeled by the thin film formula 4 t = t 12 t 23 t 34 exp͓i͑2hf/c͒n 2 ͔exp͓i͑2df/c͒n 3 ͔ 1 + r 12 r 23 exp͓2i͑2hf/c͒n 2 ͔ , ͑1͒where t ij and r ij are the complex Fresnel transmission and reflection coefficients, 4 respectively, medium 1 and 4 are vacuum, medium 3 is Si with a thickness d = 0.43 mm and with a THz frequency independent index n 3 = 3.4175 and negligible terahertz absorption, 5 and medium 2 is the metal film with a thickness of h and a complex index 4,6 of n 2 = ͑1 + i͒ ͱ /͑4 0 f͒. ͑2͒Propagation within the metal is described by exp͓ikz− t͔ where k = ͑2f / c͒n 2 . Using Eq. ͑2͒, one obtains the ampli-Note that ␣ is simply 1 / ␦, where ␦ is the usual result for the skin depth.6,7For our case of a 88 nm Al film with a conductivity of =16͑⍀͒ −1 , n 2 = ͑1+i͒370 at 1 THz, the resulting amplitude attenuation traversing a single pass within the metal film ͓i.e....

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Section: ͑2͒mentioning

confidence: 99%

Terahertz conductivity of thin metal films

Laman¹,

Grischkowsky²

2008

Applied Physics Letters

Self Cite

183

109

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“…In the terahertz regime, SPs have recently attracted much attention and become an attractive new area [10][11][12][13][14][15][16][17][18][19]. Due to a drastic increase in the value of dielectric function ε m = ε rm + iε im , most metals become highly conductive at terahertz frequencies.…”

Section: Introductionmentioning

confidence: 99%

Resonant terahertz transmission in plasmonic arrays of subwavelength holes

Zhang

2008

Eur. Phys. J. Appl. Phys.

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Abstract.A review of transmission properties of two-dimensional plasmonic structures in the terahertz regime is presented. Resonant terahertz transmission was demonstrated in arrays of subwavelength holes patterned on both metals and semiconductors. The effects of hole shape, hole dimensions, dielectric function of metals, array film thickness, and a dielectric overlayer were investigated by the state-of-the-art terahertz spectroscopy modalities. Extraordinary terahertz transmission was demonstrated in arrays of subwavelength holes made even from Pb, a generally poor metal, and having optically thin thicknesses less than one-third of a skin depth. We also observed a direct transition of a surface plasmon resonance from a photonic crystal minimum in a photo-doped semiconductor array. According to the Fano model, transmission properties of such plasmonic arrays are characterized by two essential contributions: resonant excitation of surface plasmons and nonresonant direct transmission. Plasmonic structures will find fascinating applications in terahertz imaging, biomedical sensing, subwavelength terahertz spectroscopy, and integrated terahertz devices. PACS

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“…For our Al sheet we assume the handbook value of dc conductivity 0 = 3.54ϫ 10 5 / ͑⍀ cm͒, and at 0.4 THz obtain the complex dielectric constant from Drude theory as = r + i i = −3.3ϫ 10 4 + i1.6ϫ 10 6 . 20,21 For this case k r = k 0 =2 / 0 to an accuracy of 1 part in 10 5 , where 0 is the free-space wavelength, and the relationship for k i = ␣ simplifies to ␣ = k 0 / ͑2 i ͒. At 0.4 THz we obtain the extremely small value ␣ = 0.000 026 cm −1 , corresponding to the very large, surface-wave propagation length of L ␣ =1/␣ = 38 500 cm, 900 times larger than experiment ͑L ␣ = 43.5 cm͒.…”

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THz Zenneck surface wave (THz surface plasmon) propagation on a metal sheet

Jeon

Grischkowsky

2006

Applied Physics Letters

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195

107

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We present an experimental study of the propagation of the THz Zenneck surface wave on an aluminum sheet, now more commonly denoted as the THz surface plasmon ͑TSP͒. Here, the TSP pulse is generated by coupling the THz pulse from a metal parallel-plate waveguide onto the aluminum sheet; the propagated TSP pulse is detected at the output end of the sheet using a standard photoconductive dipole antenna. We separate the associated free-space THz pulse from the TSP pulse using a curved sheet. The observed weakly guided TSP propagation has the expected low group velocity dispersion, but also has anomalously high attenuation and much tighter binding to the metal surface than predicted by Zenneck theory. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2171488͔ Surface electromagnetic ͑EM͒ waves have now been studied for more than a century, starting with Sommerfeld's study of EM propagation on a single metal wire, 1 and including Zenneck's description of EM propagation on a flat metal surface.2 An excellent overview of the early EM surface wave investigations is the work by Barlow and Cullen.3 A more recent work also provides a good description of EM surface wave measurements and experimental techniques.4 A good description of EM surface waves from the equivalent point of view of surface plasmons is given in Ref. 5. Most recently, the study of surface plasmons has been stimulated by the observation of unusually high transmission resonances through thin metal subwavelength hole arrays at optical frequencies, 6 which have now been studied in the optical, infrared and THz regions. Here, we describe an experimental study of THz pulses propagating as surface waves, or equivalently as THz surface plasmons ͑TSP͒, on a metal sheet. We measure a much higher attenuation of the propagating TSP pulses and a much reduced spatial extent of the TSP evanescent field than predicted by theory. In previous work, such pronounced disagreement between theory and experiment has resulted in a long standing and unresolved controversy. [8][9][10][11][12][13][14][15] It has been experimentally difficult to distinguish between freely propagating EM radiation along the surface and the guided surface wave. [8][9][10][11][12][13][14][15] Due to the collinear propagation and equal phase velocities, power transfer between the two waves easily occurs. Extremely flat and optically smooth surfaces appear to be required to obtain the predicted large propagation distances; 9,12 surface roughness has been predicted to bind the wave more tightly to the surface and thereby increase the attenuation.12 Submicron layers of high-index, low-loss dielectrics on the metal surface can reduce the extent of the evanescent field by an order of magnitude, causing much higher propagation loss. 10,11,14 Our experimental study involved measuring TSP propagation on 10-cm-wide by 51-m-thick Al sheets of different lengths with smooth, but not polished surfaces. As shown in Fig. 1͑a͒, the initially freely propagating THz pulses were collimated and focused into the wavegui...

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Observation of a New Type of THz Resonance of Surface Plasmons Propagating on Metal-Film Hole Arrays

Cited by 67 publications

References 38 publications

Terahertz conductivity of thin metal films

Terahertz conductivity of thin metal films

Resonant terahertz transmission in plasmonic arrays of subwavelength holes

THz Zenneck surface wave (THz surface plasmon) propagation on a metal sheet

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