Tunable plasmonic crystals of heavily-doped conducting polymers promise novel devices
The research field known as plasmonics has become an area of increasing interest recently due to the highly-developed nature of micro-fabrication technologies. Using nano-structured metallic materials, it is possible to manipulate surface plasmon polariton (SPP) excitations on the surface of metallic films, creating a broad range of opportunities to fabricate novel optical devices.1 In 1998, Ebbesen et al.reported the SPP-assisted phenomenon of `anomalous transmission' through optically-thick metallic films perforated with two-dimensional (2D) subwavelength hole arrays. They found that the optical transmission through such films exceed unity when normalized by the aperture fill fraction.2 Since this discovery, numerous studies have been carried out that explore both fundamental issues and potential device applications.
To date, ‘anomalous transmission’ has been investigated primarily using metallic or doped semiconductor thin films. Heavily-doped organic conducting polymers—such as polyacetylene, polyaniline, and polypyrrole (PPy)—are another, more exotic class of conductors. These materials exhibit a metal-insulator transition at high doping levels of a few percent.3 We have recently demonstrated the observation of ‘anomalous transmission’ through thin films of heavily-doped PPy perforated with a 2D subwavelength aperture array.4 Using conducting polymers with properties that can be widely tuned by chemical synthesis and doping, it is possible to fabricate a variety of novel optical devices with different characteristic properties and responses than plasmonic crystals using more traditional metallic and doped semiconductor films.
In our studies, we used a highly-conductive polymer: PPy doped with PF6 [PPy(PF6)]. The PPy films were polymerized and doped electrochemically with a measured DC conductivity of ∼ 200S/cm. The films were free-standing and has a thickness of ∼ 25 m. It has been postulated that heavily-doped PPy(PF6) has two plasma frequencies. The lower of these two frequencies appears to be caused by a Drude free-electron response and occurs in the terahertz (THz) spectral range.5 We therefore fabricated 2D aperture arrays in the PPy(PF6) films with lattice periodicities in the millimeter range, so that the expected SPP-assisted transmission band occurs in the THz range. The film area was about 3 × 3 cm2 and the fractional aperture area was ∼ 20%. The optical transmission was measured using a conventional THz time-domain spectroscopy technique.
Figure 1 shows the transmission spectra through two PPy(PF6) films with a hole-array lattice constants of 1mm and 1.5mm, respectively, compared to an unperforated film. Without the hole array, the PPy(PF6) film is nearly opaque. However, several transmission peaks are observed in the spectra of the fabricated plasmonic-hole array. For the film with 1mm periodicity, the transmittance at the first peak (∼0.27 THz) is almost 60%. This is much larger than the fractional aperture area, implying that ‘anomalous transmission’ was obtained. Moreover, the frequencies of the other transmission peaks agree very well with the conventional model.2 We therefore conclude that the so-called anomalous transmission peaks are related to resonant interactions with SPP. Our results indicate that, in spite of the intrinsic disorder, PPy(PF6) contains a large density of free carriers that allow direct excitation of SPPs in a manner that is similar to conventional metals.
From the viewpoint of device applications, we believe that our findings are quite exciting because of the possibility of in situ spectral tunability. Using an electrochemical process, the doping level can be readily controlled in a reversible manner using an externally-applied voltage. As a result, it would be possible to control the electrical conductivity of the doped, conducting polymer films, and consequently also the SPP transmission bands. This in situ project is currently being studied.
In conclusion, we have observed, for the first time, ‘anomalous transmission’ through heavily-doped organic conducting polymer films that were perforated with 2D subwavelength hole arrays. By using organic conducting polymers with characteristic properties that are widely tunable via chemical synthesis or doping level variations, we should be able to fabricate a variety of novel optical devices that may not be possible using metallic and doped semiconductor films.
