ACTIVE SURFACE PLASMONS: Tuning of surface plasmons leads to new optoelectronic devices
Surface plasmons can be thought of as light waves trapped and traveling along the interface between a metal and a dielectric material.
Surface plasmons can be thought of as light waves trapped and traveling along the interface between a metal and a dielectric material. However, the surface plasmon (SP) is not merely a guided light wave but, in fact, a hybrid excitation consisting of collective oscillations of electrons in the metal coupled to an electromagnetic wave at the metal surface. Interest in surface plasmons has surged recently because of their potential in applications ranging from displays to sensing to on-chip communication. The common thread in each of the proposed SP applications is the desire to exploit the dual nature of the surface plasmon itself. As a hybrid electronic and optical wave the surface plasmon represents-not only symbolically but practically-a merging of electronics and photonics. As electronic components shrink, and as we strive to observe and detect ever-smaller amounts of matter, the control and enhancement of the interaction between light and this subwavelength-scale matter becomes an essential part of the next generation of electro-optical devices.
Surface plasmons are generated when incident radiation, at certain frequencies, couples to the surface mode at the metal/dielectric interface. By solving Maxwell’s equations at such a boundary, for a given wavelength and angle of incidence of the incoming radiation, we can derive the allowed frequencies for this coupling. The geometry of a metal structure or the patterning of a metal film can alter the properties of the allowed surface plasmons, resulting in unique absorption or transmission properties (see Fig. 1).
Surface plasmons are the basis of well-established technologies, such as surface-enhanced Raman spectroscopy (SERS), which has been used to demonstrate the detection of single molecules.1 In SERS, radiation is coupled to surface plasmons in metallic nanostructures attached to molecules, and the interaction between the surface excitation and the molecule of interest is enhanced in the near field, making SERS many orders of magnitude more sensitive than traditional Raman spectroscopy. Surface plasmons, it is argued, are also responsible for the novel phenomenon known as extraordinary optical transmission (EOT), which has shown significant promise for detector, display, and sensing applications.2
In EOT, orders of magnitude more light can be transmitted through a periodic array of subwavelength apertures in a metal film than is predicted by classical aperture theory. Incident light couples to surface-plasmon modes at the interface between the patterned metal and the dielectric, and is then re-emitted on the opposite side of the metal film. The spacing of the subwavelength apertures determines the frequency of the light transmitted through the metallic grating. In an EOT grating designed for transmission of light at a wavelength of 9 µm, metal covers 80% of the surface area of the device. However, the primary transmission peak at 9 µm (approximately 1100 cm-1) shows a transmission of 25%, indicating that light incident on the normally opaque metal portion of the structure is transmitted through the grating (see Fig. 2).
Despite the dynamic nature of current surface-plasmon research, state-of-the-art devices based on surface plasmons are almost all passive. To truly integrate surface-plasmon technology with current photonic and electronic technologies, and to maximize the utility of SP-based devices, active devices are desired. In addition, these devices would ideally be integrable with the semiconductor technology that is the basis of modern photonics and electronics. Recent research has demonstrated the feasibility of integrating surface-plasmon structures with semiconductors and taking advantage of semiconductor properties to develop a new class of active surface-plasmon devices.3, 4
This research utilized EOT gratings as the vehicle to demonstrate active control over surface-plasmon resonances. The devices operate in the mid-infrared (mid-IR) spectral range, a wavelength region of vital importance for numerous sensing and security applications. The mid-IR also provides an optimal test bed for the development of novel structures at other shorter wavelengths. Because the device features scale as a function of operating wavelength, mid-IR SP devices can be fabricated using traditional optical lithography. This allows device fabrication and characterization to proceed at a much faster rate than in the visible/near-IR, where time-consuming and expensive patterning techniques, such as focused ion-beam milling and electron-beam lithography, are required.
To demonstrate active tuning of plasmonic structures, our research group deposited patterned metallic films upon epitaxially grown gallium arsenide (GaAs) semiconductor structures. Tuning the permittivity of the GaAs substrate upon which the metal films are fabricated controls the surface-plasmon resonance. Initial work investigated the transmission through EOT grating structures grown on GaAs epilayers of varied doping. In the mid-IR, the permittivity of GaAs is strongly dependent on the free-carrier concentration in the material, which can be controlled by changing the doping concentration of the semiconductor. Thus, by changing the doping of the semiconductor, the frequency of the SP allowed at the metal/GaAs interface is shifted. The transmission spectra for an EOT grating designed to transmit at 9 µm, deposited upon four separate epitaxially grown wafers with varying doping concentrations shows that the peak of the transmission shifts significantly with doping (by approximately 37 cm-1), indicating a change in the properties of the surface-plasmon resonance responsible for the EOT effect (see Fig. 3, left).
The demonstration of carrier-concentration tuning of surface-plasmon resonances is a significant step toward active control of plasmonic devices. The results from the above experiment do not simply verify the effect of the permittivity of the dielectric material on the surface-plasmon resonance, they offer a direct path toward electronically tunable surface-plasmon-based devices. After all, the control of carrier densities is the basis of modern electronics, as exemplified by the field-effect transistor (FET), in which a gate bias determines the carrier concentration, and thus the conductivity of a semiconductor material between a source and drain contact (see “Plasmon grating-gate devices have potential as tunable terahertz detectors,” p. 131). When we apply a bias to an EOT grating that also serves as a gate contact, we can control the carrier density, and thus the permittivity of the underlying semiconductor, shifting the resonance of the surface plasmon at the dielectric/gate interface. Such a device, known as an extraordinary-transmission field-effect transistor (ETFET), would allow high-speed electronic control of plasmonic devices.
As a first step toward the demonstration of an ETFET, we have fabricated EOT grating devices with source and drain contacts. In these devices, current is run between the source and drain contact through a thin (100 nm), highly doped GaAs layer. The doped GaAs effectively acts as a strip resistor, and as current flows through the layer, it heats up. The temperature change results in a change in the permittivity of the GaAs, which in turn alters the frequency of the surface-plasmon resonance at the interface between the heated GaAs and the metal grating. By actively controlling the source-drain current, the transmission spectrum of the EOT device can be tuned (see Fig. 3, right). We have observed peak shifts in such EOT grating devices of over 25 cm-1 at the highest powers (corresponding to 200 mA current and a local device temperature of approximately 600°C). Not only is such a device among the first electrically tunable plasmonic structures, but its tuning mechanism can be utilized in the mid-IR, as well as at shorter wavelengths of interest.
To use these tunable plasmonic devices as active optical components, research must address the transmission losses. Fortunately, research has seen only a small decrease in transmission peak strength (approximately 25%) as the device temperature increases. These relatively minor losses are most likely due to increased absorption in both the thick (and now heated) GaAs substrate and at the interface between the conducting GaAs epilayer and the metal grating. Burying the highly doped GaAs layer under an undoped surface layer would decrease the overlap of the SP field with the lossy free carriers of the doped layer, while still allowing for resistive heating of the sample. Losses in the GaAs substrate can be minimized by moving toward thinner devices, which, at the same time, will reduce the thermal mass and electrical power requirements for operation.
The demonstration of electrically controlled temperature tuning of surface-plasmon-based EOTs devices is an important step in the development of plasmonic technology. The development of semiconductor-based tuning mechanisms for mid-IR SP resonances is important for a variety of active mid-IR photonic devices including filters, emitters, and detectors. Furthermore, the research has implications for a wide range of frequencies, as the tuning mechanisms could be used at near-IR and visible wavelengths. The results may be a new class of plasmonic devices with increased utility in numerous applications.
1. K. Kneipp et al., Phys. Rev. Lett. 78, 1667 (1997).
2. T.W. Ebbesen et al., Nature 391, 667 (1998).
3. D. Wasserman et al., Appl. Phys. Lett. 90, 191102 (2007).
4. E.A. Shaner, et al, Appl. Phys. Lett., 91, 181110 (2007).
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DAN WASSERMAN is assistant professor of physics and assistant director of the Photonics Center at the University of Massachusetts Lowell, One University Ave., Lowell, MA 01854; e-mail: email@example.com; www.uml.edu/physics.