Surface Plasmons/Optical Antennas

Coupling between Surface Plasmons

Plasmon sensors, waveguides and filters usually employ collections of nanoparticles. Compared to single particles, collections of particles allow the averaging of a signal over several particles, thereby increasing intensity and mitigating against defects. Practical plasmon nanodevices therefore usually rely heavily on arrays of noble metal nanoparticles. Interactions between nanoparticles, however, are often overlooked, despite their importance. We have recently measured the dependence of the surface plasmon frequency of metal nanoparticle chain arrays on the k -vector of the incident illumination.

The illumination of a metal nanoparticle results in the excitation of surface plasmons. Electrons in the metal move under the influence of the incident electric field, creating regions of positive and negative surface charge. This results in the generation of dipole fields around the nanoparticle. As shown in Fig. 1a, this leads to coupling between the surface plasmons of closely-spaced metal nanoparticles. This can significantly modify the plasmon frequencies and the local field distributions. We have recently measured the dispersion relations of metal nanoparticle chain arrays. The dispersion relation specifies the dependence of the surface plasmon frequency of metal nanoparticle chain arrays (Fig. 1b) on the k -vector of the incident illumination (Fig. 1c) [1]. This is important because of the prevalence of coupling phenomena in plasmonics, with examples that range from molecular rulers to optical antennas, surface enhanced Raman scattering (SERS) and metal nanoparticle chain waveguides.

Fig. 1 . a). Plasmon resonances of metal nanoparticle modified by fields from nearby nanoparticle. b). Fabricated gold nanoparticles. c). Dispersion relations: experiments & theory.

Optical Antennas

Optical techniques are well established at the macro-scale, but difficult to apply on the nanoscale. This is due to the mismatch between the wavelength of light, and the dimensions of nanostructures. On the other hand, the possibility of photonics-based tools for observation, measurement and manipulation are an exciting prospect for nanotechnology and nanoscience. Surface plasmon nanostructures functioning as antennas (“optical antennas”) present an opportunity to bridge the length scales of the wavelength of light, and the dimensions of nanostructures. Optical antennas enable electromagnetic energy to be concentrated into deep sub-wavelength regions. As such, they present a number of opportunities for ultra-sensitive spectroscopy, near-field scanning optical microscopy and compact subwavelength light sources.

Our investigations of optical antennas began in the mid-infrared wavelength range [1]. Numerical simulations were carried out using the finite-difference time domain (FDTD) method. Key results are shown as Fig. 2. The simulated optical antenna (Fig 2a) was 1.56 micron long, made from gold (60 nm thick), and on a Si substrate. It was illuminated with a plane wave incident from within the Si substrate at lambda = 10.375 micron. In Fig. 2b, the calculated electric field intensity ( <|E|^2 > ) on the top surface of the antenna is shown. The intensity at the tip is ~3905 times that of the illumination, and confined to a region at the tip of ~280 nm (lambda/37) by ~160nm (lambda/66).

Fig. 2 (a). Simulation geometry of optical antenna, & polarization of illuminating plane wave. (b). Calculated electric field intensity at top surface of antenna. (c). Zoom-in of antenna tip.

Gold antennas were fabricated on silicon substrates (Fig. 3a). To determine the antenna resonant wavelength, we measured the transmission spectra of fabricated antennas. From Fig. 3b, it can be seen experimental and FDTD-calculated extinction spectra are in excellent agreement [1].

Fig. 3 a). Scanning Electron Micrograph of gold optical antenna fabricated on silicon substrate. b). Extinction cross section of optical antenna. Black – experiment, green – FDTD model

Our experimental results at mid-infrared wavelengths encourage us to scale our antennas to smaller dimension to allow them to function at shorter wavelengths, and to develop new applications and structures.

In the area of spectroscopy, we are developing periodic arrays of optical antenna structures for surface-enhanced Raman scattering. We envisage that these structures will offer exceptionally high Raman enhancements, but with the reproducibility needed in practical sensor applications.

For near-field scanning optical microscopy, we are developing microfabricated optical antenna probes. These consist of atomic force microscope (AFM) – type probes with optical antenna nanostructures fabricated at the ends of the tips.

In collaboration with Professor Capasso's research group, a new device termed a plasmonic laser antenna has been developed. Plasmonic laser antennas consist of semiconductor lasers with optical antenna structures fabricated on the facets. These devices have been demonstrated at near-infrared and mid-infrared wavelengths. A schematic of the near-infrared device is shown in Fig. 4a, and consists of a pair of gold nanorods (Fig. 4b) fabricated on the facet of a laser. Light from the laser strikes the antenna, exciting the surface plasmon resonance. This generates an enhanced field in the antenna gap.

Fig. 4 a). Schematic of near-infrared plasmonic laser antenna. b). Gold nanorod optical antenna. From Cubukcu et al [3].

To characterize the field confinement capabilities of the antennas, near-field scanning optical microscopy (NSOM) was performed. A near-field image is shown as Fig. 5a, and a linescan through the data is shown as Fig. 5b. The results demonstrated a spot in the antenna gap of ~40 nm ~100 nm. The area of the spot is more than fifty times smaller than a diffraction-limited spot at the same free-space wavelength (lambda = 830 nm).

Fig. 5 a). Near-field image of near-infrared plasmonic laser antenna (from photodiode signal). b). X-axis line section of data. From Cubukcu et al [3].

An important attribute of the optical antenna approach is that it does not place any constraints on the design of the laser. In collaboration with Professor Capasso's group, the integration of optical antennas onto lasers operating in the mid-infrared spectral range has been demonstrated [4, 5]. Gold nanorod antennas were fabricated on the facets of quantum cascade lasers and characterized using near-field microscopy (Fig. 7a). AFM and NSOM images of the device are shown as Fig. 6b. From the NSOM image, it can be seen that the antenna concentrates the fields to the ~100 nm –wide gap. This work was selected for the October 2007 cover of Applied Physics Letters (Fig. 6c).

Fig. 6. Quantum cascade laser with gold nanorod optical antenna fabricated on facet for field concentration. a). Experimental set-up for imaging near-fields of antenna. b). Top: AFM scan of antenna, consisting of two gold rods (each 1.2 m m long) separated by a ~100 nm gap. Middle: Near-field optical image of antenna, showing ~100nm wide spot in gap. Bottom: Linescan of optical near-field signal along long axis of antenna. c). Paper chosen for APL cover, Oct 2007. From N. Yu et al , [4]

REFERENCES

[1] K.B. Crozier, E. Simsek, E. Togan and T. Yang, “Experimental Measurement of the Dispersion Relations of the Surface Plasmon Modes of Metal Nanoparticle Chains,” Optics Express, vol. 15 , 17482 (2007)

[2] K.B. Crozier, A. Sundaramurthy, G.S. Kino and C.F. Quate, “Optical antennas: resonators for local field enhancement,” Journal of Applied Physics 94 , pp. 4632-4642 (2003)

[3] E. Cubukcu, E.A. Kort, K.B. Crozier, and F. Capasso, “Plasmonic Laser Antenna,” Applied Physics Letters vol. 89 , 093120 (2006)

[4] N. Yu, E. Cubukcu, L. Diehl, M.A. Belkin, K.B. Crozier, F. Capasso, D. Bour, S. Corzine, and G. Hofler, “Plasmonic Quantum Cascade Laser Antenna, “ Applied Physics Letters 91 , 173113 (2007)

[5] N. Yu, E. Cubukcu, L. Diehl, D. Bour, S. Corzine, J. Zhu, G. Hofler, K.B. Crozier and F. Capasso, “Bowtie Plasmonic Quantum Cascade Laser Antenna, “Optics Express 15 , 13272 (2007)