Tunable Nanoscale Plasmon Antenna for Localization and Enhancement of Optical Energy. Douglas Howe

Transcription

1 Tunable Nanoscale Plasmon Antenna for Localization and Enhancement of Optical Energy Douglas Howe Applied Optics Spring 2008

2 Table of Contents Abstract... 3 Introduction... 4 Surface Plasmons... 4 Nano Fabrication... 5 Optical Antenna... 6 Analogy to dipole antenna... 6 Tunable Aspects... 7 Conclusion... 7

3 Abstract This paper explores the method upon which nanoscale silver arrays are being engineered to provide a localized response that is dependant on the wave length of the incident light being used. The coupling of optical energy with the surface plasmons that occur on the surface of metals is discussed as a method to confine light in a way that circumvents the fundamental diffraction limit to sub wave length dimensions. The operation of these nanoscale arrays will be described by using the analogous of how dipole-array radio antennas operate. The nanoscale fabrication technique and the tunable design implications of these nanoscale arrays are examined.

4 Introduction Currently, in applied optics, the resolution of controlled light is limited by the fundamental diffraction limit. That is that light can not be focused in to a dimension that is smaller than the wavelength of the light. This practical limit is a current roadmap barrier that many industries are battling. Breaking through this fundamental diffraction limit will enable great advances across many industries, from increased resolution for optical imaging to non-linear optics 1 to plasmonic circuits. 2 Optical antennas are being developed as a way to localize the optical energy that couples with surface plasmons that exist on nanoscale metal structures. Gold and silver nanoparticles are of particular interest to the biomedical industry due to the compatibility with the human body. 3 The physics of the optical antenna is analogous to the multi element array antenna that works at radio wavelengths. The ability to control the geometry of the nanoscale particles of metal to dimensions smaller than the wavelength of the incident light is critical for tuning the device and controlling the localization of the energy. Modern manufacturing methods such as electron beam lithography and ion beam milling are being used in the laboratory setting to produce the devises. I chose this topic because of the spot size limitations experienced when working as a mechanical engineer on the application of lasers for micromachining. Any way to circumvent the diffraction limit that currently limits spot size of lasers and allows production of laser spots diameters less than the wavelength will greatly reduce the cost of micro and nanomachining application that are currently done with more expensive lithography methods. Surface Plasmons The ability to focus or confine light into dimensions smaller than the wavelength of light relies on surface plasmons that exist on the surface of metals. Generally speaking, when electromagnetic waves couple with a electric or magnetic dipole, a polariton is produced. Then the coupling is between light and surface plasmons the polaritons and called surface plasmon polaritons (SSP s.) Prior to understanding SPP s, the use of metals to transmit light did not look promising due to the associated losses. 4 These losses are mitigated by confining the SSP s in a thin layer between the metal and a dielectric material. At this location there are minimal free electrons (when compared to the losses into the bulk material) for the waves to collide with. 2 For surface plasmons to convey light energy the incident electromagnetic waves at optical wavelengths must be coupled with the surface plasmon. The initial incident light can trigger an electron density wave or surface plasmon. 5 Depending on the metal being used, some frequencies exactly fit the sphere and set up a resonant normal mode of vibration. At this scale they are predominantly dipole resonances.

5 The application of SSP s is not new. Without realizing their existence or understanding the physics behind surface plosmons, the creator of the Lycurgus Cup (4 th century A.D.) used their strange properties, when interacting with light, to create a shift in color. When the cup is illuminated from the outside it is green in color. (Figure 1) When illuminated for the inside, however, the cup glows red. (Figure 2) This is due to the presence of nanoscale gold particles that absorb light a 520 nm. 5 Figure 1: Lycurgus Cup illuminated from the outside Figure 2: Lycurgus Cup illuminated from inside Nano Fabrication The oriention of the nanosize metal particles in the array is explored as a means of transmitting and enhancing the field created from the coupling of the light into the SSP s. A general diagram of the optical antenna design is illustrated below. (Figure 3) The material used by Waele et al 1 was silver. The silver dots were fabricated by electron beam lithography and lift off techniques on fused silica. Other techniques such as focused ion beam milling can also be used for fabrication. 2,3 The particle array is composed of 10 silver particles with a 110 nm diameter. The spacing is less that the incident wavelength of the light at 150 nm from centerline to centerline. This is critical to obtain the constructive interference needed to propagate the energy along the chain of particles. 1,2

6 Figure 3: Overview of optical antenna. Optical Antenna Analogy to dipole antenna As a tool to help understand how nano particle arrays act to enhance and localize light into sub wavelength spaces, the analogy to dipole array radio antennas is made with the optical antenna operating at the nanoscale. Radio dipole antennas rely on array elements that act as dipole scatters. These elements are spaced at a distance such that constructive interference, or geometric resonance, occurs around the operating frequency. With the optical antenna, the resonance occurs as a material response. 1 The location of the enhanced field is dependent upon the material, geometry and incident wavelength of light. 1,6 Figure 4: a. Multi element array antennas concentrate electromagnetic wave to a point. b. optical antenna localize and enhance the coupled light to a point smaller than the wavelength of the incident light. c. The location is highly dependent on the incident wavelength of the light.

7 Tunable Aspects In the design shown in figure 3 there is a cross over frequency that occurs where the enhanced field emission occurs. Below about 670 nm incident light the localization occurs to the left of the array. Above 670 nm the location switches to the right side of the array. (Figure 4c) The physics responsible were modeled by Waele at el with good results (Figure 5). Phase retardation and interference are crucial for the phenomenon in Figure 5 to occur. 1 The theory had to take into account both near and far field interactions to accurately model the phenomenon. 1,4,6 Figure 5: Comparison of Experimental and Theoretical modeling of the optical antenna. Conclusion Nano photonics research and understanding has increased dramatically in the past decade with the promise of broad applications. With the advancing ability to direct and control the movement and intensity of the couple light described above applications in communication and imaging are especially intriguing. Having the ability to focus light into sub optical wavelength spaces could allow for the development of optical imaging devices that provide the resolution currently obtained by X-ray wavelength devises but at optical frequencies. There is also research and development of a SPASER (Surface Plasmon Amplified by Stimulated Emission of Radiation) that, in theory can produce a beam 10,000 times smaller than the human hair. 7 (Figure 6)

8 Figure 6: Characters machined in a human hair with a UV laser spot 15 times smaller than the diameter of the hair. Image a spot 10,000 times smaller.

9 References 1 Rene de Waele, A. Femius Koenderink, and Albert Polman, Tunable Nanoscale Localization of Energy on Plasmon Particle Arrays, Nano Letters 7, 2004 (2007). 2 Harry A. Atwater, The Promise of Plasmonics, Scientific American, March Boris Khlebtsov, Vitaly Khanadeev, Vladimir Bogatyrev, Lev Dykman, and Nikolai Khlebtsov, Engineering of plasmon-resonant nanostructures for biomedical applications, Tenth International Conference on Light Scattering by Non-spherical Particles, Conway, J. A., Plasmons and Nonofocusing Plasmonic Focus April 26, Park, Sung Yong and Stroud, D., Theory of melting and the optical properties of gold/dna nanocomposites Phys. Rev. B 67, (2003) 6 Niek F. van Hulst, "Light in chains " Nature 448, 141 (2007). 7 Bergman, D., Stockman, M., SPASER as Ultrafast Nanoscale Phenomenon and Device Ultrafast Phenomena XIV-Proceedings of the 14th International Conference, Niigata, Japan, July 25 30, 2004