Optical Properties of Metal Nanogratings

Research project P 14292 Optical Properties of Metal Nanogratings Franz R. AUSSENEGG 06.03.2000 Up to now the optical properties of surface plasmons in individual `0,1 and 2D` metal structures (i.e., particles, wires and thin films) have been investigated and a consistent framework for their theoretical description has been developed. The same holds for the effect of near-field coupling. In contrast, optical phenomena connected to ensembles of regularly arranged metal nanostructures with distances between the structures that exceed the distance that allows near-field coupling remain experimentally unexplored. However, there is theoretical evidence that grating effects which rise on regularly patterned samples influence the plasmon characteristics of the individual nanostructures strongly. On the other hand such systems where a large number of nanostructures is implied are of paramount interest for practical applications. In this project the theoretical findings of Meier et al. [J.Opt.Soc.Am.B2,931(1985)] should be verified experimentally. For metal nanostructures arranged regularly in a grating, they predict a significant influence of grating effects on the resonance of the particle plasmon (= localized surface plasmon in the metal nanoparticle), if the grating constant is in the range of the resonance wavelength. In particular a red shift of the resonance wavelength is predicted, if the resonance wavelength is approaching the grating constant, due to an almost in-phase addition of the .scattered light fields of the respective nanoparticles. If the grating constant is larger than the resonance wavelength, a significant broadening of the extinction band of the plasmon resonance should occur, due to the additional radiation damping caused by the radiative character of the first non-trivial grating order. The predicted phenomena will be investigated experimentally on electron-beam-lithographically fabricated 2-D- nanoparticle-arrays of gold and silver, respectively. Both, the red shift and the spectral broadening, will be investigated with a spectroscopy setup. The enhanced near-field due to the in-phase addition of the scattered light fields of the grating elements will be investigated with a Photon Scanning Tunneling Microscope (PSTM). The change in damping will additionally be examined by fs-time resolved measurements of the plasmon decay time by an appropriate autocorrelation technique. The understanding of grating induced changes of particle plasmon parameters promises a wide range of applications in the field of micro- and even nanooptics.

Within the framework of this project we studied the interaction of visible light with regularly arranged metal nanostructures. We determined the optical near-field and far-field properties of such metal nanogratings, accomplished by time-resolved measurements. Our results led to a comprehensive knowledge of the influence of the grating geometry on the interaction of light with such metal nanostructures. Advancing miniaturization implies to meet scientific and technological limitations. Concerning the defined manipulation of visible light, the step from micro- to nanooptics demands optical devices with dimensions smaller than the light wavelength, which for the visible spectral range translates to two thousendths of one millimeter. This requirement is fulfilled by metal nanostructures. Even with a diameters in the range of a few millionths of a millimeter they interact with visible light, as the light waves excites resonant motions of the conduction electrons of the metal, so called plasmons. In this project we investigated optical phenomena connected to ensembles of regularly arranged metal nanostructures. This is interesting regarding fundamental research as there exists theoretical evidence of new physical effects, that promise subsequently also a high application potential. Thus, we optimized the geometry of gold and silver nanowires and nanoparticles applying the method of electron-beam-lithography. With complementary experimental techniques we investigated the influence of the grating constant (i.e. the particle distance) on the interaction of the metal nanogratings with visible light, where main emphasize was laid on the excited plasmon resonances. The determination of the extremely short lifetime of such plasmon resonances of a few femtoseconds (a few millionths of a billionth second) demands elaborate methods. With a laser measuring technique we could verify a theoretically predicted grating effect, that augments the plasmon lifetime significantly. As the augmentation results in enhanced optical near-fields in the very vicinity of the metal nanostructures, this effect allows to improve existing methods of surface enhanced excitation of molecules, which is in particular interesting for biochemical sensorics. Detailed studies of the optical near-field distribution of such metal nanogratings have been performed with high spatial resolution by means of a PSTM (photon scanning tunneling microscope). The experimentally obtained intensity profiles agree excellently with our model calculations and led to a comprehensive knowledge of the near- field properties of such systems. Beyond the scope of this project we were able to investigate physical phenomena as the excitation of multipolar plasmon resonances and the physical fundamentals for a quasi two dimensional (chip like) plasmon optics. Our research results do not only attract interest among physical experts but also in public discussion. They are rated to be trend-setting for the development of nanostructured opto-electronic devices (see, e.g., The Economist, 24-10- 2002 ).