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Plasmonic Resonances in Metallic Nanoarrays Bachelor Thesis

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Plasmonic Resonances in Metallic Nanoarrays Bachelor Thesis Plasmonic resonances in metallic nanoarrays Bachelor Thesis Jana Huber Spring/Summer 2015 Faculty of Science Department of Physics First Evaluator: Prof. Dr. Elke Scheer Second Evaluator: Ass. Prof. Dr. Vassilios Kapaklis Department of Physics and Astronomy Materials Physics Supervisor: Ass. Prof. Dr. Vassilios Kapaklis Abstract The optical and magneto-optical response of plasmonic resonances in metallic nanoarrays out of square structures, either in holes or islands, were investigated. The excitation of the Bragg Plasmons (BP) takes place within a grating. Signicant dierences in the excited plasmon modes were seen by using p- or s-polarized light as well between the holes and islands sample. In order to investigate magneto-optical response from the magnetic nanostrucures, transverse magneto-optical Kerr eect (TMOKE) measurements were done with the result that there is a dierence in holes and islands sample. Contrary to what is generally expected for the polarization dependence of TMOKE, a TMOKE signal for s-polarized light on the holes sample was measured. Deutsche Zusammenfassung Plasmonen sind quantisierte Schwingungen des freien Elektronengases in einem Metall. Diese können an der Grenzäche zu einem Dielektrikum Oberächenplasmonen ausbilden. Dabei sind die Dichteschwingungen im Metall an das elektromagnetische Feld im Dielektrikum gekoppelt. In der folgenden Arbeit wurden plasmonische Resonanzen in verschiedenen Nanostrukturen untersucht. Diese unterscheiden sich in der Struktur (Löcher oder Inseln) wie auch in der Gröÿe. Da die Anregung über ein periodisches Gitter passiert, wurde die Periodizität der Nanostrukturen in allen Fällen gleich gehalten. Die Resonanzen wurden mittels optischen Messungen in Reektivität und Transmission untersucht. Dabei wurde jeweils p- sowie s-polarisiertes Licht verwendet. Auallend hierbei ist, dass es möglich ist mit den zwei verschieden Polarisierungsmöglichkeiten unterschiedliche Resonanzen anzure- gen, die jeweils in die Polarisierungsrichtung des elektrischen Feldvektors laufen. Ebenfalls wurden markante Unterschiede zwischen Löcher- und Inselstrukturen gefunden. Allerdings sind diese Strukturen nicht komplementär in ihren Gröÿen, weshalb die unterschiedliche Reaktion nicht nur eine Ursache der unterschiedlichen Struktur, sondern auch der unterschiedlichen Gröÿenverhältnissen sein könnte. Um darüber hinaus Aussagen über die magneto-optische Reaktion machen zu können wurden transversale magneto-optische Kerr-Eekt Messungen durchgeführt. Diese zeigen ebenfalls deutliche Unterschiede zwischen Löcher- und Inselstruktur. Für den transversalen magneto-optischen Kerr-Eekt wird das Magnetfeld parallel zur Probe und senkrecht zur Einfallsebene des Lichtes ausgerichtet, weshalb nur für p-polarisiertes Licht ein Signal er- wartet wird. Dementgegen wurde für die Löcherstruktur ein deutliches Signal mit s-polarisiertem Licht gemessen. Die Ursache dafür ist noch nicht erklärt. Contents 1 Introduction 1 2 Theoretical Background 2 2.1 Optics . 2 2.1.1 States of Light . 2 2.1.2 Light in Matter . 3 2.1.3 Optical Properties . 4 2.1.4 Reection and Transmission . 5 2.1.5 Babinet's Principle . 6 2.2 Plasmons . 7 2.2.1 Volume Plasmons . 7 2.2.2 Localized Plasmons (LSPs) . 8 2.2.3 Surface Plasmons Polaritons (SPPs) . 9 2.2.4 Excitation of Surface Plasmon Polaritons . 12 2.3 Magnetism . 13 2.3.1 Ferromagnetism . 14 2.3.2 Magneto-Optical Kerr Eect . 16 2.4 Sample . 20 2.4.1 Deposition . 20 2.4.2 Patterning . 20 2.4.3 Structure . 21 2.4.4 Grating Coupling for Square Structures . 22 2.5 Measurement Techniques and Equipment . 24 2.5.1 AFM . 24 2.5.2 TMOKE . 25 2.5.3 Experimental Set-Up . 26 3 Measurements, Results and Discussion 27 3.1 AFM-Measurements . 27 3.1.1 Calculation of the Amount of Metal . 28 3.2 Optical-Measurements . 28 3.2.1 Strength of Resonances γ ........................ 29 3.2.2 Reectivity . 29 3.2.3 Transmission . 36 3.2.4 Absorption . 38 3.2.5 Babinet's Principle . 39 3.3 Magneto-Optical Measurements . 40 4 Conclusion and Outlook 44 5 Acknowledgment 45 6 List of Figures 46 7 List of Tables 48 8 References 49 1 Introduction Optics, the science of light, has amounted to plenty of useful tools for the humanity. Ev- ery morning we take a look in the mirror, switch on the light and get the news from the internet over a glass ber directly to the breakfast table. Unfortunately the size of optical devices, like glass bers, are constrained by the diraction limit [1]. In order to avoid interference eects between two neighboring light waves, the width of the glass ber must be at least half of the light's wavelength [1]. For devices used in computer technology the sizes of optical components are a lot bigger than that of electronic devices. However they can carry a lot more data and are also faster than the electronic devices. If it would be possible to squeeze down the size of optical devices it would be possible to transfer a huge amount of data quickly across a computer chip. Indeed researchers have found a technology to squeeze light down to a sub-wavelength scale. This is possible if one shines light at the interface between a dielectric and a metal. Under certain circumstances the oscillation of the free electrons in the metal can couple to the electromagnetic eld in the dielectric and a coupled state between these two oc- curs, called a Surface Plasmon Polariton (SPP). This charge density wave travels along the metal/dielectric interface, like ripples on a water pond. The SPP waves can be used in future applications to transfer data very fast on computer chips. [1] In order to use SPPs in such applications it is very important to know how to inuence and control these resonances. Material properties as well as the structure on which the excitation takes place is a big playground to gure out how the resonances could be con- trolled optimally. This work focuses on the inuence of dierent structures to the plasmon resonances. But one can not only tune the light with optical devices. As it was shown by M. Faraday and J. Kerr in the 19th century also magnetic elds can inuence the properties of light. Magneto-optics was born and is used nowadays in plenty of applications for example in data storage devices. To use magneto-optical materials in plasmonics opens the possibility to use external magnetic elds in order to control the plasmon resonances. An interesting application might be the improvement of magneto-optic data storage. [2] To understand the physics of such a wave one has to take a closer look at the optical and magneto-optical interplay between a metal and a dielectric. 1 2 Theoretical Background 2.1 Optics Light and optics is omnipresent in our everyday life. We wear glasses to see better, use light to send data around the world and even our life rhythm is adjusted to the sun circulation. It is no wonder that optics is one of the oldest parts of physics. Since the antiquity people have been curious about the nature of light. Already in this time simple optical devices like mirrors and magniers were used. In the 17th century the development of the telescope lead to a revolution of the concept of the world. The desire to explain the nature of light became huge and two competitive theories were discussed. Is light a particle or a wave? This question was rst answered with the development of the quantum mechanics in the 20thcentury. New optical devices like the LASER inuences our life deeply and it would look completely dierent without this invention. Nowadays optics is hidden and gets smaller and smaller. In the bar-code scanner in the supermarket, in the CD player or remote controls. But there are a lot more scenarios thinkable how we could use light to improve our life. They reach from the improvement of computer chips to cancer treatment or even invisibility cloaks. To understand the idea behind these potential future applications one has to look closer at the interplay between light and matter for dierent materials. J. C. Maxwell made it with his development of the Maxwell equations evident that light can be described as an electromagnetic wave. The curl-Maxwell equations state that a time varying electric eld E~ causes a magnetic eld B~ which is perpendicular to the changing direction of the electric eld. The same is true for a changing magnetic eld which generates an electric eld. In this way they can recreate each other in an endless cycle. [3] The electromagnetic wave can be considered as a plane wave, traveling in the direction of the wave vector ~k with the angular frequency ! and an amplitude E0. The connection between the absolute value of the wave vector ~k and the angular frequency ! is named as the dispersion relation and is given for an electromagnetic wave by: [3] ! k = c where c is the speed of light in vacuum. 2.1.1 States of Light Electromagnetic waves are transversal meaning electric and magnetic elds are oscillating perpendicular to the propagation direction ~k. One can dene dierent polarization states depending on the orientation of the electric eld vector to the wave vector, sketched in gure 1. When the electric eld vector has a xed orientation in the space one speaks about linear polarized light. Whereas one speaks about circularly polarized light when the electric eld vector describes a circular path. The most general case is elliptically polarized light. Here the direction and the strength of the electric eld is changing with time. The electric eld vector describes an ellipse. 2 Figure 1: Assuming the electromagnetic wave is travelling in z direction and the electric eld vector E is oscillating in the xy-plane, the three dierent kinds of polariza- tion states are sketched. 2.1.2 Light in Matter When an electromagnetic wave propagates through matter its electric eld will interact with the elds of the atoms in the material.
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