Time-resolved plasmonics in designed nanostructures

• A metal nanoparticle can be considered as consisting of a base of positive ion cores and a sea of free electrons. When the free electrons are displaced, for example, by an incident electric field, a restoring force acts on the electrons. The electrons may then oscillate back and forth until equilibrium is reached. This oscillation occurs at the natural frequency, or eigenfrequency, of the system. By matching the driving frequency with this frequency, the amplitude (the maximum electron displacement) can be made large - the system is in resonance. This resonance mode is a plasmon. The separation of charge on that small length scale will result in a large field in the vicinity of the nanoparticle. This large field, often oscillating at optical frequencies, on the spatial scale of nanometers, has many potential applications, such as high-resolution microscopy, photo-voltaics, light emission and coherent control. Because of the interest in manipulating light on the nanoscale, particles having their resonances in the optical domain are often used. The collective electron oscillation, when resonantly excited, therefore occurs on the femtosecond timescale. Due to this ultrashort timescale, the dynamics are difficult to follow in time. The spatial confinement of the oscillation to the nanometer scale makes it challenging to also image them. This thesis explores ways of studying the ultrafast dynamics of plasmons spatially and temporally, simultaneously. Two types of experiments are discussed. The first is autocorrelation experiments where the induced and enhanced field is autocorrelated with itself. For one of these experiments, bowtie nanoantennas were manufactured, using the focused ion beam technique. In the second kind of experiment an infrared laser pulse is used to excite the plasmon, and a short attosecond pulse probes it. The work described in this thesis deals with the fabrication of nanostructures and the implementation of attosecond pulse generation schemes suitable for this purpose.

• Popular Abstract in English Where are we? And what are all the things we have around us? These are questions that we ask as children when we start to discover the world around us. But just because we grow up does not mean that we find the answers to those questions. Around us, we see things as red buses, blue oceans and yellow flowers. But according to science, it is not these things that we fundamentally have around us. Let us start with the concept of color. Physicists like Galilei and Newton argued that the objects around us, as well as the light reflected from them, actually lack color and that colors are created in our consciousness. This point of view is today so established within psychology that it is presented in modern textbooks about the subject. Color is seen as something which only exists in our consciousness, just like for example pain. If we assume this point of view to be true, what does it tell us about the places we observe around us? The colors are at specific locations in our field of view. If the colors are created in our brain, so should the places in our visual field carrying those colors be. This means that our visual world, everything we see, is created in the brain. Maybe, however, based on physical input to our eyes, such as light. Some researchers talk about the perceived world as a projection or a user interface - we interact with fundamental reality through what we experience, in the same way as we interact computers by clicking on icons on the computer screen. As an explanation for us being equipped with this user interface, natural selection is often suggested. The user interface helps the organism to note and initiate responses, and recognize objects in its environment. So, if we can't directly access reality through our senses, what should we do to gain knowledge about the fundamental reality behind what we see? We have to turn to physics. Physics describes how electrons in atoms oscillate and, when they do so, cause an electric field, which in turn makes other electrons oscillate. So when we think we ``see'' an object illuminated, for example, by a light bulb, it is really electrons in the light bulb that oscillate, cause the electrons in the object to oscillate, which in turn should make the electrons in our eye photoreceptors oscillate. A signal is sent to the brain which then creates a representation of what is around us - it creates what we are seeing. To understand the physical basis of what we see around us, we therefore have to understand atoms, and how they interact with each other through electric fields. A metal nanoparticle can be regarded as a model of an atom, or a ''toy atom''. Both systems have a heavy positively charged part. In the case of the atom it is a nucleus, while in the case of the nanostructure it is the fixed ion core background. Both systems contain easily movable, negatively charged, electrons. Atoms only have a few electrons, while metal nanostructures contain a ''sea'' of free electrons. If the electrons are slightly displaced with respect to the core or cores, (for example, by an applied electric field) the electrons will swing back towards the positive part, overshoot, swing back again to the positive part, and so on until the oscillation finally stops. The electrons will oscillate at the natural frequency of the system. This electron oscillation occurs in the same way as when a swing is pushed. The swing will oscillate back and forth at its natural frequency until it reaches its equilibrium, that is to hang straight down. As seen above, the electron oscillation will cause an electric field. This field will propagate as a wave with the same frequency as the oscillation. For many metal nanostructures and many types of atoms, the electrons naturally oscillate at frequencies on the order of 10^14bHz, which leads to an electromagnetic wave with the same frequency. Our eyes are sensitive to electromagnetic waves of this frequency, and we call these waves light or visible radiation. This light can, in turn, make electrons in other systems oscillate, possibly resulting in absorption of the light there. Both atoms and metal nanostructures can thus both emit and absorb light. Although there are similarities between metal nanostructures and atoms, there is an important difference in that nanostructures can today be designed and fabricated so that they have specific properties, for example, a certain natural frequency. Additionally, they can be studied individually. By studying nanostructures, we may be able to gain a better understanding of atoms, the interaction of which forms the basis of what we see. There are also many technical applications of metal nanostructures, as they can both enhance and focus incoming light. Because the electrons are displaced on such a small spatial scale and so rapidly, their motion is very difficult to follow. This thesis discusses two ways of still doing this. Both involve the use of an electron microscope. The first method uses two pulses consisting of only a few cycles, and with a wavelength in the infrared part of the electromagnetic spectrum. The pulses are sent into the microscope where they impinge on the nanostructure. Each pulse drives the electrons a few cycles back and forth, whereafter they oscillate freely. From areas where many electrons are densely packed the electrons can leave the structure due to an applied static electric field. Images can then be obtained of the areas from which electrons leave, and areas where they do not. The electron emission from a particular point can be made to vary by varying the time between two light pulses. The electrons may oscillate in phase as a result of the two pulses, leading to densely packed electrons and high emission, or they may oscillate out of phase, leading to lower emission. The way in which the emission from a given point varies with the time between the pulses provides information about the oscillation of the electrons. The other way of studying electron oscillations in the nanostructure is to first drive it with a short infrared pulse consisting of only a few cycles, and then let the electrons oscillate freely, without high emission. An even shorter pulse, an attosecond pulse (10^-18 s) will then be seen at different times relative to the infrared pulse. The attosecond pulse provides a kind of snapshot, and only causes electron emission during its short duration. In this way, it is possible to take ''photographs'' of where the electrons are during and after the infrared pulse. This thesis describes experiments using the first technique, and steps towards experiments using the second. Hopefully, these experiments, and similar ones, will help us understand nanoparticles and their applications, atoms, and maybe also, where we are, a little better. Popular Abstract in Swedish Var är vi någonstans? Var har vi hamnat? Dessa är frågor vi kan ställa som barn när vi börjar upptäcka världen omkring oss. Men bara för att vi växer upp och lär oss mer om världen betyder det inte att vi helt får svar på de frågorna. Omkring oss ser vi röda bussar, blå hav och gula blommor. Men enligt vetenskapen är det inte sådana saker vi fundamentalt har omkring oss. Vi börjar med aspekten färg. Fysiker som Galilei och Newton menade att objekten omkring oss, samt ljuset som reflekteras från dem, saknar färg och att färg är något som först skapas i vårt medvetande. Denna hållning är också så etablerad inom psykologin att den står med i moderna läroböcker i ämnet. Färg anses vara något som bara finns i vårt medvetande, precis som till exempel smärta. Om vi antar denna hållning, vad säger det om platserna vi ser omkring oss? Färgerna finns på vissa platser i vårt synfält. Om färgerna skapas av vår hjärna bör rimligtvis de platser i synfältet som bär färgen också skapas av vår hjärna. Det betyder att hela vår visuella värld, det vill säga det vi ser, helt skapas av hjärnan. Kanske dock baserat på fysiska input till ögonen, såsom ljus. Man talar ibland om det vi ser som en projektion eller ett användargränssnitt - vi interagerar med den bakomliggande verkligheten genom det vi upplever, analogt med hur vi interagerar med datorns hårdvara genom att klicka på ikoner på datorskärmen. Som skäl att vi har ett sådant här användargränssnitt brukar man ange det naturliga urvalet. Genom detta gränssnitt får organismen hjälp att notera och initiera responser och känna igen saker i sin omgivning. Så nu när vi inte verkar kunna lita på våra sinnen för direkt åtkomst av verkligheten, hur ska vi göra för att få kunskap om verkligheten bakom det vi ser? Vi får här vända oss till fysiken. I fysiken beskrivs hur elektroner i atomer svänger och när de gör det, orsakar ett elektriskt fält som i sin tur får andra elektroner att svänga. Så när vi ''ser'' ett objekt, upplyst av till exempel en glödlampa, är det egentligen elektroner i glödlampan som vibrerar, vilket sätter elektroner hos objektet i vibration, vilket i sin tur bör få elektroner i våra synreceptorer att börja vibrera. Utefter denna vibration skickas sedan en signal till hjärnan som då skapar en representation av vad som finns omkring oss - den skapar det vi ser. För att förstå den fysikaliska basen till vad vi ser omkring oss behöver vi alltså förstå atomer och hur de interagerar med varandra genom elektriska fält. En metallnanopartikel kan ses som en modell av en atom, en leksaksatom. Båda system består av en tyngre positivt laddad del. I atomens fall är det en kärna, i nanostrukturens fall är det den fixa jonkärnebakgrunden. Båda system innehåller negativt laddade och lättrörliga elektroner. I atomen är det ett fåtal elektroner medan det i metallstrukturen är ett ''hav'' av fria elektroner. Om elektronerna flyttas lite grand i förhållande till kärnan eller kärnorna, (genom att ett elektriskt fält, till exemp

Ämnesord

NATURVETENSKAP  -- Fysik -- Atom- och molekylfysik och optik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Atom and Molecular Physics and Optics (hsv//eng)

Nyckelord

Plasmon

Nanostructure fabrication

High-order harmonic generation

Photoemission electron microscopy

Focused ion beam

Fysicumarkivet A:2016:Lorek

Publikations- och innehållstyp

dok (ämneskategori)

vet (ämneskategori)