The terehertz radiation potentially has many interesting applications. From air port security, non-destructive evaluations of electronics and space shuttle panels, to non-ionizing photon energies with the potential to detect cancer growths and quality control of pharmaceutical tables, the list of potential applications is vast as shown in chapter 1. However, there is a lack of cheap, robust and efficient THz sources, detectors and modulators. Further, the long wavelengths render micron sized details unseeable with far-field imaging techniques. This has rendered most imaging applications unusable in the real world. This thesis is based around demonstrating an imaging technique that uses a near-field THz modulator to obtain sub-wavelength images. There are five distinct experimental demonstrations that show the full capacity of the imaging technique developed here. Chapter 2 gives an outline of the background physics knowledge needed to understand the entirety of the thesis. An outline of the mathematics used for modellingis given in the latter part of the chapter as well. Chapter 3 gives a background on the THz generation and detection techniques used in our THz-TDS system, optical rectification and electro-optic sampling in ZnTe. Further more, our system is capable of photoexciting a sample in conjunction to it being probed with a THz pulse. For the most part, we photoexcite a silicon wafer in order to use its photoconductive properties to modulate our THz pulse. Our photoexcitation pulse is spatially modulated, via a digital micromirror device, which in turn spatially modulates our THz pulse. This patterned THz pulse can then be used with a single-element detector to perform imaging. How to do this and the type of patterns needed is described in the latter part of chapter 3. Chapter 4 is the first demonstration that photo-induced conductivity in silicon can be used to manipulate evanescent THz fields for sub-wavelength imaging. For this, we imaged a 1D sub-wavelength slit and were able to obtain the slit profile with 65μm (λ/6 at 0.75T Hz) resolution. Chapter 5 demonstrates what limits the resolution in our imaging system. Namely, the distance which the patterned THz pulse propagates to the object from where itwas spatially modulated. We demonstrate 9μm (λ/45 at 0.75T Hz) resolution using an ultra-thin (6μm) silicon wafer. At such sub-wavelength scales polarization becomes an important factor. We show how one can use polarization in order to detect 8μm breaks in a circuit board hidden by 115μm of silicon. Chapter 6 concerns itself with showing how noise affects our images. Further more, our imaging system is compatible with compressed sensing where one can obtain an image using fewer measurements than the number of pixels. We investigate how different under-sampling techniques perform in our system. Note under-sampling at sub-wavelength resolutions, as is done here, is rather unusual and is of yet to be demonstrated for other part of the electro-magnetic spectrum. Chapter 7 shows that one does not need to photoexcite silicon. One can in principle illuminate any material, hence we photoexcite graphene with our spatially modulated optical pulses. This allows us to obtain the THz photoconductive response of our graphene sample with sub-wavelength resolution (75μm ≈ λ/5 at 0.75T Hz). We compare our results with Raman spectra maps. We find a clear correlation between THz photoconductivity and carrier concentration (extracted from Raman). Chapter 8 exploits the full capacity of our imaging system by performing hyper-spectral near-field THz imaging on a biological sample. For this, in our imaged field of view, we measured the full temporal trace of our THz pulse at a sub-wavelength spatial resolution. This has allowed us to extract the frequency dependent permittivity of our biological sample, articular cartilage, over our spectral range (0.2-2T Hz). We find the permittivity to change on a sub-wavelength scale in correlation with changes in the structure of our sample. However, the permittivity extraction procedures that have been developed make a far-field approximation. We mathematically show the presence of the THz near-fields to render the long wavelength spectral parts of our extracted permittivity to be wrong. Chapter 9 is where we conclude and point out the main problem that needs to be addressed in order to make the measurements presented here more accessible to others. Namely, the cost of the laser system powering the THz-TDS and how to further reduce the acquisition time.

The subject of the thesis is analysis and modeling of the entry, transport, and atmospheric precipitation of solar wind ions, H+ and He2+, into the induced magnetosphere of Mars. The solar wind is a flow of charged particles emitted by the Sun. The solar wind carries with it a magnetic field, the interplanetary magnetic field (IMF). The IMF piles up on the dayside of the non-magnetized Mars and is then convected towards the nightside. The solar wind ions can normally not cross the magnetic barrier, formed by the pile up IMF. However, in situ observations by the Mars Express spacecraft reveal that downward moving solar wind H+ and He2+ are sometimes present in the Martian ionosphere, below the magnetic barrier. The gyroradii of shocked solar wind ions may be comparable to the size of the dayside Martian magnetic barrier and for certain circumstances, these ions can gyrate through. Observations by Mars Express are used to analyze H+ and He2+ penetrating through the magnetic barrier and precipitating into the Martian ionosphere, identified by the presence of ionospheric photo-electrons. A case study shows evidence of narrower energy distributions for H+ (with energy ≥ solar wind energy), as the spacecraft moves down in altitude. From this, the study concludes that the magnetic barrier prevents the lower energy H+, from reaching low altitudes. The thesis also describes a statistical study of precipitating H+ fluxes, which indicate that H+ precipitation is rare (detected during 3% of the dayside observation time only) and carries on average 0.2% of the upstream solar wind particle flux. In another statistical study, the thesis shows that the precipitation of H+ and He2+ decreases even further when Mars encounters solar wind pressure pulses. A possible explanation is that the enhanced mass loading of the magnetic field flux tubes by planetary heavy ions, while the tubes drag through the ionosphere at lower altitudes, slows down their velocity and allows more magnetic flux to pile up. The magnetic barrier becomes a more effective obstacle to the solar wind ion precipitation. Furthermore, the thesis describes a model of H+ precipitation onto the Martian upper atmosphere using a hybrid code of the Mars solar wind interaction. The spatial patterns of the precipitation depend on the H+ energy, on the H+ origin (solar wind or generated from the hydrogen corona) and on the altitude. Some features of the observed H+ distributions are reproduced by simulations, while others are not, indicating a more complex physics than in the model. The thesis also describes amodel study of transport of H+, fast H atoms and He2+ through the atmosphere using a Direct Simulation Monte Carlo model. This study demonstrates the crucial role of the magnetic field in determining the energy deposition of the solar wind ions in the topside atmosphere. For instance, a horizontal magnetic field with strength of 50 nT backscattered almost all H+, thus preventing these particles to deposit their energy at lower altitudes. The conclusion of the thesis work is that although some solar wind ions do precipitate, the magnetic barrier effectively protects the onospherefrom precipitating solar wind ions. / Ämnet för avhandlingen är analys och modellering av inflödet av solvindsjoner, H+ och He2+, genom Mars inducerade magnetosfär. Solvinden är ett flöde av laddade partiklar från Solen. Solvinden bär med sig ett magnetfält, det så kallade interplanetära magnetfältet (IMF). IMF packas ihop framför dagsidan av planeten innan det tar sig vidare mot nattsidan. Solvindsjoner kan vanligtvis inte passera denna magnetiska barriär som skapas då IMF packas ihop. Dock avslöjar in situ-observationer av rymdsonden Mars Express att nedåtflödande H+ och He2+ från solvinden ibland påträffas inuti Mars jonosfär, nedanför den magnetiska barriären. Gyroradierna hos solvindsjoner i shockregionen kan vara jämförbara med storleken av den magnetiska barriären over Mars dagsida och i vissa fall kan jonerna gyrera igenom barriären. Observationer från Mars Express används för att analysera H+ och He2+ som tar sig igenom den magnetiska barriären och ner i Mars jonosfär, vilken identifieras genom närvaron av jonosfäriska fotoelektroner. En fallstudie visar tecken på smalare energifördelningar av H+ (med energi ≥ solvindens energi), ju lägre rymdsonden tog sig. Från detta slutleder studien att den magnetiska barriären reflekterar H+ med lägre energi och förhindrar dem från att nå lägre altituder. Avhandlingen beskiver även en statistisk studie av inflödande H+, vilken indikerar att inflödet av H+ är sällsynt (observeras enbart under 3% av observationstiden över dagsidan) och bär i genomsnitt med sig 0.2% av partikelflödet som finns uppströms i solvinden. I en annan statistisk studie visar avhandlingen att inflödet av solvindsjonerna H+ och He2+ minskar ytterligare när Mars möter tryckpulser i solvinden. En möjlig förklaring är att den ökade masslastningen av magnetfältets av tunga planetära joner, då magnetfältet släpas genom jonosfären på lägre höjd, bromsar upp magnetfältet och orsakar ytterligare hoppackning av magnetfältet. Det gör den magnetiska barriären till ett mer effektivt hinder för inflödet av solvindsjoner. Vidare beskriver avhandlingen en modell för inflöde av H+ till Mars övre atmosfär genom att använda en hybridkod för Mars växelverkan med solvinden. Mönster i utbredningen av inflödet beror på energin hos H+, på källan till H+ (solvinden eller skapad från vätekoronan), och på altituden. Vissa egenskaper hos H+-fördelningarna återskapas av simuleringar, medan andra inte gör det, vilket tyder på en mer komplicerad fysik än i modellen. Avhandlingen beskriver också en modellstudie av transport av H+ , snabba H atomer, och He2+ genom atmosfären med en Direct Simulation Monte Carlo modell. Denna studie demonstrerar den avgörande roll som magnetfältet har i att bestämma energin som solvindsjoner avlämnar i den övre atmosfären. Till exempel reflekterade ett horisontellt magnetfält på 50 nT nästan allt H+, och förhindrade dessa partiklar från att avlämna sin energi på lägre altituder. Slutsatsen av avhandlingen är att även om vissa solvindsjoner tar sig igenom, så är den magnetiska barriären ett effektivt skydd av jonosfären mot infallande solvindsjoner. / <p>Godkänd; 2012; 20121009 (catdiv); DISPUTATION Ämne: Rymdteknik/Space Technology Opponent: PhD Christian Mazelle, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France Ordförande: Professor Stas Barabash, Svenska institutet för rymdfysik, Kiruna Tid: Fredag den 14 december 2012, kl 10.00 Plats: Aula, Svenska institutet för rymdfysik, Kiruna</p>

An ionic liquid (IL) is a salt, which usually is in the liquid state at normal temperature and pressure. The properties of ILs can be adjusted for various processes and applications by choosing different combinations of ions. Similar to other salts, ILs contain only ions with positive (cations) and negative (anions) charges in equal proportions. However, to prevent solidification, ions in ionic liquids usually contain bulky organic chemical groups, which, apart from electrostatic interactions, promote other types of interactions between ions, such as: (i) van-der-Waals interactions; (ii) hydrogen bonding; (iii) - stacking, etc., depending on the particular chemical structure of the ions. All these interactions, in combination, may lead to formation of specific microstructures in ILs, which may vary with temperature caused by changing thermal rotational and translational energies of the ions. Ions in these microstructures may have preferential orientations relative to each other, maintain anisotropic properties similar to those in liquid crystals or, in some specific cases, may even separate into microscopically organised liquid phases. Therefore, the dynamics of ILs may also be dependent on their microstructure. In many practical applications ionic liquids are placed on surfaces or in confinements. Solid surfaces introduce extra forces, which may be specific to the charge of the ions or/and to functional groups in the ILs. The geometry and interactions of ions in confinements or/and pores of materials may also disrupt specific bulk microstructures of ILs. Both confinement effects and interactions of ions with surfaces are manifested in the translational dynamics of the ions. One of the most direct and informative methods to study translational dynamics of ILs is pulse-field-gradient nuclear magnetic resonance (PFG-NMR).In this thesis the results of PFG-NMR studies on a few classes of ILs are reported: (i) the historically “standard” (since Walden’s discovery in 1914) ionic liquid, the ethylammonium nitrate (EAN) and (ii) halogen-free orthoborate-based phosphonium, imidazolium and pyrrolidinium ILs with varied structure and lengths of alkyl chains in cations, and varied structures of orthoborate anions. These ILs were studied in bulk at different temperatures, and also in confinements, such as between parallel glass and Teflon plates and in mesoporous Vycor glass. It was found that diffusion coefficients of cations and anions in EAN, phosphonium and pyrrolidinium orthoborate ILs in bulk are different, but according to the standard Stocks-Einstein model, they correspond to diffusion of ions in homogeneous liquids. A change in the chemical structure of one of the ions results in a change in both the diffusion coefficient of the oppositely charged ion and the activation energy of diffusion for both ions in an IL. Similar effects were observed from the chemical shifts and diffusion coefficients measured by NMR for imidazolium orthoborate ILs dissolved in polyethylene glycol solutions, in which imidazolium cations strongly interact with PEG molecules, further affecting the diffusion of orthoborate anions via electrostatic interactions. A liquid-liquid phase separation was suggested for a few phosphonium and pyrrolidinium bis(mandelato)borate ILs, in which a divergence of diffusion coefficients and activation energies of diffusion for cations and anions was detected at temperatures below ca 50 °C. In addition, a free-volume theory was invoked to explain the dependences of density of ILs on the alkyl chain length in cations.It was also found that for a phosphonium bis(salicylato)borate IL confined in 4 nm mesoporous Vycor glass the diffusion coefficients of ions increase by a factor of 35! This phenomenon was explained by the dynamic heterogeneity of this IL in micropores and empty voids of the Vycor glass. For EAN IL in confinements between glass and Teflon plates, the diffusion of ethylammonium cations and nitrate anions is significantly anisotropic, i.e. slower in the direction of the normal to the plates and faster along the plates compared to diffusion of the ions in bulk. A plausible explanation of this PFG NMR data is that EAN forms layers near polar and non-polar solid surfaces. A similar phenomenon, to a lesser extent, was also observed for phosphonium cations of bis(mandelato)borate, bis(salicylato)borate and bis(oxalato)borate confined between glass plates. The results of these studies may have implications in modeling tribological performance, i.e., friction and wear reduction for contact pairs of different materials lubricated by various classes of ionic liquids. / För godkännande; 2016; 20160420 (andfil)