<p>An active optical shutter for infrared light (3-5 μm) has been designed, exploiting the phase transition in thermochromic vanadium dioxide (VO₂). A spin coating processing route for VO₂ films has been adapted to manufacture reproducible depositions onto sapphire (Al₂O₃) substrates. The VO₂ films have been characterized by X-ray powder diffraction (XRPD) and infrared spectroscopy (FTIR), showing 55 % transmittance in the open mode and 0.1 % in the closed mode.</p><p>The VO₂ film temperature determines the operating mode of the shutter, and a resistive circuit of gold was deposited on top of the film for heating purposes. Switching times from the open to the closed mode down to 15 ms have been measured.</p><p>This work is a part of a comprehensive project at the Swedish Defence Research Agency (FOI), aiming to improve active components for protection against lasers. The shutter within this work is at this stage an early prototype, and needs further development and complementary systems such as a control unit to be implemented in an optical system.</p>

An active optical shutter for infrared light (3-5 μm) has been designed, exploiting the phase transition in thermochromic vanadium dioxide (VO2). A spin coating processing route for VO2 films has been adapted to manufacture reproducible depositions onto sapphire (Al2O3) substrates. The VO2 films have been characterized by X-ray powder diffraction (XRPD) and infrared spectroscopy (FTIR), showing 55 % transmittance in the open mode and 0.1 % in the closed mode. The VO2 film temperature determines the operating mode of the shutter, and a resistive circuit of gold was deposited on top of the film for heating purposes. Switching times from the open to the closed mode down to 15 ms have been measured. This work is a part of a comprehensive project at the Swedish Defence Research Agency (FOI), aiming to improve active components for protection against lasers. The shutter within this work is at this stage an early prototype, and needs further development and complementary systems such as a control unit to be implemented in an optical system.

The behaviour of surface plasmon polaritons (SPPs) in nanoscale geometries is studied using numerical methods supported by theory and experiment. First, we derive the behaviour of SPPs at graded metal-dielectric interfaces, including dispersion relations, field profiles, propagation velocities, losses, and cutoff wavelength. Numerical simulations show excellent agreement with analytic solutions. In the second part of the thesis we design hybrid vanadium dioxide-plasmonic based absorption switches. The switches are designed and optimized using optical, electrical and thermal simulations. 5 $\mu$m switch designs have extinction ratios exceeding 30 dB and require powers of 10 mW. A switch is fabricated based on the proposed design. A 7 $\mu$m experimental switch reaches 16.4 dB of extinction and requires 64 mW of power, making it one of the most efficient optical switches ever demonstrated in terms of extinction and power consumption. Numerical simulations predict experimental results with a high degree of accuracy.

The behaviour of surface plasmon polaritons (SPPs) in nanoscale geometries is studied using numerical methods supported by theory and experiment. First, we derive the behaviour of SPPs at graded metal-dielectric interfaces, including dispersion relations, field profiles, propagation velocities, losses, and cutoff wavelength. Numerical simulations show excellent agreement with analytic solutions. In the second part of the thesis we design hybrid vanadium dioxide-plasmonic based absorption switches. The switches are designed and optimized using optical, electrical and thermal simulations. 5 $\mu$m switch designs have extinction ratios exceeding 30 dB and require powers of 10 mW. A switch is fabricated based on the proposed design. A 7 $\mu$m experimental switch reaches 16.4 dB of extinction and requires 64 mW of power, making it one of the most efficient optical switches ever demonstrated in terms of extinction and power consumption. Numerical simulations predict experimental results with a high degree of accuracy.

The scientific relevance and potential for technological applications of complex materials have made them the focus of active investigation in order to fully charac- terize the competition and interactions between their electronic, structural, orbital, and spin degrees of freedom. Optical and terahertz (THz) spectroscopy provide ac- cess to electronic and low frequency quasiparticle responses, and therefore play a key role in understanding the fundamental mechanisms which dictate the macroscopic properties of complex materials. Time-resolved experiments, in turn, have the po- tential to disentangle the various coexisting energy scales through a careful selection of the pump and probe characteristics. This work investigates the role played by the electronic, structural and magnetic excitations in the insulator-to-metal transi- tions (IMT) of VO2, V2O3 and NdNiO3, through studies under different conditions of temperature, strain, doping and photoexcitation. Our work shows that a complete understanding of the IMT in VO2 requires sev- eral length scales and time scales to be considered. Indeed, epitaxial strain leads to anisotropy in the IMT characteristics of thin films of (100) and (110) VO2/TiO2, measured using THz spectroscopy, which can be explained by strain induced modi- fications both in the (microscopic) V3d orbitals and in the geometry of mesoscopic metallic domains. On the other hand, ultrafast studies which track, with femtosecond resolution, the electronic and structural dynamics of VO2 thin films following THz excitation reveal a delay in the onset of the structural response with respect to the electronic one, lending support to the correlation rather than Peierls driven picture of the IMT in this material. As for V2O3, the IMT is seen to occur via nucleation and growth of metallic domains, as previously reported in VO2. However, a scaling of the photoinduced conductivity dynamics rise time is further identified, which reveals the temperature and fluence dependence of the nucleation and growth process. Finally, strained NdNiO3 films exhibit a two step dynamical conductivity response following optical excitation, different from that of the vanadates with which they share a complex, albeit more tunable, phase diagram. This hints at a significant role being played by the magnetic structure during the IMT in NdNiO3.

abstract: This research focuses mainly on employing tunable materials to achieve dynamic radiative properties for spacecraft and building thermal management. A secondary objective is to investigate tunable materials for optical propulsion applications. The primary material investigated is vanadium dioxide (VO2), which is a thermochromic material with an insulator-to-metal phase transition. VO2 typically undergoes a dramatic shift in optical properties at T = 341 K, which can be reduced through a variety of techniques to a temperature more suitable for thermal control applications. A VO2-based Fabry-Perot variable emitter is designed, fabricated, characterized, and experimentally demonstrated. The designed emitter has high emissivity when the radiating surface temperature is above 345 K and low emissivity when the temperature is less than 341 K. A uniaxial transfer matrix method and Bruggeman effective medium theory are both introduced to model the anisotropic properties of the VO2 to facilitate the design of multilayer VO2-based devices. A new furnace oxidation process is developed for fabricating high quality VO2 and the resulting thin films undergo comprehensive material and optical characterizations. The corresponding measurement platform is developed to measure the temperature-dependent transmittance and reflectance of the fabricated Fabry-Perot samples. The variable heat rejection of the fabricated samples is demonstrated via bell jar and cryothermal vacuum calorimetry measurements. Thermal modeling of a spacecraft equipped with variable emittance radiators is also conducted to elucidate the requirements and the impact for thermochromic variable emittance technology. The potential of VO2 to be used as an optical force modulating device is also investigated for spacecraft micropropulsion. The preliminary design considers a Fabry-Perot cavity with an anti-reflection coating which switches between an absorptive “off” state (for insulating VO2) and a reflective “on” state (for metallic VO2), thereby modulating the incident solar radiation pressure. The visible and near-infrared optical properties of the fabricated vanadium dioxide are examined to determine if there is a sufficient optical property shift in those regimes for a tunable device. / Dissertation/Thesis / Doctoral Dissertation Aerospace Engineering 2020

Vanadium dioxide (VO_2) is a material of particular interest due to its exhibited metal to insulator phase transition at 68°C that is accompanied by an abrupt and significant change in its electronic and optical properties. Since this material can exhibit a reversible drop in resistivity of up to five orders of magnitude and a reversible drop in infrared optical transmission of up to 80%, this material holds promise in several technological applications. Solid phase crystallization of VO_2 thin films was obtained by a post-deposition annealing process of a VO_{x,x approx 2} amorphous film sputtered on an amorphous silicon dioxide (SiO_2) layer. Scanning electron microscopy (SEM) and electron-backscattered diffraction (EBSD) were utilized to study the morphology of the solid phase crystallization that resulted from this post-deposition annealing process. The annealing parameters ranged in temperature from 300°C up to 1000°C and in time from 5 minutes up to 12 hours. Depending on the annealing parameters, EBSD showed that this process yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. In addition to these films on SiO_2, other VO_2 thin films were deposited onto a-, c-, and r-cuts of sapphire and on TiO_2(001) heated single-crystal substrates by pulsed-laser deposition (PLD). The temperature of the substrates was kept at ~500°C during deposition. EBSD maps and orientation imaging microscopy were used to study the epitaxy and orientation of the VO_2 grains deposited on the single crystal substrates, as well as on the amorphous SiO_2 layer. The EBSD/OIM results showed that: 1) For all the sapphire substrates analyzed, there is a predominant family of crystallographic relationships wherein the rutile VO_2{001} planes tend to lie parallel to the sapphire's {10-10} and the rutile VO_2{100} planes lie parallel to the sapphire's {1-210} and {0001}. Furthermore, while this family of relationships accounts for the majority of the VO_2 grains observed, due to the sapphire substrate's geometry there were variations within these rules that changed the orientation of VO_2 grains with respect to the substrate's normal direction. 2) For the TiO_2, a substrate with a lower lattice mismatch, we observe the expected relationship where the rutile VO_2 [100], [110], and [001] crystal directions lie parallel to the TiO_2 substrate's [100], [110], and [001] crystal directions respectively. 3) For the amorphous SiO_2 layer, all VO_2 crystals that were measurable (those that grew to the thickness of the deposited film) had a preferred orientation with the the rutile VO_2[001] crystal direction tending to lie parallel to the plane of the specimen. The use of transmission electron microscopy (TEM) is presented as a tool for further characterization studies of this material and its applications. In this work TEM diffraction patterns taken from cross-sections of particles of the a- and r-cut sapphire substrates not only solidified the predominant family mentioned, but also helped lift the ambiguity present in the rutile VO_2{100} axes. Finally, a focused-ion beam technique for preparation of cross-sectional TEM samples of metallic thin films deposited on polymer substrates is demonstrated.

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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24011号 / エネ博第447号 / 新制||エネ||84(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 平藤 哲司, 教授 土井 俊哉, 教授 藤本 仁 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

<p>The fascinating optical properties of metallic nanostructures, dominated by collective oscillations of free electrons known as plasmons, open new opportunities for the development of devices fabrication based on noble metal nanoparticle composite materials. This thesis demonstrates a low-cost and versatile technique to produce stimuli-responsive ultrafast plasmonic nanostructures with reversible tunable optical properties. Albeit challenging, further control using thermal external stimuli to tune the local environment of gold nanoparticles embedded in VO2 host matrix would be ideal for the design of responsive functional nanocomposites. We prepared Au-VO2 nanocomposite thin films by the inverted cylindrical reactive magnetron sputtering (ICMS) known as hollow cathode magnetron sputtering for the first time and report the reversible tuning of surface plasmon resonance of Au nanoparticles by only adjusting the external temperature stimuli. The structural, morphological, interfacial analysis and optical properties of the optimized nanostructures have been studied. ICMS has been attracting much attention for its enclosed geometry and its ability to deposit on large area, uniform coating of smart nanocomposites at high deposition rate. Before achieving the aforementioned goals, a systematic study and optimization process of VO2 host matrix has been done by studying the influence of deposition parameters on the structural, morphological and optical switching properties of VO2 thin films. A reversible thermal tunability of the optical/dielectric constants of VO2 thin films by spectroscopic ellipsometry has been intensively also studied in order to bring more insights about the shift of the plasmon of gold nanoparticles imbedded in VO2 host matrix.</p>