Browsing by Author "Kaivola, Matti, Prof., Aalto University, Department of Applied Physics, Finland"

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  • Direct digital manufacturing: projection stereolithography and incremental sheet forming
    (2017) Lehtinen, Pekka
     School of Science | Doctoral dissertation (article-based)
    The drastic development of advanced manufacturing technologies coupled with consumer demands for more customized products are causing the manufacturing industry to move from mass production to small volume and a wide range of products. These kinds of requirements are met by direct digital manufacturing (DDM) techniques that seamlessly produce three-dimensional objects directly from digital data in a single step regardless of the complexity of the products to be built. DDM techniques include additive manufacturing and forming techniques where highly specified products are fabricated in layers. Due to the high versatility of DDM systems and low tooling costs, DDM is outstanding in one-off production. However, DDM is still an immature technology that will make a real impact across a large range of industries as it is developed further. This thesis aims to develop two DDM techniques: (i) projection stereolithography and (ii) incremental sheet forming. While the first one is an additive manufacturing technique where a plastic part is fabricated by selectively curing liquid polymer in layers, the second one is a metal forming technique where the desired shape is created through a series of small incremental deformations. First, a projection microstereolithogarphy apparatus that can produce high quality miniature objects with a resolution of a few micrometers was built and some specific issues related to the printing process were solved. The research involved increasing the manufacturing speed, developing suitable polymer solutions (resolution, conductivity, biocompatibility, penetration depth control), and providing new insight and methods to control the curing mechanisms of liquid polymers. Secondly, the effect of local heating on the formability of metal sheets in incremental sheet forming was investigated. In this research, metal sheets were formed by a round tipped tool that was attached to a 3-axis positioning system and the investigated materials were aluminum, copper and steel. To increase the formability of metal sheets, a laser light source was used to irradiate the bottom side of the sheet. With this method it was possible to produce shapes that were impossible to form without heating.
  • Electromagnetic Coherence Theory, Universality Results, and Effective Degree of Coherence
    (2013) Blomstedt, Kasimir
     School of Science | Doctoral dissertation (article-based)
    Electromagnetic coherence is central to modern optics and photonics. Three topics from classical coherence theory are considered: 1) coherence of blackbody radiation within a cavity and at a cavity aperture, 2) universality of spatial coherence of fields created by homogeneous and isotropic sources, and 3) the effective degree of coherence. Blackbody sources and fields have played a pivotal role in the development of quantum physics. The cross-spectral density (CSD) and degree-of-coherence of the blackbody field at an opening in a blackbody cavity wall and in the far field are computed, and it is shown that the aperture CSD obtained in previous works is, in fact, erroneous. The effect of the cavity wall on the CSD of the field is also studied. It is found that the wall indeed has an appreciable influence, but that this does not affect the CSD of the field in the aperture. The coherent-mode representation of the CSD function of the vector-valued blackbody field is derived for a spherical volume, and the importance of the constituent coherent modes is assessed. This expansion marks the first time that the coherent-mode representation is determined within a three-dimensional region. The blackbody results can be used to model thermal sources and propagation of natural light. In prior works it has been shown that the degree-of-coherence functions of scalar optical fields, which are produced by stochastically homogeneous and isotropic sources, all have the same universal form in lossless infinite systems. Here that result is extended to electromagnetic fields. Additionally, it is proven that for actual systems, which necessarily are finite and lossy, the universal character disappears, regardless of how negligible the losses are or how large the source region is. These considerations apply to optical media and sources of all sizes, from nanoscopic to macroscopic. It is shown that the effective degree of coherence can be extended from field representations in volumes to arbitrary (Hilbert) spaces. In addition, it is demonstrated that the effective degree of coherence is an intrinsic property of the electromagnetic field, and that it can be computed from almost any field representation. In fact, what is proven is that the effective degree of coherence is invariant to scaled unitary mappings. Finally, it is shown that of all Hilbert space functionals with this property, only the effective degree of coherence is additive.
  • Imaging of Surface Vibrations Using Heterodyne Interferometry
    (2014) Kokkonen, Kimmo
     School of Science | Doctoral dissertation (article-based)
    The research summarized in this dissertation focuses on the development of a heterodyne scanning laser interferometer and of data-analysis techniques for the characterization and analysis of the surface vibration fields in microacoustic devices and test structures. The heterodyne laser interferometer enables a phase-sensitive, absolute-amplitude detection of the out-of-plane component of a surface vibration field, with a minimum detectable amplitude of less than a picometer, while the lateral resolution is better than 1 micrometer. The instrument features a flat frequency response up to 6 GHz. The phase-sensitive absolute-amplitude data enables the visualization of the actual wave behavior in electromechanical components and test structures, but more importantly, it is the basis for further analysis. The research instrument is applied to the study of electroacoustic devices based on surface acoustic wave (SAW) and bulk acoustic wave (BAW) technologies. Two novel SAW devices are studied in detail: a phononic crystal (PnC) structure and a scattering structure resulting in a random wave field. PnCs are acoustic metamaterials that can provide engineered material properties. The laser interferometric measurements were amongst the first to directly characterize the wave interaction with the PnC. SAW slowness curves of an anisotropic substrate material are extracted by measuring and analyzing the scattered random wave field. Data analysis methods are developed further in the context of BAW research by experimentally addressing two important aspects of device design: the correct operation of the acoustic reflector used to confine the energy in the resonator, and investigation of the role of the dispersion and standing wave resonances to the spurious responses often observed in high-Q resonators. Fourier transform techniques are used for selective visualization of wave fields and for the extraction of the dispersion characteristics of the plate-waves. The dispersion data are then further used to analyze the transfer characteristics of an acoustic reflector and to study the properties of lateral eigenresonances and the lateral energy confinement in detail. The research described in this thesis provides a detailed characterization of the operation of SAW and BAW devices and effects within, highlighting and further developing the experimental characterization capabilities and data-analysis methods.
  • Interaction of light with functional spatially dispersive nanomaterials
    (2019) Kivijärvi, Ville
     School of Science | Doctoral dissertation (article-based)
    Progress in nanotechnology has enabled systematic development of artificial nanomaterials with extraordinary optical properties, such as zeroth and negative index of refraction, perfect absorption, and enhanced nonlinearity, anisotropy and temporal dispersion. Another property that is common for designed nanomaterials is spatial dispersion. It makes the refractive index and wave impedance depend on light propagation direction, which complicates the description, but also makes it possible to obtain previously unreachable capabilities for the materials. For example, the material can be made to reflect or absorb light differently by its different facets. In spite of a high potential of spatially dispersive nanomaterials in science and technology, only few theoretical models and design tools for these materials can be found in the literature. The results presented in this thesis can pave the way to comprehensive characterization and more efficient design of spatially dispersive nanomaterials, including metamaterials and nanostructured optical waveguides. We develop novel theoretical methods and numerical calculation techniques to characterize and design nanostructured materials and devices made of them. Furthermore, we put forward a novel approach to characterize nanostructured optical media, for which electromagnetic modes cannot be introduced due to the effect of polarization conversion by spatial dispersion. We find that a large variety of designed optical nanomaterials belong to this class of optical media. In addition, the thesis contains an efficient semi-analytical technique to model the interaction of optical beams with spatially dispersive materials. Making use of the tools of Fourier optics, the method allows treating the beam-propagation phenomenon in large-sized nanomaterial samples. Compared to conventional numerical approaches, the technique is computationally light and provides a deeper understanding of the light-matter interaction picture. Using the developed methods, we design novel optical nanoscatterers and nanomaterials composed of them. As an example, we demonstrate both theoretically and experimentally a spatially dispersive metasurface that shows primarily quadrupole optical response when illuminated from one side, and primarily dipole response when illuminated from the other side. Furthermore, we design novel diffraction-compensating optical metamaterials, whose anisotropy and spatial dispersion prevent focused light beams from spreading upon propagation. We also demonstrate a diffraction-compensating metamaterial slab waveguide with low reflection and absorption losses and a broadband operation. The designed materials and optical components based on them may find applications in efficient solar cells, novel laser resonators, and high-speed photonic integrated circuits, and together with the presented theoretical tools, contribute to the progress of nanostructured optical media towards real-life applications.
  • Nanophotonic control of optical emission, propagation, and all-optical modulation
    (2021) Nyman, Markus
     School of Science | Doctoral dissertation (article-based)
    The field of nanophotonics is concerned with the use of nanoscale structures and systems to control light for purposes such as miniaturization of optical components and control over the emission of light by quantum emitters. The research compiled in this dissertation focuses on artificial optical nanomaterials, generation of light, and all-optical modulation. Most of the artificial optical materials examined in this dissertation are metamaterials. These consist of scatterers, such as nanorods, in a lattice with a sub-wavelength period. For light passing through a metamaterial, the material appears as a homogeneous medium but with extraordinary optical properties. Typically, the optical properties also vary depending on the direction of light propagation; this effect is known as spatial dispersion. To analyze and design spatially dispersive metamaterials for different purposes, this dissertation adopts a wave parameter-based approach where a refractive index and wave impedance is assigned to each plane wave propagating in the metamaterial. We refine the previously-developed wave parameter theory to describe the movement of optical energy inside nanomaterials. We then develop a Fourier transform-based method to analyze optical emission inside spatially dispersive materials. We also discuss the outcoupling of light from inside nanomaterials, and show how emitted light can efficiently escape even from inside materials that exhibit strong attenuation. We then design and experimentally demonstrate nanostructures that drastically increase the brightness of a fluorescent film by combining the enhancements of radiation directivity and pump light absorption. The methods and ideas put forward in this part are useful for understanding and designing nanomaterials and structures for the purpose of controlling optical emission, with an eye on applications such as sensors and light sources. The dissertation also discusses nanomaterials for controlling the polarization of light. We theoretically and experimentally demonstrate a wave plate that relies on the strong birefringence of a sub-wavelength metal-dielectric structure. The wave plate combines low loss, broad bandwidth, and operation in the transmission mode, and we also discuss how it can be fabricated on large areas, making mass production feasible. Finally, we present a method of ultrafast all-optical modulation, as well as a method of ultrafast detection. Both methods rely on an optical gain medium and a Fourier transform pulse shaper. The on-resonance gain medium facilitates strong nonlinear interaction of two optical signals. The pulse shaper is used as a wavelength splitter and recombiner (in all-optical modulation) and a time-to-space mapper (in detection). In our experiments, modulation and detection reach sub-picosecond speed and resolution, respectively, despite the overall nanosecond-scale response of the gain medium.
  • Optical imaging of surface dynamics in microstructures
    (2016) Lipiäinen, Lauri
     School of Science | Doctoral dissertation (article-based)
    The research summarized in this thesis covers the design, implementation, and use of optical techniques for characterizing surface movements in microstructures. The main focus of the work has been on developing instrumentation and data analysis methods for investigating surface vibrations in micromechanical components that are based on, e.g., microelectro-mechanical systems (MEMS), and surface and bulk acoustic waves.   All the scanning single-point and full-field vibration detection setups and methods developed in this work enable phase-sensitive, absolute amplitude measurement of surface vibrations. An unstabilized homodyne interferometry concept is presented for detecting out-of-plane (OP) vibrations with a scanning single-point Michelson laser interferometer. A noninterferometric detection method for measuring in-plane (IP) vibrations is also described that is implemented in this scanning system. The setup enables vibration measurements for frequencies up to 2 GHz, with typical minimum detectable amplitudes of even less than 1 pm and 10 pm for the OP and IP components, respectively. Furthermore, novel methods based on these scanning techniques were implemented to allow for studies of the nonlinear behavior of surface vibrations, which serve to advance the understanding of such effects in microacoustic components. The scanning-based optical imaging methods were applied to two research studies in MEMS resonators that showed unexpected behavior.The full-field interferometric techniques and analysis methods developed in this thesis work push the performance of the camera-based detection of OP vibrations into new limits. The work advances the stroboscopic white-light interferometric technique to be applicable for characterizing high-frequency devices with vibration amplitudes down to less than 100 pm and with frequencies up to 1 GHz. In addition, a stabilized full-field stroboscopic detection concept was developed and the implemented setup was demonstrated to allow for detecting surface vibrations with minimum detectable amplitudes of less than 30 pm. The stabilized full-field interferometer was also developed further for imaging surface dynamics on microstructures in the time domain with even subnanometer vertical resolution.  The optical imaging methods described in this thesis contribute substantially to the research and development of micromechanical devices as they offer direct information of the underlying device physics. The benefits of these advanced optical characterization methods are clearly highlighted in the two MEMS resonator study cases, in which the optical characterization revealed the physical mechanisms that adversely affect the device performance.
  • Photolithographic fabrication of periodic nanostructures for photonic applications
    (2015) Kravchenko, Aleksandr
     School of Science | Doctoral dissertation (article-based)
    Artificial micro- and nanostructures have already found numerous applications in various sectors of optics and photonics. Periodic patterns are used as diffraction gratings, photonic crystals, ultrathin polarizers and wave retarders, antireflection coatings, optical filters, plasmonic waveguides, optical antennas and sensors, as well as substrates for surface enhanced fluorescence and Raman spectroscopy (SERS). Surface micro- and nanostructures have also been demonstrated to exhibit superhydro- and superoleophobicity and, for example, the so-called structural colors that do not use any dyes. These properties can lead to fascinating applications, e.g., in self-cleaning eyeglasses and touchscreens, as well as in various types of displays. This thesis describes the development of a set of nanofabrication techniques for manufacturing nanoscale optical components. One of the key ideas was to switch from conventional photolithography based on photoresist to a new type of maskless lithography making use of azobenzene-containing polymers (azo-polymers). This transition fundamentally changes the fabrication process, for example, eliminating wet processing steps, such as photoresist development and stripping. The azo-polymer-based interference lithography developed in this thesis is a fast and simple technique to pattern large-area arrays of perfectly ordered nanofeatures. In addition, the azo-polymers are insensitive to humidity and temperature fluctuations, as well as to stray light. These properties make them an attractive alternative to traditional photoresists. The invented nanofabrication technique was shown to be capable of patterning various materials, such as semiconductors, glass and metals. Using the technique we have fabricated various optical elements, such as plasmonic filters, ultrathin polarizers, all-metal reflective waveplates and substrates for surface enhanced Raman scattering with various degree of complexity. Maskless lithography allows for fast adjustment of the pattern parameters and nearly instant prototyping. The scaling up capability of the technique, meanwhile, opens up the door to industrial applications.
  • Piezoelectrically transduced temperature compensated silicon resonators for timing and frequency reference applications
    (2016) Jaakkola, Antti
     School of Science | Doctoral dissertation (article-based)
    Reference oscillators are used in a wide range of electronic devices for timing and for providing the frequency reference signals for wireless communications. Typically, an oscillator has to be based on a mechanical resonator, and for many decades, quartz crystals have served for this purpose. With the progress of microelectromechanical system (MEMS) technologies, silicon resonators have been developed for providing similar functionality as quartz. A silicon MEMS resonator can offer several advantages over quartz, such as smaller device size, decreased costs, and integration with other electronics. This work focuses on two challenges in silicon resonators: First, electromechanical transduction of silicon resonators has typically been achieved with electrostatic coupling, which is inherently quite weak and requires DC biasing of the devices and tends to complicate fabrication. Transduction based on a piezoelectric thin film on top of the resonator has been investigated as an alternative. Second, the resonance frequency of a silicon resonator is orders of magnitude more sensitive to temperature variations than that of a quartz crystal. Degenerate doping of silicon can be used to drastically reduce this effect. The first part of the work concentrates on the design, fabrication and characterization of piezoelectrically transduced silicon resonators. An oscillator based on a width extensional resonator operating at a frequency 24 MHz is demonstrated to have a phase noise -128 dBc/Hz at a 1-kHz offset from the carrier. An experimental test is conducted on piezoelectrically transduced square extensional mode resonators, whose dimensions are varied so that the main resonance mode occurs at a frequency range of f = 13 ... 30 MHz. As a result, an anchor coupling effect is identified and a subharmonic nonlinear coupling mechanism is discovered. In the second part of the work, the effect of degenerate doping on the elastic parameters of silicon is investigated experimentally, with a focus on temperature compensation. Resonance modes that can be temperature compensated using doping are identified, and design rules for the optimization of the frequency stability are developed. The elastic parameters of silicon are determined as a function of temperature and n-type doping up to a level of n = 7.5x1019cm-3, enabling modelling of the frequency-vs-temperature characteristics of an arbitrary resonator design. Extrapolation from the results yields a prediction of full second order temperature compensation in optimally designed resonators for n-type doping level above 1020cm-3. The prediction is experimentally verified by the demonstration of piezoelectrically transduced resonators with frequency stability within +/- 10 ppm on a temperature range ofT = -40 ... +85C, on par with the best quartz crystals.
  • Theoretical Description and Design of Optical Nanomaterials
    (2014) Grahn, Patrick
     School of Science | Doctoral dissertation (article-based)
    Recent developments in nanotechnology have made it possible to create a large variety of nanoparticles with predefined shapes. Nanoparticles that are much smaller than the optical wavelengths can appear as artificial atoms to light and, when assembled into a periodic three dimensional lattice, they form a crystalline optical nanomaterial. The main difference from natural materials is that the optical response of the material can be purposefully tailored by designing the constituting nanoparticles. In fact, optical nanomaterials can exhibit extraordinary optical properties that cannot be found in natural materials. However, in order to successfully design such artificial media, advanced theoretical methods are required. The main goal of the research described in this thesis was to develop a theoretical basis for a comprehensive treatment of optical nanomaterials. In the developed formalism, the microscopic optical response in the unit cell of a nanomaterial is characterized in terms of elementary electric current multipoles. These multipoles are fundamentally connected to the nanoparticles' geometry, which provides an efficient way to adjust and tune the optical response. The influence of higher-order multipole excitations is demonstrated by designing a nanoscatterer in which light cannot excite any electric dipole moment. In the thesis, it is shown that the macroscopic optical properties of nanomaterials can straightforwardly be described in terms of the interaction of optical plane waves with the planar arrays of nanoscatterers that compose the medium. Effective material parameters, such as the refractive index and wave impedance, naturally appear in this description in the form of simple analytical expressions. In contrast to existing theories, the introduced approach correctly handles also spatially dispersive materials, including those composed of noncentrosymmetric nanoscatterers. In such materials, two counterpropagating waves can experience the medium differently. The developed theory reveals the fundamental role of higher order multipoles and spatial dispersion in realizing extraordinary optical properties with designed nanomaterials. In particular, materials composed of asymmetric nanoparticles may find novel light-guiding and light-harvesting applications. Furthermore, nanomaterials can be designed to suppress optical reflection at certain interfaces only, which can be exploited, e.g., in new interferometric optical devices. Most of the introduced theory can be straightforwardly applied to other artificial media, such as radiofrequency metamaterials and, in some cases, photonic and phononic crystals.
  • Towards calorimetric detection of individual itinerant microwave photons
    (2016) Govenius, Joonas
     School of Science | Doctoral dissertation (article-based)
    This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions.  We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, single-photon microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation.  The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses.  In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.