Umeå universitets logga

Characterizing the energy performance of building components and locating anomalies is necessary for effectively refurbishing existing buildings. It is often challenging because defects in building envelopes deteriorate without being visible. Passive infrared thermography (PIRT) is a powerful tool used in building inspection. However, thermal image interpretation requires significant domain knowledge and is prone to artifacts arising from a complex interplay of factors. As a result, PIRT-based inspections require skilled professionals, and are labor-intensive and time-consuming. Artificial intelligence (AI) holds great promise to automate building inspection, but its application remains challenging because common approaches rely on extensive labeling and supervised modeling. It is recognized that there is a need for a more applicable and flexible approach to leverage AI to assist PIRT in realistic building inspections. In this study, we present a label-free method for detecting anomalies during thermographic inspection of building envelopes. It is based on the AI-driven prediction of thermal distributions from color images. Effectively the method performs as a one-class classifier of the thermal image regions with a high mismatch between the predicted and actual thermal distributions. The algorithm can learn to identify certain features as normal or anomalous by selecting the target sample used for training. The proposed method has unsupervised modeling capabilities, greater applicability and flexibility, and can be widely implemented to assist human professionals in routine building inspections or combined with mobile platforms to automate the inspection of large areas.

We use a vortex retarder approach to generate few optical cycles light pulses carrying orbital angular momentum and arbitrary polarization. The optical vortices' structure is then reconstructed in the spatio-temporal domain.

Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing.

We use a vortex retarder-based approach to generate few optical cycles light pulses carrying orbital angular momentum (OAM) (known also as twisted light or optical vortex) from a Yb:KGW oscillator pumping a noncollinear optical parametric amplifier generating sub-10 fs linearly polarized light pulses in the near infrared spectral range (central wavelength 850 nm). We characterize such vortices both spatially and temporally by using astigmatic imaging technique and second harmonic generation-based frequency resolved optical gating, respectively. The generation of optical vortices is analyzed, and its structure reconstructed by estimating the spatio-spectral field and Fourier transforming it into the temporal domain. As a proof of concept, we show that we can also generate sub-20 fs light pulses carrying OAM and with arbitrary polarization on the first-order Poincaré sphere.

In the past 20 years, we have reached a broad understanding of many light-driven phenomena in nanoscale systems. The temporal dynamics of the excited states are instead quite challenging to explore, and, at the same time, crucial to study for understanding the origin of fundamental physical and chemical processes. In this review, we examine the current state and prospects of ultrafast phenomena driven by plasmons both from a fundamental and applied point of view. This research area is referred to as ultrafast plasmonics and represents an outstanding playground to tailor and control fast optical and electronic processes at the nanoscale, such as ultrafast optical switching, single photon emission, and strong coupling interactions to tailor photochemical reactions. Here, we provide an overview of the field and describe the methodologies to monitor and control nanoscale phenomena with plasmons at ultrafast timescales in terms of both modeling and experimental characterization. Various directions are showcased, among others recent advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics, in which charge, spin, and lattice degrees of freedom are exploited to provide active control of the optical and electronic properties of nanoscale materials. As the focus shifts to the development of practical devices, such as all-optical transistors, we also emphasize new materials and applications in ultrafast plasmonics and highlight recent development in the relativistic realm. The latter is a promising research field with potential applications in fusion research or particle and light sources providing properties such as attosecond duration.

Pulses of light that are both ultrashort and ultraintense are often generated using optical parametric amplifiers (OPA). These are capable of driving highly non-linear interactions with matter, which are interesting when studying the fundamental laws of our universe. Furthermore, they are also used in many scientific and industrial applications, such as particle accelerators, inertial-confinement nuclear fusion, and medical diagnostics and treatment. This thesis explores the diagnostic and optimization of pulses of light with extreme properties and utilizes them to drive electron acceleration.

The applied light pulses with very short duration (<5 fs) and high peak power (>10 TW) are sensitive to develop spatio-temporal aberrations. These are color-dependent distortions that can significantly degrade the pulse properties, like peak-intensity, and affect their applicability. Furthermore, in most cases they are not easy to correctly diagnose, with current tools failing to provide widely applicable solutions. In this thesis, we describe a new type of spatio-temporal coupling that is especially relevant for optical parametric synthesizers (OPS), systems that coherently combine multiple OPA stages. To do this, we have contributed to the development of two methods for the characterization of such aberrations, the so-called simplified-INSIGHT and HASO multispectral. These enabled us to further improve the structure of our OPS and laser systems.

We also explored the applicability of light pulses to drive relativistic electron acceleration in vacuum. To this end, an injection system using nanotips is presented, capable of inserting electrons spatially in the focus and temporally in the most intense light-cycle. This way, vacuum laser accelerated electrons of up to 14 MeV were detected using a tight focusing configuration (f#1) and their properties characterized. Furthermore, we investigated the dependence of the acceleration process when the focusing geometry is relaxed (f#3). This resulted in the unexpected outcome of similar electron energies in both cases, although the intensity was ten times reduced. This indicates that the decrease in accelerating field strength is compensated by longer acceleration lengths, which is not predicted by currently existing analytical models. 

Vacuum laser acceleration (VLA) of electrons has been an intense field of research for a long time due to the extremely high (>1 TV/m) accelerating fields. However, it is very challenging to realize and only a few promising experiments have been performed which have demonstrated the principle. Here, we report on the interaction of relativistic intensity (10-20 Wcm-2) sub-two optical cycle (<5 fs) laser pulses with nanotips to realize and optimize VLA. Various properties of accelerated electrons (angular distribution, charge, and electron spectrum) are measured with different intensities and carrier envelope phases of the laser pulse. Among others, waveform dependence of the electron propagation direction is observed. Furthermore, comparable or even higher electron energies beyond 10 MeV are detected with lower laser intensity, i.e., longer focusing, than with high intensity. These surprising results are reproduced using Particle-In-Cell simulations, which indicate a nanophotonics electron emission from the nanotip followed by VLA. In fact, the unexpected observations are a direct proof of the VLA process and provide a way to optimize it towards higher energy, isolated, attosecond electron bunch generation.

Optical parametric amplification (OPA) is a powerful tool for the generation of ultrashort light pulses. However, under certain circumstances, it develops spatio-spectral couplings, color dependent aberrations that degrade the pulse properties. In this work, we present a spatio-spectral coupling generated by a non-collimated pump beam and resulting in the change of direction of the amplified signal with respect to the input seed. We experimentally characterize the effect, introduce a theoretical model to explain it as well as reproduce it through numerical simulations. It affects high-gain non-collinear OPA configurations and becomes especially relevant in sequential optical parametric synthesizers. In collinear configuration, however, beyond the direction change, also angular and spatial chirp is produced. We obtain with a synthesizer about 40% decrease in peak intensity in the experiments and local elongation of the pulse duration by more than 25% within the spatial full width at half maximum at the focus. Finally, we present strategies to correct or mitigate the coupling and demonstrate them in two different systems. Our work is important for the development of OPA-based systems as well as few-cycle sequential synthesizers.

Vacuum laser acceleration (VLA) is a paradigm that utilizes the strong fields of focused laser light to accelerate electrons in vacuum. Despite its conceptual simplicity and a large existing collection of theoretical studies, realizing VLA in practice has proven remarkably challenging due to the difficulties associated with efficient injection: the electrons to be accelerated must be pre-energized and temporally compressed below an optical half-cycle before timely entering the rapidly oscillating fields of the laser. Therefore, only a handful of experiments have been published up to date, and a knowledge gap remains [1-3].

For many applications of extreme ultraviolet (XUV) and X-ray pulses, a small focus size is crucial to reach the required intensity or spatial resolution. In this article, we present a simple way to characterize an XUV focus with a resolution of 1.85 µm. Furthermore, this technique was applied for the measurement and optimization of the focus of an ellipsoidal mirror for photon energies ranging from 18 to 150 eV generated by high-order harmonics. We envisage a broad range of applications of this approach with sub-micrometer resolution from high-harmonic sources via synchrotrons to free-electron lasers.