This research covers the development and application of multiple low- noise high-bandwidth lockboxes to position the mirrors of four cascaded optical cavities using piezo actuators for frequency...Show moreThis research covers the development and application of multiple low- noise high-bandwidth lockboxes to position the mirrors of four cascaded optical cavities using piezo actuators for frequency locking with a preci- sion of 40 Hz. The design and fabrication of a custom printed circuit board that hosts two ARM-based microcontrollers for signal processing are elucidated. By measuring the impedance of the piezo actuators over a large range of frequencies, we found several electromechanical resonances ranging from a few kHz to a main resonance at 80 − 100 kHz. It was found that these resonances greatly impact the ability to lock the cavity to the laser source and thereby impose bandwidth limitation on the feedback. By avoiding excitation of such resonances by reducing the feedback bandwidth to below the first prominent resonance at 2.5 kHz, we were able to achieve a high quality lock of a single optical cavity. Using a reduced bandwidth of 250 Hz and a reduced modulation frequency of 1.9 kHz, we demonstrated the locking of four cascaded cavities and achieved an optical transmission of T ≈ 40%, limited mostly by optical alignment. Finally, we show an initial lock freeze procedure for three cascaded cavities in which 90% of the transmission during the locked state is retained for a period of 4.9 seconds, while providing no feedback on the piezos.Show less
Surface acoustic wave (SAW) resonators can confine and enhance the displacement associated with SAW phonons. SAW resonators are useful in quantum technology, where they are used to enhance the...Show moreSurface acoustic wave (SAW) resonators can confine and enhance the displacement associated with SAW phonons. SAW resonators are useful in quantum technology, where they are used to enhance the coupling between a single phonon and a semiconductor quantum dot (QD). In this thesis, the fabrication process of SAW resonators on GaAs with acoustic mirrors based on aluminum Bragg reflectors, and an investigation into the relation between the finesse of a resonator and the thickness of the aluminum mirrors are detailed. For this purpose, three resonators identical in design apart from the thickness of their aluminum mirrors (35 nm, 50 nm, and 100 nm) are fabricated. The finesse of these resonators is derived by examining their acoustic resonance spectra and displacement maps. Both types of measurements are performed with a fiber-based scanning Michelson interferometer. It is found that losses associated with the resonator limit the finesse. The maximal finesse is found to be F ≈ 11 for the 100 nm resonator. Based on the measurement results, it is hypothesized that reducing the resonator length will lead to a decrease in propagation loss, thereby raising the upper limit of the finesse. This project has been a step towards the optical detection of thermal phonons, with its final goal to detect single phonons.Show less
Surface acoustic waves (SAWs) propagate in piezoelectric materials and are generated by interdigital transducers (IDTs). SAW devices have applications in a variety of fields, under which...Show moreSurface acoustic waves (SAWs) propagate in piezoelectric materials and are generated by interdigital transducers (IDTs). SAW devices have applications in a variety of fields, under which telecommunication, biology and chemistry. The context of SAW devices in this research project is in quantum acoustics, in which the underlying idea is that quanta of sound waves (single phonons) can be used to transfer quantum information between different systems. As these systems often operate at GHz frequencies and are fabricated on Gallium Arsenide (GaAs) substrates, the goal of this research project is to fabricate (using electron-beam lithography) and characterize 1 GHz SAW devices on GaAs. The SAW emission by the IDTs is measured in the frequency domain, where good agreement is found with the coupling of modes model, allowing us to extract the SAW amplitude, the SAW velocity and the reflection coefficient of the IDT fingers. The IDTs are also measured in the time domain, were good agreement is found with the findings of the frequency domain measurements. Finally, a first step is made towards SAW resonators, which can enhance the SAW amplitudes significantly. This amplitude enhancement is crucial for quantum acoustics experiments, as high coupling between quantum dots and SAWs is needed. This project can therefore be thought of as the foundation for future quantum acoustics experiments.Show less
Sub-microsecond optoplasmonic detection has been utilized to measure the interactions of single ferritin, single apoferritin, and single gold nanospheres (GNSs) with an immobilized gold nanorod ...Show moreSub-microsecond optoplasmonic detection has been utilized to measure the interactions of single ferritin, single apoferritin, and single gold nanospheres (GNSs) with an immobilized gold nanorod (GNR) of dimensions 40 x 112 nm2. If a protein enters the near-field region of the GNR, a redshift in localized surface plasmon resonance (LSPR) occurs, which is quantified as amplitude changes in volts (V) in the performed time trace measurements of the scattered light by the GNR. This resulted in a relative change in scattering cross section (∆σ/σ) of 1.2% and 1.9% for the mean and maximum burst amplitude of the GNS, respectively. When applying the correlation between GNS and apoferritin found with the boundary element method (BEM) simulation, an expected mean burst amplitude of 0.27 mV (∆σ/σ = 0.24%) is anticipated, which falls within the achieved signal-to-noise ratio during the performed measurements. For ferritin ∆σ/σ was 0.73% and 1.04%, representing two measurements performed on different GNRs. These ∆σ/σ can be compared to 5.8% and 1.25% for GNS and ferritin, respectively, obtained from the BEM simulations, resulting in a good comparison between the measured and performed BEM simulations for the GNS and Ferritin.Show less
The imaging of biological tissue is an important aspect to understand the human body and the processes taking place at small scales. Existing optical and electron microscopy techniques face trade...Show moreThe imaging of biological tissue is an important aspect to understand the human body and the processes taking place at small scales. Existing optical and electron microscopy techniques face trade-offs between resolution and beam damage. Optical near-field electron microscopy (ONEM) is a newly developed technique combining optical microscopy with electron microscopy. A sample is illuminated from the back with light in the visible spectrum, a near-field shadow is cast on a low workfunction photocathode. Photons are converted into electrons by the photoelectric effect and finally, the electrons exit the photocathode towards the detection optics. As a result, ONEM allows the imaging of (biological) samples without exposure to the damaging electron beam without being diffraction limited. We explore the preparation of samples suitable for ONEM experiments consisting of a fused silica substrate with an indium tin oxide (ITO) coating and a slice of biological tissue from a mouse tail. We create a cap over the biotissue from either single-layer graphene or amorphous carbon and finally, we grow a thin caesium photocathode in situ to lower the workfunction. Using a blue laser with a wavelength of 450 nm, we show for the first time that optical near-field electron microscopy can be used to image biological tissue. We can distinguish features in ONEM matching features in PEEM when overlapping the images. The resolution of these features is in the order of 100 nm when using a 5.7 μm field of view. Furthermore, we show that a carbon or graphene layer is required for ONEM imaging, yet this layer decreases the resolution of the PEEM image.Show less
In the quest for finding a room temperature stable single photon source (SPS), defects in hexgonal boron nitride (hBN) have gained more and more attention in the previous years. This thesis studies...Show moreIn the quest for finding a room temperature stable single photon source (SPS), defects in hexgonal boron nitride (hBN) have gained more and more attention in the previous years. This thesis studies the single photon emit- ting properties of defects in a multi-layer hBN flake with a home-build confocal microscopy setup. As a reference system, nitrogen vacancy (NV) centers in nanodiamonds (NDs) are measured first, as there luminescent properties are well documented in literature. In order to improve on pre- vious work in this group, polarization selective elements are included in the optical setup. This inclusion allowed measurements of the absorption and emission dipoles of the SPSs and improved our results by (1) reduc- ing the background and (2) allowing more efficient excitation. We found that defects in hBN showed promising results in terms of single photon purity (g2(0) =0.09 ± 0.06) and brightness (35 KHz), outperforming the best single NV center (g2(0) =0.29 ± 0.07, brightness: 4 KHz). We also found that the hBN defects showed long term (up to 3000 ns) bunching behaviour, possibly related to the excitation power during measurements. This, together with the measurement of different hBN flakes, constitute interesting topics for future investigation.Show less
Microcavities play a significant role in the study of cavity quantum electrodynamics (CQED), as they induce efficient coupling of light and matter. Confining matter within a cavity, increases the...Show moreMicrocavities play a significant role in the study of cavity quantum electrodynamics (CQED), as they induce efficient coupling of light and matter. Confining matter within a cavity, increases the probability of deterministic interactions between e.g. a (quantum) particle and the light cast into the cavity by a laserbeam. Cavity parameters such as cavity length and radius of curvature of the micromirror define the cavity regime and consequently the physical phenomena that can be observed, from the bad-cavity (Purcell) regime to the strong coupling regime. We are looking for a highly controllable and repeatable way of producing microcavities with a small mode volume and a high finesse. This translates into the creation of micromirrors with radii of curvature between 10 μm and 50 μm and a depth of up to 1 μm. We offer an overview of micromirror production methods; CO2 laser ablation, focused ion beam milling (FIB), direct laser writing and current controlled curvature. For CO2 laser ablation the key parameters to control the dimensions of the ablated structure, are the power of the laser and the beamwaist. We perform simulations of the ablation process and show that micromirrors with the desired dimensions can be obtained by using a CO2 laser in a range between 420-440 mW and a beamwaist of 40 μm. We find that for these parameters clipping loss is negligible. Therefore, CO2 laser ablation meets the criteria and proves to be a reliable way of producing micromirrors.Show less
We successfully produce Josephson junctions by stacking NbSe2 flakes, where the twist angle and strain are responsible for a weak link. Field sweep measurements are taken to characterize these...Show moreWe successfully produce Josephson junctions by stacking NbSe2 flakes, where the twist angle and strain are responsible for a weak link. Field sweep measurements are taken to characterize these devices. These measurements highlight the importance of the direction of the applied magnetic field. Prompted by this, simulations of the critical current given an arbitrarily shaped interface area are presented and compared to measurements.Show less
There is a demand for high bandwidth down links from space to earth. A cubesat in a GEO could function as a relatively cheap access point to a high bandwidth communication channel with earth. This...Show moreThere is a demand for high bandwidth down links from space to earth. A cubesat in a GEO could function as a relatively cheap access point to a high bandwidth communication channel with earth. This thesis explores new ways to increase the bandwidth by identifying bottlenecks in the GEO- Earth communication channel and how to circumvent them. The diffrac- tion limit causes large beam spreading at GEO distance, holding back ad- vanced modulation techniques due to the inability to capture the whole wave front. In this case, a modulation scheme using only a few bits should be chosen, allowing to modulate as fast as possible. The low signal inten- sity can be detected with more sensitivity by making use of a quantum enhanced receiver. From GEO to Earth, data rates around 50 Gbps are possible. Additionally, the atmosphere introduces spatial incoherence. To mitigate the effects of the atmosphere, a modulation scheme should be chosen that exploits modulation vectors which are orthogonal to the spa- tial dimension, such as polarization or wavelength. This gives a modula- tion scheme with many degrees of freedom. To deal with the complexity, a variational auto-encoder deep neural network is used to act as the modu- lator and demodulator. The variational distribution is chosen to match the noise introduced by an atmospheric channel. Using this scheme, we were able to find encodings that increase the density of symbols in phase space relative to the noise. This approach is especially promising in a bandwidth limited channel.Show less
Inelastic electron tunneling spectroscopy (IETS) is a tool used to research the vibrational modes in a tunneling junction. This report describes the setup of a cryogenic scanning tunneling...Show moreInelastic electron tunneling spectroscopy (IETS) is a tool used to research the vibrational modes in a tunneling junction. This report describes the setup of a cryogenic scanning tunneling microscope (STM). Potentially this STM can be used in the research to the edge modes of graphene edge junctions. Gold samples were used for the calibration of the z stage of the setup by using two different methods (histogram based/line cut based). Experiments at cryogenic temperatures seemed to have difficulties measuring topographic images at 77K. These difficulties are most likely caused by condensation of atmospheric gasses still present in the system. To solve this problem, a heater is required to prevent material condensing on the sample or tip. In order to implement IETS in the system it is proposed to used a combination of a heater and thermometer in order to keep the system clean.Show less
Using the ARRES technique with a low-energy electron microscope researchers can measure a part of the electronic bandstructure of the surface of a sample material. Specifically the unoccupied...Show moreUsing the ARRES technique with a low-energy electron microscope researchers can measure a part of the electronic bandstructure of the surface of a sample material. Specifically the unoccupied electron states above the vacuum level can be scanned by firing electrons at the sample which can be absorbed if they have the same energy as those unoccupied states. By tilting the electron beam the in plane momentum of the incoming electrons can be varied, thus mapping both dimensions of the bandstructure. This report presents an expansion of the software used to create those ARRES bandstructure maps. The new tool uses those local measurements of the surface bandstructure to map numerical properties which can be derived from those bandstructures. To demonstrate the concept the number of layers, bandgap and effective electron mass are mapped of a sample consisting of a few atomic layers of MoS2.Show less
This works investigates the possibility of building a full-field X-ray fluorescence microscope using Silicon Pore Optics (SPO) technology in the Advanced Kirkpatrick-Baez (AKB) configuration. AKB...Show moreThis works investigates the possibility of building a full-field X-ray fluorescence microscope using Silicon Pore Optics (SPO) technology in the Advanced Kirkpatrick-Baez (AKB) configuration. AKB system focuses the X-ray beam by reflection from four curved mirrors. Such an imaging system is an aberration-free and relatively easy to manufacture optics. We start with a simple two-mirror Kirkpatrick-Baez (KB) system as a first step and a basis for understanding and modelling of Advanced KB. Using the derived equations, a KB system is parameterized and characterized. A sensitivity study is conducted to find the optimal system parameters under constraints enforced by microscopy application and SPO technology. Performance of an optical system is assessed on the basis of solid angle, tightness of focal spot and field of view. The conclusions made for the KB system are relevant for the AKB system as well. SPO technology was initially developed to enable light-weight large area telescopes with resolution of a few arc seconds. It uses the concept of stacking of slightly curved reflective mirrors of high quality. We were interested to explore whether SPO can improve X-ray microscopy. However, it was found that using SPO technology for the case of full-field X-ray microscopy presents no benefits in terms of increase of solid angle. Nonetheless, the technology can be used to provide self-standing stacks with only upper reflecting plate for AKB-based microscopy setup.Show less
Within the diverse order of beetle species that produce color through structural coloration, some beetles produce light with a strong left-handed circular polarization caused by a chiral structure...Show moreWithin the diverse order of beetle species that produce color through structural coloration, some beetles produce light with a strong left-handed circular polarization caused by a chiral structure in their outer shells. From an evolutionary perspective, there should be no substan- tial benefit to left-handed over rather-handed polarization. This raises the question why no beetles have been found showing right-handed polarization. Large-scale beetle surveys are required to investigate this question further. A systematic beetle classification scheme based on polarization would aid in such a survey and highlight species with interesting optical prop- erties for further investigation. To this end we constructed a setup capable of measuring the Stokes parameters of a beetle specimen for angles of incidence and observation. We tested this setup on the beetle species protaetia speciosa jousselini and verified that the setup produces accurate results. We are however sceptical this method will result in identifying beetle-specific Stokes parameters, as their values vary with respect to both location on the beetle cuticle as well as observational angles. Furthermore we provide quantitative evidence for earlier re- ported inversion of the polarization handedness in beetle species jousselini at large angles of observation. We detect such inversion at angles of observation of at least 70◦.Show less
This work investigates hexagonal Boron Nitride (hBN), being a promising source of single photons, with a home-built confocal microscope. Using colloidal quantum dots as a reference, the setup was...Show moreThis work investigates hexagonal Boron Nitride (hBN), being a promising source of single photons, with a home-built confocal microscope. Using colloidal quantum dots as a reference, the setup was optimized and characterized. All in all it showed decent efficiency and resolution, close to the limit of the current objective. The hBN sample, provided by a scientific group in Denmark, was investigated in two stages. First, primary investigation revealed that there are two types of flakes present on the sample; we call them flat and fluffy flakes. They were observed using pump reflection, fluorescence and darkfield imaging. All measurements present clear distinction between the flakes including difference in form, structure, fluorescent and reflective properties. Fluffy flakes show larger absolute strength of fluorescence signal and pump reflection. Bleaching measurements indicate greater stability of flat flakes compared to fluffy flakes. In the second stage, special defects on the flat flakes were studied by means of spectral and lifetime measurements. These defects were found using position-resolved spectral scans. Some of the defects show spectra with two-peak structure and prominent spectral lines, resembling those reported in literature. Spectra of fluffy flakes present uniform distribution of bulk fluorescence. This research is a stepping stone to a better understanding of single-photon fluorescent defects in 2D materials.Show less
In order to curb the growing energy demand and environmental impact of data centers, breakthrough technologies bringing increased energy efficiency are required. Superconducting computing promises...Show moreIn order to curb the growing energy demand and environmental impact of data centers, breakthrough technologies bringing increased energy efficiency are required. Superconducting computing promises massive energy savings through near-dissipationless operation. So far, multiple designs for superconducting memory are proposed, but none meets all requirements for an implementable device. To this end, we propose a novel design of superconducting memory (SCM) device, using two stable magnetic spin textures in a single-layer elliptical SFS Josephson junction. We observe a change in critical current between the two states by a factor of five at remanence. Using a novel technique to quantify the effects of stray fields using micromagnetic simulations, we conclude that the shift in critical current is caused by a large contrast in stray field strength between the two magnetic states. Furthermore, we verify that the switching process is deterministic and unambiguous during read-out. Future pathways to scale up this memory device are proposed.Show less
