Microtechnology and Nanoscience

Title: Investigation and Optimization of Low FSR Photonic Molecule Microcombs
Date and time: 15 August 2025, 14.00 Location: Fasrummet A820, MC2
Supervisor: Carmen Haide López Ortega Examiner: Victor Torres Company Opponent: Saba Kheviashvili
Abstract: Frequency combs are a key technology for many applications, such as optical clocks, precision spectroscopy, and massive parallelization in optical communications. Integrated frequency combs leveraging Kerr nonlinearity can operate with milliwatt-level pump powers, though their conversion efficiencies (CE) typically remain below 10%. In recent years, photonic molecules have been introduced to overcome the problem of generally low CE of microcombs. Reported efficiencies have exceeded 50% by transitioning from single cavities to photonic molecule configurations. However, this improvement has only been shown for photonic molecules with a free spectral range (FSR) of 100 GHz. For low FSR photonic molecules this improvement is challenging, due to higher intrinsic losses and a power distribution across the larger cavity volume.
In this thesis we used Ikeda map-based simulations to identify critical parameters for achieving higher CE in low FSR configurations. We characterized existing chips and compared measured comb spectra with simulations to extract key parameters and quantify the currently achievable CE for different FSRs. This data was used to analyze the influence of multiple parameters on the CE, including coupling factors, input power, and comb detuning.
Based on these investigations, we developed improved parameter sets for microresonators with FSRs of 25 GHz and 50 GHz. The simulated CE increased from 25% (simulation of former devices) to over 45% for the 25 GHz design. For the 50 GHz design, we present a parameter set that achieves a CE over 65%. These CE values can be achieved over a range of commonly used input powers in the milliwatt regime. These results demonstrate the potential to design photonic molecules with tailored FSRs, enabling greater flexibility across applications.

Title: Circuit Waveguide Electrodynamics with Fluxonium Qubits
Date and time: 22 August 2025, 11.00 Location: Kollektorn, MC2
Supervisor: Aamir Ali Examiner: Simone Gasparinetti Co-promotor: Kristiaan De Greve (KU Leuven) Referee: Thilo Bauch
Abstract: Fluxonium circuits are emerging as a promising alternative to transmons for quantum information processing. In addition to the standard capacitor and Josephson junction, a fluxonium includes a superinductor, which suppresses charge noise, increases coherence, and provides large anharmonicity. Its rich transition spectrum, governed by non-trivial selection rules sensitive to external flux, enables dissipation engineering with potential applications that include fluorescence-based resonator-free readout, electromagnetically induced transparency, and $\Lambda$-systems. Previous studies have mostly relied on three-dimensional waveguides or cavities, which limit circuit scalability and integration of elements such as Purcell filters. In this work, we fabricate fluxoniums using Josephson junction arrays as superinductors and couple them to both coplanar-waveguide resonators and semi-infinite transmission-line waveguides for comparative studies. We characterize dispersive coupling to resonators, demonstrating tunability via external flux. Using spectroscopy, we identify transitions among the first four energy levels (0-1, 0-2, 1-2, 0-3) in a fluxonium coupled to a waveguide. We perform power-dependent measurements to extract coupling rates, which are consistent with theoretical predictions. We identify a parameter regime where the 0-1 transition is suppressed while 0-2 and 1-2 remain comparable, providing a pathway to implement a $\Lambda$-system within a single artificial atom. Future device iterations aim to optimize this regime, with the ultimate goal of realizing a robust single-atom microwave photodetector.

Title: <span style="font-size: 12.0pt; font-family: 'Helvetica',sans-serif; mso-fareast-font-family: Aptos; mso-fareast-theme-font: minor-latin; color: black; mso-ansi-language: SV; mso-fareast-language: SV; mso-bidi-language: AR-SA;">Feedback-driven optimization of hot-carrier solar cell
Date and time: 28 August 2025, 14.00 Location: Luftbryggan, MC2
Supervisor: Bruno Bertin-Johannet Examiner: Janinie Splettstösser Co-promoter: Bart Sorée (KU Leuven) Referee: Thilo Bauch
Abstract: Solar cells convert energy absorbed from the sun into electrical power and they are, therefore, of high interest for green energy solutions. Ongoing research to improve solar cells deals with a broad range of aspects from improved light absorption to pushing the limits of efficiency searching for keys in materials research and device&nbsp;design.
Hot-carrier solar cells, which are at the focus of this thesis aim at improving the output power of a solar cell by exploiting the excess energy of the light-generated charge carriers, which is lost in fast relaxation processes in standard solar cells. The drawback of these hot-carrier solar cells is however that low-energy charge carriers are not exploited for power generation.
In this thesis, we propose and theoretically analyze a feedback mechanism to adapt the energy-filtering scheme for the extraction of hot carriers to the nonequilibrium conditions under which the solar cell is operated. This is done by coupling a quantum dot to the electron collector, which thereby measures the collector’s potential. This quantum dot is furthermore capacitively coupled to the energy filter, so that it modifies its properties depending on the collector potential. We demonstrate that this feedback can improve the charging time, when the solar cell is used to charge a battery, and it can improve the power generated, when the solar cell is connected to a circuit with a load.
In this thesis we use a scattering matrix approach to model the solar cell operation. The load resistance or battery capacitance, as well as the filter properties enter the model through general variables, which could in the future be adapted to experimentally relevant conditions, in order to test the opportunities of this proposal for realistic devices.

Title: C-axis transport in YBCO nanostructures
Date and time: 1 September 2025, 13.00 Location: Kollektorn, MC2
Supervisor: Floriana Lombardi Co-supervisor: Alessia Garibaldi Examiner: Thilo Bauch Co-promotor: Bart Sorée (KU Leuven) Referee: Samuel Lara Avila
Abstract: Despite nearly four decades since the discovery of high-temperature superconductivity (HTS) in cuprates, the underlying microscopic mechanisms remain one of the most profound open questions, in Condensed Matter Physics. The presence of multiple, intertwined electronic orders, driven by strong electron-electron correlations, adds further complexity to their behavior. This is reflected by a highly intricate phase diagram. Specifically we focus on YBa2Cu3O7−δ (YBCO), as our choice of HTS. While in-plane transport (ab-plane) has been extensively studied, little is known about the out-of-plane (c-axis) transport, especially in thin film systems. This is due to the challenges associated with growing the film in other, suitable orientations (most convenient and highest quality YBCO films are c-axis oriented) or fabricating suitable nanostructures to probe it. In addition, the out-of-plane conduction pathway is significantly more resistive than the in-plane one. Despite these challenges, thin film platforms offer the ability to investigate properties that are inaccessible in bulk, single crystals. Modifications of the YBCO ground state by an engineered superpotential can only manifest in thin film systems, where the substrate morphology has been patterned into a superstructure (for example, nanofacets). This superpotential has been shown to induce changes such as enhancement of superconductivity, unidirectional Charge-Density Wave (CDW) and even nematicity. Therefore, developing a platform to gain access to c-axis transport properties in thin films (where the effect of such superpotential modifications on c-axis can also be observed) offers a complementary perspective to the ab-plane studies. Together, they may hold the key to unlocking the mystery of HTS. Previous studies on c-axis transport in YBCO are mostly performed on bulk single crystals. They have revealed that charge transport along the c-axis differs fundamentally from that in the ab-plane. In underdoped samples, a crossover from metallic to semi conducting behavior is observed with decreasing temperature. This behavior could be related with the opening of the pseudogap. Moreover, the anisotropy between the in-plane and out-of-plane responses becomes more pronounced at lower dopings, suggesting a quasi-two-dimensional character. This thesis focuses on creating a reliable platform for probing the out-of-plane transport by mesa devices. These devices are made on thin film YBCO, which is grown by pulsed-laser-deposition (PLD). Furthermore, we describe two fabrication routes for the mesa devices using single-lithography (but with larger areas), or by utilizing a more intricate three-lithography method, which grants much smaller areas. Then we use them to probe the c-axis transport on 50nm thick, YBCO films.
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Presentation of master thesis

Master and Bachelor thesis presentations at Microtechnology and Nanoscience

15 August 2025

14:00 Nora Dzieia, MPWPS

22 August 2025

11:00 Saba Kheviashvili, EMNAN

28 August 2025

14:00 Krishna Lyn Delima, EMNAN

1 September 2025

13:00 Klinti Sinomjeri, EMNAN