Student Projects

Physics experiments are becoming more and more complex, and the technical requirements they call for are pushing the limits of technology ever further. For example, the very rapid (ns to  s) switching on and off of signals is now an absolute necessity for certain experiments such as those carried out in the Photonics Laboratory [1,2]. For example, after demonstrating the cooling of a levitating nanoparticle to its ground state [2], the next step is to observe macroscopic quantum mechanics [3]. To achieve this goal, protocols consisting of rapid changes in the signals sent to it are required. Devices such as solid-state switches [4] make this possible. However, a single experimental set-up often requires a large number of these switches, and the multiplication of devices makes experimental set-ups tedious to operate (both from a hardware and software point of view). A compact "all-in-one" system is therefore highly desirable. Commercial implementations already exist, but they are very expensive, so the development of an in-house system is promising to improve a maximum of laboratory-scale experimental set-ups. The aim of this semester project is to design a compact, fast and low-noise multi-switch system. You will design and create the PCB (printed circuit board) system from existing single-switch components, which you will organize into a single 10-input to 10-output device. You will pay particular attention to ensuring that the characteristics of the single switches are not affected (target switching speed in the order of a few ns) and that the whole system is low-noise. You must have a solid understanding of electronics and a keen interest in printed circuit board design. In addition, you should have a quantitative approach and be a critical, independent thinker who enjoys contributing to a broader research effort as part of a team. References: [1] Tebbenjohanns et al. Nature 595.7867 (2021): 378-382. [2] Militaru et al., Physical Review Letters 129.5 (2022): 053602 [3] Gonzalez-Ballestero, et al. Science 374.6564 (2021): eabg3027. [4] https://www.minicircuits.com/pdfs/M3SWA-2-50DRB+.pdf Prerequisites: Good knowledge of and strong interest in PCB design, electronics skills, quantitative approach. FPGA and Python programming is a plus.

Supervisor: Andrei Militaru (andreimi@ethz.ch), Louisiane Devaud (ldevaud@ethz.ch)

Two-dimensional semiconductors are some of the most promising materials for future optoelectronics and photonics. The strong Coulomb interaction and the quantum confinement lead to the formation of strongly bound electron-hole pairs, so called excitons. Recently, van der Waals magnets such as CrSBr gained attention due to the additional coupling of excitons with the magnetic order, while still strongly interacting with light [1,2]. However, most studies rely on optical excitation, whereas direct electrical access still remains a challenge. The goal of this semester project is to electrically excite excitons in atomically thin CrSBr and measure both the electrical and optical response at temperatures down to 4 K. Therefore, you will first build tunnel junctions from metallic graphene and isolating hBN by exfoliation from bulk crystals. After electrically characterizing these structures you will add thin flakes of CrSBr on top in order to electrically generate excitons. The device is then mounted inside a closed-cycle cryostat and measured across the whole temperature range with focus on the Néel temperature at around 132 K. The project will combine optical spectroscopy with electrical measurements in a challenging cryogenic environment. Furthermore, you will gain insight into fabrication of van der Waals heterostructures by building and exfoliating your own devices. Finally, applying our knowledge of solid state physics and optics will allow us to gain more insight into these new and exotic atomically thin layers. References: [1] Wilson, N.P., Lee, K., Cenker, J. et al. Interlayer electronic coupling on demand in a 2D magnetic semiconductor. Nat. Mater. 20, 1657–1662 (2021). [2] Dirnberger, F., Quan, J., Bushati, R. et al. Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons. Nature 620, 533–537 (2023). [3] Wang, L., Papadopoulos, S., Iyikanat, F. et al. Exciton-assisted electron tunnelling in van der Waals heterostructures. Nat. Mater. 22, 1094–1099 (2023).

Supervisor: Jonas D. Ziegler (Zieglerj@ethz.ch)

Silicon nanoparticles are interesting scatterers in the optical regime due to their high refractive index, giving rise to the rich physics of Mie scattering for rather small particle diameters [1]. While the synthesis of Si nanoparticles of high quality has been challenging until recently, these materials are now available and can be investigated and put to use by the photonics community [2]. The high surface-to-bulk ratio and the geometrical confinement, as in nanoparticles, often leads to different properties as compared to bulk material. One of these properties is the carrier lifetime after optical excitation by a short laser pulse. The ultra-fast optical response of these particles is governed by the decay of excited electrons. The goal of this semester thesis is to acquire a statistic of the carrier lifetime of different sized SiNP, which are then compared to the bulk carrier lifetime of silica. You will start by fabrication of a sample, that allows to measure individual SiNPs. You will characterize the sample geometry by means of atomic force microscopy and measure the carrier lifetime in a state-of-the-art time-domain pump-probe setup. You should have a solid understanding of optics and interest to learn the basics of sample fabrication and characterization. Furthermore you should have basic knowledge of data analysis. You should have a quantitative approach and be a critical and independent thinker who enjoys to contribute to a larger research effort of a team. References: [1] Sugimoto et al., Adv. Opt. Mat. 8, 200033 [2] Thiessen et al., Chem. of Mat. 31, 678 Prerequisites: Good knowledge of and strong interest in optics and electronics, practical lab skills, quantitative approach.

Supervisor: Moritz Cavigelli (cmoritz@ethz.ch)

Quantum control over the motional degrees of freedom of a levitated object has been recently achieved both using passive (i.e. cavity-based) [1] and active feedback schemes [2,3]. Asymmetric levitated objects also possess well defined rotational degrees of freedom that experience restoring optical torques. As a result, the object orientation performs small angle oscillations (librations) about some equilibrium position determined by the beam polarization and by the particle optical susceptibility tensor. A growing control over the librations is underway, yet the preparation of a high purity quantum state (e.g. the groundstate) has remained an elusive task. In this project the student will work with an existing standing-wave optical trap where librational modes frequencies reach the MHz frequency band. The student will need to: 1. Design a homodyne receiver to perform quantum limited measurements of the particle orientation and determine the absolute measurement efficiency of the apparatus. 2. Characterize an existing fast light polarization controller based on an electro optical modulator (EOM) and show that the polarization controller can be used to exert a dynamical torque on the levitated object. 3. Implement and optimize a digital feedback controller and demonstrate the capability to reduce the amplitude of the thermal librational motion in moderate vacuum. 4. Characterize the feedback cooling performance while gradually approaching ultra-high vacuum. Provided that the radiation torque shot noise limited regime can be reached for pressures about 10-8 mBars and the measurement efficiency is above 1/9, this will allow the student to prepare the nanoparticle in the librational groundstate. Fig.1 - Sketch of the experimental apparatus. A Sagnac interferometer based optical tweezer traps an asymmetric nanoparticle. The outcomes of efficient quantum limited measurements of the object angular displacement are fed to a digital filter producing a feedback signal that is transduced into an optical torque by modulating the linear polarization of the trapping laser. The synthesised optical torque counteracts angular fluctuations effectively cooling the librational degree of freedom. References: [1] U. Delić et al. Science 367 (6480), 892-895 (2020) [2] L. Magrini et al. Nature 595 (7867), 373-377 (2021) [3] F. Tebbenjohanns et al. Nature 595 (7867), 378-382 (2021) Prerequisites: Optics, Electronics, Quantum measurement theory.

Supervisors: Nicola Carlon Zambon (carlonn@ethz.ch)