Cavity-Based Membrane Scanning Force Microscopy
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2024
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Doctoral Thesis
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Abstract
Highly-stressed silicon nitride resonators are versatile mechanical elements that are an ideal platform for force sensing. Their size, geometry, mass, and frequency can be optimized to suit a wide range of applications. They are especially promising as sensors for scanning force microscopy, including magnetic resonance force microscopy (MRFM), a three-dimensional non-destructive imaging technique. So far, MRFM has achieved imaging of a virus sample with nanometer resolution in three dimensions and subnanometer resolution in a single dimension. Most experiments in MFRM have used low-spring constant cantilever as mechanical elements, leading to technical challenges such as "snap-in" and "non-contact friction". High-stress silicon nitride resonators have much higher spring constants compared to cantilevers, making them auspicious candidates for beyond-cantilever MRFM systems. In this thesis, we present the first steps towards a new generation of membrane and string MRFM.
As a first step, we present characterization of soft-clamped silicon nitride strings at millikelvin temperatures. Cryogenic temperatures are beneficial for most sensing applications – including MRFM – due to the reduced thermal motion of the oscillators. We measured the quality factor Q depending on the sample stage temperature and the duty cycle of our readout laser. We observed an increase in Q for temperatures below 200 mK, which can be explained by the freezing-out of two-level systems. Our result will lead to better force sensitivity once we perform MRFM measurement at low temperatures.
Next, we investigated the interaction between a scanning tip and a silicon nitride membrane at room temperature. We observed that the membrane resonator’s quality factor was not drastically affected even in the closest vicinity of the scanning tip, i.e., there was no degradation of the membrane’s mechanical properties. Scanning a sharp tip over the membrane allowed us to map gold nanoparticles and tobacco mosaic virus particles deposited on the membrane. The double-defect membrane serves as a sample plate and a sensor simultaneously.
Finally, we realized an optical cavity for displacement readout of a soft-clamped silicon nitride membrane, which included a scanning tip. We measured the thermal force noise acting on the resonator, showing that our bandwidth is limited by laser phase noise. A feedback loop was implemented to reduce the laser phase noise by 25 dB. This led to an increase of the thermally-limited bandwidth of the resonator by a factor of 3. Preliminary imaging measurements demonstrated the scanning capabilities of the cavity-based membrane scanning force microscope.
The presented work takes an important step towards a functional implementation of a silicon nitride resonator in a scanning force microscope capable of detecting nuclear spins.
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Examiner : Degen, Christian
Examiner : Eichler, Alexander
Examiner : Schliesser, Albert
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ETH Zurich
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03906 - Degen, Christian / Degen, Christian