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Quantum Limited Atomic Force Microscopy

Periodic Reporting for period 2 - Q-AFM (Quantum Limited Atomic Force Microscopy)

Reporting period: 2020-01-01 to 2021-06-30

The field of Atomic Force Microscopy (AFM) is limited by the sensitivity of the resonant mechanical force sensor. Our vision is a radical new design of the force sensor, which will result in radical improvement of the AFM. The AFM is important to society. Being one of the most important tools of nanotechnology, the AFM offers very high resolution, quantitative images of surfaces, helping scientists and engineers to design better materials, catalysts, and understand fundamental physical and chemical process at the nanometer scale.

We will adopt a new paradigm for the resonant mechanical force sensor used in Low Temperature Atomic Force Microscopy (LT-AFM). Our ultimate goal is a Quantum-limited Atomic Force Microscope (Q-AFM), where the force sensor is working at the fundamental limit of action and reaction set by quantum physics. Achieving this limit will result in three orders of magnitude improvement in force sensitivity, and five orders of magnitude in measurement bandwidth, beyond the current state-of-the-art. This gain in performance will translate to a radical increase in imaging speed and in the information content of images. Our sensors will lead to a revolution in SPM, where multi-dimensional data sets are acquired in seconds, as opposed to several days as is the current practice.

The main objectives of the QAFM project are:

1. The development of strong-coupling resonant mechanical force sensors.

2. Adapting Low-Temperature Atomic Force Microscopy (LT-AFM) so multifrequency techniques so that multifrequency methods can be used with the existing qPlus sensor technology.

3. Integrating the new strong-coupling sensor designs in a LT-AFM platform.
In the first year we spent a lot of time on literature study, to better understand what has been done and to find inspiration for our sensor design. We do not recount on this study here. After much reading and discussion, we settled on a design concept and have worked to build up simulations tools and calculate basic design parameters. We identified the critical dimensions and features of the design and we have begun fabrication to see what we can achieve in a real device. We developed a classical nonlinear model to simulate and study the dynamics of our sensor and readout concept. We have also made preparations for low temperature and high-frequency measurements. This work was carried out in WP1, and a detailed description is given in the technical report part B.

In the first year we also worked to adapt the multifrequency AFM methodology to the case of very high Q resonance, typical of AFM in vacuum and at low temperature. We wrote simulation code and checked out different drive and measurement schemes in simulation. We have also worked on the theory force reconstruction and have checked our ideas using simulated data. We made a deeper analysis of qPlus sensor readout using an RF tank circuit. Our analysis should that this scheme will not work, so we have scratched this idea in our original proposal. Our experiments with Intermodulation AFM and the qPlus sensor pointed to unforeseen difficulties with the traditional multifrequency feedback method. We have therefore defined a new task; to implement a fully programable and very flexible approach to AFM feedback. This work was carried out in WP2, and a detailed description is given in the technical report part B.

In the second period of the project, up to the end of year 2.5 we settled on a few different designs and we devoted much of our effort on the fabrication and testing of these designs. Our designs fall in to two catagories: designs based on capacitive electro-mechanical coupling, and designs based on strain-coupling. With strain-based coupling we have fabricated and tested samples which demonstrate a new type of electro-mechanical coupling, where surface strain influences the kinetic inductance and therefore the resonant frequency of a superconducting resonator. We have also demonstrated and studied strain-based mechanical-to-mechanical coupling in standard Cantilevers. We have designed and fabricated samples where high-frequency surface acoustic wave (SAW) will couple to a low-frequency flexure mode of a cantilever. We have made good progress with the design and simulation and we have developed all the necessary fabrication methods for capacitive coupling.

In the second period of the project we also worked on the adaptation of multi-frequency AFM to the case of very high Q resonant mechanical force sensing. We greatly improved over our measurements in the first period, and can now achieve 300 times faster acquisition rate for 4D data sets (force, x, y, z), in comparison with the traditional methods. We have developed force reconstruction methods appropriate to this high Q case and these methods have been validated on simulated data. We are in the process of applying it to real data and discover that we need somewhat better signal-to-noise (larger dynamic range) in order faithfully reconstruct from real data.

The second phase of the project has also seen the start of the integration work package. We are making the necessary background work, equipment purchase and validation, to prepare for integrating our sensors in to a low temperature platform, with scanner, and appropriate for UHV surface science. The final period of this project will focus on this work in Work Package 3.
We are trying to solve a problem in the LT-AFM community and not by an incremental amount. By introducing a new measurement paradigm to the LT-AFM community, our project has the potential to result in enormous advancement. We are merging ideas from the superconducting quantum circuit community, augmenting these with designs and techniques from the MEMS sensors and actuators community, and applying them to LT-AFM. Implementing our sensor requires expertise in microwave analog electronics. In order to use the massive improvement in sensitivity and bandwidth, multifrequency digital acquisition techniques using Field Programmable Gate Arrays (FPGA) is required. Achieving our goals requires an interdisciplinary team with skills in diverse areas of applied physics and cutting-edge technology.

The social and economic impacts of our research are direct for a smaller community of scientist, but far reaching in their indirect impact. If we are successful, a smaller community of LT-AFM scientists will get a much better instrument. To the larger society who daily use high-tech electronic devices, sensors, which rely advanced materials and measurement techniques, our project will have impact by pushing the development of signal transduction to the quantum limit. Our approach of measuirng in the frequency domain brings the technical advantage of advanced signal processing methods which are in daily use for communication technology (both radio and optical fiber). Our research adapts and brings these methods to sensing and actuating.
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