Passive and active cooling of cavity optomechanical torque sensors for magnetometry applications

• Author / Creator

  Kim, Paul H.

• Cavity optomechanics, the study of mechanical effects from resonant light, has recently shown remarkable sensitivities in displacements and forces (torques) --- which opened up new avenues for scientific and technological developments. One of the interesting prospects of cavity optomechanics includes torque magnetometry, where mesoscopic effects of magnetism can be explored through sensitive nanomechanical detection. In this thesis, I drastically improved the sensitivity of optomechanical torque sensors using advanced nanofabrication techniques and our unique dilution refrigerator. Looking through a cryogenic compatible microscope objective, a versatile dimpled microfibre can access any optical microdisk. Arriving at the optimized device, thermalized at 25 mK, the torque sensitivity was 2.9 yNm per root-Hz, just eleven times above the standard quantum limit.

  Furthermore, I have demonstrated hybrid systems using hard and soft magnets, namely iron needle and permalloy disk. Utilizing a device with a 0.6 zNm per root-Hz sensitivity at room temperature, I have successfully driven torque using a magnetized sample and an AC magnetic field, where I extracted bulk remanence of iron. I have also shown feedback cooling as well --- highlighting mode-damping below 12 K. My final experiment implements torque-mixing magnetic resonance spectroscopy to study gyrotropic spin modes of a 1.1 micron diameter permalloy disk by mixing the two AC fields: in-plane and out-of-plane. I was able to track spin modes and vortex pinning, which peaks at a higher frequency above 1 GHz. In summary, this platform, where optomechanics, cryogenics, and torque magnetometry are brought together, can be a powerful tool to study vortex-dynamics, phases of matter, and mesoscopic properties of superconductors.

• Subjects / Keywords

  • Torque magnetometry
  • Nanofabrication
  • Cryogenics
  • Cavity optomechanics

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-cw1x-z582

• License

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