This dissertation reports on sensing enhancements enabled by nanomaterials, focusing on twoareas: magnetic field sensing and imaging, and fluorescence microscopy. In both domains, we find that nanomaterials lead to sensors which outperform bulk and microscale materials.

Motivating the magnetic field sensing project, we observed that most existing magnetic field sensing techniques follow a trend where their magnetic field sensitivity is inversely proportional to sensor volume. This presents a challenge when attempting to perform high-sensitivity, spatially resolved magnetic field measurements.

The primary research question investigates whether this paradigm could be broken. Because the sensitivity of Faraday rotation magnetometry (FRM) depends on optical power rather than intensity, magnetic field sensitivity to be decoupled from spot size.

To leverage this benefit, two key improvements were required. First, we developed a new ma- terial to enhance sensitivity potential. Second, we optimized the sensing architecture to maximize performance using this newly developed transducer.

We discovered that a nanocomposite of terbium-doped magnetite nanoparticles embedded in a polymer host produced an extremely high Verdet constant while maintaining good optical clarity. However, the material exhibited a low optical damage threshold, which limited performance when tightly focused optical spots were used. Since magnetic field sensitivity can be improved by increasing optical power, this low optical damage threshold posed a problem.

To address this issue, we employed a non-common path heterodyne detection scheme. This approach allowed most of the light to bypass the sample, reducing illumination power while mixing the beams on the detector. This amplified the signal above the electronic noise floor, enabling shot-noise-limited measurements even with low optical illumination power. Using this method, we iii achieved a sensitivity of 568 nT/√Hz in our magnetometer.

Further exploration, inspired by polarization-sensitive optical coherence tomography, led us to propose an alternative and novel detection architecture called dual-balanced heterodyne detection (DBHD). This approach altered the power scaling of the measurement, providing additional signal amplification at low magnetic fields. This technique initially had potential to achieve pT/Hz sensitivity. A proof-of-concept device was implemented to validate the hypothesis. However, after extensive experimental work, I concluded that the sensitivity potential was not realizable due to nonlinear scaling of the noise. Though we explore some potential advantages of the technique.

The final part of this dissertation investigates the use of upconversion nanoparticles (UCNPs). These particles are employed in fluorescence microscopy, an imaging technique commonly used in biology to stain samples. UCNPs convert two low-energy photons into one higher-energy photon, enabling background-free measurements because the illumination light differs from the detection light. However, UCNPs are limited by their low photon absorption probability.

We hypothesized that the brightness of UCNPs could be increased using paired photons to enable instantaneous upconversion attempts. A rate-equation model was developed to simulate the dynamics of the nanoparticles. We found that the threshold for brightness enhancement between biphoton and conventional illumination depends on the lifetime of singly excited electrons. In our material, this lifetime is on the order of milliseconds. The threshold for enhancement was so low that it would produce fewer than one photon per second, rendering experimental pursuit impractical.

In summary, this dissertation examines two different systems, both with imaging applications, that leverage the properties of nanoparticles to enhance sensing. The magnetite nanoparticles enabled highly sensitive measurements when combined with a novel detection configuration. Meanwhile, the UCNP nanoparticles proved too efficient to justify the use of paired photon sources for illumination, as their performance could not be practically enhanced.