Optical toroid resonators are doughnut-shaped silica devices made on a silicon pillar. Due to their smooth surface and isolation from the silicon substrate, toroids make excellent optical resonators with quality factors on the order of 100 million. These high quality factors enable silica toroids to efficiently confine light in circular orbits for long periods of time. As a result, optical toroids can be used in many key applications, including integrated optics, lasers, and even as biosensors capable of detecting single molecules.Enhancing the Sensitivity of Toroid-based Optical Sensors
The goal of my current research project is to develop improved sensors based on toroid resonators, and use these sensors to study biological systems. Recently, I developed a sensor based on a toroid microlaser platform. By heterodyning the toroid microlaser's 1064nm emission with a 1064nm tunable reference laser, the sensitivity, signal to noise, and time resolution could be improved by over 50x. This work was recently published in Applied Physics Letters.
Since the heterodyned toroidal microlaser sensor has greatly improved sensitivity, signal to noise, and time resolution compared to traditional optical toroid-based sensors, it is a promising device for studying biological systems.
This research is currently in progress. Please check back soon for more updates.Optical Materials
To improve the performance of silica optical devices, it is often necessary to change the material properties. One approach I have used to tailor the properties of silica devices is fabricating custom silica materials with sol-gel silica. For example, titanium can be added to sol-gel silica to tune the refractive index of the resulting silica. By controlling the refractive index and material properties, we can tailor the behavior of light in silica devices for different applications.
I am also doping sol-gel silica with other transition metals and rare earth elements to develop improved lasers. For example, in order to develop the heterodyned toroidal microlaser sensor, I fabricated toroids using custom sol-gel silica films doped with neodyumium and alumina.Optical Waveguides
To develop effective waveguide sensors, we must first fabricate waveguides which trap light very effectively and thus have low optical loss. My first research project in the Armani Lab focused on developing a low loss waveguide usable for biosensing and integrated optics applications.
Optical waveguides are devices which trap and transfer light. They effectively confine light by total internal reflection. Total internal reflection is an optical phenomenon which causes light to be trapped between a core of refractive index n1 surrounded by another medium with lower refractive index n2 (cladding).
Some light escapes through the waveguide’s cladding and forms an exponentially decaying electromagnetic wave, known as the evanescent field. For waveguide sensors, the evanescent field can be used to probe the area surrounding the waveguide for changes in refractive index.
In a biosensor, the waveguide’s surface may be covered with antibodies or other materials to which specific biomolecules can bind. When an analyte molecule binds to the waveguide’s surface, the refractive index of the surface changes. This refractive index change alters the waveguide’s evanescent field, causing either a power change or a phase shift in the light traveling through the core.
A detector at the end of the waveguide measures these changes, from which the concentration of the analyte can be determined. In this way, the waveguide can be used as a label-free sensing device to detect the presence of specific analytes.