We have developed a new platform for sensing that incorporates photonic crystal cavities affixed to the tips of optical fibers. Ordinary photonic cavities are limited to bulky semiconductor chips that can only be measured with complicated free space optics. This prevents the full utilization of these sophisticated optical devices in remote environments, such as in the human body. By relocating our photonic crystal cavities to the tips of optical fibers, we are able to realize the many benefits of high quality factor cavities now in a convenient plug-and-play platform which we call a fiberPC.
The method for transferring cavities uses a simple and rapid epoxy process whereby fiber tips are carefully 'painted' with adhesive before being bonded to thin semiconductor membranes. The membranes are then removed from their host semiconductor substrates and firmly bound to the fiber facets. Cavities can be coupled directly to fiber cores simply by physical overlap and light can transfer easily from fiber to cavity to free space, and vice versa. All semiconductor materials, active or passive, may be incorporated in this fashion containing quantum dots, quantum wells, or other.
As an example application of the fiberPC, we show that these cavities can sense various nanoparticles that are of high interest in biomedicine. We find that when we withdraw our functionalized fiber tips from solutions of nanoparticles, a combination of evaporative concentration and fluid dynamics focuses an aggregate of nanoparticles directly on top of our cavities. Due to the absorptive and refractive index properties of the nanoparticles, the cavity modes are highly perturbed in wavelength and quality factor, which provides a convenient and simple readout of the nanoparticle concentration. This method can be applied to gold nanoparticles (used in molecular imaging studies for cancer detection) as well as iron oxide nanoparticles (used as magnetic resonance contrast agents) and can be used in a wide variety of sensing applications. The fiberPC provides an interesting alternative for sensing which might be useful in endoscopic fiber-based detection.