We describe and demonstrate a physical mechanism that substantially enhances the label-free sensitivity of a
Whispering-Gallery-Mode biosensor for the detection of individual nanoparticles in aqueous solution. It involves the
interaction of dielectric nanoparticle in an equatorial carousel orbit with a plasmonic nanoparticle bound on the orbital
path. As a 60 nm dielectric particle parks on plasmonic hot spots we observe frequency shifts that are considerably
enhanced consistent with a simple reactive model. Using the same model the label free detection of a single bovine
serum albumin (BSA) molecule is projected.
We report on the development of versatile, miniaturized optofluidic ring resonator (OFRR) dye lasers that can be
operated regardless of the refractive index (RI) of the liquid. The OFRR is a piece of a thin-walled fused silica capillary
that integrates the photonic ring resonator with microfluidics. In an OFRR dye laser, the active lasing materials (such as
dye) are passed through the capillary whereas the circular cross section forms a ring resonator and supports whispering
gallery modes (WGMs) that provide optical feedback for lasing. Due to the high Q-factors (> 10<sup>9</sup>), extremely low lasing
threshold can be achieved. The operation wavelength can conveniently be changed by using different dye and fine-tuned
with solvent. The laser can be out-coupled through a fiber taper in touch with the capillary, thus providing an easy
guiding for the laser emission. Our experiments demonstrate lasing through direct excitation and through efficient
energy transfer (ET). Theoretical analysis and experimental results for OFRR lasers are presented.
The liquid core optical ring resonator (LCORR) integrates an array of optical ring resonators into a microfluidics
channel. The LCORR is made of a micro-sized glass capillary; the circular cross-section of the capillary acts as an
optical ring resonator while the resonating light interacts with the fluid sample passing through the core. Q-factors
larger than 10<sup>7</sup> have been achieved in LCORRs on the order of 100 micrometers in diameter. This implies an effective
interaction length between the evanescent field of the resonator and the fluidic core of over 10 cm.
The novel integrated architecture and excellent photonic performance lead to a number of applications in sensing,
analytical chemistry, and photonics. For the last decade, optical ring resonators have been explored for label-free
bio/chemical detection. The LCORR architecture possesses the same capabilities as other optical ring resonator
bio/chemical sensors while also integrating micro-capillary-based fluidics with the sensor head. The integrated fluidics
design in combination with the micro-sized sensor head and pico-liter sample volume lead to a lab-on-a-chip sensor for
biomolecules, such as biomarkers and specific DNA sequences. Also, because the ring resonator creates a high-intensity
field inside the microfluidic channel, the LCORR is an excellent microfluidic platform for surface-enhanced
Raman scattering (SERS) detection in silver colloids. Finally, the high quality factor of the capillary-based resonator
enables novel opto-fluidic devices, such as dye lasers. We will discuss the details of these concepts and present our
research results in each of these applications.
We describe the synthesis of gold nanorods (NRs) nucleated by HgTe nanoparticles (NPs) of average size 3 nm in diameter. Growth of ~200 nm by ~50 nm NRs on various surfaces is achieved by using an intermediary polyelectrolyte layer. A poly(dimethyldiallylammonium) chloride (PDDA) monolayer on the surface attracts the thioglycolic acid (TGA) capped HgTe NPs and assists in one-dimensional gold growth. Rod morphology is observed for approximately one third of the resulting features. Confirmation of Au deposition is obtained with x-ray photoelectron spectroscopy and optical absorption measurements that show an increase in the Au plasmon band with time spent in gold growth solution. Au NRs were grown directly on the surface of high quality factor (Q) optical resonators (microspheres and microcylinders). Although the coating procedure reduces the Q of the resonators, whispering gallery modes are sustained. This seeding technique, amenable to many different surfaces, may result in semiconductor-metal nanocomposites with novel electronic and optical properties.
Chemical absorption is investigated using the evanescent fraction of a whispering-gallery mode (WGM). An effective absorption path length of about 50 cm is obtained in methane sensing and about 100 cm in a liquid dye solution.