The effect of the discrete values of the refractive index of the surrounding medium on the spectral behavior of the whispering-gallery modes (WGMs) in the elastic scattering spectra of high-refractive-index silica microspheres submerged in fluids, such as air, water, and glycerol, is studied. The elastic scattering spectral measurements, as well as the spectral autocorrelation analysis of these elastic scattering spectra show that the spectral-mode spacing, the spectral-mode density, and the spectral-mode definition of the WGMs decrease as the refractive index of the surrounding fluid increases. We believe that this work opens up the way for optofluidic applications of high-refractive-index silica microsphere-based guided wave optics.
The idea of creating photonics tools for sensing, imaging and material characterization has long been pursued and many achievements have been made. Approaching the level of solutions provided by nature however is hindered by routine choice of materials. To this end recent years have witnessed a great effort to engineer mechanically flexible photonic devices using polymer substrates. On the other hand, biodegradability and biocompatibility still remains to be incorporated. Hence biomimetics holds the key to overcome the limitations of traditional materials in photonics design. Natural proteins such as sucker ring teeth (SRT) and silk for instance have remarkable mechanical and optical properties that exceed the endeavors of most synthetic and natural polymers. Here we demonstrate for the first time, toroidal whispering gallery mode resonators (WGMR) fabricated entirely from protein structures such as SRT of Loligo vulgaris (European squid) and silk from Bombyx mori. We provide here complete optical and material characterization of proteinaceous WGMRs, revealing high quality factors in microscale and enhancement of Raman signatures by a microcavity. We also present a most simple application of a WGMR as a natural protein add-drop filter, made of SRT protein. Our work shows that with protein-based materials, optical, mechanical and thermal properties can be devised at the molecular level and it lays the groundwork for future eco-friendly, flexible photonics device design.
Whispering-Gallery-Mode (WGM) resonators are emerging as an excellent platform to study optical phenomena resulting from enhanced light-matter interactions due to their superior capability to confine photons for extended periods of time. The monolithic fabrication process to achieve ultra-high-Q WGM resonators without the need to align multiple optical components, as needed in traditional design of resonators based on precise arrangement of mirrors, is especially attractive. Here we explain how to process a layer of thin film doped with optical gain medium, which is prepared by wet chemical synthesis, into WGM structures on silicon wafer to achieve arrays of ultra-low threshold on-chip microlasers. We can adjust the dopant species and concentration easily by tailoring the chemical compositions in the precursor solution. Lasing in different spectral windows from visible to infrared was observed in the experiments. In particular, we investigated nanoparticle sensing applications of the on-chip WGM microlasers by taking advantages of the narrow linewidths and the splitting of lasing modes arising from their interactions with nano-scale structures. It has been found that a nanoparticle as small as ten nanometers in radius could split a lasing mode in a WGM resonator into two spectrally separated lasing lines. Subsequently, when these lasing lines are photo-mixed at a photodetector a heterodyne beat note is generated which can be processed to signal the detection of individual nanoparticles. We have demonstrated detection of virions, dielectric and metallic nanoparticles by monitoring the changes in this self-heterodyning beat note of the split lasing modes. The built-in self-heterodyne method achieved in this monolithic WGM microlaser provides an ultrasensitive scheme for detecting and measuring nanoparticles at single particle resolution, with a theoretical detection limit of one nanometer.
We demonstrate electro-optical tuning and modulation of the optical resonances of a silicon microsphere placed on an
optical fiber half coupler and immersed in a nematic liquid crystal. The relative refractive index between the microsphere
and the liquid crystal is controlled by an applied external AC electric field. The tuning and modulation of the
microsphere optical resonances is monitored with the transmission and elastic scattering signals.