The optical signal of the microring in response to an ultrasound pulse; and frequency response of a polystyrene microring resonator to a 50 MHz  ultrasound pulse.

 

 

   

 

 

 

Schematic shows interaction between the waveguide and an in-coming ultrasound pulse. The pulse stresses the structure, deforming the waveguide dimension and simultaneously modifying the refractive index of the core. Ultrasound detection using the optical resonance of a microring sensor.

 

Photonic Microresonator Sensors

 

Optical waveguide sensors using evanescent wave to interrogate the presence of analytes on waveguide surface or in surrounding environment typically rely on the detection of effective refractive index change. A great demand exists for highly sensitive devices to detect biomolecules with very low concentrations. Toward this end, our group has investigated polymer microring resonators as sensitive and compact biosensors to take the advantage of polymers’ rich surface functionalities for specific attachment of biomolecules as well as low cost and simple processing. We have developed a process to fabricate polymer microring resonators using a direct imprinting technique, which can create microring devices in a variety of polymer materials (JVST. B. 20, 2862, 2002), and can stamp out arrays of microring devices in a single step. We also developed techniques to reduce the surface roughness caused scattering loss (Appl. Phys. Lett. 84, 2479, 2004) to increase the cavity Q’s and improve the device performance (IEEE Photon. Tech. Lett. 16. 1498, 2004). We exploited a Fano-resonance feature to achieve improved slope sensitivity, and measured glucose concentration in aqueous solution (Appl. Phys. Lett. 83, 1527, 2003). We also obtained results on the detection of protein molecules (BSA and streptavidin) as well as small molecules (biotin) by specific surface binding to demonstrate the potential use of the device for different applications (SPIE proceedings, 5517-44, 2004, and IEEE Spec. Top. Quan. Electron., 12, 134, 2006). We proposed and theoretically studied active micro-cavity sensors with gain medium to create sensors with greatly improved sensitivity (IEEE Spec. Top. Quan. Electron., 12, 143, 2006); and established the theoretical design rules for improving the device sensitivity (J. Lightwave Tech. 24, 1395, 2006). Recently we studied silica micro-tube resonators by using the prism-coupled method and observed record high refractive index sensitivity of ~ 600nm/RIU in the resonator based sensors (Opt. Exp. 2007). This is attributed to a new type resonance mode, which has the highest optical field present in the low index fluid region, and maximizes the interaction of light with the analyte solution flowing though the micro-tube. 

 

We have demonstrated another new sensor application of the polymer microring device by detecting high-frequency ultrasound signals with very high sensitivity (Appl. Phys. Lett. 85, 5418, 2004), and demonstrated broadband response of the device with initial 2D scan results (IEEE Ultrasound, 2007). Recently we have achieved record high sensitivity in terms of noise-equivalent pressure and also demonstrated wavelength multiplexing of 4-ring elements for detecting ultrasound via a single bus waveguide (IEEE Spec. Top. Quan. Electron. on Biophotonics 2008). Recently we have demonstrated broadband response and record high sensitivity of ultrasound transducers of similar size (Appl. Phys. Lett., 2008).

 

Sensors based on optical microcavities offer a unique advantage for detecting low concentrations of biomolecules as well as studying their binding kinetics by using small sample amount and without fluorescent labels. The demonstrated ultrasound detection lead to an exciting application in high-frequency ultrasound imaging for medical diagnosis by providing high-density detector arrays for greatly improved spatial resolution.

Guo Research Group

 

Experimental setup of a prism-coupled capillary microtube sensor

Radial field intensity distribution of  the super high sensitivity mode

 Resonance cure shift due to the lipid bilayer absorbed on the inner wall of the tube

(a) Optical micrograph of the fabricated four-microring device. (b) Four rings in serial create four distinct resonance peaks that can be probed from the same waveguide. Two dimensional spatial sensitivity of the microring array for each of the 4 wavelengths tested by using a scanning ultrasound transducer.

 

(a)