Hesam Oveys

Liquid-core optical ring-resonator sensors.

We have demonstrated a novel sensor architecture based on a liquid-core optical ring-resonator (LCORR) in which a fused silica capillary is utilized to carry the aqueous sample and to act as the ring resonator. The wall thickness of the LCORR is controlled to a few micrometers to expose the whispering gallery mode to the aqueous core. Optical characterization with a water-ethanol mixture shows that the spectral sensitivity of the LCORR sensor is approximately 2.6 nm per refractive index unit. A model based on Mie theory is established to explain the experimental results. The LCORR takes advantage of the high sensitivity, small footprint, and low sample consumption with the ring resonator, as well as the efficient fluidic sample delivery with the capillary, and will open an avenue to future multiplexed sensor array development.

Integrated multiplexed biosensors based on liquid core optical ring resonators and antiresonant reflecting optical waveguides

The authors demonstrate integrated biosensors utilizing antiresonant reflecting optical waveguides (ARROWs) to excite the whispering gallery modes of a liquid core optical ring resonator (LCORR) sensor. Because this architecture is based on integration, it is robust and well suited for dense multiplexing of sensors. They analytically and experimentally characterize the coupling between the ARROW structure and the LCORR and show good agreement between the coupling theory and experimental results. The multiplexing capability is experimentally demonstrated by exciting multiple ring resonators along a single LCORR using the ARROW array. Also, they show the label-free detection of bovine serum albumin with this LCORR-ARROW system.

Refractometric Sensors for Lab-on-a-Chip Based on Optical Ring Resonators

We demonstrate refractive index measurement of liquids using two sensor system designs, both based on microring resonators. Evanescent sensors based on microrings utilize the resonating nature of the light to dramatically decrease the required size and sample consumption volume, which are requirements of lab-on-a-chip sensor systems. The first design, which utilizes an optical microsphere, exhibits a sensitivity of 30 nm/RIU and a resulting detection limit on the order of 10-7 RIU. The second approach is a novel design called a liquid core optical ring resonator (LCORR). This concept uses a quartz capillary as the fluidics and as the ring resonator and achieves a sensitivity of 16.1 nm/RIU. The detection limit of this system is around 5times10-6 RIU. Both of these systems have the potential to be incorporated with advanced microfluidic systems for lab-on-a-chip applications. In particular, the LCORR combines high sensitivity, performance stability, and microfluidic compatibility, making it an excellent choice for lab-on-a-chip development

Overview of novel integrated optical ring resonator bio/chemical sensors

In parallel to a stand-alone microsphere resonator and a planar ring resonator on a wafer, the liquid core optical ring resonator (LCORR) is regarded as the third type of ring resonator that integrates microfluidics with state-of-the-art photonics. The LCORR employs a micro-sized glass capillary with a wall thickness of a few microns. The circular cross section of the capillary forms a ring resonator that supports the whispering gallery modes (WGMs), which has the evanescent field in the core, allowing for repetitive interaction with the analytes carried inside the capillary. Despite the small physical size of the LCORR and sub-nanoliter sensing volume, the effective interaction length can exceed 10 cm due to high Q-factor (106), significantly improving the LCORR detection limit. The LCORR is a versatile system that exhibits excellent fluid handling capability inherent to capillaries and permits non-invasive and quantitative measurement at any location along the capillary. Furthermore, the LCORR uses the refractive index change as a transduction signal, which enables label-free detection. Therefore, the LCORR is a promising technology platform for future sensitive, miniaturized, lab-on-a-chip type sensors. In this paper, we will introduce the concept of the LCORR and present the theoretical analysis and the experimental results related to the LCORR sensor development.

Tuning whispering gallery modes in optical microspheres with chemical etching

We demonstrate a new method to tune the resonance of whispering gallery modes in a fused silica optical microsphere resonator by removing atomic layers from the sphere surface with low concentrations of hydrofluoric acid. Our results show that the WGMs can be tuned over 660 pm (430 GHz), more than one free spectral range of the microsphere resonator, with a tuning precision better than 0.2 pm (130 MHz). Both atomic force microscope images and a Q-factor measurement performed in air suggest that no additional degradation in Q-factor due to surface roughness is introduced during this etching process.

Universal coupling between metal-clad waveguides and optical ring resonators

We demonstrate excitation of whispering gallery modes in optical ring resonators using a gold-clad pedestal planar waveguide structure. The gold-clad structure provides a strong evanescent field for light-coupling into the resonator while enabling low transmission loss throughout much of the visible and near-infrared region. This is advantageous compared to the previously demonstrated anti-resonant reflecting optical waveguide (ARROW) structure, which can only transmit a narrow wavelength band. We show that the height of the pedestal waveguide can be designed to optimize the coupling conditions for the ring resonator. This technology enhances the practicality of optical ring resonators for sensing devices, laser systems, and many other important applications.

Liquid core optical ring resonator label-free biosensor array for lab-on-a-chip development

Label-free optical biosensors offer advantages for many applications due to their simplicity and low cost compared to fluorescence detection. Thus, it is desirable to develop label-free sensors that can be integrated with advanced microfluidic systems into dense, multi-purpose biosensor arrays. One candidate technology is ring resonators, which utilize the resonating whispering gallery modes to create a strongly enhanced optical field in the sensing volume. Because of the high Q-factor of ring resonators, the optical field can be enhanced by 2-3 orders of magnitude, which leads to much smaller required light-matter interaction length and sensing volume. These are critical characteristics for dense integration into lab-on-a-chip systems. We have developed a novel label-free ring resonator sensor based on a liquid core optical ring resonator (LCORR). This system uses a glass capillary as both the fluidics and the ring resonator. With the LCORR, we have demonstrated a measurable whispering gallery mode spectral shift of 30 pm/refractive-index-unit (RIU), which leads to a detection limit of approximately 10-6 RIU. Additionally, we have achieved an estimated detection limit for protein molecules of 10 pg/mm2. These experimental demonstrations of this novel sensing system will lead to the development of highly sensitive label-free sensors that are well-suited for dense integration with advanced microfluidics for lab-on-a-chip systems.

Demonstration of a liquid core optical ring resonator sensor coupled with an ARROW waveguide array

The liquid core optical ring resonator (LCORR) sensor is a newly developed capillary-based ring resonator that integrates microfluidics with photonic sensing technology. The circular cross-section of the capillary forms a ring resonator that supports whispering gallery modes (WGM). The WGM evanescent field is exposed to the capillary core and detects the aqueous samples conducted by the capillary using a label-free protocol. The high-Q of the WGM allows for repetitive light-analyte interaction, resulting in excellent sensitivity. Recently a detection limit of the LCORR on the order of 10-6 refractive index units was reported. In this work, we have further integrated the LCORR with an anti-resonant reflective optical waveguide (ARROW) array for multiplexed sensor development. The ARROW, with an array of 8 waveguides separated by 250 microns each, consists of a core and a lower reflective double-layer with alternating high and low refractive index, and thus has a significant evanescent field above the waveguide. The WGM is excited at each LCORR/ARROW junction simultaneously when the LCORR is brought into contact with the ARROW array. We experimentally investigated the optimal waveguide geometry for WGM excitation using a range of waveguide heights from 2 to 5 microns. Furthermore, the LCORR/ARROW system is utilized for a biomolecule sensing demonstration. The LCORR/ARROW system is not only essential for assembling a robust, practical, and densely multiplexed sensor array, but also enables on-capillary flow analysis that has broad applications in capillary electrophoresis, chromatography, and lab-on-a-chip development.

Development of Label-Free Microsphere Optical Resonator Bio/Chemical Sensors

Whispering Gallery Modes (WGMs) in microsphere ring resonators enable excellent sensitivity due to the high Q-factor (> 106) that significantly increases the light-matter interaction. The analytes attached to the sphere surface change the local refractive index, leading to a spectral shift in the resonances of the WGM. A practical microsphere-based sensor must detect minute changes in the refractive index. In addition, the microsphere surface should be functionalized for subsequent binding of bio/chemical molecules. Both functionalization and binding processes should be monitored in order to better anchor the captured molecules and to acquire quantitative and kinetic binding information. In this work, we have carried out a series of experiments towards developing highly sensitive fused silica microsphere based sensors. A fiber prism is used to couple the light from a tunable diode laser to the sphere. A fluidic well is built to allow for injection and withdrawal of samples. The sensor sensitivity of refractive index is characterized by using the mixture of water and alcohol. It is shown that our system is able to detect changes in refractive index as low as 10-7. We further monitor the kinetics of layer deposition when the sphere surface is functionalized with silane solution. Finally we monitor the protein binding to and peptide cleavage from the functionalized microsphere. Our results should lead to highly sensitive microsphere bio/chemical sensor arrays with applications in biomedical sciences, environmental monitoring, and drug discovery.

Applications of the liquid core optical ring resonator platform

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 107 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.