Chung-Yen Chao

Polymer microring resonators for biochemical sensing applications

Polymer microring resonators were demonstrated for sensing biomolecules without using fluorescent labels. The microring devices, fabricated by a direct imprinting technique, possess high Q factors of /spl sim/20000. This feature provides high sensitivity and a low detection limit for biochemical sensing applications. With these properties, the devices were used to detect and quantify the biomolecules present either in a homogeneous solution that surrounds the microring waveguide (homogeneous sensing) or specifically bound on the waveguide surface (surface sensing). In the former sensing mechanism, the current devices can detect an effective index change of 10/sup -7/ refractive index units (RIU); in the latter, they can reach a detection limit of /spl sim/250 pg/mm/sup 2/ of biomolecular coverage on the microring surface. In addition, the experiments show that the devices can detect both small and large biomolecules.

Design and optimization of microring resonators in biochemical sensing applications

Microring resonators can be exploited for biochemical sensing applications. To gain a better understanding of the design and optimization of microring sensors, the authors analytically derive the detection limit and the sensitivity. Other important parameters, including the ON-OFF contrast ratio and the signal-to-noise ratio (SNR), are also considered. In this paper, the combination of two sensing mechanisms and two sensing schemes are analyzed. These calculations provide a guideline for determining the microring geometry to satisfy the desired sensing requirements. In addition, the results can provide insights on how to enhance the sensitivity and lower the detection limit

High-frequency ultrasound sensors using polymer microring resonators

Polymer microring resonators are demonstrated as high-frequency, ultrasound detectors. An optical microring resonator consists of a ring waveguide closely coupled to a straight bus waveguide, serving as light input and output. Acoustic waves irradiating the ring induce strain, deforming the waveguide dimensions and changing the refractive index of the waveguide via the elasto-optic effect. These effects modify the effective refractive index of the guided mode inside the waveguide. The sharp wavelength dependence of the microring resonance can enhance the optical response to acoustic strain. Such polymer microring resonators are experimentally demonstrated in detecting broadband ultrasound pulses from a 50 MHz transducer. Measured frequency response shows that these devices have potential in high-frequency, ultrasound detection. Design guidelines for polymer microring resonators forming an ultrasound detector array are discussed

Ultrasound detection using polymer microring optical resonator

Application of polymer waveguide microring resonators for high-frequency ultrasound detection is presented. The device consists of a microring optical resonator coupled to a straight optical waveguide which serves as input and output ports. Acoustic waves irradiating the ring waveguide induce strain modifying the waveguide cross section. As a consequence, the effective refractive index of optical waves propagating along the ring is modified. The sharp wavelength dependence of the high Q-factor resonator enhances the optical response to acoustic strain. High sensitivity is demonstrated experimentally in detecting broadband ultrasound pulses from a 10MHz transducer. Methods of extending the technique to form multi-element ultrasonic arrays for imaging applications are proposed.

Reduction of surface scattering loss in polymer microrings using thermal-reflow technique

A thermal-reflow technique is used to greatly reduce surface roughness scattering loss in polymer microring devices. Smoother surfaces were achieved and verified by scanning electron microscopy. Reduction in surface roughness scattering was further supported by the loss measurement of both straight waveguides and microring resonators. The variance of surface roughness in a polymer microring device is estimated to decrease by 35-40 nm after the thermal-reflow treatment, corresponding to a loss reduction of /spl sim/74 dB/cm.

Fabrication of photonic nanostructures in nonlinear optical polymers

Two experimental techniques have been developed for creating photonic structures in nonlinear optical (NLO) polymers with precisions down to nanoscale. The first technique uses nanoimprinting technology to directly pattern the guest-host NLO polymers. It can be applied to the fabrication of photonic bandgap structures in NLO materials, as well as many other photonic structures in both linear and nonlinear polymers. The second technique utilizes self-assembly of NLO polymer monolayers onto a nanostructured template. This approach provides a highly effective means to implement practical waveguide devices using high performance self-assembled polymers with large electro-optic activity and inherent long-term stability. An optical switching device is proposed, based on nanofabricated NLO polymers.

Nonlinear micro-ring resonators for optical switching applications

We propose to use nonlinear optical (NLO) polymer to fabricate microring resonator device for all-optical switching application. In the proposed device NLO polymer provides the saturable absorption nonlinearity and microring resonator provides the feedback needed for optical bistability. Waveguide confinement and field intensity build-up in the ring resonator both facilitate the nonlinear optical process, making it possible to achieve low switching intensity. Moreover, the size of microrings is in the range of several tens of micro-meters, which is promising for high-speed optical switching as well as for high-density integration. We present detailed analysis of the device operation and identify key facts for the optimization of the devices. We propose to use nanoimprinting lithography technique to create microring resonator structures in NLO polymers, and show our initial results that prove the feasibility of this approach.

Label-Free Biochemical Sensors Based on Optical Microresonators

Biochemical sensors play a significant role in extensive applications that have tight relationship with human life. These sensors require high sensitivity and low detection limit. In this chapter, two optical sensors that meet the requirement will be discussed: polymer microring biochemical sensors and microtube resonator sensors. Both have advantages of high sensitivity, label-free detection capability, low cost, robustness, and simple fabrication process. The former devices show high sensitivity over 70 nm per RIU and low detection limit as 250 pg mm−2, while the latter ones can push sensitivity to 600 nm per RIU. Moreover, polymer microring biochemical sensors are compact and integrable in an array on a substrate; while microtube-based sensors have built-in fluidic handling capability. These features facilitate the development of miniature and highly sensitive lab-on-a-chip sensors.

5J-5 High-Frequency Ultrasound Transduction Using Polymer Microring Resonators

High-frequency ultrasound detection using polymer microring optical resonators is demonstrated. An optical microring resonator consists of a ring waveguide closely coupled to a straight bus waveguide, serving as light input and output. Acoustic waves irradiating the polymer ring waveguide can induce strain to deform the waveguide dimensions and change the refractive index of the waveguide via the elasto-optic effect. These effects lead to the modification of the effective refractive index of the guided mode inside the waveguide. The sharp wavelength dependence of the microring resonator can enhance the optical response to acoustic strain. A polymer microring resonator with a width of 2.4 mum and a height of 1.85 mum, and a diameter of 95 mum was fabricated using nano-imprint technique. Its frequency response measured using a 50-MHz transducer shows that the microring resonator can detect ultrasound at least up to 55 MHz. A calibrated 10 MHz transducer generating a peak pressure of about 5 MPa was used to estimate the sensitivity and the noise equivalent pressure of the microring resonator to be 21 pm/MPa and 150 kPa, respectively. Furthermore, a two-dimensional scan was performed and the effective detecting area of the microring was estimated to be 1.4 times the ring size, implying that array operation would be possible with minimal cross talk between elements. These results suggest polymer microring resonators may be well suited to high-frequency ultrasound detection arrays

Resonant optical ultrasound transducer (ROUT) arrays for high resolution photoacoustic imaging

Multi-dimensional, high frequency ultrasound arrays are extremely difficult to fabricate from conventional piezoelectrics. For over a decade, our lab has explored optical detection as an alternate technology for high frequency applications. We have developed several different types of acoustically coupled optical resonators to provide the sensitivity and bandwidth required for biomedical imaging. Waveguide and fiber lasers, thin Fabry-Perot etalons constructed from polymers, and thin microring resonators imprinted into polymers have all been used as ultrasound transducer arrays. Their performance rivals the theoretical conversion efficiency of piezoelectric devices but with bandwidths approaching 100 MHz, array element dimensions approaching 10 um, and no electrical interconnects. In this paper we present results on several resonant optical ultrasound transducer (ROUT) arrays, emphasizing their potential use in photoacoustic imaging. These results strongly suggest that a high resolution photoacoustic microscope can be constructed using a ROUT in a footprint appropriate for endoscopic and minimally invasive applications.