L.J. Guo

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

Reversal imprinting by transferring polymer from mold to substrate

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. “Embossing” occurs at temperatures well above the glass transition temperature (Tg) of a polymer; “inking” occurs at temperatures around Tg with nonplanarized polymer coating surface on the mold; and “whole-layer transfer” occurs at temperatures around Tg but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the s...

Nanoimprinting over topography and multilayer three-dimensional printing

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patte...

Optical sensors based on active microcavities

We propose an active optical sensor based on a microcavity with gain. Greatly improved sensitivity can be achieved in active microcavities as compared with passive high-Q microcavities. We show that an active sensor using a gain-doped microsphere can provide 10/sup 4/-fold narrower resonance linewidth than does a passive microcavity in the transmission spectrum. Such highly sensitive microcavity optical sensors can be used to detect low concentrations of chemicals or biomolecules in their surroundings. Our analysis shows that this type of compact active microcavity is sensitive to an effective refractive index change of the order of 10/sup -9/.

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

Toward Low-Cost, High-Efficiency, and Scalable Organic Solar Cells with Transparent Metal Electrode and Improved Domain Morphology

We review our recent progress toward realizing future low-cost, high-efficiency, and scalable organic solar cells (OSCs). First, we show that the transparent electrodes based on metallic nanostructure is a strong candidate as a replacement of conventional indium tin oxide (ITO) electrode due to their superior properties, such as high optical transparency, good electrical conductivity, and mechanical flexibility, and the versatility that these properties can be adjusted independently by changing the linewidth and thickness of the metal grid structure. Furthermore, we exploited the unique optical properties due to the excitation of surface plasmon resonance by the metallic nanogratings to enhance the light absorption of organic semiconductors, and demonstrated enhanced power conversion efficiency than devices made using ITO electrode. In addition, we also investigated a new device fabrication process with a focus on the photoactive layer formation, which produces the most optimum bulk-heterojunction morphology compared with conventional annealing-based methods. Finally, we successfully demonstrated that these approaches are scalable to large-area and high-speed roll-to-roll processes. We believe that the works highlighted in this paper represent one step forward to realizing low-cost, high-efficiency, and large-area OSCs.

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.

Polymer Microring Resonators for High-Frequency Ultrasound Detection and Imaging

Polymer microring resonators fabricated by nanoimprinting are presented as a means of ultrasound detection. Acoustic waves impinging on a ring-shaped optical resonator cause strain in the ring dimensions, modulating optical output. Basic acoustic and optical characteristics of the microring sensor are presented. Measurements at several frequencies show a high sensitivity and low noise-equivalent pressure. The angular response is determined by sensing the optoacoustic excitation of a 49 mum polyester microsphere and shows wide-angle sensitivity. A 1D array consisting of four microrings is demonstrated using wavelength multiplexing for addressing each element. The high sensitivity, bandwidth, and angular response make it a potentially useful sensor platform for many applications including high-frequency ultrasonic and photoacoustic imaging.

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.