Terry L. Smith

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.

II–VI semiconductor color converters for efficient green, yellow, and red light emitting diodes

II–VI compound semiconductor quantum-well heterostructures were fabricated for use as efficient, narrow-spectrum, photoluminescent color converters to generate green, yellow, or red light when photopumped with blue GaInN light emitting diodes (LEDs). This approach promises high efficiencies in a wide range of wavelengths that includes the green-yellow portion of the spectrum where conventional LEDs offer relatively low efficiency. External quantum conversion efficiencies of 60%–70% and output spectra with full width at half maximum of 15 nm were achieved using CdZnSe–CdMgZnSe quantum wells grown by molecular beam epitaxy on InP substrates.

Reflection-mode sensing using optical microresonators

The authors present an approach for applying optical microring resonators to sensing. The approach relies on the interaction of the resonator with a nanoparticle taggant, which can induce coupling between forward and backward propagating traveling-wave resonant modes of the device. In the conventional four-port Add/Drop filter configuration, such coupling results in significant build-up of output intensity at ports that would normally be “dark.” Because the increased intensity occurs across a wide range of device resonances, the sensor may be interrogated using broadband excitation and with a broadband detector, enabling a low-cost, robust system.

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.

Effects of strain on defect structure in II-VI green color converters

State-of-the-art green emission efficiency has been achieved with light-emitting diodes incorporating CdMgZnSe color-converting quantum well heterostructures, although dark line defects (DLDs) limit the device reliability. We have determined that misfit strain plays an important role in the formation of extended stacking faults (SFs) and DLDs in II-VI green converters. Even small strain causes SFs to extend to accommodate misfit strain and extended SFs further give rise to DLDs when they intersect active regions. Detailed strain relaxation mechanisms for both tensile and compressive strain have been investigated, which may apply for other semiconductor heterostructures with an fcc lattice. Careful control of the layer strain via close lattice matching prevents the extension of SFs and leads to DLD-free converters.

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.

Simple model of errors in chirped fiber gratings

A simple etalon based model is presented to show the origin of the wavelength-dependent ripples in the group delay and phase, and in the intensity of optical signals reflected from chirped fiber gratings. The simplicity of the model allows intuitive understanding of the effects, and quantitative predictions. We derive accurate scaling laws that allow the experimenter to make quantitative connections between the grating writing process parameters and grating performance.

Enhanced efficiency of CdMgZnSe down-converted light emitting diodes using light extraction features fabricated by laser-speckle lithography

We report a method of making a wavelength converted, light-emitting diode (LED) having light extraction features based on coherent speckle patterns. These patterns have random feature size, random feature distribution, and random feature shape. The features were produced using a maskless lithography process based on exposing photoresist with a laser-induced coherent optical speckle pattern. A wet chemical etching process was then used to transfer the resist pattern into the underlying surface of the semiconductor wavelength converter layer. The external quantum efficiency of a wavelength converter with emitting surfaces having such extraction features showed a twofold increase compared to a wavelength converter without extraction features. While demonstrated here using semiconductor wavelength converters, this approach could also be applied to light extraction in conventional LED structures.

OLED light extraction with roll-to-roll nanostructured films

Improving light extraction for OLED devices will be pivotal for their acceptance into the marketplace. Incorporating nanostructures within the high refractive index regions of the OLED multi-layer stack results in an over two-fold improvement in light extraction efficiency. Such nanostructures were made using roll-to-roll fabrication processes. We will also discuss the performance characteristics of the nanostructures on color-angularity and blurring of high-resolution pixels.

Nanoparticle Sensing Utilizing Back Scattering Reflection Mode of an Integrated Micro-Resonator

We have demonstrated an optical sensing device based on back scattering signal changes in the transfer functions of an integrated silicon-based optical resonator, which are induced by the presence of one or more nanoparticles in the resonators evanescent field. The nanoparticles may be used as taggants to label other species to be detected. Our work offers several potential advantages to optical sensing, including improved detection limits, relaxed resonator fabrication requirements, CMOS compatibility and the use of simpler, lower cost optical sources and detectors.