Integrated optical microcavities for enhanced evanescent-wave spectroscopy

Simulation of reconfigurable double-input optical gates based on a microring flower-like structure. part I. basic gates

In this paper, a simulation and general analysis of nonlinear microring resonators (MRRs) as all-optical switches are discussed in a flower-like structure. These MRRs are modulated through an optical pump beam, which receives temporal shift of MRRs' resonance wavelengths. These shifts are a result of the refractive index changing due to carrier injection. A green laser is used as the optical pump to shift the resonant wavelength of each MRR. This proposed structure can operate as all-optical logic gates. Of those innovations are reconfigurable structure to achieve various gates, integrating capability, and a high extinction ratio between zero and one logical levels. This configuration would be useful to design optical integrated circuits.

Applications of Optical Microcavity Resonators in Analytical Chemistry

Optical resonator sensors are an emerging class of analytical technologies that use recirculating light confined within a microcavity to sensitively measure the surrounding environment. Bolstered by advances in microfabrication, these devices can be configured for a wide variety of chemical or biomolecular sensing applications. We begin with a brief description of optical resonator sensor operation, followed by discussions regarding sensor design, including different geometries, choices of material systems, methods of sensor interrogation, and new approaches to sensor operation. Throughout, key developments are highlighted, including advancements in biosensing and other applications of optical sensors. We discuss the potential of alternative sensing mechanisms and hybrid sensing devices for more sensitive and rapid analyses. We conclude with our perspective on the future of optical microcavity sensors and their promise as versatile detection elements within analytical chemistry.

Silicon hybrid plasmonic microring resonator for sensing applications

A novel silicon hybrid plasmonic microring resonator consisting of a silver nanoring on top of a silicon-on-insulator ring is proposed and investigated theoretically for possible applications in sensing at the deep subwavelength scale. By using the finite-element method, insight into how the mode properties (Q factor, effective mode volume, energy ratio, sensitivity) depend on the geometric structure of the hybrid microring resonator is presented. Simulation results reveal that this kind of hybrid microcavity maintains a high Q factor ∼600, an ultrasmall mode volume of 0.15  μm³, and high sensitivity of 497  nm/refractive index unit for refractive index sensing. The hybrid plasmonic microcavity with optimized geometric structures presented provides the potential for ultracompact sensing applications.

Observation of two-photon fluorescence for Rhodamine 6G in microbubble resonators

We report an observation of multi-photon excitation of organic chromophores in microbubble whispering gallery mode resonators. High-Q microbubble resonators were formed by heating a pressurized fused silica capillary to form a hollow bubble that was then filled with liquid. In this work, the microbubble was filled with a solution of Rhodamine 6G dye. The resonator and dye were excited by evanescently coupling continuous wave (CW) light from a 980 nm laser diode using a tapered optical fiber. The two-photon fluorescence of the dye can be seen with pump powers as low as 700 μW.

Enhancing the detection limit of nanoscale biosensors via topographically selective functionalization

Nanoscale biosensors have remarkable theoretical sensitivities but often suffer from suboptimal limits of detection in practice. This is in part because the sensing area of nanoscale sensors is orders of magnitude smaller than the total device substrate. Current strategies to immobilize probes (capture molecules) functionalize both sensing and nonsensing regions, leading to target depletion and diminished limits of detection. The difference in topography between these regions on nanoscale biosensors offers a way to selectively address only the sensing area. We developed a bottom-up, topographically selective approach employing self-assembled poly(N-isopropylacrylamide) (PNIPAM) hydrogel nanoparticles as a mask to preferentially bind target to only the active sensing region of a photonic crystal (PhC) biosensor. This led to over an order of magnitude improvement in the limit of detection for the device, in agreement with finite element simulations. Since the sensing elements in many nanoscale sensors are topographically distinct, this approach should be widely applicable.

Optical ring resonators for biochemical and chemical sensing

In the past few years optical ring resonators have emerged as a new sensing technology for highly sensitive detection of analytes in liquid or gas. This article introduces the ring resonator sensing principle, describes various ring resonator sensor designs, reviews the current state of the field, and presents an outlook of possible applications and related research and development directions.

Chip scale integrated microresonator sensing systems

Medicine, environmental monitoring, and security are application areas for miniaturized, portable sensing systems. The emerging integration of sensors with other components (electronic, photonic, fluidic) is moving sensing toward higher levels of portability through the realization of self-contained chip scale sensing systems. Planar optical sensors, and in particular, microresonator sensors, are attractive components for chip scale integrated sensing systems because they are small, have high sensitivity, can be surface customized, and can be integrated singly or in arrays in a planar format with other components using conventional semiconductor fabrication technologies. This paper will focus on the progress and prospects for the integration of microresonator sensors at the chip scale with photonic input/output components and with sample preparation microfluidics, toward self-contained, portable sensing systems.

Loss determination in microsphere resonators by phase-shift cavity ring-down measurements

The optical loss of whispering gallery modes of resonantly excited microresonator spheres is determined by optical lifetime measurements. The phase-shift cavity ring-down technique is used to extract ring-down times and optical loss from the difference in amplitude modulation phase between the light entering the microresonator and light scattered from the microresonator. In addition, the phase lag of the light exiting the waveguide, which was used to couple light into the resonator, was measured. The intensity and phase measurements were fully described by a model that assumed interference of the cavity modes with the light propagating in the waveguide.

Sensitive optical biosensors for unlabeled targets: A review

This article reviews the recent progress in optical biosensors that use the label-free detection protocol, in which biomolecules are unlabeled or unmodified, and are detected in their natural forms. In particular, it will focus on the optical biosensors that utilize the refractive index change as the sensing transduction signal. Various optical label-free biosensing platforms will be introduced, including, but not limited to, surface plasmon resonance, interferometers, waveguides, fiber gratings, ring resonators, and photonic crystals. Emphasis will be given to the description of optical structures and their respective sensing mechanisms. Examples of detecting various types of biomolecules will be presented. Wherever possible, the sensing performance of each optical structure will be evaluated and compared in terms of sensitivity and detection limit.

Multiplexed DNA quantification by spectroscopic shift of two microsphere cavities

We have developed a novel, spectroscopic technique for high-sensitivity, label-free DNA quantification. We demonstrate that an optical resonance (whispering gallery mode) excited in a micron-sized silica sphere can be used to detect and measure nucleic acids. The surface of the silica sphere is chemically modified with oligonucleotides. We show that hybridization to the target DNA leads to a red shift of the optical resonance wavelength. The sensitivity of this resonant technique is measured as 6 pg/mm(2) mass loading, higher as compared to most optical single-pass devices such as surface plasmon resonance biosensors. Furthermore, we show that each microsphere can be identified by its unique resonance wavelength. Specific, multiplexed DNA detection is demonstrated by using two microspheres. The multiplexed signal from two microspheres allows us to discriminate a single nucleotide mismatch in an 11-mer oligonucleotide with a high signal-to-noise ratio of 54. This all-photonic whispering gallery mode biosensor can be integrated on a semiconductor chip that makes it an easy to manufacture, analytic component for a portable, robust lab-on-a-chip device.