Thermal effects need to be accurately measured and/or controlled to generate continuous kinetic binding curves with whispering gallery mode (WGM) microcavity sensors. We use a high spatial resolution optical frequency domain reflectometry system at 780 nm to capture the Rayleigh backscattering signal within a microtoroid optical resonator for temperature calibration. It is shown that this system has a temperature detection accuracy of 30 mK. This technique characterizes thermal effects in the microcavity and the surrounding environment, thus enabling lower limits of detection to be achieved.
We use a system known as FLOWER (frequency locked optical whispering evanescent resonator) to rapidly detect trace amounts of the chemical warfare agent surrogates DIMP and DMMP. We use sorbent polymer layers covalently bound to the surface of the microtoroid cavity to selectively adsorb target gases. As the target gas diffuses into the polymer layer, a measurable change in the resonance frequency of the toroid occurs. We demonstrate 80 ppt (part per trillion) sensing of DIMP; two orders of magnitude better than what can be achieved using mass spectroscopy.
Whispering-gallery mode (WGM) microtoroid optical resonators have been used for highly sensitive biological and chemical sensing. With the help of auto-balanced detection and data processing techniques, frequency locked optical whispering evanescent resonator (FLOWER) has successfully reduced external noise and achieved detection of single macromolecules. FLOWER, however, can only analyze the size of particles and not their shape. To determine the shape of non-spherical particles interacting with different polarized WGM modes, we built a dual-FLOWER system to perform multi-mode locking. We demonstrate the ability of this system to detect particle shape by detecting gold nanorods and spheres.
Most biosensors rely on immobilized antibodies or aptamers. In contrast, receptor proteins exist naturally in lipid bilayers and are highly specific to small molecules. We use a frequency-locked optical whispering evanescent resonator (FLOWER) system for real-time quantification of rhodopsin incorporation into an artificial lipid membrane and observe photo-induced molecular transformations upon light activation. Our study of proteolipid membrane coated microtoroids for probing the local activity of G-protein coupled receptors was further expanded to kappa-opioid receptors and their endogenous ligand Dynorphin-A. G-protein coupled receptor signaling probed by a microtoroid-proteolipid system will facilitate drug discovery and therapeutic interventions.
The light confinement properties of high quality (Q) factor microtoroid whispering-gallery mode (WGM) optical resonators prevent efficient coupling between far-field radiation and the WGM. Instead, light is most commonly evanescently coupled to the WGM using optical fibers that have been tapered to micron-scale thickness. These tapers, however, break easily and are sensitive to environmental vibrations and fluid flow fluctuations. This limits their effectiveness in mass-produced and/or field-portable biochemical sensing applications. Here we present a gold nanorod grating as an experimentally-feasible alternative for robust coupling of free-space light to a microtoroid resonator, and we simulate its performance with a novel finite-element 3D beam envelope method. 3D simulations of the full system are not tractable due to its large size. Previously, simulations of nanostructures on microtoroids have been performed on a thin wedge of the 3D system with perfect electrical conductor (mirror) boundary conditions. While these simulations provided some insight, they do not accurately model typical travelling-wave WGM experiments because they can only simulate standing waves. The standing wave nodes and antinodes significantly alter interactions between the WGM and the nanostructure. In our new method, we use a small wedge domain with custom boundary conditions that accurately simulate the travelling wave and nanophotonic interactions. Using this approach, we have designed and simulated a grating for far-field WGM coupling. With the grating, it is possible to maintain a high Q-factor of 3×10^6. We anticipate that our proposed modeling approach can solve a variety of other nanoparticle-microtoroid coupled systems in the future.
Subwavelength systems such as optical nanoantennas are widely used for optical sensing due to their ultrahigh field localization. Compared to isolated nanoantennas, hybrid sensor systems composed of optical nanocavities and microcavities enjoy higher quality factor (Q) plasmonic-cavity modes, as well as larger resonance shifts for any given sensing target. We have shown that rational engineering of the coupling between nanoantennas can maximize the system’s sensitivity. This can be achieved through near-field optimization of the system to maximize the field enhancement and suppression of the far-field radiation to maintain the highest possible Q. Finite element eigenvalue analysis shows that a trimer plasmonic nanoantenna coupled to a whispering gallery mode (WGM) of a microtoroid cavity supports higher Q and field enhancement than single nanorods that are randomly scattered on the surface of microcavity. We have studied the robustness of this system against any possible perturbation in geometry of trimers such as length, angle or gap between the nanoantennas. On the basis of this study, a general design approach is introduced, which helps engineers to enhance the efficacy of plasmonic-photonics based biosensors.
Local field enhancement of plasmonic nanoantennas below the diffraction limit plays an important role in a variety of applications, including surface-enhanced Raman scattering, spontaneous emission enhancement, nanolasing, enhanced nonlinear effects and biosensing. Yet due to the radiation and ohmic loss of these nanocavities, their quality factor (Q) is much smaller than a typical optical microcavity Q factor, such as that of a microsphere or microtoroid. Coupling a nanoantenna to an optical microcavity increases the Q of the hybrid plasmonic-photonic system, however, this dramatically degrades the Q of the original microcavity. Here, we propose a judicious hybridization of a plasmonic dark mode of a gold nanoring and whispering gallery mode (WGM) of a microtoroid. It is shown through finite element simulation that the hybridized WGM and dark mode of the proposed plasmonic gold nanoring solves the aforementioned issues in two ways. First, the small radiation loss of the dark mode minimizes Q degradation and allows the system to maintain its ultra-high Q. Second, the nanoring enhances the field on the microcavity’s surface which in turn increases the interaction between, for example, a biomolecular target and the WGM. We have shown that the proposed system generates larger resonance shifts compared to a microcavity loaded with same volume of conventional linear gold nanoantennas . This results in significant enhancement in the system’s sensitivity. We have repeated the same simulations for different materials and volumes.
Optical whispering gallery mode (WGM) biochemical sensors operate by tracking changes in resonant frequency as materials enter the evanescent near-field of the resonator. To achieve the smallest limit of detection, it is desirable for WGM sensors to exhibit as large a frequency shift as possible for a material of a given size and refractive index, as well as the ability to track as small a frequency shift as possible. Previously, plasmonic nanoantennas have been coupled to WGM resonators to increase the magnitude of resonance shifts via plasmonic enhancement of the electric field, however this approach also results in increased scattering from the WGM, which degrades its quality factor, making it less sensitive to extremely small frequency shifts. This degradation is caused by the ohmic and scattering dissipation caused by metallic nanoantennas. Using simulations, we show here that the precise positioning of nanoantennas coupled to a microtoroid WGM resonator can be used to overcome this drawback and achieve ultrahigh-Q plasmonic cavity modes simultaneously with electric field enhancement. It is shown that a nanoantenna composed of two similarly coupled nanorods supports higher Q modes than a single nanorod antenna. Our results have immediate application in the context of optical sensing.
The design of a near-IR spectrometer for the Gemini 8m telescopes is described. This instrument, GNIRS, provides coverage from 0.9 to 5.5 micrometers at several spectral resolutions and two pixel scales. Capabilities include an imaging mode intended primarily for acquisition, a cross- dispersed mode covering wavelengths from 0.9 to 2.5 micrometers , and provisions for an integral field unit. The design of the GNIRS is conservative, as it must meet tight schedule and resource constraints; it nonetheless provides high throughput and operational efficiency, minimal flexure, and the flexibility needed to support queue observing. The optics are a combination of diamond-turned metal optics for the fore-optics and collimator, and refractive optics for the cameras. The mechanism include a two-axis grating turret; all mechanism are deposited by means of internal detents. The instrument achieves low flexure within its weight budget by the use of a modular structure composed of cylindrical light-weighted sections into which individual mechanisms and optics modules are mounted. Extensive analyses of mechanical and optical performance have been performed. The GNIRS has passed its critical design review, and fabrication is now underway.
The internal support structure of the Gemini Near IR Spectrograph (GNIRS) comprises a series of substructures which are interconnected to support the optical components and their mechanisms.A very stable support structure is required in the GNIRS to exploit the high image quality rings; this type of structural did not provide sufficient stiffness. This concept was replaced by a novel type of structure employing lightweight cylindrical models, with each module produced by numerically controlled machining from a solid. Finite element analysis is combined with 3D layout techniques to develop an optimized structural configuration for each module. A parametric process was performed for the design optimization to produce the highest fundamental frequency for a given weight, as well as to deal with the normal concerns about global deformation and stress.
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