We use an optical positioning and linking (OPAL) platform based on optical tweezers to fabricate a grating coupler on a microtoroid-shaped optical resonator for highly efficient coupling of light into the resonator, which acts as a chemical sensor. Light is coupled into and out of the microtoroid through a grating fabricated from 0.5 µm polystyrene particles on the rim of the microtoroid, which acts as a fiber-free coupler. Unlike a tapered fiber coupler, this particle grating is highly robust and not susceptible to mechanical and vibrational noise, paving the way for field-portable sensing devices.
More sensitive toxic gas sensors can provide earlier warning by detecting lower concentrations at greater distances from the source than conventional technologies. Recently, microtoroid whispering gallery mode optical resonators with selective polymer coatings have demonstrated part-per-trillion sensitivity to several gases, making them one of the most sensitive gas detection technologies. However, these sensors are currently coupled to laser sources via fragile and vibration-sensitive tapered optical fibers, hindering their translation from the laboratory into the field. Here we design and assemble periodic nanostructures onto the rim of microtoroids to improve free-space coupling efficiency, obviating the need for the tapered optical fiber.
Whispering gallery mode microtoroid optical resonators are one of the most sensitive and rapid sensors in existence. However, the need for a tapered optical fiber to couple light into these sensors hinders translation out of laboratory as the fiber is fragile and suffers from mechanical vibration. Here, we developed a free space coupling system together with a compatible fluidic platform which allows monitoring of molecular interactions in real time. We obtained Q-factors as high as 1.6×10^8 with this approach. The large effective coupling area, ~10 μm in diameter for a numerical aperture of 0.14, provides high coupling stability during biosensing experiments.
Microtoroid resonators are one of the most sensitive chemical sensing technologies. However, coupling light into microtoroids typically requires equipment such as vibration-isolation tables and piezoelectric nanopositioning stages. Translating microtoroids to platforms with small size, weight, power, and cost for chemical vapor sensing remains a challenge. We demonstrate an approach to position photonic nanostructures on the surface of microtoroids to facilitate free-space coupling via inexpensive optics. We have designed the nanophotonic coupler using finite element simulations with novel boundary conditions to accommodate a large simulation domain. The nanophotonic coupler is assembled using a custom manufacturing platform based on automated optical tweezers.
Label-free detection techniques are widely used in biological and environmental sensing. In particular, whispering gallery mode microtoroid resonators have been used to detect single nanoparticles and molecules by relying on the ultrahigh quality factor of these resonators together with frequency locking for ultra-precise resonance tracking. However, material property characterization on a particle-by-particle basis remains a challenge in these label-free schemes. Here we characterize single particle material properties using microtoroid optical resonators combined with spectroscopic techniques.
Vaccines for the COVID-19 pandemic are limited and so effective drugs are needed. The binding affinity of several SARS-CoV-2 variants to human ACE2 receptors was measured using a frequency-locked optical whispering evanescent resonator (FLOWER) system. The advantage of FLOWER is that it is label-free and so drug candidates do not need to be labeled and it is ultra-sensitive so drugs over a wide range of binding affinities can be tested. The dissociation equilibrium constants of spike-RBD wild type as well as two variants, were analyzed and compared. Several drug candidates which inhibit the spike-RBD binding to ACE2, predicted by in-silico simulation, were screened using a competitive binding assay and the corresponding inhibitor constants were measured.
Whispering gallery mode microtoroid optical resonators are one of the most sensitive and rapid optical sensors in existence. These sensors can detect extremely low analyte concentrations down to attomolar levels on the order of seconds. How these sensors have such a rapid response time even at these low concentrations is a question in the field. Here, we show that total analyte arrival and binding time can occur in seconds using flow visualization and finite-element simulations.
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 present the design and feasibility testing of a multimodal co-registered endoscope based on a dual-path optical system integrated with a scanning piezo. This endoscope incorporates three different imaging modalities. A large field-of-view (FOV) reflectance imaging system enables visualization of objects several millimeters in front of the endoscope, while optical coherence microscopy (OCM) and multiphoton microscopy (MPM) are employed in contact with tissue to further analyze suspicious areas. The optical system allows multiple different imaging modalities by employing a dual optical path. One path features a low numerical aperture (NA) and wide FOV to allow reflectance imaging of distant objects. The other path features a high NA and short working distance to allow microscopy techniques such as OCM and MPM. Images of test targets were obtained with each imaging modality to verify and characterize the imaging capabilities of the endoscope. The reflectance modality was demonstrated with a 561 nm laser to allow high contrast with blood vessels. It achieved a lateral resolution of 24.8 μm at 5 mm and a working distance from 5 to 30 mm. OCM was performed with a 1300 nm super-luminescent diode since this wavelength experiences low relative scattering to allow for deeper tissue imaging. Measured OCM lateral and axial resolution was 4.0 and 14.2 μm, respectively. MPM was performed with a custom 1400 nm femtosecond fiber laser, a wavelength suitable for exciting multiple exogenous, and some endogenous fluorophores, as well as providing information on tissue composition through harmonic generation processes. A 4.0 μm MPM lateral resolution was measured.
High Q whispering gallery mode optical resonators are capable of rapid and ultra-sensitive biological detection at attomolar concentrations in under 30 seconds. One main question in the field is how these sensors detect such low concentrations of molecules so quickly. Calculations based on diffusion alone suggest that transport to these sensors should take hours to days. Here, we show using bromothymol blue dye flow visualization methods that transport to a microtoroid optical resonator can take place in seconds. We reconcile these results with finite element simulations.
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