Ultra-high-quality-factor (UHQ) optical resonators have enabled numerous fundamental scientific studies and advanced integrated photonic device technology. While free-standing devices can be fabricated from many different materials, only silica (SiO2) devices have been successfully integrated onto silicon wafers in large arrays. However, the UHQs (Q > 108) are transient, gradually decaying over time due to the presence of hydroxyl groups on the silica surface that attract water. Here, we overcome this challenge by using silicon oxynitride (SiOxNy) instead of silica. Unlike SiO2, SiOxNy presents a mixture of −OH and −F groups to the environment, thus inhibiting the formation of a high optical loss water layer. As a result, quality factors in excess of 100 million are able to be maintained for longer than 14 days with no environmental controls on device storage. Over the same time frame, quality factors for SiO2 devices stored in the same manner degraded by approximately an order of magnitude.Keywords: integrated photonics; microcavity; optical resonator; silicon oxynitride; whispering gallery mode;

Optical frequency combs are high repetition rate, broad spectral bandwidth coherent light sources. These devices have numerous applications in many fields, ranging from fundamental science to defense. Recently, low-threshold and small-footprint frequency combs have been demonstrated using ultrahigh quality factor (Q) whispering gallery mode resonant cavities. The majority of research in cavity-based combs has focused on optimizing the Q. An alternative strategy is to engineer the cavity material to enhance the underlying nonlinear process for comb generation. In this work, we demonstrate that gold nanorods coated with a nonlinear material reduce the comb generation threshold when decorated on the surface of the resonant cavities. The enhancement mechanism is explored with finite element method modeling and can be explained in terms of photonic–plasmonic mode hybridization. A comb span of ∼300 nm in the near-IR range is observed with incident intensity <2 GW cm–2. The required threshold for parametric oscillation directly scales with nanorod concentration and ranges from 148 μW to 1.5 mW, which is 15 times lower than uncoated silica devices with similar optical performance.Keywords: gold nanorods; mode hybridization; nonlinear optics; optical microcavity; plasmon−polariton; whispering gallery mode resonators;

Flexible, light-emitting materials have shown promise in a wide range of applications. Here, we develop an inverse soft-lithography process for embedding zinc oxide nanotetrapods (ZnO NTP) uniformly and nondestructively into a host matrix. The crystalline NTPs were synthesized using a catalyst-free, environmentally friendly chemical vapor transport method. The fluorescent emission of the ZnO NTPs was measured before and after the embedding process. Cyclical mechanical bend tests (N > 100) were performed. The emission of the nanomaterial remains throughout.Keywords: fluorescence nanomaterials; functional nanomaterials; soft lithography; ZnO nanotetrapods;

One critical challenge facing society is balancing the positive and negative effects of UV exposure. While UV exposure contributes to Vitamin D production, in excess, UV exposure is linked to skin cancer. Therefore, methods to monitor UV exposure and help society achieve this delicate balance have been the focus of numerous research efforts. Here, we leverage advances in functional materials to create a wearable ultraviolet light sensor. The flexible, lightweight trilayer sensor, which is composed of a UV responsive polymer layer sandwiched between two transparent protective layers made from FDA-approved polymers, changes color from transparent to yellow upon UV exposure. Notably, the entire trilayer system is less than 250 μm thick, allowing it to maintain mechanical flexibility. The UV responsive material leverages the photocleavable ortho-nitrobenzyl (ONB) moiety. Because a singular ONB cleaving group is centrally located along the polymer backbone, the colorimetric response is very controlled. Using an AM1.5 solar simulator, the sensor’s linear working range is demonstrated to cover the medically relevant range of 15 min to 1 h. Additionally, we determine the sensor’s robustness to potential environmental and mechanical stress as well as long-term storage. After mechanical bending and water immersion, the performance is unchanged. Storage in ambient conditions also does not degrade the behavior. When combined with sunscreen, the sensor’s response is predictably decreased due to the attenuation of the UV light. This flexible colorimetric sensor based on a smart polymer system can greatly aid society in achieving a balance in sufficient UV exposure.Keywords: ortho-nitrobenzyl; power-free; smart polymer; ultraviolet sensor; wearable

We developed and verified a method to create a photocleavable smart surface. Using the grafting to approach, we covalently attached an intelligently designed tailor-made diblock copolymer to a silicon wafer. The photocleavable moiety, o-nitrobenzyl (ONB) ester, was integrated into the copolymer at the junction point between the hydrophilic poly(ethylene oxide) (PEO) and the hydrophobic polystyrene (PS) chains. The well-defined azide bearing amphiphilic block copolymer was synthesized via a general stepwise strategy that combines atom transfer radical polymerization (ATRP) and copper(I)-catalyzed azide–alkyne cycloaddition reaction (CuAAC), ending with azidation. The azide end-functionalized copolymer chains were covalently bound to the alkyne-immobilized silicon wafer by CuAAC. The smart surface was exposed to UV irradiation, resulting in photocleavage of the grafted ONB linker. As a result of the photocleavage and subsequent removal of the o-nitrosobenzaldehyde bearing PEO, the PS layer remained on the surface. To confirm the behavior, film thickness and wettability changes were investigated before and after UV irradiation using AFM and contact angle measurements. Integration of photocleavable polymers through covalent grafting to solid surfaces contributes responsiveness to such materials that can find a wide array of applications in advanced devices.

Raman lasers form a particularly unique and versatile class of optically pumped laser. Because they rely on stimulated Raman scattering (SRS), which is governed by the vibrational frequency of the gain material and not electronic transitions, the emission wavelength is determined by the pump wavelength, enabling broad tunability. Ultrahigh quality factor (Q) silica optical resonators are an ideal platform for Raman lasers because the long photon lifetime results in large circulating optical intensities. Previous work has demonstrated extremely low thresholds using these devices. However, only moderate Raman lasing efficiencies have been achieved in silica devices. In the present work, we demonstrate that a zirconium (Zr)-doped silica sol–gel coating can improve the performance of a silica microcavity Raman laser. Several concentrations of Zr-doped sol–gel are synthesized. The intrinsic Raman gain of the Zr-doped silica is measured using Raman spectroscopy, and the values show a clear dependence on Zr dopant concentration. Subsequently, ultrahigh-Q silica toroidal microcavities are coated with the different Zr-doped sol–gels. The Raman lasing performance is characterized using a 765 nm optical pump, and the first order and cascaded Raman emissions for the coated devices are detected starting at 790 nm. The Raman lasing emission and characteristic threshold curves are quantified using both an optical spectrum analyzer and an optical spectrograph. The unidirectional pump-to-Raman conversion efficiency exhibits a marked enhancement from 3.37 to 47.43% as the Zr concentration increases.Keywords: microcavity laser; Raman laser; silica sol−gel; ultrahigh-Q silica toroid; whispering gallery mode resonator; Zr dopants;

We characterized the kinetics of a photocleavable o-nitrobenzyl (ONB)-modified poly(methyl acrylate) in solution and in film. Using atom transfer radical polymerization, a single photolabile ONB group was incorporated into the center of the polymer chain, resulting in a clearly defined photoresponse. The photocleavage behavior was characterized in three difference solvents and in film, and it was fit to an exponential model. The dependence of the reaction rate constants and quantum efficiency on the interaction parameter between the solvent and the polymer (χ12) and the molecular weight was investigated. Interestingly, as χ12 decreased, the reaction rate increased and the quantum efficiency increased.

•Detection and differentiation of two structural methylated cytosine analogs, 5′mC and 5′hmC, at sub-pM concentration levels.•PCR-free detection based on a label-free, real-time integrated optical sensor.•Epoxy–silane based antibody bioconjugation method developed to enable selective detection.•Specificity greater than 3:1 at nM concentrations.Significant research has been invested in correlating genetic variations with different disease probabilities. Recently, it has become apparent that other DNA modifications, such as the addition of a methyl or hydroxymethyl group to cytosine, can also play a role. While these modifications do not change the sequence, they can negatively impact the function. Therefore, it is critical to be able to both read the genetic code and identify these modifications. Currently, the detection of hydroxymethylated cytosine (5′hmC) and the two closely related variants, cytosine (C) and 5′methylcytosine (5′mC), relies on a combination of nucleotide modification steps, followed by PCR and gene sequencing. However, this approach is not ideal because transcription errors which are inherent to the PCR process can be misinterpreted as fluctuations in the relative C:5′mC:5′hmC concentrations. As such, an alternative method which does not rely on PCR or nucleotide modification is desirable. One approach is based on label-free optical resonant cavity sensors. In the present work, toroidal resonant cavity sensors are functionalized with antibodies to enable label-free detection and discrimination between C, 5′mC, and 5′hmC in real-time without PCR. Specifically, epoxide chemistry is used to covalently attach the 5′hmC antibody to the surface of the cavity. Subsequently, to thoroughly characterize the sensor platform, detection of C, 5′mC, and 5′hmC is performed over a concentration range from pM to nM. At low (pM) concentrations, the hydroxymethylated cytosine produces a significantly larger signal than the structurally similar epigenetic markers; thus demonstrating the applicability of this platform.

Plasmonic–photonic interactions have stimulated significant interdisciplinary interest, leading to rapid innovations in solar design and biosensors. However, the development of an optically pumped plasmonic laser has failed to keep pace due to the difficulty of integrating a plasmonic gain material with a suitable pump source. In the present work, we develop a method for coating high quality factor toroidal optical cavities with gold nanorods, forming a photonic–plasmonic laser. By leveraging the two-photon upconversion capability of the nanorods, lasing at 581 nm with a 20 μW threshold is demonstrated.

Ultra-sensitive, label-free biosensors have the potential to have a tremendous impact on fields like medical diagnostics. For the majority of these Si-based integrated devices, it is necessary to functionalize the surface with a targeting ligand in order to perform specific biodetection. To do this, silane coupling agents are commonly used to immobilize the targeting ligand. However, this method typically results in the bioconjugation of the entire device surface, which is undesirable. To compensate for this effect, researchers have developed complex blocking strategies that result in selective patterning of the sensor surface. Recently, silane coupling agents were used to attach biomolecules to the surface of silica toroidal biosensors integrated on a silicon wafer. Interestingly, only the silica biosensor surface was conjugated. Here, we hypothesize why this selective patterning occurred. Specifically, the silicon etchant (xenon difluoride), which is used in the fabrication of the biosensor, appears to reduce the efficiency of the silane coupling attachment to the underlying silicon wafer. These results will enable future researchers to more easily control the bioconjugation of their sensor surfaces, thus improving biosensor device performance.Graphical abstractHighlights► Achieved selective bioconjugation of integrated silica-on-silicon biosensors. ► Verified selective patterning with XPS, AFM and fluorescence microscopy. ► Demonstrated that device fabrication chemistries enabled selective patterning. ► Selective patterning approach is broadly applicable to other biosensor geometries.

The optical properties of polymeric materials, such as transmission loss and the thermo-optic coefficient, determine their utility in numerous applications, ranging from nanotechnology to the automotive and aerospace industries. However, because of the wide variation in the physical properties of polymers, many are unsuited for characterization using conventional techniques; consequently, their optical properties are unknown. One such polymer is polyisobutylene, which is viscous at room temperature and therefore is not compatible with conventional transmission loss and the thermo-optic coefficient characterization techniques because they rely on contact measurements. To overcome this, we have developed an integrated, microscale optical sensor that relies on an evanescent wave to study the material’s optical behavior. Using this device, we successfully determined the refractive index, the transmission loss, and the thermo-optic coefficient of ultrathin films of polyisobutylene. The films are deposited on the sensor’s silica surface using either spin coating or surface-initiated cationic polymerization, demonstrating the flexibility of this approach.

Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.