A new route for fabricating polymer optical microcavities

Temperature Sensitivity Control of an Inkjet-Printed Optical Resonator on Pillar

We report a whispering gallery mode resonator on a pillar using inkjet printing combined with traditional microfabrication techniques. This approach enables several different polymers on the same chip for sensing applications. However, polymers inherently exhibit sensitivity to multiple stimuli. To mitigate temperature sensitivity, careful selection of design parameters is crucial. By precisely tuning the undercut-to-radius ratio of the resonator, a linear dependence in temperature sensitivity ranging from -41.5 pm/°C to 23.4 pm/°C, with a zero-crossing point at 47.6% is achieved. Consequently, it is feasible to fabricate sensing devices based on undercut microdroplets with minimal temperature sensitivity. The lowest measured temperature sensitivity obtained was 5.9 pm/°C, for a resonator with an undercut-to-radius ratio of 53%.

Roadmap on optical sensors

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

From Whispering Gallery Mode Resonators to Biochemical Sensors

Optical biosensors are frontrunners for the rapid and real-time detection of analytes, particularly for low concentrations. Among them, whispering gallery mode (WGM) resonators have recently attracted a growing focus due to their robust optomechanical features and high sensitivity, measuring down to single binding events in small volumes. In this review, we provide a broad overview of WGM sensors along with critical advice and additional "tips and tricks" to make them more accessible to both biochemical and optical communities. Their structures, fabrication methods, materials, and surface functionalization chemistries are discussed. We propose this reflection under a pedagogical approach to describe and explain these biochemical sensors with a particular focus on the most recent achievements in the field. In addition to highlighting the advantages of WGM sensors, we also discuss and suggest strategies to overcome their current limitations, leaving room for further development as practical tools in various applications. We aim to provide new insights and combine different knowledge and perspectives to advance the development of the next generation of WGM biosensors. With their unique advantages and compatibility with different sensing modalities, these biosensors have the potential to become major game changers for biomedical and environmental monitoring, among many other relevant target applications.

Large-scale flexible-resonators with temperature insensitivity employing superoleophobic substrates

Whispering gallery mode polymer resonators are becoming competitive with devices made of other materials, however, the inherent thermal sensitivity of the materials and the small size limit their applications, such as high-precision optical gyroscope. Here, a method is proposed for fabricating large-scale NOA65 resonators with quality factors greater than 10⁵ on a chip employing superoleophobic. The sandwich structure as the core layer of resonator is used to present the flexible remodeling characteristics, the surface roughness remains below 1 nm when the diameter changes by more than 25%. Importantly, theoretical and experimental results show that under the tuning action of external pressure, the equivalent thermal expansion coefficient of the resonator gradually approaches the glass sheet on both sides with the variation of 2 × 10^-4 /°C∼0.9 × 10^-4 /°C, and the corresponding temperature response range of 0.12 nm/°C∼-0.056 nm/°C shows the promise of temperature insensitivity resonators on a chip.

On-chip high-sensitivity photonic temperature sensor based on a GaAs microresonator

We demonstrate an on-chip high-sensitivity photonic temperature sensor based on a GaAs microdisk resonator. Based on the large thermo-optic coefficient of GaAs, a temperature sensitivity of 0.142 nm/K with a measurement resolution of 10 mK and low input optical power of only 0.5 µW was achieved. It exhibits great potential for chip-scale biological research and integrated photonic signal processing.

A multifunctional skin-like wearable optical sensor based on an optical micro-/nanofibre

Multifunctional skin-like sensors play an important role in next-generation healthcare, robotics, and bioelectronics. Here, we report a skin-like wearable optical sensor (SLWOS) enabled by a stretchable, flexible, and attachable patch embedded with an optical micro-/nanofibre (MNF), which is highly compatible with human skin, a curved surface, or cloth. Based on the transition from radiation modes into guided modes around the bending area of the MNF, the SLWOS embedded with a wavy MNF is highly sensitive to weak strain, achieving a gauge factor as large as 675 (strain <1%). The flexible SLWOS is also capable of monitoring the bending angle in a broad dynamic range with tunable sensitivity. In addition, temperature measurements in the range of -20 to 130 °C are realized by taking advantage of PDMS's large negative thermo-optic coefficient. The superior sensing performance together with mechanical flexibility enables the real-time monitoring of respiration, arm motion, and body temperature. This SLWOS will have great potential in wearable optical devices ranging from ultrasensitive sensors to photonic healthcare devices.

Ultrasensitive optofluidic enzyme-linked immunosorbent assay by on-chip integrated polymer whispering-gallery-mode microlaser sensors

Optical whispering-gallery-mode (WGM) microcavities offer great promise in ultrasensitive biosensors because of their unique ability to enable resonant recirculation of light to achieve strong light-matter interactions in microscale volumes. However, it remains a challenge to develop cost-effective, high-performance WGM microcavity-based biosensing devices for practical disease diagnosis applications. In this paper, we present an optofluidic chip that is integrated with directly-printed, high-quality-factor (Q) polymer WGM microlaser sensors for ultrasensitive enzyme-linked immunosorbent assay (ELISA). Optical 3D μ-printing technology based on maskless ultraviolet lithography is developed to rapidly fabricate high-Q suspended-disk WGM microcavities. After deposition with a thin layer of optical gain material, low-threshold WGM microlasers are fabricated and then integrated together with optical fibres upon a microfluidic chip to achieve an optofluidic device. With flexible microfluidic technology, on-chip, integrated, WGM microlasers are further modified in situ with biomolecules on surface for highly selective biomarker detection. It is demonstrated that such an optofluidic biochip can measure horseradish peroxidase (HRP)-streptavidin, which is a widely used catalytic molecule in ELISA, via chromogenic reaction at the concentration level of 0.3 ng mL-1. Moreover, it enables on-chip optofluidic ELISA of the disease biomarker vascular endothelial growth factor (VEGF) at the extremely low concentration level of 17.8 fg mL-1, which is over 2 orders of magnitude better than the ability of current commercial ELISA kits.

Design and analysis of visible photonics resonators coated with CuO thin film

The optical and structural properties of CuO film deposited on n-Si via spin-coating method have been ascertained for diverse annealing times. The characterizations were made using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-vis spectroscopy, ellipsometry spectroscopy and photoluminescence. A detailed analysis revealed the favorable behavior of CuO film for visible photonics resonators such as Fabry-Perot (FP) and ring resonators. The best suitable property was obtained for a film annealed for 15 min. Accordingly, the CuO-film-coated resonators were simulated and analyzed theoretically using the MODE Solutions tool by Lumerical and MATLAB. In the FP resonator, the transmission intensity, contrast factor and finesse were computed for different annealing times and angles of light incidence. Further, for the CuO ring resonator, an eigenmode solver was incorporated (in the wavelength range 300-900 nm) to compute the effective refractive index, propagation constant, group velocity, losses, dispersion and transmission intensity. Additionally, utilizing the basic expressions, the free spectral range, full-width at half-maximum and quality factor were derived.

Tunable polymer lasing in chirped cavities

Continuously tunable polymer lasing was achieved in one-dimensional, two-dimensional, and compound chirped cavities. The chirped cavity was simply fabricated by using interference lithography and spin coating. Two-dimensional and compound chirped cavities were obtained by employing oblique exposure and double exposure, respectively. The tunability range of two-dimensional chirped cavities was much wider than that of one-dimensional chirped cavities, which varied from 557 nm to 582 nm. The interaction between lasing modes was studied in the compound cavity by introducing an additional nanostructure into the two-dimensional chirped cavities. The threshold of the compound chirped cavities changed with the coupling strength between lasing modes. These results may be helpful for designing compact polymer laser sources.

Micro-/Nanofiber Optics: Merging Photonics and Material Science on Nanoscale for Advanced Sensing Technology

Micro-/nanofibers (MNFs) are optical fibers with diameters close to or below the wavelength of the guided light. These tiny fibers can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields, and surface intensity, which is very attractive to optical sensing on the micro-/nano scale. In this review, we first introduce the basics of MNF optics and MNF optical sensors from physical and chemical to biological applications and review the progress and current status of this field. Then, we review and discuss hybrid MNF structures for advanced optical sensing by merging MNFs with functional structures including chemical indicators, quantum dots, dye molecules, plasmonic nanoparticles, 2-D materials, and optofluidic chips. Thirdly, we introduce the emerging trends in developing MNF-based advanced sensing technology for ultrasensitive, active, and wearable sensors and discuss the future prospects and challenges in this exciting research field. Finally, we end the review with a brief conclusion.