Tunable optofluidic microbubble lens.

Optofluidic microlenses are one of the crucial components in many miniature lab-on-chip systems. However, many optofluidic microlenses are fabricated through complex micromachining and tuned by high-precision actuators. We propose a kind of tunable optofluidic microbubble lens that is made by the fuse-and-blow method with a fiber fusion splicer. The optical focusing properties of the microlens can be tuned by changing the refractive index of the liquid inside. The focal spot size is 2.8 µm and the focal length is 13.7 µm, which are better than those of other tunable optofluidic microlenses. The imaging capability of the optofluidic microbubble lens is demonstrated under a resolution test target and the imaging resolution can reach 1 µm. The results indicate that the optofluidic microbubble lens possesses good focusing properties and imaging capability for many applications, such as cell counting, optical trapping, spatial light coupling, beam shaping and imaging.

Ultrasensitive optofluidic coupled Fabry-Perot capillary sensors.

Refractive index (RI) measurements are pertinent in concentration and biomolecular detection. Accordingly, an ultrasensitive optofluidic coupled Fabry-Perot (FP) capillary sensor based on the Vernier effect for RI sensing is proposed. Square capillaries integrated with the coupled FP microcavity provide multiple microfluidic channels while reducing the complexity of the fabrication process. The incoherent light source and spectrometer used during measurement facilitate the development of a low-cost sensing system. An ultrahigh RI sensitivity of 51709.0 nm/RIU and detection limit of 2.84 × 10-5 RIU are experimentally demonstrated, indicating acceptable RI sensing performance. The proposed sensor has significant potential for practical and low-cost applications such as RI, concentration, or biomolecular sensing.

Hyperboloid-Drum Microdisk Laser Biosensors for Ultrasensitive Detection of Human IgG.

Whispering gallery mode (WGM) microresonators have been used as optical sensors in fundamental research and practical applications. The majority of WGM sensors are passive resonators that require complex systems, thereby limiting their practicality. Active resonators enable the remote excitation and collection of WGM-modulated fluorescence spectra, without requiring complex systems, and can be used as alternatives to passive microresonators. This paper demonstrates an active microresonator, which is a microdisk laser in a hyperboloid-drum (HD) shape. The HD microdisk lasers are a combination of a rhodamine B-doped photoresist and a silica microdisk. These HD microdisk lasers can be utilized for the detection of label-free biomolecules. The biomolecule concentration can be as low as 1 ag mL-1 , whereas the theoretical detection limit of the biosensor for human IgG in phosphate buffer saline is 9 ag mL-1 (0.06 aM ). Additionally, the biosensors are able to detect biomolecules in an artificial serum, with a theoretical detection limit of 9 ag mL-1 (0.06 aM ). These results are approximately four orders of magnitude more sensitive than those for the typical active WGM biosensors. The proposed HD microdisk laser biosensors show enormous detection potential for biomarkers in protein secretions or body fluids.

Optofluidic microbubble Fabry-Pérot cavity.

An optofluidic microbubble Fabry-Pérot (OMBFP) cavity was investigated. In contrast to plane-plane FP (PPFP) cavities, the optical mode confinement and stability in an OMBFP were significantly enhanced. The optical properties of the OMBFP cavity, including the quality (Q) factor, effective mode area, mode distribution as a function of the core refractive index, microbubble position, and mirror tilt angle, were investigated systematically using the finite element method. In optofluidic lasing experiments, a low lasing threshold of 1.25 µJ/mm2, which was one order magnitude lower than that of the PPFP, was achieved owing to improved modal lateral confinement. Since the microbubble acts as a micro-lens and microfluidic channel in the parallel FP cavity, mode selection and cell-dye laser were easily realized in the OMBFP cavity.

High-Q guided mode resonance sensors based on shallow sub-wavelength grating structures.

We present a systematic investigation on the enhancement of the quality (Q) factors for guided-mode resonance (GMR) sensors with shallow subwavelength grating structures. By introducing the coupled-mode model, a theoretical high-Q factor can be achieved by choosing the proper geometric structure. Based on this method, a GMR sensor with a Q factor up to 8000, which is an order of magnitude larger than those of typical GMR sensors with Q factors within 100 ∼ 300, was demonstrated experimentally. Besides, the approached GMR sensor achieved a bulk sensitivity of 135 nm RIU-1 with a high signal to noise ratio, which supports a detection limit of 1 ng ml-1 for bovine serum albumin detection. This high performance GMR sensor paves the way towards high-throughput industrial mass production, and shows great potential for other applications, such as optical filters, spectrometer, and bio-imaging.

Ultra-sensitive biomolecular detection by external referencing optofluidic microbubble resonators.

We propose an effective method for biomolecular detection based on an external referencing optofluidic microbubble resonator system (EROMBRS), which possesses good long-term stability and low noise. In this study, EROMBRSs were used for nonspecific detection of bovine serum albumin (BSA) molecules and specific detection of D-biotin molecules. Ultra-low practical detection limits of 1 fg/mL for nonspecific and specific biomolecular detection were achieved.

Packaged microbubble resonator optofluidic flow rate sensor based on Bernoulli Effect.

A novel flow sensor based on dynamic fluid pressure changing in a packaged microbubble resonator without additional modification on its structure has been proposed and experimentally demonstrated. The results of sensing performance under both tunable laser source and broadband light source are presented. The flow rate sensitivity can reach up to 0.0196 pm / (µL/min). The fluid pressure variation caused by Bernoulli Effect is also analyzed theoretically.

Symmetric guided-mode resonance sensors in aqueous media with ultrahigh figure of merit.

Optical sensors with a high figure of merit (FOM) for refractive index measurement can substantially enhance detection performance. For guided mode resonance (GMR) sensors, previous works mainly focused on the sensitivity enhancement rather than FOM optimization; therefore, the state-of-the-art FOM is limited within the range of 100. To address this, we propose a low-index, ultraviolet-curable resin (n = 1.344) to form a simple, stable, symmetric, GMR sensor, with enhanced sensitivity, narrowed resonant linewidth, and substantially improved FOM, in aqueous media. The influence of structural parameters was systematically investigated, and optimized FOM values as high as tens of thousands were obtained using numerical calculation. Using low-cost, nanoimprinting technology, we experimentally demonstrated a spectral linewidth as narrow as 56 pm, a bulk refractive index sensitivity of 233.35 nm / RIU, and a low detection limit 1.93 × 10-6, resulting in a FOM value up to 4200 (48 times typical GMR sensors). The proposed symmetric GMR sensor exhibits great potential in a variety of applications, including label-free biosensing, bio-imaging, and optical filters.

Design for Distributed Feedback Laser Biosensors Based on the Active Grating Model.

The distributed feedback (DFB) laser is widely used in sensing because of its portable size, simple fabrication and high sensitivity. Most theoretical design models are based on passive Bragg gratings. However, passive grating models cannot be used to predict sensor performance using the important indicator of figure of merit (FOM) through theoretical calculations. To solve this problem, we replaced the passive grating with an active grating by using the imaginary part of the coupling constant that represents the value of the gain. As a comparison, the influence of the full width at half maximum (FWHM) and sensitivity were analyzed for different grating duty cycles and depths in the passive grating sensors. To obtain a higher FOM in the active grating sensors, we systematically investigated the effects of duty cycle and gain value through numerical simulations. We found that the redshift caused by a duty cycle increase can improve the sensitivity of biomolecule detection by 1.7 times.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface.

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.