Zhihe Guo

Mechano-sensitive hair-like sensilla (MSHS) have an ingenious and compact three-dimensional structure and have evolved widely in living organisms to perceive multidirectional mechanical signals. Nearly all MSHS are iontronic or electronic, including their biomimetic counterparts. Here, an all-optical mechano-sensor mimicking MSHS is prototyped and integrated based on a thin-walled glass microbubble as a flexible whispering-gallery-mode resonator. The minimalist integrated device has a good directionality of 32.31 dB in the radial plane of the micro-hair and can detect multidirectional displacements and forces as small as 70 nm and 0.9 μN, respectively. The device can also detect displacements and forces in the axial direction of the micro-hair as small as 2.29 nm and 3.65 μN, respectively, and perceive different vibrations. This mechano-sensor works well as a real-time, directional mechano-sensory whisker in a quadruped cat-type robot, showing its potential for innovative mechano-transduction, artificial perception, and robotics applications.

The packaged microbubble resonators (PMBRs) provide a promising platform for investigating the interaction between fluid and optical microcavity. In this work, PMBRs have been applied to study the resonance wavelength shift with temperature in a hollow PMBR and a water-filled PMBR. The mode sensitivity is 0.0213 nm/°C and -0.0573 nm/°C, respectively. After filling the PMBR with liquid metal (LM), the phase transition process is monitored by using the mode sensitivity change. Below the melting point (MP) (around 15.2 °C), the optical mode sensitivity is -0.0083 nm/°C. While above the MP, the sensitivity increase to -0.0841 nm/°C. The PMBR provides a new method to study the phase transition process of phase change materials. Moreover, LM greatly improves the thermal sensitivity of the optical mode of the PMBR. The thermal optical coefficient of LM is around -3.4×10 ^-2 RIU/°C (liquid phase) according to theoretical model estimation. The coefficient of 2-3 orders of magnitude higher than the traditional dielectric material, which will open a new chapter in the application research of LM in thermo-optical response.

Bound states in the continuum (BICs) have attracted considerable attentions for biological and chemical sensing due to their infinite quality (Q)-factors in theory. Such high-Q devices with enhanced light-matter interaction ability are very sensitive to the local refractive index changes, opening a new horizon for advanced biosensing. In this review, we focus on the latest developments of label-free optical biosensors governed by BICs. These BICs biosensors are summarized from the perspective of constituent materials (i.e., dielectric, metal, and hybrid) and structures (i.e., grating, metasurfaces, and photonic crystals). Finally, the current challenges are discussed and an outlook is also presented for BICs inspired biosensors.

Bound states in the continuum (BICs) have attracted a lot of interest in the field of nanophotonics, and provide an important physical mechanism to realize high quality (Q) factor resonance. However, in practice, manufacturing error will greatly affect the Q factor. In this paper, we propose an asymmetric metasurface supporting near merging BIC under normal incidence. Such near merging BIC can achieve a higher Q factor (>10⁷) than common structures (Q ~ 10⁵) with the same degree of asymmetry in the structure. Moreover, the near merging BICs also show higher surface sensitivity than other resonant modes. Our work provides a promising approach for the realization of a high-performance biosensing platform.

The measurement of cardiac troponin‐I (cTnI) is widely used for diagnosing acute myocardial infarction (AMI) diseases because of its myocardial specificity. Packaged microbubble resonators with thin wall are utilized for label‐free and specific detection of cTnI based on whispering gallery mode (WGM). This packaged structure can provide a good protection for the biosensor, improve the anti‐interference ability of the sensor and reduce the system noise. The theoretical detection limit of the biosensors for cTnI in phosphate buffer saline (PBS) is 0.4 ag mL⁻¹ (0.02 a_M). Furthermore, we demonstrated that the biosensors can be used to detect cTnI molecules in simulated serum and the theoretical detection limit is also 0.4 ag mL⁻¹ (0.02 a_M). These results are much far below the clinical cut‐off value and show a huge application potential for the detection of cardiac biomarkers of AMI.

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

Whispering gallery mode micro-bubble optical cavities are asymmetrical ellipsoids in experimental settings, which makes their modes nondegenerate. A complicated dense spectrum is thus generated. Overlapping and coupled resonances exist in this dense spectrum. In this study, we determined that the orthogonal demodulation Pound-Drever-Hall technique can be used to analyze complicated resonances. Using this method, overlapping weak and strong coupling resonances can be analyzed. Compared to spectrum simplification and the ab initio theory of Fano resonances, this method is repeatable, simple, sensitive, and accurate. The method can increase the measurement range of differential resonance sensing, thus allowing the differential sensing of overlapped resonances.

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 a_M). Additionally, the biosensors are able to detect biomolecules in an artificial serum, with a theoretical detection limit of 9 ag mL⁻¹ (0.06 a_M). 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.

We present the spectral modulation of an optofluidic microdisk device and investigate the mechanism and characteristics of the microdisk laser in aqueous media. The optofluidic microdisk device combines a solid-state dye-doped polymer microdisk with a microfluidic channel device, whose optical field can interact with the aqueous media. Interesting phenomena, such as mode splitting and single-mode lasing in the laser spectrum, can be observed in two coupled microdisks under the pump laser. We modulated the spectra by changing the gap of the two coupled microdisks, the refractive indices of the aqueous media, and the position of a pump light, namely, selective pumping schemes. This optofluidic microlaser provides a method to modulate the laser spectra precisely and flexibly, which will help to further understand spectral properties of coupled microcavity laser systems and develop potential applications in photobiology and photomedicine.

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.