Zhenmin Chen

Flexible smart sensing skins have attracted remarkable attention for their potential applications in human-machine interfaces and target detection. Here, a flexible smart sensing skin based on optical microfiber knot resonator (MKR) is proposed for underwater acoustic wave detection. By optimizing the fabrication processes, we have produced MKRs with high Q-factors (1×10⁵) and high coupling depths (0.95), which can ensure the flexible skin has extremely high sensitivity. By designing the optical and mechanical resonant dual resonant structures, the proposed sensors have an underwater acoustic pressure sensitivity level as high as -145.6 dB re 1V/μPa at 1250 Hz, which is ~ 40.9 dB higher than a commercial hydrophone. The NEP is as low as 58.8 dB re 1 μPa/√Hz, which is equivalently 0.9 mPa/√Hz. A piece of smart sensing skin consisting of a quartet of MKR sensors is demonstrated to determine the spatial position of an underwater acoustic source in the three-dimensional (3D) space. The experimental results indicate that the average errors for the three orthogonal directions are -3.3%, 0.0%, and 2.4%, respectively. This study offers new insights into the accurate underwater target detection and positioning using flexible MKR sensors, which are promising for the future smart sensing skin technology.

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 much thicker intrinsic absorption layer (IAL) in normal-incidence Ge-on-Si photodetectors (NIPD) usually causes a contradiction between responsivity and bandwidth. In response to this issue, here, we simulate the design of an NIPD with geranium (Ge) layers based on a “fishnet” metasurface, leading to a reduced device thickness as thin as 380[ ]nm. The optical simulation results show that the light field can be perfectly localized in the 210[ ]nm IAL, and the absorptivity is as high as 99.45% at 1550[ ]nm, which is even better than bulk materials. Moreover, the electrical simulation results suggest that the horizontal size of the photosensitive region can be reduced to 11.2 [ ]m, while the responsivity of the photodetector is close to 1 A/W at −1[ ]V bias voltage, which is nearly 23 times that of a bulk device with the same thickness, and the 3dB bandwidth is up to 40[ ]GHz, which can be compared with waveguide photodetectors. Besides, this device also demonstrates a high signal-to-noise ratio with a low dark current of 28.68 nA, making it an excellent PD for opto-electrical communication.

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.

We demonstrate an optical underwater acoustic sensor with high sensitivity and broad bandwidth (10 Hz to 100 kHz), based on the packaged optical microbubble whispering gallery mode resonator, for the first time. The underwater acoustic pressure sensitivity level is as high as −159.3 dB re 1 V/μPa at 1 kHz that benefits from the high optical Q-factor and low refractive index (RI) polymer cladding. The corresponding noise equivalent pressure (NEP) is 66.8 dB re 1 μPa/Hz ^1/2 , which is equivalently 2.2 mPa/Hz ^1/2 . The sensitivity is hardly affected by the changes of static pressure and temperature in practical applications. The wide underwater acoustic response directivity is measured and the angular responses have – 6 dB angle range of 75.6° and 105.5° in two orthogonal directions, respectively. This work provides a potential miniature optical acoustic sensor for the broadband underwater acoustic wave detection.

Recently, Vernier effect is introduced into fiber optic interferometric sensors to enhance the sensitivity. To achieve a high-quality of the Vernier spectrum, the match of the free spectral range (FSR) and the extinction ratio (ER) in two arms is required. Furthermore, the reference arm should be designed to be different from the sensing arm to avoid crosstalk. In this work, a compact in-line Mach-Zehnder interferometer (MZI) by sandwiching a hollow capillary fiber (HCF) between two sections of no-core fiber (NCF) is proposed for the reference arm in Vernier effect based sensing system. FSR of the in-line MZI can be tuned by modulating the length of the HCF, while the ER can remain high up to ~20 dB, which is equivalent to that of the polarization-maintaining fiber (PMF) based interferometers. Experimental results show that the MZI exhibits low sensitivity to temperature, strain, refractive index (RI) and torsion. When employed as the reference arm cascaded with PMF Sagnac interferometer (SI), the upper and lower envelope are with high extinction ratio (ER). The proposed MZI is a promising candidate for the reference arm in Vernier effect based sensing system with the merits of compact size, good spectral characteristics and insensitivity to ambient environment.

In this work, we carry out theoretical investigation of the dispersion relationship both with the radius of the microsphere cavity and wavelength for the first two radial-order mode based on the theory of microcavity dispersion. The results show that the size of the microcavity corresponding to the zero dispersion point of the higher order mode is larger than that of lower order mode at the certain wavelength. The microsphere cavities with slight deformations, which have rich modes and high Q values, have been fabricated and optical frequency comb (OFC) in them with small size is realized by means of mode coupling. We have achieved OFC in cavities as small as 105 μm in diameter by assistance of the theoretical study. The total dispersions are −7.1 MHz in 1550 nm and −14.2 MHz in 1310 nm, which are both in the normal dispersion region.