All inorganic Cs₃Bi₂Br₉ perovskite NCs were synthesized at room temperature, demonstrating blue dual-emission peaks and high photoluminescence quantum yield.

Location error and false alarm are noticeable problems in fiber distributed acoustic sensing systems based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). A novel method based on signal kurtosis is proposed to locate and discriminate perturbations in Φ-OTDR systems. The spatial kurtosis (SK) along the fiber is firstly obtained by calculating the kurtosis of acoustic signals at each position of the fiber in a short time period. After the moving average on the spatial dimension, the spatial average kurtosis (SAK) is then obtained, whose peak can accurately locate the center of the vibration segment. By comparing the SAK value with a certain threshold, we may to some degree discriminate the instantaneous destructive perturbations from the system noise and certain ambient environmental interferences. The experimental results show that, comparing with the average of the previous localization methods, the SAK method improves the pencil-break and digging locating signal-to-noise ratio (SNR) by 16.6 dB and 17.3 dB, respectively; and decreases the location standard deviation by 7.3 m and 9.1 m, respectively. For the instantaneous destructive perturbation (pencil-break and digging) detection, the false alarm rate can be as low as 1.02%, while the detection probability is maintained as high as 95.57%. In addition, the time consumption of the SAK method is adequate for a real-time Φ-OTDR system.

Phase-sensitive optical time-domain reflectometry ($\varphi$-OTDR) based on coherent detection is one of the most widely used schemes to achieve fiber distributed acoustic sensing. In previous studies, a fairly high data acquisition speed is essential for the phase-measuring coherent $\varphi$-OTDR, thus leading to a severe computational burden. In this Letter, we first analyze the power spectrum of the beat signal and then propose the use of undersampling theory to reduce the need for high sampling frequency. Then we give the principle of selecting the matched sampling frequency and bandpass filter bandwidth so that the beat signal can be sampled without aliasing. The experimental results show that, when the central frequency of the beat signal is 200 MHz, its phase signal can be correctly demodulated even using a sampling rate as low as 71 MSa/s. This method can be extended to all existing coherent $\varphi$-OTDR systems with no or only a few modifications on them.

A surface acoustic wave based passive temperature sensor capable of multiple access is investigated. Binary Phase Shift Keying (BPSK) codes of eight chips were implemented using a reflective delay line scheme on a Y-Z LiNbO₃ piezoelectric substrate. An accurate simulation based on the combined finite- and boundary element method (FEM/BEM) was performed in order to determine the optimum design parameters. The scaling factor ‘s’ and time delay factor ‘τ’ were extracted using signal processing techniques based on the wavelet transform of the correlation function, and then evaluated at various ambient temperatures. The scaling factor ‘s’ gave a more stable and reliable response to temperature than the time delay factor ‘τ’. Preliminary results show that the sensor response is fast and consistent subject to ambient temperature and it exhibits good linearity of 0.9992 with temperature varying from 0 to 130 °C.

Free-standing colloidal MoSe ₂ nanoflowers were synthesized by a phosphine-free solution-processing approach, which showed good electrocatalytic activities.