Microwave frequency measurement using a silicon integrated microring resonator.

Photonics-assisted instantaneous frequency measurement of a microwave signal using a silicon integrated microring resonator (MRR) is proposed and experimentally demonstrated. The frequency of a microwave signal has a unique relationship with the power ratio between the two microwave signals at the outputs of two microwave photonic filters (MPF) with complementary frequency responses. The key device to implement the MPFs is a silicon integrated MMR, which is employed to convert a phase-modulated optical signal to an intensity-modulated optical signal by placing two optical carriers at the complementary slopes of the MRR. For a given frequency measurement range and resolution, an MRR is designed and fabricated, and its use for instantaneous microwave frequency (IMF) measurement is implemented. For the fabricated MRR, an IMF measurement range of 14-25 GHz with a measurement accuracy of ±0.2GHz is achieved.

Engineered endolysin-based "artilysins" for controlling the gram-negative pathogen Helicobacter pylori.

Helicobacter pylori infection can cause a variety of gastrointestinal diseases. In severe cases, there is a risk of gastric cancer. Antibiotics are often used for clinical treatment of H. pylori infections. However, because of antibiotic overuse in recent years and the emergence of multidrug-resistant bacteria, there is an urgent need to develop new treatment methods and drugs to achieve complete eradication of H. pylori. Endolysins and holins encoded by bacterial viruses (i.e., phages) represent a promising avenue of investigation. These lyase-based antibacterial drugs act on the bacterial cell wall to destroy the bacteria. Currently, a type of endolysin that has been studied more frequently acts on the amide bond between peptidoglycans, and holin is a transmembrane protein that can punch holes in the cell membrane. However, as a Gram-negative bacterium, H. pylori possesses a layer of impermeable lipopolysaccharides on the cell wall, which prevents endolysin interaction with the cell wall. Therefore, we designed a genetic linkage between an endolysin enzyme and a holin enzyme with a section of polypeptides (e.g., polycations and hydrophobic peptides) that enable penetration of the outer membrane. These complexes were designated "artilysins" and were efficiently expressed in Escherichia coli. In vitro bacteriostasis experiments showed that the purified artilysins had strong bacteriostatic effects on H. pylori. In addition, the surface of H. pylori was perforated and destroyed, as confirmed by electron microscopy, which was proved that artilysins had bacteriolytic effect on H. pylori.

Bamboo-like microfiber structures fabricated by one-step-tapering a fiber preform.

The microfiber-based optical structures have been attracting increasing research interests in communications and sensing fields. However, the fabrication of forming structures on fragile microfibers requires delicate operations, which limits the developments of their practical applications. In this work, a one-step-tapering technique is proposed to manufacture structures on microfibers. As a demonstration, the fiber preform, consisting of sawtooth shaped solid-air interfaces with designed dimensions, is obtained using a femtosecond laser milling technique. By one-step tapering the preform, periodic bumps are formed, resulting in a bamboo-like microfiber device. The fabricated structure shows spectral characteristics of a long-period grating, of which extinction ratio is up to 18.2 dB around 1553.3 nm. The response to refractive index is measured to be ∼875.02 nm/RIU and the temperature coefficient is ∼5.78 pm/°C. The theoretical analysis shows good agreement with the experimental results. The microfiber-based structure fabricated using the one-step-tapering-preform technique is featured with flexibility of design, reproducibility, and structural stability.

Polarimetric parity-time symmetry in a photonic system.

Parity-time (PT) symmetry has attracted intensive research interest in recent years. PT symmetry is conventionally implemented between two spatially distributed subspaces with identical localized eigenfrequencies and complementary gain and loss coefficients. The implementation is complicated. In this paper, we propose and demonstrate that PT symmetry can be implemented between two subspaces in a single spatial unit based on optical polarimetric diversity. By controlling the polarization states of light in the single spatial unit, the localized eigenfrequencies, gain, loss, and coupling coefficients of two polarimetric loops can be tuned, leading to PT symmetry breaking. As a demonstration, a fiber ring laser based on this concept supporting stable and single-mode lasing without using an ultranarrow bandpass filter is implemented.

Superstructure microfiber grating characterized by temperature, strain, and refractive index sensing.

Microfiber gratings with diameters in the subwavelength scale have recently attracted much attention for developments of sensitive sensors; however, a specific structure is usually chosen for sensing one parameter according to the optical response. In this work, a superstructure microfiber grating combined with microfiber Bragg grating and long-period microfiber grating is reported for the first time. The proposed superstructure is formed by ultraviolet laser inscription and femtosecond laser scratching techniques, which simultaneously endows the unique properties of the two individual gratings. The reflection and transmission spectral characteristics differing to conventional counterparts are demonstrated. The responsivities of the two gratings to temperature, strain and refractive index are investigated, providing a possibility for simultaneous multi-parameter sensing.

Noise suppression of mechanical oscillations in a microcavity for ultrasensitive detection.

Optical microcavities have been widely applied as sensitive detectors due to ultrahigh quality factors and small mode volumes. Besides considering the optical mode as the sensing signal, the optomechanical oscillations induced by the optical spring effect also perform as an elegant sensing signal. However, the minimal size of a detectable analyte is limited by the relatively weak light-matter interaction compared to the experimental noises. To improve the detection limit, many methods have been developed to either enhance device sensitivities or suppress experimental noises. In this work, we present a way to lower the detection limit by suppressing experimental noises of the mechanical frequency by 3 orders of magnitude. Utilizing a fiber tip as a benchmark analyte attaching onto the cavity, the mechanical frequency shift reflects the changes of the optical mode detuning of the cavity, predicting an effective tool for ultrasensitive detection.

Optically sizing single atmospheric particulates with a 10-nm resolution using a strong evanescent field.

Although an accurate evaluation of the distribution of ultrafine particulate matter in air is of utmost significance to public health, the usually used PM2.5 index fails to provide size distribution information. Here we demonstrate a low-profile and cavity-free size spectrometer for probing fine and ultrafine particulate matter by using the enhanced particle-perturbed scattering in strong optical evanescent fields of a nanofiber array. The unprecedented size resolution reaches 10 nm for detecting single 100-nm-diameter nanoparticles by employing uniform nanofibers and controlling the polarizations of the probe light. This size spectrometry was tested and used to retrieve the size distribution of particulate matter in the air of Beijing, yielding mass concentrations of nanoparticles, as a secondary exercise, consistent with the officially released data. This nanofiber-array probe shows potential for the full monitoring of air pollution and for studying early-stage haze evolution and can be further extended to explore nanoparticle interactions.

Single Nanoparticle Detection Using Optical Microcavities.

Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.