Molecularly imprinted polymer-coated hybrid optical waveguides for sub-aM fluorescence sensing.

The sensitivity of fluorescent sensors is crucial for their applications. In this study, we propose a molecularly imprinted polymer (MIP)-coated optical fibre-hybrid waveguide-fibre sensing structure for ultrasensitive fluorescence detection. In such a structure, the MIP coated-hybrid waveguide acts as a sensing probe, and the two co-axially connected optical fibres act as a highly efficient probing light launcher and a fluorescence signal collector, respectively. For the dual-layered waveguide sensing probe, the inner hybrid waveguide core was fabricated using a hollow quartz nanoparticle-hybridized polymer composite with a low refractive index, and the outer MIP coating layer possesses a high refractive index. Simulations showed that this dual-layer configuration can cause light propagation from the waveguide core to the MIP sensing layer with an efficiency of 98%, which is essential for detection. To validate this concept, we adopted a popular fluorescent dye, rhodamine B, to evaluate the sensing characteristics of the proposed system. We achieved an extremely low limit of detection of approximately 1.3 × 10-19 g ml-1 (approximately 0.27 aM).

Lasing properties of cesium lead halide perovskite nanowires fabricated by one-drop self-assembly and ion-exchange methods.

Lead halide perovskites are attracting intense research interest due to their high energy conversion efficiency and tunable optoelectronic properties. In this study, we demonstrate an environment-friendly one-drop self-assembly and ion-exchange methods for preparation of CsPbBr3 perovskite nanowires (NWs). High-quality NWs can be obtained with very small doses of required material. In order to expand the emission band of the NWs, an ion exchange process was utilized to substitute the bromide component in CsPbBr3 NWs with other halogens, and emission over a band of 420-710 nm was successfully achieved. The NWs realized lasing in the range of 420-560 nm, and the typical thresholds for CsPbBr3 and CsPbCl3 NWs were 63.86 µJ/cm2 and 68.2 µJ/cm2, respectively. In addition, the NWs also showed robust stability under the continuous irradiation with the high energy laser pulses in ambient atmosphere.

0.1-20 THz ultra-broadband perfect absorber via a flat multi-layer structure.

An ultra-broadband perfect absorber based on graded-index mechanism is designed and fabricated. The perfect absorber is comprised of a heavily-doped silicon absorption substrate and a flat six-layer antireflective structure. The refractive index of each layer was widely tuned by hollow polystyrene microsphere and TiO2 nanoparticle dopants, which can offer a gradually changed refractive index profile from 1.3 to 2.9. The experimental results show that 98% absorption can be achieved within the range of 0.1-20 THz. Moreover, the high absorption efficiency as well as the ultra-broad range can maintain for incident angle from 0 to 75° by the theoretical simulation.

Molecularly Imprinted Polymer-Coated Optical Waveguide for Attogram Sensing.

Ultrahigh sensitivity and selectivity are the ultimate goals of sensor development. For such purposes, we propose a sensing platform in which an optical fiber-waveguide-fiber (OFWF) structure is integrated with a molecularly imprinted polymer (MIP). The OFWF works as a highly efficient probe light launcher and signal light collector, and the MIP layer acts as a highly selective and sensitive sensing interface. In the MIP design, a high-molecular refractive index monomer (2-phenylphenoxyethyl acrylate) was copolymerized with a MIP functional monomer (acrylic acid). The resulting high-refractive index MIP layers could effectively extract the probe light from the waveguide and send it to the MIP sensing interface. Moreover, a highly elastic cross-linker (poly(ethylene glycol) 600 diacrylate) was employed to increase the MIP mesh size, which could effectively increase the penetrability of the analyte. Rhodamine B (Rh B) is widely used in the textile industry, and its contamination may lead to serious public health problems. As a proof of concept, the Rh B chromophore was used as a molecular template, and the thin MIP layer was cured on the waveguide surface by utilizing the evanescent wave of the 405 nm propagating light in the waveguide. The MIP-OFWF sensing platform afforded highly selective monitoring of the absorption spectra of the components in a mixture solution of Rh B and methyl blue. It also afforded an extremely low detection limit of approximately 6.5 × 10-17 g/mL, with an absolute mass of 20-30 ag.

One-Drop Self-Assembly of Ultra-Fine Second-Order Organic Nonlinear Optical Crystal Nanowires.

In this study, we propose a one-drop self-assembly method, which proved capable of successfully preparing 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) single-crystalline nanowires (NWs). The apparent roughness of the DAST NWs was determined to be less than 100 pm by using a high-resolution atomic force microscope, indicating their ultrafine quality. The DAST NWs also exhibited excellent nonlinear optical properties, including two-photon excited fluorescence and second harmonic generation, which could enable the production of low-cost, low-power-consumption wideband wavelength conversion devices. Thus, the described method may provide a new avenue for organic NW fabrication.

Ultra-Broadband THz Antireflective Coating with Polymer Composites.

Achieving an ultra-broadband range is an essential development direction in terahertz techniques; however, a method to cover the full terahertz band by using a highly efficient antireflection (AR) coating that could greatly increase the efficiency of terahertz radiation is still lacking. It is known that structures possessing a graded-index profile can offer a broadband AR effect, and such structures have been widely used, especially in the visible range. In this paper, first, we tuned the refractive index of a cyclo-olefin polymer (COP) by using a TiO2 dopant, and a polymer⁻TiO2 composite with a refractive index of 3.1 was achieved. We then fabricated a surface-relief structure with a graded-index profile by using a hot-embossing method. The structure on the silicon substrate can provide an excellent AR effect, but the working band is still limited by its scale of sag and swell. To obtain an ultra-broadband AR effect, we then proposed a flat six-layer structure; a graded-index profile was obtained by casting epoxy⁻TiO2 composites in the order of a high index to lower indices. With a very well controlled refractive index and thickness of each layer, we achieved an AR effect of <2% in the ultra-broadband of 0.2⁻20 THz.

Research on the Terahertz Absorption Spectra of Histidine Enantiomer (L) and its Racemic Compound (DL).

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption spectra of polycrystalline L- and DL-histidine in the frequency range of 10-100 cm-1. The spectra exhibit distinct differences in peak frequencies between the enantiomer (L-histidine) and racemic compound (DL-histidine). The observed spectral differences are attributed to the intermolecular interactions. With the density function theory (DFT) method, the frequencies of vibrational modes of L-histidine and DL-histidine in the THz range are calculated and well assigned according to the measured spectra. The origin of the observed vibrational modes is found to be non-localized and of a collective (phonon-like) nature, which points to the lattice and skeleton vibrations mediated by the hydrogen bond. Furthermore, we propose and demonstrate a method for determining the composition ratio of histidine mixtures based on the THz absorption spectra.