Wearable bioelectronic systems are one of the most important tools for human health and motion monitoring.

Photovoltaics (PV) are a promising source of clean renewable energy, but current technologies face a cost-to-efficiency trade-off that has slowed widespread implementation. 1,2 We have developed a PV architecturescreening-engineered field-effect photovoltaics (SFPV)that in principle enables fabrication of low-cost, high efficiency PV from virtually any semiconductor, including the promising but hard-to-dope metal oxides, sulfides, and phosphides. 3 Prototype SFPV devices have been constructed and are found to operate successfully in accord with model predictions.

Here we demonstrate an inexpensive, simple, and ultra-sensitive refractive index sensor based on a tapered tip optical fiber combined with a straightforward image analysis method. The output profile of this fiber exhibits circular fringe patterns whose intensity distribution dramatically changes even with ultra-small refractive index variations in the surrounding medium. The sensitivity of the fiber sensor is measured using different concentrations of saline solutions with a transmission setup consisting of a single wavelength light source, a cuvette, an objective lens, and a camera. By analyzing the areal changes in the center of the fringe patterns for each saline solution, we obtain an unprecedented sensitivity value of 24,160 dB/RIU (refractive index unit), which is the highest value reported so far among intensity-modulated fiber refractometers. The resolution of the sensor is calculated to be 6.9 ×10−9. Moreover, we measure the sensitivity of the fiber tip in the backreflection mode using salt-water solutions and obtained a sensitivity value of 620 dB/RIU. This sensor is ultra-sensitive, simple, easy to fabricate, and low-cost, which makes it a promising tool for on-site measurements and point-of-care applications.

Based on a simply-built setup, we demonstrated an inexpensive refractive index sensor. An unprecedented sensitivity value was obtained, which is the highest value reported so far among intensity modulated fiber refractometers.

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