Nitrogen-Vacancy color center in diamond-emerging nanoscale applications in bioimaging and biosensing.

Nitrogen-Vacancy (NV) color center in diamond is a flourishing research area that, in recent years, has displayed remarkable progress. The system offers great potential for realizing futuristic applications in nanoscience, benefiting a range of fields from bioimaging to quantum-sensing. The ability to image single NV color centers in a nanodiamond and manipulate NV electron spin optically under ambient condition is the main driving force behind developments in nanoscale sensing and novel imaging techniques. In this article we discuss current status on the applications of fluorescent nanodiamonds (FND) for optical super resolution nanoscopy, magneto-optical (spin-assisted) sub-wavelength localization and imaging. We present emerging applications such as single molecule spin imaging, nanoscale imaging of biomagnetic fields, sensing molecular fluctuations and temperatures in live cellular environments. We summarize other current advances and future prospects of NV diamond for imaging and sensing pertaining to bio-medical applications.

In-line fiber optic interferometric sensors in single-mode fibers.

In-line fiber optic interferometers have attracted intensive attention for their potential sensing applications in refractive index, temperature, pressure and strain measurement, etc. Typical in-line fiber-optic interferometers are of two types: Fabry-Perot interferometers and core-cladding-mode interferometers. It's known that the in-line fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, we review two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques. Also, some recently reported specific technologies for fabricating such fiber optic interferometers are presented.

Nanocomposite polyacrylamide based open cavity fiber Fabry-Perot humidity sensor.

A humidity sensor with a low temperature sensitivity is proposed and demonstrated by coating a nanocomposite hygrometer polyacrylamide in an open interferometric cavity of a fiber Fabry-Perot interferometer. In this paper the Fabry-Perot structure is formed by splicing one short section of single mode fiber between two sections of single mode fiber with a larger offset fusing method. Experimental results show that relative humidity (RH) sensitivity of the sensor is ∼0.1 nm/(1% RH) in the range of 38% to 78% RH and ∼5.868 nm/(1%RH) in the range of 88% to 98% RH, respectively.

Microbubble based fiber-optic Fabry-Perot interferometer formed by fusion splicing single-mode fibers for strain measurement.

We demonstrate an all-fiber optical Fabry-Perot interferometer (FPI) strain sensor whose cavity is a microscopic air bubble. The bubble is formed by fusion splicing together two sections of single-mode fibers (SMFs) with cleaved flat tip and arc fusion induced hemispherical tip, respectively. The fabricated interferometers are with bubble diameters of typically ~100 µm. Strain and temperature sensitivities of fabricated interferometers are studied experimentally; a strain sensitivity of over 4 Pm/µÎµ and a thermal sensitivity of less than 0.9 Pm/°C is obtained.

Refractive index sensing based on Mach-Zehnder interferometer formed by three cascaded single-mode fiber tapers.

We report a highly sensitive refractive index (RI) sensor based on three cascaded single-mode fiber tapers, in which a weak taper is sandwiched between the two tapers to improve the sensitivity of the sensor. Experimental results show that the sensitivity of the device is 0.286 nm for a 0.01 RI change, which is about four times higher than that of the normal two-cascaded-taper-based Mach-Zehnder interferometer. In addition, the sensitivity of the device could be enhanced by tapering a longer and thinner middle weak taper. Such kinds of low-cost and highly sensitive fiber-optic RI sensors would find applications in chemical or biochemical sensing fields.

Micro Fabry-Perot interferometers in silica fibers machined by femtosecond laser.

Micro Fabry-Perot (F-P) interferometers (MFPIs) are machined in a single-mode fiber (SMF) and a photonic crystal fiber (PCF) by using a near-infrared femtosecond laser, respectively. The strain and temperature characteristics of the two MFPIs with an identical cavity length are investigated and the experimental results show that the strain sensitivity of the PCF-based MFPI is smaller than that of the SMF-based MFPI due to their different waveguide structures, while the two MFPIs have close temperature sensitivities due to their similar host materials. These MFPIs in silica fibers are compact, stable, inexpensive, capable for mass-production and easy fabrication, offering great potentials for wide sensing applications.