Surface nanoscale axial photonics at a capillary fiber

Abstract

We present the theory and first experimental demonstration, to the best of our knowledge, of a sensing platform based on surface nanoscale axial photonics (SNAP) at a capillary fiber. The platform explores optical whispering gallery modes, which circulate inside the wall of a capillary and slowly propagate along its axis. Due to the small thickness of the capillary wall, these modes are sensitive to spatial and temporal variations of the refractive index of the media adjacent to the internal capillary surface. In particular, the developed theory allows us to determine the internal effective radius variation of the capillary from the measured mode spectra. Experimentally, a SNAP resonator is created by local annealing of the capillary with a focused CO laser followed by internal etching with hydrofluoric acid. The comparison of the spectra of this resonator in the cases when it is empty and filled with water allows us to determine the internal surface nonuniformity introduced by etching. The results obtained pave the way for a novel advanced approach in sensing of media adjacent to the internal capillary surface and, in particular, in microfluidic sensing.

Optical bottle microresonators

“…As another example, of special interest are microfluidic BMR sensors [41] which enable monitoring the physical properties of liquids propagating along microchannels [81]. Potentially these sensors can be used for characterization as well as manipulation of microfluidic components [97,27,28]. The future development and applications of BMRs may boost based on the SNAP technology which enables fabrication of BMRs with complex shape and unprecedented subangstrom precision.…”

“…A super-precise method for fabrication of nanometer shallow BMR was developed in SNAP technology [20][21][22][23][24][25][26][27][28].The first demonstration of SNAP BMRs fabricated with angstrom precision was published in 2011 [22]. Originally, two fabrication methods were developed.…”

“…1(c)), the axial radius can be made much greater than the azimuthal radius, ax az r r >> . For example, BMRs fabricated in SNAP (Surface Nanoscale Axial Photonics) technology [19][20][21][22][23][24][25][26][27][28] may have the axial radius exceeding a kilometre [23,24]. For this reason, the characteristic variation length of WGMs along the axis of a BMR can be much greater than that of spherical and toroidal microresonators.…”

Localization of light in an optical microcapillary induced by a droplet

“…As an example, the immediate positions and velocities of an individual and, presumably, a few micro/nanoparticles in a colloidal liquid can be determined from the dynamics of WGM spectrum measured at a fixed microfiber position. In contrast to the SNAP microfluidic sensor 28,29 , the microresonator enabling this nonlocal characterization of the droplet is naturally introduced and does not have to be fabricated.…”

“…In fact, while SNAP considers WGMs controlled by nanoscale deformation of the outer surface of an optical fiber, similar control can be performed in microcapillaries by small and slowly varying internal nonuniformities. Recently we showed that the introduction of SNAP resonators is possible by deformation of the external as well as the internal capillary surfaces 29 . Alternatively, here we consider liquid situated in a uniform microcapillary.…”