Lasing of whispering gallery modes in optofluidic microcapillaries.

This paper demonstrates lasing of the whispering gallery modes in polymer coated optofluidic capillaries and their application to refractive index sensing. The laser gain medium used here is fluorescent Nile Red dye, which is embedded inside the high refractive index polymer coating. We investigate the refractometric sensing properties of these devices for different coating thicknesses, revealing that the high Q factors required to achieve low lasing thresholds can only be realized for relatively thick polymer coatings (in this case ≥ 800 nm). Lasing capillaries therefore tend to have a lower refractive index sensitivity, compared to non-lasing capillaries which can have a thinner polymer coating, due to the stronger WGM confinement within the polymer layer. However we find that the large improvement in signal-to-noise ratio realized for lasing capillaries more than compensates for the decreased sensitivity and results in an order-of-magnitude improvement in the detection limit for refractive index sensing.

Refractometric micro-sensor using a mirrored capillary resonator.

We report on a flow-through optical sensor consisting of a microcapillary with mirrored channels. Illuminating the structure from the side results in a complicated spectral interference pattern due to the different cavities formed between the inner and outer capillary walls. Using a Fourier transform technique to isolate the desired channel modes and measure their resonance shift, we obtain a refractometric detection limit of (6.3 ± 1.1) x 10-6 RIU near a center wavelength of 600 nm. This simple device demonstrates experimental refractometric sensitivities up to (5.6 ± 0.2) x 102 nm/RIU in the visible spectrum, and it is calculated to reach 1540 nm/RIU with a detection limit of 2.3 x 10-6 RIU at a wavelength of 1.55 µm. These values are comparable to or exceed some of the best Fabry-Perot sensors reported to date. Furthermore, the device can function as a gas or liquid sensor or even as a pressure sensor owing to its high refractometric sensitivity and simple operation.

Linearly polarized emission from an embedded quantum dot using nanowire morphology control.

GaAs nanowires with elongated cross sections are formed using a catalyst-free growth technique. This is achieved by patterning elongated nanoscale openings within a silicon dioxide growth mask on a (111)B GaAs substrate. It is observed that MOVPE-grown vertical nanowires with cross section elongated in the [21̅1̅] and [1̅12] directions remain faithful to the geometry of the openings. An InGaAs quantum dot with weak radial confinement is realized within each nanowire by briefly introducing indium into the reactor during nanowire growth. Photoluminescence emission from an embedded nanowire quantum dot is strongly linearly polarized (typically >90%) with the polarization direction coincident with the axis of elongation. Linearly polarized PL emission is a result of embedding the quantum dot in an anisotropic nanowire structure that supports a single strongly confined, linearly polarized optical mode. This research provides a route to the bottom-up growth of linearly polarized single photon sources of interest for quantum information applications.