Nonlinear optics in wavelength-size waveguides: how far can the conversion efficiency be pushed?

When their size approach the wavelength of light propagating in them, waveguides exhibit favorable properties such as high nonlinearity and strong confinement. These properties have been used for a variety of nonlinear effects, including supercontinuum generation, second-, third- and one-third- harmonic generation, pulse shaping and nonlinear switching.Although simulations predicted conversion efficiencies well in excess of 30% for most of harmonic generation, experiments showed conversions of a fraction of a percent, at best.This talk will discuss harmonic generation in wavelength-size waveguides and limitations to the practical achievement of theoretical efficiency. In particular, intrinsic surface roughness due to thermal surface waves frozen during fabrication provide a constantly changing detuning from the ideal phase matching conditions, considerably reducing the overall efficiency for waveguides longer than 1mm

Parametric up- and down-conversion in sub-wavelength waveguides: coherent sources in the UV and IR

Parametric up- and down-conversion require phase matching between the generating and generated wavelengths, introducing strict requirements on the refractive index dispersion of materials used for this nonlinear processes. Optical microfibers can exploit the different modal overlap with core and cladding materials to phase match different modes at the required wavelengths. Intermodal phase-matching has been successfully exploited in optical microfibres to generate second- and third-harmonics, both in straight waveguides and related resonators. The simultaneous generation of guided higher harmonics also allowed to exploit other nonlinear effects like four wave mixing to generate light at short wavelengths in the UV part of the spectrum. Parametric down-conversion allows to generate entangled photons, but with efficiencies significantly smaller than their up-conversion processes because of their reliance of spontaneous photons generation from vacuum. The use of more complex architectures, based on multiple coupled waveguides, allows to achieve a quasi-phase matching condition, thus a sizeable improvement on the overall conversion efficiency

χ(3) processes in high numerical optical fibers and fiber tapers

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity can be used to efficiently generate UV and mid-IR wavelength regions in solid core silica optical fibers and optical fiber tapers. We theoretically study the waveguide parameters and practical considerations required for optimum conversion

Light generation in solid silica optical fibres: beyond their perceived limits

Solid silica fibres can be used to generate light in the ultraviolet (down to 180 nm) and mid-infrared (up to 6 µm) if suitable fibre designs and nonlinear processes are used

Broadband near-infrared spectroscopy of organic molecules on compact photonic devices

We demonstrate a nanophotonic approach for broadband near-infrared spectroscopy of organic molecules. Waveguides, tapered microfibers and gold nanoparticles enable ultra-sensitive miniature spectrometers for highly sensitive detection in ultra-low sample volume

Glass nanowires for nonlinear optics and sensing: a top-down approach

Optical microfibers (OMs) are waveguides of diameters comparable to the wavelength of the light propagating in them. Fig.1a shows a schematic of a typical microfiber: it consists of a region with thin waist connected to two conventional optical fibres by transition regions. Their sub-micron size, easy connectivity to conventional optical fibres and relatively high mechanical strength have made them ideal for a variety of applications including sensors, lasers and light sources. Moreover, their strong evanescent field, good light confinement and high nonlinearity have raised the interest in exploring the possibility to use these microfibers for cheap, compact optical devices

Comparison of detection limit in fiber-based conventional, amplified, and gain-clamped cavity ring-down techniques

Relative performance and detection limit of conventional, amplified, and gain-clamped cavity ring-down techniques (CRDT) in all-fiber configurations are compared experimentally for the first time. Refractive index measurement using evanescent field in tapered fibers is used as a benchmark for the comparison. The systematic optimization of a nested-loop configuration in gain-clamped CRDT is also discussed, which is crucial for achieving a constant gain in a CRDT experiment. It is found that even though conventional CRDT has the lowest standard error in ring-down time (Δτ), the value of ring-down time (τ) is very small, thus leading to poor detection limit. Amplified CRDT provides an improvement in τ, albeit with two orders of magnitude higher Δτ due to amplifier noise. The nested-loop configuration in gain-clamped CRDT helps in reducing Δτ by an order of magnitude as compared to amplified CRDT whilst retaining the improvement in τ. A detection limit of 1.03×10−4 RIU at refractive index of 1.322 with a 3 mm long and 4.5 μm diameter tapered fiber is demonstrated with the gain-clamped CRDT.</p

UV and IR light generation in silica-based optical fibre tapers

Optical fibre lasers offer significant benefits in comparison to other laser sources, such as extremely low thermal lensing, extraordinary good beam quality and very high plug efficiency. Up to date only near-IR sources have been manufactured in silica in fiberized forms. The challenge is to develop new fiberized UV and IR sources in a silicate fibre host.Here we show that up- and down-conversion in tapers made from silicate optical fibres can efficiently be used to generated light both in the UV and in the mid-IR. Simulations show that efficiencies in excess of 50% can be potentially achieved for lengths of the order of tens of millimetres

Parametric χ³ light generation in subwavelength waveguides

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity can be used to efficiently generate UV and mid-IR wavelength regions in solid core silica optical fibers and optical fiber tapers. We theoretically study the waveguide parameters and practical considerations required for optimum conversion

Silica nanowires for UV light generation and sensing

Because of the strong confinement and large refractive index contrast, silica nanowires can efficiently generate light in the UV through intermodal phase matching. Similarly, the fraction of power in the evanescent field allows for many applications in sensing