IMPACTO DE LOS ANTIHELMÍNTICOS EN LAS PASTURAS Y EL MEDIO AMBIENTE

We use a Ti-Sapphire laser at 780nm with a linewidth of 500MHz which could be tuned 0-200Ghz but not fully continuously over this range as discussed in chapter 3. Smooth tuning over a 12GHz band was easily achievable. For the majority of guiding attempts this band was chosen to start on-resonance. Occasionally this was shifted to tune from 10GHz to 22GHz and further, from 20GHz to 32GHz, red of resonance. However guide calibration was difficult to maintain at such detunings, so the bulk of work was performed over a regular 1 to 12GHz detuning. Tuning jumps of 200GHz were achieved with an intra-cavity birefringent filter and were occasionally applied to remove atom heating in transit.

Guide powers and detunings were chosen to sufficiently confine transverse momentum and reduce spontaneous emission of propagating atoms. As a general starting point an intensity of 54.57MW/m2 propagating in the a 6.6cm long hollow- core, with a 6GHz red-detuning, was estimated as the threshold where confining dipole potential overcame heating from spontaneous emission. This level was generally achieved with around 10mW beam input to the coupling lens.

Figure 7.1 illustrates a simplified experimental setup for a fibre-trap. The vacuum system and input beams are shown (the hyperfine beam would be split to enter with the cooling beams, or would enter in a slightly diverging beam in on any one of the arms. The guide beam was telescoped and coupled into the fibre from an

f=38.1mm lens mounted external to the vacuum system (on a Newport XYZ translator), this is not shown in Figure 7.1 as its mount would completely obscure the MOT chamber.

Figure 7.1 : The 2nd QWP trap with cooling beams superimposed. The MOT magnetic coils are shown wrapped around the MOT chamber. Note, a key omission here is the guide coupling lens, this would be positioned between the MOT chamber and the upper QWP mount, it was removed for this picture.

Throughout all fibre-guiding experiments guide light coupling into the fibre’s hollow-core was paramount, so guide-beam waist was located at fibre entrance, and any MOT beams and magnetic fields were arranged around this to ensure the atomic source was located in the guide path. Efficient coupling into 12μm-28μm diameter hollow cores with minimal power loss from scatter and coupling was difficult to arrange. It was extremely sensitive to fibre collinearity and numerical aperture matching. Even weak thermal lensing of optical components was sufficient to completely inhibit coupling.

Individual atom guiding attempts were performed in one of three ways, 1. with a continuous guide of fixed power and detuning, (incrementally increased for successive attempts), 2. with a ‘flashed’ guide of fixed power and detuning, 3. with a continuous fixed-power and slowly increasing detuning (typically from 1GHz to 12GHz red). Each power and detuning setting was applied ~15 times in order to average-out background noise.

Dipole Potential Along the Flux Path

Unlike free-space guide beams, the intensity, and hence dipole potential, of fibre-guide light, does not vary smoothly along the entire atom flux path. In a fibre guide an atom will experience a dipole-potential, with a steep gradient on approach to the fibre, a discontinuous drop (~30-50%) upon fibre coupling, and power attenuation within the fibre (see figure Figure 7.2). Such variation of confining potential forces consideration of which stage to optimise for efficient atom throughput. Experiments herein employed multiple guide intensities in attempts to optimise each stage, ultimately intensities capable of exceeding the required potential depth at all stages simultaneously were applied.

Figure 7.2 : Schematics depicting a capillary fibre (or Q-PCF) and a PCF. Beneath each an approximate depiction of the dipole-potential experienced at the coupling, core- propagating and emission stages. The curves here are not calculated nor experimental, they are intended to give a relative understanding of the power-loss profiles of each of the 3 separate regions worthy consideration when selecting input guide power, (However the coupling loss values are experimental values). Image courtesy of Dr.D. Rhodes.

It was expected a ~3mm diameter guide of 10mW power, 6GHz red-detuning would yield a potential depth of 0.127mK at a position 6mm above beam waist/fibre entrance. This would be able to capture atoms released from a MOT cloud with transverse velocities up to 19.3cms-1 (average atom velocity in a ~100μK MOT is ~13cms-1). On entry to a 12μm fibre hollow core, the guide suffers coupling loss (50%) but is now tightly focused, so its potential depth improves to 175mK in a beam of ~10.8μm full width (this is the mode diameter supported in a 12μm hollow-core). Once within the fibre, the guide was expected to be able to confine atoms with transverse velocities up to 7ms-1.

The earliest fibre guiding trap built in St Andrews used a quasi-PCF segment and hence suffered ~70% guide attenuation over its length. However later builds incorporated photonic crystal fibres in which minimal power loss occurred along their length, hence guiding potential remained approximately constant.