CONTROL DE LA RESISTENCIA A LOS ANTIHELMÍNTICOS

A typical fluorescence probe beam was held at an intensity a few times Isat (up to a max of ~50Isat) and had a flat, 5mm 1mm, light sheet profile (see Figure 3.21). It was composed of ~85% cooling light tuned to the 5S1/2(F= 3) 5P3/2 (F = 4) transition and ~15% hyperfine light tuned to the 5S1/2 (F= 2) 5P3/2 (F = 3) transition. A small portion of hyperfine light was sampled directly from the MOT hyperfine beam, while the cooling component was sampled from the MOT cooling source by a 70/30 beam splitter (70% continued to trap) and then passed through two AOMs, to achieve strong atomic resonance. Two AOMs were necessary due to the 12MHz offset of the cooling beam and the 80MHz baseline shift of the AOMs. The cooling

and hyperfine components were combined in a polarising beam splitter (PBS) cube and ultimately yielded ~1.5mW of probe light.

Figure 3.21 : Fluorescence probe setup for Gaussian guidance of a dropped cloud, (Guide shown in Blue here for contrast only; not to be confused with blue-detuning).

The probe beam could be directed to enter (via a flip mirror mount) along either of two perpendicular paths, both shown in Figure 3.22. Both on the horizontal plane, one ran normal to the large viewport windows entering just underneath the MOT beam opto-mechanics and the other ran through the small veiwports at 90 to the first. The latter was used when probing short distances beneath MOT centre (up to 2.5cm below) which were inaccessible in the other axis due to the MOT optics mounts. The probes could not be used simultaneously since atoms would be heated and disrupted by the first probe encountered.

When collecting fluorescence the probe beam was retro-reflected through a quarter wave plate (QWP) (replacing photo-detectors shown in Figure 3.22) to prevent a large radiation pressure imbalance kicking atoms from the probe beam. However a slight intensity imbalance remained due to the number of components in the probe path; effects of this are discussed in chapter 8.

The low-light-level detector used for fluorescence collection was an Hamamatsu, H7710-03 photomultiplier tube (PMT). This was mounted off-axis on a

Figure 3.22 : A photograph of the Drum trap with cooling MOT beams and both probe paths indicated. The Drum trap has been stripped of all components in order to simplify the image. The distance a probe beam could reach from the MOT was limited by the size of the viewport accessed, and the awkward placement of external optics (not shown). The photodiode used for absorption probe measurements is shown in wire-frame at the end of the absorption probe beam, this was replaced with a retroreflector and QWP when using this axis for fluorescence probe measurements.

skew angle from the probe (as shown in Figure 3.21). The probe region, where atoms were expected to fall into the probe, was imaged through an f=75mm (f=38.1mm in the case of the fibre-guiding trap) lens positioned as close as possible (~100mm distant) and focused just prior to the PMT such that the signal light filled the entire detection surface (to improve detector efficiency). The PMT, its mount and collecting lens are shown in Figure 3.23. External lab light and, in particular, stray guide-light reflections inside the vacuum system forced numerous measures to prevent swamping of the expected low intensity signal. A 1mm (adjustable) aperture was positioned at the focus to block unwanted light from inside the UHV system caught in the PMT s field of view. Extraneous lab light was suppressed by wrapping the optical path to the PMT with a tailored section of opaque laser-safety curtain and by mounting an interference notch filter (with 75% transmission at 780nm) immediately in front of the PMT sensor surface. Guide light reflected from the trap walls was at ~same wavelength as the probe-scatter and of far greater intensity, thus it was especially difficult to filter without suppressing the desired signal. Hence a chopper was installed in the probe beam and a phase sensitive detection system used to further suppress noise from the guide. The output signal was finally displayed and recorded on a digital oscilloscope (Tektronix, TDS 360) with hard-copy output ability.

Figure 3.23 : A photograph showing the detection system for fluorescence probes used for the majority of dropped-MOT and fibre-guiding experiments performed herein. From left to right ; the PMT, an interference filter (passing a narrow notch around 780nm), a shadowing tube and an f=38.1mm collecting lens. All these components and the fluorescence path were light-shielded when taking data.

3.6.2 Absorption Probes

The absorption detection system employed a custom built photodiode to monitor a beam of similar proportions to the fluorescence probe; 1mm 5mm. The probe intensity was reduced (through an ND wheel) to ~1/2 Isat and directed under the MOT through the smaller viewport windows (see Figure 3.22). The retro-reflector was replaced with a photodiode. Since this system collected a relatively high intensity level many noise concerns were removed; no collection optics were necessary since the probe was sufficiently collimated to propagate ~0.5m from trap centre to the photodiode. Thus opaque shielding around the probe beam path was the only significant noise suppressor since all other light sources did not propagate along the narrow path to the photodiode.

Unlike the fluorescence probe, the absorption probe diminishes in strength as atoms pass through it, thus the chief concern was to obtain optimum contrast between atoms and no-atoms signal levels. Optimisation required substantial patience as multiple tweaks to its intensity must be implemented over numerous cloud drops while also maintaining a dense MOT cloud and good guide-MOT-probe alignment, which is difficult to judge until the probe has good contrast; a cyclic path to achieve good signal strength and good contrast.

In order to align the absorption probe (when launched through the smaller viewports) it was tilted up to hit the MOT cloud and frequency locked on resonance such that atoms in the cloud would cast a shadow on a piece of card positioned just above the photodiode. Once the shadowing was confirmed the beam would be pivoted back down onto the horizontal plane up to 5cm beneath the cloud. The photodiode and final probe-launch mirror were vertically translated in-sync so as to maintain horizontality. Occasionally this alignment was purposefully broken so as to prevent vertically skew reflections interacting with atoms before they fall into the main probe beam; in these cases the shift from horizontal was no more than 2 , and the distance of MOT to beam (at point of closet approach) was used as the defining probe height.

3.6.3 CCD Detection

A simplistic detection method was used in early experiments on the 10-way trap whereby video streams from the MOT-observing cameras (Pulnix NIR PE2015) were collected and separated into single digital stills images (at 25fps). The video stream was initially recorded on S-VHS magnetic-tape cassettes and converted to digital stills on a Studio DC10+ computer capture card. Subsequent analysis was performed by National Instruments Vision Builder software. This process is simplistic, with only basic imaging optics (described above) required to image MOT fluorescence onto the CCD chip. The only significant complexity came from the need to synchronise camera output with experiment initiation and with the first recorded frame of the cassette recorder. With the intention of arranging the first output frame to coincide with guide input, a re-sync command was sent to the cameras at the same time as a command was sent to the guide shutter to cause it to open. This primer signal had the undesirable effect of causing a loss of sync between the camera and S- VHS recorder, thus losing an indeterminate number of precious frames (each 40ms apart) during an effect which lasted <100ms. However, a simple solution was presented by pre-initiating the recorders and cameras whereby the re-sync command was issued sufficiently in advance to have the recorder catch up with the CCD frame rate. During the separation of the video stream into its constituent 25fps images, knowledge of the time window allowed accurate matching of captured images to known experiment timings.

3.7 Conclusion

The laser systems used for all atom guiding experiments in St Andrews have been described. Their construction, frequency stabilisation, tuning, and applicability towards guiding and probing have been discussed. The following table summarises the activities performed by the author with the equipment detailed in this chapter.

Equipment Activity

Extended Cavity Diode Laser, (ECDL)

Atom cooling, hyperfine re-pumping, seeding (a slave laser) and fluorescence/absorption probing. These have been built from scratch many times, stabilised on a narrow-linewidth output and rendered frequency tuneable. Cavity tweaking and re-stabilising continued on a daily basis to compensate for thermal variation and gradual laser deterioration.

Titanium Sapphire Laser

Atom guidance in a Gaussian beam profile and generation of alternate spatial-profiles (eg. Bessel, Laguerre-Gaussian). This laser was re-aligned with its Millenia pump source, arranged to 780.24nm and calibrated to tune smoothly over a 12GHz range. Continual monitoring and correction of mode-jumping was necessary. Realignment, re-calibration and coolant maintenance, were regular duties.

Neodymium doped Yttrium Orthvanadate Laser, (Nd:VO4)

Far-off resonance atom guiding in high-quality Gaussian beam profiles. No frequency tuning nor maintenance were required in the short time this was available. Beam profiling and frequency characterisation performed daily.

Electronic Control Circuits

Design, assembly and maintenance of circuits performed. These were necessary for AOM frequency-shift control, Newport-shutter control and speaker-shutter control.

Results of guiding experiments reported in the following chapters will refer to the laser systems detailed herein. Each vacuum trap will have further details regarding the arrangement of these lasers discussed in context with each experiment.

3.8 Bibliography

1

M. W. Fleming and A. Mooradian, Spectral characteristics of external-cavity controlled semiconductor lasers", IEEE Journal of Quantum Electronics, QE-17 44, (1981).

2

M. A. Clifford, J. Arlt, J. Courtial and K. Dholakia, High-order Laguerre-Gaussian laser modes for studies of cold atoms , Opt. Commun. #156, p300 (1998).

3

G. P. T Lancaster, Experimental Studies of Diode Lasers and Cold Atom Guiding, PhD Thesis, University of St Andrews (2001).

4

Chapter 4

Realisation of Magneto-Optic Traps for Atom

Guiding

Introduction

In this chapter a description of the construction and maintenance of atom traps for use in free-space and fibre guiding experiments is given. This is intended as an introduction to vacuum traps, additional information specific to each batch of experiments is included in later chapters (in context with the experiments).

A diverse range of vacuum systems have been built in St Andrews for the purpose of guiding atoms along optical potentials. Each successive trap was designed to address specific problems of its predecessor, most often this was an issue of optical access, or facilitating greater guiding distance or offering physical mounts for internal components. Traps built for hollow-fibre guiding differed greatly from those built for free-space guiding, namely in size and component density within their vacuum. The larger-volume free-space traps used the same vacuum pumps and ultra-high vacuum (UHV) connections, but did not require internal optics mounting and thus were a great deal simpler to construct.

Throughout this thesis the term free-space guiding is in reference to atom guiding in light beams which propagate without external influence, similarly the term

fibre-guiding pertains to atom guiding where the optical potential is maintained by an hollow optical fibre. All traps built throughout the course of this Ph.D. were designed and constructed by the author.