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In , he joined the research group of Prof Pavlos Lagoudakis , and then in moved to Cambridge to the group of Prof Jeremy Baumberg. In , he was appointed to a lectureship in the University of St Andrews. Rich, our fourth PhD student, was a Southampton graduate, and worked on the coherent manipulation of cold atoms using the phase-programmable laser and our magneto-optical trap. He was awarded his PhD in for his thesis on 'Coherent two-photon excitation within an extended cloud of Rubidium 85 for the purposes of atomic interferometry and cooling '.

Sunil was our first PhD student, joining us in , and set up most of the major experimental apparatus. Born and bred "uup noorth" in the Yorkshire Dales, where the puddings are battered and even the M1 is cobbled, he studied Physics at Imperial College , London and spent a happy year as an Erasmus exchange student in Trento , Italy. He was awarded his PhD in for his thesis 'A chirped, pulsed laser system and magneto-optical trap for rubidium' , and is now a computer guru in Cambridge.

Elle, a graduate of the Open University , joined us in to work on rotation sensing by atomic matterwave interferometry.

Atomic & Optical Physics - 8.2.10 - The magneto-optical trap - the optical Earnshaw theorem

She now works with the university's Doctoral College , and in her spare time is a talented singer. Andrew joined us in to work on the development of laser-based water vapour detection in collaboration with Qrometric Ltd. Oliver spent the academic year with us, working on composite pulse techniques for the year-long project of his Physics with a Year of Experimental Research degree.

Claire spent the academic year with us, working on the fabrication of flexible photonic crystals for the year-long project of her Physics with a Year of Experimental Research degree. Martin spent the academic year with us, working on enabling technologies for integrated atom chips for the year-long project of his Physics with a Year of Experimental Research degree. Toby spent the academic year with us, carrying out his Physics with a Year of Experimental Research research project on 'Tuning an external cavity diode laser with a spatial light modulator'.

Ultracold Publications

Matt was the group's third PhD student, having spent his undergraduate years in Southampton. His specialization in the Quantum Control group was the stabilization and locking of semiconductor diode lasers, and the spectroscopy and manipulation of atomic rubidium.

Matt was awarded his PhD in for his thesis 'Coherent manipulation of ultracold rubidium'. Following a year as a post-doc in Axel Kuhn's group in Oxford from , Matt was awarded a 5 year research fellowship from the Royal Academy of Engineering, and returned to Southampton in to establish his own group developing enabling technologies for integrated atom chips. He was appointed to a lectureship in , and in took up a position at Dstl. Matt continues to collaborate on our quantum control experiments, and we look forward to using his atom chips for our miniature matterwave gyroscopes. James joined the group in as our second PhD student, following his degree at Imperial College , London.

His PhD research addressed the coherent manipulation of ultracold rubidium, the development of a phase-programmable laser source and many aspects of experimental control, together with theoretical studies of the fidelity of the coherent manipulation scheme, all described in his thesis 'Novel schemes for the optical manipulation of atoms and molecules'.

Ultracold Publications

Following his PhD, James remained in the group and obtained a PhD Plus award to work on the theory of mirror-mediated cooling, and he co-instigated our project on optically driven photonic crystal actuators, for which he was granted an Innovation Award from the Southampton Metamaterials Programme in After a period in the group of Prof Malgosia Kaczmarek , James was a research assistant with Dr Hendrik Ulbricht , before taking up a lectureship at the University of Swansea in He continues to collaborate in particular on our work on optically-driven actuators. Jonathan joined us, from Exeter via Manchester, in to work on a new generation of frequency-stabilized lasers for our atomic physics experiments.

He was awarded his PhD in for his thesis on 'A mode-locked diode laser frequency comb for ultracold atomic physics experiments.

By detuning a laser beam to a frequency less than the resonant frequency also known as red detuning , laser light is only absorbed if the light is frequency up-shifted by the Doppler effect , which occurs whenever the atom is moving towards the laser source. This applies a friction force to the atom whenever it moves towards a laser source.

Laser Cooling and Trapping of Neutral Atoms

For cooling to occur along all directions, the atom must see this friction force along all three Cartesian axes; this is most easily achieved by illuminating the atom with 3 orthogonal laser beams, which are then reflected back along the same direction. Magnetic trapping is created by adding a spatially varying magnetic quadrupole field to the red detuned optical field needed for laser cooling.


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This causes a Zeeman shift in the magnetic-sensitive m f levels, which increases with the radial distance from the centre of the trap. Because of this, as an atom moves away from the centre of the trap, the atomic resonance is shifted closer to the frequency of the laser light, and the atom becomes more likely to get a photon kick towards the centre of the trap. The direction of the kick is given by the polarization of the light, which is either left or right handed circular, giving different interactions with the different m f levels.

The correct polarisations are used so that photons moving towards the centre of the trap will be on resonance with the correct shifted atomic energy level, always driving the atom towards the centre. Because of this, if an atom is to be laser cooled, it must possess a specific energy level structure known as a closed optical loop, where following an excitation-spontaneous emission event, the atom is always returned to its original state.

Many atoms that do not contain closed optical loops can still be laser cooled, however, by using repump lasers which re-excite the population back into the optical loop after it has decayed to a state outside of the cooling cycle.

Magneto-optical trap

If it falls back to the dark state, the atom stops cycling between ground and excited state, and the cooling and trapping of this atom stops. All magneto-optical traps require at least one trapping laser plus any necessary repumper lasers see above. These lasers need stability, rather than high power, requiring no more than the saturation intensity, but a linewidth much less than the Doppler width, usually several megahertz.

Because of their low cost, compact size and ease of use, laser diodes are used for many of the standard MOT species while the linewidth and stability of these lasers is controlled using servo systems, which stabilises the lasers to an atomic frequency reference by using, for example, saturated absorption spectroscopy and the Pound-Drever-Hall technique to generate a locking signal.

Equilibrium trapping of cold atoms using dipole and radiative forces in an optical trap

By employing a 2-dimensional diffraction grating it is possible to generate the configuration of laser beams required for a magneto-optical trap from a single laser beam and thus have a very compact magneto-optical trap. The MOT cloud is loaded from a background of thermal vapour, or from an atomic beam, usually slowed down to the capture velocity using a Zeeman slower. However, the trapping potential in a magneto-optical trap is small in comparison to thermal energies of atoms and most collisions between trapped atoms and the background gas supply enough energy to the trapped atom to kick it out of the trap.

If the background pressure is too high, atoms are kicked out of the trap faster than they can be loaded, and the trap does not form.

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The minimum temperature and maximum density of a cloud in a magneto-optical trap is limited by the spontaneously emitted photon in cooling each cycle. While the asymmetry in atom excitation gives cooling and trapping forces, the emission of the spontaneously emitted photon is in a random direction, and therefore contributes to a heating of the atom.

The density is also limited by the spontaneously emitted photon.