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Birkhäuser Boston eBooks

The Impossible Observed

David Wick

Published Jan 1, 1995• 0 citations
Publisher

Abstract

In 1979, in Heidelberg, Germany, Dr. Werner Neuhauser looked up from JL the eyepiece of a low-power microscope and fancied he heard the ghost of Mach whispering: “Now I believe in atoms.” Neuhauser had just glimpsed what appeared to be a bright blue star floating in the void; it was a single Barium ion, caught in an electromagnetic trap and fluorescing in a laser’s beam. Thus transpired the first observation of an isolated atom using a lens; soon one would be glimpsed with the naked eye as well. [140]

DOI
10.1007/978-1-4612-5361-7_15

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In 1979, in Heidelberg, Germany, Dr. Werner Neuhauser looked up from JL the eyepiece of a low-power microscope and fancied he heard the ghost of Mach whispering: “Now I believe in atoms.” Neuhauser had just glimpsed what appeared to be a bright blue star floating in the void; it was a single Barium ion, caught in an electromagnetic trap and fluorescing in a laser’s beam. Thus transpired the first observation of an isolated atom using a lens; soon one would be glimpsed with the naked eye as well. [140]
​Richard Feynman, in his famous Lectures on Physics, toyed with the idea of all scientific knowledge being wiped out and facing the choice of passing one single idea down to future generations (Feynman et al. 1989). He believed it should be the hypothesis that all things are made of atoms. The idea that matter is built up from discrete fundamental units can be traced back to several ancient cultures (the word atom deriving from the Greek word atomos ‘cannot be divided’). It is only within the last century or so, however, that theories for atomic structure were developed to explain a series of phenomena observed throughout the 18th–19th Centuries. Many of these phenomena were optical, such as Melvill’s observation in 1752 that a mixture of sea salt and alcohol would burn with a yellow flame (see Figure 1) or Wollaston’s observation in 1802 that sunlight contains black lines in its spectrum (Cajori 1962). Further refinement and characterisation from people like Fraunhofer, Kirchoff, Bun-sen, Balmer, and Zeeman followed, but it was not until 1913, with the introduction of the Rutherford-Bohr atomic model, that a cornerstone was laid for what would become a quantum mechanical description of the atom.
In this thesis we study: the dynamics leading to a near-deterministic preparation of single atoms in a microscopic optical dipole trap; the non destructive imaging and counting of trapped atoms; and a system for sorting single atoms between optical dipole traps using a high numerical aperture lens and a spatial light modulator. \n \nWe near-deterministically prepare trapped single Rubidium atoms by exposing them to a tailored light field which induces light assisted collisions. By preparing pairs of Rubidium atoms in an optical micro-trap we can study the outcome of light assisted collisions at the single event level. Using this knowledge we tailor the light such that each collision releases only enough energy for one atom of a colliding atom-pair to escape the trap. This thereby yields an 83% single atom loading efficiency. \n \n \nWe demonstrate a method to count small numbers of atoms held in a deep, microscopic optical dipole trap by collecting fluorescence from atoms exposed to a standing wave of light that is blue-detuned from resonance. While scattering photons, the atoms are cooled by a Sisyphus mechanism that results from the spatial variation in light intensity. The use of a small blue detuning limits the losses due to light assisted collisions, thereby making the method suitable for counting several atoms in a microscopic volume. \n \nWe create two overlapping one-dimensional optical lattices using a single laser beam, a spatial light modulator and a high numerical aperture lens. These lattices have the potential to trap single atoms, and using the dynamic capabilities of the spatial light modulator may shift and sort atoms to a minimum atom-atom separation of 1.52 µm. We show how a simple feedback circuit can compensate for the spatial light modulator's intensity modulation.
Trapped cold ions are tools which we used to approach two very disparate areas of physics, strong coupling between Ba+ ions and optical resonators, and investigations of a low-energy nuclear isomer of 229-Th. The first part of this thesis describes our progress towards the integration of a miniature Paul (rf) ion trap with a high finesse (F=30000) optical cavity. Ba+ ions were trapped and cooled for long periods and a new scheme for isotope selective photoionization was developed. The second part of this thesis describes our progress towards controlled excitation of the low energy nuclear isomer of 229-Th, which may provide a bridge between the techniques of cold atomic and nuclear physics. As a step towards this goal, 232-Th3+ ions were confined in rf ions traps and cooled via collisions with a buffer gas of helium. A sophisticated scanning program was developed for controlling ion trap loading, tuning lasers, and running a CCD camera to look for fluorescence. The low-lying electronic transitions of Th3+ at 984 nm, 690 nm and 1087 nm were observed via laser fluorescence.
Using a visible probe laser exciting the 2p3P2→3d3D3 transition (λ=588 nm), we have demonstrated a visible observation of metastable He atoms magnetooptically trapped by an infrared laser resonant with the 2s3S1→2p3P2 transition (λ=1083nm). Detecting visible resonance fluorescence, only 104 atoms confined in a trap with a diameter of 1 mm have been sensitively observed even with the naked eye. It has been found that under a given condition the visible laser operates as a trapping laser and considerably extends the capture range of infrared laser detuning.