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arXiv:physics/0007069 · Journal of Modern Optics

Ultracold metastable calcium ensembles, a medium for matter wave amplification?

Jan Grünert, G. Quehl, Vladimir Elman, Andreas Hemmerich

Published Nov 1, 2000• 7 citations
arXivHTMLPDF

Abstract

We propose an experimental implementation of matter wave amplification by optical pumping with metastable calcium atoms. First experimental results indicate that pumping rates can be significantly higher than in previous experimental schemes so that it appears promising that the threshold condition for the generation of degeneracy can be reached.

DOI
10.1080/09500340008232193

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We propose an experimental implementation of matter wave amplification by optical pumping with metastable calcium atoms. First experimental results indicate that pumping rates can be significantly higher than in previous experimental schemes so that it appears promising that the threshold condition for the generation of degeneracy can be reached.
We discuss the prospects for a class of laser-like sources of atoms where the stimulated accumulation of atoms in the fundamental mode of a matter wave resonator is achieved by optical pumping. Precooled atoms in an internal state a which is not affected by the resonator trapping potential are excited into a state e by means of laser light. Once the atom is in state e it decays into the electronic ground state g by means of spontaneous emission, thereby occupying one of the resonator modes v equals 0, 1, ... Neglecting reabsorption of the spontaneously emitted photon, the probability for a transition into mode v is proportional to 1 + nv (`gain'), where nv is the number of atoms in mode v prior to the transition. Including linear losses this model shows threshold behavior in the fundamental mode v equals 0, mode competition and Poissonian atom statistics above threshold. We also analyze the `small-signal gain' including reabsorption of the spontaneously emitted photons. We demonstrate that the threshold for matter wave amplification can only be reached in small 3D resonators of linear size comparable to the wavelength of the spontaneous photon, or in resonators with effectively reduced dimensionality.
An experiment is described in which large numbers ( approximately 10 16 ) of calcium atoms are placed in the 4s4p 3 P metastable level by optical pumping with a tunable dye laser and then studied spectroscopically. Equations governing the degree of excitation by the laser are derived. Predictions of the theory are shown to be in good agreement with experimental results. In particular, at low optical depths produced by 10 15 ground state atoms cm -2 the dye laser raises approximately 50% of the calcium atoms into the metastable state. At higher optical depths associated with 10 17 ground state atoms cm -2 the number of excited states is limited only by the laser energy and line width. For the dye laser used the largest number of metastables produced was approximately 10 16 . The absorption spectra of the 4s4p 2 P level is seen out to 4s4p 3 P to 4s22d 3 D. The relaxation rate between the excited state fine structure is studied directly with an optically delayed dye laser continuum. Other applications of this technique of producing large numbers of excited states are briefly discussed.
I describe the construction of a calcium matter-wave interferometer. The interferometer is based on a Ramsey-Borde scheme, and uses a thermal beam of atoms excited by an optical-frequency transition in calcium. In our experiment four pi/2 pulses of light are delivered to the atoms, which split and recombine the wave functions of the atoms. Our experimental design minimizes first-order Doppler shifts, and allows for the cancellation of systematic errors including phase shifts due to rotation and acceleration. I describe the individual components of the interferometer and its assembly. The requirements for the electronics used in the experiment as well as their design and performance are described in great detail. I also give an overview of the techniques used to passively stabilize the laser and optical components. Finally, I report on the current status of the experiment as well as detail future work to be done on the apparatus.
Phase-coherent matter-wave amplification was demonstrated using Bose- Einstein-condensed rubidium-87 atoms. A small seed matter wave was created with coherent optical Bragg diffraction. Amplification of this seed matter wave was achieved by using the initial condensate as a gain medium through the superradiance effect. The coherence properties of the amplified matter wave, studied with a matter-wave interferometer, were shown to be locked to those of the initial seed wave. The active matter-wave device demonstrated here has great potential in the fields of atom optics, atom lithography, and precision measurements.