Slow and fast light by spectral hole-burning in the solid state

Abstract

The generation of slow (subliminal) and fast (superluminal) light has attracted a great deal of attention over recent years because of potential applications in optical data storage, optical switching, quantum data processing and pulse regeneration. Slow and fast light can be achieved in materials that have very large normal or anomalous dispersion. In this thesis, methods that can be used to generate subliminal and superluminal light, especially the spectral hole-burning technique are reviewed and applied in the experiments. The experimental studies conducted are based on transient and persistent spectral hole-burning and the solid state materials emerald (Be₃Al₂Si₆O₁₈:Cr³⁺), ruby (Al₂O₃:Cr³⁺) and NaMgAl(oxalate)₃.9H₂O:Cr(III) containing chromium(III) ions were used to burn spectral holes in to the so called R1-line within the temperature range of 1.5 to 8 K.

Spectral holes in the absorption spectrum of a crystal can generate strongly frequency-dependent dispersion that in turn results in slow and fast light. Conventional spectroscopy was applied to characterize the systems and then laser spectroscopy was applied to conduct hole-burning in the R1-line. Steeper normal or anomalous dispersion results in larger delay or advancement of the light pulse propagating through the crystal. Therefore, the homogeneous width of the sample was controlled by external magnetic fields. A magnetic field reduces the spectral hole width compared to zero field by reducing the electronic spin-flip rates and hence fluctuations of the local fields. Magnetic fields up to 4.5 T were applied in these studies. The linear filter theory was applied to rationalise the observed slow/fast light phenomena based on spectral holes. However, experiments in strong magnetic fields (4-5 T) revealed that in the coherent limit the linear filter theory fails because of coherent transients such as Rabi oscillations, free-induction decay and self-induced transparency (SIT).

Switching between slow and fast light was obtained by transient hole-burning, which was reported for the first time. These results could pave the way to applications such as all optical switches. In addition, light could be delayed by action of light in low magnetic fields, which in turn can be used as a spectroscopic method to determine spectral hole widths as a function of time, i.e. spectral diffusion. Moreover, re-investigation of the phenomenon of self-induced transparency (SIT) lead to the observation of the longest pulse delay so far by SIT.