Four-wave mixing in rubidium vapour with structured light and an external cavity

  • Rachel Frances Offer

Student thesis: Doctoral Thesis

Abstract

Thermal atomic vapours are an experimentally simple and efficient system in which to study wave mixing processes. We investigate a resonantly enhanced four-wave mixing (FWM) process in rubidium vapour, which coherently converts 780nm and 776nm light to 5.2 µm and 420 nm. Firstly, we use this system to explore the coherent frequency conversion of structured light, in particular Laguerre-Gauss (LG) beams. These modes, and more generally the orbital angular momentum (OAM) that they carry, are important research tools for optical manipulation, imaging and communication. Previous qualitative studies have demonstrated OAM transfer from the near-infrared pump beams to the generated 420nm light. We investigate this further by making the first quantitative measurements of the 420nm transverse mode for a range of values of pump OAM. Our results indicate that the FWM process is likely to be an efficient source of OAM-entangled 5.2 µm and 420nm light, with a spiral bandwidth that increases with increasing pump OAM. Using independently shaped pump beams, we also study FWM for more general pump modes, including beams carrying opposite handedness of OAM, coherent superpositions of LG modes, and for the first time in this system, radial LGmodes. This work shows the importance of OAM conservation and Gouy phase matching in the FWM process, and is relevant for similar schemes involving the inscription and storage of transverse modes in atomic vapours. Finally, we report the first use of a ring cavity to both increase the output power and narrow the linewidth of the generated 420nm light. For Gaussian pump beams, the low-finesse cavity, which is singly resonant with the 420nmlight, increases the maximum 420nm output power from 340 µW (single pass) to 940 µW (cavity-enhanced), and narrows the linewidth from 33MHz (single pass) to < 1MHz (cavity-enhanced), resulting in a narrow linewidth, tunable light source suitable for near resonant rubidium studies.
Date of Award1 Oct 2018
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorAidan Arnold (Supervisor) & Erling Riis (Supervisor)

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