Research On Non-degenerate Four-wave Mixing In Rubidium Vapor And 3-D Laser Cooling At The Doppler Limit

The theory about the interaction between light and matter has been widely applied in the field of nonlinear optics and laser cooling. Double-A non-degenerate four-wave mixing in hot rubidium vapor has been used to generate squeezed state and entanglement. Doppler cooling, which is based on the interaction between light and two-level atom is a fundamental method to cooling atoms. This thesis consists of two main parts, two-mode phase-sensitive amplifier and ultralow-light-level all-optical transistor based on four-wave mixing in hot rubidium vapor cell, and three-dimensional laser cooling at the Doppler limit using the special properties of helium, such as its small mass and narrow transition linewidth. The main researches are as follows:1. By seeding two weak probe and conjugate beams, one strong pump beam into the hot rubidium vapor cell, a phase-insensitive amplifier becomes two-mode phase-sensitive one. The input-output relation predicts several interesting and new quantum properties, such as the maximal degree of intensity difference squeezing can be enhanced by nearly 3 dB compared to a phase-insensitive amplifier with the same gain, the possibility to observe intensity sum squeezing, and quadrature entanglement which can be manipulated by the phase of the input beams. The amplification and de-amplification have been observed, also investigated how the visibility changes with a lot of parameters in experiment. The experimental results agree well with the theory predictions. We will study this type of amplifier in Chapter 2.2. The conical emission (CE), which is observed in four-wave mixing in hot rubidium vapor, is used to construct an ultralow-light-level all-optical transistor (AOT). A switching efficiency of 50% at the output probe port with 0.9 mW is achieved with a probe beam of as low as 180 pW. It means that we can manipulate a light beam with power of 5.0×106more, which proves the cascade of the AOT. This system here is different with the other all-optical devices. The amplification far exceeds those demonstrated by other configurations. The CE occurs in a single pass without feedback. It will be investigated in Chapter 3.3. In chapter 4, we will focus on the apparatus used to cooling the metastable helium atoms and the experiment of three-dimensional laser cooling at the Doppler limit. The special properties of helium, such as its small mass and narrow transition linewidth, prevent sub-Doppler cooling. The Magneto-optical Trap and optical molasses generated by the transition 23S1â†'23P2 is well explained by the Doppler theory. The experimental results such as the temperature of MOT and red detuned optical molasses, the size of MOT cloud, the drift velocity in optical molasses with unbalanced power between counter-propagating laser beams, agree very well with the prediction of Doppler theory. It confirms the model of Doppler cooling. In particular, the minimum temperature obtained at a laser detuning of δ=-Γ/2 is 1.3 TD(TD is the Doppler limit), very close to the Doppler limit.