Excitation of High Optical Energy Levels

Anne Débarre

The experimental study of a laser-induced energy transfer for a dipole-dipole collisional interaction in a Na-Ca mixture is reported. A specific excitation scheme is used in this experiment for which the virtual level is optically excited. A comparison with the case of a dipole-dipole interaction in the Eu-Sr mixture where the virtual level is excited by collisional energy transfer is presented.

Jia-Ming Liu

POPULATION RATE EQUATIONS From the discussion in the preceding chapter, it is clear that population inversion is the basic condition for an optical gain. For any system in its normal state in thermal equilibrium, a low-energy level is always more populated than a high-energy level, hence there is no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy level to a high-energy level. If left alone, the atoms in a system relax to thermal equilibrium. Therefore, population inversion is a nonequilibrium state that cannot be sustained without active pumping. To keep a constant optical gain, continuous pumping is required to maintain population inversion. This condition is clearly consistent with the law of conservation of energy: amplification of an optical wave leads to an increase in optical energy, which is possible only if the required energy is supplied by a source. Pumping is the process that supplies energy to the gain medium for the amplification of an optical wave. There are many different pumping techniques, including optical excitation, electric current injection, electric discharge, chemical reaction, and excitation with particle beams. The use of a specific pumping technique depends on the properties of the gain medium being pumped. The lasers and optical amplifiers of particular interest in photonic systems are made of either dielectric solid-state media doped with active ions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP. For a dielectric gain medium, the most commonly used pumping technique is optical pumping using either an incoherent light source, such as a flashlamp or a light-emitting diode, or a coherent light source from another laser. A semiconductor gain medium can also be optically pumped, but it is usually pumped by electric current injection. In this section, we consider the general conditions for pumping to achieve population inversion. Detailed pumping mechanisms and physical setups are not addressed here because they depend on the specific gain medium used in a particular application. The net rate of increase of the population density in a given energy level is described by a rate equation. As we shall see below, pumping for population inversion in any practical gain medium always requires the participation of more than two energy levels.

David A. Cardimona

Tadashi Kawazoe, Shunsuke Tanaka, Motoichi Ohtsu

We demonstrated a high-quality single-photon emitter based on excitation energy transfer between two different-sized CuCl quantum dots (QDs). It is operated under triple blockade mechanisms. The mechanisms are tuning the incident light to the smaller QD and using an electric dipole forbidden excitation energy level in the larger QD, using the optical near-field interaction to transfer energy from the smaller QD to the electric dipole forbidden level of the larger QD, and using a single exciton emission level in the larger QD. These mechanisms are supported by the large binding energy of the exciton molecule. A 99.3% plausibility of single-photon emission was confirmed with 99.98% accuracy based on a photon correlation experiment with 80-MHz repetition frequency using an optical fiber probe.