Stimulated emission by two-level atoms pumped to the upper level

Erik J. Bochove, Peter W. Milonni

The properties of a phase conjugate mirror (PCM) interacting radiatively with other objects are investigated. The spontaneous emission rate of a two-level atom (TLA) is attributed to the dipole emission of the upper energy level into vacuum field modes incident on the front face of the PCM, the emission into vacuum modes transmitted from the rear port of the PCM, enhanced by a factor 1 + R (R = reflectivity), and the emission of a photon while the atom is in the lower level, accompanied by annihilation of two pump photons, weighted by a factor of R per emitted photon. These processes lead to an enhancement factor of 1 + R for the spontaneous decay rate and a steady state upper level occupation probability R/(2R + 2), both in agreement with solution of the Heisenberg equations of TLA operators.1 The momentum transfer is zero, and the steady state emission rate is A12R(R + 2)/(2R + 2) photons/s atom. The stimulated emission and absorption constants are enhanced in a similar way. Contrary to the case of a conventional mirror, the Lippman fringes are present only for coherent radiation states.

Constantine Mavroyannis

Nackchin Sung, Jin J. Kim

Recently we reported the observation of stimulated emission in atomic copper vapor that is excited by a resonant tunable laser beam.1 One important result obtained in the experiment is that the two upper levels involved in the laser transitions of the atomic copper vapor are collisionally mixed in stimulated emission. This observation is particularly important for studies of high-pressure transverse-discharge copper vapor lasers. We conducted an optical pumping experiment to measure the collisional mixing cross sections (CMCSs) below the threshold of stimulated emission. We observed all four transitions: 2P1/2−2S1/2 (327.4 nm); 2P3/2−2S1/2 (324.8 nm); 2P1/2−2D3/2 (578.2 nm); and 2P3/2−2D5/2 (510.6 nm), when either one of the two levels 2P3/2,1/2 was excited. From the measurements of the intensities of the resonantly excited and sensitized fluorescences, we estimate the CMCSs Q34(2P1/2→2P3/2) and Q43(2P1/2←2P3/2) at 2 × 10−16 and 1.3 × 10-16 cm2 for Cu + Ne collisions, respectively. We also measured the linewidths of the optically pumped green and yellow lasers as a function of the buffer gas pressure and of detuning of the pump frequency. We observe that the linewidths change in an interesting way, which can be explained in terms of the gain narrowing.

Sergey G. Rautian, T. A. Germogenova

T. A. B. Kennedy, Daniel F. Walls

The quantum noise properties of atomic and semiconductor lasers are compared. The intensity fluctuation spectrum of the output field is calculated for the case of regularly, i.e., sub-Poissonian, pumped lasers. The effects of spontaneous emission on the output amplitude fluctuations may be suppressed in regularly pumped atomic lasers by requiring the lifetime of the lower lasing level to be short in comparison with that of the upper laser level. The output amplitude fluctuations then tend to zero at the laser frequency. In regularly pumped semiconductor lasers, net stimulated emission of photons dominates over spontaneous emission because of the combination of pumping and rapid intraband thermalization due to Coulomb scattering.

Constantine Mavroyannis