Coherent Spontaneous Emission Research Articles

Coherent Spontaneous Emission

Coherent spontaneous emission from a system of $N$ two-level atoms interacting with a quantized radiation field is treated for the case in which Dicke's cooperation number $r$ is macroscopically large. It is shown how to modify the quasiclassical approach to this problem to incorporate quantum effects that are lost in the self-consistent-field approximation. The statistics of the emitted radiation are found to vary markedly with the initial state of the system of atoms. The photon statistics tend to that typical of blackbody radiation when the initial state of the atomic system is that which would result from incoherent pumping ($m\ensuremath{\sim}r$). When, on the other hand, the atoms are initially in a superradiant state ($m\ensuremath{\ll}r$), the emitted radiation may be represented approximately by a coherent state.

Quantum-Mechanical Amplification and Frequency Conversion with a Trilinear Hamiltonian

A description of the parametric amplifier and frequency converter is presented without introducing the classical (i.e., parametric) approximation for the pumping field. Constants of the motion are found which reduce the solution of the Schr\odinger equation to the diagonalization of a matrix. This diagonalization is accomplished numerically, and the eigenvalues and eigen-functions of a system with fixed energy are calculated. The time-dependent behavior of the mean number of photons in the amplified or frequency up-converted field is presented. The time evolution of the probability distributions is illustrated. The technique is extended to the problem of coherent spontaneous emission from a system of $N$ two-level atoms interacting with the radiation field where both the atomic system and the radiation field are quantized.

Superradiant Effects on Pulse Propagation in Resonant Media

A theoretical model for the propagation of coherent light pulses in resonant media where a large number of atoms cooperate in spontaneous emission (superradiance) is discussed. The medium is supposedly free of inhomogeneous broadening, and no lossy relaxation mechanisms are taken into account. The conditions for the emission of all spontaneous radiation in the direction of pulse propagation are described. A semi-classical model is used and, at each point in the medium, the propagation field and the spontaneous damping field are considered separately. The equation governing the evolution of a superradiant system in the presence of an applied pulse is derived. A mechanism of energy transfer from the traveling pulse to the medium is described which includes superradiant spontaneous emission effects. As a result of the neglect of all lossy relaxation processes, all energy stored in the medium is eventually returned to the pulse by coherent spontaneous emission. Therefore, in this theory, all pulses travel through the medium without energy loss. Numerical results for square input pulses are discussed which show two possible types of propagation behavior: (a) Low input intensities produce a negative tail to the input (positive) pulse and sharpening of the tail with the advance in the medium; (b) higher input intensities produce, after sufficient penetration, a steady-state pulse envelope somewhat similar to the $2\ensuremath{\pi}$ pulses of self-induced transparency. The steady-state solutions of the equations of motion are found numerically and the pulse shapes are computed for different degrees of superradiance. In the limit of strong superradiance, very asymmetric pulse envelopes of large area are found. For negligible superradiance, approximately $2\ensuremath{\pi}$ hyperbolic-secant pulses are obtained. The effects of inhomogeneous broadening are briefly considered, and the consequences of important superradiant damping in photon-echo experiments are qualitatively discussed.

Quantum-mechanical description of coherent spontaneous emission

In this paper we consider the general problem of coherent spontaneous emission of a system of N two-level atoms coupled to a single-mode e.m. field (1). The system shall be considered enclosed in a travelling-wave cavity in order to avoid the complications due to the spatial degree of freedom of the e.m. field. Following DICKE (1), the initial state is assumed to have definite population difference 2d, and co-operation number J ; no photon is initially present. The first assumption on the initial state implies that the atoms radiate coherently and the second that the system starts evolving via purely spontaneous emission. The first quantum-mechanical s tatement of this problem has been lucidly made by DICK]~ (1) several years ago, and the solution was found in the framework of first-order t ime-dependent perturbation theory. In the following our aim is to remove such limitation, giving a full quantum solution, at least in the case where the co-operation number J is much bigger than 1. The interaction Hamiltonian is

Comments on beam maser type spectrometers using coherently radiating states

It is pointed out that the technique used by Dicke and Romer of pulse excitation of a volume of gas to a coherently radiating state can be applied to high-resolution spectroscopy using molecular beams. A two-cavity maser-like system without a state separator is proposed, where the molecular beam is polarized by a pump field in the first cavity and the coherent spontaneous emission radiated in a second pick-up cavity detected by a superheterodyne receiver.

Coherent Spontaneous Microwave Emission by Pulsed Resonance Excitation

This paper describes an investigation of the coherent microwave emission from pulse-excited ammonia molecules. Coherent and periodic pulses of near resonance frequency and 1-µsec duration excited the gas from its initial thermal equilibrium condition. Self-induced coherent emission (molecular ringing) continued after the excitation field was removed. This radiation was observed during a period of 10 µsec. In an actual experiment performed, a new Doppler bandwidth reduction method was used in the gas cell. The observed spectral width of the ammonia 7,7 line was about 5 kc. The emission was used to stabilize the excitation signal source to a short-term frequency stability of 2 X 10/sup -10/.