We present a quantitative study of radiation amplification without population inversion based on models recently proposed by Harris [Phys. Rev. Lett. 62, 1033 (1989)]. We discuss both two-level and three-level arrangements in terms of the density matrix. Our calculations, which are fully time dependent, incorporate an incoherent pumping mechanism that is essential for amplification. We discuss in detail the range of and conditions on atomic parameters for which amplification is possible. Its feasibility is demonstrated through the direct calculation of the change of the intensity of a probe beam interacting with the atom.
We consider how externally coupled (e.g., dc-field-coupled) autoionizing states can be useful in producing gain without population inversion. The dc fields produce new absorption minima that enable one to obtain laser action in different frequency regions depending on the strength of the coupling. We calculate absorption and emission profiles assuming that one of the autoionizing states is incoherently pumped as in most conventional laser models. We present numerical results that demonstrate gain regions without population inversion.
We consider laser action on a transition involving an autoionizing state \ensuremath{\Vert}a〉 and an initially unpopulated state \ensuremath{\Vert}f〉, the population of which builds up because of spontaneous emission and Raman processes. The autoionizing state is pumped by a coherent field. We discuss the transient amplification of the probe field on the lasing transition for a range of parameters. The coherent field on the pumping transition leads to minima in the absorption spectra if the atomic population were in the state \ensuremath{\Vert}f〉. Such regions of minima can be used for obtaining gain even for long times.
Light-induced absorption with population inversion and amplification without population inversion in a coherently prepared closed three level V-type system are investigated. This study is performed from the point of view of a two color dressed-state basis. Both of these processes are possible due to atomic coherence and quantum interference contrary to simple intuitive predictions. Merely on a physical basis, one would expect a complementary process to the amplification without inversion. We believe that absorption in the presence of population inversion found in the dressed-state picture utilized in this study, constitutes such a process. We derive approximate analytic time-dependent solutions, for coherences and populations that are compared with full numerical solutions exhibiting good agreement. Steady-state quantities are also calculated, and the conditions under which the system exhibits absorption and gain with and without inversion, in the dressed-state representation are derived. It is found that for weak input probe-laser field absorption with inversion and amplification without inversion may occur, for a range of system parameters. These take place at resonance and the generalized Rabi ac Stark-shifted frequencies, in agreement with exprimental investigations, and at beat frequencies, depending on the relevant parameters.
We have shown that (1+1\ensuremath{'})-photon lasing without population inversion (LWOPI) can be achieved in a
H2 molecule for emission from the lowest doubly excited autoionizing (AI) state of $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}$ symmetry to the ground X
1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}$ (v=0,j=1) state in the presence of the resonant B
1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}$ (v=4,j=2) level. We have considered both coherent and incoherent pumping of the AI state, which is the upper lasing level in this scheme. Coherent pumping of the AI state can be done by two-photon excitation from the ground to the AI state. Incoherent pumping of the AI state can be done by charge-transfer collision of a
H2+ ion with atomic and molecular species. For a weak coherent pumping field of intensity 6.4\ifmmode\times\else\texttimes\fi{}
107 W/
cm2, a lower probe field [for the X
1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}$ (v=0,j=1)\ensuremath{\rightleftarrows}B
1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}$ (v=4,j=2) transition] of intensity 9.6\ifmmode\times\else\texttimes\fi{}
106 W/
cm2 is required to obtain an appreciable gain. However, for stronger coherent fields of intensity 6.4\ifmmode\times\else\texttimes\fi{}
109 W/
cm2, the lower probe intensity can be lowered by orders of magnitude without significant loss in gain. In this case, using a lower probe field of intensity 9.6\ifmmode\times\else\texttimes\fi{}
104 W/
cm2, we have calculated the gain by the perturbative method. However, for a lower value of coherent field strength, 6.4\ifmmode\times\else\texttimes\fi{}
107 W/
cm2, and a higher intensity of the lower probe field, 9.6\ifmmode\times\else\texttimes\fi{}
106 W/
cm2, we have done nonperturbative calculations and have shown that LWOPI can be achieved only in a short range of probe pulse duration less than or equal to 3.45 ps. In all these cases, the optimum intensity for the upper probe field [for the B
1${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}$ (v=4,j=2)${\ensuremath{\rightleftarrows}}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}$ transition] is 9.6\ifmmode\times\else\texttimes\fi{}
107 W/
cm2. We have also compared the evolution of gain for the
sech2(t/T)-shaped pulse with that for the square pulse, the pulse duration T=3.45 ps being the same in both cases.
This thesis is concerned with the amplification of coherent radiation from an active medium under conditions which require no population inversion between the energy levels. Gain without population inversion may be obtained by creating a sufficiently strong correlation between pairs of atomic states, usually with the help of external sources of resonant electromagnetic radiation. Our main objective is to investigate two possible selections for the active medium: a three level atom in a lambda configuration and a Raman-driven four level model. For both we identify the conditions that lead to inversionless laser action, we calculate the small signal gain and investigate the physical mechanisms which are responsible for the emergence of gain. While the ultimate cause of the gain is quantum interference, we propose attractive alternative pictures based on the dressed states of the coupled atom-driving field systems. We address the important practical issue of the effect of atomic motion and related Doppler broadening on the inversionless gain profile and we also analyze the transient response of each laser model with the help of the appropriate Maxwell-Bloch equations. Finally we develop a fully quantum mechanical description of the lambda model and find that the radiated field can display unique quantum features which manifest themselves with sub-Poissonian photon fluctuations.