Optical switching and inversionless amplification controlled by state-dependent alignment of molecules

Abstract

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.PACS numbers: 42.70.Hj Amplification of light is determined by the difference between the net absorbtion and stimulated emission of radiation. Usually, this requires larger population of the upper quantum state then the lower one that are coupled by the induced transitions. However, the amount of absorbed and emitted photons depend not only on the populations of the resonant energy levels but also on the probabilities of the induced transitions and on distribution of the populations over the energy-degenerated states. Therefore, the inversion of population is, generally speaking, only a special case, in which the amount of the emitted light prevails over the absorbed one. Based on this fact, a variety of realizations of amplification without population inversion (AWI) was proposed in the early years of quantum electronics. Such possibility based on a possible difference in velocity distribution in upper and lower states was discussed in [1]. Nonreciprocity of probabilities of induced emission and absorbtion caused by nonlinear interference affect in the field of the auxiliary radiation was investigated for two-level atoms in [2] and for three-level systems in [3]. Corresponding AWI at transitions of neon was predicted and analyzed in details in [4] and proved in the experiments [5]. The asymmetry in the lineshape of net emission and absorption at the transitions to the autoionizing states and related possibility of AWI was considered in [6]. A possible AWI in dichroic molecules was pointed out in [7]. The feasibility of AWI of short pulses in three-level system was considered in [8]. A review of later publications on AWI is given in [9]. An asymmetry in absorption and emission lineshape of two-level system caused by its interaction with a thermostat is discussed in recent publication [10]. In this paper we propose and discuss a method of achieving lasing without population inversion by the anisotropic molecules. It bases on selective alignment and consequent decoupling the lower-state molecules from the polarized resonant radiation, while the upper-state molecules remain amplifying this radiation through its stimulated emission.In electrodipole approximation, the probability of induced transitions between levels m and g, W mg , depends on the factor d mg cos θ, which is the projection of the electrodipole transition matrix element on a direction of a vector of a resonant oscill...

Amplification of Light by Molecules with Selectively Oriented States in the Pulsed Electric Field

“…The possibility of inversionless amplification of light in a system of dichroic molecules selectively oriented over states in an external field was considered in [2,3]. These ideas were subsequently developed by the authors in [4,5] in which on the basis of the coupled system of kinetic Boltzmann equations for the ground and excited states the effect of the relation between τ ef f 2 and τ 0 on gain α is numerically analyzed.…”

On the possibility of inversion-free amplification of light by dichroic molecules in an external orienting field

“…The inversion free amplification may result, for example, from the difference in the velocity distributions of the excited and nonexcited atoms [2], the difference in the probabilities of transitions with absorption and emission due to nonlinear interference effects [3][4][5], or it may occur on the transitions between discrete levels and with the participation of continuum [3,6]. The possibility of inversion free amplification of light in a system of dichroic molecules selectively oriented over states in an external field was considered in [7,8]. It was assumed that the lifetime of the excited state is much longer than time τ 0 of stabilization of orientational equilibrium (see also [9]).…”

“…In [7,8], axially symmetric molecules were consid ered, in which the direction of constant dipole moment μ coincided with the direction of the symme try axis of the molecule, which formed angle ϑ 0 with electric field E 0 . The latter enabled us to considerably simplify the problem of evaluating the amplification factor (2) under the assumption that time τ 0 of stabili zation of orientation equilibrium is much shorter than the lifetime of the excited state.…”

“…The above assumptions obviously limit consider ably the domain of applicability of relations derived in [7,8], which necessitates the development of a model for calculating the values of the distribution functions appearing in relation (2). In this connection, we con sider a model based on the system of Boltzmann kinetic equations, which makes it possible to solve the problem formulated above [11].…”

“…This system with the boundary conditions (8) and the normalization condition (9) combined with relation (2), makes it possible to solve the problem of calculation of the amplification factor formulated above. For this purpose, we consider the following two cases, in which the effect of inversion free amplification can be manifested most strongly [7,8].…”