|
Nonlinear Optics and Laser Physics:
Intense
Field Interactions
Multilevel
radiation interactions:
In nonlinear optics the interaction of optical waves with matter is
commonly described by Schrodinger equation of quantum mechanics as long as
the initial state of the system is known.
In general, apart from simple two or three level, the wave equation
of motion cannot be solved exactly for physical systems of interest. For this reason the perturbation
expansion technique was developed and from this the dressed atom approach
for dealing with such simple interactions in the limit of a single strong
field. However the problem remains
unresolved even for these level systems when all fields are strong or for
more complex systems such as resonant or near-resonant excitation of
multi-level systems by a single strong laser field.
This theoretical work addresses this problem through an alternative
approach, which extends conventional perturbation theory to accommodate
arbitrary field strengths by introducing the concept of self-regulated
partitioning of the Hamiltonian. The
treatment gives a clear physical insight into field-induced splitting of
the states, called here partially dressed states, and also accounts for the
transition of energy between the states.
In general these states are characterised by the strength and form
of the field-matter interaction and determined by comparison with density
-matrix analysis. Using these states
as base states, this theory allows for a perturbation-expansion analysis
for arbitrary interaction strength.
This treatment is being developed to provide an analytic basis for
describing multilevel radiation interactions, a simple example being its
application to third harmonic generation in atoms.
Lasing
without inversion:
Lasing without population inversion is at first site a paradox and yet
in recent years a variety of schemes have been proposed and today the field
is a most active area of quantum optics.
Popular three level schemes have been analysed in which this
phenomenon arises from either phase-dependent quantum interference or
electro-magnetically induced interference in lifetime-broadened
systems. While for both systems a
coherent pump is required to drive the systems, additional pumps, coherent
or incoherent, are also required either for coherent coupling of the levels
or for redistribution of the thermal population, respectively.
In this work we research through theory, lasing without inversion in a
V system driven by a single pump field, the simplest system considered to
date in terms of both the level scheme and its interaction with the optical
field. Here the role of the single
optical field is both to drive the system and, more significantly, to
modify the states with which it interacts.
The latter through state splitting, forms a coherent coupled pair of
states which can result in a trapped state, similar to that formed by a
degenerate or near- degenerate pair of real ground-state levels in the more
familiar schemes. This is the
underlying physical mechanism responsible for lasing without inversion in
this scheme. Results of our model
description so far show lasing without inversion to occur for a broad range
of laser intensities dependant on the relaxation rates and photon
degeneracy parameters of the system.
Back to Research Reviews'
Page
|
