Summary form only given. Intensive theoretical work, carried out in the past decade, has indicated that optical properties of matter can be dramatically affected by atomic coherence and interference effects. In particular, atomic coherence and interference was shown to lead to such intriguing phenomena as electromagnetically induced transparency, lasing without population inversion (LWI), and resonant enhancement of refractive index accompanied by vanishing absorption. In the present paper we report the first experimental observation of LWI oscillation and the first demonstration of index enhancement.
Summary form only given. Intensive theoretical work, carried out in the past decade, has indicated that optical properties of matter can be dramatically affected by atomic coherence and interference effects. In particular, atomic coherence and interference was shown to lead to such intriguing phenomena as electromagnetically induced transparency, lasing without population inversion (LWI), and resonant enhancement of refractive index accompanied by vanishing absorption. In the present paper we report the first experimental observation of LWI oscillation and the first demonstration of index enhancement.
Quantum coherence and correlations in atomic and radiation physics have led to many interesting and unexpected consequences. For example, an atomic ensemble prepared in a coherent superposition of states yields the Hanle effect, quantum beats, photon echo, self-induced transparency, and coherent Raman beats. In fact, in Section 1.4, we saw that the quantum beat effect provides one of the most compelling reasons for quantizing the radiation field. A further interesting consequence of preparing an atomic system in a coherent superposition of states is that, under certain conditions, it is possible for atomic coherence to cancel absorption. Such atomic states are called trapping states †. The observation of nonabsorbing resonances via atomic coherence and interference impacts on the concepts of lasing without inversion (LWI),‡ enhancement of the index of refraction accompanied by vanishing absorption, and electromagnetically induced transparency. In lasing without inversion, the essential idea is the absorption cancellation by atomic coherence and interference. This phenomenon is also the essence of electromagnetically induced transparency. Usually this is accomplished in three-level atomic systems in which there are two coherent routes for absorption that can destructively interfere, thus leading to the cancellation of absorption. A small population in the excited state can thus lead to net gain. A related phenomenon is that of resonantly enhanced refractive index without absorption in an ensemble of phase-coherent atoms ( phaseonium ). In a phaseonium gas with no population in the excited level, the absorption cancellation always coincides with vanishing refractivity.
The study of an ensemble of phase–coherent atoms has recently lead to interesting theoretical innovations and experimental demonstrations of counter–intuitive effects such as electromagnetically induced transparency (EIT), lasing without inversion (LWI), enhancement of index of refraction and ultra–large nonlinear susceptibility. In the present notes, we report a couple of new effects along these lines. In fact, it would be fair to call them surprises even in the repertoire of ‘counter–intuitive effects’, i.e. ‘counter–counter–intuitive effects’. Specifically, we will show that the LWI concept, which is based on quantum coherence, has a surprising counterpart in the classical physics of free electron laser operation. In other current work, we find that it is possible to ‘lock’ atoms in an excited state via atomic coherence. It is, by now, not surprising that such a phase–coherent ensemble can show holes or dark lines in the emission spectrum. It is surprising that we can lock atoms in an excited (normally decaying) state via atomic coherence and interference. Such a phase–coherent collection of atoms, i.e. ‘phaseonium’, is indeed a novel new state of matter.
The most commonly used lasers operate with population inversion. This requires an incoherent pumping power (from lamps, electrical discharges, etc.) that rapidly scales with the frequency of the generated laser radiation. In practice, this is the main obstacle for the operation in the continuous-wave regime of short-wavelength lasers in the UV, X-rays, etc. But lasers can also operate without population inversion using atomic coherences excited in the laser medium by coherent pumping with another laser. We present in this paper a short review of the field of lasing without inversion (LWI). In particular, we summarize the basic physics of LWI in the framework of both semiclassical theory and quantumjump formalism to describe light-matter interaction. We also review the experimental state-of-the-art in LWI. Most proof-of-principle demonstrations of LWI have used atomic vapours, so far. But recently gain without inversion (a pre-requisite for lasing) has been observed also in a technologically relevant medium: semiconductor nanostructures that can be tailored to have “artificial atom” electron states.
Coherent interactions of laser fields with resonant atomic and molecular transitions significantly modify optical characteristics of the medium via quantum interference between transition amplitudes. This interference can result in reduction or enhancement of the laser absorption, decrease of the group velocity of the laser pulse etc. The origin of all these processes is electromagnetically induced transparency. This effect is intensively investigated in the connection with its multiple applications: lasing without inversion, group velocity control, quantum optical storage and quantum computing, effective nonlinear interactions of low field intensities, high-precision measurements, spectroscopy and metrology, plasma diagnostics, atomic clock etc.
The control of the optical properties of a medium by atomic coherence and quantum interference effects is an important area of research, not only for its fundamental aspects in the understanding of radiation and matter interaction, but also for its potential applications in the design of optical devices. Atomic coherence can emerge in a medium through the coupling of a strong coherent field to an atomic transition. This coupling affects the optical properties of the medium in a very interesting ways. For example, it may yield light amplification without population inversion. Also, it can modify the traditional dispersion of an optical medium in such a way that maximum variations of the refractive index occur around a spectral region of vanishing absorption. In the first part of this work we investigate some fundamental aspects of this coherent dynamics. We investigate how this non-inversion gain is affected by the presence of atomic motion. The coherent interaction between atom and radiation can lead to the interference between optical paths of decay. We report on an atomic setting where the interference structures depend upon the phase of the coherent field, an new effect which is absent in other resonance fluorescent phenomena. As a potential application, we investigate a novel approach for the modulation of optical signal. This approach consists of injecting atomic coherence in the medium in order to tailor its optical properties so it is a more suitable medium for modulation.
Summary form only given. Atomic coherence and interference of quantum pathways provides the means to manipulate the spectral features of absorption-gain and refractive indexes as well as the nonlinear-optical response of a medium. These open opportunities for lasing without inversion, forming laser-induced transparency, for manipulating by phase matching, and for increase in output of frequency-mixing generation. Theory and first experimental evidence of laser-induced continuum structures (LICS) have been reported. In this paper we develop the theory of LICS on the case of two strong fields coupled to bound-free transitions.