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Telecommunications and Radio Engineering

OPTICAL RETUNING OF SPECTRAL CHARACTERISRICS OF QUANTUM- CASCADE STRUCTURES USING FOR SPECTROSCOPIC DETECTION OF EXPLOSIVES

S. I. Petrov, M.V. Klimenko, Шулика Алексей Владимирович

Published Jan 1, 2012• 0 citations
Publisher

Abstract

In this paper we come up with a new approach to retuning the spectral characteristics of quantum-cascade structure amplification. The process of retuning proceeds using the continuous-wave optical pumping and it is strongly dependent upon the phase space filling effect. As suggested by the results from the mathematical simulation of the quantum-cascade structure optical properties, this particular method allows the spectral amplification characteristics to be retuned within the limits of 1.1 meV with no substantial change in the plan gain magnitude. The results thus obtained is of certain interest for spectroscopic detection of explosive.

DOI
10.1615/telecomradeng.v71.i14.80

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In this paper we come up with a new approach to retuning the spectral characteristics of quantum-cascade structure amplification. The process of retuning proceeds using the continuous-wave optical pumping and it is strongly dependent upon the phase space filling effect. As suggested by the results from the mathematical simulation of the quantum-cascade structure optical properties, this particular method allows the spectral amplification characteristics to be retuned within the limits of 1.1 meV with no substantial change in the plan gain magnitude. The results thus obtained is of certain interest for spectroscopic detection of explosive.
It is shown that when an optically allowed transition is coherently excited in an impurity atom in a photonic crystal under conditions where one of the normal relaxation channels is suppressed by the spectral characteristics of the photonic crystal, new relaxation mechanisms are activated involving a coherent field quantum. These mechanisms substantially alter the dynamics of the atomic system, leading to filling of levels of the impurity atom which do not belong to the coherently excited atomic transition. Under certain conditions this leads to population inversion as a result of an optically allowed transition which does not interact with the coherent pump and at a frequency where no photonic band gaps can exist.
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.
The amplification before detection of the carrier of a modulated optical signal by a narrow-band quantum amplifier enhances the signal-to-noise ratio, particularly when the signal wave-front is distorted. A further improvement is obtained by using a combination of wide-band and narrow-band quantum amplifiers. The practical application of these schemes requires a degenerate regenerative ring-type amplifier capable of amplifying arbitrary transverse field configurations. Experiments show that such an amplifier with a gain of 24 dB and a bandwidth of 1 MHz is feasible. The incident beam axis can be displaced by as much as ten times the beam-waist radius without losing more than 4 dB in gain. Frequency modulation may be converted into amplitude modulation by the phase shift introduced in the carrier.
The Xe(L) system at ? ~ 2.9 ? uniformly exhibits all of the canonical attributes of a strongly saturated amplifier on the full ensemble of single-vacancy Xeq+ transition arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The key observables are (1) sharp spectral narrowing, (2) the detection of a narrow directed beam (??x200 ?rad), (3) an increase in the amplitude of the emission and the development of an intense output (?106 enhancement) and (4) the observation of deep spectral hole-burning on the inhomogeneously broadened spontaneous emission profile. Experimentally determined by two methods, (a) line narrowing and (b) signal enhancement, the observations for several single-vacancy 3d?2p transitions indicate a range of values for the effective small signal (linear) gain constant given by go25?100 cm?1. Quantitative analysis shows that this result stands in clear conflict with the corresponding upper bound go40?80 cm?1 that is based on available spectroscopic data and estimated with conventional theory. Overall, the observed values deviate substantially from expectations scaled to the spectral density of the measured Xe(L) spontaneous emission profile; they are systematically too high. The most extreme example is the heavily saturated Xe32+ transition at ? = 2.71 ?, a case that fails to reconcile the lower bound of the measured signal strength with the corresponding theoretically predicted maximal value; the former falls above the latter by a factor exceeding 400 giving an enormous gap. Moreover, although saturation is a prominent characteristic of the amplification at ?2.71 ?, as demonstrated by spectral hole-burning, the theoretical upper bound of go given for this transition is far too small for saturation to be reached. The Xe31+ transition at ?2.93 ? exhibits comparably pronounced anomalous behaviour. This double paradox is resolved with the Ansatz that the amplification is governed principally by the saturated gain gs, not the conventionally described small signal value go. This interpretation is further supported by the observation of deep spectral hole-burning, the signature of strong saturation, that occurs uniformly across the spectrum of the spontaneous emission profile. The effective amplification exhibits an anomalously weak dependence on the spectral density; saturation is the rule, not the exception. A lucid manifestation of the saturation is the recording of spectrally resolved x-ray yields on the Xe31+ array that are sufficiently high to produce gross structural damage to the material in the film plane of the spectrograph. The behaviour of the amplifier can be best described as an explosive supersaturated amplification. The source of this exceptionally strong amplification can be traced to the dynamically enhanced radiative response of the excited Xe hollow atom states located in the clusters that are mode coupled to the plasma waveguide forming the amplifying channel.
The theoretical formulation for pump-probe spectroscopic analysis of free induction decay has been made using semiclassical time dependent perturbation technique in GaAs/AlxGa1-xAs quantum well structure. The pump pulse is considered to be tuned below the band edge and the excitation intensity is assumed to be in the moderate power regime so that the only many-body effect that dominates is the excitonic effect. The probe transmission spectrum features the characteristic resonance frequencies of the system concerned. The temporal nature of the transmitted probe signal is also found to characterize the ultrafast free induction decay of the pump induced polarization, which is recorded by the low intensity probe transmission in the system.