Sep 18, 2013 - Kvantovaya Elektron. (Moscow) 6, 1971-1976 (September 1979). A theoretical investigation is made of the coherent interaction between ...
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Coherent interaction between high-power light pulses and exciton–impurity centers in molecular crystals
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1979 Sov. J. Quantum Electron. 9 1160 (http://iopscience.iop.org/0049-1748/9/9/A17) View the table of contents for this issue, or go to the journal homepage for more
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/, MW/cn·/
FIG. 2. Dependences I(t) calculated using Eq. (6) for turning on (curve 1) and off (curve 2) of a pump source of power W = 714W/cm3; the other parameters are the same as in Fig. 1. SO 120 №0 ί,/cec
lay time τ0 of stimulated emission after the turning on of the pump source: τ
' I
τ
2[m+l
°~
c
A ~ ayNco)
\-i
(24)
•
The threshold pump power W can be obtained from Eq. (24) by allowing r0 to approach infinity: (25) When the threshold is exceeded significantly, so that C^>A(/rt-|-l)/(0yJVco)»
\"bJ
the delay time of stimulated emission becomes equal to the time rm of arrival of an excitation wave at the point m: xm=(m+l)/(2C).
(27)
Figure 2 shows, by way of example, the results of
calculations of the stimulated emission intensity I(t) carried out using Eq. (6) for the case of turning on and off of a pump source and ignoring gas heating. Our formulas can be used to describe selective stimulated emission also from other diatomic molecules such as HC1, HF, etc., and they can easily be generalized to the case of selective stimulated emission due to overtones of diatomic molecules. 'A. P. Napartovich, I. V. Novobrantsev, and A. N. Starostin, Kvantovaya Elektron. (Moscow) 4, 2125 (1977) [Sov. J. Quantum Electron. 7, 1216 (1977)]. 2 Yu. B. Konev, I. V. Kochetov, and V. G. Pevgov, Abstracts of Papers presented at Second Ail-Union Seminar on Physical Processes in Gas Lasers, Uzhgorod, 1978 (in Russian), p. 58. 3 C. A. Brau, Physics (Utrecht) 58, 533 (1972). 4 B. F. Gordiets, Sh. S. Mamedov, and L. A. Shelenin, Preprint No. 28 (in Russian), Lebedev Physics Institute, Academy of Sciences of the USSR (1974). 5 B. F. Gordiets and Sh. S. Mamedov, Zh. Prikl. Mekh. Tekh. Fiz. No. 3, 13 (1974). e S . A. Zhdanok, A. P. Napartovich, and A. N. Starostin, Abstracts of Papers presented at Second All-Union Seminar on Physical Processes in Gas Lasers, Uzhgorod, 1978 (in Russian), p. 46. 7 S. A. Zhdanok, A. P. Napartovich, and A. N. Starostin, Zh. Eksp. Teor. Fiz. 76, 130 (1979) [Sov. Phys. JETP 49, 66 (1979)]. Translated by A. Tybulewicz
Coherent interaction between high-power light pulses and exciton-impurity centers in molecular crystals L. Valkunas, £. Galzhauskas, I. A. Poluoktov, and Yu. M. Popov P. N. Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow and Institute of Physics, Academy of Sciences of the Lithuanian SSR, Vilnius (Submitted January 23, 1979) Kvantovaya Elektron. (Moscow) 6, 1971-1976 (September 1979) A theoretical investigation is made of the coherent interaction between high-power optical radiation and excitons in impurity-doped molecular crystals. A self-consistent system of equations is obtained for the medium and the optical field. It is shown that, under certain conditions, the distances over which coherence effects are observed, decrease sharply. An analysis is made of the specific conditions for the experimental observation of these effects. PACS numbers: 7L35. + z, 61.70.Rj, 42.60.He
1. Coherent propagation of high-power ultrashort light pulses under conditions of resonance interaction with exciton transitions in ideal crystals has been the subject of many recent investigations.1"4 Introducing impurities into a crystal gives rise to new absorption bands and also alters the oscillator strengths of the optical transitions and distorts the natural exciton spectrum. Thus, if the difference between the energy levels of the impurity and crystal molecules is comparable with the exciton band width, resonance interaction between the impurity and the crystal molecules has a substantial in1160
Sov. J. Quantum Electron. 9(9), Sept. 1979
fluence on the optical properties of the impurity center. In particular, an anomalous increase in the absorption coefficient at the frequency of the impurity level may be found in the crystal as this level approaches the bottom of the exciton band.5 In addition, a similar effect may 8 be found in the absorption of light by local excitons. The present paper reports an analysis of the coherent propagation of an ultrashort laser pulse ( τ ο « Γ 2 , where T0 is the pulse duration and T2 is the phase relaxation time of the medium) under conditions of resonance in-
0049-1748/79/091160-04$02.40
© 1980 American Institute of Physics
1160
teraction (one-photon and two-photon) with an excitonimpurity center in a molecular crystal. 2. We shall assume that the concentration of impurities is low and their interaction can be neglected. The crystal can then be considered to be a collection of impurity centers, each comprising an impurity molecule surrounded by molecules of the crystal "solvent." We shall express the Hamiltonian of an impurity center in teracting with the electromagnetic wave field E{z, t) in the second quantization representation: 6 Η-
Σ
fl,
n'?'
[(e o +D)8 B ,,
(1) where n is the cell coordinate; q is the number of a molecule in the cell; .BJ, and £ m 4 are the creation and annihilation operators of the excitation of a molecule nq; ε 0 is the free molecule excitation energy; D is the energy difference between the interactions of excited and unexcited molecules with all the other crystal molecules; iW«e,»'
