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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Space-temporal transient phenomena under coherent excitation of quantum systems in anisotropic crystals

Evgueni F. Martynovich, Evgueni F. Martynovich, S. I. Polityko

Published Oct 21, 1998• 0 citations
Publisher

Abstract

Non-stationary mechanisms of spatial modulation of luminescence intensity in anisotropic quantum systems within birefringent crystals under their interaction with ultrashort polarized laser pulses were studied. We have found connection between this phenomenon and phase relaxation of excited quantum systems. We have as well grounded a new method for investigation of coherency of quantum systems based on study of characteristic properties of modulation phenomena. It was shown that the influence of the frequency dispersion on the clarity of the modulation picture in insignificant at durations of 100 fs and greater, even for crystals with strong dispersion. Due to the existence of chirp in femtosecond pulses exciting the luminescence, modulation period in instant distribution of absorbed power decreases over distance. In the time-integrated picture of spatial distribution of luminescence intensity, the rate of decrease of modulation depth grows. This leads to new possibilities in experimental investigation of the value of chirp of femtosecond pulses.

DOI
10.1117/12.328225

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Journal of the Optical Society of America B•Jun 1, 1989•3 citations
Non-stationary mechanisms of spatial modulation of luminescence intensity in anisotropic quantum systems within birefringent crystals under their interaction with ultrashort polarized laser pulses were studied. We have found connection between this phenomenon and phase relaxation of excited quantum systems. We have as well grounded a new method for investigation of coherency of quantum systems based on study of characteristic properties of modulation phenomena. It was shown that the influence of the frequency dispersion on the clarity of the modulation picture in insignificant at durations of 100 fs and greater, even for crystals with strong dispersion. Due to the existence of chirp in femtosecond pulses exciting the luminescence, modulation period in instant distribution of absorbed power decreases over distance. In the time-integrated picture of spatial distribution of luminescence intensity, the rate of decrease of modulation depth grows. This leads to new possibilities in experimental investigation of the value of chirp of femtosecond pulses.
The spatial dynamics of variations in the intensity of the luminescence of quantum systems in crystals of the medium and highest categories of symmetry upon excitation with opposing optical pulses of an arbitrary shape is investigated by numerical simulation methods. It is established that the material dispersion, which is responsible for the dispersion spreading of pulses, has no effect on the shape and width of the envelope of the spatial distribution of the luminescence intensity, as well as on the depth of the luminescence intensity modulation associated with the interference of opposing pulses.
Changes in the modulus and phase of a slowly varying envelope of an ultrashort pulse are calculated for the case of propagation across a saturable absorber. Vibronic relaxation of the optical transition in the absorber is allowed for. A study is made of the dependences of the duration of a pulse transmitted by a nonlinear absorber on the vibrational relaxation time of dye molecules and on the detuning of the carrier frequency of a pulse relative to the maximum of the equilibrium absorption spectrum. It is shown that a chirp appears even in the absence of such detuning provided the energy of lf quanta of optically active vibrations is comparable with the thermal excitation energy. This chirp originates from partial completion of the vibrational relaxation process during a pulse, so that absorption and refraction occur in a system which is not in a vibrational equilibrium.
A spatially periodic distribution of the luminescence intensity of F2 and F 3 + color centers is observed in LiF crystals with induced anisotropy. The period of spatially periodic modulation of the luminescence intensity generally coincides with the period of the change in the polarization state of excitation light. For F2 centers, the experimental axial spatially periodic dependence of the luminescence intensity is close to the calculated dependence obtained in the description of the absorption and radiative transitions in the centers by linear oscillators oriented along six C 2 axes of the cubic crystal. However, for F 3 + centers, the same experiment shows a spatially periodic distribution of luminescence intensity with a modulation period shorter by a factor of 2. It is shown that this effect of modulation frequency doubling in the spatially periodic pattern of the luminescence of color centers arises due to the nonlinear dependence of the luminescence intensity on the excitation light intensity. For F 3 + centers, upon luminescence excitation in the absorption band with λmax = 452 nm, the modulation frequency doubling is due to the saturation of the metastable triplet state of such a center.
The time dependence of the frequency shift (δω) due to colliding-pulse phase modulation (CPPM) is computed, for collision of femtosecond optical pulses, both in nonresonant and in saturable media. In particular, thin material slabs (up to several pulse widths τ) are considered, and although the results are more generally applicable, special attention is given to a typical saturable-absorber jet in a colliding-pulse mode-locked laser. Results show that CPPM in nonresonant media causes noticeable effects on the chirp, both for thin slabs with L ≈ laser wavelength, where the coherence of the pulses leads to interference effects, and for thicker slabs with L ≈ cτ. Complications to CPPM by saturable media are discussed, and it is shown that the consequences of dispersion by the ground-state population, and finite phase-relaxation time, may be important for short, low-energy pulses. The results show, for parameters typical of contemporary colliding-pulse mode-locked lasers, what conditions are favorable for short, stable pulse formation in solitonlike pulse shaping. It is also shown that there are conditions for which the pulses produced would be spread by any sign of group-velocity dispersion.