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

Decay of the spatial intensity modulation of luminescence in crystals excited by ultrashort pulses

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

Published May 10, 1996• 0 citations
Publisher

Abstract

Spatial intensity modulation of luminescence in crystals excited by ultrashort pulses decays with respect to distance due to outrunning of the wave packets corresponding to the ordinary and extraordinary waves. The dispersion spreading of the packets introduces distortions into this picture. It is shown that these distortions become significant at pulse duration less than 0.1 ps.

DOI
10.1117/12.239807

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Spatial intensity modulation of luminescence in crystals excited by ultrashort pulses decays with respect to distance due to outrunning of the wave packets corresponding to the ordinary and extraordinary waves. The dispersion spreading of the packets introduces distortions into this picture. It is shown that these distortions become significant at pulse duration less than 0.1 ps.
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.
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.
Here, I discuss the propagation of an ultrashort pulse through a collection of harmonic multilevel systems. In the limit of weak excitation and a large number of excited states, I show that the amplitude of the input driving pulse decays exponentially with propagation distance. The absorption coefficient associated with this decay is determined by the characteristic time of the manifold of excited states, instead of the polarization decay time as in the conventional absorption coefficient of a two-level atom. The input ultrashort pulse creates in the excited states a wave packet, which oscillates emitting secondary pulses in the process. Analytic solutions are obtained that describe the propagation of individual wave-packet re-emission pulses, and it is shown that their phase depends on the detuning of the input pulse.
Laser propagating through resonance media taking into account of spatial effect was discussed by simultaneously solving the Maxwell-Bloch equations with definite differential method. Results indicate that the propagation of a 2π pulse form would no longer be a soliton, its profile would expand, and the degree of distortion depends on the interaction intensity of laser and material: stronger interaction causes more distortion till the profile split. The interaction intensity was decided by such elements as the laser frequency, laser intensity, the dipole of material, detuning and the concentration of resonance particles．
We have studied the dispersion of ultrafast pulses in a photonic crystal waveguide as a function of optical frequency, in both experiment and theory. With phase-sensitive and time-resolved near-field microscopy, the light was probed inside the waveguide in a non-invasive manner. The effect of dispersion on the shape of the pulses was determined. As the optical frequency decreased, the group velocity decreased. Simultaneously, the measured pulses were broadened during propagation, due to an increase in group velocity dispersion. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. The asymmetry increased as the group velocity decreased. The asymmetry of the pulses is caused by a strong increase of higher order dispersion. As the group velocity was reduced to 0.116(9) .c, we found group velocity dispersion of -1.1(3) .10(6) ps(2)/km and third order dispersion of up to 1.1(4) .10(5) ps(3)/km. We have modelled our interferometric measurements and included the full dispersion of the photonic crystal waveguide. Our mathematical model and the experimental findings showed a good correspondence. Our findings show that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion.