Hidden spatially periodic oscillations of energy absorption by orientation groups of anisotropic quantum systems in birefringent crystals were calculated. Conditions favoring experimental detection of modulation phenomena were determined, and their features have been studied under nonstationary excitation by ultrashort light 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.
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 comparative analysis of two approaches to the description of the light modulation pulse in a saturable absorber is presented. According to the simplest model, the delay of the optical pulse is a result of the distortion of its shape due to the self-modulation of absorption in the nonlinear medium. The second model of the effect, arisen at the beginning of our century, relates the pulse delay to the so-called “slow light,” attributing this delay to the group velocity reduction under the condition of coherent oscillations of the population. It is shown that the data of all known experiments on the light pulse delay in saturable absorbers can be comprehensively described in terms of the simplest model of saturable absorber and do not require invoking the effect of population coherent oscillations with spectral hole-burning and anomalous modifications of the light group velocity. It is concluded that the effect of the group velocity reduction under the condition of population coherent oscillations has not received so far any experimental confirmation, and the assertions about real observation of the slow light based on this mechanism are unfounded.
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.
Recent experimental and theoretical results on the optical properties of globular photonic crystals coauthored by the author are presented. The dispersion relation for electromagnetic waves in a 1D photonic crystal that simulates the properties of a selected direction in the globular photonic crystal is calculated. The spectral ranges that are characterized by the anomalous slowing of electromagnetic waves in the photonic crystal and correspond to the stop-band edges are determined. A method for the measurement of the transmission and reflection spectra of the broadband radiation in photonic crystals is proposed. The method enables one to find the characteristics of the stop bands. The features of the secondary emission that emerges in opals due to the UV and visible excitation are reported. The conditions for the low-threshold lasing in opals filled with rareearth elements are presented. The experimental results on the induced-globular light scattering are demonstrated. Such a scattering implies the coherent excitation of vibrational states of the globules in a globular photonic crystal. A new phenomenon (slow light scattering) which involves the excitation of slow photons (slowtons) that correspond to the stop-band edges of the photonic crystal is observed. The conditions for the measurement of the slow light scattering in opals excited using the ruby and nitrogen lasers are experimentally determined. The experimental and theoretical results open up the prospects for low-threshold nonlinear optical processes in material media.