Roman V. Markov

Optical and Nonlinear-Optical Properties of Pseudoisocyanine J-Aggregates with the Controlled Exciton Absorption Linewidth

Thin films of molecular J-aggregates of pseudoisocyanine with different linewidths of exciton absorption (from 75 to 210 cm–1) were obtained. A change in the absorption linewidth of J-aggregates was achieved by introducing organic salts into a sample. As an alternative method of varying the exciton absorption linewidth, variation in sample temperature was used. For the samples obtained, the third-order nonlinear optical susceptibility χ(3) was measured at temperatures of 80 and 293 K by the Z scan and pump–probe spectroscopy techniques and the dependence of χ(3) on the absorption linewidth W was found to follow a power law. It was revealed that the character of the function χ(3)(W) is almost independent of the method for measuring the absorption linewidth to have the form χ(3) ∼ Wδ with δ ≈ –(1.36–1.58). Based on this finding, the relation of the exciton delocalization length in the aggregate to the absorption linewidth and optical nonlinearity is discussed.

Lasing on the resonance transition in sodium atoms under nonresonant optical excitation

Coherent emission on the 3P–3S resonance transition (D line) of nonresonantly excited sodium atoms in a buffer gas atmosphere is studied experimentally and theoretically. Both forward and backward coherent emission on the D lines is observed relative to the propagation direction of a pump beam whose frequency is blue-shifted from resonance. The divergence of the emitted radiation does not exceed that of the pump beam. The emission is due to the population inversion created on the “operating” transition when the pump is far detuned from resonance and the buffer gas pressure is sufficiently high. It is found that both emission intensity and the detuning range where this phenomenon is observed increase with the buffer gas pressure.

Population inversion on transitions to the ground state of atoms upon nonresonance absorption of laser radiation

Lasing at the resonance transitions (D1− and D2−lines) of sodium was observed in the superradiance mode upon nonresonance optical excitation in the presence of a buffer gas. The dependences of the lasing intensity on the exciting radiation intensity and on the detuning of its frequency from the frequencies of resonance transitions were studied. It is found that, under specific conditions of the experiment (high pressure of a buffer gas and a rather high radiation intensity), in the case of a large positive detuning of the exciting radiation frequency from the resonance (“working”) transition frequency, the population inversion is produced at the “ working” transition, which results in lasing.

Enhancement of resonant bleaching of J-aggregates upon lengthening of an exciting radiation

Thin films of J-aggregates of a new amphiphilic thiacarbocyanine dye of the benthiazole series are prepared and the nonlinear optical response of molecular J-aggregates is studied for femto-and nanosecond exciting radiation pulses. It is found that the nonlinear optical response of J-aggregates exhibits substantial enhancement upon an increase in the pulse duration, which cannot be described by the saturation effect in the model of a two-level system. This effect is considered using a three-level model taking into account the formation of self-trapped exciton states in molecular J-aggregates.

Third-order nonlinearity optical properties of the films of cyanine dye with borate anion

The formation of thin-solid J-aggregated films Pseudoisocyanine prepared by spin-coating method is described. The process of dye aggregation was stimulated by addition of anions B10H10-2 and B12H12-2. It is shown that only addition of decahydro-closo-decaborate dianion (B10H10-2) to initial solution of dye give rise the J-aggregation with high efficiency. Obtained films have long storage time and suitable for non-linear optical measurements. The third-order susceptibility close to J-peak of PIC aggregates in films was studied by Z-scan method. Prepared films have a large magnitude of (chi) (3) to approximately 10-5 esu. The thermal contribution to the optical nonlinear response of dye films was estimated by use Kramers-Kronig relationship.

Nonlinear optical properties of two types of PIC J-aggregates in thin films

Optical properties of molecular J-aggregates of cyanine dyes are investigated intensively at present time. The most characteristic peculiarity of J-aggregates is presence of narrow peak (J-peak) red shifted from the peak of single molecule in absorption spectrum. J-peak is due to exciton absorption (Frenkels exciton) and its width is defined by effect of motional narrowing) The interest to J-aggregates of pseudoisocyanine (PlC) has been aroused recently by discovery of their huge cubic susceptibility with l)iCOSecOnd decay time.24 The developing of preparation methods for J-aggregates PIG in thin films is actual for practical application. J-aggregates PlC in films of Langmuir-Blodgett or in a polymer matrix have been described recently.58 Temperature and radiation stability of such films allows to carry out investigations of structure, exciton dynamics and their nonlinear-optical properties. One of the methods for study of nonlinear properties of thin films is z-scan technique when real and imaginary parts of third order susceptibility are measured independently . Nonlinear optical properties of two types of i-aggregates prepared in thin films and studied by z-scan technique are described in this paper.

Thin film of J-aggregates a saturable absorber for passive mode locking in a YAG:Nd3+ laser

Thin films of molecular J-aggregates of polymethyn dyes were obtained. Linear and nonlinear optical properties of the films were investigated. These thin films of J-aggregates were successfully used as saturable absorbers for passive mode-locking in YAG:Nd3+ laser for the first time. The reproduced generation of ultra short pulses with a duration of ~13 ps was obtained for two types of dyes forming the J-aggregates.

Observation of optical nonlinearity size enhancement in one-dimensional molecular aggregates

Thin films of molecular aggregates of pseudoisocyanine were obtained. Delocalization length of exciton wave function was measured with an assumption that an absorption line width is defined by disorder induced mixing of the lowest states of the one-exciton band. Predicted effect of size enhancement of the nonlinear optical properties for molecular aggregates was observed. The assumption used to determine the delocalization length is shown to coincide with the observed optical nonlinearity size enhancement phenomenon.

Population inversion in two-level system under continuous optical excitation

Lasing on the sodium resonance transitions (D1 and D2 lines) at the superluminosity regime was observed upon the nonresonance excitation in the presence of a buffer gas. Dependences of the lasing intensity on the pump radiation intensity and its frequency detuning from the frequencies of resonance transitions were examined. It was found that under the specific experimental conditions (high buffer gas pressure, sufficiently high intensity of pump radiation) upon the large positive frequency detuning of pump radiation with respect to the frequency of resonance (working) transition, contrary to ingrained conceptions, population inversion for the working transition is raised. That results in observed phenomena.

Optical properties of thin solid films of J-aggregated dyes with long alkyl substituents

Thin solid films of low-dimensional J-aggregates of amphyphylic cyanine dyes with various lengths of the N-alkyl substituents were obtained. It was shown that the chemical modification of dye helps obtaining the stable thin films of J-aggregates by spin-coating method without any stabilization polymer. The optical properties of the thin films are described.