Third-order Nonlinear Processes Research Articles

Effects of cross relaxation and line mixing on third-order nonlinearities of resonant systems.

The effect of cross relaxation, i.e., the transfer of coherence between transitions coupled by inelastic collisions, on third-order nonlinear processes, namely, saturated absorption and four-wave mixing, is studied within the context of a model four-level system. Numerical results displaying changes in the resonant structures in four-wave-mixing signals for a range of collisional parameters are presented.

Full intensity profile analysis of ultrashort laser pulses using four-wave mixing or third harmonic generation

New methods are proposed for the measurement of the full temporal intensity profile I( t) of ultrashort laser based on recording of a triple intensity-autocorrelation function. The possibilities offered by two third-order nonlinear processes, degenerate four-wave mixing (DFWM) and noncollinear third-harmonic generation (THG) are investigated from this aspect. In both cases double time-delay methods are considered first, where three replica pulses of profile I( t) with adjustable delays between them are used as input and the integral output intensity is monitored. The same information may be obtained in single-delay or single-shot schemes as well by using appropriate input beam expansions and a one- or two-dimensional detector array respectively, The methods considered have at least subpicosecond resolution. DFWM methods, in particular, are very convenient due to their wavelength-independence in a broad interval including UV.

Third-order optical mixing of frequencies of a strong bichromatic field interacting with a three-level atom in the “Lambda” configuration

We have considered the excitation spectra of three-photon processes arising from the interaction of a three-level atom, in the “lambda” configuration, with two strong electromagnetic fields whose initially populated modes ω a and ω b are at resonance with the two atomic transition frequencies, respectively. In the limit of high photon densities, expressions for the spectral functions are derived describing third-order nonlinear processes near the frequencies ω = ± (ω a − 2ω b) and ω = ± (ω b − 2ω a), respectively. Analytical calculations are then performed and the computed spectra are presented graphically for different values of the Rabi frequencies Ω a and Ω b, respectively. Expressions for the spectral function near the frequence ω = ω a − 2ω b are derived for specific values ω ⪡ η b, η b ≈ 2η a, η a ≈ η b, η a ≈ 2η a and η a ⪢ η b, where η a = Ω a/γ 0 and η b = Ω b/γ 0 are the reduced Rabi frquencies of the laser fields, respectively, and γ 0 is the s pontaneous emission probability. The computed spectra are compared with the corresponding ones for an atom in the “V” configuration.

Effects of focusing on third-order nonlinear processes in isotropic media

A theoretical analysis of third-order nonlinear interactions of focused laser beams is performed for the processes \omega_{1} + \omega_{2} + \omega_{3} \rightarrow \omega_{4}, \omega_{1} + \omega_{2} - \omega_{3} \rightarrow \omega_{4} , and \omega_{1} - \omega_{2} - \omega_{3} \rightarrow \omega_{4} . The total power and far-field beam profile of the generated radiation is related to the total powers of the fundamental beams, to the tightness and location of the focus, and to the value of the difference between the wave vectors of the generated radiation and driving polarization. The optimum degree of wave-vector mismatch as a function of tightness and location of focus is determined for each of the three processes. The process \omega_{1} + \omega_{2} - \omega_{3} \rightarrow \omega_{4} is found to be unique in that it is always optimized by focusing as tightly as possible. Experimental results, which verify the theory for the processes \omega_{1} + \omega_{2} + \omega_{3} \rightarrow \omega_{4} and \omega_{1} + \omega_{2} - \omega_{3} \rightarrow \omega_{4} , are presented.