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Optics Letters

Second-harmonic generation in absorptive media

Gilad Almogy, Amnon Yariv

Published Nov 15, 1994• 15 citations
Publisher

Abstract

The solution of the coupled-wave equations for second-harmonic generation in a near-resonant three-level system is extended to include absorption. It is shown, within second-order perturbation theory, that double resonance is the optimal conversion condition, despite absorption enhancement. We extend the solution numerically, using nonperturbative susceptibilities derived within the rotating-wave approximation, to saturating intensities and discuss the modifications to the perturbative conclusions as well as the regimes of validity for the various approximations.

DOI
10.1364/ol.19.001828

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The solution of the coupled-wave equations for second-harmonic generation in a near-resonant three-level system is extended to include absorption. It is shown, within second-order perturbation theory, that double resonance is the optimal conversion condition, despite absorption enhancement. We extend the solution numerically, using nonperturbative susceptibilities derived within the rotating-wave approximation, to saturating intensities and discuss the modifications to the perturbative conclusions as well as the regimes of validity for the various approximations.
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A theoretical analysis is made of the influence of the pump intensity on the nonlinear susceptibility of the third harmonic excited under conditions of two-photon resonance between the pump wave and the Doppler-broadened transition. Pumping by both traveling and standing waves is considered. For traveling wave pumping, an expression for the nonlinear harmonic susceptibility in an arbitrary field is derived and analyzed. The optimal detunings of twice the pump frequency relative to the two-photon transition frequency are determined. For standing wave pumping an expression for the nonlinear harmonic susceptibility is obtained from perturbation theory up to and including the third order. It is shown that an important factor in the reduction of the harmonic susceptibility is the saturation of the transition due to Doppler-free two-photon absorption of opposite traveling waves.
We theoretically study intensity-saturation effects in resonant harmonic generation and multiphoton ionization. A first laser excites a two-photon resonance which is coupled by a second laser to an autoionizing state. Starting from an effective Hamiltonian for the three resonant atomic states, we derive a set of equations for the density matrix of the gaseous medium and the electromagnetic field within a semiclassical framework. We present and discuss analytical and numerical solutions of these equations which show a variety of line profiles depending critically on the intensities of the incident laser pulses.
The effect of saturable absorption on second-harmonic generation in a quasi-phase-matched structure is studied, including both plane-wave and diffractive coupling of the fields and detuning of the pump field from resonance. In the regime of large Fresnel numbers, the main effect of saturable absorption is to suppress the parametric instability, which limits the efficiency of the second-harmonic generation and places a bound on the device length in order to fit sufficient harmonic power. The optical limiting nature of resonance absorption places an upper bound on the gain that quasi-phase-matching can achieve. In the diffractive regime, a variety of output transverse structures may be initiated by modulation of the relative phase and mutual displacement between the input beams of the pump and seeding harmonic signal. In particular, second-harmonic dragging, i.e., the asymmetric interaction between initially overlapping pump and second-harmonic beams propagating at different angles, is observed such that a pump beam can drag a second-harmonic one out a spatial aperture, thereby implementing a colored logical switch with gain.