Sn. Volkov et al., 2ND-HARMONIC GENERATION IN THE INTERIOR OF AN ISOTROPIC MEDIUM WITH QUADRATIC NONLINEARITY BY A FOCUSED INHOMOGENEOUSLY POLARIZED PUMP BEAM, Journal of experimental and theoretical physics, 86(4), 1998, pp. 687-695
This paper presents a theoretical study of second-harmonic generation
(SHG) by a focused pump beam in the interior of an isotropic medium, e
xperimentally observed earlier. It shows that the spatial dispersion o
f the quadratic optical response of the substance can be responsible f
or this nonlinear process even when a macroscopic inversion center is
present in the medium. It is established that this effect, which is im
possible in the plane-wave approximation, also does not occur when a G
aussian pump beam with uniform distribution of the polarization state
of the wave field over the cross section is used, but that the presenc
e in the pump beam of higher transverse modes with polarization differ
ent from the fundamental mode can cause an SHG signal to appear. The c
onditions for a wave to appear at the doubled frequency are found, ana
lytical dependences for its electric field and total power on the prop
agation coordinate, the degree of focusing, and the other parameters o
f the problem are obtained, and the requirements on the optimum experi
mental geometry are formulated. The dependence of the signal-beam powe
r on the wavevector detuning is studied,and it is shown that interfere
nce effects can cause the wave at the second harmonic to disappear whe
n a normal dispersion law is obeyed in the region between the fundamen
tal and doubled frequencies. (C) 1998 American Institute of Physics.