Explicit solutions of the inhomogeneous paraxial wave equation in a linear and quadratic approximation are applied to wave fields with invariant features, such as oscillating laser beams in a parabolic waveguide and spiral light beams in varying media. A similar effect of superfocusing of particle beams in a thin monocrystal film, harmonic oscillations of cold trapped atoms, and motion in magnetic field are also mentioned. Green function and generalized Fresnel integrals. In the context of quantum mechanics, a one-dimensional (1D) linear Schrödinger equation for generalized driven harmonic oscillators,(a, b, c, d, f , and g are suitable real-valued functions of time t only), can be solved by the integral superposition principle:Gx; y; tψy; 0dy;where Gx; y; t 2πμ 0 t
−1∕2× expiα 0 tx 2 β 0 txy γ 0 ty 2 δ 0 tx ε 0 ty κ 0 t; (3) for certain initial data ψx; 0 φx (see [1][2][3][4] and the references therein for more details). The intrinsic connection between Hamiltonian mechanics and the process of wave propagation is anything but a new idea [5,6]. Yet, in paraxial optics, when the time variable t represents the coordinate, say s, in the direction of wave propagation, Eqs. (2) and (3) can be thought of as a generalization of the Fresnel integral [7][8][9][10].In the paraxial approximation, a 2D coherent light field in a parabolic inhomogeneous medium with coordinates r; s x; y; s is described by the following equation for the complex field amplitude: iA s −aA xx A yy bx 2 y 2 A