We discuss representation of certain functions of the Laplace operator ∆ as Dirichlet-to-Neumann maps for appropriate elliptic operators in half-space. A classical result identifies (−∆) 1/2 , the square root of the d-dimensional Laplace operator, with the Dirichlet-to-Neumann map for the (d + 1)-dimensional Laplace operator ∆ t,x in (0, ∞) × R d . Caffarelli and Silvestre extended this to fractional powers (−∆) α/2 , which correspond to operators ∇ t,x (t 1−α ∇ t,x ). We provide an analogous result for all complete Bernstein functions of −∆ using Krein's spectral theory of strings.Two sample applications are provided: a Courant-Hilbert nodal line theorem for harmonic extensions of the eigenfunctions of non-local Schrödinger operators ψ(−∆) + V (x), as well as an upper bound for the eigenvalues of these operators. Here ψ is a complete Bernstein function and V is a confining potential.