On multiple frequency power density measurements

We shall give a priori conditions on the illuminations φi such that the solutions to the Helmholtz equation and their gradients satisfy certain non-zero and linear independence properties inside the domain Ω, provided that a finite number of frequencies k are chosen in a fixed range. These conditions are independent of the coefficients, in contrast to the illuminations classically constructed by means of complex geometric optics solutions. This theory finds applications in several hybrid problems, where unknown parameters have to be imaged from internal power density measurements. As an example, we discuss the microwave imaging by ultrasound deformation technique, for which we prove new reconstruction formulae.