A differential polarization interferometer is provided. An interferometer performs direct measurement of phase shift of a light wave passed under an arbitrary angle through a sample composed of a transparent substrate holding a thin deposited test film, for metamaterial testing. An example apparatus has a laser source and a first polarizer, and two optically connected arms. A first arm creates orthogonally polarized components of a single output beam for a broadband non-polarizing beam splitter. A second arm has a controllable phase retarder to introduce a phase shift into one polarization component of the reflected single output beam, and a second polarizer to equalize and mix the polarization components of the reflected single output beam. This transforms the reflected single output beam into a beam resulting from interference of polarization components of the reflected single output beam. A photodetector transforms an intensity of the beam into an electric signal for output.