We propose an incoherent interferometer which can partly modify the spatial correlation property of thermal light. Applying the interferometer to the two-photon double-slit interference with thermal light, we find that the subwavelength interference patterns appear in the intensity correlation not only between two same polarized photons at different positions but also between two orthogonally polarized photons at the same position. The latter can correspond to the two-photon double-slit interference using a two-photon entangled source. Our experimental result suggests that a thermal light source is capable of accomplishing subwavelength lithography technique to surpass the Rayleigh resolution limit.
We propose a two-photon subwavelength interference scheme for classical light in which multiple quantum-like entangled two-photon paths play an essential role. These entangled two-photon paths are introduced through a specially designed source composed of many point sources
j with
j's complex amplitude being a superposition of modes ${e}^{i{\ensuremath{\phi}}_{j}}$ and ${e}^{i{\ensuremath{\phi}}_{j}^{(1)}}$, where ${\ensuremath{\phi}}_{j}$ and ${\ensuremath{\phi}}_{j}^{(1)}$ are temporally random phases but satisfying ${\ensuremath{\phi}}_{j}+{\ensuremath{\phi}}_{j}^{(1)}={\ensuremath{\phi}}_{0}$, with ${\ensuremath{\phi}}_{0}$ being either a constant or a random phase in time. Interference between the entangled two-photon paths could lead to second-order subwavelength interference of an object put in front of the source plane. In a proof-of-principle experiment, by using a spatial light modulator to modulate the wave front of a coherent light, we have generated such a source and observed subwavelength interference of a double-slit mask via two-photon measurement.