Sub-Diffraction Near-Field Imaging with Undetected Photons using Thin Sources of Photon Pairs

Abstract

Quantum imaging with undetected photons allows for imaging an object by detecting photons that never interacted with it, using the correlation between photon pairs generated in the process of spontaneous parametric down conversion (SPDC) [1] . Through SPDC in a χ (2) material, a pump (P) photon of wavelength λ P can split into a pair of signal (S) and idler (I) photons, such that 1/ λ P = 1/ λ S + 1/ λ I . In such imaging schemes, the object’s optical properties at λ I , such as its absorption profile, can be inferred by only imaging the signal photons. Thus, one can image in challenging spectral ranges, such as mid-IR for λ I , using detectors in the visible for λ S [1] . In this scheme, the resolution is fundamentally diffraction-limited to the wavelength that interacts with the object, λ I [2] . We show that one can go beyond this diffraction limit by placing the object in the near-field of a subwavelength thin SPDC source, as shown schematically in Fig. 1(a) , through transferring the near-field information at λ I into propagating modes at λ S that can be detected in the far field. In this scheme, the resolution of imaging the object at λ I is determined by the detected signal wavelength λ S . As an application, we numerically demonstrate how two closely-spaced small scatterers at mid-IR can be resolved with a visible-range resolution.