Nonlinear quantum interferometer: generating and sensing mid-infrared radiation for cancer diagnostics

In this work, a miniaturized nonlinear interferometer module for applications in cancer diagnostics is presented. Utilizing spontaneous parametric down-conversion (SPDC), photons in the mid-infrared and near-infrared are generated and the concept “sensing with undetected photons” is applied. The module comprises a periodically poled potassium titanyl phosphate (ppKTP) crystal that is polished and coated to form a monolithic enhancement cavity for the pump. The SPDC-generated MIR photons can be used for label-free bio-marker detection. The required pump radiation at 720 nm is coupled into the module by a polarization maintaining optical fiber, while an optical tapered amplifier (TPA) inside the module ensures beyond-watt-level pump power entering the crystal. All components are hybrid-integrated as a robust module and precise temperature control of the crystal and TPA is possible. The main design considerations and simulations regarding the ppKTP cavity crystal and corresponding module development as well as experimental results are described.