Doubly resonant nanocavities for molecular frequency upconversion at the single-photon level

As applications in fields like security or medicine require sensitive schemes in order to detect IR photons, an interesting strategy consists in converting weak IR signals into the optical domain. In the conversion process suggested here, an incoming IR field drives resonantly a molecular vibration and modifies its excited state population, which is mapped onto the scattered Raman signal produced during the interaction between the same vibrational mode and an optical pump beam. The modified vibrational population gives rise to an additional emission of coherent optical photons on the Raman sidebands that can be detected with existing single photon counting techniques. To insure an optimal overlap between the two beams and the molecular system, doubly resonant nanocavities confine the fields into similar mode volumes and increase the efficiency of the conversion process.

Our first experimental nanocavities show 13 orders of magnitude enhancement of the conversion efficiency per molecule with respect to free space and enable to clearly demonstrate the coherency of the conversion process. These first results pave the way for the development of novel technological platforms harnessing the coherent nature of the conversion process for advanced IR sensing and imaging applications.