Upconversion Device for THz to LWIR to MWIR Imaging

Phys.org: Physical Review paper "Molecular Platform for Frequency Upconversion at the Single-Photon Level" by Philippe Roelli, Diego Martin-Cano, Tobias J. Kippenberg, and Christophe Galland from EPFL and Max Planck Institute proposes a way to convert photons with wavelength of 60um to 3um to 0.5-1um ones.

"Direct detection of single photons at wavelengths beyond 2um under ambient conditions remains an outstanding technological challenge. One promising approach is frequency upconversion into the visible (VIS) or near-infrared (NIR) domain, where single-photon detectors are readily available. Here, we propose a nanoscale solution based on a molecular optomechanical platform to up-convert photons from the far- and mid-infrared (covering part of the terahertz gap) into the VIS-NIR domain. We perform a detailed analysis of its outgoing noise spectral density and conversion efficiency with a full quantum model. Our platform consists in doubly resonant nanoantennas focusing both the incoming long-wavelength radiation and the short-wavelength pump laser field into the same active region. There, infrared active vibrational modes are resonantly excited and couple through their Raman polarizability to the pump field. This optomechanical interaction is enhanced by the antenna and leads to the coherent transfer of the incoming low-frequency signal onto the anti-Stokes sideband of the pump laser. Our calculations demonstrate that our scheme is realizable with current technology and that optimized platforms can reach single-photon sensitivity in a spectral region where this capability remains unavailable to date."

"A promising way to further reduce the gated dark-count level consists in designing an array of molecular converters, sufficiently distant from each other so as not to interact by near-field coupling. We assume that the array is illuminated by a spatially coherent IR signal and optical pump beam, which is achievable when using a high f-number lens due to the subwavelength dimensions of the antennas. The key advantage of this scheme is that the anti-Stokes fields of thermal origin from different antennas would not exhibit any mutual phase coherence; they will add up incoherently in the far field. On the contrary, the up-converted (sum-frequency) anti-Stokes fields would be phase coherent and interfere constructively in specific directions, in analogy with a phased emitter array [44,45].

Considering a simple linear array, as demonstrated in Appendix F, this effect would jointly decrease the thermal contribution to the dark-count rate and dilute the intracavity photon number per device, enabling single-photon operation with improved sensitivity. A configuration with multiple converters within the IR spot could alternatively be leveraged for on-chip IR multiplexing [46–49] with distinct converters responding to distinct IR frequencies by the proper choice of molecular vibrations and antenna design, thereby bypassing the limited detection bandwidth of a single converter. This subwavelength platform benefits from the coherent nature of the conversion process and opens the route to IR spectroscopy, IR hyperspectral imaging, and recognition technologies."

Thanks to TG for the link!