Abstract:

Nonlinear interferometry allows phase and amplitude information to be passed between light at one wavelength that interacts with an object and another that is detected. The technique uses correlated photon beams created in a nonlinear material, either via difference frequency generation or parametric fluorescence. When implemented with nondegenerate beams, there are wide-reaching applications where passing information between wavelengths is desirable; for example, where the technical and fundamental limitations of mid-infrared (MIR) detectors can be circumvented with silicon-based sensors for label-free biological imaging, environmental monitoring and medical imaging.

Recent research has predominantly focused on a quantum mechanical variant of nonlinear interferometry known as imaging with undetected light; where spontaneous parametric down conversion (SPDC) is created in phase-matched bulk nonlinear crystals and interferes by induced coherence without induced emission. Indistinguishable photon pairs are generated at two instances, and the paths of respective colours are overlapped. The lack of “which path” information induces coherence; placing an object in the longer wavelength path restores path knowledge and destroys interference at the shorter wavelength. Classical nonlinear interferometry achieves a similar effect by stimulating the parametric fluorescence, coherently transferring amplitude and phase across wavelengths, and offers advantages such as an improved signal to noise ratio, greater coherence of interference and reduced reliance on highly nonlinear material.

This talk will present quantum and classical implementations and showcase phase-matching-free classical nonlinear interferometry using plasmonic metasurface sources. Inside the interferometer, stimulated four-wave mixing (FWM) or harmonic generation is emitted at resonant metasurfaces under femtosecond-pulsed illumination to achieve sensing in the near-IR and detection in the visible. Through engineering of the thin-film sources, a degree of control is offered over the interferometric technique not previously demonstrated with conventional nonlinear materials.

Bio:

Toby Severs has recently joined TMOS as a research fellow in Prof. Ann Roberts’ group at The University of Melbourne. Toby received his PhD from Imperial College London in 2025 under the supervision of Prof. Rupert Oulton, where his research focused on plasmonic metasurfaces, nonlinear optics and nanolasers. Prior to PhD study, he worked as a research assistant in the Low Dimensional Structures and Devices (LDSD) group at The University of Sheffield investigating the optical properties of two-dimensional materials.