Theoretical modelling and experimental demonstration of a mid-infrared femtosecond upconversion system

Summary form only given. Mid-infrared (mid-IR) imaging and spectroscopic techniques have been rapidly evolving in recent years, primarily due to a multitude of application within diverse fields like biomedical imaging, chemical sensing and cancer diagnostics [1]. Most complex chemicals have unique spectral signatures in the mid-IR region, facilitating unambiguous identification based on their absorption features. Owing to the limitations of conventional detectors in the mid-IR spectral range, sum frequency generation (SFG) in a second order nonlinear crystal can be used to convert the mid-IR signal to the visible/near-IR, enabling easy detection using silicon-based detectors. Ultrashort-pulsed upconversion is of particular interest for performing pump-probe experiments and studies of fast relaxation dynamics of chemicals and molecules [2]. In this work, we report the first demonstration of femtosecond mid-IR upconversion imaging using SFG in the Fourier plane of a 4f imaging setup. The theory developed in [3] is not valid in the short -pulsed regime (below approximately 1 picosecond) because it does not account for the reduced interaction length in the crystal (temporal walk -off between the interacting pulses) due to group velocity mismatch (GVM). We develop a theoretical model to describe femtosecond upconversion imaging, considering the broad spectrum associated with femtosecond pulses and reduced interaction length associated with GVM. This model enables the calculation of key parameters of a short -pulsed upconversion imaging system. We fmd a significant increase in angular and spectral acceptance bandwidth for mid-IR signal in the short -pulsed regimes, due to drastic reduction in interaction length, resulting in exceptionally large upconversion imaging fi eld of view (FoV) as depicted in Fig 1 (a). The resolution of the system is studied experimentally by illuminating a USAF resolution target with a mid-IR signal followed by detection of the upconverted sign