title: Molecule Detection by Sum Frequency Mixing of Mid-Infrared Laser Pulses
Creators: name: Burger, Benedikt
Publisher: name: UNSPECIFIED
Year: 2024

description: The research project dealt with the detection of molecules using resonantly enhanced sum frequency mixing (SFM) at rovibrational resonances with mid-infrared transition frequencies.
Such resonances cause characteristic molecular spectra, making it possible to identify the molecules.
Using mid-infrared radiation close to rovibrational transitions offers a large nonlinear susceptibility as multiple transitions have simultaneously small detunings, enhancing the overall detection sensitivity.

Previously, our group successfully implemented resonantly enhanced third harmonic generation (THG) for hydrogen chloride (HCl) detection, achieving a detection limit of 1 mbar.
In this work, we explored both SFM within the medium itself and up-conversion of the THG signal via SFM to improve the signal-to-noise ratio (SNR) and detection limit of spectroscopic measurements.
While sensitive detection of the mid-infrared THG signal is challenging, the use of photomultiplier tubes (PMTs), which offer high gain in the visible range, promises more sensitive detection.

We successfully implemented resonantly enhanced SFM of intense narrowband mid-infrared laser pulses and near-infrared laser pulses in HCl with a PMT based detection setup as an alternative to THG spectroscopy.
Similarly to THG, SFM of mid-infrared laser pulses is close to one- and two-photon resonances with detunings typically in the range of a few hundred GHz.
Our experimental comparison demonstrated that SFM outperforms THG in terms of sensitivity, offering a lower detection limit of 0.1 mbar, compared to 0.32 mbar, and a significantly higher SNR of 4 500, compared to 190.
Although SFM requires more complex alignment due to the additional beam, its performance outweighs this challenge, making it a promising method for future applications.

Furthermore, we improved the numerical simulations for THG by incorporating electronically excited states and a more accurate refractive index calculation.
These improvements resulted in a better alignment between the simulated and experimental data.
The simulations reveal that both THG and SFM signals exhibit a pressure dependence slightly below the quadratic relationship, with an exponent of 1.85 due to phase-matching effects.
This aligns closely with our experimental findings, where we observed an exponent of 1.6 for THG and 1.7 for SFM.
The simulations predict only a slightly larger number of generated photons for SFM, compared to THG.
Therefore, we attribute the superior performance of SFM primarily to the detection setup.
The higher internal gain of the PMT, compared to the avalanche photodiode (APD) used for THG, accounts for this advantage.
Notably, a more efficient PMT could further enhance the SNR of SFM.

Besides using SFM inside the medium, we successfully up-converted up to 12 % of the THG signal photons into the visible spectrum using SFM in a nonlinear crystal.
We found that up-conversion improves the detection limit from 8.5 mbar, for direct detection of the THG signal, to 0.7 mbar, and it increases the SNR by a factor of 26.
While this up-conversion setup requires an additional SFM stage and, in our case, a second harmonic generation (SHG) stage to generate the pump wave, the alignment of the up-conversion setup was simpler compared to THG detection, which was hindered by the small area of the APD.

Ultimately, both approaches – SFM in the medium itself and up-conversion of the THG signal – demonstrated an increased sensitivity by shifting the detection to the visible spectrum, where more sensitive detectors are available.
This highlights the potential of these techniques to significantly improve molecular detection in the mid-infrared regime.