Detection of HCl molecules by resonantly enhanced sum-frequency mixing of mid- and near-infrared laser pulses

We perform experimental studies of resonantly enhanced sum-frequency mixing (SFM), driven by tunable, spectrally narrowband mid-infrared and fixed-frequency nanosecond laser pulses, aiming at applications in molecular gas detection. The mid-infrared pulses are tuned in the vicinity of two-photon rovibrational transitions in the electronic ground state to provide strong resonance enhancements of the nonlinear susceptibility, while a probe laser at shorter wavelength uses an off-resonant single-photon coupling to excited electronic states. This SFM approach benefits from the advantageous combination of typically small detunings among the mid-infrared, vibrational transitions and the typically large transition dipole moment for couplings of electronic states. Moreover, compared to resonantly enhanced third harmonic generation (THG), a signal wave at much shorter wavelength permits simple and efficient detection. We demonstrate resonantly enhanced SFM via rovibrational states in gaseous hydrogen chloride molecules and compare its features to THG. The SFM spectra offer a large signal-to-noise ratio of 4 orders of magnitude and a detection limit down to a pressure of 0.1 mbar, corresponding to a particle density of $0.35\times {10}^{15}$ per ${\mathrm{cm}}^3$.

Figure 1

Coupling schemes for (a) CARS, (b) THG, and (c) SFM. For simplicity, we show only the vibrational states $|\nu \rangle$ of the electronic ground state, but not their rotational substructure, nor details of the excited electronic state(s) $|e\rangle$. Arrows indicate laser fields at the fundamental (${\omega }_{\mathrm{f}}$, red), probe (${\omega }_{\mathrm{p}}$, green, in the case of CARS also called “pump”), Stokes (${\omega }_{\mathrm{st}}$, blue), and generated signal (${\omega }_{\mathrm{s}}$, purple) frequency while ${\delta }_i$ indicates the $i$-photon detuning. We assume the lasers are tuned to two-photon resonance with vibrational states in the electronic ground state.

Figure 2

Experimental setup with (cw) fiber pump laser, (cw) optical parametric oscillator (OPO), pre-amplifier (OPA1), power amplifier (OPA2), Nd:YAG pump laser, longpass filter (LP), lenses (L), photodiode (PD), dichroic mirror (DM), Pellin-Broca prism (PBP), photomultiplier tube (PMT), avalanche photodiode (APD), and energy meter (EM). Note the angle between the beams in the gas cell to indicate a noncollinear geometry.

Figure 3

(a) SFM and (b) THG signal pulse energy vs fundamental wavelength (or wave number on the upper axis). The signal pulse energy is normalized to the maximum value in each data set. The fundamental peak intensity was $360\mathrm{MW}/{\mathrm{cm}}^2$ (pulse energy 1.2 mJ) for SFM and $300\mathrm{MW}/{\mathrm{cm}}^2$ (pulse energy 1 mJ) for THG. The probe peak intensity was $2.9\mathrm{GW}/{\mathrm{cm}}^2$ (pulse energy 1.2 mJ). Blue dots indicate experimental data, averaged in wavelength bins of 1 pm. The red line shows a numerical simulation with the mean background (dashed black line) as offset. For simplicity and faster calculation, we assumed a Lorentzian line shape instead of a Voigt profile. Markers indicate the central wavelengths of rovibrational two-photon resonances ($\nu =0\to 2$, Q-branch with $\mathrm{\Delta }N=0$ in the electronic ground state), with rotational quantum numbers $N$ of the ground state for the transition (orange squares for $\mathrm{H}{}^{35}\mathrm{Cl}$, red diamonds for $\mathrm{H}{}^{37}\mathrm{Cl}$). Note that the logarithmic scale on the pulse-energy axes limits the presentation of the background to positive values, though it contains negative values due to electronic noise as well.

Figure 4

SFM signal pulse energy vs (a) fundamental and (b) probe pulse energy (or peak intensity on the upper axis). The pulse energy of the other beam was kept at $500\mu \mathrm{J}$. Collinear beam geometry (probe beam waist of $175\mu \mathrm{m}$). (c) THG signal pulse energy vs fundamental pulse energy. The HCl pressure was 57 mbar for all measurements. Blue points show experimental data. The red line shows an exponential fit.

Figure 5

(a) SFM and (b) THG signal photon count per pulse vs HCl pressure. The blue line represents experimental data from continuous variation of the HCl pressure at fixed fundamental wavelength (3533.12 nm), binned in 0.01 logarithmic decade pressure intervals. The blue shaded area indicates the standard deviation. Orange hollow circles show the maxima of Voigt fits to spectral lines at selected pressures. The red dashed line is an exponential fit to the latter data. Black dotted lines mark the noise level and detection limit. Note the logarithmic scales on both axes. The insets show the spectral lines [photon count per pulse vs two-photon (2P) detuning] at the detection limit, for (c) SFM at 0.1 mbar and for (d) THG at 0.32 mbar. For SFM, we distinguish between the electronic background in blue and the optical signal as orange dots. The black dashed line is added to guide the eye.