MIR-Pump NIR-Probe Fiber-Optic Photothermal Spectroscopy With Background-Free First Harmonic Detection

We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS-<inline-formula> <tex-math notation="LaTeX">$1{f}$ </tex-math></inline-formula>) detection of formaldehyde, one of the most common volatile organic compounds (VOCs), in a silica hollow-core negative curvature fiber (HC-NCF). The photothermal gas sensor adopts a mid-infrared interband cascade pump laser at <inline-formula> <tex-math notation="LaTeX">$3.6~\mu \text{m}$ </tex-math></inline-formula> and a near-infrared fiber probe laser at <inline-formula> <tex-math notation="LaTeX">$1.56~\mu \text{m}$ </tex-math></inline-formula>. At the optimal modulation frequency (8 kHz) and modulation index (1.8) of the pump laser, we obtained a normalized noise equivalent absorption (NNEA) coefficient of <inline-formula> <tex-math notation="LaTeX">$4\times 10^{-9}$ </tex-math></inline-formula> cm⁻¹WHz<inline-formula> <tex-math notation="LaTeX">$^{-1/2}$ </tex-math></inline-formula>. The use of HC-NCF with an inner diameter of <inline-formula> <tex-math notation="LaTeX">$65~\mu \text{m}$ </tex-math></inline-formula> enables the sensitive photothermal detection even for a very low pump power of micro-watts. The background-free PTS-<inline-formula> <tex-math notation="LaTeX">$1{f}$ </tex-math></inline-formula> detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic (<inline-formula> <tex-math notation="LaTeX">$2{f}$ </tex-math></inline-formula>) detection. A theoretical model was established in this work to interpret the experimental results.