Abstract
We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS- $1{f}$ ) 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 $3.6~\mu \text{m}$ and a near-infrared fiber probe laser at $1.56~\mu \text{m}$ . 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 $4\times 10^{-9}$ cm−1WHz $^{-1/2}$ . The use of HC-NCF with an inner diameter of $65~\mu \text{m}$ enables the sensitive photothermal detection even for a very low pump power of micro-watts. The background-free PTS- $1{f}$ detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic ( $2{f}$ ) detection. A theoretical model was established in this work to interpret the experimental results.
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