With an achromatic Mach-Zehnder-type interferometer, our team has shown a first proof-of-principle of differential molecular fingerprinting in the near infrared spectral range. The spatial separation and therefore direct experimental access to the small difference between two molecular fingerprints will benefit many applications of broadband mid-infrared spectroscopy because it tackles two longstanding issues in spectroscopy: excess source noise and detection dynamic range.
When including two similar samples in the interferometer arms, only the light field associated with the differences in the light-matter interactions occurring in the two arms survives the destructive interference. Thus, excess source noise is cancelled and the dynamic range of the signal reduces to the amplitude of the process under scrutiny.
The challenge in designing an interferometer for differential molecular fingerprinting is to introduce a wavelength-independent phase shift of π between the two interferometer arms. In our case, the phase shift is caused by the fundamental property of light waves undergoing no phase change when reflected off a boundary to a medium with lower refractive index. With our setup, we achieved achromatic subtraction of optical fields with an intensity extinction of 4×10-4 over a super-octave band (950 – 2100 nm). Our calculations predict a sensitivity improvement over traditional spectroscopy of more than one order of magnitude for this extinction ratio.
Figure: The main plots show (a) the spectrally resolved destructive (blue) and constructive (green) interference of the achromatic interferometer and the according extinction ratio (dashed orange) with a RMS of 6.2×10−4 and (b) methane resonances in the direct (red) and the differential (light blue) measurement configuration, at ~1 bar. For comparison, the inverted theory curve for the differential measurement is shown (dark blue).
Original publication:
T. Buberl, P. Sulzer, A. Leitenstorfer, F. Krausz, and I. Pupeza, "Broadband interferometric subtraction of optical fields," Opt. Express 27, 2432 (2019).
