Fourier transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

We introduce a birefringent interferometer for Fourier transform (FT) spectroscopy in the mid-infrared, covering the vibrational fingerprint region (5-10 µm, 1000-2000 cm^-1), which is crucial for molecular identification. Our interferometer employs the crystal calomel (Hg₂Cl₂), which combines high birefringence (n_e-n_o≈0.55) with a broad transparency range (0.38-20 µm). We adopt a design based on birefringent wedges, which is simple and compact and guarantees excellent delay accuracy and long-term stability. We demonstrate FTIR spectroscopy, with a frequency resolution of 3 cm^-1, as well as two-dimensional IR (2DIR) spectroscopy. Our setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy.