Linear and nonlinear fourier-transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer

Summary form only given. Fourier transform (FT) spectroscopy [1] is a powerful technique to measure spectra in the time domain. With respect to frequency-domain spectrometers, FT spectrometers have the advantage of higher signal to noise ratio and optical throughput, and the possibility to adjust the frequency resolution. FT spectroscopy requires to generate two field replicas whose delay must be scanned with accuracy of a small fraction of the optical cycle. Typically the replicas are generated by a Michelson interferometer in which the stability of the delay requires very robust mechanical setups and optical feedbacks. This difficulty is overcome by birefringent interferometers [3], in which two cross-polarized field replicas travel a common optical path and hence have high long-term stability. FT spectroscopy is particularly advantageous in the mid-infrared (mid-IR) from 5 to 10 μm (corresponding to 1000-2000 cm
^-1
), the so-called fingerprint region [2], where array detectors for spectrometers are noisy and very expensive. We recently demonstrated a birefringent interferometer for FT spectroscopy [4], based on the Translating-Wedge-based Identical pulses eNcoding System (TWINS) [5] with alpha barium borate and lithium niobate. However the transparency of these crystals does not extend to the mid-IR fingerprint region.In this work we extend the TWINS interferometer to the fingerprint region by using mercurous chloride (Hg2Cl2, calomel) [6]. This crystal combines a very large birefringence (Δn ≈ 0.55) with a transparency ranging from 400 nm to 20 μm. We successfully apply the calomel TWINS to both FT infrared (FTIR) spectroscopy, achieving a frequency resolution of 2.7 cm-1, and 2D spectroscopy. We find that calomel TWINS translates to the mid-IR the advantages (compactness, long-term stability, delay reproducibility) of its versions in the visible [5,7].