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

J Réhault,G Cerullo,A Oriana,C Manzoni,C P Hauri,J Helbing,R Borrego-Varillas

doi:10.1364/oe.25.004403

Abstract

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 (ne-no≈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.

Highlights

Fourier transform (FT) spectroscopy [1] is a powerful technique to measure spectra in the time domain, by recording the interferogram of two delayed replicas of an optical waveform and performing the FT of the delay-dependent signal
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 setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy

Summary

Introduction

Fourier transform (FT) spectroscopy [1] is a powerful technique to measure spectra in the time domain, by recording the interferogram of two delayed replicas of an optical waveform and performing the FT of the delay-dependent signal. Coherent 2DIR spectroscopy is best understood as a pump-probe technique, which measures the absorption changes after vibrational excitation of a sample as a function of both probe and excitation frequency [8]. This yields 2D-correlation maps, which provide information on molecular structure and dynamics: for example, a sample excited at the frequency of normal mode a may instantaneously show an absorption change at the frequency of mode b, as an indication of the (orientation and distance-dependent) coupling of the two modes. The excitation frequency can be obtained by FT when the probe intensity is recorded as a function of the delay between two replicas of a spectrally broad excitation pulse

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Conclusion