HEterodyne Receivers OptimizEd for Synchrotron sources – HEROES

The goal of our project “HEROES” is to provide to the national and international scientific community two ultra-high resolution spectrometers in the THz (0.1 THz-1.1 THz) and far-IR (1 THz-6 THz) spectral regions using the heterodyne techniques. These instruments will be set-up on the AILES beamline of SOLEIL facility.

The AILES beamline extracts synchrotron radiation emitted by various operating modes of the SOLEIL machine in a broad spectral range covering the THz and far-IR regions. The AILES beamline aims at characterizing the spectral properties of various materials and molecular species using the synchrotron radiation. While in the IR several light sources and spectrometers are highly performant, THz technologies still suffer of either limited power, or low spectral purity or narrow bandwidth despite the large technological advances realized in the last years (e.g. opto-electronic conversion, quantum cascade lasers, multiplication chains pumped by microwave synthesizers, …). Since ten years the AILES team made important efforts to optimize the performances of the THz and far-IR radiation extracted by the beamline. As every synchrotron-based far-IR spectroscopy stations over the world, the AILES beamline makes use of Fourier transform interferometry to acquire high resolution absorption spectra. Due to the limited optical delay line obtained using mechanical displacement of the moving mirror, such technique has inherent limited spectral resolution (30 MHz is the highest spectral resolution available from commercial interefrometers). Since its opening, AILES beamline had a federative role in the scientific community leading to the creation of several consortia dedicated to instrumental developments and specific spectroscopic applications. The present project is a consortium of four different laboratories having very complementary skills ISMO (UMR 8214 – U-PSUD), IEMN (UMR 8520 – U-Lille), LPCA (EA 4493- ULCO), la ligne AILES (SOLEIL Saclay). Our consortium relies on two recent discoveries: the implementation on QCL-based new molecular far-IR lasers on one side and the dense THz frequency comb (FC) emitted by the Coherent Synchrotron Radiations (CSR) operating modes.

In the present project, the first realization we intend to set-up is based on the recent advances performed by the IEMN laboratory concerning THz molecular lasers pumped by mid-IR QCLs. The patent describing these new far-IR laser sources is pending and the HEROES consortium propose to use such devices as Local Oscillators (LO) in a heterodyne receiver for the far-IR synchrotron continuum. The huge number of far-IR laser emission lines of the LO (available from such optical excitation), and the coupling with the bright far-IR continuum source will permit a full frequency coverage in the 1-5 THz spectral range with unprecedented resolution (better than 1 MHz). This original approach will open new, broad perspectives for high resolution molecular spectroscopy in terms of size of the molecular families to be studied, observation of fine coupling between angular momenta, energy levels splittings due to large amplitude motions …

The second realization proposed in this project follows our recent discovery of the discrete nature of the CSR in various operation modes. We demonstrated that CSR is emitted through a very stable THz frequency comb with 846 kHz spacing between comb teeth. Thanks to this very interesting spectral characteristic, we propose to develop a dual-comb spectrometer. Our idea is to probe the CSR-FC with a second FC having a repetition rate close to 846 kHz. This leads to detect the beating between the two FCs in the radiofrequency domain where electronic devices are extremely performant. This new THz spectrometer will combine high brilliance and spectral coverage from CSR with the high resolution (846 kHZ), and ultra-fast spectral analysis of the dual-comb approach which revolutionized molecular spectroscopy in the mid- and near-IR regions.