The SOLEIL team is embarking on an ambitious upgrade project for a storage ring [1], aiming to achieve an emittance of the X-rays photon beam approximately 50 times smaller than that of the present synchrotron facility. The future facility, SOLEIL-II, is set to maintain a broad spectrum of photons, ranging from THz to hard X-rays, while preserving the number of available beamlines (29). However, the new accelerator's characteristics, advantageous for X-rays, present challenges for extracting the IR/THz emission while ensuring the exceptional flux and brilliance for these beamlines.

We will showcase the design of an optimized extraction port for THz/IR light, featuring large solid angles that enable high flux and brilliance across the entire range, based on preliminary simulations conducted with SRW (Synchrotron Radiation Workshop). Additionally, we will present the beamline project, which will comprise four branches, facilitating a rich scientific program. Three branches, equipped with Fourier transform interferometers, will be dedicated to: - (i) Optical studies of condensed matter under extreme pressures and temperatures [2], - (ii) High-resolution spectroscopy of isolated molecules at various temperatures, crucial for atmospheric and astrophysical research, - (iii) Reaction kinetics for in situ studies of battery materials, catalysis, and electrochemistry [3]. A fourth branch, dedicated to white light extraction, will be utilized for SNOM studies [4], heterodyne detection for ultra-high molecular resolution [5], and electron beam diagnostics through photonic time stretch [6].

The Millimeter Wave (mmW) and Terahertz (THz) domains are very dynamic fields in term of applied research, particularly with a view to using them for high-frequency telecommunications. In this context, the characterization of new ranges of materials is becoming a topical issue. THz Time Domain Spectroscopy (THz-TDS) technique is commonly used for precise characterization of materials in a frequency range from 100 GHz to around 5 THz. On the other hand, with increasing frequency of radiofrequency (RF) characterization tools, such as FSM-CW (Free Space Method with Continuous Wave), new characterization methods are now available to probe the lower part of this spectral range, up to several hundreds of GHz. In this work, we compare the two main techniques, THz-TDS and FSM-CW, in terms of characteristics and performance (bandwidths, dynamics, limits, uncertainties, reproducibility…). The THz-TDS is a time-domain method that is intrinsically very broadband as the whole spectra is reached with only one measurement. The FSM-CW method is a frequency-domain approach that utilizes a Vector Network Analyzer (VNA). Thus, to span a frequency range as broad as that covered by the THz-TDS method, multiple setups are required. In this study we probed the following bands: W (75–110 GHz) and J (220–330 GHz). By using these different techniques and setups, we characterized a set of different materials to cover a wide range of permittivity values, for which we also varied the thickness. Figure 1 exhibits results we obtained on HDPE, PA6 and Al2O3 with 3 different thicknesses (uncertainties on the real part of the permittivity have not been plotted for sake of clarity). Due to the low value of the imaginary part of the permittivity, the extraction process sometimes gives negative values, that must be considered in regards to all the uncertainties. All the results will be presented and commented at the conference.

This work is supported by the France 2030 programs, grant ANR-22-PEFT-0006 (NF-SYSTERA, PEPR 5G and beyond - Future Networks).

PbTe is a well-known thermoelectric material that exhibits a soft phonon mode related to its ferroelectric instability [1]. The tunability of the critical temperature through carrier concentration, along with the material's ionic nature, highlights its complex lattice dynamics and the significance of electron-phonon coupling [2,3]. These properties can be adjusted through doping to achieve a topologically non-trivial state. While the parent compound is a conventional semiconductor, doping Pb-sites with Sn leads to band inversion at the L-points of the Brillouin zone, resulting in a topological crystalline insulator (TCI) that features intriguing surface states and massive Dirac fermions in the bulk [4]. The critical level of doping is temperature-dependent; however, a crossover is observed for Sn concentrations exceeding 0.3 at low temperatures.

In this study, we report on the results of terahertz time-domain spectroscopy (THz-TDS) experiments conducted on the pseudobinary alloy Pb_1−xSn_xTe for various Sn concentrations. We investigate the charge dynamics and the soft mode across the transition from trivial to TCI. Sn-doping inevitably alters the carrier concentration, and the THz spectra are predominantly influenced by the free-carrier Drude contribution at higher doping concentrations. Meanwhile, the soft mode exhibits a similar trend until the topological crossover is reached. Additionally, a high-energy mode associated with structural disorder quickly diminishes with even a small amount of Sn-doping.

Reference:

[1] M. P. Jiang, et al, Nat. Commun. 7, 12991 (2016).

[2] A. Baydin, et al., Phys. Rev. Lett. 128, 075901 (2022).

[3] F. G. G. Hernandez, et al, Sci. Adv. 9, eadj4074 (2023).

[4] T. Liang, et al, Nat. Commun. 4, 2696 (2013).

Quantum Cascade Lasers (QCLs) are devices emitting highly coherent light in the mid-infrared spectral region. Amplitude and phase of the generated electric field are coupled through the refractive index of the active medium broadening the Schawlow-Townes linewidth by a factor (1 + 𝛼²). These effects are investigated in a directly modulated 9.3𝜇𝑚 DFB-QCL as a function of the modulation frequency by measuring its heterodyne beating with a highly stabilized optical frequency comb. From the acquired temporal traces a Hilbert transformation of the data provides a simultaneous measurement of the two quadratures, as a function of the modulation frequency. A decrease of the phase modulation as a function of the modulation frequency is observed and modeled as a thermal low-pass filter. Moreover, at high modulation frequency (higher than 1GHz), where thermal contributions are negligible, it is possible to measure the linewidth enhancement factor, which for our laser is 𝛼 = 0.3. We have also realized this experiment on a harmonic comb laser emitting at 7.9μm, where we could estimate the alpha factor 0.4 < a < 0.9 for 6 distinct optical modes.

The use of continuous wave (CW) laser pairs and optoelectronic devices for difference frequency generation in the terahertz range is an established technique to address broad bandwidths required in CW terahertz spectroscopy systems. However, the resolution in both the generation and detection of terahertz radiation is constrained by the lasers’ linewidth and their inherent frequency fluctuations. For free running lasers, this can lead to a frequency fluctuation on the order of 10 MHz. In addition, these systems commonly rely on lookup tables for the laser settings, exploiting knowledge of the laser diode characteristics to deduce the laser frequencies and therefore the frequencies of the detected or emitted terahertz radiation. However, the use of lookup tables may result in frequency uncertainties of Δf = 1 GHz. To mitigate these frequency fluctuations, active frequency stabilization of the CW lasers can be employed.

In [1], we presented a concept for terahertz spectrum analysis with high frequency precision based on an optical phase-locked loop (OPLL) using a photoconductive antenna for terahertz detection, while stabilizing optoelectronic CW terahertz difference frequency generation via an electronic reference source. Since the use of a single OPLL has similar bandwidth limitation as the electronic reference source, we cascaded multiple CW lasers to increase the usable bandwidth of the terahertz difference frequencies. We demonstrated a laser-locked signal by cascading three lasers with a 500 GHz electronic source, achieving a lock at 1 THz [1]. A promising aspect of our system concept is its compatibility with commercially available vector network analyzers (VNAs). In [2], we implemented the concept as an optoelectronic frequency extender for a commercial VNA and demonstrated its application in free-space VNA measurements.

We present our recent achievements of frequency and phase-stabilization of CW lasers for network analysis scenarios, gaining increasing importance, e.g., for the characterization of high-frequency devices for future 6G bands in the sub-1THz range. Nevertheless, the system still requires various stabilized electronic sources, as well as an additional laser per OPLL, to achieve seamless bandwidth, which is costly. To address these challenges, we are currently pursuing two lines of research. First, a locking solution that involves outcoupling terahertz signals after each multiplier stage in a stabilized electronic multiplier chain for multiple sub-harmonic OPLL reference signals. Second, we are investigating higher stabilized bandwidths

without the exceeding use of multiple electronic sources by four-wave mixing of the stabilized laser outputs

in optical amplifiers, resulting in the generation of additional frequency-locked optical signals. In an optical filter setup, we can then select a combination of optical components to address further stabilized difference frequencies at δf, 2δf and 3δf.