A new setup for probing condensed matter in the Far IR to THz ranges at sub-Kelvin temperatures on the AILES beamline A new spectroscopic setup dedicated to the development is needed, including the implementation of optical optical study of materials in the sub-Kelvin apertures to permit the light beam to probe the sample. The source provided by AILES beamline is characterized by a high temperature range, is now available on the brilliance in the THz and FIR spectral ranges [4], as well as a AILES beamline of synchrotron SOLEIL. small divergence, allowing a minimal source optical étendue This ensemble, based on an adiabatic ideally suited to be coupled to such cooling ensemble. demagnetization refrigerator, has been THE REQUIREMENTS designed for spectroscopy in the far-infrared The adaptation of this sub-Kelvin cryostat to optical measurements and terahertz domains. The combination requires modifications at three levels: mechanical, optical and of the cryogenic set-up with synchrotron cryogenic. These developments are interdependent and have radiation allows for reflectivity and been considered simultaneously. The main specifications are described below: transmission measurements down to 95mK - The sample must be able to reach temperatures down to and 144mK, respectively, with reproducibility 250 mK for a time sufficient to record the required spectra, better than 0.5%. with a temperature varying no more than 5%. - The vacuum must be sufficient to limit condensation of ice or atmospheric species on the sample. A vacuum of 10 mbar or-5 For studying condensed matter, the fine control of physical less is required before starting the cooling, which then allows parameters such as temperature or pressure is required to cryo-pumping on the cryogenerator upper parts. explore different structures of a phase diagram or to verify some- The spectral range of the light reaching the sample has theoretical models. When this control is extended towards sub-to be adjustable and filtered to the region of interest for Kelvin temperatures, remarkable quantum effects can occur in the spectroscopic study. For the phenomena targeted in materials, such as transitions to superconducting states or thecondensed matter, most of the excitations occurring at sub- appearance of a quantum magnetic ordering. In these two cases,Kelvin temperatures are expected in the THz or FIR range, the low energy involved makes infrared (IR) and terahertz (THz)defined here as 0 – 100 cm and 0 – 600 cm, respectively.-1 -1 spectroscopies well suited to probe these excitations [1, 2]. The- The beam focal point must be at most 2 mm diameter at the control of the temperature is also important to characterize thesample plane with a cone opening of about 30°. optical properties of detectors having an interest for astronomical- Two different optical configurations are required, to perform applications [3]. Indeed, in many space missions, the detectoreither reflectivity measurements or transmission measurements must be cooled down to sub-Kelvin temperature in order to on transparent materials. Also, aligning in situ the optical beam reduce significantly the thermal noise, essential to study thewith either visible part of the SR beam or an external light universe background radiation or low intensity THz sources. source using a camera should be possible. THE APPROACH TESTS AND PERFORMANCES To reach this sub-Kelvin temperature range, two methods are The ADR cryogenerator has a cyclic operation and, as it does not exploited: dilution of 3He in 4He or adiabatic demagnetizationprovide continuous cooling power, no constant “measurement refrigerators (ADR). For the present development, the latter,temperature” can be maintained. Rather it is realistic to define ADR technology has been chosen, as it presents several cruciala “hold time” during which a limited temperature drift can be advantages in terms of size, operation costs and simplicity guaranteed. We thus rather arbitrarily define our hold time as the of operation. The ADR uses a magnetic cycle to cool down time during which data can be collected within a temperature samples at temperatures as low as 50 mK. In order to couple adrift of 2.5 mK, half the absolute accuracy of our temperature commercial ADR instrument to a spectrophotometer, an importantsensing system. 80