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The experimental expertise and equipements of the MIL Group is divided in two main parts :
- Crystal growth equipments and materials
- Linear, Nonlinear Spectroscopy techniques
Crystal growth equipments and materials
The group specialized in the growth of crystals with low phonon energies such as fluorides, chlorides and bromides, by using standard techniques such as the Czochralski and Bridgman techniques as well as, more recently, the LPE (Liquid Phase Epitaxy) technique, to grow crystals in the form of layers of several µm thickness. More specific efforts are presently devoted to the growth and the implementation of bulk crystals and LPE layers of rare-earth doped fluorides for the RGB and high-power laser applications.
Contact : Abdelmjid Benayad, Gurvan Brasse
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Linear, Nonlinear Spectroscopy techniques
In addition to the standard spectroscopy techniques, the group specialized in the measurements of ESA (Excited-State Absorption) spectra and refractive index variations resulting from polarisability changes in highly pumped materials.
ESA spectra are measured by using two kinds of techniques.
The first technique is a double lock-in amplifier technique which consists in extracting an excited-state difference spectrum through the measurement of the intensity of a weak probe beam chopped at a high frequency (typically 1kHz) and transmitted by the sample while it is pumped and unpumped by a pump beam chopped at a low frequency (typically 10Hz). This technique is preferentially used for rare-earth doped materials in which the absorbing state is a metastable level with more than a few microseconds lifetime and for which the 4f-4f optical transitions have relatively low oscillator strengths.
The second technique consists again in recording some ESA difference spectrum, but resulting from pulsed pump and probe light sources. Details on this time-resolved pump-probe technique and on the procedure which is used to extract the data can be found in Refs 2. With this second technique, ESA difference spectra can be registered up to about 180 nm. It is preferentially used for materials doped with transition-metal ions and to investigate near-UV charge-transfer and 4f-5d interconfigurational transitions with relatively high oscillator strengths.
Refractive index changes which can be observed in highly pumped laser materials due to thermal and/or excited-state population lensing effects are measured and discriminated by using two kinds of pump-probe techniques (Ref 3).
The first technique is based on a Jamin-Lebedev interferometer. It consists in measuring the transient change in the interference signal given by a weak He-Ne probe beam when it passes through the crystal under investigation along two different optical paths, one being pumped by a nanosecond pulsed Ti-Sapphire or a QCW fiber-coupled diode laser.
The second technique is a pump-probe transient grating technique. A nanosecond pulsed Ti-sapphire laser is split in two beams with the aid of the adequate transmission phase grating and the two beams (corresponding to the +1 and -1 orders of this grating) are focused inside the crystal to form a transient (thermal and excited-state population) refractive index grating. The associated refractive index change, the discrimination between the thermal and purely electronic contributions, and the underlying polarisability change are deduced by analysing the dynamics and measuring the efficiency of the weak He-Ne probe beam which is diffracted by the pump induced grating.
While both techniques give the same information for 3-levels laser systems as Yb-doped laser materials, they give complementary data in the case of 4-levels laser systems as Nd-doped laser materials.