Julie Carreaud

Gain structuration in dual-wavelength Nd:YSAG ceramic lasers

We demonstrate a dual-wavelength Nd:YSAG ceramic laser in which the gain volume is structurated into two different regions providing gain at the wavelength of 1061 nm and 1064 nm respectively. We discuss the role of the nonuniform distribution of the temperature in structurating the gain region via the Boltzmann effect. We show that the two laser wavelengths can be switched by adjusting the size of the pump beam or by slightly modifying the geometrical parameters of the laser cavity, either the length of the cavity or the orientation of a mirror. Additionally, we demonstrate that the transverse modes at the two wavelengths are shaped according to the effect of gain filtering caused by the structuration of the gain region.

High thermoelectric performance in Sn-substituted α-As2Te3

Lead chalcogenides PbX (X = Te, Se, S) have been the materials of choice for thermoelectric power generation at mid-range temperatures (500–700 K). Here, we report on a new family of chalcogenides α-As2Te3 that exhibit similar thermoelectric performances near 500 K. Sn doping in p-type polycrystalline α-As2−xSnxTe3 (x ≤ 0.075) provides an efficient control parameter to tune the carrier concentration leading to thermopower values that exceed 300 μV K−1 at 500 K. Combined with the structural complexity of the monoclinic lattice that results in extremely low thermal conductivity (0.55 W m−1 K−1 at 523 K), a peak ZT value of 0.8 is achieved at 523 K for x = 0.05. A single-parabolic band model is found to capture well the variations in the transport properties with the Sn concentration and suggests that higher ZT values could be achieved through band structure engineering. These results surpass those obtained in the sister compounds β-As2−xSnxTe3 and further show that α-As2Te3 based materials are competitive with other chalcogenides for thermoelectric applications at intermediate temperatures.

Electronic structure, low-temperature transport and thermodynamic properties of polymorphic β-As2Te3

β-As2Te3 belongs to the prominent family of Bi2Te3-based materials, which show excellent thermoelectric properties near room temperature. In this study, we report a joint theoretical and experimental investigation of its electronic and thermal properties at low temperatures (5–300 K). These results are complemented by specific heat measurements (1.8–300 K) that provide further experimental evidence of the first order lattice distortion undergone by β-As2Te3 near 190 K. Data taken on cooling and heating across this transition show that the lattice distortion has little influence on the electronic properties and further evidence a weak hysteretic behavior. Although first-principles calculations predict a semiconducting ground state, these measurements show that β-As2Te3 behaves as a degenerate p-type semiconductor with a high carrier concentration of 1020 cm−3 at 300 K likely due to intrinsic defects. Calculations of the vibrational properties indicate that the extremely low lattice thermal conductivity values (0.8 W m−1 K−1 at 300 K) mainly originate from low-energy Te optical modes that limit the energy window of the acoustic branches. This limited ability to transport heat combined with a relatively large band gap suggest that high thermoelectric efficiency could be achieved in this compound when appropriately doped.