No abstract

The trade-off between the spectral bandwidth and average output power from chalcogenide fiber-based mid-infrared supercontinuum sources is one of the major challenges towards practical application of the technology. In this paper we address this challenge through tapering of large-mode-area chalcogenide photonic crystal fibers. Compared to previously reported step-index fiber tapers the photonic crystal fiber structure ensures single-mode propagation, which improves the beam quality and reduces losses in the taper due to higher-order mode stripping. By pumping the tapered fibers at 4 μm using a MHz optical parametric generation source, and choosing an appropriate length of the untapered fiber segments, the output could be tailored for either the broadest bandwidth from 1 to 11.5 μm with 35.4 mW average output power, or the highest output power of 57.3 mW covering a spectrum from 1 to 8 μm.

Stable bulk glasses from the quaternary system Cu-As-Te-Se are investigated for thermoelectric applications. These materials exhibit a low thermal conductivity k $ 0.3 W K À1 m À1 which is appealing for raising the thermoelectric figure of merit ZT. The addition of small amounts of selenium within the telluride amorphous matrix plays two fundamental roles. First, the increased disorder associated with the size mismatch improves glass-formation and widens the glass-formation domain, and second, it increases phonon scattering and slightly decreases the thermal conductivity. Furthermore, the addition of copper up to 32% dramatically increases the electrical conductivity without notably affecting the thermal conductivity. This permits us to obtain bulk glass samples with promising thermoelectric properties, which could be manufactured through conventional low-cost glass casting methods. While addition of copper permits the increase of electrical conductivity by more than six orders of magnitude, another three orders of magnitude are required to obtain thermoelectric materials with competitive ZT.Nevertheless, predicted values of ZT > 1.2 are estimated which would constitute some of the highest reported figure of merit for a bulk solid at room temperature. The effect of glass annealing on thermoelectric properties is also discussed.

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2  dB at 1550 nm in the case of free-space in-coupling, which is ∼7  dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.