Physical properties of chalcogenide glasses in the AsxSe1$-${}x system have been measured as a function of composition including the Young's modulus E, shear modulus G, bulk modulus K, Poisson's ratio $\nu${}, the density $\rho${}, and the glass transition Tg. All these properties exhibit a relatively sharp extremum at the average coordination number ⟨r⟩=2.4. The structural origin of this trend is investigated by Raman spectroscopy and nuclear magnetic resonance. It is shown that the reticulation of the glass structure increases continuously until x=0.4 following the ``chain crossing model'' and then undergoes a transition toward a lower dimension pyramidal network containing an increasing number of molecular inclusions at x>0.4. Simple theoretical estimates of the network bonding energy confirm a mismatch between the values of mechanical properties measured experimentally and the values predicted from a continuously reticulated structure, therefore corroborating the formation of a lower dimension network at high As content. The evolution of a wide range of physical properties is consistent with this sharp structural transition and suggests that there is no intermediate phase in these glasses at room temperature.

Telluride glasses possess the widest infrared window of all amorphous materials and are key to a number of long-wavelength applications such as bio-sensing. However they are not intrinsically good glass formers and require significant materials engineering for device fabrication. Strategies for stable glass engineering are presented and the fabrication of far infrared optical fibers is described. A new type of optical sensor based on electrophoretic capture of protein is also presented. This sensor is based on a conducting telluride glass which can act as both a capture electrode and an infrared optical element for collecting vibrational signatures of target molecules such as proteins.

A series of physical properties have been measured throughout and above the glass transition for the whole AsxSe1$-${}x system, including the activation for viscous flow E$\eta${}, the activation energy for enthalpy relaxation EH, and the activation energy for structural relaxation Ea obtained by specific heat spectroscopy. All properties show a double minimum at an average coordination number $\langle${}r$\rangle${} = 2.3 and $\langle${}r$\rangle${} = 2.5 with a local maximum at $\langle${}r$\rangle${} = 2.4. This is in stark contrast to the physical properties previously measured on the same samples at room temperature and which instead show a single minimum centered at $\langle${}r$\rangle${} = 2.4. The observed trend is consistent with the dimensionality of the network derived from structural data obtained by nuclear magnetic resonance. An analysis of the complex heat capacity also reveals a bimodal relaxation process in As-rich glasses, which explains why they are kinetically fragile but appear thermodynamically strong. Finally, these results demonstrate that previous observations of an ``intermediate phase'' in AsxSe1$-${}x glasses near $\langle${}r$\rangle${} = 2.3 is associated with the high temperature behavior of the glassy network and should be interpreted in terms of the temperature dependence of structural constraints rather than the number of constraints in the room-temperature glass.