Chalcogenide glasses (ChGs) are an important class of amorphous semiconductors used in phase-change memories, solar cells, sensors and photonics. These glasses are highly nonlinear and have been shown to have a range of unique optical properties such as high transmission in the Mid-IR, high linear refractive indices (2-3 at 1550) and high nonlinear refractive indices n₂ (about 500 times the value for silica). The femtosecond response time of their nonlinearity makes them suitable for ultra-fast all optical processing.

In this Optical Materials Express paper, Wang and colleagues report the measurement of the third order optical nonlinearity of 51 different ChGs in the near-IR. In order to quantify the third order nonlinearities the team used the Z-scan technique. This technique is based on the principle of spatial beam distortion and it measures both the sign and magnitude of the third order optical nonlinearities. The authors demonstrate that the experimental data conform very well to the semi-empirical Miller’s rule, a relation between linear and nonlinear susceptibility. The so-called Miller’s coefficient depends only on the linear refractive index n₀, thus providing a very simple and useful way of predicting the nonlinear refractive index n₂ of ChGs. However, the uncertainties of the measurements of n₂ are high, almost up to ±16%, and this limits the precision with which Miller’s rule can be verified.

The frequency dependence of the nonlinear absorption β and refraction n₂ coefficients is studied by utilising two different models, which describe respectively the case of direct and indirect-gap semiconductors. The experimental data are in better agreement with the indirect gap-semiconductor model. These models predict the values of β and n₂ as functions of the normalised photon energy; as both n₂ and β tend to grow together, the models’ predictions define an upper limit for the nonlinearity above which two-photon absorption (2PA) becomes significant. In other words, the models predict a threshold below which 2PA is not significant, which is a highly desirable material property for telecommunications applications. The models require two parameters, n₀ and E_g as opposed to the simpler Miller’s rule which requires the knowledge of onlyn₀. Furthermore, E_g needs to be determined accurately as the scaling of β and n₂ is proportional to E_g³ and E_g⁴ respectively. As a result even the slightest error in the estimation of E_g will lead to large uncertainties in the values of of β and n₂. The authors compare their measurements to both models, find that the data are closer to the indirect-gap semiconductor model, and discuss possible reasons for the differences between the data and the model predictions.

Another fact that the measurements highlight is that by substituting more polarisable elements, such as Se for S and Sb for As, the nonlinearity of these glasses increases, up to almost four times in some occasions. The main drawbacks that accompany this increase in nonlinearity are a lower laser damage threshold, reduced glass transition temperatures, increased two-photon absorption, and large photosensitivity.

The addition of other glass-forming elements can help in compensating some of the undesirable changes induced in ChGs when replacing elements aiming for higher polarisability. For example the addition of four-fold co-ordinated Ge helps in raising the glass transition temperatures and creates a glass system that has an exceptionally wide glass-forming range.

This work points to the direction of substituting for more polarisable elements in ChGs, in combination with the addition of a range of glass-forming elements, in order to tune the properties of the glasses. In this way, one hopes to provide a combination of the best nonlinearity and material properties with a view to Chalcogenide-based next-generation devices for telecommunications and photonics applications.

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