A study of the electro‐optic shielding effect in II/VI semiconductors

Abstract

In this paper the effect of working temperature on the low frequency electro-optic characteristics of some II/VI crystals is studied. It is found that, by heating the modulators above 100 °C, it is possible to overcome the so called field shielding effect, i.e. the strong shielding of the externally applied electric field caused by a local field produced by photo-generated carriers. The presented measurements also show that previously suggested models for the mechanism responsible for the field effect are not valid. A new explanative model is proposed. 1 Introduction Many II/VI compounds like CdTe, CdS, and related alloys, show a high electro-optic figure of merit. This feature can be exploited in semi-insulating compensated samples, which can sustain strong electric fields. Moreover, the above mentioned materials typically show a high transparency in the IR range down to 15 microns, which allowed, for example, cadmium tellurite crystals to be employed as intracavity modulators for CO 2 lasers since the seventies [1]. In more recent years, there has been a renewal of interest towards CdTe crystals, used as electro-optic switches operating in the third window for telecommunications at λ = 1.5 µm [2].However, the electro-optic exploitation of II/VI crystals in the telecommunication range is hindered by the formation of space charge regions in the bulk material, due to carrier photo-generation from intragap levels. This results in decreasing of the electro-optic yield, through the shielding of the externally applied field [3]. This effect is particularly effective whenever the energy of the modulated optical signal approaches the band gap energy and at low modulation frequencies, where some optical switching devices, like for instance cross-connects, should preferably operate [2]. Kaminow [4] proposed an explanation for the shielding effect and linked the speed of response of the phenomenon to the carrier relaxation time, hence to the resistivity of the material. More recently, some of the authors showed that Kaminow's model could properly account for the dependence of the shielding effect on the optical power, provided that not only equilibrium carriers but also the population of photogenerated carriers is accounted, thus extending the model to the high-injection regime [3].The aim of this paper is to show that the shielding effect can be strongly reduced in electro-optic modulators based on some II/VI crystals, by heating the devices to a temperature of 100 °C. This extends effective exploitation of these modulators to the whole optical spectrum below the absorption edge of the material.