## 1 Introduction
In the case of GaAs and shallow impurity transitions, the response is composed of a few lines corresponding to different transitions which are spread over quite a large range of \(B\), typically of a few Tesla [10]. This makes interpretation of measured spectrum very difficult. A high quality GaAs, typically required to fabricate a CR photocurrent detector, shows a complicated response as a function of magnetic field which is caused by Shubnikov-de Haas oscillations [12]. To decrease the amplitude of these oscillations one has to increase the temperature of the detector which decreases its response at the CR. In the case of InSb, a very strong magnetoresistance, resulting from a small effective mass of electrons in this material, makes these detectors non-resonant for frequencies less than about 1 THz which corresponds to a CR transition in InSb at 0.5 T [11]. On the other hand, at magnetic fields of a few T (i.e., not very high in typical THz experiments), the resistance of InSb is so high that its application as a detector becomes questionable because it is short-circuited by the input impedance of a lock-in amplifier. These remarks allow us to conclude that are no ideal magnetic-field-tuneable detectors and looking for new solutions is justified. Results of the present paper indicate that detectors based on CdTe quantum wells can be an interesting option in some cases. Cadmium Telluride is a direct band-gap semiconductor of a zinc-blend structure which has been used for decades in basic studies and applications. This compound possesses a number of distiguishing properties. For instance, it is an ideal platform to study the electron-phonon interaction in solids due to a high Frohlich constant, equal to 0.286 [13], the highest one among semiconductors. Also, bulk CdTe crystals are used as nuclear detectors due to their exceptional stopping power of gamma rays [14]. Broad studies have been related to mixed crystals of CdTe and other II-VI compounds. Begginnings of this research date back to the middle of the last century, and most of the fundamental properties of these materials were discovered and described a few decades ago [15]. However, the interest in such crystals has outbursted recently due to their relation to the Dirac matter and topological insulators [16; 17; 18]. The physics of CdTe-based low-dimensional structures is equally extremely rich. It covers such areas of research as a giant spin splitting in Mn-doped quantum wells (QWs) [19], spectroscopy of single magnetic impurities in quantum dots [20] or quantum transport of a high electron mobility 2 DEG [21]. This research was the subject of many review papers and it will not be here described in detail [22]. On the other hand, interaction of THz radiation with CdTe-related materials, bulk or two-dimensional, did not attract a large interest in the past but this situation has been changing because observation of quantum effects in a 2 DEG in CdTe-based QWs became possible due to technological developments of the growth of the quantum structures [21]. A sufficiently high mobility of electrons allowed for observation of the CR and magnetoplasmon (MP) excitaitions in such high quality samples [23; 24]. These excitations can be observed at THz frequencies at quantizing magnetic fields. In our recent studies [23; 24] we concentrated on plasmonic excitations in CdTe/CdMgTe structures with one modulation doped QW. A metallic grating prepared on the surface of samples allowed to excite MPs with monochromatic THz radiation. We determined the dispersion relation of MPs and showed how it is influenced by a strong Frohlich interaction. Also, we showed a nonlinear dependence of the CR frequency on the magnetic field which appear at high \(B\) due to the polaron effect [25; 26]. In the present paper, we consider CdTe-based structures with one or two QWs and with metallic or etched gratings.