## 4 Discussion  
A few paths led us to the study described in this paper. The first one was to enlarge our previous research on plasmonic excitations in modulation doped single CdTe/CdMgTe quantum wells [23; 24] to more complicated system. We decided to study a system with two QWs separated by a large distance of about 44 nm. Thus, we were not looking for effects of interaction of the wells. This distance is at least a few times bigger than that typically considered in Coulomb drag experiments [35]. In fact, in results of our measurements, we did not observe any feature which could be attributed to interaction of these two conducting layers at THz frequencies. Second, we wanted to follow an idea put forward long ago by [PERSON] [36] who suggested that replacement of typically used metallic grating by a grating composed of quantum wires would lead to amplification of a plasmonic instability. The main argument behind this idea was to lower the frequency of plasmonic excitations in the grating towards the frequency of plasmonic waves in the quantum well. This could be achieved only by an essential reduction of concentration of electrons in the grating, i.e., by replacement a metal with a doped semiconductor. A practical realization of such a \"quantum wire\" grating leads to a structure with two quantum wells where the upper one is etched to form stripes with a 2 DEG. A relatively big separation of the two QWs in wafer B resulted also from the idea of using the upper etched QW only as a \"wire grating\" for the lower QW. The most important result which could be addressed in connection with this reasoning is presented in Figure 8 which compares transmission spectra obtained on samples with one QW and two QWs on which gratings of a period of 8 \(\upmu\)m were etched (wafers A and B, respectively). The depth of etching of sample from both the wafer A (single QW) and B (two QWs) was 40 nm. The depth of etching in the case of the sample with two QW was chosen to not to depopulate the lower QW but to form conducting wires from the upper QW (compare Figure 6). Comparison of these two curves (after normalization) shows that the main difference is the width of the line which is essentially larger in the case of the sample with two QWs. Additionally, the amplitude of MP resonances (in absolute values, normalized to the power of laser only) in the case of two QW is about five times smaller than in the case of the single QW. This could result from a greater disorder in the case of two QWs with one of them cut by etching or from a generally lower concentration \(n_{s}\) in samples after etching. Although this result does not follow the expectations related to [PERSON] [36], we think that more research is necessary in this direction. In particular, one could think about etching samples with different separation of the QWs or to study other materials which would not show so strong depopulation of 2 DEG with etching. Figure 8: Transmission spectra of samples with etched grating of the period equal to 8 \(\upmu\)m: single and two quantum wells. The wavelenght of radiation was 118.8 \(\upmu\)m. The vertical bar indicated position of the CR in unprocessed sample. The fact that one obtains sharp plasmonic resonances in the case of etched grating on a single QW makes questionable the procedure of preparing a \"wire grating\" by etching the upper QW in a sample with two wells. The results show that plasmonic resonances appear just in a single QW and do not need conducting stripes. That is why, we suggest that to follow the idea presented in [PERSON] [36], one should look for other solutions. For example, one could try to process a grating made of graphene or a similar two-dimensional conducting material with concentration of electrons comparable to that of a 2 DEG in the underlying quantum well. Third, we aimed at a broad characterization of studied samples as resonant detectors of THz radiation which would be appropriate to work at high magnetic fields and low temperatures.