Terahertz Laser Induced Ratchet Effects and Magnetic Quantum Ratchet Effects in Semiconductor Nanostructures

In the framework of this thesis, different magnetic quantum ratchet effects were firstly discovered which were generated by the excitation of dual grating gate samples with terahertz radiation in the presence of a magnetic field. These effects were observed in various CdTe and (Cd,Mn)Te samples with different parameters of the top gate structure. The observed magnetic quantum ratchet currents exhibit sign-changing magneto-oscillations with an amplitude by orders larger than the photocurrent at zero magnetic field. These oscillations are 1/B-periodic and they are in phase with the derivative of the Shubnikov-de Haas oscillations. The current amplitude and direction were controllably changed by the variation of the lateral potential asymmetry parameter, induced by the voltages applied to the thin or wide gate stripes. Moreover, the amplitude and the sign of the current oscillations were altered by the inplane orientation of the electric field in respect to the double grating gate structure for the linear magnetic ratchet effect or by the radiation helicity for the circular magnetic ratchet effect. The photocurrent formation is well described in terms of the theory developed simultaneously to the experiments. Here, the lateral dual grating gate has two tasks: First, it induces a periodical lateral potential acting on the electrons in the quantum well and second, this grating modulates the incident radiation in the near-field and hence in the plane of the two-dimensional electron gas. In fact, the generated photocurrent consists of three different contributions, namely the Seebeck (polarization independent), the linear and the circular magnetic quantum ratchet current. They are caused by different physical principles which will be briefly explained in the following. The polarization independent photocurrent is well described in terms of the semiclassical theory of the Seebeck magnetic ratchet effects. The observed effect is driven by the periodic modulation of the electron temperature in the two-dimensional electron gas which is caused by the local heating induced by the electrical near-field diffraction at the dual grating gate structure. The theory of the Seebeck ratchet effect in the presence of a quantizing magnetic field shows that the ratchet current follows the longitudinal magneto-resistance oscillations and the numerical calculations of the Seebeck ratchet current fit well to the experimental data. The theoretical explanation of the polarization dependent ratchet currents considers the different polarization states of the radiation, the static conductivity tensor and the time-dependent spatially periodic electron density oscillations which are linear in both the electric field and the lateral potential. All considered mechanisms for explaining the magnetic ratchet effect were shown to be of orbital nature. The idea of building a detector based on the ratchet effect to detect different polarization states of the radiation was realized by applying terahertz radiation to dual grating gate InAlAs/InGaAs/InAlAs/InP high electron mobility transistors resulting in a polarization sensitive photocurrent response at room temperature. This work demonstrates that high electron mobility transistors with an asymmetric lateral superlattice of gate fingers with unequal widths and spacing can be applied for generation of a photocurrent caused by linearly and circularly polarized radiation. Although the reported responsivity of field effect transistor detectors is higher than the ones obtained in this work, the measured currents change their amplitude and sign by variation of the polarization states as well as by changing the lateral asymmetry parameter which was not observed up to now. Furthermore, the obtained photocurrents can be proportional to one selected Stokes parameter simply by variation of the voltages applied to the individual gates, offering new possibilities for detecting

Magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with a lateral asymmetric superlattice

We report on the observation of magnetic quantum ratchet effect in (Cd, Mn) Te-and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field applied normal to the quantum well plane. A dc electric current excited by cw terahertz laser radiation shows 1/B oscillations with an amplitude much larger as compared to the photocurrent at zero magnetic field. We show that the photocurrent is caused by the combined action of a spatially periodic in-plane potential and the spatially modulated radiation due to the near-field effects of light diffraction. Magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization and, for (Cd, Mn) Te at low temperatures, from the exchange enhanced Zeeman splitting in diluted magnetic heterostructures. Theoretical analysis, considering the magnetic quantum ratchet effect in the framework of semiclassical approach, describes quite well the experimental results

Circular and linear magnetic quantum ratchet effects in dual-grating-gate CdTe-based nanostructures

Circular and linear magnetic quantum ratchet effects induced by alternating electric fields in the terahertz frequency range have been observed. The ratchet current shows 1/B-periodic oscillations with an amplitude, which is much larger than the photocurrent at zero magnetic field and is sensitive to the orientation of the terahertz electric field (linear ratchet) and to the radiation helicity (circular ratchet). The ratchet effects are detected in (Cd,Mn)Te quantum well structures with dual-grating-gate lateral superlattices. Theoretical analysis performed in the framework of semiclassical approach and taking into account the Landau quantization describes well the experimental data