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.
•The linear and circular magnetic quantum ratchet effects in semiconductor structures with a lateral asymmetry were observed.•The polarization dependent magneto-ratchet currents show 1/B-oscillations with giantly enhanced amplitude as compared to B = 0.•Sign and amplitude of the linear ratchet currents are controlled by the orientation of radiation electric field orientation.•Sign of the circular ratchet currents reverses upon switching the photon helicity.•A developed theory shows that the ratchet currents are caused by the Dynamic Carrier-Density Redistribution mechanism.
We report on the observation of the magnetic quantum ratchet effect in graphene with a lateral dual-grating top gate (DGG) superlattice. We show that the THz ratchet current exhibits sign-alternating ...magneto-oscillations due to the Shubnikov–de Haas effect. The amplitude of these oscillations is greatly enhanced as compared to the ratchet effect at zero magnetic field. The direction of the current is determined by the lateral asymmetry which can be controlled by variation of gate potentials in DGG. We also study the dependence of the ratchet current on the orientation of the terahertz electric field (for linear polarization) and on the radiation helicity (for circular polarization). Notably, in the latter case, switching from right- to left-circularly polarized radiation results in an inversion of the photocurrent direction. We demonstrate that most of our observations can be well fitted by the drift-diffusion approximation based on the Boltzmann kinetic equation with the Landau quantization fully encoded in the oscillations of the density of states.
We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear ...energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency
ω
and momentum relaxation time
τ
l
larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to
exp
(
−
E
0
2
/
E
2
)
, with the radiation electric field amplitude
E
and the characteristic field parameter
E
0
. As observed in experiment, it exhibits a strong frequency dependence for
ω
τ
≫ 1 characterized by the characteristic field
E
0
linearly increasing with the radiation frequency
ω
.
We report on a strong nonlinear behavior of the photogalvanics and photoconductivity under excitation of HgTe quantum wells (QWs) by intense terahertz (THz) radiation. The increasing radiation ...intensity causes an inversion of the sign of the photocurrent and transition to its superlinear dependence on the intensity. The photoconductivity also shows a superlinear raise with the intensity. We show that the observed photoresponse nonlinearities are caused by the band-to-band light impact ionization under conditions of a photon energy less than the forbidden gap. The signature of this kind of impact ionization is that the angular radiation frequency ω=2πf is much higher than the reciprocal momentum relaxation time. Thus the impact ionization takes place solely because of collisions in the presence of a high-frequency electric field. The effect has been measured on narrow HgTe/CdTe QWs of 5.7 nm width; the nonlinearity is detected for linearly and circularly polarized THz radiation with different frequencies ranging from f=0.6 to 1.07 THz and intensities up to hundreds of kW/cm2. We demonstrate that the probability of the impact ionization is proportional to the exponential function, exp(−E02/E2), of the radiation electric field amplitude E and the characteristic field parameter E0. The effect is observable in a wide temperature range from 4.2 to 90 K, with the characteristic field increasing with rising temperature.
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.
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