### Magnetoresistance oscillations induced by high-intensity terahertz radiation

#### Abstract

We report on observation of pronounced terahertz radiation-induced magnetoresistivity oscillations in AlGaAs/GaAs two-dimensional electron systems, the terahertz analog of the microwave induced resistivity oscillations (MIRO). Applying high-power radiation of a pulsed molecular laser we demonstrate that MIRO, so far observed at low power only, are not destroyed even at very high intensities. Experiments with radiation intensity ranging over five orders of magnitude from 0.1 to ${10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$ reveal high-power saturation of the MIRO amplitude, which is well described by an empirical fit function $I/{\left(1+I/{I}_{s}\right)}^{\beta }$ with $\beta \sim 1$. The saturation intensity ${I}_{s}$ is of the order of tens of watts per square centimeter and increases by a factor of 6 by increasing the radiation frequency from 0.6 to 1.1 THz. The results are discussed in terms of microscopic mechanisms of MIRO and compared to nonlinear effects observed earlier at significantly lower excitation frequencies.

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• Revised 3 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.115449

1. Research Areas
Condensed Matter & Materials Physics

#### Authors & Affiliations

• 1Terahertz Center, University of Regensburg, 93040 Regensburg, Germany
• 2Rzhanov Institute of Semiconductor Physics, 630090 Novosibirsk, Russia
• 3Novosibirsk State University, 630090 Novosibirsk, Russia
• 4Ioffe Institute, 194021 Saint Petersburg, Russia

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#### References

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##### Issue

Vol. 96, Iss. 11 — 15 September 2017

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#### Images

• ###### Figure 1

Magnetic field dependence of the photoconductivity $\mathrm{\Delta }\sigma$ induced by $f=1.07$- and 0.6-THz pulsed radiation in samples B and A (see legend). Oscillation maxima are marked by arrows and numbers denote the closest integer $\epsilon =\omega /{\omega }_{c}$ (oscillation order). Red and blue lines in panel (a) correspond to the data obtained for right- and left-handed circularly polarized radiation, respectively, while the inset sketches the experimental setup.

• ###### Figure 2

Photoconductivity $\mathrm{\Delta }\sigma$ normalized to dark conductivity $\sigma$ as a function of $\epsilon =\omega /{\omega }_{c}$. Panels (a) and (b) present data for samples B and A illuminated by radiation of frequency $f=1.07$ and 0.6 THz, respectively. In addition, the thin black line in panel (b) presents the fit function $\mathrm{\Delta }\sigma /\sigma =0.15-3.5exp\left(-\epsilon /f{\tau }_{q}\right)\epsilon sin\left(2\pi \epsilon \right)$. Here the damping parameter ${\tau }_{q}=1.5$ ps is taken from similar fitting of the low-intensity data, obtained for $I\simeq 0.11$ W/${\mathrm{cm}}^{2}$ on the same sample in Ref. [20]. The values of the effective mass ${m}_{e}$ for both samples, needed to fix the scaling of the $\epsilon =2\pi {m}_{e}f/eB$, are also taken from Ref. [20].

• ###### Figure 3

Normalized photoconductivity $\mathrm{\Delta }\sigma /\sigma$ as a function of $\epsilon =\omega /{\omega }_{c}$ for sample ${\mathrm{D}}_{S}$ illuminated by $f=1.07$-THz laser pulses. Red and blue curves correspond to the right- and left-handed circularly polarized radiation. The upper inset shows the MIRO amplitude ${\mathrm{\Delta }}_{3}{\sigma }_{\text{norm}}={\mathrm{\Delta }}_{3}\sigma \left(x\right)/{\mathrm{\Delta }}_{3}{\sigma }_{\text{max}}$ for oscillation order $\epsilon =3$ as a function of the laser spot position $x$. Here ${\mathrm{\Delta }}_{3}{\sigma }_{\text{max}}$ is the maximal value of ${\mathrm{\Delta }}_{3}\sigma \left(x\right)$. These data were obtained for sample ${\mathrm{D}}_{L}$ and $I=500$ W/${\mathrm{cm}}^{2}$. The measurement setup is shown in the lower inset: The beam spot with a radius $d/2\approx 1.25$ mm being smaller than ${r}_{o}=4.25$ mm (but larger than ${r}_{i}=0.25$ mm) is scanned across the Corbino disk. The line in the upper inset is a guide for the eye.

• ###### Figure 4

Normalized photoconductivity $\mathrm{\Delta }\sigma /\sigma$ as a function of $\epsilon =\omega /{\omega }_{c}$ for sample B excited by $f=1.07$-THz radiation with different intensity as marked.

• ###### Figure 5

Intensity dependence of the reduced oscillation amplitude ${\mathrm{\Delta }}_{3}\sigma /I$ at the oscillation order $\epsilon =3$ for sample B (see definition in the text). The hexagon presents the result of Ref. [20] obtained using a low-power cw laser operating at $f=0.69$ THz. Other data points correspond to excitation by terahertz laser pulses at three different frequencies. The inset shows intensity dependence of the non-normalized oscillation amplitude ${\mathrm{\Delta }}_{3}\sigma$ in a linear-linear plot. Solid lines are fits using ${\mathrm{\Delta }}_{3}\sigma /I\propto {\left(1+I/{I}_{s}\right)}^{-1.3}$. Saturation intensities used for the fits are indicated by arrows and are given by ${I}_{s}=15$ W/${\mathrm{cm}}^{2}$ ($f=0.6$ THz), ${I}_{s}=45$ W/${\mathrm{cm}}^{2}$ ($f=0.78$ THz), and ${I}_{s}=85$ W/${\mathrm{cm}}^{2}$ ($f=1.07$ THz). The dashed line in the inset corresponds to a linear fit ${\mathrm{\Delta }}_{3}\sigma \propto I$.

• ###### Figure 6

Intensity dependence of the reduced oscillation amplitude ${\mathrm{\Delta }}_{N}\sigma /I$. Solid lines are fits using ${\mathrm{\Delta }}_{3}\sigma /I\propto {\left(1+I/{I}_{s}\right)}^{-\beta }$. Panel (a) presents ${\mathrm{\Delta }}_{N}\sigma /I$ for different oscillation orders, $N=3$, 4, and 5, measured on sample B under $f=1.07$-THz illumination. Solid line is the fit using $\beta =1.3$ and ${I}_{s}=85$ W/${\mathrm{cm}}^{2}$. Panel (b) presents ${\mathrm{\Delta }}_{3}\sigma /I$ measured on sample A under $f=0.78$- and 1.07-THz radiation. The solid lines are fits using $\beta =1$ and saturation intensities ${I}_{s}=30\left(190\right)$ W/${\mathrm{cm}}^{2}$ for $f=0.78\left(1.07\right)$ THz.

• ###### Figure 7

The background part $\mathrm{\Delta }{\sigma }_{\text{bg}}$ of the photoconductivity normalized to the radiation intensity $I$ measured for sample B at three radiation frequencies. Dashed lines are guide for the eye.

• ###### Figure 8

Terahertz radiation-induced photoconductivity as a function of magnetic field as measured in sample B in the dark at $I=300$ W/${\mathrm{cm}}^{2}$ and after illumination with room light at $I=40$ W/${\mathrm{cm}}^{2}$. The data for the dark conditions are obtained at higher intensities to enable resolving of MIRO. The inset shows corresponding magnetotransport data.

• ###### Figure 9

Magnetic field dependence of the photoinduced voltage drop signal $\mathrm{\Delta }U$ (panel a) and the photoconductivity $\mathrm{\Delta }\sigma$ (panel b), both normalized to radiation intensity $I$, for three samples ${\mathrm{G}}_{L},\phantom{\rule{0.16em}{0ex}}{\mathrm{G}}_{M}$, and ${\mathrm{G}}_{S}$. The samples are made from the same wafer but have different inner Corbino radius, ${r}_{i}$, equal to 2.5, 1.5, and 0.5 mm, respectively.

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