Cyclotron resonance photoconductivity of a two-dimensional electron gas in HgTe quantum wells

Far-infrared cyclotron resonance photoconductivity (CRP) is investigated in HgTe quantum wells (QWs) of various widths grown on (013) oriented GaAs substrates. It is shown that CRP is caused by the heating of two-dimensional electron gas (2DEG). From the resonance magnetic field strength effective masses and their dependence on the carrier concentration is obtained. We found that the effective mass in each sample slightly increases from the value (0.0260 \pm 0.0005)m_0 at N_s = 2.2x10^11 cm^(-2) to (0.0335 \pm 0.0005)m_0 at N_s = 9.6x10^11 cm^(-2). Compared to determination of effective masses by the temperature dependence of magnitudes of the Shubnikov-de Haas (SdH) oscillations used so far in this material our measurements demonstrate that the CRP provides a more accurate (about few percents) tool. Combining optical methods with transport measurements we found that the transport time substantially exceeds the cyclotron resonance lifetime as well as the quantum lifetime which is the shortest.Comment: 3 pages, 2 figure

High intensity study of THz detectors based on field effect transistors

Terahertz power dependence of the photoresponse of field effect transistors, operating at frequencies from 0.1 to 3 THz for incident radiation power density up to 100 kW/cm^2 was studied for Si metal-oxide-semiconductor field-effect transistors and InGaAs high electron mobility transistors. The photoresponse increased linearly with increasing radiation power up to kW/cm^2 range. The saturation of the photoresponse was observed for all investigated field effect transistors for intensities above several kW/cm^2. The observed signal saturation is explained by drain photocurrent saturation similar to saturation in direct currents output characteristics. The theoretical model of terahertz field effect transistor photoresponse at high intensity was developed. The model explains quantitatively experimental data both in linear and nonlinear (saturation) range. Our results show that dynamic range of field effect transistors is very high and can extend over more than six orderd of magnitudes of power densities (from 0.5 mW/cm^2 to 5 kW/cm^2)

Highly superlinear giant terahertz photoconductance in GaAs quantum point contacts in the deep tunneling regime

A highly superlinear in radiation intensity photoconductance induced by continuous wave terahertz laser radiation with low intensities has been observed in quantum point contacts made of GaAs quantum wells operating in the deep tunneling regime. For very low values of the normalized dark conductance Gdark/G0≈10−6, with the conductance quantum G0=2e2/h, the photoconductance scales exponentially with the radiation intensity, so that already at 100mW/cm2, it increases by almost four orders of magnitude. This effect is observed for a radiation electric field oriented along the source drain direction. We provide model considerations of the effect and attribute it to the variation of the tunneling barrier height by the radiation field made possible by local diffraction effects. We also demonstrate that cyclotron resonance due to an external magnetic field manifests itself in the photoconductance, completely suppressing the photoresponse

Infrared photoresistance as a sensitive probe of electronic transport in twisted bilayer graphene

We report on observation of the infrared photoresistance of twisted bilayer graphene (tBLG) under continuous quantum cascade laser illumination at a frequency of 57.1 THz. The photoresistance shows an intricate sign alternating behavior under variations of temperature and back gate voltage, and exhibits giant resonance-like enhancements at certain gate voltages. The structure of the photoresponse correlates with weaker features in the dark dc resistance reflecting the complex band structure of tBLG. It is shown that the observed photoresistance is well captured by a bolometric model describing the electron and hole gas heating, which implies an ultrafast thermalization of the photoexcited electron–hole pairs in the whole range of studied temperatures and back gate voltages. We establish that photoresistance can serve a highly sensitive probe of the temperature variations of electronic transport in tBLG

Nonlinear intensity dependence of ratchet currents induced by terahertz laser radiation in bilayer graphene with asymmetric periodic grating gates

We report on the observation of a nonlinear intensity dependence of the terahertz radiation induced ratchet effects in bilayer graphene with asymmetric dual grating gate lateral lattices. These nonlinear ratchet currents are studied in structures of two designs with dual grating gate fabricated on top of encapsulated bilayer graphene and beneath it. The strength and sign of the photocurrent can be controllably varied by changing the bias voltages applied to individual dual grating subgates and the back gate. The current consists of contributions insensitive to the radiation's polarization state, defined by the orientation of the radiation electric field vector with respect to the dual grating gate metal stripes, and the circular ratchet sensitive to the radiation helicity. We show that intense terahertz radiation results in a nonlinear intensity dependence caused by electron gas heating. At room temperature the ratchet current saturates at high intensities of the order of hundreds to several hundreds of kWcm$^{-2}$. At $T = 4 {\rm K}$, the nonlinearity manifests itself at intensities that are one or two orders of magnitude lower, moreover, the photoresponse exhibits a complex dependence on the intensity, including a saturation and even a change of sign with increasing intensity. This complexity is attributed to the interplay of the Seebeck ratchet and the dynamic carrier density redistribution, which feature different intensity dependencies and a nonlinear behavior of the sample's conductivity induced by electron gas heating. Our study demonstrates that graphene-based asymmetric dual grating gate devices can be used as terahertz detectors at room temperature over a wide dynamic range, spanning many orders of magnitude of terahertz radiation power. Therefore, their integration together with current-driven read-out electronics is attractive for the operation with high-power pulsed sources.Comment: 11 pages, 13 figure

Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi, Sb) Te based three-dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow us to determine the room temperature carrier mobilities in the surface states despite the presence of thermally activated residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30 and 60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity

Giant Terahertz Photoconductance of Quantum Point Contacts in the Tunneling Regime

We report on the observation of the giant photoconductance of a quantum point contact (QPC) in the tunneling regime excited by terahertz radiation. Studied QPCs are formed in a GaAs/(Al, Ga) As heterostructure with a high-electron-mobility two-dimensional electron gas. We demonstrate that irradiation of strongly negatively biased QPCs by laser radiation with frequency f = 0.69 THz and intensity 50mW/cm(2) results in two orders of magnitude enhancement of the QPC conductance. The effect increases with the dark conductivity decrease. It is also characterized by a strong polarization dependence and a drastic reduction of the signal by increasing the radiation frequency to 1.63 THz. We demonstrate that all experimental findings can be well explained by the photon-assisted tunneling through the QPC. Corresponding calculations are in good agreement with the experiment

Detection of highly conductive surface electron states in topological crystalline insulators Pb1−xSnxSe using laser terahertz radiation

We suggest a method for detection of highly conductive surface electron states including topological ones. The method is based on measurements of the photoelectromagnetic effect using terahertz laser pulses. In contrast to conventional transport measurements, the method is not sensitive to the bulk conductivity. The method is demonstrated on an example of topological crystalline insulators Pb1−xSnxSe. It is shown that highly conductive surface electron states are present in Pb1−xSnxSe both in the inverse and direct electron energy spectrum