Time- and spectrally resolved terahertz photoconductivity of quantum Hall systems

Abstract

We present terahertz photoconductivity measurements on GaAs/ AlGaAs and HgTe/ HgCdTe heterostructures. The photoresponse is investigated time and spectrally resolved under quantum Hall conditions. The samples are excited by a pulsed p-Ge laser, which emits photons of frequencies around 2 THz ͑corresponding to photon energies around 10 meV͒. Corbino-shaped GaAs/ AlGaAs samples show relaxation times down to 10 ns. The dependence of on the applied source-drain voltage is explained by a two-level picture after normalizing the data. All spectrally resolved measurements show contributions of the cyclotron resonance and the bolometric effect. These results are compared to numerical calculations based on a self-consistent Born approximation method. The measurements on HgTe/ HgCdTe samples show comparable results. However, the effective mass in these samples is only m c = 0.026m 0 ͑approximately 1 / 3 of the mass in GaAs/ AlGaAs͒. Thus the cyclotron resonance is shifted to smaller magnetic fields around 2 T. This fact makes HgTe/ HgCdTe systems especially interesting for terahertz detector applications.

Terahertz photoresponse of AlInSb/InSb/AlInSb quantum well structures

“…This is qualitatively similar to our earlier observations. 3 In Fig. 3͑b͒ the photoconductivity for magnetic fields 1.0Ͻ B Ͻ 2.5 T in the range of laser energies 8.8Յ E laser Յ 10.30 meV is shown in a colored plot in comparison to the CR, measured by the transmission of THz waves through the samples in the energy range 8.8Յ E laser Յ 10.30 meV.…”

“…For the generation of THz waves we use a p-Ge laser system. 1,2 Our laser is continuously tunable in the wavelength range 120ՅՅ180 m, 3 by the magnetic field on the Ge crystal.…”

“…3 and 4͒. We use quantum-Hall ͑QH͒ detectors 3 of various materials 3,4 for the detection of the THz radiation. These QH detectors are simultaneously fast ͑re-sponse times less than 10 ns are achievable͒ and spectrally sensitive.…”

“…These QH detectors are simultaneously fast ͑re-sponse times less than 10 ns are achievable͒ and spectrally sensitive. 3 In this study we present observations of the photoresponse ͑PR͒ obtained at samples made from wafers with InSb quantum wells ͑QWs͒ embedded in barriers of AlInSb. We observed the cyclotron resonance ͑CR͒ of electrons in these wafers by the measurement of the transmission of THz waves through the sample as a function of the magnetic field B applied perpendicular to the wafer ͑Faraday configuration͒.…”

The detection of terahertz waves by semimetallic and by semiconducting materials

“…There is a variety of terahertz lasers like gas lasers, 2 quantum cascade lasers, 3 free-electron lasers, 4 or Stokes emission from Sb-doped Si samples. 5 For our studies, we use a p-Ge laser 6,7 which is continuously tunable in the wavelength range of 120 m ՅՅ180 m. 8 The generation of broadband terahertz radiation is far easier as for example by black-body radiation of the temperature of about T Х 20 K ͑heat radiation or far infrared radiation͒. The transmission of terahertz waves is partially obstructed by the strong absorption of photons possessing an energy E ph of about 10 meV by CO 2 and H 2 O molecules which have some rotation modes with corresponding quantum energy differences.…”