High-frequency impact ionization and nonlinearity of photocurrent induced by intense terahertz radiation in HgTe-based quantum well structures

Abstract

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/cm 2 . We demonstrate that the probability of the impact ionization is proportional to the exponential function, exp(−E 2 0 /E 2 ), 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.

Impact Ionization Induced by Terahertz Radiation in HgTe Quantum Wells of Critical Thickness

“…Indeed, rising the temperature results in an increase of energy losses due to emission of phonons and, subsequently, in the reduction of radiation induced electron gas heating. Note that the same temperature behavior was previously reported for light impact ionization of HgTe QWs with L w = 5.7 nm [24].…”

“…As it is shown in the inset in Fig. 4, scaling of E 0 with ω, being characteristic for the light impact ionization [4,24], is also detected. At low intensities, however, we find a deviation from this behavior and the signal is determined by the first term in the left-hand side of Eq.…”

“…For electron energies ε lower than the energy of optical phonons ε 0 , the electron distribution function is almost independent of energy and is equal to its value at ε = ε 0 . In the region of higher energies, the distribution function is given by [24] f 0 (ε)…”

“…Such a process has been observed first in bulk InSb crystals and was termed light impact ionization [4,5]. With the development of high-power THz laser systems like molecular lasers, free-electron lasers, and Ti:Sapphire based THz-systems, there has been a steady increase on experimental and theoretical research interest in the field of THz radiation-induced impact ionization, carrier multiplication, and nonperturbative nonlinearities in threeand two-dimensional semiconductor systems [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], for reviews see [25,26]. Recently it has been shown that impact ionization and Auger recombination processes can also be efficiently excited and probed by THz radiation in graphene [27][28][29][30][31].…”

Nonlinear intensity dependence of photogalvanics and photoconductance induced by terahertz laser radiation in twisted bilayer graphene close to magic angle

“…The sample temperature has been controllably varied by using an optical continuous flow cryostat, which allowed to tune the temperature between liquid helium temperature and room temperature. The radiation intensity was modified using a crossed polarizer setup [49,50] and the radiation electric field was varied using half-and quarterwave plates, see Ref. [51].…”