Response of a Bloch oscillator to a THz‐field

Abstract

Abstract. In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the TH2-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.Keywords: Superlattice; Bloch oscillator; 'hahertz radiation.According to Bloch [I], the electric conductivity by electrons moving in a periodic potential of atoms in a crystal superimposed with a potential of a homogeneous dc electric field is a consequence of scattering of the electrons due to defects and thermal vibrations of the atoms. Zener [2] pointed out that ballistic electrons, i.e. electrons moving without being scattered, should perform oscillations, today denoted as Bloch oscillations, because of the Bragg reflection of the electrons when their kinetic energy corresponds to the upper boundary of the energy band due to the periodic potential. Besides Bloch oscillations, i.e. electron motion within a band, electron transport between different bands separated by forbidden bands was recognized as origin of dielectric breakdown at high dc fields [2]. The eigen frequency of the Bloch oscillation, the Bloch frequency, is determined by the dc voltage across a spatial period of the periodic potential. Esaki and Tsu [3] proposed to study Bloch oscillations in semiconductor superlattices with periods that are, on the one hand, large enough for sustaining, without dielectric breakdown, a sufficiently large voltage across a period and, on the other hand, small enough to allow almost ballistic propagation of electrons over many periods and predicted that under the condition of the occurrence of Bloch oscillations a negative differential conductance should appear in the current-voltage characteristic * Permanent address: Institute for Physics of Microstructures, Russian Academy of Sciences, 46 UIjanov Str., 603600 Nizhny Novgorod, Russia.

Photon-assisted transport in semiconductor nanostructures

“…[29]. A similar effect was observed by Ignatov and coworkers [32,33] who reported ac-field-induced reduction of the dc current. They attributed such a reduction to a frequency modulation of the Bloch oscillations of electrons at the frequency of the external ac field.…”

“…A similar result is found for the average electron velocity using semiclassical arguments [31,32]: Considering the dispersion relation E(k) = ǫ 0 − ∆cos(kd) 2 for the undriven superlattice and a time dependent electric field: E(t) = F sinωt, the group velocity of a wave packet centered around k 0 and t = T /4 is given by:…”

“…This noise can be interpreted as generated by photon-created electron-hole pair partitioning. Without microwaves, noise in this mesoscopic system can be understood as follows: the left reservoir emits electrons at a frequency 32 For a detailed review about shot noise see Ref. [6].…”

The non-linear terahertz response of hot electrons in low-dimensional semiconductor superlattices: Suppression of the polar-optical phonon scattering

“…In recent years, semiconductor superlattices have received considerable attention from the viewpoint of the creation of new detectors [1][2][3][4][5] , oscillators [6][7][8][9][10] and frequency converters [11][12][13] of the terahertz and sub terahertz radiation. In this regard, the study of new transport phenomena associated with the Bloch electron dynamics and the electron heating in superlattices appears to be very interesting.…”

Current responsivity of semiconductor superlattice THz-photon detectors

“…[20][21][22][23][24][25] In a number of papers Esaki-Tsu negative differential conductance was found to be experimentally realizable 26,27 and has been carefully examined [28][29][30] in the context of development of new millimeter-wave band ͑0.03-0.3 THz͒ oscillators. On the other hand, the recent observations of a strong dc current suppression indicating dynamical localization of electrons, 31 absolute negative conductance, 32,33 and Shapiro steps on the dc current-voltage curve of the THzfield irradiated superlattices 34 open the prospects for applications of the superlattices as novel solid state detectors operating in 1-10 THz-frequency band which the Bloch frequency in the superlattices normally belongs to. 35 It has recently been estimated 36 that the room temperature current responsivity of a superlattice detector ideally coupled to the THz photons can nearly reach the quantum efficiency e/ប ͑where is the incident radiation frequency͒ in the limit of high frequencies ӷ .…”