Ultrafast shift and injection currents observed in wurtzite semiconductors via emitted terahertz radiation

Abstract

Shift and injection currents are generated in the wurtzite semiconductors CdSe and CdS at 295 K using above-band-gap ͑ប Ͼ E g ͒ femtosecond pulses and detected via the emitted terahertz radiation; the optical beams are normally incident on samples with the optic axis in the plane of the surface. For optical intensities up to 75 MW cm −2 ͑or carrier density Ͻ10 18 cm −3 ͒ the terahertz radiation amplitude shows the expected linear dependence and also varies with optical polarization and sample orientation consistent with the third-rank tensors that govern the current generation processes in the wurtzite structure. The largest shift currents are generated along the optical axis for light polarized along that axis. In CdSe with ប = 1.80 eV ͑690 nm͒, the electron shift distance is ϳ40% of the 0.25 nm bond length and the peak current density is 5 kA cm −2 for an optical intensity of 10 MW cm −2 ; for CdS the corresponding experiment at ប = 3.0 eV ͑410 nm͒ gives a shift distance ϳ80% of the 0.26 nm bond length with a peak current density of 50 kA cm −2 for an incident intensity of 75 MW cm −2 . For injection current produced in CdSe with circularly polarized 690 nm excitation, electrons are injected with an average speed of 9 km s −1 ; this is ϳ3% of the group velocity for electrons excited with the same energy. The corresponding values for CdS excited at 410 nm are 20 km s −1 and 2%. From the temporal characteristics of the terahertz emission for injection currents in CdS we deduce that the electron momentum scattering time is Ͻ100 fs, consistent with mobility studies.

Ultrafast Photovoltaic Response in Ferroelectric Nanolayers

“…(We measure an absorption constant α PTO = 9 x 10 5 cm -1 at λ = 266 nm.) This observation precludes optical rectification, which is expected to depend primarily on incident intensity, as a possible mechanism for these effects, consistent with previous work [22].…”

“…Such nonlinear responses include (non-resonant) optical rectification [21]; shift currents (also referred to as the "linear photovoltaic effect" [22]), associated with an anisotropic displacement of electrons and holes; and injection currents [22]. Little is known about the coupling of these photogenerated currents to the structural response of ferroelectrics.…”

“…medium [21,22]. Such nonlinear responses include (non-resonant) optical rectification [21]; shift currents (also referred to as the "linear photovoltaic effect" [22]), associated with an anisotropic displacement of electrons and holes; and injection currents [22].…”

Linear photogalvanic current excited in sillenite crystals by femtosecond laser pulses

“…In contrast with previous publications [2][3][4][5], the generated electric current was detected not via the emitted terahertz radiation but by charge accumulation on the electrodes.…”

“…One of the important practical impetuses for these investigations is that excited current pulses of femtosecond time scale are capable of inducing terahertz emission. The terahertz radiation arising due to generation of ultra-short pulses of so-called shift and/or injection currents, which are also referred to as linear and circular photogalvanic currents, respectively [1] (in this paper the last terms are used), were observed both in bulk [2,3] and quantum-well (QW) samples [4,5]. Drawing away from particular physical origins of the currents, it should be emphasized that the first type of current arises only under excitation by linearly polarized light while the second type arises under circularly polarized light.…”