Density-dependent exciton dynamics and L -valley anisotropy in germanium

Abstract

Optical pump-THz probe spectroscopy is used to investigate the exciton formation dynamics and its intensity dependence in bulk Ge. Associated with the intra-excitonic 1s-2p transition, the gradual build-up of an absorption peak around 3.1 meV (0.75 THz) signifies the delayed exciton formation after optical pump which is accelerated for higher excitation densities. Analyzing the spectral shape of this THz absorption resonance, two distinct resonances are found which are attributed to the mass-anisotropy of L valley electrons via a microscopic theory.

Exciton ionization by THz pulses in germanium

“…Via the optical excitation of the Ge sample by an ultrashort optical pulse with a central photon energy of 0.8 eV, we create an unbound electron-hole plasma. On a timescale of several ns, this plasma condenses into bound electron-hole pairs as discussed in detail in [10]. This configuration is the initial condition for the THz probe pulse which has a 9 ns delay and a low field strength of 0.12 kV cm −1 .…”

“…We use an excitation density of 8.3×10 13 photons cm −2 . For the THz pulses, we choose a delay of 9 ns since previous experiments [10] have shown that at those experimental conditions an almost pure incoherent population of 1s-excitons has formed. The THz pulses are generated using a LT-grown large aperture GaAs antenna that is excited by the fundamental laser pulse emitted by the femtosecond amplifier system, which delivers pulses with a duration of 35 fs and a central energy of 1.55 eV (800 nm).…”

“…Since the anisotropy plays only a minor role for our current studies, we simplify the calculations by assuming m e =1.25×0.0815 m 0 =0.1019 m 0 in all directions. In other words, we ignore the L-valley anisotropy with the consequence that the computed spectra show only the main 1s-2p absorption peak at 3.1 meV without the shoulder at 2.4 meV [10,18]. In our calculations, the THz response follows from Maxwellʼs wave equation…”

“…In the last decade, intra-excitonic transitions have been studied in quantum wells [7] as well as in the bulk semiconductors silicon [8], germanium [9,10], gallium arsenide [11] and cuprous oxide [12]. In contrast to probing excitons by means of weak THz fields, strong THz fields lead to the excitation of high energy states and even to the ionization of the quasi-particles [11,[13][14][15].…”

Advances in Functional Nanomaterials Science

“…THz spectroscopy evolved from an exotic technique two decades ago to a widely used and commercially available method, [199] covering the far-IR spectral range between approximately 3-300 wavenumbers, which corresponds to the frequency range of about 0.1-10 THz. Among others, Martin Koch and his group (Philipps-Universität Marburg) demonstrated that THz spectroscopy is not only used for basic research, for instance to study fundamental physical mechanisms such as the formation and decay of excitons [200,201] (in the fashion of previous exciton formation studies [202] ) or the formation of crystals out of a solution, [203] but also for non-destructive testing. Recently, Stuebling et al combined a THz system with a robotic arm to achieve and demonstrate a THz tomography apparatus for the analysis of arbitrarily shaped samples.…”