We have characterized the structural and magnetic properties of low-temperature molecular-beam epitaxy grown Ge: Mn by means of high-resolution transmission electron microscopy ͑HR-TEM͒, energy dispersive x-ray spectroscopy, and superconducting quantum interference device ͑SQUID͒ magnetometry. We find a coherent incorporation of Mn 5 Ge 3 clusters in an epitaxially grown Ge: Mn matrix, which shows the characteristics of a diluted magnetic semiconductor phase of Mn-doped Ge. The clusters are preferentially oriented with the hexagonal ͓0001͔ direction parallel to the ͓001͔ growth direction of the Ge: Mn matrix, as determined from both HR-TEM and SQUID measurements.
We present a detailed study of the magnetic properties of low-temperature molecular beam epitaxy grown Ge:Mn dilute magnetic semiconductor films. We find strong indications for a frozen state of Ge 1−x Mn x , with freezing temperatures of T f = 12 K and T f = 15 K for samples with x = 0.04 and x = 0.2, respectively, determined from the difference between field cooled and zero-field cooled magnetization. For Ge 0.96 Mn 0.04 , AC susceptibility measurements show a peak around T f , with the peak position T ′ f shifting as a function of the driving frequency f by ∆T ′ f /[T ′ f · ∆logf ] ≈ 0.06, whereas for sample Ge 0.8 Mn 0.2 a more complicated behavior is observed. Furthermore, both samples exhibit relaxation effects of the magnetization after switching the magnitude of the external magnetic field below T f which are in qualitative agreement with the field and zero-field cooled magnetization measurements. These findings consistently show that Ge:Mn exhibits a frozen magnetic state at low temperatures, and that it is not a conventional ferromagnet.
Geometry, nonlinearity, dispersion and two-photon absorption figure of merit of three basic silicon-organic hybrid waveguide designs are compared. Four-wave mixing and heterodyne pump-probe measurements show that all designs achieve high nonlinearities. The fundamental limitation of two-photon absorption in silicon is overcome using silicon-organic hybrid integration, with a five-fold improvement for the figure of merit (FOM). The value of FOM = 2.19 measured for silicon-compatible nonlinear slot waveguides is the highest value published.
Gain and phase dynamics in InAs/GaAs quantum dot semiconductor optical amplifiers are investigated. It is shown that gain recovery is dominated by fast processes, whereas phase recovery is dominated by slow processes. Relative strengths and time constants of the underlying processes are measured. We find that operation at high bias currents optimizes the performance for nonlinear cross-gain signal processing if a low chirp is required.
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