Using field ionization combined with the direct detection of excited neutral atoms we measured the distribution of principal quantum number n of excited He Rydberg states after strong-field excitation at laser intensities well in the tunneling regime. Our results confirm theoretical predictions from semiclassical and quantum mechanical calculations and simultaneously underpin the validity of the semiclassical frustrated tunneling ionization model. Moreover, since our experimental detection scheme is spin sensitive in the case of He atoms, we show that strong-field excitation leads to strong population of triplet states. The origin of it lies in the fact that high angular momentum states are accessible in strong-field excitation. Thus, singlet-triplet transitions become possible due to the increased importance of spin-orbit interaction rather than due to direct laser induced spin-flip processes.
We investigate designed InN/GaN superlattices (SLs) grown by plasma-assisted molecular beam epitaxy on c-plane GaN templates in situ by line-of-sight quadrupole mass spectroscopy and laser reflectivity, and ex situ by scanning transmission electron microscopy, X-ray diffraction, and photoluminescence (PL). The structural methods reveal concordantly the different interface abruptness of SLs resulting from growth processes with different parameters. Particularly crucial for the formation of abrupt interfaces is the Ga to N ratio that has to be bigger than 1 during the growth of the GaN barriers, as Ga-excess GaN growth aims at preventing the unintentional incorporation of In accumulated on the growth surface after the supply of InN, that extends the (In,Ga)N quantum well (QW) thickness. Essentially, even with GaN barriers grown under Ga-excess yielding to 1 monolayer (ML) thick QWs, there is a real discrepancy between the designed binary InN and the actual ternary (In,Ga)N ML thick QWs revealed by the above methods. The PL emission line of the sample with atomically abrupt interfaces peaks at 366 nm, which is consistent with the In content measured to be less than 10%.
In this work, we discuss theoretically the formation of the Tamm plasmon/exciton-polariton hybrid states in an (Al,Ga)As microcavity and their modulation by surface acoustic waves. The modulation of the Tamm plasmon/exciton-polariton states origins in the change of the excitonic band gap energy and the thickness change of the sample structure layers due to the induced strain fields by surface acoustic waves. The frequency fSAW of the acoustic modulation of the Tamm plasmon/exciton-polariton states is limited by the thickness of the upper distributed Bragg reflector. For the Tamm plasmon/exciton-polariton states in AlxGa1−xAs/GaAs structures fSAW is in the range of 370 MHz while fSAW in GHz range is possible for the parametric Tamm plasmon/exciton-polariton states. In both cases, the acoustic modulation is several meV for typical acoustic power levels.
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