77 research outputs found
INTERNAL FEEDBACK MECHANISMS OF OPTOELECTRONIC BISTABILITY IN CADMIUM SULFIDE
No full textTwo mechanisms of internal feedback in cadmium sulfide in electric field have been studied, namely, thermal shift of absorption lines and electron state transformation due to multiparticle interaction in the high carrier concentration system
Polarization revival of a Bloch oscillating wave packet in conjunction with resonant Zener tunneling
No full textWe investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands
Nonlinear transport in superlattices: Bloch oscillations and Zener breakdown
No full textSemiconductor superlattices have been intensively used as a model system for the investigation of high field transport in solids. Here, we discuss ultrafast optical experiments which allow us to monitor the electron wave packet motion in detail. First, we discuss a novel technique which directly traces the center of mass spatial motion. We use this method for the detection of a linear motion of the wave packets, which is superimposed on the harmonic Bloch oscillations. A comparison with theory shows that this coherent analog to the Shapiro effect in superconductors is associated with gain at THz frequencies. In a second part, we discuss recent results which address the Zener breakdown due to tunneling to higher bands by linear spectroscopy, which directly shows the wave function delocalization. We also trace the dynamics of the Zener tunneling by time-resolved spectroscopy and show the damping of Bloch oscillations due to Zener tunneling
Vidurio sunkumai: Aristotelio ‘vidurio samprata’ (mesotes) ir buddhos ‘vidurio keuas’ (majjhima patipad)
No full textThe author compares Aristotle with the Buddha who, though incompatible with one another in their mode of thought (Aristotle was a theoretical thinker, inquiring into those very matters which the Buddha, as a practical religious thinker, considered to be completely futile), agreed at least about one point: they clearly realised the extreme difficulty of attaining the mean (or the middle), both understood it as something more complex than an equal distance from opposite ends, or an arithmetical mean, or a mechanical equilibrium (equipoise). They presented the mean regarding human beings as a state which is never given a priori, established spontaneously, or found by pure chance, but, on the contrary, is the subject of a constantly renewable creative search. The comparison is based on the analysis of Aristotle’s texts (Eud. Eth., Nic. Eth., etc.) and the Buddhist texts from the Pali Canon (Vinaya pitaka and Sutta pitaka).Analizuodama pirminius Aristotelio tekstus ir ankstyvuosius budistinio pali kanono tekstus autorė siekia palyginti jų ‘vidurio’ principo sampratas. Atkreipiamas dėmesys į tai, kad nors Aristotelis ir Buddha esmiškai nesuderinami (pirmasis, kaip teorinis mąstytojas, samprotavimu sprendė tuos klausimus, kuriuos antrasis, kaip praktinis religinis mąstytojas, laikė visiškai beverčiais), jiedu vieningai pritaria nuomonei, jog praktiškai įgyvendinti vidurio taisyklę yra itin sunku. Abu jie ją suvokė kaip sudėtingą sistemą, kuri negali būti vertinama kaip paprastas aritmetinis vidurkis, mechaniška pusiausvyra ar aprioriškai priimta nuostata, o kaip nuolatos kūrybiškai atnaujinamos paieškos objektas
Supersonic exciton gratings: coherent inter-polariton scattering in semiconductor microcavities
No full textWe report on a coherent nonlinear phenomenon in a semiconductor microcavity (SMC), which has no parallel for QW excitons. When two different polariton modes of the SMC are impulsively excited they undergo normal mode oscillations (NMOs) with coherent energy exchange between the exciton and the cavity mode. In our experiment the two polaritons are excited with slightly different angles resulting in a travelling wave exciton grating. When a test polariton mode is excited it will scatter in the travelling grating producing amplitude modulation sidebands. This phenomenon produces a transient four-wave mixing (TFWM) signal, which is shifted in frequency from that of the test beam by the NMO frequency, in our case, in the THz range corresponding to a grating velocity = 1 /spl times/ 10/sup 7/ m/s, which is four orders of magnitude larger than the sound velocity. The sample under investigation is a GaAs/AlGaAs /spl lambda/ cavity with a single 25 nm GaAs QW at the center
Rekonstruktion der Wellenfunktion in einem gestaffelten Halbleiter-Übergitter
No full textS.441-445We reconstruct a 'test' wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the distance between 'test' and 'probe' wave functions. By spatially tuning one wave function with respect to the other and recording the amplitude and the sign of the modulation of the spectrally resolved four-wave-mixing (FWM) signal with respect to delay, we are able to reconstruct the 'test' wave function. Our numerical simulation of the third-order response of an inhomogeneously broadened system reproduces the experimental data in great detail. The wave function used for the modelling is computed by a one-dimensional transfer matrix model including electron-hole Coulomb interaction. Our experimental scheme inherently allows us to quantitatively distinguish between non-linear mechanisms leading to the FWM signal, namely phase-space filling and excitation-induced dephasing.78Nr.
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