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We report observations of an electric-field threshold conduction and of related ac voltage (broad-band noise) generation in low-disorder two-dimensional electron systems in the extreme magnetic quantum limit. We interpret these phenomena as definitive evidence for formation of a pinned quantum Wigner crystal and determine its melting phase diagram from the disappearance of threshold and noise behavior at higher temperatures.

We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum wells by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation processes play an important role in exciton spin relaxation. PACS numbers: 78.47.+p, 71.35.+Z, 7l.70.Ej, 71.70.Gm Investigations of polarization of interband optical transitions provide considerable information about the symmetry of electron, hole, and excitonic wave functions in semiconductors. Such studies have led to the identification of diflferent spin relaxation processes [1,2] in bulk semiconductors. In quantum wells, cw measurements of linear [3] and circular polarization [4,5] of near-bandgap luminescence have been reported and various theories have been proposed to explain these results [6-12]. Time-resolved spectroscopy of luminescence polarization has also been reported recently for very high (lO'^ cm ~^) carrier densities [13], for different samples at intermediate carrier densities with 150-fs time resolution [14,15], and for low densities [16,17].In spite of this intense activity, spin relaxation in quantum wells is poorly understood. The quality of samples is important since spin dynamics can be strongly influenced by localization or defects. On a more fundamental level, the differences in the behavior of electrons, holes, and excitons must be recognized and carefully investigated. The prediction of slower hole spin relaxation in quantum wells compared to bulk semiconductors [10] has not been investigated. Finally, the influence of the formation dynamics of excitons and of many-body effects must be considered.In this Letter, we discuss new insights into many of these unresolved issues obtained through our investigations of spin dynamics of electrons, holes, and excitons in quantum wells. Using «-modulation-doped quantum wells, we obtain the first measurement of hole spin relaxation time in a semiconductor. The measured ^=: 4 ps demonstrates that the usual assumption of instantaneous hole spin relaxation is incorrect. We also show that the electron Fermi sea can be spin polarized under certain excitation conditions. In /?-modulation-doped quantum wells, we measure electron spin relaxation time of ^^^ 150 ps, approximately a factor of 4 shorter than that in comparably /7-doped bulk GaAs. We will show that an electron-hole exchange is responsible for this reduction. For intrinsic quantum wells, we show that polarized and unpolarized spectra exhibit an unusual splitting that depends on excitation density and time delay. This splitting results from many-body exchange interactions between excitons and contributes to an increase in the exciton spin relaxation time at higher density. Finally, we explain why spin relaxation in nonresonantly created excitons is faster than in resonantly created excitons. We believe that our quantitative measurement...

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We have systematically studied the electron transport in the vicinity of the transitions from the n 1 and 1͞3 quantum Hall liquids to the Hall insulator, in a wide variety of samples. Our results indicate that the diagonal resistivity at the transition is universal and close to the quantum unit of resistance e 2 ͞h.

We present thermally tunable silicon racetrack resonators with an ultralow tuning power of 2.4 mW per free spectral range. The use of free-standing silicon racetrack resonators with undercut structures significantly enhances the tuning efficiency, with one order of magnitude improvement of that for previously demonstrated thermo-optic devices without undercuts. The 10%-90% switching time is demonstrated to be ~170 µs. Such low-power tunable micro-resonators are particularly useful as multiplexing devices and wavelength-tunable silicon microcavity modulators.

%'e show that excitons form with a time constant~~20 ps following the creation of electron-hole pairs by subpicosecond optical excitation. The excitons are initially formed in large-wave-vector states. At low temperatures, these nonthermal excitons relax in =400 ps to the K =0 states, which couple directly to light by interaction with other excitons and acoustic phonons. This leads to a slow rise of exciton luminescence and an unusual dependence of this rise time on temperature, excitation density, and excitation energy.The optical properties of excitons in quantum wells have been the subject of intense research in recent years for fundamental' and applied reasons. Many fundamental properties of excitons have been investigated by using ultrafast laser spectroscopy. For example, the dynamics of exciton ionization and the ac Stark effect of excitons induced by an intense optical field ' have been investigated using excite-and-probe spectroscopy. The homogeneous linewidth of excitons, and the influence of temperature and various collisions on this linewidth, have been investigated by four-wave-mixing experiments. The recombination dynamics of excitons has been investigated by time-resolved luminescence spectroscopy.In spite of this intense interest in excitons, one important aspect of excitons, the dynamics of formation of bound states of excitons following photoexcitation of electron-hole pairs, has remained essentially unexplored.The recent results of Kusano et al. are largely dominated by extrinsic effects. The complex process of formation of intrinsic excitons has been identified as an important problem since the introduction of the concept of excitons in solids, but has not been addressed either experimentally or theoretically. Photoexcited electron and hole (either from a geminate or a nongeminate pair) can form an exciton by interaction with acoustic and optical phonons, and also by carrier-carrier interactions. The relaxation of photoexcited pairs within the bands proceeds simultaneously with the exciton formation process. Also, the excitons can be initially formed in the ground as well as excited states, and in the singlet as well as triplet states (corresponding to the orthohydrogen and parahydrogen states). Furthermore, the excitons are very likely created with a large total momentum wave vector E, corresponding to the center-of-mass motion of excitons in the quantumwell planes. The relaxation of these nonthermal excitons into the singlet K =0 state (the only state that can directly couple to the photons) must also be considered. It is clear that an understanding of these aspects of excitons is a fundamental importance in the physics of elementary excitons in solids.We present in this article the first results on the dynamics of exciton formation and relaxation, which provide insight into many facets of exciton physics that have not been considered before. All results presented in this letter deal with intrinsic excitons. We probed exciton formation dynamics in GaAs quantum wells as a function of temperat...

We report a high-speed ring modulator that fits many of the ideal qualities for optical interconnect in future exascale supercomputers. The device was fabricated in a 130 nm SOI CMOS process, with 7.5 μm ring radius. Its high-speed section, employing PN junction that works at carrier-depletion mode, enables 25 Gb/s modulation and an extinction ratio >5 dB with only 1V peak-to-peak driving. Its thermal tuning section allows the device to work in broad wavelength range, with a tuning efficiency of 0.19 nm/mW. Based on microwave characterization and circuit modeling, the modulation energy is estimated ~7 fJ/bit. The whole device fits in a compact 400 μm2 footprint.