Electrical pulses shorter than 0.6 ps were generated by photoconductively shorting a charged coplanar transmission line with 80 fs laser pulses. After propagating 8 mm on the line the electrical pulses broadened to only 2.6 ps.

Silicon Mach–Zehnder waveguide modulators have been fabricated and operation characterized at the wavelength of 1.3 μm. Device operation is based on the free-carrier-induced change in the refractive index of silicon. Modulation depths of −4.9 dB and response times τresponse<50 ns have been achieved at λ=1.3 μm for an injected carrier density of 6.5×1017 cm−3.

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Low-temperature time-resolved Faraday-rotation measurements in Cdi_ x Mn x Te allow direct observation of the time evolution of the interaction between an electron spin and the magnetic moments of dilute impurity ions. The experiment displays real-time formation and evolution of magnetically organized states suggestive of the bound magnetic polaron.PACS numbers: 71.35 + z, 71.38. + i, 78.20.Ls One of the many interesting problems in elementary magnetism concerns the fundamental exchange interaction of an electron with the spins of localized magnetic neighbors. Such a system can be made available for study by carefully diluting a normal semiconductor with magnetic ions, producing the so-called "dilute magnetic semiconductor." Recent studies of these materials 1 have stimulated interest in local magnetic phenomena which have been explained with the concept of the bound magnetic polaron, 2 where electrons or holes localized at impurity sites interact with the dilute magnetic ions. It has been suggested that a local ordering of the magnetic moments of these ions would arise from the Heisenberg exchange between the spin of the charged carrier and the paramagnetic ions within its Bohr orbit. This exchange interaction creates the dramatic spin-dependent effects observed experimentally, such as very large Zeeman splittings and giant dc Faraday rotations. Moreover, this "spinball" has a diameter which varies as a function of binding energy, and provides a unique opportunity to study microscopic spin organization.The dynamics of polaron binding were first studied by Harris and Nurmikko 3 through transmissionmodulated spectroscopy. However, in order to witness dynamically the formation and evolution of bound magnetic polarons, it is useful to employ a magnetically sensitive probe along with a high-speed timeresolved detection scheme. Consequently, we have recently developed new techniques to measure timedependent magnetization on picosecond time scales. These involve the exploiting of the Faraday effect as a probe of the magnetic state of the system and use of optical fibers to conduct tunable laser pulses into high magnetic fields at low temperatures. We have chosen to study Cdi_ x Mn x Te because its ease of crystal fabrication, extensive characterization, direct optical gap, and cubic structure result in an isotropic sample with favorable optical properties., Studies of this material have shown that the dominant subgap transitions are due to acceptor bound excitons. 1 A complete description of the experimental technique will be given elsewhere, 4 but the key features are described below.The experimental configuration for time-resolved Faraday rotation relies on ultrashort optical pulses from a synchronously pumped, wavelength-tunable dye laser. Pumped by an argon ion laser, the dye laser produces pulses of 5 psec duration, widely tunable within the visible region of the spectrum. The pulse train of the dye laser is split into a strong pump beam and a much weaker probe beam. The pump pulses promote approximately 10 14 ...

Potential distributions across Si(001)p-n junctions have been studied using cross-sectional scanning tunneling microscopy, spectroscopy, and potentiometry. A clear transition between p- and n-type material can be seen across each junction, and variations in the energy of the conduction-band edge can be detected with a spatial resolution of better than 100 Å. Current-voltage characteristics have been measured in both unbiased and electrically biased structures, and measurements under both conditions are consistent with calculated potential distributions.