A giant asymmetry in the magnetoresistance was revealed in high-mobility, two-dimensional electron gas on a cylindrical surface. The longitudinal resistance along the magnetic-field gradient impressed by the surface curvature was found to vanish if measured along one of the edges of the curved Hall bar. If the external magnetic field is reversed, then the longitudinal resistance vanishes at the opposite edge of the Hall bar. This asymmetry is analyzed quantitatively in terms of the Landauer-Büttiker formalism.

We demonstrate the operation of surface-acoustic-wave (SAW) transducers fabricated on AlN/SiC structures at frequencies as high as 19 GHz. The high SAW velocity of the AlN film is enhanced by the even higher sound velocity of the SiC substrate, enabling us to achieve these frequencies with a SAW wavelength of 400 nm.

Size effects on a first-order phase transition in submicron disks is demonstrated using a high-resolution imaging method. In MnAs disks on GaAs ͑001͒ substrates having sizes smaller than twice the film thickness, an individual disk consists of either entirely ␣-MnAs or entirely ␤-MnAs as a consequence of the nucleation initiation of the first-order phase transition. A phase coexistence occurs on macroscopic scales as the two kinds of the disks can be present simultaneously. In slightly larger disks, the stripe structure due to the phase coexistence in films is modified to a core-shell-type phase segregation.

The authors introduce an alternative approach for acousto-optical light control based on the interference of light propagating through several waveguides, each subjected to a periodic refractive index modulation induced by a surface acoustic wave. The feasibility of the concept is demonstrated by the realization of an optical switch for arbitrary time intervals with an on/off contrast ratio of 20. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2768889͔The acousto-optical interaction provides a wellestablished process for controlling light beams in solids. 1 With the advent of integrated optics, approaches have been sought after to implement the required phase matching between the acoustic and optical waves propagating in waveguide ͑WG͒ structures. Examples are the acoustically induced coupling between neighboring WGs ͑Ref. 1͒ and the use of Umklapp scattering in photonic crystals. 2,3 As an alternative approach, Gorecki et al. proposed the combination of surface acoustic waves ͑SAWs͒ with MachZehnder interferometers ͑MZIs͒ for WG modulators. 4,5 Here, the strain field of a SAW impinging perpendicularly to one of the interferometer arms introduces a periodic modulation of the transmitted light. More recently, de Lima et al. 6 introduced an approach for acoustically driven MZI, where a single SAW simultaneously modulates the refractive index of both MZI arms with opposite phases, as illustrated in Fig. 1͑a͒. The opposite phase modulation of amplitude max per arm is achieved by simply spacing the WGs by an odd multiple of the half acoustic wavelength SAW .In this letter, we demonstrate an alternative concept for WG acousto-optical elements based on the modulation of several interferometer arms by a single SAW beam. These acousto-optic multiple interference devices ͑AOMIDs͒ provide integrated optics building blocks for functionalities such as switching, harmonic generation, and pulse shaping. 7 The feasibility of the AOMID concept is established by the realization of a WG on/off switch for arbitrary time periods.The AOMID concept is an extension of the acoustically driven MZIs of Fig. 1͑a͒ for a number N P ജ 2 of WG arms connected in parallel. 8 The configurations for N P =2 ͑corre-sponding to the simple MZI͒, N P = 4, and N P = 8 are illustrated in Figs. 1͑a͒-1͑c͒. In each case, the single mode WGs ͑numbered by the index p =1, ... ,N P ͒ are assumed to be identical and much narrower than SAW . The WGs are aligned perpendicularly to the SAW propagation direction and displaced laterally so as to experience SAW phases differing by a multiple of SAW =2 / N P . The amplitude max of the light phase shift upon propagation through the WGs is related to the WG length ᐉ and to the refractive index modulation ␦n induced by the SAW strain by max =2 ␦nᐉ / L , where L is the light wavelength. The use of symmetric Y-splitters in Fig. 1 ensures unity transmission in the absence of a SAW. Under the previous assumptions, the transmission T P of an AOMID with N P arms becomes:where f SAW is the SAW frequency. Figure 1͑d͒ comp...

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