Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs/i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [1[over ¯]10]-oriented cantilever, whereas the damping (deamplification) is observed in the [1[over ¯]10] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.

Magneto-optic Kerr microscopy was employed to investigate the spin-orbit interactions of electrons traveling in semiconductor quantum wells using surface acoustic waves (SAWs). Two-dimensional images of the spin flow induced by SAWs exhibit anisotropic spin precession behaviors caused by the coexistence of different types of spin-orbit interactions. The dependence of spin-orbit effective magnetic fields on SAW intensity indicates the existence of acoustically controllable spin-orbit interactions resulting from the strain and Rashba contributions induced by the SAWs.

. In general, ESR requires two external magnetic fields: a static field (B 0 ) to split the spin states in energy and an oscillating field (B 1 ) with the frequency resonant to the splitting energy. However, spin manipulation methods relying on real magnetic fields-much broader than the size of individual electrons-are energetically inefficient and unsuitable for future device applications. Here we demonstrate an alternative approach where the spin-orbit interaction 7 of trajectory-controlled electrons induces effective B 0 and B 1 fields. These fields are created when electron spins surf on sound waves [8][9][10] along winding semiconductor channels. The resultant spin dynamics-mobile spin resonance-is equivalent to the usual ESR but requires neither static nor time-dependent real magnetic fields to manipulate electron spin coherence.In the electron systems in inhomogeneous magnetic fields or systems with spin-orbit coupling, the motion of electrons is converted to time-dependent effective magnetic fields. The latest studies have reported that ESR can be driven by effective B 1 fields, which are produced by the reciprocating motion of electrons with spatially dependent spin splitting [11][12][13] or with spin-orbit coupled systems [14][15][16] , but these techniques still need external B 0 fields. A promising approach for incorporating effective B 0 fields is the use of long-distance spin transport, which induces spin precessions around the static spin-orbit magnetic fields 9,17 . In addition, spin-qubit operations using moving quantum dots have been theoretically proposed 18 . Thus, we expect all of the magnetic fields needed for ESR to be replaced with spin-orbit effective magnetic fields (B SO ) by using trajectorycontrolled travelling electrons.Mobile spin resonance is based on the dependence of B SO on the electron momentum vector k. For simplicity, we assume a two-dimensional electron system in a (001) III-V quantum well with only k-linear terms in the Dresselhaus spin-orbit interaction 19 (SOI). The effective Dresselhaus field is described bywhere we used a coordinate system with base vectorsx Now we consider electrons travelling along the sinusoidal channel shown in Fig. 1b. At each position of the channel, the deflection angle of the path determines the direction of the k-vector. As a result, the moving electrons experience an effective magnetic field that swings with the frequency f = v y /λ, where v y is the time-averaged y-component of the electron velocity and λ is the period of the winding channel. In the reference frame moving with the electrons, the time-dependent effective magnetic field can be expressed as the sum of a static field (B To control the trajectory of travelling electrons, we adopted acoustically induced moving dots 21,22 produced in an undoped 20-nm-thick GaAs/AlGaAs (001) quantum well, where the SOI acting on the two-dimensionally confined electrons is dominated by the Dresselhaus term 10 . The piezoelectric field induced by a surface acoustic wave (SAW) beam propagating alongŷ (...

We report the synthesis and optical properties of GaAs nanowires with very small diameters. We grew the GaAs nanowires by using size-selective gold particles with nominal diameters of 5, 10, 20, 40, and 60 nm. The diameter-controlled nanowires enable us to observe blueshifts of the free exitononic emission peak from individual nanowires with decreasing gold-particle size due to the two-dimensional radial quantum-confinement effect. We also analyze the absorption and emission polarization anisotropies of these bare GaAs quantum nanowires.