Atomic polarization in ferroelectric compounds is manipulated to control local electronic structure and influence chemical reactivity. Ferroelectric domains are patterned with electron beams or with probe tips, and electron exchange reactions occur preferentially on positive or negative domains. Using photo reduction from aqueous solution, metal nanoparticles are produced in predefined locations on an oxide substrate. Subsequently, organic molecules are reacted selectively to the particles. The process can be repeated to develop complex structures consisting of nanosized elements of semiconductors, metals, or functional organic molecules.

of the device: the cell is assembled in a way that allows a gradient of the dopant concentration. Consequently, a smooth variation of the helical pitch was obtained inside the same cell. The main aspect of this second method is that a simple translation of the cell with respect to the exciting beam enables the fine-tuning of the laser wavelength. In both methods, a shift of the lasing wavelength by 30±40 nm was obtained. ExperimentalInvestigations on photoexcitation were performed using the third harmonic of a Q-switched Nd:YAG laser (Continuum, Surelite II) as the source. The pulse wavelength, width, and repetition rate were 355 nm, 6 ns, and 5±10 Hz, respectively. The excitation beam energy was strongly attenuated, combining a k/2 waveplate, a polarizer, and several neutral density filters. The laser beam was focused using a cylindrical lens (f = 150 mm) to reduce the spot size on the cell to a few hundred micrometers. The pump beam irradiated the sample at an angle of 45 with respect to the cell normal, the usual experimental geometry for this kind of experiment [8,13]. An optical fiber coupled to the spectrometer (an Avantes Fiberoptics mod. AVS-S2000, with a resolution of 1.5 nm) collected the light emitted from the sample.