We present a comparative investigation of the morphological, structural, and optical properties of vertically aligned ZnO nanowires (NWs) before and after high energy argon ion (Ar(+)) milling. It is found that the outer regions of the as-grown sample change from crystalline to amorphous, and ZnO core-shell NWs with ZnO nanocrystals embedded are formed after Ar(+) milling. Optical properties of the ZnO NWs have been investigated systematically through power and temperature dependent photoluminescence measurements, and the phenomenon of exciton localization as well as the relevant favorable photoluminescence characteristics is elucidated. Interestingly, under high density optical pumping at room temperature, coherent random lasing action is observed, which is ascribed to exciton localization and strong scattering. Our results on the unique optical properties of localized exciton in ZnO core-shell nanostructures shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Continuous wave ultraviolet laser irradiation at = 244 nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100 nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution microstructuring approach when followed by chemical etching. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2884185͔ Domain engineering 1,2 of lithium niobate ͑LN͒ is a subject of extensive research and a simple, cheap, and robust method of fabrication of well-defined periodic domaininverted structures on submicron scales is highly desirable. Spatial domain engineering is used for many optical processes in bulk crystals and waveguides and can also allow for the creation of both freestanding 3 and surface relief structures 4 through the differential etching characteristics of the polar z faces of the crystal. If achievable on the submicron scale, surface structuring through differential etching will allow the implementation of a range of interesting applications such as tunable photonic crystals, ridge waveguide lasers, and multifunctional micromachines.Previous work has shown that ultraviolet ͑UV͒ and visible laser light can either directly invert 5 or assist the domain inversion process in LN. [6][7][8][9] In this paper, however, a different effect is presented whereby illumination of the +z face with UV light at = 244 nm ͑with photon energy greater than the LN band gap͒ inhibits domain inversion in illuminated areas during subsequent electric field poling ͑EFP͒. Of major importance, the inhibited domains are not restricted in their shape or alignment with the crystal x or y axes, hence, arbitrarily shaped domains can be formed. Some initial results of this effect and its applicability in the creation of micro/nano structures in LN are presented.A beam from a frequency-doubled Ar-ion laser was focused to a spot size of ϳ2.5 m on the +z or −z face of either an undoped congruent or 5 mol % MgO-doped LN crystal. Positioning and exposure control of the crystal was achieved by a computer-controlled, three-axis stage system coupled with a mechanical shutter.For dynamic exposures, sets of parallel lines were drawn on the z faces of the crystals along the crystallographic x or y directions by moving the stages at speeds ranging from 0.05 to 0.3 mm s −1 . For static exposures, arrays of illuminated spots with identical exposure times, ranging from a few milliseconds to a few tens of seconds, were formed. The separation between the edges of adjacent illuminated spots in the arrays varied from 0 to 6 m which permitted us to verify if any proximity effect existed such as that observed in pulsed laser direct poling 5 where the closest approa...

Ferroelectric domain reversal has been achieved by scanning a tightly focused, strongly absorbed UV-laser beam across the x-and y-faces of lithium niobate crystals. The domains were investigated by piezoresponse force microscopy. The emergence and width of any domain was found to depend on the scanning direction of the irradiating laser beam with respect to the polar z-axis. Full width and half width domains, or no domain formation at all could be achieved for scanning along specific directions. We interpret the results by a direct correlation between the local temperature gradient and the resulting polarization direction.PACS numbers: 77.65.-j, 68.37. Ps, 77.80.Dj Lithium niobate (LiNbO 3 ) has recently become the material of first choice for nonlinear optical applications in the visible and near-infrared wavelength range. 1In many cases patterning of ferroelectric domains at a length scale of sub-to-few microns is required. For instance frequency conversion employing quasi-phase matching relies on a periodically poled domain pattern. 2Domain patterning is in general performed by electric field poling (EFP) with structured electrodes, which reverses the direction of polarization by locally applying an electric field along the polar z-axis of the crystal, exceeding the so-called coercive field (E c ). 3A different approach for domain patterning uses direct UV-laser beam writing.4 It was found that irradiating the −z-face of LiNbO 3 with a tightly focussed, strongly absorbed UV-laser beam creates ferroelectric domains along the laser-written track without any application of an external electric field. The very same procedure on the +z-face results in poling inhibition of the irradiated area.

Given that a ferroelectric domain is generally a three dimensional entity, the determination of its area as well as its depth is mandatory for full characterization. Piezoresponse force microscopy ͑PFM͒ is known for its ability to map the lateral dimensions of ferroelectric domains with high accuracy. However, no depth profile information has been readily available so far. Here, we have used ferroelectric domains of known depth profile to determine the dependence of the PFM response on the depth of the domain, and thus effectively the depth resolution of PFM detection. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3126490͔ During the past decade piezoresponse force microscopy ͑PFM͒ has become a standard tool for the investigation of ferroelectric domains.1,2 This is mainly because of its ease of use ͑no specific sample preparation͒ combined with its capability for imaging ferroelectric domains with high lateral resolution of Ͻ20 nm.3 Furthermore, PFM is not limited to specific crystallographic orientations of the sample, and hence ferroelectric domains can be visualized with PFM on all faces of the crystal. 4 Being an all-purpose analytical tool, and therefore advantageous with respect to many other relevant techniques used for the investigation of ferroelectric domains, 5 it is often ignored that PFM produces twodimensional maps only of the domain patterns. The question that arises is: up to what depth below the surface is PFM sensitive? While some earlier attempts at addressing this problem were performed using thin films, 6,7 to date, however, there are no reports on measurements using single crystals. Such samples are needed therefore as they uniquely allow for a defined domain configuration, and thus to quantitatively determine the depth resolution of PFM.The goal of the investigations which are presented in this paper was to obtain a direct correlation between the depth of a surface domain 8 and the corresponding contrast obtained in PFM measurements. The first challenge was thus to fabricate a sample with ferroelectric surface domains of known depth. A method that can produce such domains in lithium niobate is UV laser-induced inhibition of poling, 9 a brief description of which is given here for clarity. It was found that irradiation of the +z polar surface of lithium niobate crystals with UV laser radiation locally increases the coercive field. Hence, a preirradiated area of the crystal surface will maintain its original polarity after a subsequent uniform electric field poling step. The depth d 0 of those poling inhibited domains is of the order of a few microns, depending on the specific UV-writing conditions, such as the illuminating laser light ͑wavelength and intensity͒ and scan speed used.10 Linear ferroelectric domain tracks several millimeters long were produced by scanning the crystal sample in front of the focused laser beam.In order to obtain surface domains of different depth d 0 the sample was wedge polished at a shallow angle ͑␣ =5°͒. For a domain of d 0 =3 m depth we thereby obtai...

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