LETTERS Tunable nanowire nonlinear optical probe Yuri Nakayama1,6*, Peter J. Pauzauskie1,5*, Aleksandra Radenovic2,4*, Robert M. Onorato1*, Richard J. Saykally1,4, Jan Liphardt2,3,4 & Peidong Yang1,5 Onecrucialchallengeforsubwavelengthopticshasbeenthedevelop- ment of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near- field imaging techniques using fibre-optic scattering probes1,2 have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion3, structured illumination4, and photoactivated localization microscopy5, have enabled impres- sive, theoretically unlimited spatial resolution of fluorescent bio- molecular complexes. Previous work with laser tweezers6���8 has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substan- tially below the wavelength of visible light and have electronic and optical properties9,10 that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual pot- assium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allow- ing the local synthesis of a wide range of colours via sum and differ- ence frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nano- wire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology. Nanometre-scale photonics is emerging as a key ingredient in novel sensing and imaging applications, as well as for advanced information technology, cryptography, and signal processing circuits. A versatile and useful nonlinear circuit element for integrated optical networks must be able to double the frequency of light using second harmonic generation (SHG), a second-order nonlinear optical phenomenon. In this process, two photons with the fundamental angular frequency v1 are converted through a nonlinear crystal polarization into a single photon v2 at twice the fundamental frequency (v2 5 2v1). We have recently demonstrated and characterized harmonic generation11, waveguiding, and optically pumped lasing in single nanowires of zinc oxide12 and gallium nitride13. Despite the growing availability of build- ing blocks such as light-emitting diodes10,14 lasers13,15,16, photodetec- tors17, and waveguides18, the field still lacks sufficiently small devices that efficiently generate tunable coherent photons. Here we show that the large second-order susceptibility x(2) of KNbO3 nanowires facil- itates the generation of tunable, coherent visible radiation that is suf- ficient for in situ scanning and fluorescence microscopy. We chose the perovskite oxide KNbO3 as the nanowire material because of its low toxicity, chemical stability, large effective nonlinear optical coefficients (deff 5 10.8��� 27 pm V21 at l 5 1,064 nm) at room temperature (298 K)19, large refractive indices (n 5 2.1���2.5)20, as well as its transparency in a wide range of wavelengths including the visible spectral region21. Single-crystalline KNbO3 nanowires were synthesized using a hydrothermal method22, and characterized as orthorhombic phase (Amm2) with the growth axis parallel to the [011] direction (Fig. 1a���e): the polar c axis23 is therefore 45u off the nanowire���s growth axis. The SHG response of single KNbO3 nanowires were characterized first using femtosecond pulses described elsewhere24 and compared with measurements of ZnO nanowires which have been studied previously11. The nanowires were supported on amorphous silica coverslips and aligned such that the growth axis was orthogonal to the pump beam. The maximum SHG signals for both KNbO3 and ZnO nanowires (l 5 502 nm) are shown in Fig. 1f, generated by introducing the fundamental beam (l 5 1,004 nm, 18 kW cm22). A direct comparison of the SHG signal collected from single KNbO3 and ZnO nanowires is complicated by the anisotropic scattering related to their respective rectangular and hexagonal cross-sections. However, a rough estimate for the deff of KNbO3 nanowires based on the relative ratio of integrated signals is possible and we found it to be ,9.1 pm V21. This illustrates that the nonlinear polarizability of KNbO3 nanowires is larger than that for ZnO, as expected from the consideration of bulk values. The second key requirement for a versatile nonlinear circuit element for use in nano-photonics is wave mixing, specifically sum frequency generation (SFG) (v3 5 v1 1 v2) and difference frequency generation (v3 5 jv1 2 v2j).Fig.1gshowsSFGsignals(l 5 423 nm, 454 nm)and SHG signals (l 5 525 nm, 700 nm), obtained from a single KNbO3 nanowire by introducing fundamental beams at a variety of different frequencies via the tunable femtosecond pump. This demonstrates the ability of nanowire frequency converters to create four different waves from two fundamental input frequencies v1 and v2: 2v1, 2v2, and jv1 6 v2j. We did not observe the difference frequency generation signal corresponding to the SFG at 423 nm here because the expected wavelength (l 5 7,200 nm) is outside current instrumental limits. The SHG signal at 400 nm was weak, owing to photoabsorption within the nanowire. This set of experiments demonstrates the ability of KNbO3 nanowires to generate continuously tunable and coherent light throughout the visible spectrum via nonlinear wave mixing. This cap- ability, as well as the nanowire���s subwavelength cross-section, enables the development of a novel form of scanning light microscopy. Recently, laser trapping was used to optically manipulate nanowires in closed aqueous chambers8,25. We hypothesized that a single KNbO3 nanowire may, when opticallytrapped,be abletodouble thefrequency of the trapping light and then waveguide this locally generated light to its ends. Single KNbO3 nanowires were optically trapped using a home-built infrared8 optical tweezers instrument (Fig. 2a) with the trapwavelengthat1,064 nm,awavelengthpopularforopticaltrapping of wet samples owing to the tolerance of living cells to infrared laser irradiation26. We used an electron-multiplying charge-coupled device (CCD) to search for visible light radiating from trapped nanowires. *These authors contributed equally to this work. 1Department of Chemistry, 2Department of Physics, and 3Biophysics Graduate Group, University of California, 4Physical Biosciences Division and 5Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 6Materials Laboratories, Sony Corporation, 4-16-1 Okata Atsugi-shi, Kanagawa 243-0021, Japan. Vol 447|28 June 2007|doi:10.1038/nature05921 1098 NaturePublishing ��2007 Group

As we hoped, light was observed to radiate from the distal end of trapped KNbO3 nanowires. We charted the radiation profile as a function of position along the nanowire���s long axis by coarsely changing the focus of the top objective mounted on a micrometre stage (Fig. 2b). A diffraction-limited spot was observed at the distal end of the wire, revealing optical waveguiding away from the site of photon conversion and emission from the aperture defined by the nanowire���s cross-section. Measurements with a colour CCD camera show that SHG output from the nanowire varies less than 0.2% from the mean over 20 s of collection (Supplementary Fig. 3a). The light emitted from the nanowire���s end was collected through the lower trapping objective and spectrally analysed. Spectral analysis revealed that the light was green, with a wavelength of 531 6 1.8 nm (Fig. 2c), matching well with the expected SHG signal given a trap- ping/pump wavelength of 1,064 nm. As a control, we also trapped ZnO and Si nanowires. ZnO (not shown) and Si nanowires (Fig. 2c, black line) did not produce visible light, supporting our conclusion that the green light from the KNbO3 nanowires was light generated inside the nanowire by SHG and indicating negligible SHG from symmetry breaking at the wire���water interface. Assuming a non-depleted plane-wave pump geometry (Supple- mentary Information) and a typical trap irradiance of ,108 W cm22, we calculate the total two-photon conversion efficiency g2v to be at least 1025. Estimations from this simple theoretical model are in agreementwithsecondharmonicoutputpowersof ,10 nWmeasured with the electron multiplying CCD (Supplementary Information). Although we used a 1,064nm laser, there is no theoretical limitation to extending both trapping and SHG to the range of wavelengths demonstrated above with femtosecond pulses. Unlike other nonlinear optical geometries in which alignment is accomplished manually via transducers, here the nonlinear crystal (the nanowire) spontaneously orients itself to the optical axis of the trap/pump laser, resulting in detectable SHG signal along the [011] growth axis.This favourable index matching allowstheentire nanowire cavity to participate in the production of second harmonic photons. It is possible that index matching could be further improved by control- lingthetemperatureofthebuffer,althoughwedidnotexplorethishere. Having in hand a nanometric, raster-scannable source of coherent visible light, we wondered whether it could be used to image objects. We used a simple transmission geometry analogous to near-field scanning optical microscopy (NSOM)27,28, in which the sample is scanned through a beam, modulating the fraction of light arriving at a detector. The ultimate resolution of such a transmission micro- scope depends on the radiation characteristics of the illumination source. In this approach the resolution is of the order of the cross- section of the illumination aperture but many other factors are also important, such as the quality of the probe���s end-facets, far-field collection optics and implemented feedback control. c a 1 ��m 2 nm 3 nm 200 nm d e b c b a b c f 3 nm g Intensity (a.u.) SHG intensity (a.u.) Normalized intensity SFG (423 nm) SHG (525 nm) SFG (454 nm) SHG (700 nm) 510 500 490 60 50 40 30 20 011,100 111 022 200 113 131 122 211 70 520 600 500 400 700 ZnO max KNbO3 max Wavelength (nm) �� 2 (degrees) Wavelength (nm) Figure 1 | KNbO3 nanowires and their structural analysis. a, Scanning electron microscopy image of KNbO3 nanowires. b, X-ray powder diffraction pattern of KNbO3 nanowires. The inset shows the unit cell structure of this material, with spontaneous polarization parallel to the c-axis. (a.u., arbitrary units.) c, Transmission electron microscope image of a KNbO3 nanowire and its electron diffraction pattern (inset) with [100] zone axis and [011] growth direction. d, e, High-resolution TEM images of single [011] growth-direction KNbO3 nanowires and an electron diffraction pattern (e, inset) with the zone axis of [100] (d) and [2233] (e). f, Maximum SHG spectra of single KNbO3 and ZnO nanowires reflecting the larger nonlinear polarizibality of KNbO3. g, Panchromatic wavelengths generated by the nonlinear optical processes within individual KNbO3 nanowires. Set of infrared filters 1,064 nm notch filter Lens 50 �� air objective Beam expander Continous -wave laser Colour or Andor camera Andor CCD camera Spectrometer Intensity of KNbO 3 wire (a.u.) 532 nm 1,064 nm 60 �� water objective Collection plane Trap plane Variable 800,000 600,000 400,000 200,000 0 650 600 550 500 450 800 600 400 200 Wavelength (nm) Intensity of Si wire (a.u.) Si nanowire KNbO3 nanowire a b c 532 nm bandpass filter Figure 2 | Radiation from optically trapped single KNbO3 nanowires. a, Detailed set-up for the single-beam optical trapping instrument. b, Bright field (left) and SHG (right) images of the trapped KNbO3 nanowire. Waveguiding of the SHG signal (green) leads to diffraction rings at the distal (top) end of the nanowire which acts as a subwavelength aperture. c, Observed spectra for KNbO3 and Si nanowires. Strong SHG signal at l 5 532 nm is collected from the trapped KNbO3 nanowire (green, left axis), while no signal was observed from Si nanowires (black, right axis). NATURE|Vol 447|28 June 2007 LETTERS 1099 NaturePublishing ��2007 Group