REPORTS B black-and-white dark-field PL of two coupled ribbons 250 nm, 630 um total length Light is incident on the right ter- collected at the left terminus of otes the location of the junc ion. An SEM image(inset)re Raw emission spectra of the left 2m1 bbon before (red) and after ming the junctio bon and the junction lowers the utput light intensity by 50% whereas its modulation is SnO2 field PL images of a three-ribbon ring structure that functions as a bon (135 um by 540 nm by 175 nm) is flanked by two linear rib- bons (left, 120 um by 540 nm Zno um by PL Sn 420 nm by 235 nm). Light input at branch 1 exits preferentially 25 um Wavelength(nm at branch 3(left), whereas light input at branch 2 exits branc (right). See fig S2 for a dark-field image of the structure. (D)A true-color bottom: unguided PL of the nanowire, waveguided(wG)emission from rk-field PL image of a Zno nanowire(56 um long, at top, pumped at the Zno wire collected at the bottom tern 3.8 ev)channeling light into a SnO2 nanoribbon(265 um long, at waveguided emission from the Sno2 ribbon excite below the botto he arrow denotes the location of the junction. (E)An SEM junction and collected at its bottom terminus, and pl of the nage of the wire-ribbon junction ( F)Spectra of upled struct Zno nanowire is me uring its aken at different excitation and collection locations. From top to transit through the nanoribbon cavity because a change in cavity curvature and/or ribbons together by van der Waals forces, as a detector in this case because it can weakly cavity-substrate coupling alters the interfer- often simply by draping one over another, absorb sub-band gap light(28). We used ence pattern of propagating waves, resulting to create a robust optical junction. Figure 4, NSOM tip to excite the nanoribbon locally and in the enhancement of some modes and the A and B, shows an example of two-ribbon thereby provide sufficient spatial resolution to partial quenching of others. Our data also coupling. More functional geometries(26), detect waveguided light amid the background indicate that the emission pattern from a typ such as y junctions, branch networks, laser light scattered onto the wire detector. ical nanoribbon is spatially heterogeneous, as Mach-Zehnder interferometers, and ring Scanning the NSOM tip on and off the ribbon shown previously in ZnO nanowires(24). As oscillators can also be constructed. The caused photocurrent oscillations within the de- a consequence, the far-field spectrum chang- three-ribbon ring structure in Fig. 4C oper- tector. Moreover, the oscillations ceased when es somewhat with collection angle, though ates by circulating light that is injected the end of the ribbon was moved away from the not enough to account for the complex modal from one branch around a central cavity, nanowire detector. These results demonstrate variations seen in response to distortions of which can be tapped by one or more output the feasibility of nanowire-based photonic cir- the cavity shape channels to act as an optical hub. With cuitry. Practical devices will require the integra Nanoribbon waveguides can be coupled further integration, it should be possible to tion of ribbon waveguides with electrically together to create optical networks that may create optical modulators based on nanor- driven nanowire light sources and a variety of form the basis of miniaturized photonic ibbon assemblies that use the electrooptic high-performance detector elements with dif- circuitry. Because light diffracts in all di- effect for phase shifting(27) ferent band ga rections when it emerges from a subwave- Nanowire light sources and optical detec- Single-crystalline nanoribbons are in- ngth aperture, nanoribbons must be in tors can be linked to ribbon waveguides to triguing structures with which to manipu close proximity, and preferably in direct create input and output components for future late light, both for fundamental studies and physical contact, to enable the efficient photonic devices. Figure 4D shows light in- photonics applications. As passive ele- transfer of light between them. We tested jection into a ribbon cavity by an optically ments, they are efficient UV and visible various coupling geometries and found that pumped ZnO nanowire. The ribbon imprints waveguides and filters that can be assem- a staggered side-by-side arrangement, in its mode profile on both the UV and visible bled into optical networks and components. which two ribbons interact over a distance emission of the nanowire(Fig. 4F), demon- Being semiconductors or, in their doped of several micrometers, outperforms direct strating the propagation and modulation of a state, transparent metals, oxide nanorib end-to-end coupling, which relies on scat- quasi-Gaussian light beam in a subwave- bons are well suited to combine simulta- tering between end facets. Staggered rib- length optical cavity In the opposite config- neous electron and photon transport in ac- bons separated by a thin air gap can com- uration, PL from a nanoribbon can be detect- tive nanoscale components. Key challenges municate via tunneling of evanescent ed electrically by a Zno nanowire positioned to the wider use of these materials include waves(25). It is also possible to bond two at its end (fig. $3). It is possible for Zno to act narrowing their size dispersity and devel 1272 27AuguSt2004Vol305ScieNcewww.sciencemag.orgbecause a change in cavity curvature and/or cavity-substrate coupling alters the interfer￾ence pattern of propagating waves, resulting in the enhancement of some modes and the partial quenching of others. Our data also indicate that the emission pattern from a typ￾ical nanoribbon is spatially heterogeneous, as shown previously in ZnO nanowires (24). As a consequence, the far-field spectrum chang￾es somewhat with collection angle, though not enough to account for the complex modal variations seen in response to distortions of the cavity shape. Nanoribbon waveguides can be coupled together to create optical networks that may form the basis of miniaturized photonic circuitry. Because light diffracts in all di￾rections when it emerges from a subwave￾length aperture, nanoribbons must be in close proximity, and preferably in direct physical contact, to enable the efficient transfer of light between them. We tested various coupling geometries and found that a staggered side-by-side arrangement, in which two ribbons interact over a distance of several micrometers, outperforms direct end-to-end coupling, which relies on scat￾tering between end facets. Staggered rib￾bons separated by a thin air gap can com￾municate via tunneling of evanescent waves (25). It is also possible to bond two ribbons together by van der Waals forces, often simply by draping one over another, to create a robust optical junction. Figure 4, A and B, shows an example of two-ribbon coupling. More functional geometries (26), such as Y junctions, branch networks, Mach-Zehnder interferometers, and ring oscillators can also be constructed. The three-ribbon ring structure in Fig. 4C oper￾ates by circulating light that is injected from one branch around a central cavity, which can be tapped by one or more output channels to act as an optical hub. With further integration, it should be possible to create optical modulators based on nanor￾ibbon assemblies that use the electrooptic effect for phase shifting (27). Nanowire light sources and optical detec￾tors can be linked to ribbon waveguides to create input and output components for future photonic devices. Figure 4D shows light in￾jection into a ribbon cavity by an optically pumped ZnO nanowire. The ribbon imprints its mode profile on both the UV and visible emission of the nanowire (Fig. 4F), demon￾strating the propagation and modulation of a quasi-Gaussian light beam in a subwave￾length optical cavity. In the opposite config￾uration, PL from a nanoribbon can be detect￾ed electrically by a ZnO nanowire positioned at its end (fig. S3). It is possible for ZnO to act as a detector in this case because it can weakly absorb sub–band gap light (28). We used a NSOM tip to excite the nanoribbon locally and thereby provide sufficient spatial resolution to detect waveguided light amid the background laser light scattered onto the wire detector. Scanning the NSOM tip on and off the ribbon caused photocurrent oscillations within the de￾tector. Moreover, the oscillations ceased when the end of the ribbon was moved away from the nanowire detector. These results demonstrate the feasibility of nanowire-based photonic cir￾cuitry. Practical devices will require the integra￾tion of ribbon waveguides with electrically driven nanowire light sources and a variety of high-performance detector elements with dif￾ferent band gaps. Single-crystalline nanoribbons are in￾triguing structures with which to manipu￾late light, both for fundamental studies and photonics applications. As passive ele￾ments, they are efficient UV and visible waveguides and filters that can be assem￾bled into optical networks and components. Being semiconductors or, in their doped state, transparent metals, oxide nanorib￾bons are well suited to combine simulta￾neous electron and photon transport in ac￾tive nanoscale components. Key challenges to the wider use of these materials include narrowing their size dispersity and devel￾Fig. 4. Nanoribbon coupling and optical components. (A) A black-and-white dark-field PL image of two coupled ribbons (both ribbons are 750 nm by 250 nm, 630 m total length). Light is incident on the right ter￾minus of the right ribbon and collected at the left terminus of the left ribbon. The arrow de￾notes the location of the junc￾tion. An SEM image (inset) re￾solves the junction layout. (B) Raw emission spectra of the left ribbon before (red) and after (black) forming the junction. The addition of the second rib￾bon and the junction lowers the output light intensity by only 50%, whereas its modulation is retained. (C) True-color dark- field PL images of a three-ribbon ring structure that functions as a directional coupler. The ring rib￾bon (135 m by 540 nm by 175 nm) is flanked by two linear rib￾bons (left, 120 m by 5 40 nm by 250 nm; right, 275 m by 420 nm by 235nm). Light input at branch 1 exits preferentially at branch 3 (left), whereas light input at branch 2 exits branch 4 (right). See fig. S2 for a dark-field image of the structure. (D) A true-color dark-field PL image of a ZnO nanowire (56 m long, at top, pumped at 3.8 eV ) channeling light into a SnO2 nanoribbon (265 m long, at bottom). The arrow denotes the location of the junction. (E) An SEM image of the wire-ribbon junction. (F) Spectra of the coupled structures taken at different excitation and collection locations. From top to bottom: unguided PL of the nanowire, waveguided (WG) emission from the ZnO wire collected at the bottom terminus of the ribbon, waveguided emission from the SnO2 ribbon excited just below the junction and collected at its bottom terminus, and unguided PL of the ribbon. The emission from the ZnO nanowire is modulated during its transit through the nanoribbon cavity. R EPORTS 1272 27 AUGUST 2004 VOL 305SCIENCE www.sciencemag.org