REPORTS Fig. 1. Optical waveguiding in a xcited ∧A of the waveguiding nanoribbon ion. The laser is focused to a spot size of -50 um (30 incidence angle) at the tra of the emission from the botto mperature and 5 K The mode tructure does not change sub- 50400450500550600650700750 on index of refraction variations Wavelength(nm) a higher resolution emission profile(inset) shows fine struc- ture in three of the central peaks. This fine structure is present in every peak. a u, arbitrary units aspect ratios (1000) act as excellent Fig. 2. Panchromatic waveguides of their visible PL emission. waveguiding in a 425- I wide band gap(3.6 eV)ser ductor characterized by defect-related Dark-field image. Cross sectional dimensions are bands at 2.5 ev(green)and 2. 1 eV (orange) 520 nm by 275 nm(B8)50 um and finds application in gas sensors and trans- PL image with the UV parent electrodes. We used a thermal trans- excitation spot center- port process(17)to synthesize single-crystal- ed near the middle of line nanoribbons of Sno with lengths of to 1500 um. These structures possess fairly waveguided emission D uniform(+ 10%)rectangular cross sections Magnified dark-field with dimensions as large as 2 um by I um view of the right end and as small as 15 nm by 5 nm. Many of the with the laser focused ribbons are 100 to 400 nm wide and thick, an on the left end. A wide optimal size range for efficient steering of (-1 um) ribbon lies visible and ultraviolet (UV) light within a terest (Inset)A scant subwavelength cavity. The waveguiding behavior of individual the right terminus ofthe nanoribbons dispersed on SiO, and mica nanoribbon, showing its substrates was studied with the use of far- rectangular cross sec Figures I and 2 show representative data ages of the guided emis. collected from single ribbons with lengths excitation with monochromatic red of 715 and 425 um, respectively. When we spot, caused by scattering at m ne focused continuous wave laser light (3.8 removing the wide ribbon with ev) onto one end of a ribbon, the generated PL was strongly guided by the cavity to such modulation is attributed to longitudi- sectional size and orientation(through bend emanate with high intensity at its opposite nal Fabry-Perot-type modes, with a mode losses, substrate coupling, and variations in end, mimicking a conventional optical spacing, AA, given by AA=A/(2L[n- refractive index) as well as on light intensity ber. Ribbons that possessed sizeable sur- A(dn/dA)]3, where A is the wavelength, L is and end facet roughness. We note that the face defects (i.e, large step edges or at- the cavity length, and n is the index of existence of a mode structure indicates that tached particulates) scattered guided light refraction (2. I for SnO,). The nanoribbons, nanoribbon cavities can have high finesse and in a series of bright spots along their however, are so long that AA is below the show that the transmission at given wave lengths. Contact points between ribbons resolution limit of our instrument(0. 1 nm). lengths can be modified by distorting the vere often dark, although overlying rib- In addition, SnO, cavities are unlikely to cavity shape. Numerical simulations are now bons sometimes acted as scattering centers show longitudinal modes, because the re- being applied to better understand the origin Fig. 2C) flectivity of their end facets is low(<13%) of this wavelength modulation Typically, an emission spectrum collect- and there is no gain to compensate fo In general, one would expect a subwave ed from the end of a ribbon features com- scattering and output-coupling losses. We length waveguide to show a aspenden. plex, quasi-periodic modulation(Fig. IC) have yet to identify the observed modula- loss that is highly wavelength-dependent that results from the interference of elec- tion with specific transverse or bow-tie (19) with better confinement of shorter wave- tromagnetic waves resonating within the modes. A systematic study of the spectral length radiation. We illuminated single na- rectangular cavity (i.e, an optical mode structure is complicated by the intricate noribbons with monochromatic red, green, structure). In short nanowire waveguides, dependence of the modes on ribbon cross- and blue light and established that red 1270 27AuguSt2004Vol305ScieNcewww.sciencemag.orgaspect ratios (1000) act as excellent waveguides of their visible PL emission. SnO2 is a wide band gap (3.6 eV) semicon￾ductor characterized by defect-related PL bands at 2.5 eV (green) and 2.1 eV (orange) and finds application in gas sensors and trans￾parent electrodes. We used a thermal trans￾port process (17) to synthesize single-crystal￾line nanoribbons of SnO2 with lengths of up to 1500 m. These structures possess fairly uniform ( 10%) rectangular cross sections with dimensions as large as 2 m by 1 m and as small as 15 nm by 5 nm. Many of the ribbons are 100 to 400 nm wide and thick, an optimal size range for efficient steering of visible and ultraviolet (UV) light within a subwavelength cavity. The waveguiding behavior of individual nanoribbons dispersed on SiO2 and mica substrates was studied with the use of far￾field microscopy and spectroscopy (18). Figures 1 and 2 show representative data collected from single ribbons with lengths of 715 and 425 m, respectively. When we focused continuous wave laser light (3.8 eV) onto one end of a ribbon, the generated PL was strongly guided by the cavity to emanate with high intensity at its opposite end, mimicking a conventional optical fi￾ber. Ribbons that possessed sizeable sur￾face defects (i.e., large step edges or at￾tached particulates) scattered guided light in a series of bright spots along their lengths. Contact points between ribbons were often dark, although overlying rib￾bons sometimes acted as scattering centers (Fig. 2C). Typically, an emission spectrum collect￾ed from the end of a ribbon features com￾plex, quasi-periodic modulation (Fig. 1C) that results from the interference of elec￾tromagnetic waves resonating within the rectangular cavity (i.e., an optical mode structure). In short nanowire waveguides, such modulation is attributed to longitudi￾nal Fabry-Perot–type modes, with a mode spacing, , given by   2 /{2L[n – (dn/d)]}, where  is the wavelength, L is the cavity length, and n is the index of refraction (2.1 for SnO2). The nanoribbons, however, are so long that  is below the resolution limit of our instrument (0.1 nm). In addition, SnO2 cavities are unlikely to show longitudinal modes, because the re￾flectivity of their end facets is low (
13%) and there is no gain to compensate for scattering and output-coupling losses. We have yet to identify the observed modula￾tion with specific transverse or bow-tie (19) modes. A systematic study of the spectral structure is complicated by the intricate dependence of the modes on ribbon cross￾sectional size and orientation (through bend losses, substrate coupling, and variations in refractive index) as well as on light intensity and end facet roughness. We note that the existence of a mode structure indicates that nanoribbon cavities can have high finesse and show that the transmission at given wave￾lengths can be modified by distorting the cavity shape. Numerical simulations are now being applied to better understand the origin of this wavelength modulation. In general, one would expect a subwave￾length waveguide to show a large optical loss that is highly wavelength-dependent, with better confinement of shorter wave￾length radiation. We illuminated single na￾noribbons with monochromatic red, green, and blue light and established that red Fig. 1. Optical waveguiding in a 715- m-long SnO2 nanoribbon. (A) A dark-field image of the meandering ribbon (350 nm wide by 245nm thick) and its surroundings. (B) The PL image of the waveguiding nanoribbon under laser excitation. The laser is focused to a spot size of
50 m (30° incidence angle) at the top end of the ribbon. (C) Spec￾tra of the emission from the bottom terminus of the waveguide, collected at room temperature and 5K. The mode structure does not change sub￾stantially with temperature, suggesting minimal dependence on index of refraction variations. A higher resolution emission profile (inset) shows fine struc￾ture in three of the central peaks. This fine structure is present in every peak. a.u., arbitrary units. Fig. 2. Panchromatic waveguiding in a 425- m-long ribbon. (A) Dark-field image. Cross￾sectional dimensions are 520 nm by 275 nm. (B) PL image with the UV excitation spot center￾ed near the middle of the ribbon, showing waveguided emission from both ends. (C) Magnified dark-field PL view of the right end, with the laser focused on the left end. A wide (
1 m) ribbon lies across the ribbon of in￾terest. (Inset) A scanning electron micrograph of the right terminus of the nanoribbon, showing its rectangular cross sec￾tion. (D to F) Digital im￾ages of the guided emis￾sion during nonresonant excitation with monochromatic red, green, and blue light, respectively. The leftmost emission spot, caused by scattering at the ribbon-ribbon junction, can be eliminated by selectively removing the wide ribbon with the micromanipulator. R EPORTS 1270 27 AUGUST 2004 VOL 305SCIENCE www.sciencemag.org