SPONSORED BY R&D SYSTEMS INC. Expiring RD ww.rndsystems. com/pathways Science Integrated Compact Optical Vortex Beam Emitters Xinlun Cai et a/ Science338,363(2012); O:10.1126/ scIence.1226528 AAAS This copy is for your personal, non-commercial use only If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here mission to republish or repurpose articles or portions of articles can be obtained by g the guidelines he The following resources related to this article are available online at www.sciencemag.org(thisinformationiscurrentasofOctober23,2012) Updated information and services, including high-resolution figures, can be found in the online version of this article at http://www.sciencemag.org/content/338/6105/363.fullhtml Supporting Online Material can be found at http://www.sciencemag.org/content/suppl/2012/10/17/338.6105.363.Dc1.html http://www.sciencemag.org/content/suppl/2012/10/17/338.6105.363.dc2.html A list of selected additional articles on the science Web sites related to this article can be ooEmggo3s found at http://www.sciencemag.org/content/338/6105/363.fullhtml#related This article cites 29 articles 1 of which can be accessed free http://www.sciencemag.org/content/338/6105/363.fullhtmlref-list-1 Science(print ISSN 0036-8075: online ISSN 1095-9203)is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is a egistered trademark of AAAs
DOI: 10.1126/science.1226528 Science 338, 363 (2012); Xinlun Cai et al. Integrated Compact Optical Vortex Beam Emitters This copy is for your personal, non-commercial use only. colleagues, clients, or customers by clicking here. If you wish to distribute this article to others, you can order high-quality copies for your following the guidelines here. Permission to republish or repurpose articles or portions of articles can be obtained by www.sciencemag.org (this information is current as of October 23, 2012 ): The following resources related to this article are available online at http://www.sciencemag.org/content/338/6105/363.full.html version of this article at: Updated information and services, including high-resolution figures, can be found in the online http://www.sciencemag.org/content/suppl/2012/10/17/338.6105.363.DC2.html http://www.sciencemag.org/content/suppl/2012/10/17/338.6105.363.DC1.html Supporting Online Material can be found at: http://www.sciencemag.org/content/338/6105/363.full.html#related found at: A list of selected additional articles on the Science Web sites related to this article can be http://www.sciencemag.org/content/338/6105/363.full.html#ref-list-1 This article cites 29 articles, 1 of which can be accessed free: registered trademark of AAAS. 2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is a American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on October 23, 2012 www.sciencemag.org Downloaded from
REPORTS the formation of sn ngle-domain, elongated phors to build polarized light-emitting diodes(Fig 4, 15. L Carbone et al., Nano Lett. 7, 2942 (2007) e superparticles with /=11+ 4 um G to J, and fig S27) 16. D. V. Talapin et al, Nano Left. 7, 2951(2007) 1.1±03μm(Fg.4, B and c). These Our results show that anisotropic interactions 17. See supplementary materials on Science Online single-domain superparticles have a different mor- of CdSe-Cds nanorods can be used to synthesize Angew. Chem. Int. Ed. 47, 220B(2008)do 18.]. Q Zhuang, H M. Wu, Y. G. Yang, Y.C. phology from those multidomain superparticles colloidal superparticles with multiple well-defined 19. D B. Williams, C.B. Carter, Transmission Electron made from identical nanorods without incubation supercrystalline domains under thermodynamic treatment(fig. S21, C to E). equilibrium. Functionality-based anisotropic inter- during the superparticle Montagne, Y C. Cao 40% and are indefinitely stable in solvents with ing to the formation of single-domain, needle-like ma8.843(2012) strong polarity, such as water or ethanoL. However, rticles. We anticipate that these findings can 23. P. Ball, The sel -Made Tapestry: Pattern Formation m these particles can undergo intraparticle ripening be extended for the self-assembly of nano-objects 24. Y. N. Xia, Y. D. Yin, Y. Lu, L. McLellan, Adv. Funct. Mater. in lower-polarity solvents such as ethylene glycol, having other anisotropic shapes, as well as the 13, 907(2003) demonstrating that the needle-like morphology is self-assembly of two or more types of anisotropic 25. D. V. Talapin et al, Nano Lett. 3, 1677(2003) not an equilibrium shape(fig. S23). The meso- nano-objects into well-defined mesoscopic and 26. A Sitt, A Salant, G. Menagen, U Banin, Nano Lett. 11. scopic sizes of these needle-like superparticles macroscopic complex architectures(1-3) 27.]. F. Wang, M. S. Gudiksen, X. F. Duan, Y. allow them to be easily aligned into unidirec- C. M. Lieber, Science 293, 1455 (2001) tional line patterns on Si, N4 substrates through rces wnich can References and notes readily transferred into uniform and removable G. M Whitesides, B Grzybowski, Science 295, 2418 Acknowledgments: Supported by Office of Naval Research thin films of polydimethylsiloxane(PDMS)with 4-09-1-0441 ( Y CC), NSF Career Awar 2.5.Mann,Nat.Mate.8,781(2009) DMR-0645520, and the Cornell High Energy Synchrotro sizes as large as 5.0 cm x 5.0 cm(Fig 4, D andE, 3. S C. Glotzer, M 1. Solomon, Nat. Mater. 6. 557(2007). Source through NSF award DMR-0936384We thank SZou for and fig S25). The resulting thin films exhibit strong 4. M.R. Jones et al, Nat. Mater. 9, 913(2010) inearly polarized PL at 579 nm with a typical emission polarization ratio [p=(n-I+I, 6. M. Rycenga, ]. M. McLellan, Y. N. Xia, Adv. Mater. 20. cted at the Major Analytical Instrumentation Center University of Florida. A PCT Intermational Patent where /y and l, are the intensities parallel (/p) and perpendicular(1)to the nanorod long axis) of 7. A Salant, E. Amitay-Sadovsky, U Banin, Am. Chem. Application has been filed(docket no.995xC1PCT,"Laterally 0.88(Fig. 4F and fig S26), which is substantially 8.M.L. Tang, N Liu, J.A. Dionne, A P. Alivisatos, 1 Am. PCT/US2012/0429,ods Assemblies, "serial no 0C.128,10006(2006). Aligned Colloidal Na hem5a133,1320(2011). o5巴o higher than that of individual single CdSe-Cds 9. 5. Wei ef al, Am Chem. Soc. 131, 9728(2009) inside the elongated needle-like superparticles 12. J.J. Urban, D V. Talapin, E V Shevchenko, CR.Kagan, table? to s277 rials nanorods[0.75(25, 26)]. This PL anisotropy 10. C T Black, C.B. Murray, R.L. Sandstrom, S H Sun, enhancement can be attributed to a combinatio Materials and Method of dielectric effect and collective electric dipole 11. A. Courty, A Mermet, P. A. Albouy, E.Duval, MP.Pileni coupling effects among the CdSe-Cds nanorods Nat. Mater.4,395(2005) embedded in PDMS films(26-28). In addition, 13. Y Yamada et al. Nat. Chem. 3. 372(2011) 品E we show that the superparticle-embedded PDMS 14.1 Q. Zhuang et al. I. Am. Chem. Soc. 131, 6084 3 May 2012: accepted 6 September 2012 Integrated Compact Optical Vortex miniaturization, and scalability compared with bulk optics(13). Compact, robust, and efficient Beam emitters ceivers are critical elements, as they can be in- s planar waveguide-based OAM emitters and re- tegrated in large numbers and interconnected via Marc Sorel, Jeremy L. o'Brien,2 Mark G. Thompson, Siyuan Yu4 ris, Jiangbo Zhu, 4,5 Xinlun Cai, Jianwei Wang, 2 Michael ] Strain, Benjamin Johnson-Mc waveguides with each other and with lasers and detectors to form photonic integrated circuits (PICs). Recently, a waveguide-based device has Emerging applications based on optical beams carrying orbital angular momentum(OAM) will probably been reported for multiplexing and demultiplex require photonic integrated devices and circuits for miniaturization, improved performance, and ing of oAM beams as a means of realizing multi- enhanced functionality. We demonstrate silicon- integrated optical vortex emitters, using angular gratings channel optical communication(5, 14). However to extract light confined in whispering gallery modes with high OAM into free-space beams with ts large size(2.5 by 1.5 mm)and phase-sensitive well-controlled amounts of oAM. The smallest device has a radius of 3.9 micrometers. Experimental rayed waveguide structure do not yet support characterization confirms the theoretical prediction that the emitted beams carry exactly defined and large-scale integration. Here, we report micrometer- adjustable OAM Fabrication of integrated arrays and demonstration of simultaneous emission of multiple sized silicon photonic waveguide OAM devices identical optical vortices provide the potential for large-scale integration of optical vortex emitters complementary metal-oxide-semiconductor compatible silicon chips for wide-ranging applications Photonics Group, Merchant ers School of K.Department of Elec- T the discovery that photons in optical vortices involve passing free-space light beams tronics and Electrical Engineering, University of Glasgo vortices-light beams with helical phase through optical elements, including computer- UK. State Key Laboratory of Optoelectronic Materials and fronts and an azimuthal component of the generated holograms(4, 8), spiral phase plates Technologies and School of Physics and Engineering, Sun wave vector-can carry orbital angular momentum (9), inhomogeneous birefringent elements(10), Applicaton-Specific Integrated Circuits and Systems and De- (OAM)()may lead to wide-ranging applications subwavelength gratings(ID), and nanoantennas(12) rtment of Communication Saence and Engineering, Fudan in optical microscopy(2), micromanipulation (3), Photonic integration has been a major pro- University, Shanghai, China Gbe-space communication(4, 5, and quantum in- pellant for widespread application of photonic "To whom correspondence should be addressed. E-mail nation(6, 7). Techniques for generating optical technologies due to advantages in reliability, s. u@bristol. acuk www.sciencemag.orgScieNceVol33819ocTober2012 363
leading to the formation of single-domain, elongated needle-like superparticles with l = 11 T 4 mm and d = 1.1 T 0.3 mm (Fig. 4, B and C). These single-domain superparticles have a different morphology from those multidomain superparticles made from identical nanorods without incubation treatment (fig. S21, C to E). The needle-like superparticles from unoptimized syntheses exhibit a PL quantum yield of ~40% and are indefinitely stable in solvents with strong polarity, such as water or ethanol. However, these particles can undergo intraparticle ripening in lower-polarity solvents such as ethylene glycol, demonstrating that the needle-like morphology is not an equilibrium shape (fig. S23). The mesoscopic sizes of these needle-like superparticles allow them to be easily aligned into unidirectional line patterns on Si3N4 substrates through capillary forces (24) (fig. S24), which can be readily transferred into uniform and removable thin films of polydimethylsiloxane (PDMS) with sizes as large as 5.0 cm × 5.0 cm (Fig. 4, D and E, and fig. S25). The resulting thin films exhibit strong linearly polarized PL at 579 nm with a typical emission polarization ratio [r = (I|| – I⊥)/(I|| + I⊥), where I|| and I⊥ are the intensities parallel (I||) and perpendicular (I⊥) to the nanorod long axis] of 0.88 (Fig. 4F and fig. S26), which is substantially higher than that of individual single CdSe-CdS nanorods [0.75 (25, 26)]. This PL anisotropy enhancement can be attributed to a combination of dielectric effect and collective electric dipole coupling effects among the CdSe-CdS nanorods inside the elongated needle-like superparticles embedded in PDMS films (26–28). In addition, we show that the superparticle-embedded PDMS films can be used as energy downconversion phosphors to build polarized light-emitting diodes (Fig. 4, G to J, and fig. S27). Our results show that anisotropic interactions of CdSe-CdS nanorods can be used to synthesize colloidal superparticles with multiple well-defined supercrystalline domains under thermodynamic equilibrium. Functionality-based anisotropic interactions between these nanorods can be kinetically introduced during the superparticle synthesis, leading to the formation of single-domain, needle-like particles. We anticipate that these findings can be extended for the self-assembly of nano-objects having other anisotropic shapes, as well as the self-assembly of two or more types of anisotropic nano-objects into well-defined mesoscopic and macroscopic complex architectures (1–3). References and Notes 1. G. M. Whitesides, B. Grzybowski, Science 295, 2418 (2002). 2. S. Mann, Nat. Mater. 8, 781 (2009). 3. S. C. Glotzer, M. J. Solomon, Nat. Mater. 6, 557 (2007). 4. M. R. Jones et al., Nat. Mater. 9, 913 (2010). 5. K. Miszta et al., Nat. Mater. 10, 872 (2011). 6. M. Rycenga, J. M. McLellan, Y. N. Xia, Adv. Mater. 20, 2416 (2008). 7. A. Salant, E. Amitay-Sadovsky, U. Banin, J. Am. Chem. Soc. 128, 10006 (2006). 8. M. L. Tang, N. Liu, J. A. Dionne, A. P. Alivisatos, J. Am. Chem. Soc. 133, 13220 (2011). 9. Q. S. Wei et al., J. Am. Chem. Soc. 131, 9728 (2009). 10. C. T. Black, C. B. Murray, R. L. Sandstrom, S. H. Sun, Science 290, 1131 (2000). 11. A. Courty, A. Mermet, P. A. Albouy, E. Duval, M. P. Pileni, Nat. Mater. 4, 395 (2005). 12. J. J. Urban, D. V. Talapin, E. V. Shevchenko, C. R. Kagan, C. B. Murray, Nat. Mater. 6, 115 (2007). 13. Y. Yamada et al., Nat. Chem. 3, 372 (2011). 14. J. Q. Zhuang et al., J. Am. Chem. Soc. 131, 6084 (2009). 15. L. Carbone et al., Nano Lett. 7, 2942 (2007). 16. D. V. Talapin et al., Nano Lett. 7, 2951 (2007). 17. See supplementary materials on Science Online. 18. J. Q. Zhuang, H. M. Wu, Y. G. Yang, Y. C. Cao, Angew. Chem. Int. Ed. 47, 2208 (2008). 19. D. B. Williams, C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials Science (Springer, New York, ed. 2, 2009). 20. L. D. Marks, D. J. Smith, Nature 303, 316 (1983). 21. L. D. Marks, J. Cryst. Growth 61, 556 (1983). 22. Y. Nagaoka, T. Wang, J. Lynch, D. LaMontagne, Y. C. Cao, Small 8, 843 (2012). 23. P. Ball, The Self-Made Tapestry: Pattern Formation in Nature (Oxford Univ. Press, Oxford, 2000). 24. Y. N. Xia, Y. D. Yin, Y. Lu, J. McLellan, Adv. Funct. Mater. 13, 907 (2003). 25. D. V. Talapin et al., Nano Lett. 3, 1677 (2003). 26. A. Sitt, A. Salant, G. Menagen, U. Banin, Nano Lett. 11, 2054 (2011). 27. J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, C. M. Lieber, Science 293, 1455 (2001). 28. S. Y. Wang et al., Nat. Commun. 2, 364 (2011). Acknowledgments: Supported by Office of Naval Research grant N00014-09-1-0441 (Y.C.C.), NSF Career Award DMR-0645520, and the Cornell High Energy Synchrotron Source through NSF award DMR-0936384. We thank S. Zou for helpful discussions and X. Liu for providing line-patterned Si3N4 substrates. Transmission electron microscope work was conducted at the Major Analytical Instrumentation Center at the University of Florida. A PCT International Patent Application has been filed (docket no. 995XC1PCT, “Laterally Aligned Colloidal Nanorods Assemblies,” serial no. PCT/US2012/042958). Supplementary Materials www.sciencemag.org/cgi/content/full/338/6105/358/DC1 Materials and Methods Supplementary Text Figs. S1 to S27 Table S1 References (29–32) 3 May 2012; accepted 6 September 2012 10.1126/science.1224221 Integrated Compact Optical Vortex Beam Emitters Xinlun Cai,1 Jianwei Wang,2 Michael J. Strain,3 Benjamin Johnson-Morris,1 Jiangbo Zhu,4,5 Marc Sorel,3 Jeremy L. O’Brien,2 Mark G. Thompson,2 Siyuan Yu1,4* Emerging applications based on optical beams carrying orbital angular momentum (OAM) will probably require photonic integrated devices and circuits for miniaturization, improved performance, and enhanced functionality. We demonstrate silicon-integrated optical vortex emitters, using angular gratings to extract light confined in whispering gallery modes with high OAM into free-space beams with well-controlled amounts of OAM. The smallest device has a radius of 3.9 micrometers. Experimental characterization confirms the theoretical prediction that the emitted beams carry exactly defined and adjustable OAM. Fabrication of integrated arrays and demonstration of simultaneous emission of multiple identical optical vortices provide the potential for large-scale integration of optical vortex emitters on complementary metal-oxide–semiconductor compatible silicon chips for wide-ranging applications. The discovery that photons in optical vortices—light beams with helical phase fronts and an azimuthal component of the wave vector—can carry orbital angular momentum (OAM) (1) may lead to wide-ranging applications in optical microscopy (2), micromanipulation (3), free-space communication (4, 5), and quantum information (6, 7). Techniques for generating optical vortices involve passing free-space light beams through optical elements, including computergenerated holograms (4, 8), spiral phase plates (9), inhomogeneous birefringent elements (10), subwavelength gratings (11), and nanoantennas (12). Photonic integration has been a major propellant for widespread application of photonic technologies due to advantages in reliability, miniaturization, and scalability compared with bulk optics (13). Compact, robust, and efficient planar waveguide–based OAM emitters and receivers are critical elements, as they can be integrated in large numbers and interconnected via waveguides with each other and with lasers and detectors to form photonic integrated circuits (PICs). Recently, a waveguide-based device has been reported for multiplexing and demultiplexing of OAM beams as a means of realizing multichannel optical communication (5, 14). However, its large size (2.5 by 1.5 mm2 ) and phase-sensitive arrayed waveguide structure do not yet support large-scale integration. Here, we report micrometersized silicon photonic waveguide OAM devices 1 Photonics Group, Merchant Venturers School of Engineering, University of Bristol, Bristol, UK. 2 Centre for Quantum Photonics, University of Bristol, Bristol, UK. 3 Department of Electronics and Electrical Engineering, University of Glasgow, UK. 4 State Key Laboratory of Optoelectronic Materials and Technologies and School of Physics and Engineering, Sun Yat-sen University, Guangzhou, China. 5 State Key Laboratory of Application-Specific Integrated Circuits and Systems and Department of Communication Science and Engineering, Fudan University, Shanghai, China. *To whom correspondence should be addressed. E-mail: s.yu@bristol.ac.uk www.sciencemag.org SCIENCE VOL 338 19 OCTOBER 2012 363 REPORTS on October 23, 2012 www.sciencemag.org Downloaded from
REPORTS that emit vector optical vortices carrying well- concept introduced in(20), we conclude that th their resonance associated with / =0 to be defined, quantized, and tunable OAM, as well as the radiated beams are vector vortices with a near the center of our tunable laser's wavelength integrated OAM emitter arrays that emit multiple topological Pancharatnam charge of l=p-q. range(1470 to 1580 nm). Figure 1, D and e, optical vortices simultaneously. The topological Pancharatnam charge, similar shows scanning electron microscopy(SEM) Circular optical resonators, such as micro- to the topological charge in scalar vortices, is ages of the device with R= 3.9 um. rings or microdisks(15), support whispering gal- directly related to the oam of vector vortices: Both types of devices have been characterized lery modes(WGMs)carrying high OAM(16, I7). The amount of OAM carried by the radiated by launching continuous-wave light from a tun- To extract the confined WGMs into free-space beam is Ih per photon, where h is Planck,s able laser into the access waveguide to excite the emission, we embed angular grating structures constant h divided by 2r. uasi-TE mode. The near-field intensity of the into the WGM resonator(Fig. 1A)with a pe We therefore have a very simple yet robust radiated beam from the devices, with 1=0, is riodic modulation of refractive index in the azi- OAM emitter scheme, in which I can only take annular with a dark center, as imaged on an in- muthal direction. values, being solely determined by the dif- frared camera(Fig. 2A), and is predominant The working principle of the angular grating between integers p and g. Equation 1 in- azimuthally polarized ( Fig. 2, B to e) is analogous to that of second-order gratings wide- dicates that the angular grating diffracts the light The emission spectrum of the device with R ly used in straight waveguides as input/output confined in a pth order WGM [carrying a OAM 7.5 um and q=72 is shown in Fig 3A. Each couplers(18), in which the guided wave is scat- of ph per photon (16)] into a free-space beam, resonance corresponds to a distinctive WGM tered by the grating elements collectively and an changing the oam by an amount of gh per pho- (p value). The emission efficiencies at various appreciable fraction of power is diverted in a cer- ton in the process. For a fabricated device, g is a resonances are measured to be 3 to 13%(16)and tain direction ( in which constructive interfer- structural constant, whereas the value of p can be are higher at the longer wavelength side due to ence occurs. The wavefront of the radiated light changed by exciting selected WGMs. Hence, the wavelength-dependent coupling between the is a plane tilted at angle ( p(Fig. 1B). If the wave- variable OAM can be generated by tuning the access waveguide and the resonator guide with grating is curved to form a loop so injected laser wavelength to various cavity reso- We used an interference scheme to character- that the guided wave forms WGMs, by way of nances or, alternatively, by tuning cavity reso- ize the wavefront structure. We observe that the Huygens'Principle, the wavefront of the radiated nances with respect to a fixed injection wavelength radiated CV vortex beam can be described as the ight should skew in the azimuthal direction and by changing the cavity refractive index. The in- superposition of two orthogonal scalar vortices, as y transform into a helix, suggesting the creation of teger p can also take negative values correspond- Ecv in Eq. 2 can be further decomposed into(22) OAM-carrying beam(Fig. IC). Rigorous the- ing to the opposite WGM propagating direction. oretical derivation shows that a Gm only emits We designed and fabricated two types of mi- Ecy into a free-space beam when the following an- coring devices(R=3.9 um, =36 and R=7.5 um. gular phase-matching condition is satisfied (16) q=72)on the licon-on-insulator chip(16 x expi(l-1)e here p is the azimuthal order of the WGM A involved, or the number of optical periods around the resonator, and q is the number of grain mal""1 ements around the resonator vrad is the azimuth 5oooEoo3ssE9 propagation constant(phase shift per unit azi- R=3.9p muthal angle)of the radiated beam, which gives rise to the azimuthal component of the wave vec- Incoming w tor and, hence, OAM. The z component of the propagation constant of the radiation mode is by Bad: =V(2n/)"-(mad R),where c a is the vacuum wavelength and R is the resonator radius(Fig 1A) Because the state of polarization(SOP)of the source WGMs and the angular grating structure Fig. 1. (A) Illustration of the device with an symmetric, the radiated beams lar grating patterned along the inner wall should maintain this symmetry and should be of a microring resonator that is coupled to cylindrical vector(CV) beams(19). In our de- access waveguide for optical input. (B) lli vices(Fig. 1A), for quasH-transverse electric (TE) and the tilted wavefront of the radiated light. (C) Illustration of an angular grating together with the WGMs, the radiated near field is predominantly helical wavefront of the radiated beam. (D and E)SEM images of a fabricated device (R=3.9 um) azimuthally polarized (16) with its Jones vector Ecy written as(20, 21) -sine E c￥ p(irade) expi(p-q)0 )ep0)(2) ed near-field intensity distribution of the radiated beam with(= 0. (B to E)Measured ons after a polarizer in the directions indicated by the arrows. a two-lobe intensity pattern wheree is the azimuthal angle. and i is the unit nal to the polarizer axis is obtained. When the polarizer is rotated, the two-lobe pattern ary number(Fig. 1A). Following the manner, confirming that the radiated beam is a Cv beam with azimuthal polarization 19octobeR2012Vol338ScieNcewww.sciencemag.org
that emit vector optical vortices carrying welldefined, quantized, and tunable OAM, as well as integrated OAM emitter arrays that emit multiple optical vortices simultaneously. Circular optical resonators, such as microrings or microdisks (15), support whispering gallery modes (WGMs) carrying high OAM (16, 17). To extract the confined WGMs into free-space emission, we embed angular grating structures into the WGM resonator (Fig. 1A) with a periodic modulation of refractive index in the azimuthal direction. The working principle of the angular grating is analogous to that of second-order gratings widely used in straight waveguides as input/output couplers (18), in which the guided wave is scattered by the grating elements collectively and an appreciable fraction of power is diverted in a certain direction ϕ, in which constructive interference occurs. The wavefront of the radiated light is a plane tilted at angle ϕ (Fig. 1B). If the waveguide with grating is curved to form a loop so that the guided wave forms WGMs, by way of Huygens’ Principle, the wavefront of the radiated light should skew in the azimuthal direction and transform into a helix, suggesting the creation of an OAM-carrying beam (Fig. 1C). Rigorous theoretical derivation shows that a WGM only emits into a free-space beam when the following angular phase-matching condition is satisfied (16) nrad ¼ p − q ð1Þ where p is the azimuthal order of the WGM involved, or the number of optical periods around the resonator, and q is the number of grating elements around the resonator. nrad is the azimuthal propagation constant (phase shift per unit azimuthal angle) of the radiated beam, which gives rise to the azimuthal component of the wave vector and, hence, OAM. The z component of the propagation constant of the radiation mode is given by brad,z ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p=l 2 − ðnradRÞ 2 q , where l is the vacuum wavelength and R is the resonator radius (Fig. 1A). Because the state of polarization (SOP) of the source WGMs and the angular grating structure are both cylindrically symmetric, the radiated beams should maintain this symmetry and should be cylindrical vector (CV) beams (19). In our devices (Fig. 1A), for quasi–transverse electric (TE) WGMs, the radiated near field is predominantly azimuthally polarized (16) with its Jones vector ECV written as (20, 21) ECV ¼ −sinq cosq expðinradqÞ ¼ −sinq cosq exp½iðp − qÞq ¼ −sinq cosq expðilqÞ ð2Þ where q is the azimuthal angle, and i is the unit imaginary number (Fig. 1A). Following the concept introduced in (20), we conclude that the radiated beams are vector vortices with a topological Pancharatnam charge of l = p – q. The topological Pancharatnam charge, similar to the topological charge in scalar vortices, is directly related to the OAM of vector vortices: The amount of OAM carried by the radiated beam is lħ per photon, where ħ is Planck’s constant h divided by 2p. We therefore have a very simple yet robust OAM emitter scheme, in which l can only take integer values, being solely determined by the difference between integers p and q. Equation 1 indicates that the angular grating diffracts the light confined in a pth order WGM [carrying a OAM of pħ per photon (16)] into a free-space beam, changing the OAM by an amount of qħ per photon in the process. For a fabricated device, q is a structural constant, whereas the value of p can be changed by exciting selected WGMs. Hence, variable OAM can be generated by tuning the injected laser wavelength to various cavity resonances or, alternatively, by tuning cavity resonances with respect to a fixed injection wavelength by changing the cavity refractive index. The integer p can also take negative values corresponding to the opposite WGM propagating direction. We designed and fabricated two types of microring devices (R= 3.9 mm, q = 36 and R= 7.5 mm, q = 72) on the same silicon-on-insulator chip (16), with their resonance associated with l = 0 to be near the center of our tunable laser’s wavelength range (1470 to 1580 nm). Figure 1, D and E, shows scanning electron microscopy (SEM) images of the device with R = 3.9 mm. Both types of devices have been characterized by launching continuous-wave light from a tunable laser into the access waveguide to excite the quasi-TE mode. The near-field intensity of the radiated beam from the devices, with l = 0, is annular with a dark center, as imaged on an infrared camera (Fig. 2A), and is predominantly azimuthally polarized (Fig. 2, B to E). The emission spectrum of the device with R = 7.5 mm and q = 72 is shown in Fig. 3A. Each resonance corresponds to a distinctive WGM ( p value). The emission efficiencies at various resonances are measured to be 3 to 13% (16) and are higher at the longer wavelength side due to the wavelength-dependent coupling between the access waveguide and the resonator. We used an interference scheme to characterize the wavefront structure. We observe that the radiated CV vortex beam can be described as the superposition of two orthogonal scalar vortices, as ECV in Eq. 2 can be further decomposed into (22) ECV ¼ i 2 1 −i exp½iðl þ 1Þq − i 2 1 i exp½iðl − 1Þq ð3Þ Fig. 1. (A) Illustration of the device with angular grating patterned along the inner wall of a microring resonator that is coupled to an access waveguide for optical input. (B) Illustration of a linear waveguide with gratings and the tilted wavefront of the radiated light. (C) Illustration of an angular grating together with the helical wavefront of the radiated beam. (D and E) SEM images of a fabricated device (R = 3.9 mm). Fig. 2. (A) Measured near-field intensity distribution of the radiated beam with l = 0. (B to E) Measured intensity distributions after a polarizer in the directions indicated by the arrows. A two-lobe intensity pattern arranged orthogonal to the polarizer axis is obtained. When the polarizer is rotated, the two-lobe pattern rotates in the same manner, confirming that the radiated beam is a CV beam with azimuthal polarization. 364 19 OCTOBER 2012 VOL 338 SCIENCE www.sciencemag.org REPORTS on October 23, 2012 www.sciencemag.org Downloaded from
REPORTS which consists of a right-hand circularly polar- indicated by the chirality of the pattern. Thus, the sisting of three identical emitters(R=7.5 um, q ized(RHCP) beam with topological charge of nine resonances correspond to 1=0, +1, #2, #3, 72)coupled to the same access waveguide(Fig 4, 7+ I and a left-hand circularly polarized (LHCp) and +4. Similar results for the device with r= A and B). Simultaneous emission of identical beam with 1-1. This indicates a new scheme of 3.9 um are given in the supplementary materials vortices has been verified, as shown in Fig 4, C measuring the value of/: When the radiated beam (16). Moreover, the spiral patterns rotate when and D. The spiral patterns rotate synchronously interferes with a copropagating circularly polar- the phase of the reference beam is changed con- when the phase of the reference beam is changed ized reference beam, spiral interference pattems tinuously(movies SI and S2). These results show (movie S3). should be produced, with the number of arms unambiguously that the wavefront of the radiated Our OAM emitters based on complementary qual to either 1-1 or 1+1, depending on the beams is helical with /=p-g Beams with larger metal oxide semiconductor compatible silicon andedness of the reference beam SoP OAM quantum numbers I can be generated from PICs produce optical vortex beams with distinc- The measured interference patterns(Fig 3, B the device. However, the observable /is limited tive and variable OAM values from a very simple nd C) have spiral arms equal to /-1(RHCP)or by the tuning range of the tunable laser. and small device, with no need for any fine ad 1+1(LHCP)as predicted in the aforementioned To demonstrate the potential of photonic in- justment of optical phase. While we have al- heme, and the sign of the topological charge is tegration, we fabricated OAM emitter arrays con- ready achieved useful emission efficiency of up to 13%, efficiency can be maximized by engineer ing the coupling ratio between the resonator and the access waveguide to the critical coupling point Mode splitting (23), at which all of the input power enters the R=7.5m resonator. As demonstrated by the integrated ar- rays, integration of a large number of devices can 7 be realized with the use of standard integrated cir- s2 uit technology to form complicated formations on silicon wafers Such scalable integration could open up truly large-scale integrated applications opportunities For example, it is possible to build oaM quan- communications channels between two chi ining the same integrated OAMPIC吕 as the oam transmitter and the other as DAM receiver(according to the principle of rec- 9 iprocity, the device emitting a specific vortex 圆國回國回國同包 electively receive the same beam). g been shown that OAM multiplex- d use of PICs(14), our device enables rapid switch- Fig. 3. (A) Radiation spectrum for a device with R= 7.5 um measured by scanning input laser wave ong oam states, as semiconductor tun- ngth Thel=0 wavelength is-1525 nm The doublets in the spectrum result from the mode splitting able lasers can already switch wavelengths in E strongest cross-coupling occurs at the wavelength with/=0 due to Bragg reflection, which is therefore been shown to tune at frequencies up to 40 GHzg associated with the largest split. (B and C) Interference patterns with LHCP and RHCP reference beams. (25). Therefore, our device provides an approach for an integrated oAM switch or modulator. An- lation of particles(26). By selectively lighting s groups of integrated emitter arrays, controllable and reconfigurable drivers can be configured for microfluidic and nanoparticle manipulation ma hines, such as lab-on-a-chip, optical tweezers, R I C Spreeuw, A45,8185(1992 2. S. Furhapter, A Jesacher, 5. Bernet, M. Ritsch-Marte pt. Lett.30.1953(2005) Fig. 4. (A)llustration of an array 3. D. G. Grier. Nature 424. 81 insisting of three identical emitters. 4. G. Gibson et al., Opt. Express The three-dimensional emission pa tern is calculated with the use of a Communication Conference, OSA Technical Dige dipole-emissionbased semianalyt ical model (16).(B)Micrograph of a fabricated array. (C)Near-field intensity patterns emitted from the array. The difference in their brightness 313(2001).1, G. Weihs, A. Zeilinger, Nature 412 is attributed to slight differences in their resonance peaks due to fabrication variations. (D)Example of an 7. G. Molina-lerriza, J. P. Torres, L. Torner, Nat. Phys. 3, interference pattern between the emitted beams from the array and the copropagating RHCP Gaussian 305(2007 beam. All beams have four arms and, therefore, the same OAM order 1=-3 The uneven brightness is due. ot et 7 221 01992),.r >min, A g winte, tensen, side vortices are somewhat deformed due to lens aberration causing phase-front distortion 1. P. Woerdman, Opt. Commun. 112, 321(1994) www.sciencemag.orgScieNceVol33819OctobeR2012 365
which consists of a right-hand circularly polarized (RHCP) beam with topological charge of l + 1 and a left-hand circularly polarized (LHCP) beam with l – 1. This indicates a new scheme of measuring the value of l: When the radiated beam interferes with a copropagating circularly polarized reference beam, spiral interference patterns should be produced, with the number of arms equal to either l – 1 or l + 1, depending on the handedness of the reference beam SOP. The measured interference patterns (Fig. 3, B and C) have spiral arms equal to l – 1 (RHCP) or l + 1 (LHCP) as predicted in the aforementioned scheme, and the sign of the topological charge is indicated by the chirality of the pattern. Thus, the nine resonances correspond to l = 0, T1, T2, T3, and T4. Similar results for the device with R = 3.9 mm are given in the supplementary materials (16). Moreover, the spiral patterns rotate when the phase of the reference beam is changed continuously (movies S1 and S2). These results show unambiguously that the wavefront of the radiated beams is helical with l = p – q. Beams with larger OAM quantum numbers l can be generated from the device. However, the observable l is limited by the tuning range of the tunable laser. To demonstrate the potential of photonic integration, we fabricated OAM emitter arrays consisting of three identical emitters (R = 7.5 mm, q = 72) coupled to the same access waveguide (Fig. 4, A and B). Simultaneous emission of identical vortices has been verified, as shown in Fig. 4, C and D. The spiral patterns rotate synchronously when the phase of the reference beam is changed (movie S3). Our OAM emitters based on complementary metal oxide semiconductor compatible silicon PICs produce optical vortex beams with distinctive and variable OAM values from a very simple and small device, with no need for any fine adjustment of optical phase. While we have already achieved useful emission efficiency of up to 13%, efficiency can be maximized by engineering the coupling ratio between the resonator and the access waveguide to the critical coupling point (23), at which all of the input power enters the resonator. As demonstrated by the integrated arrays, integration of a large number of devices can be realized with the use of standard integrated circuit technology to form complicated formations on silicon wafers. Such scalable integration could open up truly large-scale integrated applications opportunities. For example, it is possible to build OAM quantum communications channels between two chips, each containing the same integrated OAM PICs— one as the OAM transmitter and the other as the OAM receiver (according to the principle of reciprocity, the device emitting a specific vortex beam will selectively receive the same beam). Though it has been shown that OAM multiplexing and demultiplexing can be achieved with the use of PICs (14), our device enables rapid switching among OAM states, as semiconductor tunable lasers can already switch wavelengths in nanoseconds (24), and silicon microrings have been shown to tune at frequencies up to 40 GHz (25). Therefore, our device provides an approach for an integrated OAM switch or modulator. Another application area could be micromanipulation of particles (26). By selectively lighting groups of integrated emitter arrays, controllable and reconfigurable drivers can be configured for microfluidic and nanoparticle manipulation machines, such as lab-on-a-chip, optical tweezers, and optical spanners. References and Notes 1. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, J. P. Woerdman, Phys. Rev. A 45, 8185 (1992). 2. S. Fürhapter, A. Jesacher, S. Bernet, M. Ritsch-Marte, Opt. Lett. 30, 1953 (2005). 3. D. G. Grier, Nature 424, 810 (2003). 4. G. Gibson et al., Opt. Express 12, 5448 (2004). 5. N. K. Fontaine, C. R. Doerr, L. Buhl, in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, Washington, DC, 2012), paper OTu1l.2. 6. A. Mair, A. Vaziri, G. Weihs, A. Zeilinger, Nature 412, 313 (2001). 7. G. Molina-Terriza, J. P. Torres, L. Torner, Nat. Phys. 3, 305 (2007). 8. N. R. Heckenberg, R. McDuff, C. P. Smith, A. G. White, Opt. Lett. 17, 221 (1992). 9. M. W. Beijersbergen, R. P. C. Coerwinkel, M. Kristensen, J. P. Woerdman, Opt. Commun. 112, 321 (1994). Fig. 4. (A) Illustration of an array consisting of three identical emitters. The three-dimensional emission pattern is calculated with the use of a dipole-emission–based semianalytical model (16). (B) Micrograph of a fabricated array. (C) Near-field intensity patterns emitted from the array. The difference in their brightness is attributed to slight differences in their resonance peaks due to fabrication variations. (D) Example of an interference pattern between the emitted beams from the array and the copropagating RHCP Gaussian beam. All beams have four arms and, therefore, the same OAM order l = –3. The uneven brightness is due to the Gaussian distribution of the reference beam, which is only coaxial with the middle vortex. The two side vortices are somewhat deformed due to lens aberration causing phase-front distortion. Fig. 3. (A) Radiation spectrum for a device with R = 7.5 mm measured by scanning input laser wavelength. The l = 0 wavelength is ~1525 nm. The doublets in the spectrum result from the mode splitting caused by cross-coupling between the otherwise degenerate clockwise and counterclockwise WGMs. The strongest cross-coupling occurs at the wavelength with l = 0 due to Bragg reflection, which is therefore associated with the largest split. (B and C) Interference patterns with LHCP and RHCP reference beams. Each pattern in (B) has l + 1 spiral arms, whereas each pattern in (C) has l – 1 spiral arms. www.sciencemag.org SCIENCE VOL 338 19 OCTOBER 2012 365 REPORTS on October 23, 2012 www.sciencemag.org Downloaded from
REPORTS 10. L Marrucci, C Manzo, D. Paparo, Phys. Rev. Lett. 96, 20. Z. Bomzon, V Kleiner, E Hasman, Opt Lett. 26, 1424 very useful discussions and C. Railton (Merchant Venturers 163905(2006 11.G A. Niv, V. Kleiner, E. Hasman, Opt. Left. 27. Niv, G. Biene, V. Kleiner, E. Hasman, Opt. Express 14, the finite-difference time-domain simulation tool 875(2002) J.W. is funded by European Union FP7 FET-OPEN project 12. N. Yu et aL., Science 334. 333(2011) Moreno, ]. A. Davis, l. Ruiz, D. M. Cottrell, Opt. Express PHORBITEC. 13. M. Smit, ]. van der Tol, M. Hill, Laser Photon. Rev. 6, 18.7173(2010 23. A. Yariv, Electron. Left 36. 321 (2000). 14. C.R. Doerr, LL BuhL, Opt. Lett. 36, 1209(2011) 24. Y. Yu, R. ODowd, IEEE Photon. TechnoL Lett. 14, 1397 Supplementary Materials 15. K. J. Vahala, Nature 424, 839(2003). 17. A B Matsko, A A Savchenkov, D StrekaloV, L Maleki, 26. K LadavaG, D. Grier, Opt Express 12, 1144(2004). Figs. S1 to S7 Phys. Rev. Leff. 95, 143904(2005) eferences (27-31 D. Taillaert et al., Jpn. J. Appl Phys. 45, 6071 thank M. Ber University of 13230陈m University of ed 10 September 2012 (Department of Physics, Ur Glasgow, UK 10 1126/science Lethally Hot Temperatures During the reasons, such as the prolonged delay in reco ery (3), the prevalence of small taxa(4), and the Early Triassic Greenhouse absence of coal deposits throughout the Early Triassic (5). These and several facets of low- latitude fossil records shown below. includins Yadong Sun, 1,2. Michael M. Joachimski, Paul B. Wignall, 2 Chunbo Yan, Yanlong Chen,. ish, marine reptile, and tetrapod distributions Haishui Jiang, Lina Wang, Xulong Lai can be related to extreme temperatures in excess of tolerable thermal thresholds ON2ooso Climate warming long has been implicated Global warming is widely regarded to have played a contributing role in numerous past biotic crises. as one cause of the end-Permian crisis (2, 6), with Here, we show that the end-Permian mass extinction coincided with a rapid temperature rise to arbon dioxide release from Siberian eruptions exceptionally high values in the Early Triassic that were inimical to life in equatorial latitudes and and related processes providing a potential trig- suppressed ecosystem recovery. This was manifested in the loss of calcareous algae, the near-absence ger for it(7, 8). Conodont apatite oxygen isotope of fish in equatorial Tethys, and the dominance of small taxa of invertebrates during the thermal 55omE8 maxima. High temperatures drove most Early Triassic plants and animals out of equatorial terrestrial cosystems and probably were a major cause of the end-Smith State Key Laboratory of Geobiology and Environmental n csiS iences (Wuhan), Wuha A nthropogenic global warming likely is the spread of marine anoxia(2). Here, we show vironment, University of Leeds Leeds LS2 9J, UK. Geozentn contributing to the rapid loss of biolog. that lethally hot temperatures exerted a direct cal diversity currently occurring(/). Cli- control on extinction and recovery during and 91054 Erangen, Germany. "institute of Earth Sciences-Geology Ec0>s3sE mate warming also has been implicated in severe in the aftermath of the end-Permian mass ex- Graz, Austria. biotic crises in the geological past, but only as a tinction. As well as the scale of the losses, the "To whom correspondence should be addressed. E-mail corollary to more direct causes of death such as aftermath of this event is remarkable for several eeys@leeds.acuk Fig. 1. Early Triassic pa- leogeography showing arly Triassic World 252-247Ma Panthalassa fish and marine reptiles quatorial occurrence of oth groups when ich- thyosaurs had evolved in northern dimes. The global distribution of M NANPANJIANG trapods (25)indicates Paleo-Tethys occurrences almost ex- Yunnan Nuodeng clusively in higher lati- PANGEA tudes (30 N and >405) roughout the Early Tri- assic, with rare exceptions Africa ☆ Guangxi Province sp, paleolatitude-10.N) GONDWANA cality Qinzhi and Poland (paleolatitude 20°N, both probably of middle-ate Spathian age (25, 26). () Paleo- (45-47). Fish and ichthy- occurrences, see table 52. GBc 366 19OctobeR2012Vol338ScieNcewww.sciencemag.org
10. L. Marrucci, C. Manzo, D. Paparo, Phys. Rev. Lett. 96, 163905 (2006). 11. G. Biener, A. Niv, V. Kleiner, E. Hasman, Opt. Lett. 27, 1875 (2002). 12. N. Yu et al., Science 334, 333 (2011). 13. M. Smit, J. van der Tol, M. Hill, Laser Photon. Rev. 6, 1 (2012). 14. C. R. Doerr, L. L. Buhl, Opt. Lett. 36, 1209 (2011). 15. K. J. Vahala, Nature 424, 839 (2003). 16. Materials and methods are available as supplementary materials on Science Online. 17. A. B. Matsko, A. A. Savchenkov, D. Strekalov, L. Maleki, Phys. Rev. Lett. 95, 143904 (2005). 18. D. Taillaert et al., Jpn. J. Appl. Phys. 45, 6071 (2006). 19. R. Dorn, S. Quabis, G. Leuchs, Phys. Rev. Lett. 91, 233901 (2003). 20. Z. Bomzon, V. Kleiner, E. Hasman, Opt. Lett. 26, 1424 (2001). 21. A. Niv, G. Biener, V. Kleiner, E. Hasman, Opt. Express 14, 4208 (2006). 22. I. Moreno, J. A. Davis, I. Ruiz, D. M. Cottrell, Opt. Express 18, 7173 (2010). 23. A. Yariv, Electron. Lett. 36, 321 (2000). 24. Y. Yu, R. O’Dowd, IEEE Photon. Technol. Lett. 14, 1397 (1992). 25. S. Manipatruni, Q. Xu, M. Lipson, Opt. Express 15, 13035 (2007). 26. K. Ladavac, D. Grier, Opt. Express 12, 1144 (2004). Acknowledgments: We thank M. Berry and M. Dennis (Department of Physics, University of Bristol, UK), S. Barnett (Department of Physics, University of Strathclyde, UK), and M. Padgett (Department of Physics, University of Glasgow, UK) for very useful discussions and C. Railton (Merchant Venturers School of Engineering, University of Bristol, UK) for providing the finite-difference time-domain simulation tool. J.W. is funded by European Union FP7 FET-OPEN project PHORBITEC. Supplementary Materials www.sciencemag.org/cgi/content/full/338/6105/363/DC1 Materials and Methods Supplementary Text Figs. S1 to S7 References (27–31) Movies S1 to S4 25 June 2012; accepted 10 September 2012 10.1126/science.1226528 Lethally Hot Temperatures During the Early Triassic Greenhouse Yadong Sun,1,2* Michael M. Joachimski,3 Paul B. Wignall,2 Chunbo Yan,1 Yanlong Chen,4 Haishui Jiang,1 Lina Wang,1 Xulong Lai1 Global warming is widely regarded to have played a contributing role in numerous past biotic crises. Here, we show that the end-Permian mass extinction coincided with a rapid temperature rise to exceptionally high values in the Early Triassic that were inimical to life in equatorial latitudes and suppressed ecosystem recovery. This was manifested in the loss of calcareous algae, the near-absence of fish in equatorial Tethys, and the dominance of small taxa of invertebrates during the thermal maxima. High temperatures drove most Early Triassic plants and animals out of equatorial terrestrial ecosystems and probably were a major cause of the end-Smithian crisis. Anthropogenic global warming likely is contributing to the rapid loss of biological diversity currently occurring (1). Climate warming also has been implicated in severe biotic crises in the geological past, but only as a corollary to more direct causes of death such as the spread of marine anoxia (2). Here, we show that lethally hot temperatures exerted a direct control on extinction and recovery during and in the aftermath of the end-Permian mass extinction. As well as the scale of the losses, the aftermath of this event is remarkable for several reasons, such as the prolonged delay in recovery (3), the prevalence of small taxa (4), and the absence of coal deposits throughout the Early Triassic (5). These and several facets of lowlatitude fossil records shown below, including fish, marine reptile, and tetrapod distributions, can be related to extreme temperatures in excess of tolerable thermal thresholds. Climate warming long has been implicated as one cause of the end-Permian crisis (2, 6), with carbon dioxide release from Siberian eruptions and related processes providing a potential trigger for it (7, 8). Conodont apatite oxygen isotope 1 State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan 430074, People’s Republic of China. 2 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. 3 GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany. 4 Institute of Earth Sciences–Geology and Paleontology, University of Graz, Heinrichstrasse 26, A-8010 Graz, Austria. *To whom correspondence should be addressed. E-mail: eeys@leeds.ac.uk Fig. 1. Early Triassic paleogeography showing reported occurrences of fish and marine reptiles in the Smithian. Note rare equatorial occurrence of both groups when ichthyosaurs had evolved in northern climes. The global distribution of tetrapods (25) indicates occurrences almost exclusively in higher latitudes (>30°N and >40°S) throughout the Early Triassic, with rare exceptions in Utah (Parotosuchus sp., paleolatitude ~10°N) and Poland (paleolatitude ~20°N), both probably of middle-late Spathian age (25, 26). (Inset) Paleogeography of Pangea and Nanpanjiang Basin after (45–47). Fish and ichthyosaurs occurrences, see table S2. GBG, Great Bank of Guizhou. 366 19 OCTOBER 2012 VOL 338 SCIENCE www.sciencemag.org REPORTS on October 23, 2012 www.sciencemag.org Downloaded from