Qihuang Gong, Xiaoyong Hu, Jiaxiang Zhang, Hong Yang

1. Composite Materials for Ultrafast and Large Third-order Optical Nonlinearity and Photonic Applications Qihuang Gong, Xiaoyong Hu, Jiaxiang Zhang, Hong Yang Department of Physics, Peking University, Beijing, P. R. China Email: qhgong@pku.edu.cn; Fax: +86-10-62756567

2. Contents • Motivation • Enhanced ultrafast 3rd nonlinearity • using composite materials • Photonic crystal and PC optical switch • Conclusion

3. Motivation • 1980- Third-order Optical Nonlinear Materials Photonics Applications All optical device Optical switching Optical computing Integrated photonic circuits Fast and large 3rd NLO response fs NLO response large off-resonant c(3) } fs measur. • conjugated organic molecules and polymers • Semiconductors

4. Measurement on ultrafast 3rd nonlinearity • l : 760 - 850nm • : ～100fs • I1:I2 = 10:1 Femtosecond OKE System

5. OKE – four wave mixing process Es E1 P E1 Is Es E2 E2 450 I2 I1 c(3) I s= Typical OKE signal of CS2

6. 3rd optical nonlinearity of routine materials： Liquid crystal 10-7 10-6 ☆Large 3rd nonlinear susceptibility and ultrafast response are difficult to achieve simultaneously

7. II Enhanced ultrafast 3rd nonlinearity using composite materials Composite I: Coumarine 153 doped Polystyrene n2(c(3))~1/(w0 – w – iG) * Near resonant enhancement (enlarge the response time of excited state lifetime ) * Inter-molecular excited-state electron transfer

8. Coumarine 153 doped Polystyrene Inter molecular electron transfer ~ 1ps 400nm near-resonant excitation 800nm probe C153 molecule Polystyrene Polymer composite material: C153:Polystyrene

9. Composite Material II: Nano-Ag doped MEH-PPV surface plasmonics enhanced 3rd optical nonlinearity The effective third-order nonlinear optical susceptibility of the composite material can be written as and are permittivity for host material and metal nanoparticles and are third-order optical susceptibility of host material and metal nanoparticles p is the volume fraction of Ag nanoparticles a very large nonlinear coefficient In the SPR peak

10. Nano-Ag doped MEH-PPV Energy transfer ～ps SPR resonant excitation MEH-PPV Ag nanoparticle

11. III. Photonic crystal and PC optical switch ★ Photonic crystal is a novel photonic material with a periodic dielectric distribution One-dimensional Photonic crystal Two-dimensional photonic crystal Three-dimensional photonic crystal ★Photonic crystal possesses photonic bandgap and can control the propagation states of photons

12. Defect states When a structure defect is introduced in the photonic crystal, the defect states will appear in the photonic bandgap Frequency Dielectric Defect Air Defect Air Band Photonic Bnadgap Dielectric Band Defect Radius

13. Photonic Bandgap Wavelength Pump Light Probe Light Bandgap or Defect state shift ---------- change the refractive index ☆Third-order optical nonlinear photonic crystal Pump Beam Intensity Photonic Bandgap Shift Defect State Shift Transmittance Photonic Bandgap Transmittance Defect State Wavelength Probe Light Pump Light Light beam controlled Shift

14. Concept for All-Optical Switching effect Pump light Probe light Probe light Using Photonic bandgap shift or defect state shift by Pump Beam Photonic crystal optical switching

15. 1) PC optical switch using pure polymer Organic polymer: Polystyrene n2=1×10-13cm2/W Schematic Structure of Polystyrene Molecule

16. Two-dimensional Polystyrene Photonic Crystal Fabrication Process Spin Coating + FIB etching cylindrical air holes embodied in the polystyrene slab. Film Thickness 300nm Lattice Constant 320nm Radius of Air Hole 130nm Width of Line Defect 450nm The patterned area is about 4 μm×100 μm A line defect in the center of a two-dimensional photonic crystal to form photonic crystal filter

17. Photonic Crystal Devices: Filter, Switch line defect transmission mode Transmission spectra : (a) Measured result (b) Theoretical result of multiple scattering method * Central Wavelength 791nm, Quality Factor 500, Line width 1.6nm

18. X Prism Mode Z Air Gap Air Gap Waveguide Waveguide Guided Mode Substrate Substrate Evanescent Field Coupling System probe beam W θp Cross Section Structure Electric-field Distribution 1) Energy of the incident light is coupled into optical waveguide with the help of evanescent field 2)Coupling efficiency ～ 20%

19. Ti:sapphire Laser Diode Aperture Lens Prism Micro Lens Delay Line Computer PMT Monochromator Experimental Setup 800nm Pump beam Ti:sapphire laser: Pulse Duration 120fs Pulse Repetition 76MHz Wavelength 700nm－860nm 800nm 800nm Waveguide 100 μm×2.5 mm The patterned area is about 4 μm×100 μm

20. Switching Performance 800nm Pump beam Conclusion: An all-polymer tunable photonic crystal filter, switch with ultrafast time response is realized. Time Response ( as fast as the time-resolution of measurement system ) Pump Intensity as high as GW/cm2 * Transmittance Contrast 60％ * Time Response～120fs

21. 2) C153:Polystyrene PC optical switch Polystyrene doped with 15% Coumarin 153 Absorption peak of Coumarin 153 is around 400nm Film thichness: 300nm Lattice constant: 320nm Air hole radius: 120nm Line defect width: 440nm

22. Electric field distribution of defect mode Electric field was mainly confined in the defect structure

23. Tunability of the photonic bandgap microcavity Measured result Simulated result Transmittance spectra of the microcavity resonant mode as functions of the energy of the pump light

24. Ti:sapphire Laser Aperture Lens BBO Crystal Prism Filter Micro Lens Delay Line Computer PMT Fiber Spectrophotometer Experimental setup Near-resonant enhanced ----- 400nm Pump beam 800nm ☆ Near-resonant enhanced nonlinearity of polystyrene 400nm

25. All-optical switch effect Switching efficiency: 80% Response time: 1.2ps Pump power: 110 KW/cm2 (reduced by 4 orders) Chinese patent: 发明专利（ZL200710099383.2）“降低全光开关泵浦功率的方法、全光开关及其制备方法” Nature Photonics 2 (2008) 185-189

26. Nature Photonics ‘Controlling photons with light’ A strongly nonlinear photonic crystal with a wavelength-tunable bandgap could provide the solution to realizing all-optical switches for signal processing‘

27. IOP optics.org: ‘ Photonic crystals speed up all-optical switching’ A polystyrene photonic crystal that acts as an all-optical switch boasts picosecond response time and low power requirements. The picosecond switching time is impressive. 一种光子晶体开关以具备皮秒时间响应和低泵浦功率而值得自豪，皮秒的超快开关时间令人印象深刻。

28. Nature Asia Materials: Ultrafast Optical Switches:Now you see it, now you don’t Researchers from Peking University, China, now demonstrate fast all-optical switching in a photonic crystal made from a composite material.

29. Nature China: “Optical Switches: A New Low” Qihuang Gong and co-workers at the Peking University in Beijing have devised a strategy for making ultrafast photonic-crystal-based optical switches that can operate under low-power pump light）。

30. 3) Nano-Ag:MEH-PPV PC optical switch SPP resonant-enhancement Switching efficiency: 65% Response time: 35ps Pump power: 230 KW/cm2 Appl. Phys. Lett. 94, 031103 (2009)

31. PhysOrg.com: ‘ Nanocomposite material provides photonic switching’ The development of integrated photonic devices in tomorrow’s technology is taking place today at Peking University in Beijing, China, where a group of scientists has manufactured and tested nanocomposite material that could be used in integrated photonic devices

32. Nanomaterials World ： “Seeing the light” Nanomaterials world 5 (2009,Mar. 17) 5 Photonic devices could aid developments in computing, following research in China. The team from Peking University is working on a nanocomposite that could be integrated into photonic devices.

33. IV. Conclusion ☆New composite materials are demonstrated to develop the 3rd optical nonlinearity ☆Large 3rd nonlinear susceptibility (4-orders enhanced ) and ultrafast response time ( of ps order ) were achieved ☆An ultrafast low-power photonic crystal all-optical switch was realized by using the composite materials

34. V. Acknowledgement Financial Supported by: NNSFC, China MOST, China MOE, China, Peking Uiversity

35. THE END Thank You!