Nonlinear optical properties of composite films consisting of multi-armed CdS nanorods and ZnO

1. Nonlinear optical properties of composite films consisting of multi-armed CdSnanorods and ZnO Speaker：YouChuang Ku Adviser：Ja-Hon Lin

2. Outline • Introduction • Experimental • Z-scan • OKE • Results and discussion • Conclusion

3. Introduction • Nonlinear optical materials with large third-order nonlinear optical susceptibility χ(3) and small relaxation time of photocarrieshave been widely studied because of the potential applications in optical bistability, optical computing and optical phase-conjugation. • The nonlinear optical properties of the multi-armed nanorodsCdShave not been investigated as far as now. • Nonlinear optical properties of the composite films, which were formed by combining multi-armed CdSnanorodswith the other semiconductor material ZnOusing spin coating and pulsed laser deposition. The large optical nonlinearities of the composite films were obtained.

4. Produce CdS Synthesized via aqueous chemical growth route at room temperature Use centrifugation and washed with DI water and ethanol Purified CdS

5. Composite film fabrication ZnO • Fused quartz0.5 mm • ZnO 60~70nm • CdS 5nm CdS ZnO 210 nm Spin coating (3000rpm) CdS ZnO Pulse laser deposttion Fused quartz

6. Experimental setup for Z-scan (1) Wavelength：532 nm(Nd:YAG) Pulse duration：25 ps Repetition rate：1 Hz Intensities： 6–14 GW/cm2 Beam waists：25 μm (2)Wavelength：800 nm(Ti:sapphire) Pulse duration： ~120 fs Repetition rate：82 MHz After Ti:sapphire amplifier(TSA-10) Pulse duration： ~200 fs Repetition rate：10 Hz Output energy：~5mJ Intensities：150~300GW/cm2 Beam waists：34 μm • Len f=150mm

7. Experimental setup forfemtosecond OKE measurement • Wavelength：800 nm • Pulse duration： ~120 fs • Repetition rate：82 MHz • Beam waists：34 μm • Intensities： • Pump beam：30mW • Pro Beam：2.8 mW • Chopper ：1kHz

8. Linear optical absorption properties of ZnO and ZnO/CdS CdS Absorption band Pure ZnO ZnO/ CdS Pure CdSnanorod Absorption peak(365 nm) α= α0+βI λ=532nm α=8.4x103 cm-1 λ=800nm α≈0 CdSnanorod absorption band：450 nm Bulk CdS absorption band：512nm Blue-shift

9. Nonlinear optical properties with 532nm The Z-scan data of sample with an open-aperture a closed-aperture measured at 532 nm with pulse duration 25 ps. The solid lines indicate the theoretical fit.

10. Far-field aperture S • S • The ratio of the pulse energy passing through the aperture to the total energy • Open aperture • S=1 • Close aperture • S=0.4

11. Open-aperture Laser Peak Intensity I0 Effective thickness Diffraction length of beam Z0 b(nonlinear absorption coefficient) Imaginary part of susceptibility

12. Close-aperture ΔΦ0 the on-axis phase shift at the focus ΔTp-v the difference between the normalized peak and valley transmittance Nonlinear refraction coefficient Real part of susceptibility 0.3189 X=0.3189/(p/4)=0.406

13. Normalized transmittance with intensity l0= 532 nm • The normalized transmittance T when OA • ΔTp–v, (CA) The difference between the normalized peak and valley transmittance when CA vs peak intensity I0 for composite material. The linear relations indicate a third-order process

14. I = 6 GW/cm2 0.92 ΔTp–v=0.18

15. Nonlinear optical properties with 800 nm • λ=800nm • Single laser pulse=2μJ • Irradiance = 275 GW/cm2

16. Delay time Time resolved optical Kerr-effect signal of CdS/ZnO composite film measured at 800 nm. The symmetrical signal curve corresponds nearly to the laser pulse profile, implying that third-order nonlinear linear optical response of the sample was instantaneous. This ultrafast response was mainly attributed to nonresonant electronic nonlinearities.

17. Conclusion • The multilayer composite films consisting of CdS multi-armed nanorods and ZnO were fabricated layer by layer using spin coating and pulse laser deposition technique on fused quartz substrates • The third-order nonlinear optical properties of the composite films were determined by Z-scan method • The third-order nonlinear optical absorption coefficient band nonlinear refractive index n2, were obtained, respectively • The ultrafast nonlinear optical response was determined using OKE technique at 800 nm • The large and ultrafast response optical nonlinearity of the composite films indicated their potential applications for future nonlinear optical devices

18. Thanks for your attention