Size-dependent enhancement of nonlinear optical properties in nanocolloids of ZnO

1. Size-dependent enhancement of nonlinear optical properties in nanocolloids of ZnO Speaker：YouChuang Ku Adviser：Ja-Hon Lin

2. Outline • Introduction • Experimental • Results and discussion • Conclusion

3. Introduction • Nanosemiconductor materialshave attracted much interest from both fundamental and technological researchers • they exhibit peculiar properties which are not shown by their bulk counterparts • Wide band gap semiconductors, especially ZnO, have attractive nonlinear properties that make them ideal candidates for NLO-based(nonlinear optical) devices • NanosizedZnO are finding applications as efficient UV emitters, random lasers, field emission displays • in the form of quantum dots, nanowires

4. Introduction • Very little work has been done on NL properties of ZnO quantum dots and its nanocolloids • because semiconductor nanoparticles in solution with a well-controlled size, shape, and surface properties are difficult to obtain • the volume fraction of crystallites in a stable solution is usually very small , so the resultant nonlinear response is relatively weak • In this paper, the volume fraction of the nanoparticles increases with particle size; hence, high nonlinear absorption at a larger particle size occurs

5. Introduction • When the crystallite size is reduced to the order of an excitonBohr radius aB (aB=0.53x10-10 m ) • quantum size effects appear and drastic changes in optical properties are expected • The quantum confinement effect in semiconductor nanocrystals can be classified into two regimes, • the strong and weak confinement regimes

6. In strong-confinement regime • Photoexcitedelectron and hole are individually confined. • Theoretical and experimental works have revealed that the state-filling effect accounts for the nonlinearity in this regime • Size dependence of the third-order susceptibility c(3) has been studied, but the results are inconsistent • A larger nonlinear susceptibility for a larger size has been found for CdSxSe1−xnanocrystals by the saturation spectroscopy and degenerate four-wave mixing DFWM measurements [P. Roussignol, D. Ricard, and C. Flytzanis, Appl. Phys. B: Lasers Opt. 51,437 1990.] • Roussignolet al. have shown that larger c(3)values are obtained with decreasing sizes [8D. W. Hall and N. F. Borrelli, J. Opt. Soc. Am. B 5, 1650 1988.]

7. In weak-confinement regime • Coulomb interaction between the electron and hole yields an exciton and it is confined as a quasiparticle • Theoretical studies have shown that the confinement of the excitonic envelope wave function due to the infinite barrier potential gives rise to the enhancement in oscillator strength for an exciton within the nanocrystal by a factor of R3 /aB3and c(3) depends on the crystallite size. • A giant oscillator strength effect has been confirmed for CuClnanocrystals

8. Experimental • The method of preparation involves the hydrolysis of zinc acetate dihydrate (ZnAc) in diethylene glycol medium • Diethylene glycol is chosen because it is reported to give powders with a uniform shape and size distribution • The size of the particles and hence the stability of this colloidal suspension depends on the • concentration of zinc acetate as well as on the rate of heating • The molar concentration of precursor solution is varied from 0.01 to 0.1 M and a heating rate of 4 °C per minute is employed

9. Z-scan • Q-switched Nd:YAG • Wavelength：532 nm • Pulse duration：7 ns • Repetition rate：10 Hz • Lens • Focal length ：200 mm • Beam waist ：35.4 mm • Rayleigh length： 7.4 mm (Z0=pw02/l) • Sample cuvette：1 mm • Far-field aperture S=0.21

10. Absorption spectra of ZnO colloids Blue shift can be attribute to the confinement effect Excitonic peak The first derivative curve of the absorption spectrum is taken and the point of inflection is taken as the absorption edge Blue shift

11. Optical band gap of ZnO (ahn)2=k(hn-Eg) a： absorption coefficient hn：incident light energy k：constant Eg：optical band gap (size dependent) Size decrease

12. TVB and BCB shift • The total change in the band gap of the material is contributed by • both the shifts of the valence and the conduction band edges away from each other • The shift of the top of TVB(top of valence band) is not the same as that of the BCB(bottom of conduction band) • The individual shifts in TVB and BCB as a function of the size employing various forms of high-energy spectroscopies, (such as the photoemission spectroscopies) • The shifts of the band edges decrease smoothly to zero for large-sized nanocrystals in every case • The shift in the BCB is in general much larger compared to the shift in the TVB for any given size of the nanocrystal

13. TVB and BCB shift • A larger shift for the BCB is indeed expected in view of the fact that the band-edge shifts are related inversely to the corresponding effective masses • Because the effective mass of the electron is always much smaller than that of the hole in these II–VI semiconductors • The band gap of ZnO is found to be in the range 3.5–4 eVfor the range of particles from 6–18 nm

14. Open aperture I=866 MW/cm2 q0(z,r,t)=bI0(t)Leff Leff=1-e-al/a a：absorption coefficient I0：irradiance at focus Imc(3)=no2e0c2b/w n0=2.008 The open-aperture curve exhibits a normalized transmittance valley, indicating the presence of a reverse saturable absorption in the colloids. The data are analyzed by using the procedure described by Bahaeet al [M. S. Bahae, A. A. Said, and E. W. van Stryland, Opt. Lett. 14, 955 1989]

15. The closed-aperture z-scan traces of ZnO colloids of different particle sizes I=866 MW/cm2 The enhancement of nonlinear optical properties depend on the particle size increase

16. Measured values of nonlinear absorption, nonlinear refraction, and nonlinear susceptibility of ZnO colloids at an intensity of 866 MW/cm2 • Imc(3)=no2e0c2b/w • n0=2.008 eo：permittivity of free space c ：velocity of light in vacuum w：angular frequency

17. The enhancement of nonlinear optical properties with increasing dimension in the weak-confinement regime essentially originates from the size-dependent enhancement of the oscillator strength of coherently generated excitons • Exciton is confined in a quantum dot, the confinement of the excitonic wave function is expected to give rise to an enhancement of the oscillator strength per quantum dot by a factor of R3 /aB3 • A giant oscillator strength effect will result in an enhancement of the nonlinear susceptibility

18. Third-order susceptibility as a function of particle size for ZnOnanocolloids c(3)：1.5x10-10 to 1.2x10-9esu R：6 ~ 18 nm The data show a general trend of increasing c(3) values with increasing radius The size depend is R2 m=1.891≈2

19. Open-aperture z-scan curves of ZnO colloid of size 6 nm at different input fluences Size：6nm The enhancement of nonlinear optical properties depends on the input fluences increase

20. The closed-aperture z-scan curves of ZnO colloid of size 6 nm at different input fluences. Size：6nm Nonlinear optical properties are highly fluence dependent.

21. Variation of nonlinear susceptibility with irradiance for ZnO colloids of different particle sizes • c(3) is a function of intensity of • the laser radiation which can be • written as • c(3) = c0(3)+ c1(3)I+c2(3)I2 • For a lower fluence and • colloids withsmall size, c(3) is independent • of intensity,indicating it is athird-order effect resulting from two-photon absorption • For colloids of larger particle size and at higher fluences, • c(3) becomes a nonlinear function of intensity

22. Optical limiters • Recently, nanomaterials have drawn significant attention as optical limiters for eyes or for sensor protection • Optical limiters are devices that transmit light at low input fluences, but become opaque at high inputs • Optical power limiting is operated through the nonlinear optical processes of nanomaterials • To examine the viability of nano-ZnO colloids as optical limiters, the nonlinear transmission of the colloid is studied as a function of the input fluence • The optical limiting property occurs mostly due to absorptive nonlinearity, which corresponds to the imaginary part of third-order susceptibility

23. Influence of nanoparticle size on the optical limiting response • The arrow in the figure indicates the approximate fluence at which the normalized transmission begins to deviate from linearity • Colloid with a larger particle size is a better nonlinear absorber and hence a good optical limiter

24. Conclusions • Nonlinear optical properties of ZnO semiconductor nanocolloids are investigated for optical power self-limiting application • We have experimentally demonstrated optical nonlinearity as a function of the size, and an R2 dependence of nonlinear susceptibility is obtained for ZnOnanocolloids • Nonlinear susceptibility is highly fluence dependent and it becomes quadratic in nature for a large particle size. • The optical limiting response of ZnOnanocolloids, in the diameter range of 6–18 nm, increases with the increase of particle size

25. Thanks for your attention