Third-order nonlinear optical properties of ultra-thin e-beam evaporated silver films

Ag ultra-thin continuous films with thickness of 3 nm was prepared by e-beam evaporation under ultra-high vacuum conditions at room temperature on glass substrates. The Atomic Force Microscopy (AFM) imagesshowed granular... [ view full abstract ]

Ag ultra-thin continuous films with thickness of 3 nm was prepared by e-beam evaporation under ultra-high vacuum conditions at room temperature on glass substrates. The Atomic Force Microscopy (AFM) imagesshowed granular surface texture with an average grain size of 30 nm. However, the measured roughness value (R_a) was 0.08 nm only indicating a very smooth films. The third-order nonlinear optical response of the fabricated films was investigated by single beam z-scan technique using a 457 nm cw Ar⁺laser. The thermal nonlinear response of the films was reduced using an optical chopper at 1kHz. The linear absorption showed a peak at 459 nm relating the optical response to surface Plasmon behavior as expected. In the closed aperture z-scan transmission measurements the distance between the peak and the valley (Δz_p-v) was about 1.67 z₀,where z₀=πω₀²/λ, and ω₀ is the beam waist. This value indicate that the nonlinear response in these films was a third-order nonlinear optical response. The computed nonlinear refractive index (n₂) and the two photon absorption coefficient (β) were 0.31×10^-6 m²/W and 2.92 m/W, respectively.  The calculated value for χ⁽³⁾ was 4.9×10^-10esu. We believe that such strong response from these thin films is due to surface Plasmon resonance enhancements in the films.

Figure 1: a) AFM image showing the granular surface morphology. b) Closed aperture z-scan the 3 nm Ag film. The open circles are the measured data and the solid line is the theoretical fit.