Observation of four-photon absorption and determination of corresponding nonlinearities in CdS quantum dots

TL;DR

This study investigates the nonlinear optical properties of CdS quantum dots, revealing four-photon absorption and related nonlinearities using femtosecond laser techniques, and provides quantitative analysis of these effects.

Contribution

The paper reports the first direct observation of four-photon absorption in CdS quantum dots and quantifies the associated nonlinear coefficients using combined experimental and theoretical methods.

Findings

Observation of Kerr-type nonlinearity at low intensities

Detection and quantification of four-photon absorption (4PA)

Calculation of nonlinear index and refractive-index change coefficients

Abstract

Bound- and excited-state electronic nonlinearities in CdS quantum dots have been investigated by Degenerate Four-Wave Mixing (DFWM) and Z-scan techniques in the femtosecond time regime. This QD sample shows Kerr-type nonlinearity for incident beam intensity below 0.18 TW/cm$^2$. However, further increment in intensity results in four-photon absorption (4PA) indicated by open- and closed-aperture Z-scan experiments. Comparing open-aperture Z-scan experimental results with theoretical models, the 4PA coefficient $\alpha_4$ has been deduced. Furthermore, third-order nonlinear index $\gamma$ and refractive-index change coefficient $\sigma_r$ corresponding to excited-state electrons due to 4PA have been calculated from the closed-aperture Z-scan results. UV-visible absorption and photoluminescence experimental results are analyzed towards estimating band gap energy and defect state energy. Time Correlated Single Photon Counting (TCSPC) was employed to determine the decay time corresponding to band-edge and defect states. The linear and nonlinear optical techniques have allowed the direct observation of lower and higher-order electronic states in CdS quantum dots.