Correlation between Nonlinear Optical Properties and Electronic Band Modification in Cobalt-Doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mrow><mml:mi>Zn</mml:mi><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math> Nanorods
Mrinal K. Sikdar, Nitul S. Rajput, Ajanta Maity, Pratap K. Sahoo
Abstract
We report a study on the nonlinear optical properties of cobalt- ($\mathrm{Co}$) doped $\mathrm{Zn}\mathrm{O}$ nanorods (NRs) using the single-beam $Z$-scan technique. In the open-aperture measurements, the undoped sample shows two-photon absorption (TPA) characteristics, whereas the doped samples exhibit a saturable absorption (SA) signature. In the closed-aperture measurements, all the samples show a self-defocusing behavior. We correlate the nonlinear absorption coefficient with the exponential reduction of the optical band gap and explain it in terms of electronic-band-structure modification because of doping. Density-functional-theory calculation is also used to understand the observed nonlinear absorption and band-gap variation. The present study can help to understand the origin of SA in $\mathrm{Zn}\mathrm{O}$. Doped $\mathrm{Zn}\mathrm{O}$ nanorods can illustrate high SA and nonlinear refraction by appropriate $\mathrm{Co}$ doping, which can drive the technology for $Q$ switching and mode locking in solid-state and fiber lasers.