Two-Photon Absorption Properties of Mn-Doped ZnS Quantum Dots

We investigate the two-photon absorption and nonlinear refractive index properties of a quantum dot material based on ZnS nanocrystals doped with Mn isoelectronic impurities, using the Z-scan technique with 532\,nm picosecond laser pulses. The Mn-doped ZnS quantum dots have an average two-photon absorption cross section as high as 13600 Goeppert--Mayer units, which turn it into a very promising material for fluorescent label and imaging in biological samples. In addition, we also found that the two-photon absorption coefficient initially increases and then decreases with increasing pulse irradiance, which demonstrates the presence of the higher-order nonlinearity under the strong excitation.