Observation of enhanced two-photon absorption in ZnS quantum dots at an ultraviolet wavelength

Abstract

Two-photon absorption (2PA) in zinc sulphide (ZnS) and Mn 2+ -doped ZnS quantum dots is reported by the z-scan technique, with nanosecond pulsed laser radiation at 355 nm. The observed values of the 2PA cross section of all the samples are 10 5 times larger than that of bulk ZnS. OCIS codes: 190.0190, 160.4236, 190.4180. doi: 10.3788/COL201109.101902. Researchers have recently expanded their research interests to find highly potential multiphoton active materials [1−10] . Amongst others, zinc sulphide (ZnS) and Mn 2+ -doped ZnS (ZnS:Mn) quantum dots (QDs) have been found to be potential materials because of their visible luminescence and interesting nonlinear optical (NLO) properties [5−8,10−12] . Reports on the multiphoton absorption properties of ZnS QDs have been published previously. For example, two-photon absorption (2PA) has been reported by Zheng et al. with picosecond laser radiation at 532 nm in 0.5% Mn 2+ -doped ZnS QDs [7] . Some enhancements in 2PA have been reported in ZnSe/ZnS core/shell at 806 nm and in ZnS quantum structures at 532-nm laser pulses [13,14] . However, to date, there has been no report on the measurement of NLO properties of ZnS and ZnS:Mn QDs in the ultraviolet (UV) region. In this letter, we present our observation of 2PA in undoped ZnS and ZnS:Mn QDs at an UV wavelength of 355 nm. The obtained values of the 2PA cross section in the studied samples are ∼ 10 5 times larger than that of the available reported value of bulk ZnS [11] . The samples used in this work are freshly prepared through the chemical precipitation method [11] . The synthesized undoped ZnS and 1% and 2.5% Mn 2+ -doped ZnS samples are henceforth denoted as R I , R II , and R III , respectively, unless otherwise specified. The UV-visible absorption characteristics of the samples, as obtained from the measurement in a spectrophotometer (Hitachi U-3010), are shown in Fig. 1. An X-ray diffraction measurement of the samples ensures the average particle size (radius) to be ∼1.5 nm [10] . The experimental z-scan set-up used for the measurement of NLO properties is a standard one [2,8,15] . However, in the present experiment, the excitation source of 355-nm radiation is obtained through a third harmonic generation from a Q-switched Nd:YAG laser (10 ns, 10 Hz repetition rate). To avoid the absorption of the input beam in the lens material itself, a quartz lens with a focal length of 7.5 cm is used to focus the 355-nm radiation. The beam waist (w 0 ) and the confocal parameter (z 0 ) at the focus are 7 and 350 µm, respectively. After they are dispersed in methanol, the as-synthesized nanopowders are kept in a quartz cuvette with a path length (L) of 2 mm for the z-scan measurement, satisfying the thick sample condition (L > n 0 z 0 ) in the present experiment. Here, n 0 is the linear refractive index of the samples at a 355-nm wavelength. In Fig. 1, the values of the linear absorption coefficient (α) of R I , R II , and R III samples are 0.50, 0.33, and 0.47 cm −1 , respectively, at the excitation wavelength of 355 ...

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