Nonlinear Refraction in Colloidal Silver Sulfide Quantum Dots
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Journal of Russian Laser Research, Volume 41, Number 6, November, 2020
NONLINEAR REFRACTION IN COLLOIDAL SILVER SULFIDE QUANTUM DOTS A. I. Zvyagin,1 T. A. Chevychelova,1 I. G. Grevtseva,1 M. S. Smirnov,1 A. S. Selyukov,2,3,4∗ O. V. Ovchinnikov,1 and R. A. Ganeev1 1 Faculty
of Physics, Voronezh State University University Square 1, Voronezh 394006, Russia
2 Lebedev
Physical Institute, Russian Academy of Sciences Leninskii Prospect 53, Moscow 119991, Russia 3 Bauman
Moscow State Technical University Second Baumanskaya Street 5, Moscow 105005, Russia 4 Moscow
Polytechnical University Bolshaya Semyonovskaya Street 38, Moscow 107023, Russia ∗ Corresponding
author e-mail:
selyukov @ lebedev.ru
Abstract We analyze the features of nonlinear refraction in colloidal solutions containing Ag2 S quantum dots capped with thioglycolic acid (Ag2 S/TGA) or L-cysteine (Ag2 S/L-Cys), with the quantum-dot (QD) sizes ranging from 1.7 to 5.5 nm and their concentration in the solution being 0.1%. The Z-scans of the samples were obtained using 10 ns pulses of 532 nm radiation. For samples with various average sizes and positions of the characteristic feature in the optical density spectrum, both self-focusing and self-defocusing of radiation were observed. The contribution of thermal refraction to the recorded Z-scans was estimated, and the impact of thermal effects on the observed nonlinear refraction was shown to be negligible. The solutions of the Ag2 S/TGA QDs with the ground state exciton absorption peaked at 590 nm exhibit self-focusing. The samples with recombination luminescence and the ground state exciton absorption peaked at 750 nm reveal self-defocusing. We show that nonlinear refraction in the case of nanosecond pulses is predominantly nonthermal in nature and is due to absorption by free charge carriers, as well as due to the band filling effect in the Ag2 S QDs, which involves localized states of impurities.
Keywords: quantum dots, silver sulfide, nonlinear refraction.
1.
Introduction
In recent years, there has been an increasing interest in the study of nonlinear optical properties of colloidal semiconductor quantum dots (QDs) with average sizes of 2–10 nm [1–5]. The main advantages of such nonlinear optical media is high optical uniformity of the colloidal solution, as well as size-dependent spectral and luminescent properties. The size dependence of the transition energy in absorption and luminescence [6, 7] provides a means to achieve resonances, which can reduce the threshold for optical nonlinearities [8–10]. The use of QDs with a certain set of localized states has a favorable effect on resonant two-photon excitation and the onset of the reverse saturable absorption (RSA) effect [11, 12] Manuscript submitted by the authors in English first on August 10, 2020, and in final form on September 4, 2020.
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c 2020 Springer Science+Business Media, LLC 1071-2836/20/4106-0670
Volume 41, Number 6, November, 2020
Journal of Russian Laser Research
or nonlinear refraction due to the band filling [9, 13] caused by chan
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