Low-order optical nonlinearities of PbS quantum dot liquids and films

doi:10.1016/j.optmat.2021.111499

Optical Materials

Volume 121, November 2021, 111499

https://doi.org/10.1016/j.optmat.2021.111499 Get rights and content

Highlights

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Larger PbS QDs demonstrate stronger nonlinear optical effects compared with smaller QDs.
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PbS QDs possess the thermal-lens related negative nonlinear refraction and reverse saturable absorption.
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The ten-fold growth of the wavelength-dependent nonlinear optical response of QDs is observed.
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Strong influence of the excitonic state on the propagation effects is determined.
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QD-containing thin films and suspensions demonstrate strong saturable absorption and reverse saturable absorption.

Abstract

We demonstrate the variations of the nonlinear optical parameters in the suspensions and thin films containing PbS quantum dots of different sizes (9 and 2.5 nm). We show that these colloidal quantum dots possess the thermal-lens induced negative nonlinear refraction and reverse saturable absorption in the case of the 532-nm, 10-ns probe pulses. The larger nonlinear absorption coefficients of 9-nm quantum dots compared with 2.5-nm species (1 × 10⁻⁷ and 8 × 10⁻⁹ cm W⁻¹, respectively) in the case of 1064 nm, 10 ns probe pulses were determined. Similar relations between the nonlinear optical parameters of 9- and 2.5-nm species were maintained in the case of twice shorter wavelength (532 nm) of the probe pulses. The almost ten-fold growth of the nonlinear optical parameters in the visible range compared with the near infrared region was attributed to the stronger influence of the excitonic states on the propagation effects in the former case. We show that the thin films containing these quantum dots demonstrate saturable absorption, while the suspensions show mostly the reverse saturable absorption at 532 nm. A significant enhancement of the nonlinear refractive index of films compared with suspensions (−2 × 10⁻⁸ cm² W⁻¹ and -1 × 10⁻¹¹ cm² W⁻¹), as well as a large negative nonlinear absorption coefficient (−3.2 × 10⁻⁵ cm W⁻¹) in the former case are demonstrated. We discuss the signs of nonlinear refraction and nonlinear absorption of the quantum dots, as well as the role of the sizes of these species in variations of the nonlinear parameters.

Introduction

During last two decades, the small-sized nanoparticles, like quantum dots, became an important subject of studies. Among studied species are the depositions of metal nanoparticles in dielectrics [[1], [2], [3], [4]], semiconductor nanoparticles synthesized during laser ablation in liquids [5], chalcogenide nanoparticles doped in thin films [6], etc. Nonlinear optical properties of quantum dots (QDs) attract the attention due to their potential applications in optoelectonics and laser physics. Being presented in different forms (colloidal quantum dots, thin films, laser-induced plasmas), these QDs can notably increase the nonlinear optical response due to specific properties related with the quantum confinement and growing role of the local field, which can dramatically increase the response of the medium in the presence of laser field. The potential application can be related with the attractive properties of QDs to adjust the saturable absorption (SA), which can be used for the mode-locking of lasers and shortening the pulse duration from master oscillators. The reported large nonlinear susceptibilities of QD-containing materials allow expecting the modulation of the characteristics of laser pulses propagating through such species. One o such attractive materials is the lead sulfide.

Lead sulfide (PbS) is an important IV-VI group chalcogenide semiconductor that has attracted considerable attention due to its numerous optical and optoelectronic properties and useful applications in solar cells, optoelectronic devices, photoconductors, sensors and infrared detector devices [7,8]. Two decades of research on PbS colloidal QDs (CQDs) manifested in the development of synthetic procedures for QDs with narrow size distribution and good colloidal stability [9,10]. PbS thin films has direct optical bandgap that can be changed from 0.39 up to 5.20 eV [11]. PbS nanoparticles have been successfully examined for use in solar cells exhibiting the efficiency up to 13.8% [12,13]. PbS is sensitive to radiation at the wavelengths between approximately 1 and 2.5 μm.

The impressive third-order nonlinear optical properties make nanoscale PbS potentially useful in electroluminescent devices like light-emitting diodes and optical devices such as optical switches [14]. Special attention to the thin films containing these QDs is also related with the growth of such parameters as the nonlinear refractive indices (γ), nonlinear absorption coefficients (β) and saturation intensities (I_sat). The optical nonlinearities of nanoscale semiconductors in the quantum confinement regime have been frequently reported in the literature. Quantum confinement is found to enhance the nonlinear optical response, and a number of reports have discussed the physical processes underlying the mechanism of optical nonlinearity in QDs [15]. Particularly, the reduction of the PbS QDs to 2 nm showed the increase in band gap energy to 5.4 eV [16].

Most of previous studies of PbS QDs were carried out using ultrafast lasers. Particularly, saturable absorbers on the base of lead sulfide QDs for lasers emitting at 1, 1.3, 1.5, 2.1 μm can be used both for mode-locking and Q-switching of near IR lasers [17]. For PbS QDs, the nonlinear refraction index was found to be 5.1 × 10⁻¹⁶ cm² W⁻¹ in the case of 35 fs, 800 nm probe pulses [18]. Authors of [18] revealed that, contrary to the theory, PbS QDs’ nonlinear refractive index increases with size. The application of longer probe pulses (i.e. those in the range of a few nanoseconds) can reveal new peculiarities in the low-order nonlinear optical response of these QDs, especially in the case when they presented in liquid and thin film forms.

Here we present the results of our studies of the low-order nonlinearities of the PbS CQDs of different sizes being synthesized in liquids in the case of 10 ns pulses using different wavelengths of the probe radiation. We demonstrate the variations of the nonlinear absorption in the suspensions of PbS QDs of different sizes (9 and 2.5 nm) and show that these colloidal quantum dots possess the negative nonlinear refraction and reverse saturable absorption (RSA). The almost ten-fold growth of the nonlinear optical parameters in the visible range compared with the near infrared region was attributed to the stronger influence of the excitonic states on the propagation effects in the former case. We discuss the difference in the optical nonlinearities of 9-nm and 2.5-nm QDs, as well as analyze the signs of nonlinear refraction and nonlinear absorption of these quantum dots. Finally, we show that thin films containing these quantum dots demonstrate SA, while the suspensions show mostly RSA or two-photon absorption (TPA) depending on the wavelength of probe radiation. The enhancement of the nonlinear refractive index of films compared with suspensions, as well as a notable growth of nonlinear absorption coefficient in the former case, is demonstrated.

Section snippets

Synthesis and characterization of QDs

For the study of optical and nonlinear optical properties both small- and medium-sized PbS QDs were synthesized. The preparation of small colloidal quantum dots of lead chalcogenides (<3 nm) with good size-distribution requires special procedures [19]. In our case, the synthesis of small-sized lead sulfide QDs was performed by the modified procedure developed by Durmusoglu et al. [20]. It proceeds in the presence of dodecanethiol-1 in order to slow down the growth of nanocrystals.

9-nm sized quantum dots

The concentration of 9-nm sized QDs was C = 10⁻⁵ M. The thickness of used cell was 0.2 mm. The red filled circles in Fig. 2a correspond to CA z-scan at which negative nonlinear refraction was the dominating nonlinear optical process. These data were obtained using relatively low-energy 1064 nm pulses (E = 0.09 mJ). The four-fold growth of pulse energy (E = 0.36 mJ) led to the dominant influence of the positive nonlinear absorption attributed to TPA over nonlinear refraction (green empty

Discussion

Exciton Bohr radius for PbS is estimated to be about 18 nm. Therefore PbS quantum dots with mean size below 18 nm should demonstrate the quantum-confinement effect. These nanoparticles possess different absorption spectra, photoluminescence and photoelectric properties. 2.5 nm nanoparticles were selected as a first object for our study. Meanwhile, larger (9 nm) nanostructures could be very beneficial in comparison with smaller-sized QDs due to the difference in optical properties. As it was

Conclusions

Our studies of PbS QDs have shown that larger particles demonstrate stronger nonlinear optical effects compared with smaller QDs. The reason of such response of QDs corroborates with earlier suggested explanations of this effect. We have shown that these colloidal quantum dots possess the thermal-lens related negative nonlinear refraction and reverse saturable absorption while probing by nanosecond pulses. The larger nonlinear absorption coefficient of 9-nm quantum dots compared with 2.5-nm

Funding

European Regional Development Fund (1.1.1.5/19/A/003), State Assignment to Higher Educational Institutions of Russian Federation (FZGU-2020-0035), Russian Foundation for Basic Research (18-29-20062).

Disclosures

The authors declare no conflicts of interest.

Data availability statement

The data that support the findings of this study are available on request from the corresponding author.

CRediT authorship contribution statement

R.A. Ganeev: Conceptualization, Writing – review & editing. I.A. Shuklov: Methodology, Data curation. A.I. Zvyagin: Investigation. D.V. Dyomkin: Investigation. S.I. Bocharova: Investigation. V.S. Popov: Investigation. V.F. Toknova: Investigation. A.A. Lizunova: Investigation. O.V. Ovchinnikov: Writing – review & editing. V.F. Razumov: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Low-order optical nonlinearities of PbS quantum dot liquids and films

Highlights

Abstract

Introduction

Section snippets

Synthesis and characterization of QDs

9-nm sized quantum dots

Discussion

Conclusions

Funding

Disclosures

Data availability statement

CRediT authorship contribution statement

Declaration of competing interest

Physica B

Mater. Lett.

Int. J. Electrochem. Sci.

Joule

Mater. Today Chem.

J. Phys. Chem. Solid.

Opt. Mater.

Phys. Status Solidi B

Phys. Status Solidi B

Phys. Status Solidi B

Appl. Phys. B

J. Opt. A

Mater. Sci. Eng.

Science

J. Am. Chem. Soc.

Adv. Funct. Mater.

J. Phys. Chem.

Opt. Lett.

J. Appl. Phys.

Proc. SPIE

Photonics

Russ. Chem. Rev.

J. Phys. Chem. C

Cryst. Growth Des.

J. Am. Chem. Soc.