Femtosecond laser induced luminescence in hierarchically structured NdIII, YbIII, ErIII co-doped upconversion nanoparticles: Light-matter interaction mechanisms from experiments and simulations

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Highlights

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    Core@shell UCNP upconversion was investigated with CW and femtosecond laser.

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    For same average power, different upconversion mechanism with CW vs. femtosecond.

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    18-level system modeling of NdIIIYbIIIErIII to understand experimental results.

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    CW/femtosecond power-dependent upconversion can be treated on the same footing.

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    Excitation bandwidth of CW/femtosecond source also affects UCNP photophysics.

Abstract

Fundamental studies of light-matter interactions are important for basic knowledge and in applications. Thanks to advances in experimental and theoretical methods, nowadays it is possible to perform such studies in a broad dynamic range, covering timescales from that of elementary interactions to real time. In the present work, we perform an experimental-theoretical study of light intensity-dependent femtosecond and CW-laser induced frequency upconversion in hierarchically structured core-multishell nanoparticles co-doped with NdIII, YbIII, and ErIII. Upconversion spectra recorded with CW and femtosecond excitation are qualitatively similar whereas the intensity dependence of upconversion depends on excitation mode (CW or femtosecond). To further assess the observed intensity dependence, we perform light-matter interaction simulations in the dynamic range from 100 fs to 3 ms for 18-level system describing the UCNPs, including nine levels of the NdIII, two of the YbIII, and seven of the ErIII ions and a classical model for the excitation source. The calculated time- and intensity-dependent energy level population are compared with measured spectra to understand CW vs femtosecond laser-induced upconversion. To further discuss the differences between CW and femtosecond laser-induced light-matter interactions for the systems studied here, we perform semi-classical pulse propagation simulations and ultrafast pump-probe measurements to study how the light source bandwidth, relative to the absorption linewidth, influence light absorption and transmission and further connect these results with the intensity dependence. Overall, we report our progress toward mechanistic studies of light-matter interaction and photophysical pathways following femtosecond excitation and UCNPs.

Introduction

Thanks to remarkable advances in theory, computation, and experiments, nowadays it is possible to interrogate a variety of molecular and material processes that cover many orders of magnitude in time. Starting from the earliest fundamental events up to longer times, studies performed at multiple timescales include, for example, glass-transition [1,2], biophysics [3,4] and solar cells [5]. That the examples mentioned span multiple fields of research already speaks for the widespread interest in this growing class of interdisciplinary tools.

Within the field of spectroscopy of lanthanide ions in host matrices and in upconversion nanoparticles (UCNPs), multiple timescales are required to describe all processes in detail, from the basic selection rules to the multitude of energy levels and energy transfer photophysical pathways required to understand the luminescent properties of these materials. This is a challenging task, because the ultrashort laser pulses used to interrogate light-matter interactions at the shortest timescales exhibit high peak intensities. Upon their interaction with UCNPs, ultrashort lasers will thus give rise to both sequential and simultaneous excitation processes, adding complexity to the UCNP photophysics. Moreover, ultrashort laser pulses have broad spectral bandwidth, broader than most of trivalent lanthanide doped-material absorption linewidth, and thus the spectroscopic study must be performed taking this regime into account, thereby adding further complexity to the light-matter interaction study.

Nonetheless, considering the tremendous progress in fundamental understanding of the luminescent properties of lanthanide ions in complexes and host matrices [[6], [7], [8], [9]] as well as UCNPs [[10], [11], [12], [13]], together with the importance of UCNPs in basic science [14,15] and applications [[16], [17], [18]], understanding light-matter interactions involving femtosecond lasers and UCNPs may be useful in studies requiring higher energies, for example, in real-time applications and faster super-resolution bioimaging [19].

Several recent studies have reported fundamental advances in our understanding of energy transfer mechanisms in UCNPs and core-shell UCNPs [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. However, to the best of our knowledge, this work reports for the first time a comparison between experiments and simulations involving femtosecond and CW-induced upconversion with core-triple shell UCNPs containing with GdIII, NdIII, YbIII, and ErIII ions. In this work, we describe synthesis and characterization methodologies, experimental setup and simulation details, followed by sample characterization by X-ray diffraction, electron microscopy and intensity-dependent upconversion luminescence. We present kinetic simulation results for a model including NdIII, YbIII, and ErIII ions, as well as a realistic model for the femtosecond laser used in this work, as a function of time and excitation intensity, and compare the measured and calculated power-dependent luminescence. We present semi-classical pulse propagation simulations and femtosecond pump-probe measurements for NdIII to help isolate different contributions to the observed UCNP luminescence. We conclude by summarizing the main results reported and discuss possible directions for investigations.

Section snippets

Materials and methods

Materials, synthesis, and characterization: All reagents were analytical grade. The syntheses of hierarchically structured upconversion nanoparticles with one core and three shells (shown schematically in Fig. 1a) were carried out as described previously [31,32]. Briefly, as an example, for the synthesis of NaDyF4 core nanoparticles, 1.5 mmol of dysprosium trifluoracetate were combined with 30 mmol of 1-octadecene (Sigma Aldrich, 90%), 15 mmol of oleic acid (Sigma Aldrich, 90%) and 15 mmol of

Results and discussion

Fig. 2 shows the X-ray diffraction patterns of the UCNP. The nanoparticles were obtained successfully in hexagonal phase that shows higher intensity of upconversion emission when compared to cubic phase [31].

Fig. 3 shows TEM image and corresponding size distribution histogram of the UCNP showing a mean particle size 24.1 ± 3.7 nm The EDS data of the core-only nanoparticles is shown in ESI (Table 1S) and the results indicates a reasonable correlation with the nominal concentrations of each ion.

Conclusions

In conclusion, we have measured upconversion spectra as a function of pump power for hierarchically structured UCNPs excited by CW and femtosecond laser pulses. A nearly quadratic pump-power dependence was observed under CW excitation, consistent with low-energy density excitation regime. On the other hand, for femtosecond pulse excitation, pump-power dependence with a slope of 1.5 in the log-log plot was observed, consistent with high-energy density excitation regime. Numerical modeling of

Author statement

GH Oliveira: Conceptualization, Methodology, Investigation, Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization. FS Ferreira: Methodology, Validation, Investigation, Resources, Writing – Review & Editing, Visualization. GF Ferbonink: Methodology, Validation, Investigation, Data Curation, Writing – Review & Editing, Visualization. MP Belançon: Conceptualization, Resources, Writing – Review & Editing, Visualization. FA Sigoli: Conceptualization, Methodology,

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.

Acknowledgements

Financial support from FAPESP (grant 13/22127-2, 14/50906-9, 16/23430-9) and CNPq (the National Council for Scientific and Technological Development) INCTs: Catalysis in Molecular and Nanostructured Systems and INOMAT are gratefully acknowledged.

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    Current address: Universidade Tecnológica Federal do Paraná (UTFPR), Câmpus Pato Branco

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