Short communicationUltrafast pump-probe spectroscopy studies of CeO2 thin film deposited on Ni-W substrate by RF magnetron sputtering
Introduction
During the last several years cerium oxide (CeO2) has attracted much attention due to its wide variety of applications [1], [2], [3]. It is transparent in the visible and near-IR spectral region and highly absorbing in the ultraviolet (UV) region, which makes it a suitable material for various optical, electro-optical and optoelectronic devices [4], [5]. Also CeO2 is being used as a dopant material to improve the optical properties, such as broad absorption and luminescence with ultra short lifetime in UV and visible wavelength region, and therefore finds wide usage in laser crystals and glasses, and fluorescent lamps [6], [7], [8], [9], [10].
CeO2 is a mixed valence rather than pure 4f compound that involves Ce4+ and Ce3+ ions [11] and, in its ground state Ce has an electronic structure of [Xe] 4f1, 5s2, 5p6, 5d1, 6s2 where f electrons are shielded by 5s2, 5p6 and 6s2 orbitals and, inherently, nonbonding. The unfilled configuration of 4f electrons are known to have a dominant influence on the optical properties, which is accompanied by f–f electron transition [12]. The optical properties of the CeO2 films have been reported by several workers providing different values for the refractive index (n) and the optical band gap (Eg) in the range of 1.78–2.6 and 2.0–3.6 eV, respectively [13], [14], [15], [16]. However, no report is available on the rapid dynamical processes viz. vibrational relaxation times, and population times etc., that occur in pure CeO2 materials, in any of its form.
The introduction of ultrafast pump-probe spectroscopy has enabled coherent spectroscopy to be extended to the measurements of a wide range of rapid dynamical processes in molecules. Using femtosecond (fs) laser pulses, it is possible to probe the transitions between different states in molecular systems, allowing various hidden dynamical processes (macroscopic inhomogeneous broadening etc.) to be resolved [17]. Due to its characteristics of ultrashort pulse and ultrahigh peak power fs laser has been used in a range of scientific fields to extract information about molecular dynamics such as decoherence times, vibrational relaxation times, population lifetimes and coherence coupling etc. Thus, it has become a powerful tool to fabricate various optical elements, including optical waveguides, gratings, couplers, photon crystals and other functional optoelectronic devices [18], [19], [20], [21], [22], [23].
To the best of literature survey this is the first investigation on the rapid dynamical processes that occur in pure CeO2 thin film, using ultrafast pump-probe spectroscopy at room temperature.
Section snippets
Experimental
For the present study, we have used a single (200) oriented CeO2 thin film deposited on Ni-W substrate by RF magnetron sputtering technique and the details are published elsewhere [24]. To perform ultrafast optical pump-probe spectroscopy a train of optical pulses from a Ti: Sapphire laser amplifier (35 fs, 4 mJ/pulse, 1 KHz, 800 nm) was split into two beams (9:1) with a beam splitter. One with high intensity was used as a pump and an optical parametric amplifier (TOPAS, Light conversion) was
Results and discussion
The transient spectra recorded for pump/probe wavelengths 350 nm/500–800 nm (2.48–1.55 eV), as a function of delay time, are shown in Fig. 2(a–d). This shows an initial sharp rising transition followed by an exponential decay. The observed pump induced change in probe absorption values are about 0.002–0.012 from 500 to 700 nm, and there is no significant change from 700 to 800 nm. It is to be noted that the absolute value of the absorption signals is proportional to the transitions from
Conclusions
In conclusion, optical behavior of CeO2 thin film has been investigated using ultrafast pump-probe spectroscopy. The ultrafast transient spectra show initial sharp rise transition followed by an exponential photon decay with decay constant varying from of 171 to107.5 ps for the probe wavelengths 500–800 nm. The rise time of the signal is about 10 ps, irrespective of the probe wavelength. Further, two absorption peaks are observed at 745 and 800 nm, in ultrafast absorption spectra, and
Acknowledgement
The authors are grateful to the Director, CSIR-National Physical Laboratory for his continuous encouragement and support during this work. The financial support provided by Department of Science and Technology (DST) through SERB, New Delhi, India with reference No: SB/EMEQ-040/2014 (Project No: GAP150332) is gratefully acknowledged.
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