Effect of the band gap and the defect states present within band gap on the non-linear optical absorption behaviour of yttrium aluminium iron garnets
Graphical abstract
Introduction
Intense monochromatic laser radiation explores the unparalleled information on the optical properties of a material. Probability of simultaneous absorption of more than one photon in a material can be enhanced if the intensity of the input radiation is sufficiently high. Hence, interaction between the intense laser field and the materials can induce population redistribution leading to interesting counteractions like stimulated emission and absorption of photons, complex energy transitions and free carrier generation etc., as encountered in non-linear optics. Among these, the nonlinear absorption (NLA) of high intensity lasers plays an imperative role in materials for optical limiting applications [1]. Nonlinear optical properties of various materials and their applications for passive optical limiting devices, signal processing and optical switches have fascinated the researchers, which led to the advancement in synthesis and development of novel materials having strong nonlinear optical absorption capabilities [[2], [3], [4]]. In optical limiting devices, transmittance decreases as a function of intensity or fluence of the laser beam. These properties of materials are used in optical devices for applications, such as in smoothing, pulse compression and optical pulse shaping [5,6]. Research on optical limiting materials is quite active for past few decades [7]. In recent years, utmost amount of interest has been devoted towards protection of optical sensors from laser induced damage, human health from exposure of laser radiation and also protection of laser shields itself from high intense X-ray free electron laser beams. The efficiency of such devices depends on the NLA behaviour of the concerned materials. The mechanism of NLA is associated with many optically divergent NLA processes, depending on laser excitation wavelength, fluence and pulse-width and material properties. The commonly encountered NLA processes are: two-photon absorption (2 PA), three-photon absorption (3 PA), free-carrier absorption (FCA), excited state absorption (ESA) and saturable absorption (SA) [8].
Owing to the high band gap (~6.5 eV) of yttrium aluminum garnet (YAG, Y3Al5O12), it is transparent for visible light [9]. Hence, YAG is used as a lasing medium for various solid state lasers such as Nd, Er, Nd:Cr, Yb, Nd:Ce, Ho, Dy, Sm, Tb, Ce, Ce:Gd and Gd doped YAG lasers [10,11]. These dopants generally substitute Y at the dodecahedral site in the Ia-3d crystal structure of YAG, due to their very similar cationic size [12]. Apart from being an excellent host material for many of these rare-earth ions, YAG displays magnificent physical and chemical properties such as high optical transparency, low thermal expansion and acoustic loss, high stability against chemical and mechanical damage. It is one of the most resistant-oxide material suitable for forming high-temperature ceramic composite materials for optical device application. On the other hand, yttrium iron garnet (YIG, Y3Fe5O12) shows ferrimagnetic behaviour [13]. Epitaxial films of these ferrimagnetic garnets and bulk magnetic garnets have been most actively studied in recent decades in view of both fundamental and technological applications, particularly, for magneto-optical, acoustic, optoelectronic, EMI shielding, magnetomechanical applications [[14], [15], [16], [17]]. The non-linear optical properties of YIG have been studied in epitaxial magnetic thin films in the spectral ranges of 1.7–3.2 eV and 2.4–4.2 eV, respectively [16,17]. YIG nanoparticles display sturdy reverse saturable absorption (RSA) behaviour when irradiated by 10 Hz laser pulses in nanosecond regime, proving it as a potential candidate for optical limiting applications. Saturation absorption and nonlinear transmission in tetrahedral V3+doped YAG has also been studied [18].
Here, we investigate the NLA behaviour of iron substituted yttrium aluminium garnet ceramic samples (Y3Al5-xFexO12, with x = 0, 1, 2, 3, 4 and 5). The effect of iron substitution in YAG samples on both linear as well as non-linear optical absorption properties and their mechanism is investigated. The structural defects in the samples are considered as the deciding factors in tuning the non-linear optical properties through excited state absorption process.
Section snippets
Experimental details
Iron substituted yttrium aluminium garnet ceramic samples (Y3Al5-xFexO12 with x = 0, 1, 2, 3, 4 and 5) were synthesized by solution auto-combustion route. The detailed synthesis and characterization of the samples is given elsewhere [19]. Briefly, a typical synthesis involves combustion of the aqueous solution of stoichiometric ratios of metal nitrates (as oxidiser) and glycine (as fuel). This combustion reaction was carried out in a preheated muffle furnace at 450 °C. The reaction proceeded
Results and discussion
As shown in Fig. 1, we observed a color transition from white to olive (green) with increasing iron content in our samples, which confirms the homogeneity of the samples and the substitution of Fe for Al in the garnet structure of YAG. The XRD patterns for all our synthesized garnet samples are shown in Fig. 3. The XRD patterns reveal that all the samples are phase pure with cubic (Ia-3d) crystal structure [19]. In YAG, the Y3+ cations occupy the dodecahedral interstitial sites, whereas Al3+
Conclusion
The iron substituted yttrium aluminium garnets (Y3Al5-xFexO12, YAIG) with x = 0, 1, 2, 3, 4 and 5, were synthesized by solution combustion route and their nonlinear absorption behaviour is investigated using open aperture Z-scan technique. The observed differences in the nonlinear absorption coefficients () and saturation intensity (Is) of all our samples were explained in detail. We observed that the substitution of Fe for Al in yttrium aluminium garnet leads to a gradual decrease in band
CRediT authorship contribution statement
Ajay Kumar: Writing - original draft, Writing - review & editing. Rajeev Kumar: Formal analysis, Writing - original draft, Writing - review & editing. Nancy Verma: Formal analysis, Writing - review & editing. A.V. Anupama: Formal analysis, Writing - review & editing. Harish K. Choudhary: Writing - review & editing. Reji Philip: Project administration, Writing - review & editing. Balaram Sahoo: Conceptualization, Funding acquisition, Project administration, Writing - original draft, Writing -
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
The authors RK and HKC are grateful to CSIR, New Delhi, India, for providing Senior Research Fellowship.
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