Elsevier

Optical Materials

Volume 108, October 2020, 110163
Optical Materials

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

https://doi.org/10.1016/j.optmat.2020.110163Get rights and content

Highlights

  • Substitution of Fe for Al in yttrium aluminium garnet (Y3Al5-xFexO12, YAIG) leads to decrease in bandgap.

  • YAIG samples show coral-like morphology with high surface area with surface disorder/defects.

  • The defect energy states spread into mid-bandgap region forming the Urbach tails, for all samples.

  • Reduction in band gap helps in enhancing nonlinear absorption (NLA) through occurrence of 2 PA rather than 3 PA.

  • Defect energy states within Urbach tail facilitate strong NLA through excited state absorption (ESA).

Abstract

The effect of variation of band gap and electronic defect energy levels within the band gap on non-linear optical absorption of 532 nm (green) laser light by coral-like yttrium aluminium-iron garnet (YAIG, Y3Al5-xFexO12) ceramics prepared by solution combustion route is investigated using Z-scan technique. Our yttrium aluminium garnet (YAG) sample with band gap of ≥ ~ 6.5 eV, is transparent to visible light. The band gap of YAG was tuned by substituting Fe at the aluminium site and consequently, the surface defects in YAG were also modified. The defects in the YAIG samples introduce electronic defect states within the band gap. We observed that the intensity of the non-linear absorption (NLA) signal increases with increase in the amount of iron content. We assigned this enhancement of NLA signal to: (1) the decrease in the band gap from ≥ ~ 6.5 eV for YAG to ~ 2.6 eV for yttrium iron garnet (YIG) facilitating two photon absorption (2 PA) process rather than three photon absorption (3 PA), and (2) the increase in excited state absorption (ESA) due to the spread of the defect energy states into the band gap region. As the band gap gets closer to the photon energy of the laser, the non-linear optical absorption is enhanced due to ESA, in combination with the occurrence of the relatively stronger 2 PA process rather than the very weak 3 PA process. Our results demonstrate a method to tune the NLA performance of YAG through iron substitution at the Al sites and provide a better understanding of the non-linear absorption behaviour of YAIG ceramics for their application in optical limiting devices such as laser shields.

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 (β) 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.

References (44)

  • C. Eevon et al.

    Linear and nonlinear optical properties of Gd3+ doped zinc borotellurite glasses for all-optical switching applications

    Results Phys

    (2016)
  • M. Nikl et al.

    Energy transfer and storage processes in scintillators: the role and nature of defects

    Radiat. Meas.

    (2007)
  • D. Dini et al.

    Nonlinear optical materials for the smart filtering of optical radiation

    Chem. Rev.

    (2016)
  • M. Soljačić et al.

    Nonlinear photonic crystal microdevices for optical integration

    Opt. Lett.

    (2003)
  • V.R. Almeida et al.

    All-optical control of light on a silicon chip

    Nature

    (2004)
  • C. Zhang et al.

    Modulation of third-order nonlinear optical properties by backbone modification of polymeric pillared-layer heterometallic clusters

    Adv. Mater.

    (2008)
  • D.W. Preston

    Doppler-free saturated absorption: laser spectroscopy

    Am. J. Phys.

    (1996)
  • D.J. Harter et al.

    Power/energy limiter using reverse saturable absorption

    J. Appl. Phys.

    (1984)
  • Y. Xu et al.

    Electronic structure of yttrium aluminum garnet (Y3Al5O12)

    Phys. Rev. B Condens. Matter

    (1999)
  • G. Huber et al.

    Laser pumping of Ho-, Tm-, Er-doped garnet lasers at room temperature

    IEEE J. Quant. Electron.

    (1988)
  • R.D. Shannon

    Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides

    Acta Crystallogr. A

    (1976)
  • H.K. Choudhary et al.

    Effect of coral-shaped yttrium iron garnet particles on the EMI shielding behaviour of yttrium iron garnet-polyaniline-wax composites

    Chemistry

    (2018)
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