Comprehensive study on the nonlinear optical properties of lanthanum nanoparticles and lanthanum oxide doped zinc borotellurite glasses

Comprehensive study on the nonlinear optical properties of lanthanum nanoparticles and lanthanum oxide doped zinc borotellurite glasses

Optics and Laser Technology 127 (2020) 106161 Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.c...

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Optics and Laser Technology 127 (2020) 106161

Contents lists available at ScienceDirect

Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Comprehensive study on the nonlinear optical properties of lanthanum nanoparticles and lanthanum oxide doped zinc borotellurite glasses

T

M.F. Faznnya, M.K. Halimaha, , C. Eevona, A.A. Latifb, F.D. Muhammadb, A.S. Asyikina, S.N. Nazrina, I. Zaitizilaa ⁎

a b

Glass and Dielectric Lab, Physics Department, Faculty of Science, Universiti Putra, Malaysia Photonic Lab, Physics Department, Faculty of Science, Universiti Putra, Malaysia

HIGHLIGHTS

element doped zinc borotellurite glasses were successfully fabricated. • Lanthanum proved the existence of TeO , TeO , BO and BO in the prepared glass. • FTIR optical properties of the glasses were extensively investigated. • Nonlinear trends in the nonlinear parameters are due to structural changes. • Inconsistent • La NPs doped glass can possibly be used as efficient optical switching device. 4

3

4

3

ARTICLE INFO

ABSTRACT

Keywords: Tellurite based glasses Lanthanum Nonlinear optical properties Reverse saturable absorption Third order susceptibility Figure of Merit

Recently, the rapid growth in photonic field has increased the demand for nonlinear materials with higher performance and greater efficiency. Hence, a thorough investigation on the nonlinear optical properties of materials are essential and need to be done for possible future application of the material in photonic field as optical limiting or optical switching devices. Thus, through this research, the crucial parameters in nonlinear optical properties of zinc borotellurite glasses doped with lanthanum oxide and lanthanum nanoparticles were studied and investigated extensively. The two series of glasses were successfully fabricated via conventional melt-quenching technique. The fabricated samples were characterized by using Fourier Transform Infra-Red (FTIR) spectroscopy as well as Z-scan technique in order to study the structural and nonlinear optical properties of the glass systems. The existence of various amount of TeO4, TeO3, BO4 and BO3 in all the prepared glasses are proven through the observable absorption bands in the FTIR spectra. Inconsistent trends recorded for both nonlinear absorption coefficient and nonlinear refractive index might be associated with the simultaneous creation of bridging as well as nonbridging oxygen which eventually affect the values for both parameters. The figure of merit of the prepared glasses with values ranging from 0.055 to 0.267 which are smaller than one hints that the glass materials possess potential to be employed as all optical devices. 0.03 M fraction of lanthanum nanoparticles doped zinc borotellurite with FOM value larger than 1 has proven the ability of the respective glass to be employed as efficient all optical switching devices. The determined nonlinear optical parameters of the glasses should be able to provide sufficient and useful information on the fabricated glass samples for future application in photonic field.

1. Introduction Nonlinear optical properties of materials are one of the most potential approach used for controlling a light signal through another light beam for optoelectronic and photonic application [1]. Materials with nonlinear absorbing capability which possess various nonlinear



absorption process such as saturable absorber, reverse saturable absorption and two-photon absorption have great potential to be applied in science and technology field [2]. Meanwhile, materials possessing high third-order nonlinear susceptibility with response times of less than a subpicosecond are essential for all-optical signal processing in integrated and fiber optics [3]. Previous researches indicates that

Corresponding author. E-mail addresses: [email protected] (M.K. Halimah), [email protected] (A.A. Latif), [email protected] (F.D. Muhammad).

https://doi.org/10.1016/j.optlastec.2020.106161 Received 15 October 2019; Received in revised form 18 February 2020; Accepted 24 February 2020 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.

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glasses containing chalcogens such as Sulfur (S), Selenium (Se) and Tellurium (Te) are promising nonlinear glass systems [4]. Researcher also reported the supremacy of chalcogenide glass in which the nonlinearity of chalcogenide glass is usually 200–1000 times larger than those of silica glass [5]. Non-silica glasses, such as tellurite based, fluoride based and chalcogenide (S, Se, Te) based glasses are nominated as promising candidates to be applied as fiber glass materials for midinfrared nonlinear optical applications over conventional silica glass fiber due to their high refractive index, large optical nonlinearity and superior optical transparence in the wavelength range of 0.4–7 μm, 0.3–8 μm and 1–16 μm respectively [6,7]. Thus, the nonlinear optical properties of materials must be extensively investigated before the potential of the glass can be unleashed. Currently, tellurite based glasses that have been doped with various elements as well as dopants are being widely studied in owing to their excellent nonlinear optical properties and their potential application as ultra-fast optical switches as well as electro-optic modulator [8–10]. In this case, boron and zinc are introduced into tellurite glass as glass former and glass modifier respectively due to the good rare earth solubility of boron element as well as the ability of zinc oxide to enhance the overall glass forming ability of the glass material [11,12]. Previous researches done on the effect of gadolinium as well as erbium on zinc borotellurite glass system have reported nonlinear trends with relatively large values for all nonlinear optical properties parameters [13,14]. The investigation on erbium as well as gadolinium doped zinc borotellurite glasses verified the potential of both glass system to be employed as solid state laser and all optical switching devices. However, lanthanum element is rarely doped into glass system because of the absence of f electron that capable of enhancing the overall optical properties of a glass system. Since no research had tried to investigate the possible potential of lanthanum to be utilized as nonlinear photonic device, thus through this research, detailed study on the influence of lanthanum oxide and lanthanum nanoparticles on the structural as well as nonlinear optical properties of zinc borotellurite glass was carried out. Both nonlinear absorption coefficient and nonlinear refractive index obtained from Z-scan technique will be used for the calculation of Figure of Merit (FOM) values that is capable of verifying the potential of the prepared glass to be employed as optical switching devices.

Fig. 1. Schematic diagram of single beam Z-scan experimental setup.

hour to reduce thermal stress as well as removing air bubbles in the casted glass. The furnace was switched off after one hour and the glass was left in the furnace overnight to allow the glass to cool down to room temperature. A small portion of the glass was crushed and ground before being sent for structural characterization. Meanwhile the bulk glass was cut as well as polished to obtain parallel, flat and scratch-less surface for nonlinear optical characterization via the Z-scan technique. 2.2. Nonlinear z-scan measurement The setup for Z-scan technique utilized in this research is as shown in Fig. 1 while the actual and the clearer setup is illustrated in Fig. 2. The light source used in this research is the high intensity 532 nm continuous wave diode laser (Coherent Compass SDL-532-150T) beam. The polished glass was clamped by sample holder and positioned on a translation stage that move along Z-axis with the help of motorized translational stage which is controlled by BS101 micro-stepping controller (Thorlabs) in a computer. An aperture was located between the glass sample and detector whereby the aperture will be in open mode for open aperture Z-scan method to determine the nonlinear absorption coefficient. In contrast, the aperture will be in closed mode for closed aperture Z-scan technique in order to retrieve the nonlinear refractive index of the fabricated glass. The high intensity green laser beam propagates through a focal lens, the clamped glass sample and aperture before the beam will be detected as well recorded by detector that connected to the computer in the Z-scan setup.

2. Methodology 2.1. Fabrication of glass Lanthanum oxide as well as lanthanum nanoparticles doped zinc borotellurite glasses with chemical composition of {[(TeO2)0.7(B2O3)0.30]0.7(ZnO)0.3}1−x (La2O3)x and {[(TeO2)0.70 (B2O3)0.30]0.7(ZnO)0.3}1−x (La NPs)x where x = 0.01, 0.02, 0.03, 0.04 and 0.05 M fraction were prepared through conventional melt quenching technique. Tellurium oxide, TeO2 (Alfa Aesar), zinc oxide, ZnO (Alfa Aesar), lanthanum oxide, La2O3 (Alfa Aesar) and lanthanum nanoparticles, La NPs (Nanostructured & Amorphous Materials Inc.) chemical powders used in the sample preparation process were of high purity (99.99%) while pure analytical grade chemicals boron oxide, B2O3 has a purity of 98.5%. Conventional melt-quenching process started with the weighting of a total of 13 g of chemical powders into alumina crucible before being stirred for 30 min to ensure the homogeneity of the chemical mixture. The alumina crucible containing the stirred chemical mixture was then put into an electrical furnace at 400 °C for preheating procedure in order to remove any moisture content in the chemical powders. After one hour of preheating process, the chemical mixture was transferred into another electrical furnace for two hours at 900 °C to undergo melting process. At the same time, the mould was polished, cleaned and put into the first furnace for preheating purposes as the chemical mixture went through melting procedure. The molten was then quickly quenched into the pre-heated mould before being annealed inside the first furnace at 400 °C for one

3. Results and discussion 3.1. FTIR spectroscopy FTIR spectrometer is an important instrument in material research in order to probe and investigate the structure of a material. The

Fig. 2. Actual and clearer schematic diagram of single beam Z-scan experimental setup. 2

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Fig. 3. FTIR transmission spectra for lanthanum oxide doped zinc borotellurite glasses.

Fig. 4. FTIR transmission spectra for lanthanum nanoparticles doped zinc borotellurite glasses. Table 1 Position of FTIR absorption bands and their respective assignment for lanthanum oxide doped zinc borotellurite glasses. Number

0.00

0.01

0.02

0.03

0.04

0.05

Assignments

1 2 3 4 5

635 677 969 1229 1369

631 674 965 1239 1366

637 669 960 1241 1365

639 671 963 1239 1366

645 665 973 1241 1360

640 679 977 1243 1369

TeO4 stretching vibration [16] Formation of TeO3 trigonal pyramidal [17] B-O bond stretching vibrations in BO4 tetrahedral from tri-, tetra-, and penta- borate groups [18] Trigonal B-O bond stretching vibrations of BO3 units from boroxyl groups [19] Asymmetric stretching relaxation of the B-O band of trigonal BO3 units [20]

Table 2 Position of FTIR absorption bands and their respective assignment for lanthanum nanoparticles doped zinc borotellurite glasses. Number

0.00

0.01

0.02

0.03

0.04

0.05

Assignments

1 2 3 4 5

635 677 969 1229 1369

638 669 977 1230 1339

642 671 960 1233 1342

640 671 981 1235 1335

644 673 993 1235 1327

642 665 996 1236 1327

Stretching vibration of TeO4 [21] Formation of trigonal pyramidal TeO3 [22] B-O bond stretching vibrations in BO4 tetrahedral from tri-, tetra-, and penta- borate groups [23] Trigonal B-O bond stretching vibrations of BO3 units from boroxyl groups [24] Asymmetric stretching relaxation of the B-O band of trigonal BO3 units [25]

3

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Fig. 5. Z-scan open aperture transmission curve for lanthanum oxide doped zinc borotellurite glasses.

infrared spectrum capable of distinguishing molecules and giving detailed information on the presence or absence of functional groups in a material [15]. The ability of identifying molecules in a material is related to the vibrational mode of molecules whereby each and every molecule possesses different vibrational mode which able to differentiate a molecule from other molecules. Generally, the vibration of a molecular group in a glass matrix is independent on the vibration of other molecular groups since the wavenumber where each of the molecular group vibrates is distinct to each other. Fig. 3 as well as Fig. 4 illustrate the FTIR spectra for zinc borotellurite glass doped with lanthanum oxide and lanthanum nanoparticles respectively while the assignment for every recorded absorption bands are listed in Tables 1 and 2. The fabricated glasses were tested under room temperature at 250–1750 cm−1 whereby the recorded absorption bands unveiled the existence of TeO4, TeO3, BO4 and BO3 in the glass system. The backbone of the studied glasses which is made up of tellurite and boron still exist in the glass matrix after glass formation process as shown in Fig. 3. Generally, tellurium and boron that exist as network former can present in a borotellurite glass system as TeO4, TeO3, BO4 as well as BO3 whereby the molecules will vibrate at 600–650 cm−1, 650–700 cm−1, 800–1200 cm−1 and 1200–1400 cm−1 respectively after they absorb infrared ray [26–29]. In this case, different amount of TeO4, TeO3, BO4 and BO3 structural groups positioned at 631–645 cm−1, 665–679 cm−1, 960–996 cm−1, 1229–1369 cm−1 respectively are recorded as absorption bands with various depth in the FTIR spectra. From both Figs. 3 and 4, the incorporation of lanthanum oxide as well as lanthanum nanoparticles seems to reinforce the glass network as the dopants creates more bridging oxygen in form of TeO4 and BO4 when compared to TeO3 and BO3 with nonbridging oxygen [30]. At the same time, the formation BO3 and TeO3 in the glass matrix after the addition of dopants might be the result of the breaking of the backbone of the glass system.

3.3. Nonlinear absorption coefficient and nonlinear refractive index Generally, when a light beam propagate through a glass material, the changes in the intensity of the output light that dependent on the alteration of the absorption coefficient of the material will take place as the intensity of the incident light beam increases. The relationship between both linear absorption coefficient and nonlinear absorption coefficient can be described as the following equation:

=

0

+

=

0

+ I

(1)

where α represents the nonlinear absorption coefficient, α0 is the linear absorption coefficient, Δα stands for changes in absorption coefficient and β represents the nonlinear absorption coefficient. The values for nonlinear absorption coefficient, β of the prepared glasses are computed via the equations as follow [33]:

Leff =

1

e

L

(2) (3)

q0 (z ) = I0 Leff

where Leff is the effective length of the material, L is the thickness of the material, q0(z) represents the parameter that portrays the strength of the nonlinearity of a material towards the Gaussian beam and Io is the on-axis irradiance at the focus. The obtained data from open aperture Zscan measurement is plotted, fitted and illustrated in Fig. 5 as well as Fig. 6. On the other hand, nonlinear refraction ability of a glass material which include the converging and diverging of high intensity light beam is wholly dependent on the nature of the glass. The converging or diverging phenomena of high intensity laser light will affect the amount of light that able to propagate through the aperture before being recorded by detector in Z-scan setup. Nonlinear refractive index, n2 that express the nonlinear refraction ability of a glass can be calculated through the following relation [34]:

n2 =

3.2. Z-scan technique According to Zakery and Elliott, many technique such as twophoton absorption spectroscopy, degenerate four-wave mixing (DFWM), Z-scan, third harmonic generation (THG), optical Kerr-shutter (OKS) and self-phase modulation can be used to determine the nonlinear refractive index as well as nonlinear absorption coefficient of a chalcogenide glass [31]. However, Z-scan technique has been chosen to be utilized in this research in owing to the simplicity and accuracy of the Z-scan technique. This simple experimental method to obtain nonlinear absorption coefficient, nonlinear refractive index and third order nonlinear susceptibility of nonlinear optical materials has been proposed by Sheik Bahae in 1990 [32].

k=

0

0

(4)

kLeff Io

2

=

(5)

Tp (0.406)(1

v

S )0.25

(6)

where Δϕo represents the nonlinear phase change, k is the wavevector, λ represents the wavelength of the laser beam, S stands for the linear transmittance of the aperture and ΔTρ-v is the changes in the peak-valley transmission. Glass material possessing positive nonlinear refractive index value will have a pre-focal valley followed by post-focal peak pattern in the Z-scan closed aperture transmission curve while a pre4

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Fig. 6. Z-scan open aperture transmission curve for lanthanum nanoparticles doped zinc borotellurite glasses.

Fig. 7. Z-scan closed aperture for lanthanum oxide doped zinc borotellurite glasses.

Fig. 8. Z-scan closed aperture for lanthanum nanoparticles doped zinc borotellurite glasses.

focal peak and post-focal valley pattern will be recorded in Z-scan closed aperture transmission curve of a negative nonlinear refractive index glass [35]. The Z-scan closed aperture transmission curve for lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses are shown in Figs. 7 and 8 respectively. The trends for nonlinear absorption coefficient and nonlinear refractive index of both glass series are illustrated in Figs. 9 and 10 while the exact values for both parameters are tabulated in Table 3. From Figs. 5 and 6, valley profile which can be observed in the open aperture Z-scan curve for both glass series has hinted that the reverse saturable absorption phenomena take place in all the prepared glass

samples. The reverse saturable absorption phenomena recorded in all glass samples indicate that the prepared glasses cannot be employed as saturable absorber devices since saturable absorbing devices requires material with saturable absorbing phenomena. Inconsistent trends in nonlinear absorption coefficient for both glass series are illustrated in Fig. 9 whereby the nonlinear trends maybe contributed by the simultaneous creation of TeO4 and BO3 units in the glass matrix [13]. The sudden increment in nonlinear absorption coefficient for glass samples with 0.04 M fraction of lanthanum oxide and lanthanum nanoparticles might be attributed by the structural change that alter the amount of TeO4, TeO3, BO4 and BO3 structural units in the glass network. 5

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Fig. 9. Nonlinear absorption coefficient for lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses.

Fig. 10. Nonlinear refractive index for lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses.

the polarizability of the glass material. In this case, the larger cation tend to be easily distorted by the electromagnetic field as compared to those of smaller size and eventually causes a reduction in the nonlinear absorption coefficient values [36,37]. A pre-focal valley and post focal-peak pattern that can be seen in both Figs. 7 and 8 has suggested the occurrence of self-focusing phenomena in lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses. A variation in the trends for nonlinear refractive index is correlated to the effect of various concentration of dopants in the zinc borotellurite glass system. Generally, the small nonlinear refractive index values of zinc borotellurite glasses doped with lanthanum oxide and lanthanum nanoparticles can be related to the small linear refractive index of the glasses [34] whereby the exact values of linear refractive index values for lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses that have been reported in [26] as well as [38] lies in the range of 2.19–2.50. Since both linear and nonlinear refractive index heavily dependent on the polarizability of the glass system, the creation of low polarizability bridging oxygen might be the main reason that contributed to the small and decreasing refractive index values. When lanthanum oxide and lanthanum nanoparticles are introduced into the zinc borotellurite glass matrix as network modifiers, both lanthanum oxide and lanthanum nanoparticles assist in forming low polarizability bridging oxygen instead of breaking the glass host to create high polarizability nonbridging oxygen that are capable of increasing the overall electronic polarizability of the glass matrix. The increasing amount of bridging oxygen in form of TeO4 as well as BO4 that contributed to the decreasing nonlinear refractive

Table 3 Nonlinear absorption coefficient as well as nonlinear refractive index of lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses. Molar fraction, x

Nonlinear absorption coefficient, β (cm/GW)

Nonlinear refractive index, n2 (×10−14 cm2/W)

La 0.00 0.01 0.02 0.03 0.04 0.05

0.57 2.68 1.26 0.56 1.91 0.66

0.79 0.80 1.04 1.34 1.31 0.08

La NPs 0.00 0.01 0.02 0.03 0.04 0.05

0.57 2.31 0.56 0.04 0.60 0.62

0.79 1.32 0.79 0.38 0.18 0.23

However, the general decreasing trend of nonlinear absorption coefficient as concentration of lanthanum oxide and lanthanum nanoparticles in zinc borotellurite glasses rises is closely related to the cation size of the dopants. The decreasing nonlinear absorption coefficient values maybe related to the addition of large La3+ ions into the glass system when compared to Te4+ and B2+ with smaller sizes. The incorporation of dopants with larger cation size usually will lead to an increment of 6

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index values has been supported by the results from FTIR spectra in Figs. 3 and 4. On the other hand, the increasing nonlinear refractive index especially for lanthanum oxide doped zinc borotellurite glass might be caused by the polarizability of the constituent ions in the glass matrix [36]. The addition and substitution of La3+ ion that possess a higher cation polarizability than those of B2+ in the glass system may increase the overall polarizability and eventually lead to the increment in nonlinear refractive values as the concentration of lanthanum oxide rises in the glass matrix. The inconsistency such as sudden increment or decrement in the trend of the nonlinear parameters is due to the changes in the overall polarizability as the glass material is exposed to high intensity laser light. Any slight change in the polarizability of the overall material due to the presence of different amount of low polarizability bridging and high polarizability nonbridging oxygen will eventually alter the values for nonlinear parameter since nonlinear parameters are sensitive to small changes in polarizability. From Table 3, smaller nonlinear absorption coefficient values are recorded for lanthanum nanoparticles doped glass system as compared to those of lanthanum oxide doped glasses. The smaller nonlinear absorption coefficient values might be due to unique characteristics of nanoparticles whereby the large surface area of lanthanum nanoparticles capable of inducing the creation of high density surface states that able to trap charge carriers [39]. On the other hand, the formation of large amount of bridging oxygen in the lanthanum oxide doped zinc borotellurite glass system which will require more energy for the electron to break free from the binding and to jump to the conduction band contribute to the larger nonlinear absorption coefficient values when compared to those of lanthanum nanoparticles. Zinc borotellurite glasses doped with lanthanum oxide have larger nonlinear refractive index values than those of lanthanum nanoparticles whereby the overall higher values of nonlinear refractive index is correlated to the density of both glass system. Lanthanum oxide doped zinc borotellurite glasses with density ranging from 3.5439 to 6.6431 gcm−3 are much denser than those of lanthanum nanoparticles doped zinc borotellurite glass with 3.5669 to 3.7374 gcm−3 as their respective density values [38,39]. Since the dense packing state in a glass system has a direct proportional relationship with its refractive index [40], lanthanum oxide doped glasses with a higher density will eventually have larger nonlinear refractive index values.

Table 4 Calculated values for third order susceptibility of lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses.

n2

m2 W

=

n0 n2 [esu] 12

(7)

40 n2 cn 0

(8)

La 0.00 0.01 0.02 0.03 0.04 0.05

2.67 3.15 3.52 4.48 4.34 0.25

La NPs 0.00 0.01 0.02 0.03 0.04 0.05

2.67 5.14 2.82 1.41 0.64 0.81

3.5. Figure of merit (FOM) The capability of a material to be utilized as all-optical switching devices is quantified as figure of merits (FOM). FOM is computed by using nonlinear absorption coefficient, nonlinear refractive index and the wavelength of laser through the following formula [42]:

FOM =

n2

(9)

The calculated values of FOM are tabulated in Table 5 and the plots for the trends of FOM are depicted in Fig. 12. Inconsistent trends are recorded for FOM of both glass series whereby glass samples with 0.03 M fraction of lanthanum oxide and lanthanum nanoparticles have the highest FOM values among other glasses in the same series. The abnormal structural change that takes place in the glass matrix eventually lead to the inconsistent trend of FOM for both glass series. Despite the inconsistent trends, the FOM values for all glass samples with values less than 1 indicate that the studied glasses have the potential to be applied as all optical switching devices [43]. According to Chen et al. [44], materials with FOM > 1 can be designed as all optical switching devices while those with FOM > 10 is ideal material for efficient alloptical devices. Structural changes that occurred in zinc borotellurite glass doped with 0.03 M fraction of lanthanum nanoparticles bring about exceptional FOM value which suggest the potential of the particular glass to be applied as efficient all optical switching devices.

Other than linear and nonlinear refractive index, third order susceptibility of a material is another crucial parameter that must be considered before employing the material as optical devices. Third order susceptibility of glass materials are affected by the sign and magnitude of both real as well as imaginary part that represent the nonlinear refractive index and nonlinear absorption coefficient [32]. The third order susceptibility, χ(3) of a glass is related to both linear and nonlinear refractive index of the glass as shown in the relations below [36]:

[esu] =

Third order susceptibility, χ(3) (×10−9 esu)

the overall polarizability of the glass network. The inconsistent trend of third order susceptibility as shown in Fig. 10 maybe caused by the formation of inconsistent number of high polarizability nonbridging oxygen in the prepared glasses system. On the other hand, any decrement in the optical nonlinearity of a glass system might be caused by a lower ratio of oxygen-to-cation that will eventually decrease the polarizability and third order susceptibility of the glass material [41].

3.4. Third order susceptibility

(3)

Molar fraction, x

3.6. Comparison of lanthanum doped zinc borotellurite glass with other nonlinear media The nonlinear optical properties that can be quantify as nonlinear optical parameters of materials varies depending on the nature of the material. The composition or elements that made up the material will directly influence the optical nonlinearity of the material. Table 6 listed out the values for nonlinear parameters that include nonlinear absorption coefficient (β), nonlinear refractive index (n2), third order susceptibility (χ(3)) as well as Figure Of Merit (FOM) between the prepared glass material and materials from previous study. From Table 6, in general, lanthanum element doped zinc borotellurite glasses seem to possess smaller nonlinear optical parameters

where no stands for linear refractive index and c is the speed of light. The exact values for third order susceptibility is listed in Table 4 and the respective trend is illustrated in Fig. 10. From Fig. 11, third order susceptibility of both glass series has trends similar to those of nonlinear refractive index. Both parameters have the same trend since third order susceptibility is directly proportional to nonlinear refractive index whereby the relationship between the two parameters can be clearly seen in Eq. (7). The third order susceptibility is highly influenced by the structural properties as well as 7

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Fig. 11. Third order susceptibility of lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses.

nonlinear refractive index that will contribute to a larger figure of merit. The small nonlinear refractive index of silica fibre can be related to the absence of tellurium oxide in the chemical composition of the silica fibre. The existence of high refractive index tellurium oxide that have high polarizability tellurium ion seems to enhance the overall refractive index of the glass system and eventually resulting in higher nonlinear refractive index values [45]. From Table 6, neodymium element doped with zinc borotellurite glasses have the largest nonlinear absorption coefficient and nonlinear refractive index values as compared to other glass system as well as nonlinear medium. When comparing neodymium doped zinc borotellurite glasses are compared to glasses with the same glass system but different dopants, the difference between the values might be correlated to the number of f shell electron that the dopants of zinc borotellurite glass possess. Both neodymium and gadolinium elements have f shell electrons however lanthanum element used in this research has no 4f electron. According to Terashima et al. [46], the presence of elements with 4f electrons in the chemical composition of a glass system has been well known to help in enhancing the optical properties of glass material. The 4f electrons usually take part in reducing the energy band gap since the electrons are loosely bound and thus contribute to the formation of high polarizability nonbridging oxygen that eventually rises both linear and nonlinear refractive index of the glass material.

Table 5 Computed FOM values of lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses. Molar fraction, x

Figure of merit, FOM

La 0.00 0.01 0.02 0.03 0.04 0.05

0.260 0.056 0.155 0.448 0.128 0.023

La NPs 0.00 0.01 0.02 0.03 0.04 0.05

0.260 0.107 0.267 1.613 0.055 0.070

values when compared to other glass system as well as material. However, it is obvious that the nonlinear refractive index of silica fibre is much smaller than those of other materials. Since the main function of silica fibre is to transmit light, according n = c/v equation, the values of refractive index must be small in order for to allow the light to travel in a greater speed. On the other hand, in order for a material to be utilized as optical switching devices, the material must have a large

Fig. 12. FOM of lanthanum oxide and lanthanum nanoparticles doped zinc borotellurite glasses. 8

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Table 6 Comparison on the prepared glass with other nonlinear media. Glass system TeO2.B2O3.ZnO.La2O3 TeO2.B2O3.ZnO.La NPs TeO2.B2O3.ZnO.Nd2O3 TeO2•B2O3•ZnO•Nd2O3 NPs TeO2.B2O3.ZnO.Gd2O3 GeS2.In2S3.CsCl Bi2O3.WO3.TeO2 Na2O.B2O3.SiO2.Ni NPs Zn3Mo2O9/PMMA GeS2.Sb2S3.CdCl2 Bi2O3.H2BO3.SiO2. Au2O3 Bi2O3.H2BO3.SiO2. Dy2O3. Bi2O3.H2BO3.SiO2. Dy2O3. Au2O3 Silica fibre Telluride based fibre

n2 (cm2/W)

β (cm/W) 0.66 0.62 4.84 7.65 0.65 1.69 0.57 2.39 162 2.4 7.96

× × × × ×

−9

10 10−9 10−3 10−3 10−14

× 10−9 × 10−13

0.08 0.23 0.63 1.91 0.83 3.81 1.52 9.82 – 6.14 –

3.16 × 10−12



−12



4.35 × 10 – –

× × × × × × × ×

−14

10 10−14 10−12 10−12 10−14 10−8 10−14 10−14

× 10−13

−16

2.36 × 10 ~10−13

4. Conclusion

χ(3) (esu)

FOM

Reference

0.25 × 10 0.81 × 10−9 18.0 × 10−6 34.30 × 10−6 – – – – – – –

0.023 0.07 – – 4.153 – – – – 0.62 –

Present study Present study [47] [48] [13] [49] [34] [50] [51] [52] [53]





[53]





[53]

– –

– –

[54] [54]

−9

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The effect of doping lanthanum oxide and lanthanum nanoparticles on nonlinear optical properties of zinc borotellurite glasses has been investigated. Results from FTIR spectroscopy have revealed the formation of various amount of TeO4, TeO3, BO4 as well as BO3 at ~635 cm−1, ~670 cm−1, ~970 cm−1 and ~1300 cm−1 respectively. In general, the inconsistent trends recorded for both nonlinear absorption coefficient and nonlinear refractive index is contributed by concurrent formation of TeO4 and BO3 structural units as well as the conversion of TeO4 to TeO3 as supported by the data from FTIR spectroscopy. The trend for third order susceptibility which is similar to that of nonlinear refractive is due to the direct relationship between the two parameters whereby both parameters are greatly affected by the polarizability of the glass system. The prepared glasses with FOM values that ranges from 0.023 to 0.448 have the potential to be applied as all optical switching devices. Structural changes in 0.03 M fraction of lanthanum nanoparticles doped zinc borotellurite glass causes the breakage in the backbone of the glass network and creates a large amount of nonbridging that eventually lead to a high FOM of 1.613 which allow the usage of the glass as all optical switching devices. This work is the first research that verify the potential of lanthanum doped zinc borotellurite glass especially zinc borotellurite glass doped with 0.03 M fraction of lanthanum nanoparticles to be employed as optical switching devices based on the results from Z-scan technique. 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 gratefully appreciate the financial support from Universiti Putra Malaysia through the Geran Putra Berimpak (9597200). References [1] H. Nasu, J. Matsuoka, O. Sugimoto, M. Kida, K. Kamiya, Non-resonant type thirdorder optical nonlinearity of rare earth oxides-containing GeO2 glasses: optical materials and their applications, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 101 (1) (1993) 43–47. [2] G.S. He, K.T. Yong, Q. Zheng, Y. Sahoo, A. Baev, A.I. Ryasnyanskiy, P.N. Prasad, Multi-photon excitation properties of CdSe quantum dots solutions and optical

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