Z-scan studies of Barium Bismuth Borate glasses

Optical Materials 84 (2018) 178–183

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Z-scan studies of Barium Bismuth Borate glasses a

a,∗

b

M. Veeramohan Rao , V.V. Ravi Kanth Kumar , Nk Shihab , D. Narayana Rao a b

T b

Department of Physics, Pondicherry University, Puducherry, 605 014, India School of Physics, University of Hyderabad, Hyderabad, 500046, Andhra Pradesh, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Glass Z-scan studies Three photon absorption Optical limiting Femto second Picosecond and nanosecond Pump-probe studies

Transparent Barium Bismuth Borate glasses are synthesised with melt-quenching technique. The linear properties of the glasses are studied with UV–Vis absorption studies. Raman spectra was used to study the structural units of the BBB glasses. Linear absorption, optical band gap and Raman spectral studies reveal that the NBOs increases with the increase of Bismuth content in glass network. Nonlinear absorption properties of the glasses are measured by using z-scan technique with femto, pico and nanosecond lasers establish that three photon absorption (3PA) and free carrier absorption (FCA) are dominant in BBB glasses. Optical limiting and transient absorption characteristics of BBB glasses are also reported.

1. Introduction The development of lasers with ultrafast pulse duration paved the way to explore high-order nonlinear optical effects [1–21]. These studies are not only driven by academic interest but also to find potential applications in all optical communication and data processing. One of the most important area of research is multiphoton absorption (MPA) and related processes such as two photon (2PA), three photon (3PA) and four photon (4PA) absorptions which plays a pivotal role in fluorescence imaging [1], frequency up-conversion lasing [12], optical microfabrication [13], optical power limiting [14], optical data storage [15], photodynamic treatment [16] etc. Likewise development of passive optical limiters that protect sensitive human eye or optical components from getting damaged to exposure of intense laser beams and optical transients were reported [22–24]. Existing effectively used optical limiters are based on organic materials having efficiency at long pulse durations but have limitations due to their poor stability and large linear absorption. For all practical reasons glasses have turned out to be better candidates for this application because they are transparent even at low intensities with fast response time, good mechanical and thermal stability over organic materials. Subsequently glass is superior over crystals as glass, with appropriate composition and shape, can be easily prepared for chosen application with required explicit property. Specifically, Borate glasses with various compositions obtained remarkable importance due to their interesting linear and nonlinear properties [25,26]. Fundamentally, borate glasses are high phonon energy glasses consisting of trigonal [BO3] and tetrahedral [BO4] units that in turn combined to form various BxOy structural units. The incorporation of

∗

Corresponding author. E-mail address: [email protected] (V.V. Ravi Kanth Kumar).

https://doi.org/10.1016/j.optmat.2018.06.066 Received 23 May 2018; Received in revised form 26 June 2018; Accepted 27 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

heavy metal ions such as Bi3+ and Pb2+ ions reduces the phonon energy of borate glasses and increase the nonlinear refractive index of medium [27,28]. The nonlinear properties of the heavy metal oxide glasses can be tailored by choosing appropriate compositions. Consequently, much effort have gone in improvement of heavy metal oxide glasses having large linear refractive index, nonlinear absorption and fast response times for various prior mentioned photonic applications [24–40]. Bismuth glasses in general, and in particular Bismuth borate glasses are familiar for its effective moisture resistant, glass formation over relative large compositional region, high linear and nonlinear refractive indices [27–36]. Commonly, in triple bismuth systems where the presence of highly polarizable bismuth atoms and complex polyborate anions (BO3)3-, (BO4)5-, and (BiO4)5- would promote the formation of acentric structural groups. However, the larger nonlinear effects in bismuth borate systems predominantly originate from highly hyperpolarizable [BiO4]5 anionic units in the glass [34,35]. Specifically an electronic effects, including a transfer between O and of lone-pair electron of Bi in (BiO4)5- groups leading to an exceptionally large dij coefficients in Bismuth borate than other existing systems like BBO, LBO, Li B3O5 etc., [34,35]. This makes Bismuth borate as a promising material for effective frequency conversion. In addition to this, Bismuth Borate glasses are well suited material for optical fibres. Kityk et al. showed non-centro-symmetry and in turn large second harmonic generation in BBO fibers owing to the photo-induced electrostricted phonons because of the UV-induced electron phonon anhramonic interactions [36]. Gomes et al. studied nonlinear optical absorption and refraction properties of bismuth borate glasses by thermally managed

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Fig. 2. Raman spectra of BBB glasses with different Bi2O3 concentrations.

plate and annealed near to glass transition temperature for 2 h to remove internal stress, if any. The annealed glass samples are ground to thickness ≈1.5 mm and then optically polished for linear and nonlinear optical measurements. The absorption spectra were recorded by using a UV–Vis–NIR Shimadzu Varian 5000 spectrophotometer. The Raman spectra have been recorded in the range of 50–2000 cm−1 using Renishaw in Via Raman microscope with 488 nm argon ion laser. The nonlinear optical absorption and refraction of the glasses are studied with a laser of the wavelengths are 800 nm (fs), 532 nm (ps & ns) using standard Z-scan technique [37]. The fs laser with 110 fs pulse duration, repetition rate of 1 kHz at 800 nm, ns and ps laser with 6 ns and 30 ps pulse duration, repetition rate of 10 Hz at 532 nm wavelengths respectively. The Z-scan experiment was repeated more than once to ascertain the repeatability of obtained data and best fit of this data gives nonlinear optical coefficient value. Pump-Probe experiment was performed to the all samples using femtosecond laser with 100 fs pulses. 3. Result and discussion Fig. 1. (a) UV–Vis absorption spectra of BBB glasses and (b) The optical bandgap plots of BBB glasses.

3.1. UV-VIS absorption studies The recorded absorption spectra of the BBB glasses were shown in Fig. 1a. With the increase of Bismuth content in the BBB glass, the red shift of absorption edge and decrease of optical band gap values viz., 2.59 ev (BBB20), 2.48 (BBB25), 2.45 ev (BBB30) that are obtained from Tauc plot (Fig. 1b) are attributed to loosening of network due to formation of non-bridging oxygens (NBO's) [31,32]. These NBOs which are connected to single cation of the glass network will be easily polarizable and play a significant role in nonlinear properties of system [39]. The optical basicity of the glasses gives an account of the ability of anion to donate negative charge and thereby reduce positive charge on cation was calculated as explained in literature [40]. The increase of optical basicity (Λth) values of BBB glasses with increasing bismuth content in the order BBB20 (0.507) < BBB25 (0.5360) < BBB30 (0.557) indicates that enhancement of oxide ion polarizability due to presence of more NBOs.

eclipse and conventional Z-scan methods and optical limiting measurements [29,30]. Recently, nonlinear absorption and refraction behavior in the nanosecond (ns), picosecond (ps), and femtosecond (fs) time domains were studied by Shamugavelu et al. [31]. In the present paper, nonlinear properties of Barium Bismuth Borate glasses (BBB) glasses are studied using Z-scan and is likely to become the forerunner in realizing three photon absorption in all the glasses for three domains viz. fs, ps and ns respectively.

2. Experimental details Transparent Barium bismuth borate glasses, hereafter called as BBB glasses, with composition 10BaO - xBi2O3 – (70-x) B2O3where x = 20, 25, and 30 were prepared by melt-quenching method. The required amount of chemicals such as BaO, Bi2O3 and H3BO3 are taken in a crucible and heated in a muffle furnace at 1050 °C for 30 min. The obtained melt is quenched to room temperature by casting on steel

3.2. Raman spectra Raman spectra of BBB glasses (Fig. 2) have been studied to

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understand the local or short range arrangement of structural units in the glass network [41–44]. Generally, Bismuth is present in the form of BiO6 and BiO3 units in bismuth borate glasses [43]. The Raman spectra of BBB glasses consists of seven peaks at 73, 140, 365, 577, 650, 924, and 1300 cm−1. First two peaks belong to the boson peak (73 cm−1) of the glass [41] and heavy metal ion vibrations (140 cm−1) [42] respectively. The bridged anion modes fall in between 300 and 600 cm−1 where 365 cm−1 is attributed to the stretching vibration of Bi-O-Bi bond in [BiO3] and [BiO6] units and Bi-O bond stretching vibrations of [BiO6] unit can be seen at 577 cm−1 [42]. In ternary glasses, the weak bands around 600-700 cm−1 can be attributed to Bi-O- non-bridging oxygen stretching vibrations in [BiO6] octahedral and meta borate units [43,45–48]. The band at 924 cm−1 may be due to the higher number of boron linked units [44]. The broad peak 1300 cm−1 is a combination of 1200 and 1400 cm−1 peaks arises due to stretching vibrations of B-O and B-O- units where latter one is due to NBOs [44]. Thus Raman studies confirms the presence of NBOs in the BBB glass network and it is clearly evident from Fig. 2 that an increase in the intensity of corresponding peaks with increase of bismuth oxide content. Fig. 3. Open aperture Z-scan curves of the BBB glass sample with 800 nm, 110 fs pulses. Solid curves are the theoretical fit to the experimental data.

3.3. Z scan measurements The nonlinear absorption studies were measured by Z-scan technique using Nd:YAG laser whose specifications are mentioned above in experimental section. Third order nonlinearity of glasses were determined by using Z-scan measurements in open and closed aperture [27–31]. In a standard Z-scan setup, the sample is moved in path of a laser beam, focussed by a lens, having Gaussian profile. The intensity will be maximum at focal point and will steadily decreases in both directions. The sample thickness must be less than the Rayleigh range of the focussed beam and usually it is about 3 mm. The intensity of beam is controlled by neutral density filters while aperture is used for beam shaping. The obtained data is saved in a computer using an analog to digital card which has been collected by translating the glass through the focus and gathering data using a boxcar average. The magnitude of three photon absorption and free carrier absorption cross section were determined for all the samples is given by Equn. 1 [49,50].

dI = −αI (Z ) − γI 3 (Z ) − σe NI (Z ) dZ

(1)

and

∂N (z , t ) γ 3 = I (z , t ) ∂t 3hv

Fig. 4. Open aperture Z-scan curves of the BBB glass sample with, 532 nm, 30ps pulses. Solid curves are the theoretical fit to the experimental data.

(2)

Where α (cm−1) is the linear absorption coefficient, γ (cm3/W2) is the three photon absorption, σ (cm2) is the mean free carrier absorption cross-section for electrons and N (z , t ) is the number density of photogenerated free carrier. Under assumption that N(z,t) remains constant during the femtosecond, picosecond and nanosecond pulses respectively, Eq. (2) can be approximated as

N (z , t ) =

γτ 3 I (z , t ) 3hv

Solving Eq. (4) gives

I (z ) =

[1 + 2Io2 γeff Leff ]1/2

(5)

Where is the incident intensity, Leff = (1 − e −2αL)/2α and γeff defined as

γeff = γ +

(3)

where τ is the laser pulse width and v is laser beam frequency. By substituting Eq. (3) into Eq. (1), the resulted differential equation for propagation of laser beam in the nonlinear media would be

γτ 4 dI = −αI (Z ) − γI 3 (Z ) − I (z , t ) dZ 3hv

Io e −αz

ηIo Leff

L

e −3αL

∫ 0

⎡1 + ⎣

1/2 γIo2 (1 − e−2αL) α

⎤ ⎦

dz (6)

The transmission of incident light having intensity (Io) by the sample of thickness L can be expressed as

T=

(4)

180

I (L) e −αz = Io [1 + 2Io2 γeff Leff ]1/2

(7)

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Fig. 6. Scaling for 3PA in BBB glasses.

According to this, the slope (σ) of linear fit of plot In(1-Top) verses In (I) must be 2 for 3 P A. Fig. 6 shows the respective obtained typical plot for BB30 glass along with linear fit having slope 2 as observed in present systems clearly confirm 3PA. Though this is inconsistent with band gap values, the high order effects like collision ionization causing absorption or emission of electron among energy levels at high intensities ≥ 1012 W/cm2 is the reason for 3PA [54–58]. The modelling of this mixed high order mechanism is out of scope of present paper.

Fig. 5. Open aperture Z-scan curves of the BBB glass sample with 532 nm, 6ns pulses. Solid curves are the theoretical fit to the experimental data.

The obtained experimental data that is variation of transmittance with respective intensity for BBB glasses for all three time domains namely femtosecond, picosecond and nanoseconds are shown in Figs. 3–5. Solid lines shown in these figures are theoretical fitting obtained using Equn. 7 and is found to fit well for 3PA. The respective estimated 3PA and free carrier absorption coefficients are listed in Table 1. Estimated 3PA and mean free carrier concentration values are increasing with the increase of Bismuth content in the glass that can be attributed to the presence of more number of NBO in BBB30 glass than other two systems. Usually, devices based on non-resonant process are more popular over resonant based because latter are constricted due to requirement of functioning near the absorption band edge though both the processes contribute for nonlinearity of oxide glasses. As per anharmonic oscillator model and electronic mechanism, electronic transitions contribute to self-defocusing or negative nonlinear refractive index while positive non-resonant nonlinearity is attributed to the hyper-polarizability of glass constituents such as NBOs, heavy metal or alkaline ions [51–54]. In general, introduction of heavy metal oxides (Bi2O3) or alkali oxides (BaO) as modifiers in borate glass loosens the network and formation of NBOs which are less stable and weakly bound to boron atoms. An optical electric field can create strong anharmonic effects by distorting the valence electrons of NBOs leading to higher nonlinearity values due to hyper-polarizability of NBOs (about 0.60 × 1035 esu cm3/ion) [51,54]. J.He et al. [56], manifests the 3PA process by using below Equn. (8)

ln(1 − Top) = 2 I ln I + ln(3−3/2α3 L)

3.4. Optical limiting & transient absorption The reverse saturable absorption (RSA) nonlinear process plays a main role in fabrication of passive optical limiters. The nonlinear transmission of all the present glasses was calculated as a function of the fluence and respective optical limiting curves of the glass samples in both fs and ns regimes are shown in Fig. 7a and b. The material can be used as optical limiter if its possess dynamic value minimum two which can be calculated by dividing maximum intensity utilized in the experiment by the observed threshold intensity [29,59]. All the present studied glass at 800 nm and 532 nm exhibit dynamical range value about 5. The transient absorption is measured by pump-probe experiment to realise the dynamics of the excited states of the glasses. Since optical band gap values of studied glasses is about 2.5 eV, the pump and probe wavelength are chosen as 800 nm (1.5 eV) and 500 nm (2.4 eV) are respectively. Fig. 8 shows the transient absorption as the function of time delays in the middle of the pump and probe pulses for all the studied glasses (BBB20, BBB25 and BBB30). The observed transient absorption in the present studied samples is due to presence of localized state of Bi ions and all studied sample possess fast response i.e. about 1 ps.

(8)

Table 1 Peak intensity in fs, ps & ns, three photon absorption (α3 ), and three photon free carrier cross section (σe ) and optical limiting threshold values of BBB glasses. Sample

BBB20 BB25 BBB30 FM543 BZH229−30

Peak intensity (Io ) (GW/cm2)

λ = 800 nm τp = 100 fs

800 nm (fs)

532 nm (ps)

532 nm (ns)

γ (cm3/GW2)

σe *10−19 cm2

150 158 170

1.31 1.72 1.77

0.059 0.067 0.082

4.04*10−24 4.585*10−24 5.20*10−24 17*10−25

2.38 2.54 2.66

λ = 532 nm τp = 30ps

λ = 532 nm τp = 6ns

Optical limiting threshold (GW/cm2)

γ (cm3/GW2)

σe *1018 cm2

σe *1017 cm2

Optical limiting threshold (GW/cm2)

0.214 0.237 0.245

8.25*10−17 12.38*10−17 38.16*10−17

5.72 6.48 6.97

1.74 1.77 2.166

1.3*10−2 1.38*10−2 1.42*10−2

6.7

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Fig. 8. Transient changes of transmission of samples for pump pulses at 800 nm (1.5 eV) and probe pulses at 480 nm (2.5 eV).

Acknowledgments The authors thank Central Instrumentation Facility, Pondicherry University (India) for providing characterization facilities. Pump-Probe characterization of the glass samples was carried out at Spectroscopy and Optical Microscopy Facility at Solid State and Structural Chemistry Unit, Indian Institute of Science. This work was supported by DST-SERB (EMR/2017/320), DST-FIST (SR/FST/PS-II/2017/19), New Delhi. VVRK also acknowledge UGC-SAP-DRS-II (F.530/1/DRS-II/2015 (SAPI)). References [1] Daniel R. Larson, Warren R. Zipfel, Rebecca M. Williams, Stephen W. Clark, Marcel P. Bruchez, Frank W. Wise, Watt W. Webb, Water-soluble quantum dots for multiphoton fluorescence imaging in vivo, Science 300 (2003) 1434–1436. [2] Babu, B. Hari, Mengsi Niu, Xiaoyu Yang, Yanbo Wang, Lin Feng, Wei Qin, XiaoTao Hao, Systematic control of optical features in aluminosilicate glass waveguides using direct femtosecond laser writing, Opt. Mater. 72 (2017) 501–507. [3] Reddy, A. Siva Sesha, J. Jedryka, K. Ozga, V. Ravi Kumar, N. Purnachand, I.V. Kityk, N. Veeraiah, Laser stimulated third harmonic generation studies in ZnO–Ta 2 O 5–B 2 O 3 glass ceramics entrenched with Zn 3 Ta 2 O 8 crystal phases, Opt. Mater. 76 (2018) 90–96. [4] S.N.C. Santos, J.M.P. Almeida, K.T. Paula, N.B. Tomazio, V.R. Mastelaro, C.R. Mendonça, Characterization of the third-order optical nonlinearity spectrum of barium borate glasses, Opt. Mater. 73 (2017) 16–19. [5] B. Kulyk, V. Kapustianyk, V. Figà, B. Sahraoui, Quadratic nonlinear optical parameters of 7% MgO-doped LiNbO3 crystal, Opt. Mater. 56 (2016) 36–39. [6] Valligatla, Sreeramulu, Alessandro Chiasera, Stefano Varas, Pratyusha Das, B.N. Shivakiran Bhaktha, Anna Łukowiak, Francesco Scotognella, et al., Optical field enhanced nonlinear absorption and optical limiting properties of 1-D dielectric photonic crystal with ZnO defect, Opt. Mater. 50 (2015) 229–233. [7] Ren, Jing, Bo Li, Tomas Wagner, Huidan Zeng, Guorong Chen, Third-order optical nonlinearities of silver doped and/or silver-halide modified Ge–Ga–S glasses, Opt. Mater. 36 (2014) 911–915. [8] Bala, Rajni, Ashish Agarwal, Sujata Sanghi, Navneet Singh, Effect of Bi2O3 on nonlinear optical properties of ZnO⋅ Bi2O3⋅ SiO2 glasses, Opt. Mater. 36 (2013) 352–356. [9] W.J. Lima, V.M. Martins, A.F.G. Monte, D.N. Messias, N.O. Dantas, M.J.V. Bell, Tomaz Catunda, Energy transfer upconversion on neodymium doped phosphate glasses investigated by Z-scan technique, Opt. Mater. 35 (2013) 1724–1727. [10] Chen, Feifei, Tiefeng Xu, Shixun Dai, Qiuhua Nie, Xiang Shen, Jianliang Zhang, Xunsi Wang, Linear and non-linear characteristics of tellurite glasses within TeO2–Bi2O3–TiO2 ternary system, Opt. Mater. 32 (2010) 868–872. [11] Zimmermann, Felix, Matthieu Lancry, Anton Plech, Sören Richter, B. Hari Babu, Bertrand Poumellec, Andreas Tünnermann, Stefan Nolte, Femtosecond laser written nanostructures in Ge-doped glasses, Opt. Lett 41 (2016) 1161–1164. [12] He, S. Guang, Przemyslaw P. Markowicz, Tzu-Chau Lin, Paras N. Prasad, Observation of stimulated emission by direct three-photon excitation, Nature 415 (2002) 767. [13] Kawata, Satoshi, Hong-Bo Sun, Tomokazu Tanaka, Kenji Takada, Finer features for functional microdevices, Nature 412 (2001) 697. [14] He, S. Guang, Ken-Tye Yong, Qingdong Zheng, Yudhisthira Sahoo, Alexander Baev,

Fig. 7. Optical limiting response of Barium Bismuth Borate glass samples (a) at 800 nm wavelength with pulse duration of 110 fs pulses and (b) at 532 nm wavelength with pulse duration of 6 ns.

In summary, the present glasses can be better optical limiting materials as they possess dynamic values that are comparable with organic material and at the same time being glasses they can surmount the draw backs such as poor chemical durability, low damage threshold and high nonlinear absorption [60]. In addition, these glasses exhibit 3PA, a cubic reliance on the incident intensity and stabilization is accomplished by using two photon absorption along with fast response time.

4. Conclusions Third order nonlinear properties of the glasses were studied by Zscan technique using a laser of the wavelengths are 800 nm (fs), 532 nm (ps & ns) using standard Z-scan technique. Optical absorption and Raman studies confirmed the loosening of network with Bismuth content where NBOs play a significant role in nonlinear properties. Consequently, the 3PA and free carrier concentration values increase with Bismuth content. The localized states of Bi ions contributed to the fast transient response of the present glasses.

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