Third order nonlinearity and optical limiting behaviors of Yb:YAG nanoparticles by Z-scan technique

Optics and Laser Technology 109 (2019) 561–568

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Third order nonlinearity and optical limiting behaviors of Yb:YAG nanoparticles by Z-scan technique

T

⁎

S. Arun Kumara, J. Senthilselvana, , G. Vinithab a b

Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 25, Tamil Nadu, India Department of Physics, Vellore Institute of Technology, Chennai 127, Tamil Nadu, India

H I GH L IG H T S

time investigation of third-order NLO of Yb:YAG nanoparticles by Z-scan method. • First reverse-saturable absorption, optical-limiting for eye protection and sensor system. • Exhibit enhancement in nonlinear refractive index (n = 8.649 × 10 cm /W). • Giant thermo-optic coefficient (dn/dT = 6.864 × 10 K ). • High • High figure-of-merit (n /2βλ = 74.50) indicates effective optical switching application. 2 −6

−8

2

−1

2

A R T I C LE I N FO

A B S T R A C T

Keywords: Yb:YAG nanoparticle Nonlinear optical Z-scan Optical limiting Thermo-optic coefficient

The present report explores third order nonlinear optical behavior of phase pure Yb3+:YAG nanoparticles for the first time by Z-scan technique. The measurement was carried out using diode pumped continuous wave (CW) Nd:YAG laser at 532 nm. The Yb:YAG nanoparticles exhibit characteristic near-infrared (NIR) emission at 1030 nm under 940 nm excitation. The nanoparticles exhibit high nonlinear refractive index (n2 = 8.649 × 10−8 cm2/W) and low nonlinear absorption coefficient (β = 0.109 × 10−4 cm/W) giving an appreciable figure of merit (FOM) of ∼74.50. The excitation power (8.2–10.5 W cm−2) dependent emission spectra were recorded to study exchange energy interaction of Yb3+ ions with YAG host lattice. By utilizing the nonlinear refractive index ‘n2’ from Z-scan measurement, thermo-optic coefficient (dn/dt) was calculated to demonstrate Yb3+:YAG nanomaterial for high power compact solid state laser gain amplifier systems.

1. Introduction In recent decades, search for photonic materials for novel optical waveguide [1], frequency conversion, optical amplification, Q-switching, optical switching [2,3], optical computing devices based on nonlinear optical principles [4] is getting fascinated. Materials with high optical nonlinearities and superior third-order nonlinear response are essential for optical limiting and can be investigated by a simple Z-scan technique. It is the most versatile and sensitive method proposed by Sheik-Bahae et al. [5], to determine nonlinear optical (NLO) parameters such as nonlinear refractive index (n2), nonlinear absorption coefficient (β), nonlinear optical susceptibility (χ(3)) and figure of merit (FOM) of photonic materials for optical limiting applications. High nonlinear optical efficiency and large magnitude of third-order nonlinear susceptibility is highly demanded for optical communication and fiber amplification applications. An efficient NLO material should ⁎

possess high optical homogeneity, large nonlinear refractive index, small nonlinear absorption coefficient, large birefringence, high molecular polarizability, physico-chemical stability and high laser damage threshold [6]. A combination of these properties can be achieved generally in single crystalline materials for nonlinear optical applications [7–9]. With the advent of nanotechnology, in recent years there is widespread interest in exploring third order nonlinear optical properties in nanomaterials such as plasmonic silver and gold nanocolloids [10,11], semiconductor quantum dots [12,13], carbon nanotubes [14,15], graphene [16] and metal-oxide nanoparticles [17–20] by single beam Z-scan method. The non-linearity can be attained by twophoton absorption, multi-photon absorption and particulate induced scattering in nanomaterials. Investigation of nonlinear properties in rare earth doped nanophosphors [21,22] and ferroelectrics [23,24] are recent attraction to obtain better NLO characteristics. Ytterbium doped yttrium aluminum garnet (Yb:YAG) nanoparticles are nowadays getting

Corresponding author. E-mail address: [email protected] (J. Senthilselvan).

https://doi.org/10.1016/j.optlastec.2018.08.037 Received 24 February 2018; Received in revised form 3 July 2018; Accepted 14 August 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.

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resultant suspension was filtered and dried at 70 °C. The air dried powder was calcined at 900 °C in a microwave furnace for 10 min to obtain Yb:YAG nanoparticles.

attraction to prepare laser ceramics for compact diode pumped solid state disk laser [25]. The trivalent Yb ion possesses low-phonon energy, absence of upconversion mechanism, broad multiplet absorption and longer excited state lifetime [26,27]. The infrared lasing and second harmonic emission respectively at 1030 and 515 nm in Yb:YAG makes it a potential media to replace traditional Nd:YAG laser system. Therefore, Yb:YAG has also gained interest in the development of diode pumped mode locked femto-second (fs) oscillators [28] and in ultrashort laser generation [29] because of its superior nonlinear optical performances. The irradiance dependent nonlinear refractive index (n2) variation with absorption wavelength is one of the significant nonlinear behaviors studied by Z-scan method in Yb based lasers. Major et al. [28] performed the Z-scan measurements on rare earth Yb doped Ca4GdO (BO3)3, Ca4YO(BO3)3 and Sr3Y(BO3)3 borate crystal using Ti:Sapphire laser at the lasing wavelength of 1300 nm and observed larger nonlinear refractive index (n2) value of about 11.4 × 10−7 cm2/GW for Kerr-lens mode lock lasing system. Although the effect of nanosecond, picoseconds and femto-second laser interaction on third-order nonlinearity of materials for optical switching applications are of recent interest, highly efficient solid state laser developed by Geusic, Marcos, and Van Uitere [30], the most popular Nd:YAG solid state laser is still largely being used by Z-scan technique for the instant exploration of nonlinear optical properties of materials. Because it possesses high gain, narrow line width and low threshold properties which make it the most versatile laser for variety of photonic applications [31]. It can be used for exploring the nonlinear optical properties of nanomaterials [19,32–34] as continuous wave (CW) Nd:YAG laser interaction with nanoparticles can minimize thermal induced stress compared to fiber coupled ultra-short and acousto-optically modulated lasers [35]. In addition, second harmonic frequency-doubling from near infra-red (1064 nm) wavelength to visible green at 532 nm stimulates research interest to explore nonlinear optical property and optical limiting behaviors. The world’s first gas laser invented by Ali Javan, Benner and Herriott, the most popular He-Ne gas laser [36] is also still largely being used by Z-scan technique for the exploration of nonlinear optical properties. Basically, Nd3+ and Yb3+ doped YAG crystals are popular in laser physics and have been largely used for laser operation in pulsed modes. In this aspect, fundamental physics of rare earth doped YAG for lasing action is well reported in literatures, but nonlinear change of refractive index and third-order nonlinearity in Yb:YAG is not addressed in the published reports. To the best of our knowledge, the nonlinear optical property of Yb:YAG nanoparticles is not investigated using Z-scan technique and its third order nonlinear refractive index and absorption coefficient is not yet determined for opto-electronic device applications. In the present work, for the first time, third-order nonlinear properties in Yb:YAG nanoparticles by single beam Z-scan experimental technique is reported. The estimated nonlinear optical parameters are higher than that achieved in other nanomaterials and therefore, Yb:YAG nanoparticles can be useful for optical limiting applications.

2.1. Characterization techniques Structural parameters of Yb:YAG nanoparticles at different Yb3+ doping concentrations were determined by powder X-ray diffractometer (XRD, GE-3003-TT) using CuKα1 radiation (λ = 1.5406 Å) at a scanning rate of 0.04° s−1 in 2θ range from 10° to 70°. The surface morphology, elemental composition and inter-planar spacing’s of the nanopowders were examined using field emission scanning electron microscopy (FESEM, FEI QUANTA 200 FEG) and high resolution transmission electron microscopy (HR-TEM, JEM 2100 PLUS). The optical absorption, refractive index (n) and absorption coefficient (α) of the nanopowders were measured using ultra-violet visible-diffuse reflectance spectrophotometer (DRS, UV–Vis 2600). The characteristic near-infra-red emission and excitation powder dependent emission profile of Yb:YAG nanoparticles were analyzed by spectrofluorometer (JOBIN-YVON) excited at 940 nm. 3. Results and discussion 3.1. Structural and morphological studies The stoichiometry of Yb:YAG nanoparticles were made by varying Yb3+ doping concentrations in order to investigate the structural and optical behavior. The Yb3+ ions are doped into YAG host lattice to replace Y3+ ions in dodecahedral site. XRD pattern (Fig. 1) of Yb:YAG-1 and Yb:YAG-5 samples microwave calcined at a temperature of 900 °C resulted in formation of YAG cubic phase (JCPDS No: 33-0040) with the space group Ia−3d [37]. No impurity and secondary phases such as Y2O3, YAlO3 and Y3AlO4 were evolved. From powdercell structural refinement analysis, lattice constants of Yb:YAG-1 and Yb:YAG-5 samples are calculated to be 12.056 ± 0.004 Å and 12.024 ± 0.008 Å, respectively. Since, the ionic radius of Yb3+ ion (r = 0.985 Å for CN = 8) is smaller than Y3+ ion (r = 1.019 Å for CN = 8), the Yb3+ ions can easily substitute Y3+ ions in dodecahedral site of Y3Al5O12 host lattice and resulted in decrement of unit cell lattice parameters [38]. The crystallite size (D) is estimated from full-width at half maximum (FWHM) of X-ray line broadening by implying Scherrer formula D = 0.9λ/β cosθ; where, λ is the wavelength of X-ray (CuKα1), β is the FWHM of diffraction peaks and θ is the diffraction angle. The crystallite

2. Experimental technique The nonlinear optical properties of Yb:YAG nanoparticles with low 1.0 at% and high 5.0 at% Yb3+ doping concentrations were prepared by simple reverse strike co-precipitation method. High purity yttrium nitrate (Y(NO3)3·4H2O; Alfa Aesar, 99.99%), aluminum nitrate (Al (NO3)3·9H2O; Alfa Aesar, 99.99%), ytterbium nitrate (Yb(NO3)3·4H2O; Alfa Aesar, 99.99%) and ammonium hydrogen carbonate (AHC, Alfa Aesar) were used as starting materials. Aqueous solution of metal-cations were made by dissolving yttrium nitrate and aluminum nitrate in 3:5 mol ratio with ytterbium nitrate (low 1.0 and high 5.0 at%; here after stated as Yb:YAG-1 and Yb:YAG-5) in 100 ml distilled water under stirring for 30 min. To initiate precipitation, the metal-cation precursor solution was added drop-wise at 3 ml min−1 into 1.5 M of aqueous ammonium hydrogen carbonate solution until the pH reaches 5–7. The

Fig. 1. XRD pattern of Yb:YAG-1 and Yb:YAG-5 nanoparticles calcined at 900 °C. 562

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Fig. 2. (a) FESEM of microwave calcined Yb:YAG-5 nanoparticles. (b) EDS spectrum shows the presence of Y, Yb, Al and O elements. (c) HR-TEM image of Yb:YAG-5 nanoparticles (c. Inset) Histogram shows the particle size distribution of Yb:YAG-5 in nano-size range. (d) High resolution TEM image shows inter-planar lattice fringes.

size of Yb:YAG-1 and Yb:YAG-5 is calculated to be 57 and 53 nm, respectively. The surface morphology of Yb:YAG-5 nanoparticles prepared by simple reverse-strike co-precipitation method and microwave calcination at 900 °C for 10 min is shown in Fig. 2a. The particles are lessagglomerated and spherical in its morphology. Sergei et al. [39], reported that rare earth Nd, Yb doped Y2O3, Lu2O3, YAG and LuAG spherical nanoparticles are utilized to fabricate transparent laser ceramics by high pressure-temperature compaction techniques. Energy dispersive spectroscopic (EDS) analysis is performed on the microwave calcined nanoparticles (Fig. 2b) and it confirms the presence of Y, Al, O and Yb stoichiometric elements. Fig. 2c displays HR- TEM image of Yb:YAG-5 nanoparticles, displays the formation of spherical nano-particulates with the average particle size of ∼66 nm (Fig. 2c.inset), which is almost in agreement with the crystallite size estimated from XRD analysis. The high resolution TEM image of Yb:YAG-5 nanoparticles shows well-defined lattice fringe patterns (Fig. 2d), indicates phase pure and highly crystalline Yb:YAG nanoparticles synthesized under the influence of AHC. The calculated lattice spacing value is found to be 0.322 nm which correspond to (3 3 1) crystal plane of YAG host lattice (JCPDS No: 33-0040).

Fig. 3. Photoluminescence spectra of Yb:YAG-1 and Yb:YAG-5 nanoparticles excited at 940 nm. Inset shows DRS spectrum of Yb:YAG-5 nanoparticles displays absorption peaks at 913, 940 and 968 nm.

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Table 1 Estimated nonlinear optical parameters, linear absorption coefficient, figure of merit and thermo-optic coefficient of Yb:YAG nanoparticles from Z-scan experiment. S. no

Sample

n0

n2 × 10−8 (cm2/W)

β × 10−4 (cm/W)

Reχ(3) × 10−6 (esu)

Imχ(3) × 10−6 (esu)

χ(3) × 10−6 (esu)

α (cm−1)

Leff (mm)

FOM T

FOM B

dn/dt × 10−6 (K−1)

1. 2.

YbYAG-1 YbYAG-5

1.825 1.834

8.649 8.362

0.109 0.105

7.254 6.784

0.677 0.652

7.286 6.815

2.49 2.87

0.994 0.998

0.013 0.012

74.50 74.84

6.593 6.864

experiments during the laser beam interaction with the Yb:YAG nanoparticles, the transmitted beam intensity at each Z-position was measured using the detector placed just behind the aperture. The input reference beam energy was measured and their ratio was estimated. The normalized transmittance is estimated at different Z-positions.

3.2. Optical absorption and emission studies The room temperature optical diffuse reflectance spectrum of Yb:YAG-5 sample is shown in Fig. 3inset. It consists of three broad absorption bands in which the strong absorption peak is centered at 940 nm and other two weak bands are located at 913 and 970 nm, which are attributed to 2F7/2 → 2F5/2 electronic transitions of Yb3+ ions. The optical refractive index (no) and absorption coefficient (α) of Yb:YAG-1 and Yb:YAG-5 samples can be determined by the equations,

n0 =

−(R + 1) ± √ (−3R2 + 10R−3) and α = 2.303t−1log(I0 /I) 2(R−1)

3.3.2. Optical nonlinearity – Experimental evaluation Z-scan closed aperture, open aperture and closed-to-open aperture traces of low 1%-Yb doped Yb:YAG-1 and high 5%-Yb doped Yb:YAG-5 are shown in Fig. 5a–c respectively. In closed-aperture Z-scan profile (Fig. 5a), pre-focal peak is followed by the post-focal valley which indicates negative non-linearity. From Z-scan traces the peak to valley distance ΔTp-v closer to 1.71 times of Rayleigh length Z0 = πω(0)2/λ correlates the existence of electronic thermal non-linearity [5,41]. The measured ΔTp-v distance of 4.1 mm approximately equal to the parameter 1.71 of Z0 confirms that Yb:YAG nanoparticles possess third order nonlinear response. Z-scan curves (Fig. 5) of Yb:YAG-5 sample exhibiting post-focal valley trend indicates the high-Yb doped YAG nanoparticles may have self-defocusing property and a similar trend is reported [43,44]. The open aperture Z-scan profiles (Fig. 5b) for both the samples show maximum transmittance at high intensity focal position and it is due to reverse saturable absorption behavior. It is because of high absorption cross section of ground state, lower absorption cross section of excited state and quite long excited state life time (Yb3+-2F5/2) is in the order of milliseconds [26]. At the focal position (Z = 0) as the laser intensity increases a large photon density is available. Rapid increase in photon density could highly excite a large number of atoms at the ground state as ground state absorption cross-section is larger. Eventually, the excited state population become increases and stay quite over a long time due to its long excited state life time which increases the transmission and hence the occurrence of reverse saturable absorption. Respectively from the closed aperture and open aperture Z-scan trials, the third order nonlinear optical properties such as nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of Yb:YAG nanoparticles are evaluated. In optical materials, the intensity of the laser beam directly depends on refractive index of the materials, absorption and scattering. In Z-scan profile, the difference between the highest (peak) and the lowest (valley) value transmittance, Tp ─ Tv is termed by ΔTp–v. By numerically relating this transmittance parameter to on-axis phase shift Δφo (which is a function of n2,) the nonlinear refractive parameter (n2) can be calculated by the Eq. (4) by using closer aperture Z-scan data.

(1)

where, R is the total reflectance arises due to the light reflected from the sample, log Io/I is the common logarithm of optical density ratio and t is the sample thickness and the calculated ‘no’ and ‘α’ values of the nanopowders are given in Table 1. The room temperature photoluminescence spectra of Yb:YAG-1 and Yb:YAG-5 nanopowders excited at a wavelength of 940 nm displayed narrow bands at near infrared (NIR) regime (Fig. 3). The emission spectra reveals a strong peak at 1030 nm and three other weak peaks at 971, 1008 and 1050 nm were assigned to 2F5/2 → 2F7/2 transitions of Yb3+ ions [40]. It is also found that the emission intensity of Yb:YAG-5 is observed to be higher than that of Yb:YAG-1. This is due to the incorporation of high Yb3+ ions into YAG host lattice. The enhancement in the luminescence behavior also improves the electronic transition pathway of Yb3+ ions which could possibly minimize the laser threshold energy [30]. 3.3. Z - scan technique A simple single-beam Z-scan technique [5] was employed to study the nonlinear optical properties such as nonlinear absorption coefficient (β), nonlinear refractive index (n2) and third order nonlinear optical susceptibility (χ(3)) of Yb:YAG nanoparticles using the standard Z-scan setup [41] shown in Fig. 4. Z-scan based nonlinear optical properties have been reported mostly for metal nanoparticles of silver and gold collides, carbon nanotube and graphene, organic dye molecules, quantum dots and ZnO based nanocomposites, and the details are well documented in the excellent review by Dini et al [42]. However, hardly any reports could be traced in the Z-scan studies of rare earth doped YAG nanoparticles. 3.3.1. Nonlinear optical measurement – Z-scan experiment In this investigation we report for the first time the nonlinear optical characteristics of Yb:YAG nanoparticles using Z-scan technique under the irradiation of CW Nd:YAG laser (Coherent Compass TM215 M-50) that is commonly available in research laboratories. The experiments were conducted using CW Nd:YAG laser beam with Gaussian profile at a wavelength (λ) of 532 nm (50 mW). The laser beam was focused to fine spot (at beam waist position Z = 0) by 3.5 mm focal length lens with input power to produce a beam waist (ω0) of 1.53 mm. The Yb doped YAG nanopowders dispersed in water was taken in quartz cuvette cell of path length 1 mm, which is within the Rayleigh length range (Z0 = πω(0)2/λ) of 13.8 μm. This sample cell was positioned at the focal point (Z = 0) and moved both ways along laser beam axis in Z-propagation direction at small steps with position precision of 0.5 mm. In the

Phase shift,

|Δφo |=

ΔTp − v 0.406(1−ST )0.25

(2)

Aperture linear transmittance,

ST =

−2r 2 1−exp⎜⎛ 2a ⎞⎟ ⎝ ωa ⎠

(3)

Nonlinear refractive index, n2 =

Δφo (closed aperture) kIo Leff

(4)

Nonlinear absorption coefficient, β =

2 2 ΔT (open aperture) Io Leff

(5)

where, ST – aperture linear transmittance, that is the aperture size signified by its transmittance in the linear region when the sample is placed far away from the focus or a ratio of the aperture diameter to the 564

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respectively. The thirdorder nonlinear susceptibility obtained in the present Yb:YAG nanoparticles can be compared with rare earth doped materials such as Rh:BaTiO3 films (Reχ(3) = 3.59 × 10−7 esu and Imχ(3) = 4.01 × 10−8 esu) [46], Ce:BaTiO3 quantum dots (2.21 × 10−8 esu) [47], Yb doped phthalocyanine polymer (χ(3) = 2.53 × 10−12 esu) [48]. Recently, Samuel et al. [49]; have studied the nonlinear refractive index property of YAG and Nd:YAG ceramics and crystals by dual arm Z-scan technique, and estimated the n2 value to be 8 ± 2 × 10−16 cm2/W. Rang Li et al. [50]; have demonstrated an enhancement and modulation of nonlinear optical response in silver nanoparticles embedded Nd:YAG single crystal and reported that the nonlinear refractive index showed a high value of ∼10–12 cm2/W and four fold increment over un-implanted Nd:YAG. Yb:YAG nanoparticles presented in this work have demonstrated a giant enhancement in nonlinear refractive index (n2) value of 8.649 × 10−8 cm2/W. An optical parameter that quantifies the nonlinear properties for various materials for all-optical switching application is figure of merit (FOM). The criteria is T = 2βλ/n2 < 1 or its inverse called as figure of merit (FOM) is given as B = n2/2βλ > 1 for materials for optical switching [51–53]. Therefore for efficient nonlinear optical devices, the nonlinear absorption should be low compared to nonlinear refractive index change. Also, it has been reported that the FOM > 10 is desirable [54,55] for efficient all-optical devices and improved FOM can bring down the switching power in optical switching devices. A high value of FOM = 11 in As60Se40 and FOM > 50 in Ge11.5As24Se64.5, Ge15Sb10Se75 chalcogenide glasses [56] and even FOM > 100 in As2S3 glass fibers (by considering the effective area, length and propagation loss, refractive index, and Brillouin gain coefficient in dB) is reported [57]. In this study, Yb:YAG nanoparticles with high nonlinear refractive index (n2 = 8.649 × 10−8 cm2/W) and low nonlinear absorption (β = 0.109 × 10−4 cm/W) at a wavelength of 532 nm give an appreciable FOM of ∼74.50 and hence the nano-particulate Yb:YAG can be used for effective optical switching device applications.

Fig. 4. Z –scan optical experiment performed in open and closed aperture conditions using CW Nd:YAG laser.

area of the whole beam diameter at the aperture. It lies between 0.1 and 0.5 in closed aperture Z-scan. In open aperture Z-scan, ST = 1, refers to collecting all the transmitted light and hence becomes insensitive to any nonlinear beam distortion due to nonlinear refraction. The terms ra is radius of aperture and ωa is the beam radius at the aperture. In Eqs. (4), (5) the term k refer to wave number (k = 2п/λ), Io is laser beam intensity at the focus (Z = 0) and Leff refers to the effective propagation length inside the sample (quartz cuvette) of its length L with its linear (−αL)

1 − exp ⎤. The calcuabsorption coefficient α, it is given by Leff = ⎡ α ⎣ ⎦ lated values of α and Leff for Yb:YAG-1 and Yb:YAG-5 samples are given in Table 1. From the open aperture Z-scan, the nonlinear absorption coefficient (β) can be measured by the Eq. (5). The real and imaginary parts of the third order nonlinear optical susceptibility (χ(3)) is respectively estimated by considering the measured nonlinear refractive index (n2) and the nonlinear absorption coefficient (β) values in the following equations,

R eχ (3) (esu) = 10−4

εo C 2no2 n2 (c m2 /W) π

(6)

Im χ (3) (esu) = 10−2

εo C 2no2 λ β (c m2 /W) 4π 2

(7)

3.3.3. Optical limiting Another interesting NLO application is optical limiting and it is a nonlinear optical property in which initially the transmitted intensity increases at low incident intensities with an increase in input laser power, then the transmitted output power attains threshold value and it becomes constant even after increasing the laser input power. Generally, the optical limiting behavior depends on various nonlinear mechanisms such nonlinear absorption and nonlinear scattering, the first one is due to multi-photon absorption (mostly in organic molecules, organic crystals, quantum dots), reverse saturable absorption (fullerines, pthalocyanis, and free-carrier absorption (semiconductor nanoparticles, metal nanocomposites), and the later nonlinear scattering mostly happens in nanomaterials is due to generation of solvent bubbles, ionization of nanoparticles and refractive index discontinuity [58]. The optical limiter materials should possess low threshold value, high optical damage threshold and fast response time with stability. Fig. 6 shows the optical limiting profile and from which the optical limiting threshold (TH) of Yb:YAG nanoparticles can be determined. It reveals that output power intensity varies linearly with input laser power. A threshold is reached at an input power of 19.3 mW for both the low-Yb (Yb:YAG-1) and high-Yb (Yb:YAG-5) doped YAG nanoparticulate samples with an output power of 1.82 and 2.09 mW respectively. As the input laser fluency increases, the nonlinear scattering is dominant in the nanoparticulate samples. This might reduce the proportional intensity of the laser beam passing the aperture and hence only a meager ten percent output is obtained for the input intensity. Thus the linear transmitted output intensity is found to be clamped at a lower threshold of 19.3 mW, and it indicates that Yb:YAG nanoparticles can be good optical limiter. Present optical limiting behavior of Yb:YAG nanoparticles are compared with the well-known plasmonic silver and gold colloidal

Therefore the absolute value of third order nonlinear optical susceptibility is calculated by equation,

χ (3) (esu) =

[Reχ (3) ]2 + [Im χ (3) ]2

(8)

where, ε o is the vacuum permittivity, c is the velocity of light in vacuum, no is the linear refractive index of the sample and λ is the output wavelength of laser beam. The calculated third order nonlinear optical susceptibility of low-Yb doped YAG and high-Yb doped YAG nanoparticles is summarized in Table 1. It is found that the calculated third order nonlinear optical parameters of high-Yb doped YAG sample is less than low-Yb doped YAG. It suggests that high-Yb doped YAG can increase the operating radiation flux density and radiation damage threshold level, a beneficial factor to use Yb:YAG as effective laser gain medium. Table 1 shows a large third order nonlinearity of n2 = 8.649 × 10−8 cm2/W and χ(3) = 7.286 × 10−6 esu. The measured n2 value in Yb:YAG nanomaterial is almost comparable to the reported value of 2.31 × 10−8 cm2/W in Yb3+ doped phosphate glasses [45]. Table 1 shows the values of Reχ(3), Imχ(3) and χ(3) calculated to be 7.254 × 10−6 esu, 0.677 × 10−6 esu and 7.286 × 10−6 esu 565

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also recorded under different input excitation power from 8.2 to 10.5 W cm−2. It is found that the emission intensity increases with increase in excitation power, suggesting that it obeys I α Pn relation, where I is the NIR emission intensity, P is the excitation power density and n is the number of absorbed photons required for infra-red emission at various excitation power. The exchange energy interaction occurs between YAG host lattice and luminescent centers of Yb3+ion may be responsible for such enhanced NIR emission. The super-linear response in emission profile between the input excitation power and output intensity can be more sensitive for high power Yb:YAG femto-second laser gain amplifiers [28]. 3.3.4. Thermo-optic coefficient It is well known that rare earth doped YAG crystals are extensively used as solid state laser gain medium. Especially, Nd3+ and Yb3+ doped YAG crystals are most widely used in high power laser systems and nonlinear optical devices. In recent years, optically transparent Yb:YAG nanoceramic discs that are prepared by sintering of Yb:YAG nanopowder with high pressure - high temperature compaction by hot isostatic pressing or spark plasma sintering is getting attention for the development of diode pumped solid state laser. Also, the nanoceramic laser has become more popular due to the advent of high power InGaAs laser that emit intense photons within the broad absorption band of Yb3+ ion at 900–980 nm. During optical pumping and laser oscillation, the solid state active medium often encounters with thermal lensing problem which affects its lasing performance. Optical pumping of Yb activator ions in YAG gain medium absorb intense infrared photon, generate heat, produce temperature gradient and induce thermal lensing [62]. When Gaussian laser beam is absorbed in the sample a transient heat is generated via non-radiative decay causes refractive index variation. Therefore, the output laser power and beam quality can possibly be affected due to variation in refractive index across the laser gain medium. Such thermal induced refractive index change is nonlinear optical phenomena often called as thermal lensing and defined by the parameter called as thermo-optic coefficient. The thermal lensing property of material depends upon laser energy absorption that causes spatial change in temperature and develops subsequently an in homogenous spatial nonlinear refractive index which acts as divergent lenslike behavior causing the laser beam to defocus and produce spatial variation in output laser profile. The above theory can be mathematically indicated by the equation in which the effective refractive index ‘n’ of nonlinear medium is due to linear refractive index ‘no’ and laser beam waist intensity ‘Io’ dependent nonlinear refractive index ‘n2’, n = n0 + n2Io/2. In thermal lensing case, laser induced thermal effect causes refractive index change.

Fig. 5. Z-scan traces of Yb:YAG-1 and Yb:YAG-5 nanoparticles (a) Closed aperture scan (b) Open aperture scan and (c) Ratio of closed-to-open scan.

nanoparticles [59]. Also, graphene oxide [60] and organic single crystal [61] shows limiting at a high laser input power of about 40 mW. Interestingly, the present Yb:YAG nanoparticles demonstrate optical limiting at a much lower input laser power of 19.3 mW. Therefore, the Yb:YAG nanoparticles is advantageous for optical limiting application. By embedding the Yb:YAG nanoparticles in transparent PMMA or PVA polymer matrix, one can use it as optical limiters for human eye protection and optical sensor systems. The concentration of the nanoparticles in polymer matrix decides its use for optical limiting applications. The excitation power dependent infrared emission profile (Fig. 7) is

Fig. 6. Power dependent optical limiting curve of Yb:YAG-1 and Yb:YAG-5 nanoparticles.

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of ∼6.864 × 10−6 K−1 was achieved in high-Yb doped Yb:YAG-5 sample. Therefore, Yb:YAG nanoparticles prepared by a simple coprecipitation method that resulted in less-agglomerative spherical nanoparticles showing high optical limiting and thermo-optical coefficient characteristics can be useful to fabricate optical transparent laser active Yb:YAG ceramics for compact diode-pumped solid state lasers and high power solid state laser amplifiers. Acknowledgement The author SAK is thankful to Department of Science and Technology (DST), India for the award of DST-Inspire Fellowship (DST/ INSPIRE FELLOWSHIP/IF150908) to carry out present research. Also, we thank Mr. B. Soundararajan, Senior Technical Officer, Department of Nuclear Physics, University of Madras for his support during microwave heat treatment. The authors also thank Prof. S. Bala Kumar, Director, National Centre for Nanoscience and Nanotechnology (NCNSNT), University of Madras, Chennai for FESEM. The author SAK thanks Sophisticated Test and Instrumentation center (STIC) Kochi University for UV-DRS and HR-TEM studies.

Fig. 7. Room temperature NIR emission spectra of Yb:YAG-5 nanoparticles recorded under different excitation power from 8.2 W cm−2 to 10.5 W cm−2.

References Therefore, above equation is modified by introducing thermal induced time dependent nonlinear refraction Δn, which depend on pump power intensity and absorption coefficient n = n0+Δn and n = n0+(dn/dt)T (where T = α0ω02I0/κ). The on-axis focal position laser-irradiance dependent temperature ‘T’ rise in the medium is a linear phenomenon, and it depends on linear absorption coefficient ‘α0’, beam waist radius ‘ω0’ and thermal conductivity ‘κ’ of YAG. Therefore, the temperature dependent refractive change (dn/dt) termed as thermo-optic coefficient can be estimated by utilizing nonlinear refractive index ‘n2’ value obtained from Z-scan measurement. Therefore, in the case of thermal lensing, the relation between thermo-optic coefficient (dn/dt) and nonlinear refractive index ‘n2′ is given by the following equation [17], from which the thermo-optic coefficient of Yb:YAG nanoparticles can be estimated as (dn/dt) = 4n2 κ/α0ω02 and the estimated values are given in Table 1. The obtained thermo-optic coefficient (dn/dt) values are compared with Yb doped sesquoxide materials such as Y2O3, Lu2O3 and Sc2O3 at different temperatures [63] and found to be slightly higher at ambient temperature (296 K). Hence, the Yb:YAG nanoparticles with such high thermo-optic coefficient is a useful property for Yb:YAG gain medium to minimize the thermal energy during high power lasing action. It is well known that Nd:YAG single crystal gain medium has been used for high power solid state laser amplifiers. In recent years, high power solid state laser amplifiers are demonstrated using Yb:YAG single-crystal thin-rod [64], thin-disk [65] and single-crystal fiber [66]. Instead of Yb:YAG single crystal, optical transparent ceramic can be used as gain medium which require spherical and homogeneous Yb:YAG nanoparticles [67–69]. In this aspect, Yb:YAG spherical nanoparticles prepared by co-precipitation can be useful to fabricate Yb:YAG ceramic gain medium for high power solid state laser amplifiers.

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4. Conclusion Third-order nonlinear optical properties of Yb:YAG nanoparticles at different Yb3+ ions concentrations were studied by single beam Z-scan technique using diode pumped CW Nd:YAG laser. The calculated nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of low Yb-doped and high Yb-doped samples was found to be 8.649 × 10−8 cm2/W and 8.362 × 10−8 cm2/W and 0.109 × 10−4 cm/W and 0.105 × 10−4 cm/W, respectively. The linear transmitted output intensity of Yb:YAG nanoparticles is found to be clamped at a lower threshold of 19.3 mW, indicating that it can be employed as good optical limiting applications for human eye protection and optical sensors system. High thermo-optic coefficient (dn/dt) 567

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