Two-photon assisted excited state absorption in multiferroic YCrO3 nanoparticles

Chemical Physics Letters 529 (2012) 59–63

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Two-photon assisted excited state absorption in multiferroic YCrO3 nanoparticles Shiji Krishnan a, C.S. Suchand Sandeep b, Reji Philip b, Nandakumar Kalarikkal a,c,⇑ a

School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam – 686560, Kerala, India Light and Matter Physics Group, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore – 560080, Karnataka, India c Center for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam – 686560, Kerala, India b

a r t i c l e

i n f o

Article history: Received 19 October 2011 In final form 20 January 2012 Available online 28 January 2012

a b s t r a c t We report a novel functionality for the ferroelectric-antiferromagnet YCrO3 powder of optical limiting upon illumination by nanosecond laser pulses at 532 nm. The optical limiting properties are investigated using the open aperture z-scan technique. The obtained nonlinearity fits to a three-photon like absorption mechanism. It is proposed that this nonlinearity is caused by two-photon absorption, followed by excited state absorption. Two different sized samples were investigated and the nonlinearity is found to be size dependent. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Multifunctional materials, especially multiferroics, have received considerable attention in recent years, because of their potential applications [1,2]. To achieve high miniaturization of integrated devices, combining properties into multifunctional materials is one of innovative ways explored by the modern technology. Multiferroic materials exhibit remarkable optical properties [3]. Several of the novel optical properties observed and predicted in multiferroic materials depend on the inhomogeneous polarization and magnetization across a multiferroic domain boundary. It is known that these domains can be controlled using external magnetic and/or electric fields, and it is this controllable response that makes multiferroics attractive for tunable optical devices [4]. Moreover, nanostructured multiferroics may exhibit very different optical properties than bulk or thin film samples, precisely because of the absence of domain walls. The coexistence of ferroelectricity, ferromagnetism and optical properties suggests the possibility of new optoelectronic devices such as the memory controllable by magnetic, electric and/or optical fields [5]. Yttrium chromite (YCrO3), the focus of this study, is a p-type semiconductor with a perovskite structure [6]. Despite this, it is insulating in both ferromagnetic and antiferromagnetic ordered states like NiO and CoO [7–9]. It has high thermal, electrical, chemical and structural stability, because of its high melting temperature (2290 ± 30 °C) [10,11]. Furthermore, YCrO3 exhibit favourable sintering properties and high electrical conductivity [12]. Recently, YCrO3 has been reported to be a multiferroic, exhibiting weak ferromagnetism below 140 K (TN) and a ferroelectric transition at 473 K, ⇑ Corresponding author at: School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam – 686560, Kerala, India. Fax: +91 481 2731669. E-mail addresses: [email protected] (S. Krishnan), suchandsandeep @gmail.com (C.S. Suchand Sandeep), [email protected] (R. Philip), nkkalarikkal @mgu.ac.in (N. Kalarikkal). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2012.01.047

accompanied by a hysteresis loop [7]. These properties make YCrO3 a promising candidate for many technologically demanding and challenging applications, such as interconnect materials in solid oxide fuel cells, humidity sensor, oxidation catalysts and thermistor, in addition to the potential magnetoelectric application [13–16]. The linear optical properties of YCrO3 have been studied extensively [17,18]. Moreover, second harmonic generation (SHG) in YCrO3 crystals has been reported [19,20]. However, optical limiting properties of YCrO3 have not been investigated so far. In this Letter, we report a novel functionality of YCrO3 powders, i.e., optical limiting upon illumination of nanosecond laser pulses at 532 nm.

2. Experimental Poly vinyl alcohol (PVA) assisted sol–gel method has been reported for the synthesis of some systems [21–23]. In this study, we applied the same method to obtain YCrO3 powders. Aqueous solutions of yttrium nitrate (Y(NO3)36H2O) and chromium nitrate (Cr(NO3)39H2O) were mixed together in stoichiometric proportions (1:1 M ratio) to obtain the mixed metal nitrate solution. PVA was then dissolved into the solution. The resulting solution was slowly evaporated at 50 °C until a gel was formed. The gel was prepared with two different molar ratios of PVA to the total metal ions, 1:1 and 5:1 (named as YCO1 and YCO5, respectively). Finally, the gels were calcined in air atmosphere at 600 °C for 2 h to remove the carbonaceous materials formed during the thermal decomposition of PVA and hence to obtain phase pure YCrO3 powders. The formation of desired phase of the prepared samples was confirmed by recording the infrared spectra (Shimadzu 8400S FTIR spectrophotometer) using KBr as matrix. Crystal structure of the samples were analyzed using X-ray diffractometer (Philips X’pert PRO) with Cu Ka radiation (k = 0.154 056 nm). Powder morphology

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S. Krishnan et al. / Chemical Physics Letters 529 (2012) 59–63

was studied using scanning electron microscope (Jeol JSM 6390). The fundamental optical constants, which are useful for analyzing the z-scan data, have been obtained through optical absorption measurement (Shimadzu UV-1700 spectrophotometer). Photoluminescence measurements were carried out with spectrofluorometer (Shimadzu, RF-5301PC) using 150 W Xenon lamp as excitation source. Optical limiting properties of the YCrO3 samples were investigated at 532 nm using an automated open aperture z-scan setup [24]. The frequency-doubled output of a Q-switched Nd:YAG laser (Minilite, Continuum Inc.) was used as the light source. The laser beam, which is spatially Gaussian, was focused using a convex lens of 10.5 cm focal length and the focal beam spot size (x0) was 16 lm. The laser pulse widths (FWHM) were approximately 5 ns. The pulses were fired in the ‘single shot’ mode, allowing sufficient time between successive pulses to avoid accumulative thermal effects in the sample, which might otherwise interfere with the nonlinearity measurement. The powder samples were suspended in ethylene glycol by sonication and were then taken in a 1 mm cuvette. The sample suspensions were prepared such that all of them had the same linear transmittance of 50% at 532 nm. The cuvette was mounted on a stepper motor controlled linear translation stage and was translated along the beam axis (z-axis) through the focal region over a distance substantially longer than the Rayleigh range. In p this ffiffiffiffiffiffiffiffischeme, each z position corresponds to an input fluence of 4 ln 2Ein =p3=2 xðzÞ2 , where Ein is the input laser pulse energy. x(z) is the beam radius at position z given by x0/ [1 + (z/z0)2]1/2, where z0 ¼ px20 =k is the Rayleigh range. Thus, at each position z, the sample sees a different laser fluence and the corresponding transmission is measured using a pyroelectric energy probe (Rj7620, Laser Probe Inc.) placed after the sample. The laser pulse energy reaching the sample was approximately 90 lJ.

3. Results and discussion The FTIR analysis confirmed the formation of monophasic YCrO3 (Figure 1). Sharp peaks at 584, 494 and 444 cm1 correspond to Cr–O stretching, Y–O stretching and O–Cr–O deformation vibration, respectively [25]. XRD patterns of the samples are depicted in Figure 2. The crystalline phases were identified according to Joint Committee on Powder Diffraction Standards (JCPDS) data [26]. The diffraction lines were indexed on an orthorhombic unit cell with Pnma space group and the unit cell parameters were determined using the PowderX software. The crystallite size was calculated from the X-ray line broadening measurements using the Scherrer equation. The obtained values of unit cell parameters

Intensity (a.u.)

YCO5 YCO1

400

500

600

700 -1

Wavenumber (cm ) Figure 1. FTIR spectra of YCrO3 samples.

800

Figure 2. XRD patterns of YCrO3 samples. Inset shows the corresponding scanning electron micrographs.

and crystallite size are listed in Table 1. The determined lattice parameters are in good agreement with JCPDS data. The increase in particle size is due to the increase in the heat generated as a result of the combustion of PVA in the gel. The greater the amount of PVA used in preparing the gel precursors, the greater the combustion heat generated from PVA [27]. Similar results have been reported by Zhang et al. for cellulose assisted combustion synthesis of copper ferrite. They observed that the amount of cellulose affected the reaction temperature and thus, resulted in different particle sizes [28]. Scanning electron micrographs of the samples are shown in the inset of Figure 2. The YCO1 sample shows a relatively homogeneous grain distribution, consisting of loosely agglomerated nearly spherical grains of about 212 nm, whereas the YCO5 sample shows a heterogeneous grain distribution and morphology. The average grain size of the YCO5 sample is about 701 nm, which is almost 3 times larger than that of the YCO1sample. This is consistent with the XRD results. The absorption spectra of the samples are shown in Figure 3 and are typical of Cr3+ in octahedral symmetry involving d–d electronic transitions. They are qualitatively similar on the whole, which indicates negligible change in electronic structure with particle size. The two intense bands at approximately 606 nm and 449 nm are attributed to the parity-forbidden and spin-allowed transitions from the ground level 4A2g (4F) to 4T2g (4F) and 4T1g (4F) [4T1a], respectively. Minor bands, because of the parity and spin forbidden transitions from 4A2g (4F) to 2Eg (2G) and 2T1g (2G), are observed at approximately 730 nm and 691 nm, respectively [18]. The intense absorption bands in the UV region are assigned to the chargetransfer (CT) transitions from the oxygen 2p valence band to the chromium 3d conduction band. The two peaked structure is due to the combined effect of the Hund coupling and the ligand field splitting in the 3d orbitals [29]. Incidentally, the band corresponding to the spin allowed 4A2g (4F) to 4T1g (4P) [4T1b] transition is indiscernible in the observed spectrum, since it is overlapped by the stronger CT excitations [30]. The optical band gap was determined using the Tauc’s formula [31], ahm = A(hm–Eg)n, where a, hm, Eg and A are absorption coefficient, photon energy, band gap, and a constant, respectively. Exponent n depends on the type of transition and it may have values1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively. In the present case, the best fit of (ahm)1/n versus the photon energy was obtained for n = 1/2. Thus, the Eg values are determined by extrapolating the linear portion of the (ahm)2 versus hm plot to the point a = 0 (inset in Figure 3). Secondary edges that can be

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Table 1 Sample code, unit cell parameters, crystallite size and band gap energy of YCrO3 samples. Sample code

Unit cell parameters (Å)

YCO1 YCO5

a

b

c

5.513 5.520

7.518 7.529

5.225 5.230

-2

(αhν) (eV cm )

2 2

Absorbance

0.6

1.5

2.0

0.4 0.2

2.5 3.0 Energy (eV)

0.0 400 500 600 Wavelength (nm)

700

800

Figure 3. Linear optical absorption spectra of YCrO3 samples. Inset shows optical band gap determination of the same samples using Tauc plots.

assigned to crystal field transitions were observed at higher wavelengths for both samples. The band gap is found to be size dependent and the band gap values are shown in Table 1, which is in agreement with values from previous reports [32]. Figure 4 depicts the normalized transmittance of the samples plotted as a function of the input laser fluence. The normalized transmittance plotted as a function of the position of the sample (the z-scan curve) is shown in the insets. It is evident from these graphs that the sample behaves like an optical limiter. The observed nonlinearity fits to a three-photon like absorption process and the transmission is given by the equation,

ð1  RÞ2 expðaLÞ pffiffiffiffi  pp0  þp0 expðt 2 Þ dt;

T¼

Z

þ1

1

40 128

3.24 3.17

Imvð5Þ ¼ ce20 n3 c2 k=5p;

3.5

YC O 5 YC O 1

300

Band gap (eV)

where R is the Fresnel reflection coefficient at the sample-air interface, a is the linear absorption coefficient, and L is the sample length [33]. p0 is given by ½2cð1  RÞ2 I20 Leff 1=2 , where c is the three-photon absorption (3PA) coefficient, and I0 is the incident intensity. Leff is given by [1  exp (2aL)]/2a. The imaginary part of the third-order susceptibility, Im v(5), is related to c through the equation,

1.0 0.8

Average crystallite size (nm)

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ln 1 þ p20 expð2t 2 Þ ð1Þ

ð2Þ

where n = 1.64 is the linear refractive index of the sample, e0 is the permittivity of free space, c is the velocity of light in vacuum, k is the wavelength of the radiation used [33]. The obtained values of the nonlinear absorption coefficient, c and Im v(5) are shown in Table 2. The optical limiting response is found to increase with the increase in particle size. A similar size-dependent enhancement of nonlinearity has been observed in CuCl nanocrystals and ZnO nanocolloids [34,35]. From the absorption spectra (Figure 3), it is clear that the samples show some absorption at 532 nm, complementing the linear transmission of 50% of the samples. Obviously this spectrum allows one-photon, two-photon, and two-step absorptions to take place at 532 nm. Therefore, the c and Im v(5) given in Table 2 should be considered as effective values. For comparison, previous measurements using the same excitation wavelength and laser pulse width have given the three- photon absorption coefficient values on the order of 1019 m3/W2 in Ag nanoparticles [36], 1021 m3/W2 in Au nanoclusters [37], 1022 m3/W2 in ferrofluids [38] and 1024 m3/W2 in Bi-doped ZnO nanoparticles [39] and rare earth doped strontium barium niobate nanoparticles [40]. At the outset this looks curious, because in the present case, there are no compelling reasons for a fifth order nonlinearity to override the third order nonlinearity. Both excitation photon energy (hm = 2.33 eV) and bandgap of YCrO3 fulfil the two-photon absorption (2PA) requirement (hm < Eg < 2hm). In addition, the energy of the two photon wavelength is resonant with the peak en-

1.0

0.9

0.8

0.7 -10

-5

0

5

10

1.0

0.9

0.8

1.0

0.9

0.8

0.7 -10

Z (mm)

0.7

Normalized Transmittance

0.8

YCO5

Normalized Transmittance

0.9 Normalized Transmittance

Normalized Transmittance

YCO1

1.0

-5

0 Z (mm)

5

10

0.7 4

10 10 2 Input Fluence (J/m )

5

10

4

10

5

2

Input Fluence (J/m )

Figure 4. Nonlinear transmission in YCrO3 samples of different particle sizes under 532 nm, 5 ns laser irradiation. Insets show the corresponding open-aperture z-scan curves. Circles are data points and solid lines are numerical fits using equation 1.

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Table 2 Measured values of nonlinear absorption coefficient and imaginary part of nonlinear susceptibility for YCrO3 samples under nanosecond laser excitation. Sample code

3PA coefficient (1024 m3/W2)

Im v(5) (1036 m4/V4)

YCO1 YCO5

4.5 6.3

4.7 6.6

to 4A2g transitions of Cr3+ ions in the high crystal field, when excited at 260 nm [42]. This is the only radiative emission and the lifetime of the 2Eg state of the Cr3+ ion is longer than the pulse duration used in our experiments [43]. ESA is principally from the doublet 2Eg to the three higher energy doublets (2T1g, 2T2g and 2A1g) and the ESA processes are shown in the energy level scheme, Figure 6. ESA originating from the 2Eg doublet has been previously reported in several Cr3+ doped materials [44,45].

Intensity (a.u.)

4. Conclusions

650

700

Wavelength (nm) Figure 5. Emission spectrum of YCO1 under UV excitation.

In summary, we have investigated the nonlinear optical properties of YCrO3 for optical limiting application using nanosecond laser pulses at 532 nm. The obtained nonlinearity is found to fit well with a three-photon like absorption process. We propose that this nonlinearity is caused by two-photon absorption followed by excited state absorption. The optical limiting response is found to be size dependent, within the range of our investigations. A specific advantage of YCrO3 is that the optical properties in these materials are tuneable by an applied magnetic and/or electric field. For instance, upon the application of the magnetic field, a drastic change in the SHG intensity has been observed in YCrO3 crystals [19]. Moreover, field induced spin reorientation have been observed in YCrO3 by means of spectral changes of R exciton lines corresponding to the 4A2g–2Eg transition of Cr3+ [46]. In a typical device application, such magnetic control can turn out to be very useful. Acknowledgements Financial assistances from UGC-Govt. of India through SAP and DST-Govt. of India through FIST Program are gratefully acknowledged. References

Figure 6. Simplified energy level diagram of the Cr3+ ions in YCrO3 showing ESA processes. Solid arrows are optical transitions and the dashed arrow is nonradiative relaxation. Note that the higher doublet states such as 2Eg, 2T1g, 2T2g lie beyond the experimental energy range and hence are omitted.

ergy of the charge-transfer transition band (Figure 3), which increases the probability of the two-photon absorption. Further, nanosecond laser pulses cannot provide sufficient irradiance for the occurrence of a genuine 3PA process. Open aperture z-scan with nanosecond laser pulses usually has dominant contribution to the observed reverse saturable absorption behaviour from excited state absorption. Thus, it should be noted that even though the obtained nonlinearity appears as if it is of the fifth order, in reality this need not be the case. A two-photon absorption followed by excited state absorption (ESA), which is a sequential v(3):v(1) process also can result in nonlinear transmission curves that fit numerically to a three-photon (v(5)) process. Such an effective v(5) nonlinearity of the v(3):v(1) type has been observed in semiconductor doped glasses, when the band gap of the semiconductor crystallites becomes smaller than twice the photon energy [41]. It is very likely that the same process is acting in the present case too. In YCrO3, simultaneous absorption of two photons excites directly the Cr3+ ions into the higher excited state (4T1b), which results in a substantial population of the metastable 2Eg state being an initial state for the doublet–doublet (D–D) type ESA. This is consistent with the emission spectrum of the YCrO3 (Figure 5), where we could see the typical doublet emission corresponding to the 2Eg

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