Chapter 5

A Comparative Study on the

Optical Limiting Properties of

different Nano Spinel Ferrites

using Z-scan Technique

This chapter describes the investigations on the optical limiting properties of

five different spinel ferrites, NiFe2O4, Ni0.5Zn0.5Fe2O4, ZnFe2O4,

Ni0.5Co0.5Fe2O4 and CoFe2O4, with an average particle size of 9 nm. The

optical limiting properties were investigated using the open aperture Z-scan

technique. The obtained nonlinearity fits to a two-photon like absorption

process. Except for NiFe2O4, the observed nonlinearity has contributions

from excited state absorption. The optical limiting response is also studied

against particle size and the nonlinearity is found to increase with

increasing particle size, within the range of our investigations. On

comparing the optical limiting properties, ZnFe2O4 is found to be a better

candidate for the optical limiting applications.

136

5.1 Introduction

Advances in the development of new communication methods

and new sources of radiation, particularly optical power sources, such

as lasers had resulted in research oriented towards the protection

from exposure to such sources [1]. An ideal optical limiter should be

transparent to low energy laser pulses and opaque at high energies,

so that it can protect human eyes and optical sensors from intense

laser radiation. Over the years, various materials, such as inorganics,

organics, organometallics and semiconductors have been studied for

their optical limiting performances [2-4].

Recently, nanomaterials have drawn significant attention as

optical limiters [5]. Optical power limiting is operated through the

nonlinear optical processes of nanomaterials. However, the great

potentials of nanomaterials as optical power limiters have just begun

to be recognized.

Control of the nonlinear properties by means of an external

stimulus, such as magnetic field presents remarkable applications.

Huang et al. have theoretically evaluated the tunability of nonlinear

optical properties by an external magnetic field [6]. Several magnetic

materials are reported to have good nonlinear optical absorption

characteristics [7-9].

The ferrite nanoparticles have been a subject of much interest,

because of their unusual optical [13, 14], electrical [15, 16] and

magnetic properties [17, 18], which often differ from the bulk. The

optical limiting properties of NiFe2O4 nanoparticles have been

reported [19]. However, optical limiting properties of other spinel

ferrite nanoparticles are not studied yet. In this paper, we report the

optical nonlinearity of five different spinel ferrite nanosystems,

137 Nonlinear Optical Study

NiFe2O4, Ni0.5Zn0.5Fe2O4, ZnFe2O4, Ni0.5Co0.5Fe2O4 and CoFe2O4, upon

illumination of nanosecond laser pulses at 532 nm. These systems

have a very high shelf life and remarkable thermal stability, which are

important requirements for sustainable use with intense lasers. In

fact, it is the physical and chemical stability of ferrites, which is an

important attribute for an optical limiter, which prompted us to

investigate their optical limiting properties. To the best of our

knowledge, this is the first report, where the optical limiting

properties of spinel ferrites are compared.

5.2 Experimental

Nano spinel ferrites with five different compositions, NiFe2O4,

Ni0.5Zn0.5Fe2O4, ZnFe2O4, Ni0.5Co0.5Fe2O4 and CoFe2O4, with an average

particle size of 9 nm were synthesized by the sol-gel method, as

described in previous chapters. In addition, NiFe2O4 nanosystems of

two different particle sizes were synthesized by changing the PVA to

total metal ions ratio to 2 and 1 (named as NFO2 and NFO1

respectively). The particle size obtained for these samples are 20 nm

and 25 nm respectively. The UV–Vis absorption spectra of the samples

were recorded using spectrophotometer (Shimadzu UV-1800) at room

temperature.

Open aperture Z-scan [20] experiment was used to measure the

nonlinear transmission of the powder samples suspended in ethylene

glycol. Here, a laser beam is used for sample excitation and its

propagation direction is considered as the Z-axis. The beam is

focused using a convex lens and the focal point is taken as Z = 0. The

beam has maximum energy density at the focus, which will

symmetrically reduce towards either side of it on the Z-axis. In the

experiment, the sample is scanned along the Z-axis and the

138

corresponding transmissions are measured. The position-

transmission curve thus obtained is the open aperture Z-scan curve.

From this curve, the nonlinear absorption coefficient of the sample

can be calculated.

The intensity dependent linear absorption coefficient, α(I) can

be written in terms of linear absorption coefficient, α and two photon

absorption coefficient, β as

( )I Iα α β= + (5.1)

The irradiance at the exit surface of the sample can be written as

0( , , )( , , )

1 ( , , )

l

r

I z r tz r t

q z r teI

α−

=+

(5.2)

where ( , , ) ( , , )effeff

q z r t I z r t Lβ= (5.3)

effL is the effective length and is given in terms of sample length, l

and α0 by the relation

1l

eff

eLα

α

−

−= (5.4)

The total power transmitted, P(z,t) is obtained by integrating

equation (5.2) over z and r and is given by

0

1

0

ln 1 ( , )( , ) ( )

( , )

lz t

P z t tz t

qeP q

α−

+

= (5.5)

P1( t)and q0(z, t) are given by the equations 2

0 0

1

( )( )

2

effI t LtP

πω= (5.6)

0

0

2

0

( )( , )

1

eff effI t Lq z t

zz

β=

+

(5.7)

139 Nonlinear Optical Study

For a pulse of Gaussian temporal profile, equation (5.5) can be

integrated to give the transmission as

( )2

0

0

( ) ln 1 tCT z q e dt

q π

+∞

−

−∞

= +∫

(5.8)

For 0 1q < this transmittance can be expressed in terms of peak

irradiance in a summation form as,

[ ]

( )

0

320

( ,0)( , 1)

1

m

m

q zT z S

m

∞

=

−= =

+∑

(5.9)

The nonlinear absorption coefficient of the sample is obtained by

fitting the experimental data to equation (5.8).

Since the sample sees different laser intensities at each position,

this position dependent transmission can be easily scaled to its

intensity dependent transmission. For instance, for an incident

Gaussian beam of wavelength λ, the beam radius at position z is

given by ( ) ( ) ( )1/2

2

0 0 1 /z z zω ω = +

where ω(0) is the focal spot

radius and z0 is the Rayleigh range given by ( )( )2

0 /π ω λ . Therefore,

knowing the energy of laser pulse, the input laser fluence and

intensity can be calculated for each z value. Our automated z-scan

setup used a precision stepper motor controlled translation stage to

move the sample along the z- direction. The sample taken in a 1 mm

cuvette was translated along the beam axis through the focal region

over a distance much longer than the Rayleigh range. The sample

suspensions were prepared such that all of them had the same linear

transmittance of 52% at 532 nm. Of the two pyroelectric energy

probes (Rj7620, Laser Probe Inc.) used, one monitored the input

energy, while the other monitored the transmitted energy through the

sample. The frequency-doubled output (532 nm) of a Q-switched

140

Nd:YAG laser (Minilite, Continuum Inc.) was used for exciting the

samples. The temporal pulse width of the laser pulses was 5 ns

(FWHM). The laser pulse energy used for the experiments was 100

μJ. The pulses were fired in the “single shot” mode, allowing

sufficient time between successive pulses to avoid accumulative

thermal effects in the sample.

5.3 Results and Discussions

5.3.1 Optical Absorption Study

The optical absorption spectra of the samples are shown in Figure

5.1. A well developed maximum in the spectrum of NiFe2O4 at 745 nm

is probably caused by 3A2g to 3T1g(F) transition of Ni2+ octahedrally

coordinated by O2-ions [21, 22].

Figure 5.1. Optical absorption spectra of (a) NiFe2O4, (b) ZnFe2O4, (c)

Ni0.5Zn0.5Fe2O4, (d) Ni0.5Co0.5Fe2O4 and (e) CoFe2O4.

The band gap, Eg was determined from the expression for the

absorption coefficient near the band edge, given by the equation,

( )gnh A h Eα ν ν= − where hν is the photon energy, A is a constant and

α is the linear absorption coefficient [23]. Exponent, n depends on the

141 Nonlinear Optical Study

type of transition and n = ½ or 3/2 for direct allowed and direct

forbidden transitions, while n = 2 or 3 for indirect allowed and

indirect forbidden transitions. For all the samples, the best fit of 1

( ) nhα ν versus hν was obtained for n = ½. Thus, the Eg values are

determined by extrapolating the linear portion of the (αhν)2 versus hν

plot to the point α = 0 and are given in Table 5.1.

Table 5.1. Band gap energy of spinel ferrites calculated from Tauc plot.

Sample Band gap (eV)

NiFe2O4 2.54

ZnFe2O4 1.94

Ni0.5Zn0.5Fe2O4 1.45

Ni0.5Co0.5Fe2O4 1.06

CoFe2O4 1.18

5.3.2 Nonlinear Absorption Study

Figure 5.2 shows the optical limiting curves obtained for the

samples Ni0.5Zn0.5Fe2O4, Ni0.5Co0.5Fe2O4 and CoFe2O4. Figure 5.3 is

similar figures for ZnFe2O4 nanoparticles at different input laser

energy. The curves for NiFe2O4 samples of three different particle

sizes are shown in Figure.5.4. The Z-scan curves obtained for all these

samples are numerically fitted to the nonlinear transmission equation

for a TPA process [24] given by equation (5.8).

The obtained values of the nonlinear absorption coefficient (β) are

presented in Table 5.2. From the absorption spectra, it is obvious that

all the samples show some absorption at 532 nm (Figure. 5.1),

complementing the linear transmission of 52% of the samples.

Therefore, the β values given in Table 5.2 should be considered as

effective values. To put the obtained values in perspective, previous

measurements using the same excitation wavelength and laser pulse

142

width have given the two photon absorption coefficient values of

1.9×10-9 m/W for CdS quantum dots [25], 1.5×10-10 m/W for Ag2Te

nanowires [26], 3.28×10-11 m/W for C60 [27] and 2×10-12 m/W for Cu

nanocomposite glass [28]. Thus, spinel ferrites could be a potential

candidate for optical limiting applications.

Figure 5.2. Nonlinear absorption in samples (a) CoFe2O4, (b) Ni0.5Co0.5Fe2O4

and (c) Ni0.5Zn0.5Fe2O4. Insets show the corresponding open aperture Z-scan

curves. Circles are data points and solid lines are numerical fits using

equation (5.8).

143 Nonlinear Optical Study

Figure 5.3. Nonlinear absorption in ZnFe2O4 sample for input laser energies

of (a) 40 μJ, (b) 60 μJ, (c) 80 μJ and (d) 100 μJ. Insets show the

corresponding open aperture Z-scan curves. Circles are data points and solid

lines are numerical fits using equation (5.8).

In the present case, both the excitation photon energy (2.33 eV)

and the bandgap of ferrites fulfill the one-photon absorption

requirement ( )gh Eν > except for NiFe2O4. TPA process takes place,

when the laser energy is larger than half of the band gap of the

material2

gEhν

>

. Hence, it is very likely that excited state

absorption is acting in the present case. For a detailed understanding

of the contributions from excited state absorption, we have performed

the Z-scan measurements at different pulse energies for ZnFe2O4

(Figure 5.3). The β values thus obtained are plotted against laser pulse

energies (Figure 5.5). It is noted that the β value is increasing

144

substantially, as the input laser pulse energy increases beyond 80 μJ.

Thus at higher energies, β becomes a nonlinear function of pulse

energy indicating the occurrence of higher order nonlinear processes

such as free-carrier absorption.

Figure 5.4. Nonlinear absorption in NiFe2O4 samples of three different

particle sizes (a) 25 nm, (b) 20 nm and (c) 9 nm. Insets show the

corresponding open aperture Z-scan curves. Circles are data points and solid

lines are numerical fits using equation (5.8).

In order to determine the effect of particle size on the optical

limiting response of the spinel ferrites, the Z-scan measurements were

performed on NiFe2O4 samples of three different particle sizes (Figure

5.4). The β values thus obtained are shown in Table 5.2 and are found

to be size-dependent. A similar size-dependent enhancement of

nonlinear optical properties has been observed in CuCl nanocrystals

[29] and ZnO nanocolloids [30].

145 Nonlinear Optical Study

Table 5.2. Sample composition, sample code, nonlinear absorption coefficient

and optical limiting threshold of spinel ferrite nanoparticles.

Sample

Composition

Sample

code

β (m/W) Optical limiting

threshold (W/m2)

NiFe2O4

NFO1 1.35 ×10-10 3.4×10-12

NFO2 1.35 ×10-10 3.6×10-12

NFO 1.20 ×10-10 3.9×10-12

ZnFe2O4 ZFO 1.45×10-10 2.2×10-12

Ni0.5Zn0.5Fe2O4 NZFO 1.11×10-10 4×10-12

Ni0.5Co0.5Fe2O4 NCFO 1.40×10-10 3×10-12

CoFe2O4 CFO 1.35×10-10 3.2×10-12

In the optical limiting measurement, limiting threshold is an

important parameter. The lower the optical limiting threshold, the

better will be the optical limiting response [30]. The limiting threshold

for the spinel ferrite samples are determined from the optical limiting

curves and are presented in Table 5.2. From the table, it is obvious

that increasing the particle size reduces the optical limiting threshold

and thus, enhances the optical limiting performance.

On comparing the values of β and optical limiting threshold, it is

found that among the investigated spinel ferrites, ZnFe2O4 is the best

candidate for optical limiting applications. The presence of Fe3+ ions

in the octahedral sites is responsible for the polarization in spinel

ferrites [31]. Thus, the polarization effects will be different for

different spinel ferrite systems. From the investigations on the

cationic distributions of these nano spinel ferrites, it was found that

ZnFe2O4 possesses the maximum number of Fe3+ ions in the B site.

146

Thus, the polarization effects will be higher in the case of ZnFe2O4,

which eventually contribute to the nonlinear absorption.

Figure 5.5. Variation of two photon absorption coefficient of ZnFe2O4 with

input pulse energy.

Conclusion

In conclusion, we have investigated the optical limiting properties

of five different spinel ferrites, NiFe2O4, Ni0.5Zn0.5Fe2O4, ZnFe2O4,

Ni0.5Co0.5Fe2O4 and CoFe2O4 using nanosecond laser pulses at 532 nm.

The obtained nonlinear parameters show that these materials are

efficient optical limiters. Except for NiFe2O4, the observed

nonlinearity has contributions from excited state absorption. The

optical limiting response is found to increase with increasing particle

size within the range of our investigations. On comparing the optical

limiting properties, ZnFe2O4 is found to be a better candidate for the

optical limiting applications. The large nonlinear optical responses

observed in these ferrites open up possibilities for device applications

exploiting the features of ferrimagnetism and nonlinearity.

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