2012 Nonlinear Properties and Optical Limiting of Olive Oil by Using Z-scan Technique

8/9/2019 2012 Nonlinear Properties and Optical Limiting of Olive Oil by Using Z-scan Technique

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The First National Conference for Engineering Sciences FNCES'12 / November 7-8, 2012

Nonlinear Properties and Optical Limiting ofOlive Oil by using Z-scan Technique

Amal F. Jaffar* 1, Ansam M. Salman ** 2 ,Israa N.Akram** 3 and Dr. Anwaar A. Al. Dergazly** 4

* Ministry of High Education & Scientific Researge, Foundation of Technical Education, Institute of Medical Technology MansourIraq-Baghdad

1 [email protected] **Al Nahrain University, college of engineering, laser and optoelectronics eng. Dep.

Iraq-Baghdad2 [email protected] 3 [email protected]

[email protected]

Abstract- A simplified and sensitive experimental techniquenamed z-scan has been used in this work, to study the optical

nonlinearity and optical limiting of olive oil. Olive oil isclassified as organic compounds which have a good nonlinearoptical properties candidate to be used in photonic applications.A high purity sample of olive oil has been subjected tospectrophotometer to determine the transmission spectrum usingUV-VIS spectrophotometer. The nonlinear optical propertiesrepresented by nonlinear refractive index and nonlinearabsorption coefficient are determined by using a CW of 532 nmin two parts. The first part has been done using a closed aperture(with two different diameter 1 mm and 2 mm) placed in front ofthe detector to measure the nonlinear refractive index whichexhibits negative refractive index (defocusing).Second part wasdone using an open aperture to measure the nonlinearabsorption coefficient, where the samples exhibit two photonabsorption behavior under the experimental conditions. Real andimaginary parts of the third-order optical nonlinearity, (3) wereevaluated. The third-order nonlinearity of olive oil is dominatedby nonlinear absorption, which leads to strong optical limiting ofthe laser.

Keywords: Olive oil, z-scan technique, nonlinear refractiveindex, nonlinear absorption coefficient, optical limiting.

I. INTRODUCTIONRapid technological advancements in optics have placed

great demand on the development of nonlinear optical (NLO)materials with prominent applications in optical limiting andall optical switching extremely large number of organiccompounds with delocalized electron and conjugated double

bond systems and a large dipole moment have beensynthesized to realize the susceptibilities far larger than theinorganic optical materials [1].

The olive oil behaves as a nonlinear material where it has a''blue shift'' in some excited wavelengths. Therefore, thenonlinear optical properties of olive oil which are of

prominent importance for photonics applications have beenfound [2]. Lyotropic liquid crystals are one of liquid crystalsobtained by varying the temperature or concentration which isobtained when an appropriate concentration of material isdissolved in some solvent. The most common systems those

formed by amphiphilic molecules (molecules posses ahydrophilic part that interacts strongly with water and a

hydrophobic part that is water insoluble).The olive oil represents the one class of lyotropic liquid

crystal in nematic phase [3]. Liquid crystals are opticallyhighly nonlinear materials in that their physical properties(temperature, molecular orientation, density, electronicstructure, etc.). These are obtained by an applied optical field.The polarized light beam from laser source can induce analignment or ordering in isotropic phase. These result in achange in refractive index. In isotropic phase the change inrefractive index is due to the density change following arise intemperature. In the nematic phase the change in the refractive

index depend directly on the temperature. The reorientation ofmolecules in liquid crystals depends on the phase of liquidcrystals. For the isotropic phase the liquid crystal moleculesare randomly oriented owing to thermal motion. So an intenselaser field will force the anisotropic molecules to alignthemselves in the direction of optical field through the dipolarinteraction [4].

Z -scan technique based on the spatial distortion of alaser beam passed through a nonlinear optical material (NLO)is widely used in material characterization because of itssimplicity, high sensitivity and well-elaborated theory. Theopportunity to conduct simultaneous measurements of various

NLO parameters in one set of experiments also makes thistechnique attractive and applicable for different materials.This method yields both the sign and the magnitude of thenonlinearity, and the value of the nonlinear refractive index n2 may be easily extracted from experimental data with aminimum of analysis [5, 6].

There were two parts of the z-scan: closed apertureand open aperture. Closed aperture z-scan helps to the signand magnitude of both real & imaginary part of third order(NLO) and nonlinear refractive index (n 2) .Open aperture z-scan eithertwo types: saturable absorption (SA) and reversesaturable absorption (RSA). Depending on the pump intensityand on the absorption cross section at the excitationwavelength. Open aperture z-scan helps to measure thenonlinear absorption coefficient 2 [7].


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The First National Conference for Engineering Sciences FNCES'12 / November 7-8, 2012 Optical limiting is a nonlinear optical process in which

the transmitted intensity of a material decreases with increasedincident light intensity. An ideal optical limiter has a lineartransmitted intensity at low input intensities, but above thethreshold intensity, its transmitted intensity becomes constant.

Nonlinear optical effects can be employed for the design and performance of optical limiter. It has been demonstrated thatoptical limiting can be used for the protection of eyes andsensors from intense lasers [8]. In this paper, the opticalnonlinearity and optical limiting action for olive oil at 100 mwCW laser power at wavelength of 532 nm was studied byusing z-scan technique , closed and open aperture. Closedaperture experiment was repeated for two different aperturediameters, (1 and 2 mm), the third order nonlinear refractiveindex, the nonlinear absorption coefficient, third ordersusceptibility was evaluated. The dependence of the thresholdvalue on the aperture size makes it convenient to optimize thethreshold intensity of in optical limiting.

II. THEORY A. CLOSED APERTURE Z- SCAN

The standard closed aperture Z-scan (i.e. apertureis placed in the far field) for determining nonlinear refractionis shown in Fig. 1, where the sample is moved along the

propagation direction z while keeping the input pulse energyfixed. The normalized transmittance of the sample through theaperture is monitored in the far field asa function of the

position Z. The normalization is performed in such a way thatthe transmittance is unity for the sample far from focus wherethe nonlinearity is negligible [9]. Intensity dependent on therefractiveindex causes the beam radius of the transmitted

beam to change while retaining the Gaussian profile.

Fig. 1: The scheme of the closed aperture z-scan[10].

The size of the aperture is signified by its transmittance(S), in the linear regime. In most reported experiments,0.1


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Io=

2

2

o

peak (5)

ois on the axis irradiance,

P peak : The peak power given by:P peak =E/t. (6)

E: the energy of the pulsed laser,T: the time duration. (with pulsed laser).

o : The beam radius at the focal point, , is the wavelengthof the beam.

Leff : The effective length of the sample (thickness), candetermined from the following formula: [13, 14]

oeff Le L o /)1(

(7)

Where,L: the sample thickness, [10]

o : Linear absorption coefficient,

)1

ln(

1

T L

(8),

where T: linear transmittance.Linear refractive index can be determined from the followingequation:

no =1/T+ [(1/T 2) - 1] 0.5 . (9)

The contribution of nonlinear refraction tonormalized transmittance affected by the diameter of theaperture where, the contribution of nonlinear refraction tonormalized transmittance decreases as the diameter of theapertureincreases and their peak-to-valley separation increasesas the nonlinear phase shift increases. [15] .

B. OPEN APERTURE Z-SCANAn open-aperture Z -Scan measures the change in

intensity of a beam, focused by lens l in Fig. 3, in the far fieldat detector PD, which captures the entire beam, and gives theestimate of the absorptive nonlinearity of a sample [16].The

change in intensity is caused by multi-photon absorption inthe sample as it travels through the beam waist. In the focal

plane where the intensity is greatest, the largest nonlinearabsorption is observed. At the tails of the Z -scan signature,where | Z | >>Z o, the beam intensity is too weak to elicitnonlinear effects. The higher order of multi-photon absorption

present in the measurement depends on the wavelength oflight and the energy levels of the sample [17].

Fig. 3: Open-aperture Z-Scan [11]

The absorptive nonlinearity can be due to either (i)saturable absorption (SA), in which the absorption coefficientdecreases resulting in thetransmittance increase with increasein the input laser intensity, and (ii) reverse saturableabsorption (RSA), in which the absorption coefficientincreases resulting in the transmittance decrease with increasein the input laser intensity .The normalized change intransmitted intensity can be approximated by the followingequation, [18]:

. (10)where, Z : is the sample position at the minimumtransmittance, m: integer, T (z):the minimum transmittance,

In the case of saturation type of nonlinear absorption can be estimate from the relation:C. SELF FOCUSING AND DEFOCUSING

Self-focusing (or Kerr-Lansing) is a consequence of thenon-uniform spatial profile of the laser Gaussian beam. If theintensity of a transmitted light beam is sufficiently high, therefractive index change will modify the light propagation notonly with respect to the polarization but in its geometrical

properties too [19].For a Gaussian beam of radius o(beam waist)the Kerr-lens

focal length is:F= a o2/4Ln 2I........................... (11)Where L is the thickness of the nonlinear medium (sample), Iis the irradiance and a is a correction term.

When n 2 is negative, the above equation shows therewill be a negative focal length and thus self de-focusing of theincident beam [10].

The variation in refractive index will produce a phaseshift in the pulse, leading to a change of the pulse's frequency

spectrum. A pulse (top curve, fig. 4 propagating through anonlinear medium undergoes a self-frequency shift (bottom

curve) due to self-phase modulation. The front of the pulse isshifted to lower frequencies, the back of the pulse to higherfrequencies. In the centre of the pulse the frequency shifted isapproximately linear [20, 21].

m

m

eff o

m

Zo Z L I

z T 0 2/32

)1(

)/(1)(

2/123233 ))(Im())(Re(


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Fig. 4: Self focusing in the laser beam due to the optical Kerr effect [20].

If the intensity of a transmitted light beam is sufficientlyhigh almost every material, gases, liquids or solids, will showa nonlinear interaction. Thus the refractive index will bechanged as:n x= K kerr inc.(3/4E 2inc.,x + E 2ext).(12)

Where K kerr is Kerr constant, inc.: incident wave length, Einc.the incident electric field, Eext is the external electric field,

if the nonlinear range of the electric field or intensity isreached.

In particular, if light beams with transverse intensity profile, as Gaussian beams, are applied this refractive indexchange will be different over the cross-section of the beams[19].

As a consequence for high-intensity beams with longinteraction lengths in the matter self-focusing can occur andfor short interaction lengths self-defocusing may be obtained[19].

Experimentally determined nonlinear refractive index n 2 and nonlinear absorption coefficient can be used in findingthe real and imaginary parts of the third-order nonlinearoptical susceptibility [ 3] according to the following: [9]

Re ( 3 )(esu) = 10 -4 0c2n02n2/ (cm 2/W) (13)

Im (3)

(esu) = 10-2

0c2

n02

/42

(cm/W) (14)

Where 0is the vacuum permittivity and c is the light velocityin vacuum. [8 ,9]The absolute value of the third-order nonlinear opticalsusceptibility is given by the relation [8]

.. (15)

D. OPTICAL LIMITING

In a material with a strong nonlinear effect, theabsorption of light increases with intensity such that beyond acertain input intensity the output intensity approaches aconstant value. Such a material can be used to limit theamount of optical power entering a system. The ideal behaviorof such a device is shown in Fig. 5, [23]. One importantcriteria in evaluating the utility of an optical limiting materialis whether it shows broad band spectral response, i.e., if it istransparent at low intensities while exhibiting a largenonlinearity at high intensities over a broad band spectralrange.

Fig. 5: An ideal optical limiter [22].

III. EXPERIMENTAL WORKThe experimental work based on the testing the olive oil

transmission spectrum using UV-VIS spectrophotometer.Fig.6 shows the transmission spectrum of olive oil. Theoptical transmission of olive oil shows a variable behavior ofthe transmission as a function of the incident wavelength. Thetransmission behavior olive oil is about (95.5) % at

wavelength of 532 nm.The Z -scan experiments were performed using a 532 nm CW.Maximum power is 100 m watt, beam Diameter: 3 mm, Beamdivergent 2.16x10 -3mrad, Ac: 220-240 volt, frequency: 50-60Hz 250 mA which was focused by 10 cm focal length lens.The laser beam waist o at the focus is measured to be 21. 6m and the Rayleigh length to be 2.75 mm. The linearabsorption coefficient of olive oil o ,and linear refractiveindex n 0 were determined form eq. (8) and eq. (9)respectively for the two aperture sizes as shown in table I. Theolive oil solution was filled into the glass cuvette of 1cm

thickness.


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Fig. 6: UV-VIS transmission of olive oil.

Table I linear absorption 0 and linear refractive index n o of olive oil.

apertureDiameter

Cuvettethickness

Transmittance%(T)

no 0 /cm

1mm,2mm

1 cm 95.5% 1.3577 0.046

A. NONLINEAR REFRACTIVE INDEXFor z-scan experiment, the transmittance through the

sample is monitored as a function of the incident laser lightintensity, with and without an aperture in the far field, whilethe sample is gradually translated along the optical axis of aconvex lens. The intensity dependent transmission through thesample measured as a function of sample position with respectto the focal plane, with an aperture (closed aperture (CA),fig.1) includes the effect of both nonlinear refraction andnonlinear absorption, if any, the division of the normalizedclosed aperture Z-scan data by the open aperture Z-scan data

generates a Z-scan profile pertaining to the purely dispersive(refractive) nonlinearity. Without an aperture (open aperture(OA),fig. 3 gives information about purely absorptivenonlinearity. The measurement of nonlinear refractive indexn2 and nonlinear absorption coefficient provide a directmeasurement of the real and imaginary parts of the third-ordernonlinear optical susceptibility (3).

The third-order nonlinear refractive index n 2 of olive oilwith different size aperture (1mm and 2 mm) and thenonlinear absorption coefficient , for the incident intensityI0= 6.83 KW/cm 2 were evaluated by the measurements of Z-scan. Fig. 7 shows the typical closed aperture Z-scan profiles.

Fig. 7 indicates to negative refractive index for the two sizes(1 and 2 mm) is attributed to a thermal nonlinearity resultingfrom the absorption of radiation at 532 nm, analogy to thetheoretical behavior in the dotted line in fig.2. The solidcurves in fig. 7 represent the best trend line.

Fig. 7.closed aperture Z-scan profile of olive oil with two different sizesaperture (1 and 2 mm)at I 0=6.83 Kwatt/cm 2.

Fig. 7 shows also that, the contribution of nonlinearrefraction to normalized transmittance decreases as thediameter of the apertureincreases and their peak-to-valleyseparation increases as the nonlinear phase shift increases,Table II. This result is good agreement with Shu-Qi Chen.[15].

Table II:Nonlinear refractive index parameters

Leff .,S,o , n 2,were calculated from equations 7,3, 2, 4respectively. T pv is the normalized peak and valleytransmittances were estimated from the relative positions ofthe peak and valley with Z as shown in Fig. 7.

B. NONLINEAR ABSORPTION COEFFICIENT:

Fig. 8 is The open aperture Z-scan was carried out todetermine the sign and magnitude of the nonlinear absorptionof olive oil for the incident intensity I 0= 6.83 KW/cm 2 showsthe typical open aperture Z-scan profiles.

Fig. 8: Normalized transmittance versus position at 532 nm wave length, 100mw input power foropen-aperture.

The linear transmittance is normalized to unity. When thesample is away from the focus Z=0, the incident laserintensity is low and the normalized transmission is close to 1;when it moves close to the focus, the open aperture Z-scansexhibits a reduction in the transmission which is symmetricabout the focus Z=0 . This is typical of an intensity dependent

Closed aperture z-sczn Technique for olive oil with different aperture sizes

0

0.5

1

1.5

2

2.5

3

3.5

-80 -60 -40 -20 0 20 40 60 80

Z in mm

T r a n s m

i t t a n c e

( a . u

)

Aperture diameter (1)mm

Aperture Diameter (2) mm

Trendline (1)mm

Trendline (2)mm

Open Aperture Z-s can for olive oil

0

0.2

0.4

0.6

0.8

1

1.2

-80 -60 -40 -20 0 20 40 60 80

Z in (mm)

T r a n s m

i t t a n c e

( a . u

)

Aperturediameter

R

3( cm/watt ) 2

T(z) (cm/watt)

Im 3

(esu) 3(esu)

1 mm4.83E-05

0.214

1.337 1.2E-03

0.001201

2 mm2.49E-05

0.214

1.337 1.2E-03

0.0012003


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The First National Conference for Engineering Sciences FNCES'12 / November 7-8, 2012 enhanced absorption termed reverse saturable absorption(RSA) or positive nonlinear absorption exhibited by anonlinear material [24], and can be exploited for protection ofeyes and sensors against radiation induced damage.

The experimentally determined values of , T pv, n2,) and 3are given in Table III.Table III: Nonlinear parameters for olive oil with two sizes aperture (1 and 2mm) by using CW laser at 532 nm.

Table III shows that the difference between the third-ordernonlinear optical susceptibility 3 values for the two apertureis very small as if it doesnt depend on the diameter of theaperture . R 3, , Im 3, 3, were determined from equations:

13, 10, 14, and 15, respectively.The experimentally measured n 2 of olive oil were compared

with other nonlinear liquid materials,such as Fuchsin dye in 1- Butanol solution. This material hada nonlinear refractive index -6.80 10 -8 cm2/W atcontinuous wave ( cw) laser illumination. [25].Qualitatively, these values were in good agreement with ourexperimental values ofolive oil. The highest nonlinearity ofolive oil can be used in photonic devices applications (e.g.Liquid-filled photonic crystal fiber).

With cw pumping we expect major contribution tothe observed third-order nonlinearities to be thermal in nature.The energy from the focused laser beam is transferred tosample through linear absorption and is manifested in terms ofheating the medium leading to a temperature gradient andthere by the refractive index change across the sample whichthen acts as a lens. The phase of the propagating beam will bedistorted due to the presence of this thermal lens. The peak valley separation of more than 1.7 times the Rayleigh range of~ 2.754 mm also suggests the presence of thermal componentin our case. It is well established that a separation of ~ 1.7z 0 indicates Kerr-type of nonlinearity.

It is worth noting that the value of 3 for the dye studied islarger than those of some representative third-order nonlinearoptical materials such as safranin O [1] dye and its derivativesand organic dyes like Mercurochrome [24].

C. OPTICAL LIMITING

The limiting effect of olive oil was studied by using a 100mW .CW laser at 532 nm. The experimental set-up for thedemonstration of optical limiting is shown in Fig. 9.

Fig 9: Experimental set-up for measuring limitingeffect.

A 1-cm quartz cuvette containing oliveoil is kept at the position where thetransmitted intensity shows a valley in

closed aperture Z -scan curve. Since the sample is a negativenonlinear material the valley point was closely behind thefocus. A polarizer was used to vary the input power. Anaperture A of variable diameter is used to control the cross-

section of the beam coming out of the sample cuvette. This beam is then made to fall on the photo detector (PD). Theinput laser intensity is varied systematically and thecorresponding output intensity values were measured by the

photo detector. At very high peak intensities (closer to thefocus) we could observe diffraction type pattern withconcentric ring structures probably due to self-phasemodulation. However, in limiting experiments we haveensured that there is no ring pattern formation by placing thesample away from focus. With open aperture experiment theaperture was removed.

The optical limiting curves obtained with a 100 mWcw laser

of wavelength 532 nm for olive oil with different aperturediameter and without aperture are shown in Fig. 10 and 11.For closed aperture experiment the output power rises initiallywith an increase in input power, but above 30 mw the output

power tends to be constant, because its nonlinear absorptioncoefficient increases with an increase in the incidentirradiance. In liquids, where the thermal expansion is large,high absorbance of the nonlinear material at the correspondingwavelength leads to an increase in temperature and density ofthe sample. Heating due to laser absorption is responsible forchanging the absorption coefficient and optical limiting effect.

The optical limiting effect shows an increase with smalldiameter aperture and that is due to large nonlinearsusceptibility. When the pinhole was removed optical limitingthreshold is above 60 mw. This means that optical limitingwith closed aperture is more effective than open aperture.

ApertureDiameter T% Leff. Tp. Tv. Tp-v S o n 2

1 mm 0.955 0.9773 3.25 0.14 3.11 0.199 8.134 1.033E-082 mm 0.955 0.9773 1.36 0.0213 1.3387 0.588 4.192 5.323E-09


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optical limiting of olive oil for open apertur e z_scan by using cw lase r at 532 nm

02468

101214161820

0 10 20 30 40 50 60 70 80 90input (mw)

o u

t p u

t ( m w

)

Fig. 10: Optical limiting effects of olive oil for closed aperture z-scan withtwo aperture dimeters (2 and 1 mm).

Fig. 11: Optical limiting effects of olive oil for open aperture z-scan.

IV. CONCLUSIONS

We have measured the nonlinear refraction index coefficient,n2, the nonlinear absorption coefficient, and susceptibility, (3) for olive oil using the Z -scan technique with 532 nm laserwith two different aperture diameters. The Z -scanmeasurements indicated that olive oil exhibited negativenonlinear optical properties. We have shown that thenonlinear absorption can be attributed to two photonabsorption process, while the nonlinear refraction leads to selfdefocusing in this oil. It is worth noting that the value of (3)for olive oil studied is larger than those of somerepresentativethird-order nonlinear optical materials such asorganic polymers and organic metal. (3)doesnt affectedwith the aperture diameter. Olive oil has large optical limiting

properties due to its largenonlinear susceptibility especiallywith small aperture size compared with the case where theaperture was removed.

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optical limiting ,closed aperture z-scan

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25 30 35 40 45 50 55 60 56input (mw)

o u

t p u

t ( m w

)

2 mm

1 mm


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The First National Conference for Engineering Sciences FNCES'12 / November 7-8, 2012 [23] Daniela Marciu, PhD Thesis, Virginia Polytechnic Institute and State

University, 1999.

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2012 Nonlinear Properties and Optical Limiting of Olive Oil by Using Z-scan Technique