IJAIEM-2013-05-20-045 by editorijettcs


									International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013                                             ISSN 2319 - 4847

    Femtosecond Z-scan measurements of three-
   photon absorption in TiO2 thin film prepared by
               pulse laser deposition
                                                      Raied K. Jamal1
                           Baghdad university, collage of science, physics department, Baghdad, Iraq

Titanium dioxide (TiO2) thin film of 2 μm thickness have been grown on glass substrate by pulsed laser deposition technique at
substrate temperature of 500oC under the vacuum pressure of 8×10-2 mbar. The optical properties concerning the absorption,
and transmission spectra were studied for the prepared thin film. The structure of the TiO2 thin film was tested with the X-Ray
diffraction and it was formed to be a polycrystalline with recognized peck oriented in (101), (110), (004), (211), and (200).
Three photon absorption (3PA) in thin film was measured by Z-scan technique using a femtosecond Titanium-sapphire laser
100 fs at wavelength 800 nm and 60 mJ/cm 2 power. We fit the data with an extended model that includes multiphoton
absorption, beam quality, and ellipticity. The extracted three-photon absorption coefficient is 0.0586 cm 3/Gw2. The results show
that the TiO2 thin film has great potential applications for nonlinear optical device.

Keywords: TiO2 thin films, Nonlinear optics, Three photon absorption

Thin films with large three-photon absorption nonlinear susceptibility X(3) and ultrafast response have been a topic of
much interest due to their potential application for all optical switching, light controlled phase, refractive index
modulation, and optical power limiting devices [1,2]. Metal oxide thin film have attracted growing interest due to the
high optical nonlinearity, fast response time, low absorption index, good thermal and chemical stability.
TiO2 is known to exist in nature in three different crystalline structures phase: brookite, anatase and rutile, among them
brookite phase has an orthorhombic crystalline structure. However this is unstable phase because of this, it is of very
little interest. Both anatase and rutile phase have a tetragonal crystalline structure. The anatase and rutile are stable
phase with mass densities of 3.84 and 4.26 gcm-3 respectively. In general rutile phase is formed at high temperature,
while anatase phase is fomed at low temperature. Anatase and rutile TiO2 films have been widely haracterized for their
potential applications in solar cells, Self cleaning coatings, photocatalysis. In fact, the nonlinear optical properties in
TiO2 films have been also studied and the values of X (3) were reported to 4×10-12 and 2.4×10-12 esu for rutile and
anatase film by the third harmonic generation method [3] respectively. The experimental results were found to be in
excellent agreement with the calculated X3 by lines model [4]. In recent year, Gayvoronsky et al. reported the large X3
(2×10-5 esu) in nanoporous anatase films prepared by sol-gel method measured using a Nd-YAG laser (1064 nm, 42 ps)
[5]. However, up to now, nonlinear optical properties of the pure anatase and rutile TiO2 films excited by ultrafast
intense laser radiation (<100 fs) have not been reported.
In this paper, the TiO2 films have been fabricated by pulse laser deposition (PLD) technique on glass substrate. The
three-photon absorption coefficient of the film was measured by open aperture (OA) z-scan method [6] using a
femtosecond laser (100 fs), and 60 mJ/cm2 maximum fluence.

2. Experimental details
The thin film was prepared from TiO2 powder. The powder was compact into pellet which was put it in oven at 600 oC
for 1 h. The experiment was carried out in a typical PLD configuration. The technique was first used by Smith and
Turner [7]. In our work , an UHV of about 109 mbar, an excimer laser LPX110i (Lambda Physik) with KrF radition
(wavelength 248 nm, pulse duration 30 ns) glass substrate. A thin film was synthesize under argon atmosphere with
power 500 mJ and working pressure were kept constant at 8×10-2 mbar. In order to avoid fast drilling, the target was
mounted onto a rotating holder, 45 mm from the substrate, which was also put onto a rotating holder to improve the
uniformity of the film.
The structure and orientation of the film were determined by X-ray diffraction (XRD, θ-2θ scan with Cu Kα radition at
1.54 Ao was used). Optical transmission spectra were performed using a UV mate SP-8001 double beam
spectrophotometer in the wave range from (190 nm to 1100) nm.

Volume 2, Issue 5, May 2013                                                                                         Page 451
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013                                             ISSN 2319 - 4847

The nonlinear absorption study at the near resonant regime was carried out using a fully computerized single beam
femtosecond (OA) Z-scan technique. A femtosecond laser of pulse duration 100 fs and maximum fluence 60 mJ/cm2
was used as a laser source. The pulse duration was measured by autocorrelation system and the energy was measured by
pyroelctric energy prob model type (PDA36A), covering the rang 350-1100 nm from THORLABS. The beam profile
was adjusted by spatial filter leading to spatial intensity profile near Gaussian with beam quality of M2 ≈1.7. The laser
beam was focused by a lens of 15 cm focal length to produce a waist of 37 μm. The sample was translated along the
beam axis (z-axis) through the Rayleigh distance 2.1 mm.

3. Result and discussion
The XRD pattern of a TiO2 thin film prepared with 2 μm thickness is illustrated in Figure 1. The spectrum indicates
that the Thin film is a polycrystalline structure. The highly oriented along the (101) direction implying that the crystal
orientation is mostly along the c-axis perpendicular to the substrate surface. The table 1 shown many dominant
strongest peaks with their d spacing, FWHM, and diffraction angle values. The mean grain size of thin film calculated
using the Scherrer’s equation [8]:

                                G =0.94 λ / β cosθ                                    (1)

Where G is the average crystalline grain size, λ is the wavelength, β represents the full- width at half maximum
(FWHM) in radian and θ is the Bragg diffraction angle in degree. The calculated values of grains size for TiO2 thin
film are shown in table 1. The optical properties of TiO2 film which was prepared on glass substrates has been studied
in this work. The absorption and transmission Spectra of the TiO2 film in the spectral range (250-900) nm are shown in
Figures 2, 3 respectively. The absorption spectrum shows low absorption in the visible and infrared regions; however,
the absorption in the ultraviolet region is high. The optical transmission spectrum of the TiO2 thin film is shown in
Figure 3. It can be noticed from this Figure that the transmission is high in the visible and infrared regions. The Z-scan
transition curve at 60 mJ/cm2 max fluence is recorded for the TiO2 thin film and it shown in Figure 4.

                              Figure 1 XRD pattern of thin film despite of glass substrate

         Table 1 shown all peaks and its Bragg's angle, interplanar distance, and full width half at maximum
                      peak      (hkl)      2 Theta        d(A0) FWHM              G (nm)
                      No.                  (deg)                  (deg)
                      1         (101)      25.37          3.50    0.41            22.23
                      2         (110)      27.49          3.24    0.39            23.46
                      3         (103)      36.12          2.48    0.32            32.03
                      4         (004)      38.65          2.32    0.61            17.3
                      5         (111)      41.29          2.18    0.31            34.98
                      6         (200)      44.05          2.05    0.40            28.65
                      7         (211)      53.94          1.69    0.42            33.22

Volume 2, Issue 5, May 2013                                                                                   Page 452
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013                                             ISSN 2319 - 4847





                                       Absorption %





                                                                   250          350         450         550       650   750       850

                                                                                                  Wavelength (nm)
                                                                        Figure 2 Optical absorption spectrum of the TiO2 thin film




                                                 Transmission %







                                                                   250          350         450         550       650   750       850

                                                                                                  Wavelength (nm)

                                                                       Figure 3 Optical transmission spectrum of the TiO2 thin film

                                                                   1                       Maximum fluence 80 mJ/cm2
                      Normalized transmittance


                                                                                                    γ=0.0586 cm3/Gw2

                                                                       -4      -3     -2           -1         0     1   2     3         4
                                                                                                    Z- position (cm)

   Figure 4 OA Z-scan measured at maximum influence 60 mJ/cm2, wavelength 800 nm, pulse duration 100 fs and
              repetition rate of 10 kHz. The solid line is the fitting curve employing the z-scan theory.

The normalized energy transmittance for 3PA of the open aperture z-scan is given by R. L. Sutherland et al [9]:

Volume 2, Issue 5, May 2013                                                                                                                 Page 453
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
       Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013                                             ISSN 2319 - 4847

                                                  ln 1  p                                              
                    T ( z) 
                                    1/ 2
                                                                o   exp( 2 x 2 )   
                                                                                    1/ 2
                                                                                            p o exp( x 2 ) dx   (2)

Where po=(2γIo2 Leff')1/2 , Io=Ioo/(1+z2/zo2) is the excitation intensity at the position z, zo=πωo2/λ, where z0 is the Rayleigh
range, ωo is the minimum beam waist at focal point (z=0), λ is the laser free-space wavelength, Leff`=[1-exp(-2αoL)]/2αo
is the effective sample length for 3PA processes; L is the sample length and αo is the linear absorption coefficient. The
open aperture Z-scan graphs are always normalized to linear transmittance i.e., transmittance at large values of |z|. The
3PA coefficient can be extracted from the best fit between equation (2) and the experiment (OA) Z-scan curve. If po<1
equation (2) can be expanded in a Taylor series as:

                                        m 1          p o m 2
                  T   (1)                                                                                        (3)
                        m 1                   (2m  1)!(2m  1)1 / 2

Furthermore, if the higher order terms are ignored, the transmission as a function of the incident intensity is given by
R. L. Sutherland [9]:

                          I o2 L'eff
                T 1                                                                                                   (4)
                               33 / 2
The sold curve in Figure 4 is the best fit for equation (4). The equation (4) shows clearly that the depth of the
absorption dip is linearly proportional to the 3PA coefficient γ , but the shape of the trace is primarily determined by
the Rayleigh range of the focused Gaussian beam. The fitted value of γ is on the order of 0.0586 cm3/GW2. This value
is ten times of magnitudes higher than the value observed with bulk TiO2 sample.

4. Conclusion
The three photon absorption has been observed in TiO2 thin film prepared by PLD method upon illuminating it by
femtosecond Titanium-Sapphire laser. The fully computerized Z-scan system was used to measure the nonlinear
absorption coefficient of the prepared samples. The value of the measured nonlinear coefficient was found to be ten
times higher than the bulk value.

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     Electron. 28(4), 875-882 (1989).
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[4.] M. E. Lines, "         ," Phys. Rev. B 43, 11978 (1991).
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     ,Appl. Phys. B 80, 97 (2005).
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[7.] H. M. Smith, A. F. Turner, Appl. Opt. 4, 147 (1965).
[8.] Patterson, A. L. (1939). Phys. Rev, 56, 978.
[9.] Sutherland, R. L., McLean, D. G., & Kirkpatrick, S. (2003). Handbook of Nonlinear Optics. Second Edition.
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             Raied K Jamal received the B.S., M.S. and PhD. degrees in Physics department from collage of science,
             Baghdad university in 1998, 2001 and 2012, respectively. During 1998-2012, he stayed in Laser and
             Molecular Laboratory (LML) also worked at Electro-optics Laboratory (EOL). Ministry of higher
             education in Iraq.

Volume 2, Issue 5, May 2013                                                                                                   Page 454

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