Nonlinear refraction in semitransparent pyrolytic carbon films

Nonlinear refraction in semitransparent pyrolytic carbon films Tommi Kaplas,1,* Lasse Karvonen,2 John Rönn,2 Muhammad Rizwan Saleem,1,3 Sami Kujala,2 Seppo Honkanen,1 and Yuri Svirko1 1 3

Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland 2 School of Electrical Engineering, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland School of Chemical and Materials Engineering, University of Sciences and Technology, Sector H-12, Islamabad, Pakistan *[email protected]

Abstract: We report on Z–scan measurements of the nonlinear refractive index of semitransparent pyrolytic carbon films. The nonlinear refractive index of the film is as high as 8.23 × 10−9 cm2/W and shows a non– monotonous dependence on the film thickness. We demonstrate that, although the linear absorption coefficient of the pyrolytic carbon films is comparable to that of crystalline graphite, the nonlinear absorption coefficient of the films is much lower than that of graphene. ©2012 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (190.4400) Nonlinear optics, materials.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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Received 10 Oct 2012; revised 1 Nov 2012; accepted 5 Nov 2012; published 27 Nov 2012

1 December 2012 / Vol. 2, No. 12 / OPTICAL MATERIALS EXPRESS 1822

18. M. Sheik-Bahae, A. Said, T. Wei, D. Hagan, and E. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). 19. H. Zhang, S. Virally, Q. Bao, L. Kian Ping, S. Massar, N. Godbout, and P. Kockaert, “Z-scan measurement of the nonlinear refractive index of graphene,” Opt. Lett. 37(11), 1856–1858 (2012). 20. J. Rönn, L. Karvonen, S. Kujala, A. Säynätjoki, A. Tervonen, and S. Honkanen, “Third-order optical nonlinearities of Ag nanoparticles fabricated by two-step ion exchange in glass,” Proc. SPIE 8434, 84341K (2012). 21. L. Karvonen (Aalto University, School of Electrical Engineering, P.O. Box 13500, FI-00076 Aalto, Finland), J. Rönn, S. Kujala, Y. Chen, A. Säynätjoki, A. Tervonen, and S. Honkanen are preparing a manuscript to be called “Nonlinear optical properties of glass doped with Ag nanoparticles”. 22. G. I. Stegeman, E. M. Wright, N. Finlayson, R. Zanoni, and C. T. Seaton, “Third order nonlinear integrated optics,” J. Lightwave Technol. 6(6), 953–970 (1988).

1. Introduction Strong and fast optical nonlinearity of carbon nanotubes (CNT) and graphene that make these materials promising for a number of photonic and optoelectronic applications has attracted a widespread attention of the photonics community [1–4]. However, recent advances in manufacturing of CNT and graphene [3,4] have somehow overshadowed other nanocarbon materials with dominating sp2 hybridization of atomic bonds. Among these materials one may recall pyrolytic carbon (PyC) [5,6], which is composed of randomly intertwined nano–sized graphene flakes [6–8]. High thermal and electrical conductivity, chemical inertness, bio– compatibility and mechanical durability of PyC are well–known for more than 50 years and made material appealing for a wide range of applications. Until very recently PyC has been observed solely in bulk form, however, chemical vapor deposition (CVD) technique has opened novel routes to produce ultrathin and almost atomically smooth PyC films [7,8]. A simple and inexpensive manufacturing process, yet resulting in good optical transparency versus electrical conductivity, makes these films attractive for applications in optics and optoelectronics [7,8]. In the PyC synthesis, gaseous hydrocarbon molecules decompose at the temperature of about 1000 °C and start to form benzene derivatives that crosslink and form continuous carbon layers consisting of polycyclic aromatic structures with a typical size of 1–5 nm [7,8]. These nano–sized graphene flakes are randomly oriented and intertwined, making the PyC highly amorphous. The electrical conductivity of ultrathin PyC films is lower than that in single– and multi– layer graphene [3,7,8]. However, the main advantage of transparent PyC films over the graphene is that the catalyst–free synthesis enables fabrication on arbitrarily–shaped dielectric substrates in a single–step process [6–8]. The films have shown excellent properties in terms of the conductivity and transmittance, i.e. they can be employed as transparent electrodes in various optoelectronic devices. Moreover, since the sp2 hybridization of the carbon orbitals dominates electronic properties of PyC, one may expect that PyC films should possess strong and fast optical nonlinearity similarly to graphene and CNT. In this paper we report on the first measurements of the third–order optical nonlinearity of PyC films. By using femtosecond Z–scan measurements we studied the dependence of the nonlinear refraction and absorption coefficients as functions of the film thickness. In particular, we demonstrate that the third order optical susceptibility of a PyC film with a thickness of 10–40 nm is comparable or even higher than that in glass–metal nanocomposites [9–11]. 2. Sample preparation In order to investigate the dependence of the PyC nonlinearity on the film thickness we fabricated PyC films on quartz substrates using the hot–wall CVD system described in details elsewhere [8]. In the CVD process, the chamber was filled with hydrogen up to the pressure of 5.5 mbar and heated to 700 °C at the rate of 10 °C/min. At 700 °C the chamber was pumped down and CH4:H2 gas mixture was injected to the chamber. The CH4:H2 ratio determines the thickness of deposited PyC films (see [8] for more details). Following the gas exchange, the chamber was heated at the rate of 10 °C/min to 1100 °C. This temperature was

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maintained for 5 minutes and then the chamber was cooled down to 700 °C in 80 minutes. Rest of the cooling to room temperature was done in a hydrogen atmosphere. After the CVD process both sides of the quartz substrate were covered by the PyC film. Before measuring the optical properties of the films, the back side of the the substrate was etched in a harsh oxygen plasma (200 W / 20 sccm / 3 min) to remove the film. 3. Results and discussion 3.1 Linear optical measurements The thicknesses of the fabricated PyC films were measured using a stylus profiler Veeco Dektak 150. We studied linear and nonlinear optical properties of the films with thicknesses of 14, 18, 25, 30 and 41 nm. The thickness was determined by averaging results of measurements in ten points of each substrate. The standard deviation was less than 1.5 nm. The surface roughness, which was measured with the atomic force microscope (Thermo Microscopes, AFM Autoprobe M5), was as low as 1 nm (see [8] for more details). The optical transmission spectra were measured with a spectrophotometer (Perkin Elmer Lambda 9) within the 200 nm to 2000 nm wavelength range at normal incidence (see Fig. 1(a)). By measuring the transmittance T of the films with a different thickness we evaluated the linear absorption coefficient α of the films. Figure 1(b) shows the fitting of the transmittance data at 800 nm wavelength using the Beer–Lambert law T = exp(–αL), where L is the film thickness. The fitting gives a linear absorption coefficient α = 4.0 × 105 cm−1, which is close to the absorption coefficient of crystalline graphite (5.0 × 105 cm−1) [12]. At the wavelength of 800 nm, the skin depth for the PyC film is 25 nm. The complex linear refractive index was measured by a variable–angle spectroscopic ellipsometer (VASE, J. A. Woollam Co) with an unpolarized light beam with a spot size of 3 mm. The wavelength scan step was 10 nm in the wavelength range from 250 nm to 1700 nm. The back–side of samples was roughened in order to avoid back–side reflections. The ellipsometric experiments were performed at incidence angles of 65° and 75°. The optical constants of the samples were evaluated using a conventional Cauchy model [13].

Fig. 1. (a) The transmittance of the films decreases as the films are thicker. (b) Transmittance at the wavelength of 800 nm as a function of the film thickness follows the Beer–Lambert law.

The measured spectra of the real and imaginary parts of the refractive index of the synthesized PyC films are presented in Fig. 2. The thickness of the film appears to have no significant impact on the imaginary part of the refractive index. On the other hand, the real part of the refractive index varies about 40% when the film thickness changes from 14 to 41 nm. The observed effect may arise from the increasing amount of defects in thicker PyC films. Among these defects are e.g. sp3–bonded amorphous carbon inclusions and graphene flakes oriented perpendicular to the substrate plane. It is worth noting that similar effects have been observed also in sub–wavelength thick metallic films [14]. One can observe from Fig. 3 that, in the spectral range from 250 nm to 600 nm, the spectra of the n and k for the 41 nm thick film are different from those of the thinner films. This result

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was surprising because scanning electron microscopy revealed that all the manufactured samples have the same morphology. We suggest that such a difference in the optical properties may originate from the intrinsic stress, which increases with the film thickness. In order to prove this, we increased the deposition time to obtain a thicker film. However, we discovered that when the thickness of the PyC film approaches 50 nm, the intrinsic tension causes the film to rip out from the substrate and roll to µm–sized tubes.

Fig. 2. Spectra of the real (a) and imaginary (b) parts of the complex refractive index of deposited PyC films. Data for graphite [15] (a collection from different experiments) are also shown for comparison.

3.2 Z–scan measurement The optical nonlinearities were characterized using the femtosecond Z–scan technique [16– 18], which enables measurements of the nonlinear refractive index and nonlinear absorption coefficient. Figure 3 shows the sketch of the experimental setup. The measurements were performed by using a tightly focused Gaussian laser beam (Coherent Mira 900 Ti:sapphire laser, wavelength 800 nm with a repetition rate of 76 MHz and with 200 fs pulse width). The beam waist at the focus of the lens was 25 µm. The intensity in the focus was kept below 500 MW/cm2 (damage threshold of the PyC) a using neutral density filters. The sample movement along the beam axis was controlled by a computer. The sample transmission was measured by a photodiode D. An aperture A was placed in front of D for closed aperture measurements. A small part of the input intensity was monitored by another photodiode Dref and the ratio D/Dref was recorded as a function of the sample position z. The transmission with and without the aperture was measured as the sample moved through the focal point, enabling the separation of the nonlinear refractive index from the nonlinear absorption. The transmittance of the aperture was S = 0.28. In the open aperture Z–scan measurements (i.e. when the aperture A in Fig. 3 was removed from the setup), no dependence of the transmitted signal on the sample position was observed up to the intensity of 500 MW/cm2. That is, no nonlinear absorption occurred as long as the light intensity was lower than the damage threshold. This allows us to conclude that the imaginary part of the third–order nonlinear susceptibility in PyC is much smaller than that in graphene [19], despite the fact that in both materials the optical nonlinearity is dominated by the 2pz electron orbitals. Such a difference in nonlinear optical properties may originate from suppressing the delocalization of π–electrons and transformation of the band structure due to the small size of graphene flakes and the high concentration of amorphous carbon inclusions in the PyC film.

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Fig. 3. Optical setup for the Z–scan experiment. Laser light is divided with a beam–splitter into reference and transmission arms and focused by a lens L into the sample. The transmission of the sample S is monitored by a photodiode D. An aperture A is placed in front of D for closed aperture measurements. A small part of the input intensity is monitored by another photodiode Dref and the ratio D/Dref is recorded as a function of the sample position z [20,21].

In a closed aperture (CA) configuration, an aperture blocks a portion of the transmitted light, which to visualize the spatial distortion of the beam in the Z–scan experiment. The transmittance profile as a function of the on–axis position z in the CA Z–scan is given by [16– 18] T ( z, Δφ ) = 1 −

4 xΔφ , ( x + 1)( x 2 + 9) 2

(1)

where x = z / z0, z0 is the Rayleigh length. Δφ is the nonlinear phase shift,

Δφ = 2π n2 ILeff λ ,

(2)

where n2 is the nonlinear refractive index, λ is the wavelength, I is the light intensity at the sample and Leff = (1 − e −α L ) α ,

(3)

is the effective sample length. By assuming that the beam is Gaussian and the sample is thin (L