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 OSA
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 . 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  for more details). Following the gas exchange, the chamber was heated at the rate of 10 °C/min to 1100 °C. This temperature was 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  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) . 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 .
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 .
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 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.
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 , 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.
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]16–18]:
Figure 4(a) shows the results of the Z–scan experiments in the CA configuration. The fitting of the experimental results with Eq. (4) allow us to obtain the dependence of the nonlinear refractive index on the PyC film thickness. Note that we obtained very similar results from different points of each sample. This was expected due to the high thickness uniformity of the films. One can observe from Fig. 4(b) that n2 increases when the film thickness is smaller than 25 nm (which is close to the obtained skin depth of PyC), and decreases for thicker films. The 41 nm thick film shows the lowest nonlinear refractive index, n2 = 6.18 × 10−9 cm2/W. Such a non–monotonous dependence of the nonlinear refractive index on the film thickness may originate from the intrinsic tension, which is due to the different thermal properties of quartz and PyC and increases with the film thickness. By varying the average power of the laser beam from 45 to 125 mW we found that ΔTpv was a linear function of the average power of the laser beam within the interval of 45–125 mW. This indicates that the observed effect originates from the third-order optical nonlinearity of the PyC film.
The highest nonlinear refractive index, n2 = 9.34 × 10−9 cm2/W, was found for the 25 nm thick PyC film. It is worth noting that it is larger than the nonlinear refractive index of composites containing metallic nanoparticles. On the other hand, n2 in PyC is about one order of magnitude lower than that in graphene [9–11,19]. Importantly however that in both glass–metal nanocomposites and graphene the nonlinear refraction is accompanied by strong nonlinear absorption, while low nonlinear absorption coefficient makes PyC film promising material for the nonlinear optical and photonic applications .
In conclusion, by performing femtosecond Z–scan measurements we measured nonlinear refraction coefficient of the CVD PyC films of different thicknesses. We showed in particular that although the nonlinear refraction in PyC films is lower than in multilayered graphene, the remarkably low nonlinear absorption coefficient makes PyC films promising for various photonics applications. The obtained dependence of the nonlinear refraction coefficient on the film thickness indicates that internal stress accumulated in the film during the single–step deposition process may strongly influence the linear and nonlinear optical properties. Along with high dc conductivity this opens an interesting opportunities in photonic applications of semitransparent PyC films deposited on e.g. planar dielectric waveguides.
This work was supported by the Academy of Finland (Grants Nos. 140387 and 252461), FP7 “CACOMEL” project, the Graduate School of Modern Optics and Photonics (L.K.), Walter Ahlström Foundation (L.K.), strategic funding Initiative TAILOR of the University of Eastern Finland (R.S.) and Higher Education Commission (HEC) Pakistan (R.S.).
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