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Abstract
This research examines the thermally-induced nonlinear optical properties of Ti-Al-oxide nano-films. The Ti-Al nano-layer system was sequentially evaporated on glass and c-Si(100) substrates at room temperature. The Ti layers were spontaneously oxidized by the residual oxygen of the background vacuum in the deposition chamber. The Al layer, on the other hand, was partially oxidized and contained metallic inclusions. However, when oxygen was deliberately introduced into the chamber, the Al layer was oxidized with smaller inclusions. We experimentally demonstrate the existence of double epsilon-near-zero (2ENZ) behavior in the oxidized Al films by directly measuring the dielectric permittivity through ellipsometric analysis. The Maxwell-Garnett theory for a composite film formed by a mixture of Al and TiO2 inclusions in Ti-Al-oxide matrix was used to predict the two cross-over points of the ENZ behavior. Enhancement in the nonlinear optical response near the ENZ points was obtained.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The motivation for the nonlinear absorptive and refractive properties studies in nanocomposite materials, as well as their various optical properties, is based on their potential applications in optoelectronics, optical limiters, and photonics [1–5]. Metallic composite glasses such as Al doped TiO2 [6] and Cu or Ag doped Al2O3 [7] exhibits both high optical nonlinearity and low absorption and generally considered promising candidates in the above mentioned applications. Enhancements in the optical nonlinearities in these films can be obtained from two sources: surface plasmon resonance (SPR) [8,9] or epsilon near zero (ENZ) phenomenon [10]. In the SPR regime, an incident electromagnetic field can generate resonant collective excitations of the electron gas in the metal nanoparticles. The enhancement of the local electric field by the collective electron oscillations near the nanoparticles leads to an increase in the absorption intensity at a specific wavelength. The absorption results in considerable amplification of the nonlinear optical response in comparison to the bulk metals [4,11]. The nonlinear properties in this case depend on the properties of the nanoparticle such as size, shape and concentration [12,13]. In the case of ENZ enhancement, the material’s real part of the permittivity vanishes at a specific wavelength and results in group velocity slowing [14,15]. The slow-light mechanism increases the time for the light-matter interactions and promotes an effective increase in the nonlinear response [16]. In the current work, the increase in the nonlinear response of Al-Ti-oxide nano-films due to the ENZ behavior was investigated using z-scan technique. The deposited composite samples contained Al and TiO2 inclusions with different size distribution. The films with smaller size (<10 nm) exhibited a double epsilon near zero (2ENZ) behavior. The behavior is attributed to the polarizability of the inclusions in the hosting dielectric matrix. The nonlinear optical behavior of the fabricated films was investigated using z-scan technique in the following regimes: (1) the wavelength of pumping radiation falls near the ENZ region and (2) pumping wavelength is far from ENZ wavelength. Enhancement of the absorption saturation was observed in the ENZ regime. This behavior is outlined and the results are discussed.
2. Experimental procedure
The Ti-Al films were prepared at room temperature by vapor-phase deposition in a high vacuum chamber (base pressure of 2×10−6 Torr) equipped with multi-pocket e-beam evaporator (AJA international, Inc.). A deposition controller unit was used to gauge the deposition rate and the film thickness. The Corning 2947-75×25 Soda-Lime plain microscope glass slides and c-Si(100) substrates were ultrasonically rinsed in acetone bath before being placed into the evaporator. Ti (99.995%) and Al (99.999%) pellets from Kurt J. Lesker were placed in separate intermetallic crucibles and inserted into the evaporation system. Each metal was deposited at 1.0 Å/sec. The metallic layers were sequentially evaporated while keeping the substrate holder at room temperature. Three sets of samples were prepared and named based on the deposited layers: TiO2, Ti-Al, and Ti-Al-oxide layer. Figure 1 depicts the sequence of the evaporated layers during the fabrication process. The Al layer was sandwiched between the two Ti layers and all were evaporated sequentially on the glass and c-Si substrates. To obtain the Al-oxide in the samples, oxygen (99.999%) was introduced into the deposition chamber at 2 sccm just after the completion of the intermediate Al layer. The films were pulled out of the evaporator and annealed in vacuum for 30 minutes at 350°C.
Fig. 1. Schematics of the prepared samples: (a) TiO2 thin layers. (b) Al layer sandwiched between two thin Ti layers. (c) Al-oxide layer sandwiched between two thin Ti layers.
The films’ topographical features were subsequently characterized by scanning electron microscopy (SEM) using INSPECT F50 system (FEI Technologies Inc., Oregon, USA). The spectral dielectric function ($\varepsilon = {\varepsilon _1} + i{\varepsilon _2}$) where ${\varepsilon _1}$ and ${\varepsilon _2}$ are the real and imaginary parts of this function, were obtained by Woolman variable angle spectroscopic ellipsometry (VASE) system, (J.A. Woollam Co., Inc., Lincoln, USA) equipped with an adjustable compensator. The ellipsometric analysis was done using a three-phase model structure (i.e., air/ effective Al-Ti-oxide film/Si substrate) to fit the measured ellipsometric Ψ and Δ parameters at different angles of incidence. The fitting procedure was done using CompleteEASE software (version 4.58) where a B-spline layer with 13 knots was used to model the effective medium in the films. [17] The B-spline model provides a useful numerical tool for data fitting and can also provide a set of Kramers-Kronig consistent basis functions. These functions can be used to describe complex structures in the absorption (${\varepsilon _2}$) spectra, while simultaneously providing the physically correct (i.e. Kramers-Krong consistent) dispersion (${\varepsilon _1}$) spectra. [18] The corresponding thicknesses of the films and optical coefficients were obtained after running the fitting process. Figure 2 shows the measured Ψ and Δ parameters and the fitting curves (Fig. S1 of the supplementary materials show the fitting curves at the different angles of incidence). Table 1 includes the thicknesses of the different films along with the Mean Square Error (MSE) resulting from the fitting procedure.
Fig. 2. The fitting of spectral ellipsometric Ψ and Δ parameters at 70° angle of incidence. The three phase model generated these parameters represented by the solid lines in the figure.
Table 1. The measured thicknesses and mean square error (MSE) of the Ti-Al nano-alloyed films.
XPS measurements were conducted using an ESCALAB 250Xi (Thermo Fisher Scientific, UK) under ultrahigh vacuum (better than 5 × 10−10 Torr). The X-ray source was a monochromated Al Kα at 1486.68 eV and the pass energy is 20 eV for narrow spectra and 100 eV for survey spectra. The binding energy scale was calibrated using adventitious carbon reference at 284.8 eV. The crystalline structure in the films was investigated using X-ray diffraction system (D2 PHASER 2G, Bruker Co. Germany). The thermally-induced optical nonlinear response in the prepared samples was obtained using the z-scan technique pumped with two cw laser diodes operating at 532 and 650 nm.
3. Results and discussion
3.1 Material characterization
SEM results: the SEM images of the surface morphology of the fabricated samples on the c-Si substrates are shown in Fig. 3. The SEM image of the TiO2 samples shown in Fig. 3(a) exhibited a smooth surface with grain sizes between 1 to 2 nm as indicated in the associated Histogram. In this case, the granular structure can be interpreted in terms of the natural metallic growth of thermally evaporated films at room temperature without oxidation.
Fig. 3. SEM images of the fabricated films at room temperature. (a) TiO2 films. the SEM inset in the image shows a zoom-in illustration of the fabricated film. (b) The Ti-Al samples. The yellow arrows point to the large Al formations in films. (c) Ti-Al-oxide samples. The blue arrows point to the Al formations in the films. The particle size Histograms are included in each image.
Ti is a strong gettering element, therefore, as the film nucleates on the surface it collects residual oxygen and water vapor from the vacuum background and grow into a continuous amorphous layer of connected TiO2 formations [19]. On the other hand, the Ti-Al fabricated films showed smooth surface but with large clustered formations. These clusters were inclusions of Al in Al-oxide/TiOx matrix as will be verified below. The average size of these inclusions was 30 nm as indicated from the associated Histogram. As they were deposited, the Ti and Al layers were alloyed through spontaneous diffusion at the Ti/Al interface. The driving force in the diffusion process was a dipole-type interaction at the interface resulting from electronic redistribution due to realignment of the Fermi levels in the two materials. The built-in voltage at the interface drive an interdiffusion process causing the atoms to intermix [20]. The interaction continues until equilibrium is reached were a new chemical structure is established. This structure reflects percentages of the participating elements leading to a new compound material. In the samples, there were several interfaces created during the deposition Ti/Al/Ti, Ti/Al-oxide/Ti, TiO2/Al/TiO2, and TiO2/Al-oxide/TiO2. As the layers intermix in the fabricated samples, they formed new compounds based on the concentration of each participating metal.
XRD results: the crystalline structure of the fabricated films was verified by using XRD analysis. The prepared films showed amorphous structure except for the Ti-Al samples where a small metallic Al (111) peak was observed at 38.33° as shown in Fig. 4. The grain size t in the films was calculated using Scherrer formula as follows:[21]
where $\mathrm{\lambda } = $ 1.5405 Å is the wavelength of the X-Ray source used, θ is the Bragg diffraction angle and B is the FWHM of the XRD peak. The peak was fitted with three Gaussian peaks as shown in the inset of Fig. 4. The central peak having the maximum intensity gave a grain size of 30 nm which corresponds to the grain size estimated from the SEM analysis. The results revealed that metallic Al was present in the Ti-Al films as small clustered formations in the prepared samples. Nevertheless, the size of these formations was subject to presence of oxygen in the background vacuum. The effect of this oxidation was limited as the Al layer was capped with Ti layer. The capping layer prevented further oxidation in the Al layer allowing large inclusions to form in the films. On the other hand, when the Al layer was subject to deliberate oxidation, the Al layer was oxidized with smaller inclusions. Nevertheless, when these films were capped with Ti layer, the spontaneous intermixing of Ti and Al layers proposed earlier liberated some of the Al from the oxide layer and formed small TiO2 and Al inclusions in the Al-oxide matrix. The formation of this composite material will be verified later with the Maxwell-Garnett theory (MGT) through the predication of the ENZ behavior in the films.Fig. 4. XRD patterns of the fabricated films on the c-Si (100) wafer. The peak at 38.33° indicate the presence of crystalline Al in the Ti-Al films. The inset shows the Gaussian fitting of this peak. The c-Si (200) peak at 32.72° is from the substrate.
XPS results: the elemental composition of the alloyed films was investigated by the XPS measurements at room temperature. The high-resolution spectra of Ti 2p, Al 2p and O 1s of the Ti-Al and Ti-Al-oxide samples are shown in Fig. 5. The peaks cantered at 458.5 and 464.2 eV in Fig. 5(a)-(d) are attributed to Ti 2p3/2 and Ti 2p1/2, respectively which correspond to TiO2 chemical state. The splitting Δ-value of 5.7 eV between these two peaks provides an additional indication about the formation of Ti–O bonds. Furthermore, the Ti 2p peaks have kept the same intensities in the two sets, which serves as an evidence of the successful alloying of TiOx in the Al/Al-oxide matrix. Figure 5(b)-(e) show the chemical state analysis for Al 2p spectra of the two sets where the two components at 72.2 and 74.5 eV represents the metallic Al and the oxide related Al, respectively. The peak intensity of the metallic Al in the Ti-Al samples is obviously higher than the oxidized film. We can understand this difference as a result of the deliberate oxidation of the Al layer in the Ti-Al-oxide samples. Nevertheless, since the thickness in these films was less than 50 nm, it is expected that metallic Al formations to be present as inclusions in the Al-oxide/TiOx matrix. As indicated in the Histograms of Fig. 3, these inclusions range in size between 30 and 10 nm for the Ti-Al and Ti-Al-oxide samples, respectively. To determine the oxidation states in the films, high resolution of O 1s XPS spectra was obtained. Figure 5(c)-(f) revealed these spectra in the two samples: The data analysis fitting of the O 1s spectra has revealed three main O groups. The strongest peak located at 529.8 eV originates from oxygen bonds with Ti. The middle peak at 531.1 eV is related to Al2O3 and to TiOx defect oxide. The third peak centred at 532.3 eV is originated from adsorbed adventitious carbon species, which are commonly present as the samples being exposed to the ambient atmospheric air. The dominant chemical bonding of O 1s are metal-oxygen bonding, as the percentage of peak area in the samples is higher than 90%. It could be stated here that, although the XPS signals came from the thin surface layer in the samples (5-10 nm), the results can be representative of the whole layer due to the spontaneous intermixing of the samples during the deposition process. The gettering effect of Ti limits the oxidation of Al and enhances the alloying process in the films. [22,23]
Fig. 5. High resolution XPS spectra for Ti-Al samples: (a) 2p Ti (b) 2p Al (c) 1s O. The high resolution XPS spectra for the Ti-Al-oxide samples: (d) 2p Ti (e) 2p Al (f) 1s O.
3.2 Absorption and spectral ellipsometric analysis
Absorption results: Fig. 6 shows the UV-vis absorption spectra (Perkin Elmer-lambda 35 UV/VIS) of the fabricated films. The Ti-Al films had a broad peak at 855 nm. This peak is related to the interband transition absorption of Al [24]. On the other hand, there is a broad peak at 650 nm in Ti-Al-oxide sample which could be related to the SPR modes found in Al nanoparticles encapsulated with Al-oxide [25,26]. The broadness of this beak can be understood on the basis of multiple size distributions of the Al inclusions in the oxidized films. The TiO2 samples, on the other hand, showed only one peak at 386 nm related to the interband transition absorption of the TiO2 compounds [27]. The spectra of the samples were regularly measured in order to ensure the stability of these compounds and that they preserved the initial optical properties.
Fig. 6. The UV–vis absorption spectrum of the fabricated samples. The peak at 386 nm in the TiO2 and Ti-Al-oxide samples are attributed to the interband transition of TiO2 compound. The peak at 855 nm is attributed to the interband transition of Al.
Ellipsometric analysis: The ellipsometric properties presented in Fig. 7(a and b), shows the different spectral behavior of the real (${\mathrm{\varepsilon }_1}$) and imaginary (${\mathrm{\varepsilon }_2}$) parts of the complex dielectric constant $\; \mathrm{\varepsilon }$ in the fabricated films. Based on the XPS analysis and SEM images, one can draw the conclusion that the reason for the different optical properties is linked to the presence of Al and TiO2 inclusions with different size distributions (as will be verified using the MGT). The ${\mathrm{\varepsilon }_1}$ spectra reveled a normal dielectric behavior for the TiO2 samples and showed metallic behavior (Al-like) for the Ti-Al samples. It should be noted also that, the presence of TiO2 in the Ti-Al samples did not affect the metallic Al behavior in the ${\mathrm{\varepsilon }_1}$ spectrum as being observed. [28] On the other hand, the Ti-Al-oxide films exhibited a unique behavior yielding a 2ENZ behavior with 2 ENZ cross-over wavelengths around 320 and 505 nm. This unusual phenomenon is not influenced by the films’ crystallinity since these films showed an amorphous structure. We relate the double ENZ appearance in the films to the mixture of metallic Al and AlTiOx/TiOx phases in the films. Below the first cross-over wavelength at 320 nm, the oxidized films showed a dielectric behavior similar to that of TiO2 films. Above 320 nm up to the second cross-over at 505 nm, the films exhibits a metallic behavior with negative ${\mathrm{\varepsilon }_1}$ similar to the one reported for the Ti-Al films. As the films pass the 505 nm cross-over wavelength, they return back to the dielectric behavior similar to the TiO2 films. Baric and his group observed similar and tunable double ENZ behavior in Ti oxynitride thin films. [29] In order to understand the influence of mixing metallic inclusions in an oxidized host material, they reverted to the Maxwell-Garnett formalism to simulate the experimentally observed optical properties. [29] following the same approach, we have simulated the effective media dielectric function using the following formula:[30]
Fig. 7. (a) Real and (b) imaginary dielectric constants of the films. The peaks in the imaginary part of the spectrum are consistent with the ones obtained from the UV-vis measurements. The dashed curves in both spectra are the dielectric constant obtained for the simulation of Eq. (2).
Although the filling and the depolarization factors used in the calculation predicted the 2ENZ cross-over points, In order to have a complete fitting of the ellipsometric ${\mathrm{\varepsilon }_1}$ and ${\mathrm{\varepsilon }_2}$ curves more terms should be incorporated in modeling Eq. (2) with different filling and depolarization factors. Nevertheless, the simple model described here predicts the double ENZ behavior with only one size and fill factor for the Al and TiO2 inclusions in Al-oxide host medium. It is therefore likely that the observed 2ENZ behavior arises from plasmonic polarizability of the inclusions. The results also suggest that the double ENZ behavior appears only in the case of small metallic inclusions in the Al-oxide/TiO2 matrix, while the films that has been oxidized with vacuum background oxygen (films with large Al inclusions) did not result in any ENZ behavior. The spectra of all samples were regularly measured in order to ensure the stability of the prepared compound in the films and make sure they maintained their features.
3.3. Thermally-induced nonlinear optical properties
The thermally-induced nonlinear optical properties of the fabricated films were obtained using the z-scan technique in the cw regime. The closed aperture z-scan traces of the nano-alloyed films are presented in Fig. 8 at two operating wavelengths: namely at 532 and 650 nm operating at 100 mW. As shown in the figure, the scan traces exhibited a negative nonlinear lensing effect indicated by a peak, followed by a valley trend. Figure 9 shows the open-aperture z-scan measurements of these films where a saturable absorber (SA) behavior is indicated at the two stimulating wavelengths. It is seen here that the open-aperture scan exhibited higher SA peak at 532 nm, in particular for the Ti-Al-oxide samples being closer to one of the 2ENZ wavelengths at 505 nm [16]. The SA behavior in the fabricated films can be explained as follows: during the irradiation process, the absorbed thermal energy of the Al and TiO2 inclusions transfers to the surrounding dielectric oxide through non-irradiative relaxation to the ground state. This relaxation leads to increase in the local temperature within the region where the laser beam is being focused [34]. The generated temperature changes the local refractive index where it becomes temperature-dependent. The accumulated heat generates large changes in the local density of the films and result in substantial increase in the optical nonlinearity of the films [35]. The time scale of these processes is limited by the thermal diffusion and heat transient mechanism. In isolated nanostructures, the generated heat during the z-scan radiation transfer mainly through evanescent photon tunneling and freely propagating modes [36]. The amount of the transferred heat (QBB) can be calculated using Stefan’s law for hot black surfaces at temperature T:[37]
where ${k_B}$ is Boltzmann’s constant and $\hbar $ is Plank’s constant, c0 is the speed of light in vacuum. Continuous composite nano-films are expected to be good heat conductors since they have many thermal conductive channels due to fluctuations in the electron density states of the composing metals [38].Fig. 8. Closed -aperture z-scan traces at the two operating wavelengths for (a) Ti-Al and (b) Ti-Al-oxide films. The pumping power at each wavelength was 100 mW.
Fig. 9. Open-aperture z-scan traces at the two operating wavelengths for (a) Ti-Al and (b) Ti-Al-oxide films. The black arrow indicates the enhancement in the nonlinear response when the 532 nm beam is used in the z-scan setup. The pumping power at each wavelength was 100 mW.
Based on the grain size and XPS analysis and the calculation made using the Maxwell-Garnett Theory of composite materials, there was two kind of metallic inclusions in the films, small and larger ones. It is highly suggested that electronic fluctuations allowed for the thermal power to concentrate inside the films within the small and large metallic regions. Many of these fluctuations make the participating electrons in the conduction band very energetic, and their temperature can rise to several hundred degrees [35]. These “hot electrons” have different Fermi-Dirac distribution and they are not in equilibrium with the lattice. The difference in the distribution of the hot electrons below and above the Fermi level modifies the absorption, giving the nonlinear absorption coefficient a positive sign in the films. However, within few picoseconds, the hot electrons relax, through electron-phonon coupling process, the heat they release dissipates into the lattice and the surrounding environment through phonon-phonon interactions [39]. In z-scan measurements of nanoparticles, when the stimulating laser wavelength is far from the plasmonic bands, the conduction band electrons oscillate at varies frequencies. However, with increase in the free electron fluctuations due to enhanced thermal absorption, resonance with the laser wavelength becomes available and the electrons start to exhibit plasmonic-type behavior [40]. This electronic behavior leads to the bleaching of the conduction band and results is the observed SA transmittance in the open-aperture runs [39].
It should be mentioned here that this behavior was observed for the films with the small and large Al inclusions since the electronic fluctuations at the laser wavelength were available in both films. It is highly suggested that this was the nonlinear mechanism available for both the Ti-Al and Ti-Al-oxide films when the 650 nm beam was used to stimulate the nonlinear behavior in these films. On the other hand, when the 532 nm beam was used pump the z-scan setup, the open-aperture scans for the Ti-Al-oxide films showed enhanced behavior over the Ti-Al films. It is expected that this enhancement in the nonlinear response of the Al-Ti-oxide samples is related to the ENZ response as this wavelength is closed to the ENZ point at 505 nm in these films. The enhancement in this case is due to improvements in the longitudinal electric fields on the boundary of ENZ media (according to the boundary conditions for electric field displacement). [28,41–43] To quantify the optical nonlinear response, the conventional z-scan relations were used:[44]
where $\Delta {T_{p - v}}$ is the transmittance difference between the peak and valley in the closed-aperture trace, $\Delta \varphi $ is the axial phase shift, $n_2^{th}$ is thermally-induced third-order nonlinear refractive index, $\lambda $ is the wavelength, $\partial n/\partial T$ is the thermo-optic coefficient, ${K_{eff}}$ is the effective thermal conductivity constant, β is the nonlinear absorption coefficient, I0 is the intensity of the beam at the focus, r is the aperture radius, ${\omega _a}$ is the beam radius on the aperture, α is the linear absorption coefficient, and L is the sample thickness, $\Delta {T_{op}}$ is the peak or valley in the open-aperture z-scan trace. The samples were fixed on a pc-controlled translation stage and moved in the propagation direction (z-axis) of a narrowly focused Gaussian beam using 50 cm focusing lens. The beam waist (${\omega _0}$) at the focal plane for the green (532 nm) and red (650 nm) lasers was calculated to be 46.1 and 50.9 µm at the two wavelengths, respectively. The transmitted beam passing the thin films as a function of its position relative to the lens focus was detected in the far field for the normalization process. The calculated values of the third-order thermally-induced nonlinear coefficients using Eqs. (5)-(10) are included in Table 2. The high nonlinear absorption coefficients obtained for the Ti-Al and Ti-Al-oxide are related to the high increase in the number of electrons available to participate in the nonlinear absorption process through the different nonlinear mechanisms [39]. An important observation can be made here: due to the unique thermal nonlinear optical properties in these films, they could be engineered as platforms in thermo-plasmonic applications, where saturable absorber is required but with limited nonlinear refraction as in beam steering applications [45–47]. The Z-scan measurements were repeated at different locations throughout the film’s surface and the obtained results were consistent.4. Conclusion
To summarize, the thermally-induced nonlinear optical properties of Ti-Al-oxide nano-films were investigated. Ti-Al nano-films were oxidized spontaneously by the vacuum background oxygen in the deposition chamber and deliberately by introducing oxygen into the deposition chamber. The oxidized films exhibited a double ENZ behavior observed by ellipsometric measurements and verified by the Maxwell-Grant theory of composite films. The calculation made using this theory was performed suggesting the films formed by inclusions of Al and TiO2 in Al-Oxide matrix. An enhancement in the saturable absorber trace of the z-scan measurements of the oxidized films was observed near the ENZ point of the fabricated films. These films furnish a material of choice for thermal optical nonlinear processes in advanced thermo-plasmonic applications.
Funding
German-Jordanian University (SBSH 2016/32).
Acknowledgments
The authors would also like to acknowledge the characterization support by QEERI Core Labs in Qatar.
Disclosures
The authors declare no conflicts of interest.
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