Waveguide Spectroscopy of Thin Films (Thin Films and Nanostructures)

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View on ScienceDirect. Authors: Alexander Khomchenko. Hardcover ISBN: Imprint: Academic Press. Published Date: 19th December Page Count: View all volumes in this series: Thin Films and Nanostructures. Moreover, the smooth surface of the planar geometry allows for a high uniformity of the polymer layer i. In an ultimate approach, one can imagine to combine the G-WET effect to the solutions proposed by Rose et al. Figure 2 c depicts an extremely thin polymer layer coated on a nanostructured active layer. Nanostructured materials present many interesting properties that can be beneficial for sensing applications.

For instance, densely stacked nanostructures, with an extremely small lattice parameter to emission wavelength i. On the other hand, nanostructures with a lattice parameter that is comparable to the emission wavelength i. For instance, such nanostructures can exhibit optical feedback properties that could help decrease lasing threshold and thus reach more efficient lasing action [ 20 , 21 ].

Coating the polymer layer on a nanostructured active layer can thus lead to an important enhancement in polymer sensitivity resulting simultaneously from the enlarged sensing surface, the optical modulation, and G-WET. However, sensing surface, optical modulation, and G-WET properties of the structure are tightly linked to the structure parameters. G-WET is mainly insured by the optical properties of the active substrate [ 19 ]. However, a fine tuning of the structure parameters is essential in order to maintain the waveguiding configuration that is crucial for G-WET.

Similarly, specific surface, which is purely a geometrical effect, and optical feedback, which results from the refractive index modulation, are defined by the structure parameters. However, sensing surface, optical modulation, and G-WET properties do not behave similarly and, therefore, present optimum values for differing structure parameters.

Therefore, the optimum structure parameters can only be the result of a compromise between the three considered effects. However, such a compromise requires a profound knowledge of the weight of each effect on polymer fluorescence. The determination of optimum structure parameters is thus a major theoretical and experimental challenge.

In addition to modeling challenges, nanostructured substrates present several processing difficulties. Nanostructuring often requires the application of complex bottom-up and top-down techniques, therefore increasing production time and cost. Moreover, nanostructured substrates can induce non-uniformity in the sensor, which requires an optimization of the polymer coating process. Herein, we present theoretical and experimental validation of the G-WET concept for planar geometries. Concerning the extended G-WET concept, we restrict our study to preliminary results on the enlarged sensing surface and photonic properties of the nanostructures.

To validate the concept, we consider the case of poly[dimethyl-co-methyl- 1,1,1-trifluoro trifluoromethyl oxy-pentyl -co- methyl- 4-pentyloxy- N- 2,5-di-tert-butylphenyl -1,8-naphthalimide ]-siloxane [ 22 ]. The polymer structure consists of a non-emissive polysiloxane backbone to which is introduced a fluorescent 1,8-naphthalimide moiety. The naphtalimide moiety presents a maximum absorption wavelength at nm and a maximum emission wavelength at nm. As reported in [ 22 ], the naphtalimide fluorescence is quenched in the presence of nitroaromatic compounds, such as DNT and TNT, due to the electron transfer between the electron-rich naphtalimide and the electron-poor nitroaromatic compound.

The polymer synthesis process is decribed in detail in [ 19 ]. Under low pumping intensities, defect-free ZnO thin films exhibit a free-exciton emission peak, denoted E ex , at a wavelength of nm 3. Moreover, room temperature stimulated UV emission can occur in high quality ZnO thin films at high pumping intensities [ 24 ]. The stimulated ZnO emission is characterized by the rapid appearance of sharp and red-shifted emission peaks, displaying a super linear increase with pumping intensity [ 23 ].

ZnO thin films can exhibit two distinct stimulated emission peaks, denoted P and N. The P -line is generally attributed to an exciton-exciton collision process in which an exciton recombines to generate a photon after transferring, by inelastic collision, part of its energy to another exciton that is scattered to a continuum of states.

The P -line exhibits a fixed red-shift of the peak energy and an 8th power PL dependence on pumping intensity. On the other hand, the N -line is associated to radiative recombination of electron-hole plasma EHP. EHP is observed at extremely high pumping intensities, capable of generating carrier densities which exceed the Mott transition density, therefore resulting in a dissociation of the excitons due to strong coulomb interactions.

The N -line is characterized by a non-constant red-shift of the peak energy and a 5th power PL dependence [ 23 ]. Stimulated emission is normally associated to optical gain. The high refractive index is another interesting feature of ZnO. Ellipsometry studies on ZnO thin films reported a real part refractive index ranging between 2 in the visible to 2.

The refractive index of sapphire is usually around 1. Thus, a ZnO thin film grown on sapphire is a dielectric planar waveguide capable of supporting guided modes. The waveguiding configuration of ZnO thin films is a keystone feature for stimulated emission, as it insures the optical feedback necessary for lasing.

This is further demonstrated by the disappearance of the stimulated emission in ZnO thin films with thicknesses below the cut-off [ 25 ]. ZnO is therefore chosen as the active material as it verifies all the criteria that are required to have an efficient G-WET effect. ZnO thin films were grown on sapphire substrates in order to realize the experimental study.

The growth process by which the ZnO thin films were obtained is detailed in [ 19 ].

Waveguide Spectroscopy of Thin Films

It is important to stress that most of the fluorescent polymers which are used for explosive chemosensing applications are excited by UV-blue light [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 22 ]. The use of ZnO thin films and nanostructures therefore presents a general photonic solution for the enhanced sensing issue. Theoretical modeling is essential prior to any experimental investigation, in order to fully understand the various physical phenomena at play and optimize structure parameters.

The theoretical modeling, presented herein, considers the case of an extremely thin 5 nm FSP layer coated on a ZnO thin film grown on a sapphire substrate. The applied model was shown to present good agreement with experimental results [ 33 ]. Section 2.


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Reprinted with permission from [ 18 ]. Figure 3 reveals that a ZnO excitation optimum exists for each of the considered wavelengths.

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At these thicknesses, the ZnO thin film is a single-mode waveguiding slab. Figure 3 shows that the gradual redshift of the ZnO emission from nm to nm results in a drastic increase in the ZnO excitation.

Volume 9 Issue 1 | Journal of Nanophotonics

Figure 3 shows that guided ZnO emission has more important impact on the FSP layer excitation compared to the outwards-emitted ZnO emission. ZnO guided emission provides the most efficient way to excite the FSP layer, as it allows for the FSP layer to be repeatedly excited via the tail of the evanescent wave. Polymer fluorescence was collected using an objective lens with a 0. Neutral density filters were used in order to control the pumping intensity.

Peak intensities are normalized to the maximum peak intensity presented by S0 i. Figure 4 shows that the FSP layer coated on quartz i. The fluorescence saturation of S0 is attributed to the absorption saturation of the FSP layer lying under direct laser excitation [ 19 ]. Reprinted with permission from [ 19 ]. Copyright American Chemical Society. Experimental investigations [ 19 ] show that the superlinear dependence of the FSP fluorescence is tightly linked to stimulated ZnO plasma emission i.

The fluorescence enhancement, in the case of S1, is attributed to an efficient excitation of the polymer layer lying outside of the area defined by the laser spot via the waveguided ZnO stimulated emission i. The geometrical effect is confirmed by photoluminescence PL measurements shown in Figure 5. Evolution of the photoluminescence PL spectra for the polymer fluorescent film coated on quartz, S0 sample a and on the ZnO layer, S1 sample b as a function of the distance d separating the center laser spot and the axis of the collection objective.

The inset between the two figures presents a sketch of the evolution of d. Figure 5 shows the evolution of the PL spectra collected on S0 a and S1 b as the misalignment d between the collection axis and the laser spot is gradually increased. The value of d is indicated in the legend of each graph and is illustrated in the inset of Figure 5.

The PL setup, considered here, has a collection spot area diameter of 3 mm and a laser spot diameter of 1mm. Figure 5 a shows that, in the case of S0, the fluorescent polymer area is solely defined by the laser spot. For S0, the polymer layer is uniquely excited by the laser beam. On the other hand, Figure 5 b clearly shows that, in the case of S1, the fluorescent polymer area is strictly larger than that of the laser spot. The polymer fluorescence, in the case of S1, results of two fluorescing areas.

A smaller area, defined by the laser spot depicted by the dark green star in Figure 1 b , that is mainly excited by the laser beam; and a larger area, defined outside of the laser spot depicted by the light green in Figure 1 b , that is mainly excited by the waveguided ZnO stimulated plasma N -line emission at nm.

Meanwhile, the spectral filtering i. The experimental investigation reveal almost one order of magnitude fluorescence enhancement by spincoating the FSP on a ZnO thin film. Furthermore, the experimental investigation proves the important role of waveguided ZnO emission in having an efficient fluorescence excitation of extremely thin FSP layers. The experimental results are in good agreement with the previously discussed theoretical results which also revealed the importance of waveguided ZnO emission. However, the theoretical modeling predicted a fold enhancement of the polymer fluorescence.

The prediction is overestimated mainly due to the fact that the theoretical modeling did not account for the real values of the polymer and ZnO quantum yield. As previously mentioned, nanostructured materials present many interesting properties which can be extremely beneficial for sensing applications. In the following, we present a preliminary theoretical study on the enlarged sensing surface and photonic band-gap of ZnO nanostructures.

The theoretical study, presented henceforth, considers the case of hexagonally distributed ZnO nanorods and perforated ZnO thin films. Hexagonal distribution is chosen as it presents the most compact packing arrangement and therefore the most interesting geometric and photonic properties. Nanostructured materials can exhibit extremely high specific surfaces which can help increase polymer sensitivity by enlarging the polymer sensing surface and enhancing fluorescence signal without diminishing their response time and quenching efficiency [ 16 ].

We define the gain in sensing surface R G as the ratio between the surface area of a FSP layer coated on a nanostructured support and that of a FSP layer coated on a smooth plan. Considering a homogeneous FSP coating over the whole coated area, R G , of any given geometry, can be determined through simple geometrical calculations. For the figure insets hereafter grey is for ZnO and white is for air.

The calculated values of R G are obtained for a fixed polymer thickness of 5 nm. However, for any given value of h and r differing from 0, R G presents a value higher than 1.

Nano-engineered coatings and thin films: from design to applications

In other words, any array of nanorods, no matter the structure parameters, will exhibit an enlarged sensing surface compared to a smooth plane. In general, Figure 6 shows that the enlarged sensing surface i. As seen in Figure 6 a,b, the optimum value of R G shifts towards higher values of r as d increases. Nonetheless, this remains an important enhancement factor. It is essential to stress that a facile technique for the growth of urchin-like ZnO nanostructures was recently reported by our team [ 35 ].

The grown urchin-like structures exhibited a high aspect ratio with ZnO nanorods of a diameter of around 15 nm and a length of nm. With such dimensions, the urchin-like nanostructures should result in a gain in sensing surface of more than two-orders of magnitude as compared to a flat surface. The luminescence properties of urchin-like nanostructures are thus also potentially interesting for achieving an efficient G-WET. Evolution of the surface gain R G for hexagonally ordered ZnO nanorods as a function of the radius r and for various nanorod height h.

The polymer thickness is again fixed to 5 nm. The calculated values of R G for the nanoholes, shown in Figure 7 , exhibit a similar behavior to the calculated values of R G for the nanorods, shown in Figure 6. As seen in Figure 7 , gains in sensing surface are expected independently of structures parameters i. Moreover, the enlarged sensing surface i. However, perforated thin films, in general, present smaller values of R G compared to nanorods. However, arrays of nanoholes with such dimensions are hard to perforate, especially in ZnO thin films. Again, more feasible nanoholes [ 20 ] lead to a fold enhancement one order of magnitude in the sensing surface.

Evolution of the surface gain R G for hexagonally ordered nanoholes as a function of the radius r and for various nanorod heights h. As previously mentioned, optical gain is an essential feature of the G-WET concept. It is clear that higher optical gains and reduced lasing thresholds are desirable for more efficient energy transfers. ZnO micro- and nanostructures can be thus important for such purposes. We recently showed [ 21 ] that ZnO microdisks exhibited a reduced lasing threshold compared to the initial lasing threshold measured for the ZnO thin film before microstructuring.

The reduced lasing threshold was attributed to the optical feedback occurring inside the microstructures. Such structures are extremely interesting as they are easy to realize, especially at a large scale, and they should allow for an efficient G-WET process at low pumping intensities.

On the other hand, templated growth offers an easy and cheap alternative for the realization of nanostructures. Recent advances on assisted sphere self-assembly in our team allow for the realization of hexagonally ordered ZnO nanostructures on a large scale [ 36 ]. Such large scale and cheap ZnO nanostructuring is interesting for many functional applications including sensors. Hexagonal arrays of ZnO nanostructures are photonic crystals PhCs and thus present interesting photonic properties.

Therefore, it is essential to determine the structure parameters that are required to obtain the desired PBG. However, the y -axis hereafter will be directly expressed in terms of the center-center periodicity a for more practical reasons. The specific requirements or preferences of your reviewing publisher, classroom teacher, institution or organization should be applied.

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Measurement techniques of determination of the absorption coefficient, refractive index and thickness of the dielectric, semiconductor or metallic films are considered. This book is highly recommended for specialists in the fields of integrated and thin film optics and for graduated students in related specialties. There are new techniques of measurement of thin-film parameters stated. Read more Show all links. Allow this favorite library to be seen by others Keep this favorite library private. Find a copy in the library Finding libraries that hold this item

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