Novel Bloch wave excitation platform based on few-layer photonic crystal deposited on D-shaped optical fiber

With the goal of ultimate control over the light propagation, photonic crystals currently represent the primary building blocks for novel nanophotonic devices. Bloch surface waves (BSWs) in periodic dielectric multilayer structures with a surface defect is a well-known phenomenon, which implies new opportunities for controlling the light propagation and has many applications in the physical and biological science. However, most of the reported structures based on BSWs require depositing a large number of alternating layers or exploiting a large refractive index (RI) contrast between the materials constituting the multilayer structure, thereby increasing the complexity and costs of manufacturing. The combination of fiber–optic-based platforms with nanotechnology is opening the opportunity for the development of high-performance photonic devices that enhance the light-matter interaction in a strong way compared to other optical platforms. Here, we report a BSW-supporting platform that uses geometrically modified commercial optical fibers such as D-shaped optical fibers, where a few-layer structure is deposited on its flat surface using metal oxides with a moderate difference in RI. In this novel fiber optic platform, BSWs are excited through the evanescent field of the core-guided fundamental mode, which indicates that the structure proposed here can be used as a sensing probe, along with other intrinsic properties of fiber optic sensors, as lightness, multiplexing capacity and easiness of integration in an optical network. As a demonstration, fiber optic BSW excitation is shown to be suitable for measuring RI variations. The designed structure is easy to manufacture and could be adapted to a wide range of applications in the fields of telecommunications, environment, health, and material characterization.

. Thus, the BSW dispersion relation is below the light line of the homogeneous dielectric, and within the photonic bandgap 13,14 . Moreover, multilayers are wavelength scalable and may be designed to sustain TE-and/ or TM-polarized BSWs at a broad range of wavelengths from near UV to IR 15 . As a BSW propagates along the surface, it couples light to the multilayer system 16 . Thus, the multilayer design has been optimized for low losses by appropriately choosing the thicknesses and number of periods. Most devices based on BSW excitation require a 1DPC with a large number of layers-higher than 12 10,12,[17][18][19][20][21][22][23][24][25] . These configurations have been used in 1DPCs consisting of bilayers of materials with refractive index (RI) differences as small as 0.2 refractive index units (RIU) 12,[17][18][19] up to moderate differences of 0.8 RIU 10,[20][21][22][23][24][25] . However, there are reported studies that show that it is possible to excite BSWs at the interface of 1DPCs with less than 10 layers 5, [26][27][28][29][30][31] . A common feature of these structures is the relatively large RI difference of the materials used in the 1DPC bilayers, ranging from values greater than 0.8 RIU 32,33 to 2 RIU and even higher 5,31 . The resonant generation of BSWs via prims or grating coupling is an active field of current research in applications such as sensors 13,21,31,34,35 , surface-enhanced Raman spectroscopy 36,37 , fluorescence-based detection 29,[38][39][40] , enhanced nonlinear effects 41,42 , and integrated optics 18,20 . However, very few investigations on the BSW excitation in optical fibers have been published, and not all of them have experimental verification 19,23,30,32,43 .
In this work, we report a BSW excitation platform that uses geometrically modified commercial optical fibers such as D-shaped optical fibers, with a few-layer 1DPC deposited on its flat surface using metal oxides with a moderate difference in RI. Indeed, it was recently demonstrated that D-shaped fibers can operate in reflective configuration 44 , which indicates that the structure proposed here can be used as a probe that can be used as a catheter, a nasogastric probe, or even for chemical mapping of surfaces. The combination of fiber-optic-based platforms with the nanotechnologies is opening the opportunity for the development of high-performance photonic devices that enhance the light-matter interaction in a strong way compared to other optical platforms. In this novel ESW excitation platform, BSWs are excited through the evanescent field of the core-guided fundamental mode. The 1DPC consist of alternating thin layers of tin oxide ( SnO 2 ) and copper oxide (CuO), which have the real part of the RI higher than that of the pure silica, with which the commercial optical fibers are manufactured, and the imaginary part low but not negligible 45 . A high real part of the RI provides strong in-plane confinement of the light and enables the development of compact and low loss integrated photonic devices. The nano-deposition technique used for generating the 1DPC was DC sputtering and a theoretical analysis was made, which permitted to analyze and validate the experimental results. Although most of the BSW excitation platforms require a large number of layers or materials with a large difference in RI, we demonstrate that even a 3-layer stack on the fiber-based excitation platform can sustain TE-polarized BSWs using a 1DPC consisting of materials that have a moderate difference in RI. To the best of our knowledge, this constitutes the first experimental observation of BSWs excitation in laterally polished commercial optical fibers. Moreover, the 1DPC is easy to fabricate and the results are promising for the development of new types of all-fiber photonic devices and sensing applications.

Results and discussion
Design of the structure. The structure designed to experimentally verify the excitation of BSWs in optical fiber consists of a commercially available single mode D-shaped fiber optic and a few-layer 1DPC coating the flat surface of the fiber, as shown in the schematic of Fig. 1a. In this figure, the two insets zoom the central region, where the multilayer deposited on the flat surface of the fiber can be identified. It is a structure that is easy to implement, manipulate, and allows to reduce associated costs 46 . There are many studies on thin-film depositions on geometrically deformed fibers, where the core-guided evanescent field can be coupled to waveguides 47 , surface gratings 48 , metal films to excite SPPs 46 , and metal oxides to generate lossy-mode resonances 49-52 , among others. The 1DPC consist of alternating thin layers of tin oxide ( SnO 2 ) and copper monoxide (CuO) that exhibit a moderate RI contrast, close to 0.3 at the near infrared region (1.84 for SnO 2 and 2.137 for CuO at 1550 nm), and relatively low extinction coefficient values (0.01 for SnO 2 and 0.02 for CuO, which were selected to match the experimental results reported below).
Because the RI difference between the SnO 2 and the CuO thin films is moderate, the thickness of the alternating layers of the 1DPC are chosen to achieve optimum BSW excitation condition and low losses. To design multilayers that match the conditions for BSW, the thicknesses of the high and low refractive index layers d are calculated based on the forbidden transmission band at the desired resonance wavelength of a 1DPC bandgap material 11 , d = /(4n) , where n is the refractive index within the high and low refractive index layers. For example, using this expression for the wavelength = 1550 nm, it is found that the thicknesses of the SnO 2 and CuO are 212 nm and 182 nm, respectively. Since the resonance wavelengths of BSWs are determined by the thicknesses of the multilayer, BSWs in the NIR region can be achieved by varying the layer thicknesses. In the present work, the thicknesses of the SnO 2 and CuO layers are 300 and 200 nm, respectively, which were chosen so that the multilayer exhibits a photonic bandgap around the near infrared, which is the common range of operating wavelengths in fiber-optic communication components (1150-1650 nm), and a TE-polarized BSW can be excited. The properties of a BSW are more easily understood from the study of the dispersion diagram of the semi-infinite multilayer. Figure 1b shows the dispersion band diagram of the proposed SnO 2 /CuO 1DPC for TE polarization, in which the radiative zones (blue) and the non-radiative zones (white) are distinguished for propagation within the multilayer stack. The figure also shows the light line (black) of the surrounding medium, which was taken to be 1.33. The dispersion band diagram of the semi-infinite multilayer was calculated with the transfer matrix method 53 . The β axis is the propagation constant of the incident light, which is parallel to the surface of the multilayer structure, and the frequency axis corresponds to the wavelength range of interest. The surface modes occur in the white area that correspond to the photonic bandgap of the dispersion band diagram. This allows us to excite the BSW modes of the structure, whose propagation properties strongly depend on the 1DPC termination. In practice, however, we have to deal with a finite multilayer, such as the one depicted in Fig. 1a. The strict sense of bandgaps with well-limited edges should be reconsidered since with so few periods they are vaguely defined. Although it seems to be a problem with the position of some cases of surface waves because they appear outside the ideal gaps, one can adhere to the criteria of the evanescent character of the fields and the ability of these systems to guide energy along the surfaces 54 , which results in a loss of energy in the transmitted light. Dispersion curves and electric field distributions for the fundamental BSW (BSW1) were numerically calculated using the commercial software package FIMMWAVE (details on the parameters used for the simulations are given in the "Materials and methods" section). For demonstration purposes, Fig. 1b also shows the relation of the BSW1 dispersion curves in the photonic bandgap for different thicknesses of the termination layer-the CuO layer adjacent to the surrounding medium-of the 6-layer 1DPC outlined in the inset of Fig. 1a. It can be observed that the BSW1 mode dispersion curve with a 24 nm CuO termination layer (red line) appears near the upper and lower edges of the bandgap, whereas the BSW1 dispersion curve with a 140 nm CuO termination layer (green line) lies near the lower edge of the bandgap. The electric-field intensity profiles in Fig. 1c-e correspond, respectively, to the orange, yellow and green dots, on the dispersion curves of the BSW1 modes in Fig. 1b. As expected, the field is highly enhanced at each surface of the multilayer and has an exponentially decaying shape inside the crystal. Interestingly, the field distributions in Fig. 1c,d, to the best of our knowledge, are the first evidence that in a finite periodic system of alternating dielectric layers bounded asymmetrically by dielectric media, the surface modes of each boundary can interact and become coupled to a variable degree depending on both the truncation of the outer layers as the total thickness of the 1DPC (see supplementary material). It is worth noting that the excitation of coupled surface modes has been reported in dielectric-metal-dielectric structures to excite long-range surface plasmon polaritons 55,56 and finite dielectric 1DPCs bounded by air 54 and SiC gratings 57 . For the surface modes that occur near the center of the bandgap (red dot), the mode is more confined and the attenuation by the multilayer stack is stronger. For the modes close to the edges (yellow dot), the evanescent field penetrates much further into the structure and the attenuation is relatively weaker. Therefore, the fiber-based BSW platform can be designed based on the desired application. Strong field enhancement is needed for sensing and nonlinear applications, while modes with longer propagation length are required for integrated optics 16 .
Similar to the prism-based BSW excitation, the fiber-based BSW can be efficiently excited as long as the effective index of the core-guided mode coincides with that of the BSW 32 . Besides sensitivity, one of the main determining factors for the resonance performance is the absorption loss of the multilayer materials. Increasing such losses yields considerably broadened resonance (see supplementary material). The number of layers in www.nature.com/scientificreports/ the stack also determines the resonance width and depth depending on the material properties. From what we learned from the 6-layer stack discussed above, we reduced the number of layers and included 3-and 5-layer SnO 2 /CuO stacks in the study. To have better knowledge of the BSW excitation phenomenon in the D-shaped fiber, theoretical simulations were performed using the commercial software package FIMMWAVE (details on the parameters used for the simulations are given in the "Material and method" section). Figure 2 shows the dispersion curves of the BSWs that can sustain the 3-, 5-, and 6-layer stacks designed in the spectral range of interest, with the thickness and material of the termination layer being 245 nm ( SnO 2 ), 461 ( SnO 2 ), and 50 nm (CuO), respectively, as outlined in the insets (the thicknesses were chosen so that the spectral position of the calculated spectra match with the experimental spectra, as will be shown later).
The phase matching conditions indicate that Bloch waves can be excited, and the power of the light can be efficiently transferred from the fiber core to the few-layer 1DPC. The structure can guide a finite number of BSW modes in a relatively narrow wavelength range. The number of guided modes, their transverse amplitude profiles and their propagation constants depend on the cross-sectional structure of the multilayer and on the optical frequency. In general, the resonance wavelength of a BSW mode of lower order occurs at a longer wavelength than a BSW mode of high order. For instance, the phase matching points for the 3- . It can be seen that the BSW phase-matching points of the 5-and 6-layer 1DPCs are closer together and the associated loss peaks in their transmission spectra of the core-guided mode (blue lines) are mostly caused by the attenuation bands of the surface modes BSW1 and BSW2 (for details see supplementary material). On the other hand, the phase matching points of the 3-layer 1DPC are more separated, making the loss peak in its transmission spectra wider since, in addition to the surface modes BSW1 and BSW2, the attenuation band of the BSW3 mode now has a higher contribution and, therefore, a higher light transfer from the fiber core to the 1DPC is observed in the transmission spectrum of the core-guided mode (see supplementary material). The optical-field intensity distributions in Fig. 2 show that the fields are highly confined to the termination layer Fabrication and device characterization. The three multilayer structures previously analyzed with the 3-, 5-, and 6-layer 1DPCs were fabricated using alternating SnO 2 and CuO on the flat surface of D-shaped standard fibers using a DC sputtering system (see "Materials and methods" section for more details). Note that in all cases, the first layer that is deposited on the flat surface of the D-shaped fiber is SnO 2 , whereby the 1DPC termination layer depends on the number of layers. As an example, a scanning electron microscopy (SEM) image of the cross-section of the D-shaped fiber with 5-layer 1DPC is shown in Fig. 3a. The characterization of the fiber shows that the distance between the core and the polished surface of the D-shaped fiber is h = 4.5 µ m. The SEM image in Fig. 3b allows to differentiate the profile of the metal oxide depositions, where the two SnO 2 / CuO bi-layers can be clearly identified. From the analysis of the image it can be corroborated that the fabricated structure is very close to the designed multilayer stack of two SnO 2 /CuO bi-layers of 300/200 nm and a SnO 2 termination layer of 461 nm (see inset in Fig. 2). In addition, thin films of SnO 2 and CuO deposited on coverslips placed at the same position of the fibers in the DC sputtering chamber were characterized with an atomic force microscope (AFM). A morphological study was performed in different zones in order to obtain the mean value of the film thickness and its surface roughness. The AFM images obtained are presented in Fig. 3c,d and show homogenous thin films of SnO 2 and CuO with average RMS roughness 4.7 nm and 2 nm, respectively. Furthermore, the thin films were also characterized with an ellipsometer to obtain the dispersion curves of the two materials. The wavelength dependence of the index of refraction and extinction coefficient of the SnO 2 and CuO thin films can be seen in Fig. 3e. The high (low) refractive index of the CuO ( SnO 2 ) reveals the great potential of these metal oxides for the development of BSW platforms with moderate optical losses in the near-IR region of light. It is worth mentioning that in this work, extinction coefficient values of 0.01 for SnO 2 and 0.02 for CuO were chosen to match with the experimental results. Here it is important to explain that the angle of incidence of the ellipsometer was set to 70 • because more accurate results are obtained for angles approaching the Brewster angle, and this value is recommended by the manufacturer, Horiba, for the metallic oxides that we have used in the experiment. Oppositely, light is guided in the fiber at grazing incidence (90 • ) related to the thin-film, which leads to a higher light scattering. This is not considered by the ellipsometer. Consequently, we attribute to this factor the discrepancy between the extinction coefficient calculated by the ellipsometer and the extinction coefficient required to fit the BSWs observed in the optical spectrum. After the 1DPCs were fabricated, the core-guided light transmission spectra were obtained as detailed in the "Materials and methods" section. The transmitted power is presented in Fig. 4 for the three multilayer structures   Fig. 2. Approximately at 1478, 1585 and 1496 nm, the surface modes BSW1, BSW2 and BSW3 of each structure are excited by the evanescent field of the core-guided mode in a relatively narrow wavelength range, which together with the moderate losses induced by the multilayer metal oxides, consequently cause in the experimental transmission spectrum an attenuation peak in the BSW spectral range. It is worth noting that the experimental results reveal that the effect of the BSW3 surface mode in the case of the 3-layer stack is much greater than that predicted by the simulations and consequently the attenuation peak is wider. This is confirmed by measuring the full width at half maximum (FWHM) of the experimental attenuation peaks, which are 104.5 nm, 73.5 nm, and 52.5 nm, for 3-, 5-and 6-layer structures. Also, as more layers are deposited, the spectral width of TE-BSW resonances tend to be narrower.

Characterization of the devices as refractometers.
When the surrounding medium is changed, the evanescent field of the BSW will be affected, and, thereby, the phase matching condition will be altered 43 .
In order to characterize the response of the devices presented in the previous section when they are used as refractometers, their 1DPCs were immersed in nine refractive index (glycerol-water) solutions and the generated transmission spectra were captured. These spectra, represented in the top row of Fig. 5, confirm that the BSWs generated by the SnO 2 /CuO multilayers are highly sensitive to surrounding RI index variations. It can be appreciated that the BSW resonances shift to the red when they are subsequently immersed in solutions with increasing RI value. The evolution of the central wavelengths of the attenuation peaks when the surrounding RI is increased are presented in the bottom row of Fig. 5. As can be seen in these figures, the sensing response exhibits a nonlinear feature, which is due to the wavelength-dependent mode coupling between the core-guided mode and the BSW mode 32,43 . Table 1 summarizes the refractometric response of the fabricated structures near to the surrounding RI n S = 1.33 and n S =1.40. The device sensitivity is defined as S n = ∂ res /∂n s , where res is the central wavelength of the attenuation peak. In addition, on the assumption that a 0.1 nm resolution detector is used, the sensor resolution is defined as RES = 0.1/S n 58 . The fabricated refractometric devices have comparable sensitivity to some ESWbased sensing devices 59,60 , including some BSW-based structures 33,61,62 , for example, 631 nm/RIU in Ref. 61 , 285 nm/RIU in Ref. 62 and -168 nm/RIU in Ref. 33 . On the other hand, our BSW-excitation platform has comparable sensitivity with a tapered fiber coated with a 1DPC, which reports, respectively, 650 nm/RIU and 930 nm/RIU for the TM and TE polarizations 30 .

Conclusion
In summary, we have proposed and experimentally demonstrated a novel BSW excitation platform based on D-shaped optical fibers with few-layer 1DPC deposited on its flat surface using alternating thin layers of SnO 2 and CuO. The fiber-optic-based platform enhances the light-matter interaction in a strong way compared to other optical platforms, thereby decreasing the complexity and costs of manufacturing. In this platform, BSWs can be efficiently excited through the evanescent field of the core-guided fundamental mode, resulting in a more compact, lightweight and robust alternative compared to the Kretschman prism-based configuration.
To investigate the effect of the properties of the multilayer stack, 3-, 5-, and 6-layer 1DPCs were designed and manufactured. Although most of the BSW excitation platforms require a large number of layers or materials with a large difference in RI, we demonstrated that even a 3-layer stack on the fiber-based excitation platform can sustain TE-polarized BSWs using a 1DPC consisting of materials that have a small difference in RI. Furthermore, we demonstrated the suitability of this platform for measuring RI variations, paving the way for the development of chemical sensors or biosensors. The proposed platform can be improved by selecting new materials, modifying the thicknesses of the dielectric layers, or by modifying design parameters such as the depth of the polished area, which can lead to sensors with higher sensitivities and resolutions.

Materials and methods
The D-shaped fibers were supplied by the company Phoenix Photonics. They consisted of standard single mode fiber (Corning SMF-28) with a side-polished length of 10 mm, and insertion losses of 4 dB at RI 1.45. The D-shaped fibers were coated using DC sputtering system K675XD from Quorum Technologies, Ltd. The thin films were deposited around the fiber, including the polished region, by using two different targets of tin oxide SnO 2 , and copper oxide CuO (both of 57 mm in diameter and 3 mm in thickness). The CuO target was purchased from ZhongNuo Advanced Material Technology Co, whereas the SnO 2 target from Plasmaterials, Inc. In addition, it is important to note that the tin oxide target was oxygen depleted and therefore strictly speaking SnO 2−x , although for the sake of simplicity we call it SnO 2 throughout the article. The parameters used for the DC sputtering depositions were argon partial pressure of 6 × 10 −2 mbar and an intensity of 90 mA.
The deposition process was carried out in short time intervals and was monitored in real-time by following the spectral position of each resonance in the transmission spectra, using the setup schematized in Fig. 6a. A broad-spectrum light source Agilent 83437A (1150-1680 nm) is connected to a linear polarizer and a polarization controller. The polarized light passes to the D-shaped fiber, which is located inside the sputtering chamber, and the output is connected to an optical spectrum analyzer (OSA) Agilent 86142A. The polarization controller allows to visualize the TE resonance in the optical spectrum. The position of the optical fiber was fixed inside the sputtering machine, which included the two targets of SnO 2 and CuO, programmed so that it is deposited one thin film of SnO 2 and one thin-film of CuO alternatively and progressively. After the coating process, the fibers were removed from the sputtering machine and stored for a minimum of 48 h in air at 25 • C before using  www.nature.com/scientificreports/ them for refractive index measurements. The coated fibers were characterized as refractometers using the experimental setup in Fig. 6b, for which they were immersed in refractive index solutions (glycerol-water) at different concentrations and the corresponding transmission spectra were recorded. It is worth mentioning that, to avoid distortion in the measurements, the fibers were carefully cleaned between measurements, since glycerol tends to adhere to exposed surfaces. The thin films of SnO 2 and CuO deposited on coverslips where characterized with AFM microscopy (Bruker Innova with RTESPA probes in tapping mode) and with a field emission scanning electron microscope (model UltraPlus FESEM from Carl Zeiss Inc.) with an in-lens detector at 3 kV and an aperture diameter of 30 µm ). The metal oxide coatings were also characterized with an ellipsometer UVISEL 2 from Horiba, with spectral range of 0.6-6.5 eV (190-2100 nm), an angle of incidence of 70 • , a spot size of 1 mm and software DeltaPsi2TM (from Horiba Scientific Thin Film Division). The thin films characterization was made on depositions over microscope glass slides, as the flat surface of the D-shaped fiber (from Phoenix Photonic LTD) has a good finish comparable to that of a coverslip. In Fig. 7, it is clear that the roughness is higher in the case of the D-shaped fiber, but it is still a subnanometric value, indicating that the results of the coverslip can be extrapolated to the D-shaped fiber.
Finally, numerical simulations were calculated using the waveguide mode solver FIMMWAVE and the optical propagation tool FIMMPROP. The simulated structure begins with the light input SMF section, which is analyzed with the finite difference method (FDM) because it is the most accurate method available for a cylindrical waveguide. In this section is adequate to find one propagating mode which corresponds to the fiber core mode. Then the light reaches the segment of the side-polished fiber on which the thin layers of SnO 2 and CuO were deposited. This segment has a much more complex geometry, so it must be analyzed using the FEM Solver, based on the finite element method. Due to the complexity of the structure, the number of modes analyzed in this section is 20 to find the surface modes that match the core mode. Finally, the light reaches the light output SMF, which is analyzed in the same way as the input section. The silica RI was calculated using the Sellmeier equation with the coefficients reported in Ref. 63 , while its extinction coefficient was neglected. In addition, the RI of SnO 2 and CuO were taken from Fig. 3e while its extinction coefficients were taken as 0.01 for SnO 2 and 0.02 for CuO 50,51,64 , to match with experimental results.