Toward Bloch Surface Wave-Assisted Spectroscopy in the Mid

Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Toward Bloch Surface Wave-Assisted Spectroscopy in the MidInfrared Region Grégoire M. Smolik,* Nicolas Descharmes, and Hans Peter Herzig Optics and Photonics Laboratory, École Polytechnique Fédérale de Lausanne, Neuchâtel, Switzerland ABSTRACT: We present the first observation of Bloch surface waves in the mid-infrared spectral range. Bloch surface waves are electromagnetic states that can propagate at the interface between a continuous dielectric medium and a periodic stack of materials. We designed a mid-infrared multilayer optical element specifically for this purpose. The structure is made of successive layers of ZnSe and YbF3 deposited on a CaF2 substrate. Surface waves can be coupled into this structure from 6.6 to 10.6 μm. We characterized the optical properties of the fabricated element using a tunable quantum cascade laser centered around 7.85 μm. We believe that Bloch surface wave based platforms are very promising components for surface sensing and spectroscopic applications in the mid-infrared. KEYWORDS: Bloch surface waves, mid-infrared, surface wave, surface spectroscopy he mid-infrared (MIR) spectral range (2.5−25 μm) is of great interest for spectroscopic applications. The existence of many fundamental rotational−vibrational resonances of chemical compounds in that region result in strong absorption lines. These electromagnetic “fingerprints” make it possible to detect, identify and perform concentration measurements of molecular species. Mid-infrared spectroscopy has thus a major role to play in the large fields of label-free sensing, bio and medical diagnostics, and environmental pollution monitoring, but also security and defense. For most of the above, the emergence of new technologies and methods is linked to the addressing of one or more of the following aspects: (i) The ability to detect or identify compounds in always smaller quantities. (ii) The ability to perform analysis with always smaller amounts of sample. (iii) The capacity to reduce the number of sample preprocessing steps. Attenuated total reflectance (ATR) is a proven technique that allows to sense variations of the electromagnetic environment in the vicinity of a reference surface. It relies on the probing of the environment using the evanescent field of an incoming wave undergoing total internal reflection (TIR). In the case of ATR, one of the main factors restricting the sensitivity is the limited path where the light probe interacts with the sample. The interaction range is of the order of magnitude of the wavelength.1 To circumvent this, many implementations rely on geometrical configurations where the probing beam is reflected multiple times on the surface. Though efficient, this approach presents a clear trade-off between sensitivity and system size or sample volume, as the sample to be probed needs to overlap with each region where TIR occurs. Standard ATR thus offers limited performance for the probing of very small samples. One way to increase sensitivity while keeping small sample volumes is the improvement of the interaction with the sample. With this

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© XXXX American Chemical Society

goal, approaches based on surface plasmon polaritons(SPPs) have been proposed. The combination of the light probe propagating along the surface with the local field enhancement linked to the plasmon resonance results in dramatically increased interaction. The demonstration of antibody absorption or ethanol concentration in water measurements are successful examples of surface wave probing using SPPs.2,3 We propose to follow this approach with the use of Bloch surface waves (BSWs). As in the standard ATR and SPP configurations, our approach would rely on probing the sample medium with an evanescent wave. Just as SPPs, Bloch surface waves are propagating modes that can display a large field enhancement. The major benefit relies in the longer distances over which they can propagate. Although Bloch surface wave substrates are already being successfully fabricated in the visible and NIR range,4,5 the mid-IR fingerprint region has, surprisingly, received no interest to this day. The absence of prior art in the mid-IR region is all the more surprising: a large fraction of the existing BSW literature focuses on sensing chemical compounds solely based on their refractive index in a spectral region where the later generally exhibit slow and monotonous changes. Although effective, this approach does not provide any mean of discrimination between two compounds of equal refractive index. Nor could it be as efficient as sensing refractive changes near a sharp absorption line. The aim of this work is thus 2-fold. First, to propose Bloch surface waves as a promising tool for highly sensitive surfacebased MIR spectroscopy. The second is to experimentally demonstrate, for the first time, the realization of a BSW Received: November 2, 2017 Published: February 7, 2018 A

DOI: 10.1021/acsphotonics.7b01315 ACS Photonics XXXX, XXX, XXX−XXX

Letter

ACS Photonics substrate in the MIR region. This effectively takes the first step toward a BSW-assisted MIR spectroscopy platform. Bloch surface waves6 refer to electromagnetic states that can exist at the interface between a continuous dielectric medium (refractive index n1) and a periodic stack of materials, also dielectric, and displaying a refractive index contrast (n2, n3 > n1 and n2 ≠ n3). The periodic arrangement constitutes a onedimensional photonic crystal. When it exists, the surface state is bound to the interface through two distinct mechanisms: total internal reflection from the continuous medium side and photonic band gap (PBG) effect from the periodic stack side. They have a nonzero wave vector along the plane of the interface and thus are inherently propagating waves. Unlike SPPs, Bloch surface states can exist for both polarization (TE or TM). Bloch surface waves belong to the larger family of surface polaritons. As such, they share many similarities with their better known counterparts: surface plasmon polaritons. These similarities are the following: (i) Their surface-state nature imposes a field maximum located near the interface and an exponential decay of the E (TE polarization) or H (TM polarization) field envelop on each side of the interface. (ii) Bloch surface waves can display very long evanescent tails within the continuous medium. (iii) The lateral confinement of the BSW mode allows the field at the interface to be greater than that of a plane wave undergoing TIR. The relative enhancement between BSWs, SPPs, and guided mode is currently debated,7 owing to the complexity of measuring the electric field generated at the interface. (iv) The presence of the field maximum of the mode near the interface and the large overlap of the surface state field with the continuous medium makes it very sensitive to any local perturbation and in particular to the presence of materials displaying absorption lines. (v) Finally, the dispersion curve of a typical Bloch surface mode is comprised between the air line and the line of the lowest refractive index medium. This makes it possible to excite BSWs through the classical and commonly used Kretschmann configuration.8 The most noticeable difference between BSWs and SPPs mostly comes from the dielectric nature of the materials used for the structure fabrication. This leads to two main advantages. First, BSW are free from absorption losses inherent to motion of the electrons in metal lattices. This leads to much longer propagation distance along the interface. Propagation lengths reaching over 2 mm have been reported in the NIR using a scanning near-field optical microscopy (SNOM).9,10 In comparison, SPP propagation lengths reported at equivalent wavelengths are typically in the order of 100 um.11 This corresponds to a 20-fold increase of the path length within the sample. Our calculations indicate a propagation length in the centimeter range at mid-infrared wavelengths. This point is of particular importance in the view of increasing the interaction between the propagating wave and the specimen to be analyzed. Second, BSW offer greater flexibility in the design and in the materials used for the structures. Precise tailoring of the periodic stack can be used in order to perform operation bandwidth optimization or dispersion engineering of the BSW modes. This is of primary importance, as compound identification imposes to be able to carry out measurements over large ranges of wavelengths. While the potential of SPP for operations in the MIR has already been promoted,12,13 and pioneering works published using graphene-based SPPs,14−16 the vast potential of Bloch

surface wave in this spectrum still remains completely unexplored. In particular, the use of BSWs for MIR spectroscopy is believed to be a promising candidate for the ultrasensitive detection and identification of trace elements on a surface. In this work, the very first BSW-supporting structures operating in the MIR is demonstrated. The structure is carefully designed and optimized using a internally developed routine. The routine developed is built upon the open source eigenmode expansion solver CAMFR.17 The optimization parameters of particular interest for the design are the maximum field value reached at the interface and the operational BSW bandwidth. It corresponds to the range of wavelengths for which light coupled into the surface mode is prohibited from radiating freely into the top medium (air line) and the set of k-vectors available through propagation inside the substrate can be matched to the k-vector of the surface mode (substrate line). A large bandwidth is a necessary condition for the detection of a large number of absorption lines and thus the capacity to specifically identify compounds. In this specific case, the sample is designed in order to reach a strong field enhancement in the 7.6−8.1 μm range. Our custom laser source is designed to operate within this range. The total bandwidth of the substrate is about 4 μm, from 6.6 to 10.6 μm. This region of the spectrum is known to include absorption lines of methane, cocaine or various explosive compounds for example. One of the main difficulty hampering the design and fabrication of a BSW platform in the mid-IR lies in the choice of materials for the periodic stack. The latter should be transparent in this part of the spectrum and exempt of toxicity, which restricts the ensemble of possibilities to a list of exotic compounds. The difficulty is made even greater by the micronsized thickness of the layers to be deposited, as it can lead to severe layer adhesion problems and peeling. In the present case, a stack of ZnSe and YbF3 layers deposited on a CaF2 substrate is used. A scanning electron microscopy picture of the fabricated structure can be seen in Figure 1. The designed stack is composed of a lattice of three ZnSe and YbF3 pairs forming a truncated Bragg mirror. An additional 0.239 μm thin ZnSe defect layer deposited on top, totaling seven layers. A structure with a lattice of five layer pairs has also successfully

Figure 1. Scanning electron microscopy picture of a fabricated multilayer stack of ZnSe and YbF3 on a CaF2 substrate. B


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ACS Photonics

Figure 2. (a) Dispersion diagram of propagating modes in the multilayer structure. The BSW mode in the region of interest is plotted in red color. Only guided modes are shown. (b) Imaginary part of the guided modes with the BSW mode in the region of interest plotted in red.

between the multilayer materials is preferred, as it results in a larger bandwidth for the BSW mode. Figure 2b represents the imaginary part of the modes. The BSW mode (red dots) exhibits a larger imaginary part than the guided modes within the substrate (blue triangles). The propagation length of a guided mode is higher for modes exhibiting lower imaginary components. Although the long propagation length associated with these guided modes might be of interest for some applications, the greater complexity involved in the coupling of light and the lower field enhancement compared to the Bloch surface mode make them less interesting candidates for surface sensing. An incoming wave can couple into the BSW mode if the parallel component of its propagation constant matches that the surface mode. This can be achieved by carefully selecting the combination of wavelength and incidence angle of the incoming light. Light coming from the substrate will be reflected by TIR at the top interface, except when the BSW coupling conditions are met. When the propagation constant matches that of the surface mode, part of the incident light couples to the Bloch surface mode and propagates along the surface. The amount of light reflected off the substrate is then reduced. Practically, this means that the presence of a mode can be assessed by changing the wavelength (respectively the incidence angle) of the excitation beam while maintaining the incidence angle (respectively the wavelength). It should be noticed that this type of characterization is only possible if the coupling between the excitation beam and the mode under investigation is large enough to produce a significant decrease of the reflected light. For example, a structure exhibiting a Bloch surface mode with little or no overlap with the substrate and the excitation field will not display any dip in the reflection spectrum. This can easily be demonstrated by fabricating a

been fabricated. Thicker layer depositions exhibit poorer adhesion to the substrate, illustrating the difficulty of material selection. Other MIR-compatible materials such as ZnS, YF3, Si, and Ge can be used depending on the desired operating range and refractive index difference. Figure 2a shows the calculated band-diagram of the designed one-dimensional photonic crystal. Each blue triangle represents a set of (k,ω) values for which a mode exists in the structure. The BSW mode is represented by red dots. The BSW coupling region is bound on one side by total internal reflection, represented by the light blue line (air line). Above this line, light propagates freely outside of the medium. The surface wave propagation can therefore only occur below. On the other side, the coupling of light through a Kretschmann-type configuration is limited by either the substrate line (here CaF2, depicted in green) or the low refractive index material light line (here YbF3, depicted in orange). Practically speaking, the bandwidth of the structure could be maximized through an appropriate matching of the substrate material and lowest refractive index material. This would lead to the superimposition of the substrate and low index lines. In our case, we do not have a better substrate alternative to CaF2, as the material must be available in crystalline form and transparent in the MIR. Two distinct regions below the air line can be observed in Figure 2a. The first region, represented in light green, encompasses the truly guided modes of the multilayer structure. These modes are characterized by a k-vector that is too large to be matched with freely radiating modes of the substrate or the top medium. Any energy coupled into one of these modes thus remains confined through total internal reflection both from the top and bottom interfaces. The second region, delimited by the high index material light line (here ZnSe, depicted in pink) and shaded in darker green corresponds to a (k,ω) space where no electromagnetic mode exist. A large refractive index difference C


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ACS Photonics

Figure 3. (a) Calculation of the reflectivity of a multilayer BSW structure as a function of illumination wavelength and incident angle. The total internal reflection (TIR) and BSW lines are pointed out by arrows. (b) Simulated electric field intensity of the Bloch surface wave across the multilayer structure at 8 μm wavelength.

Figure 4. Measurement setup: (1) tunable MIR laser setup, (2) mechanical shutter, (3) guide laser, (4) removable mirror, (5) parabolic mirror, (6) pinhole, (7) and (8) parabolic mirrors, (9) BSW and prism setup, and (10) photodetector on two-axis scanner. Inset: detail of the Kretschmann configuration with CaF2 substrate on top of CaF2 coupling prism. The prism is mounted on a rotational stage allowing to change the incidence angle.

portion of the electric field stands outside of the structure in the medium to be probed. The extension of the evanescent tail extends over 5 to 10 μm above the surface, approximately one wavelength, which might be relevant for probing cells, droplets, fine particles, fingerprint residues, and so on. The fabricated sample is characterized following the method just described and using the setup depicted in Figure 4. A custom tunable quantum cascade laser (QCL) setup in socalled Littrow configuration has been assembled for this experiment. The laser emission is centered around 7.95 μm with a 0.4 μm bandwidth. It provides several orders of magnitude higher power spectral density compared to globars and outputs a linearly polarized beam. The sensor is a photovoltaic mercury cadmium telluride (MCT) detector operating at room temperature. A lock-in amplifier is used to reduce background thermal radiation noise via synchronous detection. The use of mirrors allows to avoid chromatic aberrations over the laser tuning range. It also allows to use of a visible laser for preliminary alignment. More details on the setup are provided in the Experimental Section.

Bloch surface wave substrate with a great number of layer periods. While sufficient to indicate the existence of an optical mode within a range of wavelength or angular frequencies, the above method alone is not sufficient to unambiguously ascertain the nature of the mode (Bloch surface mode, bulk, guided). One method to circumvent this limitation is to perform a 2D mapping of attenuated reflection measurement over a large (k,ω) space. By doing so, the experimental dispersion diagram of the mode under investigation can be reconstructed and compared to the calculation, thus allowing its unambiguous identification. Figure 3a shows the simulated reflectivity of the multilayer structure of Figure 3b as a function of wavelength and incident angle inside the CaF2 substrate. It can be observed that, for angles higher than the angle of total internal reflection (TIR), all the light is reflected. However, at specific angle-wavelength combinations, the reflected intensity diminishes because of the light coupling into the surface mode. Figure 3b shows the simulated electric field intensity of a BSW propagating at the surface of a multilayer structure. It can be observed that a large D


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Figure 5. (a) Measurement of the reflected intensity at different wavelengths and coupling angles. (b) Superimposition of angular reflectivity measurements from 7887 to 8128 nm centered around the peak coupling angle.

The prism angle is first set using a rotational stage; the laser wavelength is then set to a specific value within its operating range; the photodetector is finally scanned vertically to measure the incident light intensity profile. The vertical position of the detector is converted into incident angle inside the substrate. The resulting measurement is displayed in Figure 5a. This figure is a composition of individual angular scans at different wavelengths, spread on the vertical axis. The horizontal axis represents the angular scanning range. The color brightness shows the reflected intensity. A transverse line of lower reflectivity can be observed, corresponding to the anglewavelength coupling conditions of the BSW mode. Practically, this corresponds to an experimental measurement of the dispersion curve of the BSW mode, since the coupling angle and k-vector are related. This unique signature demonstrates the existence of the designed Bloch surface mode. The interference pattern that can be observed on the measurement is due to a Fabry−Perot resonance. The optical length of this resonance corresponds to the 1.1 mm thick CaF2 substrate. The fringe contrast comes from the refractive index mismatch between the CaF2 substrate and the index-matching liquid (paraffin oil) between the prism and the substrate. The choice of index matching fluid is limited since it should be transparent and not exhibit absorption peaks in the selected operating range. While fluorinated oil provides better index matching with CaF2 than paraffin oil, it exhibits strong absorption in our working region and can therefore not be used. The interference pattern allows to observe the optical phase of the reflected light on the multilayer. A rapid change of the phase around the BSW resonance can be noticed. A thorough description of this effect can be found in the work of Abeles.18 This shift can also be used to improve the measurement sensitivity of a refractive index change close to the surface.19,20 Figure 5b shows the superimposition of individual angular measurements ranging from 7887 to 8128 nm, centered on the peak coupling angle. The apparent envelope shows the angular line width of the BSW mode. The width is about 0.4°. The uneven distribution on each side of the peak coupling angle results from the spectral emission profile of the laser.

The angle-wavelength condition at which the incident wave couples into the surface mode appears red-shifted by approximately 0.45 μm compared to the design. Although this figure may seem large, it only corresponds to a twentieth of the wavelength, that is, a 5% discrepancy. According to our measurements, this is due to identified variations in thickness of the deposited layers. The comparison between the measurement (inset) and the thickness−corrected calculation can be observed in Figure 6. Replacing the design thicknesses with the

Figure 6. Calculation of the reflectivity of a multilayer structure as a function of wavelength and incident angle. The lower intensity line across the graph indicates the coupling of the incident light into the BSW mode. The inset shows the measured values for comparison.

measured ones in the simulation yields a coupling condition much closer to actual measurement, with a residual red-shift of only 0.18 μm. The great agreement illustrates the precision of our design and calculation method. To summarize, the first observation of a Bloch surface mode in the mid-infrared region is presented. The design and fabrication of a Bloch surface mode-carrying substrate is described. The experimental measurement of the dispersion curve is carried out, revealing unambiguously the surface-bound nature of the mode. The existence of the mode is observed E


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ACS Photonics between 1234 and 1315 cm−1 (8.1 and 7.6 μm), limited by our light source. This spectral range coincides for example with strong absorption peaks of functional groups found for example in cocaine or explosive compounds such as pentaerythritol tetranitrate (PETN). Based on the good agreement between our measurements and calculations, the actual bandwidth is expected to be in the range of 950 to 1500 cm−1 (10.6−6.6 μm). The main advantage of the multilayer design is its scalability and adaptability to work from visible to MIR wavelengths, as long as the materials used are transparent. For example, multilayer structures based on Si or Ge could be used for operation at longer wavelengths, as the transmission bands of these materials extend further. Measurement of samples such as fingerprints, saliva, and breath could be realized with the proposed platform. Depending on the application, it might be interesting to extend the operating bandwidth to the entire mid-infrared region, which could simply be achieved by a combination of different multilayer structure designs. This work constitutes the first step toward BSW-assisted surface spectroscopy.

ORCID

Grégoire M. Smolik: 0000-0002-5446-4914 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Raphaël Barbey for the fruitful discussions and Dr. Gaël Osowiecki for his help with the measurement setup. The work of Dr. Descharmes is supported by a grant from the Gebert Rüf Stiftung. The authors also acknowledge the matplotlib project.21

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EXPERIMENTAL SECTION Sample Fabrication. Figure 1 shows a scanning electron microscopy image of the fabricated BSW structure. The multilayer structure is deposited on a 1.1 mm thick CaF2 substrate by ion beam assisted, electron beam-physical vapor deposition by the company Helia Photonics in Scotland. The stack is composed of alternating layers of ZnSe and YbF3 of 1.48, 1.50, 1.48, 1.40, 1.45, and 1.30 μm thickness. An additional 0.23 μm thick ZnSe defect layer is deposited on top. This structure supports TE polarized surface waves at a coupling angle of 67° at a wavelength of 8 um. Measurement Setup. The observation of the BSW is realized using the setup described in Figure 4. The substrate is fixed on a CaF2 equilateral prism in Kretschmann configuration.8 A droplet of paraffin oil is used as index matching liquid between the prism and the substrate. This liquid is used as it does not possess any absorption bands in the desired operating range. The prism is mounted on a rotational stage to allow the incidence angle of the light outside the prism to be changed. The light source is a custom tunable quantum cascade laser (QCL) setup in Littrow configuration specifically assembled for this experiment. The laser emission is centered around 7.95 μm with a 0.4 μm bandwidth. The light is TE polarized and is modulated by a mechanical shutter. The source is spatially filtered using a 300 μm diameter pinhole. The light is focused on the substrate using a parabolic mirror. The use of mirrors instead of lenses allows to keep a precise alignment across the whole laser bandwidth thanks to the absence of chromatic aberration. A 633 nm guide laser is used to simplify the alignment of the setup with the help of a removable mirror. The far-field amplitude of the reflected light is measured with a photovoltaic mercury cadmium telluride (MCT) detector operating at room temperature fixed on a two-axis motorized stage 690 mm away from the prism. Since the detector is sensitive to thermal radiation, a lock-in amplifier is used for synchronous detection.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: gregoire.smolik@epfl.ch. F


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ACS Photonics resonance and its sensor applications. Quantum Electron. 1998, 28, 835. (20) Shen, S.; Liu, T.; Guo, J. Optical phase-shift detection of surface plasmon resonance. Appl. Opt. 1998, 37, 1747−1751. (21) Hunter, J. D. Matplotlib: A 2D Graphics Environment. Comput. Sci. Eng. 2007, 9, 90−95.

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Toward Bloch Surface Wave-Assisted Spectroscopy in the Mid

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