Bloch surface wave ring resonator based on porous silicon

Confining light near the surface of a photonic structure is crucial for a number of applications relying on the interaction between light and matter, including high-sensitivity surface biosensors,^1–4 integration with two-dimensional materials,^5,6 and interaction with quantum dots.⁷ The evanescent field of waveguides, ring resonators, and photonic crystals is commonly exploited to support such applications due to the ease of integration of these optical structures with other on-chip components. Surface plasmon polaritons and Bloch surface waves (BSWs) are also well-suited for applications relying on the enhancement of light intensity near the surface of a photonic structure.⁸ While plasmonic structures support modes characterized by intense electric fields near the interface between a metal and a dielectric material, which explains why they have been used most extensively in biosensing applications, BSW structures can achieve strong field enhancement near the surface of a purely dielectric structure, thus without suffering from the absorption issues that plague plasmonic devices.⁹ Light confinement in BSWs is achieved thanks to a combination of total internal reflection (TIR) at the interface with the external medium and the photonic bandgap (PBG) supported by a periodic multilayer.^10–12 

Lateral confinement of BSWs, a necessary first step toward on-chip integration, can be achieved by means of dielectric ridges fabricated on top of 1D photonic crystal multilayers, with transverse light confinement due to TIR. Several types of guided modes have been predicted in such systems, with their general features studied theoretically¹³ and characterized experimentally.^14–16 To consider creating a photonic platform based on BSWs, for example, to develop integrated optical sensors or more generally to probe the interaction between photonic modes and electronic nanostructures, robust and reliable resonators based on BSWs must be demonstrated as a fundamental building block. Recently, it has been suggested that optical resonators based on BSWs could be obtained by exploiting light confinement in ring resonators.¹⁷ Later, Dubey et al. demonstrated a BSW-based microdisk resonator¹⁸ using a TiO₂ disk of radius R = 100 μm on top of a silicon dioxide-silicon nitride multilayer and reported a quality factor Q = 2 × 10³.

Porous silicon (PSi) is a low-cost tunable material that can be used to rapidly form multilayers of varying refractive indices to realize waveguides, Bragg mirrors, and microcavities,^19,20 and with additional lithographic patterning to form ring resonators with quality factors as high as 10⁴, as shown recently.²¹ PSi optical structures have been shown to be highly advantageous for biosensing applications due to their extremely large internal surface area.²² This platform has been used to implement BSWs, mainly for biosensing applications, in simple prism-coupled multilayers,²³ grating-coupled multilayers,^24,25 or multilayered membranes.²⁶ 

In this work, we move even further and demonstrate the integration of a BSW ring resonator with a single bus waveguide in a PSi structure, showing that this material platform is appealing for the development of low-cost BSW-based integrated biosensors and for other applications in which the combination of a guided surface mode with an integrated resonator is critical.

The structure, which is schematically shown in Fig. 1, was fabricated using a combination of electrochemical etching and lithographic processing. First, the PSi multilayer was formed by electrochemical etching of p+ (0.01 Ω cm) Si (100) in a 15% hydrofluoric acid solution, as previously reported.²⁴ The top layer was etched at a current density of 18 mA/cm² for 28 s. The next two layers were etched at current densities of 48 mA/cm² for 22 s and 18 mA/cm² for 34 s, respectively. Then, the remainder of the multilayer was formed by etching 10 pairs of high and low refractive index layers using an alternating current density of 5 mA/cm² for 63 s and 48 mA/cm² for 22 s, respectively. Next, a 1.5 mM KOH solution in ethanol was drop cast on the as-anodized PSi multilayer film for 5 min and thoroughly rinsed with ethanol in order to widen the pore diameters. Thermal oxidation was then carried out at 500 °C for 5 min in air to passivate the PSi surfaces.

The resulting multilayer consists of 24 PSi layers of alternating high and low refractive indices. The height of the top layer is h_WG = 149 nm, with refractive index n_WG = 1.69. The underlying PSi multilayer consists of three layers followed by ten repetitions of a two-layer unit cell. The first three layers are as follows: d₁ = 704 nm, n₁ = 1.24; d₂ = 231 nm, n₂ = 1.69; and d₃ = 704 nm, n₃ = 1.24. The unit cell consists of a high-index layer of height d_high = 248 nm and index n_high = 1.79 and a low-index layer of height d_low = 704 nm and index n_low = 1.24. The parameters are consistent with scanning electron microscopy images and the optical characterization of the multilayer via the reflectance measurement (not shown). The ring resonator pattern was transferred to the top layer of PSi by electron beam lithography and reactive ion etching, following previously reported protocols.²¹ The ring resonator has a radius R = 105 μm and ridge waveguide width w_WG = 6 μm. The width of the bus waveguide is also 6 μm, and the coupling gap is 100 nm.

The structure has been characterized by means of transmission measurements carried out in the telecom bands using a TE-polarized tunable laser (Santec TSL-510), tapered input and output coupling fibers (OZ Optics), and a photodiode receiver (Newport 2936-C). The experimental results are shown in Fig. 2, where we show transmission spectra of the PSi BSW ring resonator in the telecom C-band and L-band, respectively. The resonator free spectral range (FSR) is approximately 3.0 nm, and quality factors up to Q > 10³ have been observed in the L-band. Similar values for the quality factor have also been obtained by means of 3D finite-difference time-domain (FDTD) simulations, where roughness in the porous silicon structure was neglected. One can also show that in our case, this relatively low value of the quality factor does not depend on the number of periods in the multilayer.¹⁷ Thus, we believe that it should be ascribed to overcoupling, which is compatible with the relatively low resonance visibility, and/or intermode cross talk between the resonating modes and leaky modes supported by the multilayer structure. The presence of overcoupling is most likely due to our conservative choice of having a small distance between the ring and the bus waveguide to ensure mode excitation even in the case of large scattering losses, which we could not estimate through FDTD simulation. These results suggest that larger quality factors, as high as 10⁴ (thus similar to those obtained in PSi structures in which light confinement is based uniquely on total internal reflection²¹), could be obtained through a careful optimization of the structure parameters.

Transmission experiments were also carried out where light scattered from the top of the sample was collected for different input wavelengths. Images obtained with an infrared camera are shown in Fig. 3. When the incident light has a wavelength corresponding to one of the transmission dips, the ring lights up due to light scattering away from the ring [see Fig. 3(b)]. On the contrary, the ring is dark when the resonance condition is not fulfilled [see Fig. 3(a)].

A second set of simulations were carried out using the transfer matrix method to determine the dispersion relation and the field profile of the BSW mode. In these calculations, we considered the same geometrical parameters and refractive indices reported above. Light confinement in the plane was modeled by means of the effective-index method (EIM).¹³ The results are reported in Fig. 4, in which the white region corresponds to the photonic bandgap of the periodic multilayer and the dashed black line is the light line of the external medium—air, in our simulations. The structure supports guided modes only within the photonic bandgap. Other leaky modes, associated with Fabry–Pérot interference in the PSi multilayer, are visible outside this region.

The PSi photonic crystal ridge supports two guided BSW modes, whose dispersion relations are shown in Fig. 4. One of these modes, shown by a dot-dashed line, is located very close to the lower band edge. This corresponds to a subsurface wave, which is characterized by a field profile having its maximum in the third layer of the multilayer structure.²⁴ Its position in the proximity of the band edge is associated with considerable penetration in the multilayer and large propagation losses, which are not compatible with the quality factors observed experimentally. On the contrary, the second guided BSW, which has $k∥=4.80 μm−1$ at E = 0.8 eV, is almost in the center of the photonic bandgap and has a field profile with the maximum located at the interface between the truncated multilayer and the ridge. This is the guided mode that we believe is associated with the resonances shown in Fig. 2.

To verify that the resonant features observed in the transmission spectra are compatible with the computed BSW dispersion relation, we evaluate the corresponding free spectral range (FSR) and compare it with the one that was inferred from the transmission measurements. When group velocity dispersion is negligible, the FSR is directly related to the mode group index n_g = c/v_g. In particular, the resonance frequencies satisfy

where c is the speed of light, m is the order of the resonance of frequency ν_m with respect to a reference frequency ν₀, and R is the ring radius. In Fig. 5, we show the experimental points corresponding to the resonance order m with respect to the reference resonance ν₀ = 1.946 × 10¹⁴ Hz vs the frequency detuning $Δνm=(νm−ν0)$ along with the theoretical curve corresponding to Eq. (1), where we used the group index $ngsim≈1.177$ calculated from the dispersion relation. The theoretical curve is compatible with the experimental points within the experimental error bars, which correspond to the full-width-at-half-dip of the transmission resonances. It should be noticed that no other guided or leaky mode would lead to results in agreement with the experimental data.

Finally, as an initial demonstration of the promise of the PSi BSW ring for biosensing applications, we carried out a biotin-streptavidin assay in which molecule capture was detected based on changes in the transmission spectrum. In order to prepare the PSi BSW ring for the sensing experiment, the sample was functionalized with 3-aminopropyltriethoxisilane (APTES) and biotin molecules. A 4% solution of APTES in methanol and de-ionized (DI) water was placed on the sample for 15 min, and then, the sample was rinsed with methanol, thermally annealed for 10 min at 100 °C in air ambient, and soaked in methanol for an additional 10 min. Next, a 3 mM solution of biotin in DI water was drop cast on the sample for 30 min, followed by rinsing in DI water and drying with nitrogen gas. Transmission measurements were carried out after each functionalization step (not shown), and the resulting redshifts of the spectrum indicate that the molecules were attached to the PSi BSW ring. The target streptavidin molecules were then exposed to the functionalized PSi BSW ring: a 100 nM solution of streptavidin in DI water was drop cast on the sample for 30 min, followed by rinsing in DI water and drying in nitrogen gas. Figure 6 shows the experimentally measured redshift of one resonance of the PSi BSW ring after streptavidin molecule capture, suggesting that the PSi ring resonator can readily detect the attachment of streptavidin molecules at a concentration of 100 nM. The corresponding sensitivity for streptavidin detection, which is above 10 pm/nM, is larger than that measured using traditional silicon ring resonator sensors.^27,28

In conclusion, we experimentally demonstrated a BSW ring resonator in a PSi platform, in which light confinement is obtained by PBG in the substrate and by TIR in all the other directions. The experimental results are in good agreement with our simulations. The resonant mode is characterized by a quality factor Q exceeding 10³, which represents the state of the art for this kind of resonant mode. These results are promising with a view to the development of BSW-based platforms for integrated photonics.

This work was funded in part by the Army Research Office (No. W911NF-09-1-0101) and the National Science Foundation (No. ECCS-0746296). Lithography was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. SEM imaging was conducted at the Vanderbilt Institute of Nanoscale Science and Engineering. The authors thank Tengfei Cao and Sami Halimi for technical assistance.