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The metal-ligand complex tris(2,2'-bipyridine)ruthenium(II) chloride (Ru probe) displays a broad emission spectrum ranging from 540 to 730 nm. The emission spectra of Ru probe were measured when placed on top of a one-dimensional photonic crystal (1DPC), which supports both Bloch surface wave (BSW) and internal modes for wavelengths below 640 nm and only internal modes above 640 nm. The S-polarized emission spectra, with the electric vector parallel to the 1DPC surface, were found to be strongly dependent on the observation angle through the coupling prism. Also, the usual single broad-emission spectrum of Ru probe on glass was converted into two or more narrow-band-spectrum on the 1DPC, with emission band maxima dependent on the observation angle. The two S-polarized emission band peaks for Ru probe were found to be consistent with coupling to the BSW and first internal mode (IM1) of the 1DPC. The same spectral shifts and changes in emission maxima were observed by using Kretschmann and reverse Kretschmann illuminations. As the coupling requires the emitter to be in proximity with the photonic structure, we calculated near- and far-field distributions of a dipole directly located on the 1DPC surface. Finite-Difference Time-Domain (FDTD) simulations were performed to confirm fluorophore coupling to the BSW and internal modes (IMs). Both the measured and simulated results showed that IM coupled emission is significant. Coupling to the IM mode occurred at longer wavelengths where the 1DPC did not support a BSW. These results demonstrate that a simple Bragg grating, without a BSW mode, can be used for detection of surface-bound fluorophores.

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The overexpression and/or amplification of the HER2/neu oncogene has been proposed as a prognostic marker in breast cancer. The detection of the related peptide HER2 remains a grand challenge in cancer diagnosis and for therapeutic decision-making. Here, we used a biosensing device based on Bloch Surface Waves excited on a one-dimensional photonic crystal (1DPC) as valid alternative to standard techniques. The 1DPC was optimized to operate in the visible spectrum and the biosensor optics has been designed to combine label-free and fluorescence operation modes. This feature enables a real-time monitoring of a direct competitive assay using detection mAbs conjugated with quantum dots for an accurate discrimination in fluorescence mode between HER2-positive/negative human plasma samples. Such a competitive assay was implemented using patterned alternating areas where HER2-Fc chimera and reference molecules were bio-conjugated and monitored in a multiplexed way. By combining Label-Free and fluorescence detection analysis, we were able to tune the parameters of the assay and provide an HER2 detection in human plasma in less than 20 min, allowing for a cost-effective assay and rapid turnaround time. The proposed approach offers a promising technique capable of performing combined label-free and fluorescence detection for both diagnosis and therapeutic monitoring of diseases.

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The design of a photonic system for the trapping and waveguiding of ultracold atoms far above a dielectric surface is proposed and analyzed. The system consists of an optical rib waveguide deposited on a planar one-dimensional photonic crystal, which sustains two wavelengths of photonic crystal surface modes tuned in the red and blue sides relative to the atomic transition of the neutral atom. The addition of a third blue-tuned wavelength to the system allows the neutral atoms to be stabilized in the lateral dimension above the rib waveguide. Trapping atoms at relatively large distances, more than 600 nm above the dielectric surface, allows to reduce the influence of Casimir-Polder forces in this system. The detailed design methodology and specifications of the photonic system are provided. The presented design can be employed in atomic chips and quantum sensors.

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Thanks to the increasing availability of technologies for thin film deposition, all-dielectric structures are becoming more and more attractive for integrated photonics. As light-matter interactions are involved, Bloch Surface Waves (BSWs) may represent a viable alternative to plasmonic platforms, allowing easy wavelength and polarization manipulation and reduced absorption losses. However, plasmon-based devices operating at an optical and near-infrared frequency have been demonstrated to reach extraordinary field confinement capabilities, with localized mode volumes of down to a few nanometers. Although such levels of energy localization are substantially unattainable with dielectrics, it is possible to operate subwavelength field confinement by employing high-refractive index materials with proper patterning such as, e.g., photonic crystals and metasurfaces. Here, we propose a computational study on the transverse localization of BSWs by means of quasi-flat Fabry-Perot microcavities, which have the advantage of being fully exposed toward the outer environment. These structures are constituted by defected periodic corrugations of a dielectric multilayer top surface. The dispersion and spatial distribution of BSWs' cavity mode are presented. In addition, the hybridization of BSWs with an A exciton in a 2D flake of tungsten disulfide (WS2) is also addressed. We show evidence of strong coupling involving not only propagating BSWs but also localized BSWs, namely, band-edge and cavity modes.

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Nanoscale fluorescence emitters are efficient for measuring biomolecular interactions, but their utility for applications requiring single-unit observations is constrained by the need for large numerical aperture objectives, fluorescence intermittency, and poor photon collection efficiency resulting from omnidirectional emission. Photonic crystal (PC) structures hold promise to address the aforementioned challenges in fluorescence enhancement. In this review, we provide a broad overview of PCs by explaining their structures, design strategies, fabrication techniques, and sensing principles. Furthermore, we discuss recent applications of PC-enhanced fluorescence-based biosensors incorporated with emerging technologies, including nucleic acids sensing, protein detection, and steroid monitoring. Finally, we discuss current challenges associated with PC-enhanced fluorescence and provide an outlook for fluorescence enhancement with photonic-plasmonics coupling and their promise for point-of-care biosensing as well monitoring analytes of biological and environmental relevance. The review presents the transdisciplinary applications of PCs in the broad arena of fluorescence spectroscopy with broad applications in photo-plasmonics, life science research, materials chemistry, cancer diagnostics, and internet of things.

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The growing need for new and reliable surface sensing methods is arousing interest in the electromagnetic excitations of ultrathin films, i.e., to generate electromagnetic field distributions that resonantly interact with the most significant quasi-particles of condensed matter. In such a context, Bloch surface waves turned out to be a valid alternative to surface plasmon polaritons to implement high-sensitivity sensors in the visible spectral range. Only in the last few years, however, has their use been extended to infrared wavelengths, which represent a powerful tool for detecting and recognizing molecular species and crystalline structures. In this work, we demonstrate, by means of high-resolution reflectivity measurements, that a one-dimensional photonic crystal can sustain Bloch surface waves in the infrared spectral range from room temperature down to 10 K. To the best of our knowledge, this is the first demonstration of infrared Bloch surface waves at cryogenic temperatures. Furthermore, by exploiting the enhancement of the surface state and the high brilliance of infrared synchrotron radiation, we demonstrate that the proposed BSW-based sensor has a sensitivity on the order of 2.9 cm-1 for each nanometer-thick ice layer grown on its surface below 150 K. In conclusion, we believe that Bloch surface wave-based sensors are a valid new class of surface mode-based sensors for applications in materials science.

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A one-dimensional photonic crystal with an additional TiO2 layer, supporting Bloch surface waves (BSW), was used for enhanced signal sensitivity for the detection of protein interaction. To compare the optical response of BSW and photonic crystals (PC), bovine serum albumin and specific antibodies against bovine serum were used as a model system. The results obtained show the enhanced sensitivity of p- and s-BSW components for the 1D PC sample with an additional TiO2 layer. Furthermore, a higher sensitivity was obtained for the BSW component of p-polarization in the PC sample with an additional TiO2 layer, where the sensitivity of the ellipsometric parameter Ψ was five times higher and that of the Δ parameter was eight times higher than those of the PC sample. The capabilities of BSW excitations are discussed from the sensitivity point of view and from the design of advanced biosensing.

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Bloch surface waves (BSWs) with Bi2Se3 in a composite structure consisting of a coupling prism, distributed Bragg reflector (DBR) and cavity layer have been demonstrated. The design relies on the confinement of surface waves that originates from the coupling between the defective layer of plasmonic material (Bi2Se3) and DBR. The presence of the cavity layer modifies the local effective refractive index, enabling direct manipulation of the BSWs. The transfer matrix method (TMM) is used to evaluate the reflectance and absorptance responses in the spectral domain for various angles of incidence, demonstrating the presence of sharp resonances associated with the BSW. With an optimal thickness of DBR bilayers, the energy of an evanescent wave can be transferred into the periodic stack resulting in the excitation of waveguide modes (WGMs). It is believed that the proposed design possesses the advantage in terms of easy fabrication to develop integrated photonic systems, especially for biological and chemical sensing.

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Interferometric methods of optical sensing based on the phase shift of the Bloch surface waves (BSWs) and guided waves (GWs) supported by a one-dimensional photonic crystal are presented. The photonic crystal, composed of six SiO2/TiO2 bilayers with a termination layer of TiO2, is employed in the Kretschmann configuration. Under resonance condition, an abrupt phase change is revealed, and the corresponding phase shift is measured by interferometric techniques applied in both the spectral and spatial domains. The spectral interferometric technique employing a birefringent quartz crystal is used to obtain interference of projections of p- and s-polarized light waves reflected from the photonic crystal. The phase shifts are retrieved by processing the spectral interferograms recorded for various values of relative humidity (RH) of air, giving the sensitivity to the RH as high as 0.029 rad/%RH and 0.012 rad/%RH for the BSW and GW, respectively. The spatial interferometric technique employs a Wollaston prism and an analyzer to generate an interference pattern, which is processed to retrieve the phase difference, and results are in good agreement with those obtained by sensing the phase shift in the spectral domain. In addition, from the derivative of the spectral phase shifts, the peak positions are obtained, and their changes with the RH give the sensitivities of 0.094 nm/%RH and 0.061 nm/%RH for the BSW and GW, respectively. These experimental results demonstrate an efficient optical sensing with a lot of applications in various research areas.

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Micro-spectroscopy is a critical instrument for spectrum analysis in various applications such as chemical and biological analysis, environment detection, and hyperspectral imaging. However, current micro-spectral technique requires bulky and costly spectrometer. In this report, a new type of Bloch surface wave (BSW) based micro-spectrometer is proposed. A single silicon nanoparticle sitting on a dielectric multilayer substrate is used to excite the BSW which acts as a nanoscale unknown source. Taking advantage of the dispersion relations of BSWs, an abundant spectrally related database is formed that is useful for spectrum retrieval applications. Back-focal plane images are used to monitor the change of angular spectrum corresponding to the dispersion relationship of Bloch surface waves. Combined with an iterative algorithm, experimental retrieval of visible-range monochromatic and broadband light spectrums can be obtained. The resolution of the spectrometers can reach 2 nm across a wavelength range of 130 nm. The method in this work is CMOS compatible, enabling spectra retrieval for nanoscale radiators and can also be used to measure and retrieve the microscopic spectrum signal rapidly and timely without conventional scanning monochromator spectrometer.

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The coupling of fluorescence with surface electromagnetic modes, such as surface plasmons on thin metal films or Bloch surface waves (BSW) on truncated one-dimensional photonic crystals (1DPC), are presently utilized for many fluorescence-based applications. In addition to the surface wave, 1DPCs also support other electromagnetic modes that are confined within the 1DPC structure. These internal modes (IMs) have not received much attention for fluorescence coupling due to lack of spatial overlap of their electric fields with the surface bound fluorophores. However, our recent studies have indicated that the fluorescence coupling with IMs occurs quite efficiently. This observed internal mode-coupled emission (IMCE) is (similar to BSW-coupled emission) indeed wavelength dependent, directional and S-polarized. In this paper, we have carried out back-focal plane (BFP) imaging to reveal that the IMs of 1DPCs can couple with surface bound excited dye molecules, with or without a BSW mode presence. Depending on the emission wavelength, the coupling is observed with BSW and IMs or only IMs of the 1DPC structure. The experimental results are well matching with numerical simulations. The occurrence of IMCE regardless of the availability of BSWs removes the dependence on just the surface mode for obtaining coupled emission from 1DPCs. The observation of IMCE is expected to widen the scope of 1DPCs for surface-based fluorescence sensing and assays.

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Integrated photonics aims at on-chip controlling light in the micro- and nanoscale ranges utilizing the waveguide circuits, which include such basic elements as splitters, multiplexers, and phase shifters. Several photonic platforms, including the well-developed silicon-on-insulator and surface-plasmon polaritons ones, operate well mostly in the IR region. However, operating in the visible region is challenging because of the drawbacks originating from absorption or sophisticated fabrication technology. Recently, a new promising all-dielectric platform based on Bloch surface electromagnetic waves (BSWs) in multilayer structures and functioning in the visible range has emerged finding a lot of applications primarily in sensing. Here, we show the effect of multimode interference (MMI) of BSWs and propose a method for implementing the advanced integrated photonic devices on the BSW platform. We determine the main parameters of MMI effect and demonstrate the operation of Mach-Zehnder interferometers with a predefined phase shift proving the principle of MMI BSW-based photonics in the visible spectrum. Our research will be useful for further developing a versatile toolbox of the BSW platform devices which can be essential in integrated photonics, lab-on-chip, and sensing applications.

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The growth in aerosol particles caused by water uptake during increasing ambient relative humidity alters the physical and chemical properties of aerosols, which then affects public health, atmospheric chemistry, and the Earth's climate. The temporal resolution and sensitivity of current techniques are not sufficient to measure the growth dynamics of single aerosol nanoparticles. Additionally, the specific time required for phase transition from solid to aqueous has not been measured. Here, we describe a label-free photonic microscope that uses the Bloch surface waves as the illumination source for imaging and sensing to provide real-time measurements of the hygroscopic growth dynamics of a single aerosol (diameter <100 nm) containing the main components of air pollution. This specific time can be measured for both pure and mixed aerosols, showing that organics will delay the phase transition. This photonic microscope can be extended to investigate physicochemical reactions of various aerosols, and then knowing this specific time will be favorable for understanding the reaction kinetics among single aerosols and the surrounding medium.

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We report on the combined label-free/fluorescence use of one-dimensional photonic crystals to optimize cancer biomarker detection in complex biological media. The optimization of the assay working parameters permits us to maximize the final response of the biosensor. The detection approach utilizes a sandwich assay, in which one-dimensional photonic crystals sustaining Bloch surface waves are modified with monoclonal antibodies in order to guarantee high specificity during biological recognition. The multiple outcomes generated by such optimization experiments permitted us to determine the effective capture efficiency and the repeatability of the immobilization process, which was estimated to be close to 5%. By exploiting the resolution of the fluorescence operation mode, we studied non-specific interactions in different blocking agents, different analyte diluting buffers, and diverse concentrations of the detection antibody. As a clinically relevant biomarker, we selected the trans-membrane receptor tyrosine kinase HER2. HER2 regulates a variety of cell proliferation, growth, and differentiation pathways and its over-expression occurs in approximately 20-30% of breast cancer worldwide. As a final application, we transferred all the optimized working parameters to HER2 cancer biomarker assays in a complex biological environment. The label-free and fluorescence results obtained by analyzing MCF-7 (HER2 low positive) and 32D (HER2 negative) cell lysates demonstrate that we can successfully discriminate the two lysates.

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The total internal reflection ellipsometry (TIRE) method was used for the excitation and study of the sensitivity features of surface plasmon polariton (SPP) and Bloch surface waves (BSWs) resonances. For the BSWs generation distributed Bragg gratings were formed on the tops of the substrates (BK7 glass substrate), which had six bilayers of ~120 nm SiO2 and ~40 nm TiO2 and 40 nm of TiO2 on the top. The SPP sample consisted of the BK7 glass prism and a gold layer (45 nm). Numerical calculations of the optical dispersions and the experimental TIRE data have shown that SPP resonance overtake the BSWs in wavelength scanning by a factor of about 17. However, for the ellipsometric parameters Ψ and Δ in the vicinity of excitations, the BSW sensitivity is comparable with SPP. The obtained resolutions were Δ S P P = 7.14 × 10 - 6 R I U , Ψ S P P = 1.7 × 10 - 5 R I U for the SPP and Δ B S W = 8.7 × 10 - 6 R I U , Ψ B S W = 2.7 × 10 - 5 R I U for the BSW. The capabilities of both surface excitations are discussed from the sensitivity point of view in the design of these advanced biosensors.

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A combined label-free and fluorescence surface optical technique was used to quantify the mass deposited in binary biomolecular coatings. These coatings were constituted by fibronectin (FN), to stimulate endothelialization, and phosphorylcholine (PRC), for its hemocompatibility, which are two properties of relevance for cardiovascular applications. One-dimensional photonic crystals sustaining a Bloch surface wave were used to characterize different FN/PRC coatings deposited by a combination of adsorption and grafting processes. In particular, the label-free results permitted to quantitatively assess the mass deposited in FN adsorbed (185 ng/cm2) and grafted (160 ng/cm2). PRC binding to grafted FN coatings was also quantified, showing a coverage as low as 10 and 12 ng/cm2 for adsorbed and grafted PRC, respectively. Moreover, desorption of FN deposited by adsorption was detected and quantified upon the addition of PRC. The data obtained by the surface optical technique were complemented by water contact angle and X-ray photoelectron spectroscopy (XPS) analyses. The results were in accordance with those obtained previously by qualitative and semiquantitative techniques (XPS, time-of-flight secondary ion mass spectrometry) on several substrates (PTFE and stainless steel), confirming that grafted FN coatings show higher stability than those obtained by FN adsorption.

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Optical biosensors based on one-dimensional photonic crystals sustaining Bloch surface waves are proposed to study antibody interactions and perform affinity studies. The presented approach utilizes two types of different antibodies anchored at the sensitive area of a photonic crystal-based biosensor. Such a strategy allows for creating two or more on-chip regions with different biochemical features as well as studying the binding kinetics of biomolecules in real time. In particular, the proposed detection system shows an estimated limit of detection for the target antibody (anti-human IgG) smaller than 0.19 nM (28 ng/mL), corresponding to a minimum surface mass coverage of 10.3 ng/cm². Moreover, from the binding curves we successfully derived the equilibrium association and dissociation constants (KA = 7.5 × 107 M-1; KD = 13.26 nM) of the human IgG⁻anti-human IgG interaction.

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We report on the use of one-dimensional photonic crystals to detect clinically relevant concentrations of the cancer biomarker ERBB2 in cell lysates. Overexpression of the ERBB2 protein is associated with aggressive breast cancer subtypes. To detect soluble ERBB2, we developed an optical set-up which operates in both label-free and fluorescence modes. The detection approach makes use of a sandwich assay, in which the one-dimensional photonic crystals sustaining Bloch surface waves are modified with monoclonal antibodies, in order to guarantee high specificity during the biological recognition. We present the results of exemplary protein G based label-free assays in complex biological matrices, reaching an estimated limit of detection of 0.5 ng/mL. On-chip and chip-to-chip variability of the results is addressed too, providing repeatability rates. Moreover, results on fluorescence operation demonstrate the capability to perform high sensitive cancer biomarker assays reaching a resolution of 0.6 ng/mL, without protein G assistance. The resolution obtained in both modes meets international guidelines and recommendations (15 ng/mL) for ERBB2 quantification assays, providing an alternative tool to phenotype and diagnose molecular cancer subtypes.

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We report on the use of one-dimensional photonic crystals to detect clinically relevant concentrations of ERBB2/neu/Her2 in cell lysates. ERBB2 is a pivotal breast cancer biomarker and targetable oncogenic driver associated with aggressive breast cancer subtypes. To quantitate soluble ERBB2, we developed an optical platform that combines label-free and fluorescence detection modes. Such platform makes use of a sandwich assay in which the one-dimensional photonic crystals sustaining Bloch surface waves are tailored with a monoclonal antibody for highly specific biological recognition (BSW biochip). In a second step, a second antibody to ERBB2 quantitatively detects the bound analyte. The strategy of the present approach takes advantage of the combination of label-free and fluorescence techniques, making bio-recognition more robust and sensitive. In the fluorescence operation mode, the platform can attain the limit of detection 0.3ng/mL (1.5pM) for ERBB2 in cell lysates. Such resolution meets the international guidelines and recommendations (15ng/mL) for diagnostic ERBB2 assays that in the future may help to more precisely assign therapies counteracting cancer cell proliferation and metastatic spread.

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Resonant propagating waves created on the surface of a dielectric multilayer stack, called Bloch surface waves (BSW), can be designed for high sensitivity monitoring of the adjacent refractive index as an alternative platform to the metal-based surface plasmon resonance (SPR) sensing. The resonant wavelength and polarization can be designed by engineering of the dielectric layers unlike the fixed resonance of SPR, while the wide bandwidth low loss of dielectrics permits sharper resonances, longer propagation lengths and thus their use in waveguiding devices. The transparency of the dielectrics allows the excitation and monitoring of surface-bound fluorescent molecules. We review the recent developments in this technology. We show the advantages that can be obtained by using high index contrast layered structures. Operating at 1550 nm wavelengths will allow the BSW sensors to be implemented in the silicon photonics platform where active waveguiding can be used in the realization of compact planar integrated circuits for multi-parameter sensing.