Enhanced sensitivity of photonic crystal slab transducers by oblique-angle layer deposition.

Photonic crystal slabs (PCS) are one of the major transducers for label-free, optical biosensing applications. In this paper we present oblique-angle layer deposition of the high index slab material as a method to improve the PCS sensitivity. In simulations and experiments we consider PCSs composed of a high index silicon monoxide layer on a nanostructured resist layer on a glass substrate. By mounting the substrate at an oblique angle with respect to the evaporation source, the high index material distribution on the nanostructured surface is modified due to shadowing effects. Finite-difference time-domain (FDTD) simulations were performed to predict bulk and surface sensitivities. In order to verify the simulation results we fabricated PCSs at various deposition angles using nanoimprint lithography to replicate a linear grating nanostructure into the resist layer and thermal evaporation for a 60-nm silicon monoxide deposition. The bulk sensitivities of these structures were measured using water-glycerol dilutions. A sensitivity improvement of 281% was obtained for PCSs fabricated at 45° deposition angle compared to normal incidence deposition.

Photonic crystal slabs for surface contrast enhancement in microscopy of transparent objects.

In optical microscopy the contrast of transparent objects achieved with conventional methods is often not satisfactory, for example for the automated recognition of cells. In this paper we present a nano-optical label-free approach for contrast enhancement based on photonic crystal slabs (PCS) as the specimen holder. Quasi-guided modes inside these structures cause an intrinsic color of the PCS, which strongly depends on the wavelength and the quality factor of the optical mode. Objects on the surface of the PCS experience a significant color and intensity contrast enhancement, as they change properties of the optical modes.

Visual device for pressure measurement using photonic crystal slabs.

We propose and demonstrate a visual, all-optical pressure-measuring device composed of a flexible membrane dilating toward a photonic crystal slab. Due to its transparency and capability to be miniaturized, it may be integrated on the inner side of an artificial lens and directly measure the eye's intraocular pressure. Using crossed polarization filters for the readout process, we obtain a contrast enhancement for the circular contact area of the membrane with the photonic crystal slab. We demonstrate that the visible circle increases as a function of pressure.

Near cut-off wavelength operation of resonant waveguide grating biosensors.

Numerical simulations and analytical calculations are performed to support the design of grating-coupled planar optical waveguides for biological sensing. Near cut-off and far from cut-off modes are investigated, and their characteristics and suitability for sensing are compared. The numerical simulations reveal the high sensitivity of the guided mode intensity near the cut-off wavelength for any refractive index change along the waveguide. Consequently, it is sufficient to monitor the intensity change of the near cut-off sensing mode, which leads to a simpler sensor design compared to those setups where the resonant wavelength shift of the guided mode is monitored with high precision. The operating wavelength and the sensitivity of the proposed device can be tuned by varying the geometrical parameters of the corrugated waveguide. These results may lead to the development of highly sensitive integrated sensors, which have a simple design and therefore are cost-effective for a wide range of applications. These numerical findings are supported with experimental results, where the cut-off sensing mode was identified.

Low-cost label-free biosensors using photonic crystals embedded between crossed polarizers.

There is a strong need for low-cost biosensors to enable rapid, on-site analysis of biological, biomedical, or chemical substances. We propose a platform for label-free optical biosensors based on applying the analyte onto a surface-functionalized photonic crystal slab and performing a transmission measurement with two crossed polarization filters. This dark-field approach allows for efficient background suppression as only the photonic crystal guided-mode resonances interacting with the functionalized surface experience significant polarization rotation. We present a compact biosensor demonstrator using a low-cost light emitting diode and a simple photodiode capable of detecting the binding kinetics of a 2.5 nM solution of the protein streptavidin on a biotin-functionalized photonic crystal surface.

Optical characterization of photonic crystal slabs using orthogonally oriented polarization filters.

We present an experimental method for direct analysis of guided-mode resonances in photonic crystal slab structures using transmission measurements. By positioning the photonic crystal slab between orthogonally oriented polarization filters light transmission is suppressed except for the guided-mode resonances. Angle resolved transmission measurements with crossed polarizers are performed to obtain the band structure around the Gamma-point. Results are compared to mode simulations. Spatially resolved measurements in a confocal microscope setup are used for homogeneity characterizations. Stitching errors and inhomogeneities in exposure dose down to 1.3% in photonic crystal slabs fabricated by electron beam lithography are observed using this method.

Intensity interrogation near cutoff resonance for label-free cellular profiling.

We report a method enabling intensity-based readout for label-free cellular assays, and realize a reader device with the same footprint as a microtiter plate. For unambiguous resonance intensity measurements in resonance waveguide grating (RWG) sensors, we propose to apply resonances near the substrate cutoff wavelength. This method was validated in bulk refractive index, surface bilayer and G protein-coupled receptor (GPCR) experiments. The significantly reduced size of the reader device opens new opportunities for easy integration into incubators or liquid handling systems.

Handheld imaging photonic crystal biosensor for multiplexed, label-free protein detection.

We present a handheld biosensor system for the label-free and specific multiplexed detection of several biomarkers employing a spectrometer-free imaging measurement system. A photonic crystal surface functionalized with multiple specific ligands forms the optical transducer. The photonic crystal slab is fabricated on a glass substrate by replicating a periodic grating master stamp with a period of 370 nm into a photoresist via nanoimprint lithography and deposition of a 70-nm titanium dioxide layer. Capture molecules are coupled covalently and drop-wise to the photonic crystal surface. With a simple camera and imaging optics the surface-normal transmission is detected. In the transmission spectrum guided-mode resonances are observed that shift due to protein binding. This shift is observed as an intensity change in the green color channel of the camera. Non-functionalized image sections are used for continuous elimination of background drift. In a first experiment we demonstrate the specific and time-resolved detection of 90.0 nm CD40 ligand antibody, 90.0 nM EGF antibody, and 500 nM streptavidin in parallel on one sensor chip. In a second experiment, aptamers with two different spacer lengths are used as receptor. The binding kinetics with association and dissociation of 250 nM thrombin and regeneration of the sensor surface with acidic tris-HCl-buffer (pH 5.0) is presented for two measurement cycles.

Photonic crystal biosensors towards on-chip integration.

Photonic crystal technology has attracted large interest in the last years. The possibility to generate highly sensitive sensor elements with photonic crystal structures is very promising for medical or environmental applications. The low-cost fabrication on the mass scale is as advantageous as the compactness and reliability of photonic crystal biosensors. The possibility to integrate microfluidic channels together with photonic crystal structures allows for highly compact devices. This article reviews different types of photonic crystal sensors including 1D photonic crystal biosensors, biosensors with photonic crystal slabs, photonic crystal waveguide biosensors and biosensors with photonic crystal microcavities. Their applications in biomolecular and pathogen detection are highlighted. The sensitivities and the detection limits of the different biosensors are compared. The focus is on the possibilities to integrate photonic crystal biosensors on-chip.