Bloch surface waves at the telecommunication wavelength with lithium niobate as the top layer for integrated optics.

Lithium niobate (LN)-based devices are widely used in integrated and nonlinear optics. This material is robust and resistive to high temperatures, which makes the LN-based devices stable, but challenging to fabricate. In this work, we report on the design, manufacturing, and characterization of engineered dielectric media with thin-film LN (TFLN) on top for the coupling and propagation of electromagnetic surface waves at telecommunication wavelengths. The designed one-dimensional photonic crystal (1DPhC) sustains Bloch surface waves (BSWs) at the multilayer-air interface at 1550 nm wavelength with a propagation detected over a distance of 3 mm. The working wavelength and improved BSW propagation parameters open the way for exploration of nonlinear properties of BSW-based devices. It is also expected that these novel devices potentially would be able to modify BSW propagation and coupling by external thermal-electrical stimuli due to the improved quality of the TFLN top layer of 1DPhC.

Inverse photonic design of functional elements that focus Bloch surface waves.

Bloch surface waves (BSWs) are sustained at the interface of a suitably designed one-dimensional (1D) dielectric photonic crystal and an ambient material. The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW. An example of such an element is a focusing device that squeezes an incident BSW into a tiny space. However, the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order of Δn≈0.1 for practical reasons. Conventional elements, e.g., discs or triangles, which rely on a photonic nanojet to focus BSWs, operate insufficiently at such a low index contrast. To solve this problem, we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength. Selected examples of such functional elements are fabricated. Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope. Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs, e.g., to carry information, or lab-on-chip devices for specific sensing applications.

High-power modular LED-based illumination systems for mask-aligner lithography.

Mask-aligner lithography is traditionally performed using mercury arc lamps with wavelengths ranging from 250 nm to 600 nm with intensity peaks at the i, g and h lines. Since mercury arc lamps present several disadvantages, it is of interest to replace them with high power light emitting diodes (LEDs), which recently appeared on the market at those wavelengths. In this contribution, we present a prototype of an LED-based mask-aligner illumination. An optical characterization is made and the prototype is tested in a mask-aligner. Very good performances are demonstrated. The measured uniformity in the mask plane is 2.59 ± 0.24 % which is within the uniformity of the standard lamp. Print tests show resolution of 1 micron in contact printing and of 3 microns in proximity printing with a proximity gap of 30 microns.

Lippmann waveguide spectrometer with enhanced throughput and bandwidth for space and commercial applications.

This article presents an innovative high spectral resolution waveguide spectrometer, from the concept to the prototype demonstration and the test results. The main goal is to build the smallest possible Fourier transform spectrometer (FTS) with state of the art technology. This waveguide FTS takes advantage of a customized pattern of nano-samplers fabricated on the surface of a planar waveguide that allows the increase of the measurement points necessary for increasing the spectral bandwidth of the FTS in a fully static way. The use of a planar waveguide on the other hand allows enhancing the throughput in a waveguide spectrometer compared to the conventional devices made of single-mode waveguides. A prototype is made in silicon oxynitride/silicon dioxide technology and characterized in the visible range. This waveguide spectrometer shows a nominal bandwidth of 256~nm at a central wavelength of 633~nm thanks to a custom pattern of nanodisks providing a µm sampling interval. The implementation of this innovative waveguide FTS for a real-case scenario is explored and further development of such device for the imaging FTS application is discussed.

Magnetic spin-orbit interaction of light.

We study the directional excitation of optical surface waves controlled by the magnetic field of light. We theoretically predict that a spinning magnetic dipole develops a tunable unidirectional coupling of light to transverse electric (TE) polarized Bloch surface waves (BSWs). Experimentally, we show that the helicity of light projected onto a subwavelength groove milled into the top layer of a 1D photonic crystal (PC) controls the power distribution between two TE-polarized BSWs excited on both sides of the groove. Such a phenomenon is shown to be solely mediated by the helicity of the magnetic optical field, thus revealing a magnetic spin-orbit interaction of light. Remarkably, this magnetic optical effect is clearly observed via a near-field coupler governed by an electric dipole moment: it is of the same order of magnitude as the electric optical effects involved in the coupling. This opens up new degrees of freedom for the manipulation of light and offers desirable and novel opportunities for the development of integrated optical functionalities.

Binary zero-order diffractive and anti-reflective optical elements in silicon for the mid-infrared.

We propose a novel method for the design of binary two-level diffractive optical elements that provide an efficient diffractive function while reducing the reflectivity of a high-index interface. The structure can be fabricated with a single patterning step and is particularly useful for the mid-infrared spectral range. The concept is based on zero-order transmission gratings using subwavelength microstructures. To demonstrate the concept a Dammann grating has been realized in silicon and characterized in the mid-infrared by scanning the far-field intensity distribution.

Polarization controlled directional propagation of Bloch surface wave.

Bloch surface waves (BSWs) are recently developing alternative to surface plasmon polaritons (SPPs). Due to dramatically enhanced propagation distance and strong field confinement these surface states can be successfully used in on-chip all-optical integrated devices of increased complexity. In this work we propose a highly miniaturized grating based BSW coupler which is gathering launching and directional switching functionalities in a single element. This device allows to control with polarization the propagation direction of Bloch surface waves at subwavelength scale, thus impacting a large panel of domains such as optical circuitry, function design, quantum optics, etc.

Design, simulation, and quality evaluation of micro-optical freeform beam shapers at different illumination conditions.

A design method to generate thin micro-optical freeform (MOF) beam shapers by clipping or wrapping an original and much thicker freeform surface is provided. MOF elements are situated at the border between refractive and diffractive optical elements. The influence of parameters such as the clipping factor q, the peak-to-valley amplitude of the original surface, the design wavelength, and the spectrum of the light source (single wavelength and multiple wavelength lines) into the quality of the output intensity distributions has been studied. Integer q values are mandatory for good quality at monochromatic illumination. On the contrary, the quality obtained by broadband illumination oscillates with q and peaks maximally at around q=3.

Refraction limit of miniaturized optical systems: a ball-lens example.

We study experimentally and theoretically the electromagnetic field in amplitude and phase behind ball-lenses across a wide range of diameters, ranging from a millimeter scale down to a micrometer. Based on the observation, we study the transition between the refraction and diffraction regime. The former regime is dominated by observables for which it is sufficient to use a ray-optical picture for an explanation, e.g., a cusp catastrophe and caustics. A wave-optical picture, i.e. Mie theory, is required to explain the features, e.g., photonic nanojets, in the latter regime. The vanishing of the cusp catastrophe and the emergence of the photonic nanojet is here understood as the refraction limit. Three different criteria are used to identify the limit: focal length, spot size, and amount of cross-polarization generated in the scattering process. We identify at a wavelength of 642 nm and while considering ordinary glass as the ball-lens material, a diameter of approximately 10 µm as the refraction limit. With our study, we shed new light on the means necessary to describe micro-optical system. This is useful when designing optical devices for imaging or illumination.

Two-dimensional polymer grating and prism on Bloch surface waves platform.

A one-dimensional photonic crystal sustaining Bloch Surface Waves (BSWs) is used as a platform for two-dimensional integrated optics. The dielectric platform shows low loss, long propagation distance and high surface field enhancement. In order to study the potential of the platform for future photonic chips, polymer ultra-thin prisms and gratings (~100 nm) are engineered on the top of the platform. This polymer layer modifies the BSWs effective index enabling a direct manipulation of light. The BSW deflection effects caused by surface prisms are observed in the near-field and Snells law is verified; then the BSW diffractions through surface gratings are experimentally and theoretically characterized. The results show a robust platform that can be used for integrated optics that includes different optical components. One of the main advantages is that these 2D photonic devices can have arbitrary shapes, which is difficult to obtain in 3D.

Vertically coupled plasmonic slot waveguide cavity for localized biosensing applications.

We propose and study an integrated refractive index sensor which is based on a plasmonic slot waveguide cavity. In this device, a guided mode supported by a silicon photonic wire waveguide is vertically coupled to a metal-dielectric-metal cavity separated by a silicon oxide spacer. We perform an in-depth study that links the geometrical parameters of the sensor to the coupling mechanism and sensitivity of the plasmonic slot waveguide cavity. Simulation results promise that local changes of refractive index can be measured with a high sensitivity of around 600 nm/RIU in a femto-liter volume. These results are obtained with three-dimensional time and frequency domain simulations. Thanks to the high field enhancement in the slot of the plasmonic cavity, a high local sensitivity to changes of refractive index is obtained. Moreover, the high level of achieved decoupling between the bulk and the local sensitivity complies well with the requirements of biomolecular sensing.

Focusing and extraction of light mediated by Bloch surface waves.

The control of emission from localized light sources is an objective of outstanding relevance in nanophotonics. In a recent past, a large number of metallic nanostructures has been proposed to this end, wherein plasmonic modes are exploited as energy carriers on a subwavelength scale. As an interesting alternative, we present here the use of surface modes on patterned dielectric multilayers to deliver electromagnetic power from free-space to localized volumes and vice versa. Thanks to this low-loss energy transfer, proper periodic ring structures are shown to provide a subwavelength focusing of an external radiation onto the multilayer surface. By reciprocity, the radiated power from emitters within the ring center is shown to be efficiently beamed in the free-space, with a well-controlled angular divergence. This mechanism overcomes some important limitations involved in the all-plasmonic approach, while opening new opportunities for hybrid devices in photon management applications such as optical sensing and lighting.

Mid-infrared spectroscopy for gases and liquids based on quantum cascade technologies.

In this paper we present two compact, quantum cascade laser absorption spectroscopy based, sensors developed for trace substance detection in gases and liquids. The gas sensor, in its most integrated version, represents the first system combining a quantum cascade laser and a quantum cascade detector. Furthermore, it uses a toroidal mirror cell with a volume of only 40 cm(3) for a path length of up to 4 m. The analytical performance is assessed by the measurements of isotope ratios of CO2 at ambient abundance. For the (13)CO2/(12)CO2 isotope ratio, a measurement precision of 0.2‰ is demonstrated after an integration time of 600 s. For the liquid sensor, a microfluidic system is used to extract cocaine from saliva into a solvent (PCE) transparent in the mid-infrared. This system is bonded on top of a Si/Ge waveguide and the concentration of cocaine in PCE is measured through the interaction of the evanescent part of the waveguide optical mode and the solvent flowing on top. A detection limit of <100 µg mL(-1) was achieved with this system and down to 10 µg mL(-1) with a simplified, but improved system.

Plasmonic silicon solar cells: impact of material quality and geometry.

We study n-i-p amorphous silicon solar cells with light-scattering nanoparticles in the back reflector. In one configuration, the particles are fully embedded in the zinc oxide buffer layer; In a second configuration, the particles are placed between the buffer layer and the flat back electrode. We use stencil lithography to produce the same periodic arrangement of the particles and we use the same solar cell structure on top, thus establishing a fair comparison between a novel plasmonic concept and its more traditional counterpart. Both approaches show strong resonances around 700 nm in the external quantum efficiency the position and intensity of which vary strongly with the nanoparticle shape. Moreover, disagreement between simulations and our experimental results suggests that the dielectric data of bulk silver do not correctly represent the reality. A better fit is obtained by introducing a porous interfacial layer between the silver and zinc oxide. Without the interfacial layer, e.g. by improved processing of the nanoparticles, our simulations show that the nanoparticles concept could outperform traditional back reflectors.

Real-time amyloid aggregation monitoring with a photonic crystal-based approach.

We propose the application of a new label-free optical technique based on photonic nanostructures to real-time monitor the amyloid-beta 1-42 (Aß(1-42)) fibrillization, including the early stages of the aggregation process, which are related to the onset of the Alzheimer's Disease (AD). The aggregation of Aß peptides into amyloid fibrils has commonly been associated with neuronal death, which culminates in the clinical features of the incurable degenerative AD. Recent studies revealed that cell toxicity is determined by the formation of soluble oligomeric forms of Aß peptides in the early stages of aggregation. At this phase, classical amyloid detection techniques lack in sensitivity. Upon a chemical passivation of the sensing surface by means of polyethylene glycol, the proposed approach allows an accurate, real-time monitoring of the refractive index variation of the solution, wherein Aß(1-42) peptides are aggregating. This measurement is directly related to the aggregation state of the peptide throughout oligomerization and subsequent fibrillization. Our findings open new perspectives in the understanding of the dynamics of amyloid formation, and validate this approach as a new and powerful method to screen aggregation at early stages.

Leakage radiation interference microscopy.

We present a proof of principle for a new imaging technique combining leakage radiation microscopy with high-resolution interference microscopy. By using oil immersion optics it is demonstrated that amplitude and phase can be retrieved from optical fields, which are evanescent in air. This technique is illustratively applied for mapping a surface mode propagating onto a planar dielectric multilayer on a thin glass substrate. The surface mode propagation constant estimated after Fourier transformation of the measured complex field is well matched with an independent measurement based on back focal plane imaging.

Microfluidic droplet-based liquid-liquid extraction and on-chip IR spectroscopy detection of cocaine in human saliva.

We present a portable microsystem to quantitatively detect cocaine in human saliva. In this system, we combine a microfluidic-based multiphase liquid-liquid extraction method to transfer cocaine continuously from IR-light-absorbing saliva to an IR-transparent solvent (tetrachloroethylene) with waveguide IR spectroscopy (QC-laser, waveguide, detector) to detect the cocaine on-chip. For the fabrication of the low-cost polymer microfluidic chips a simple rapid prototyping technique based on Scotch-tape masters was further developed and applied. To perform the droplet-based liquid-liquid extraction, we designed and integrated a simple and robust droplet generation method based on the capillary focusing effect within the device. Compared to well-characterized and commonly used microfluidic H-filters, our system showed at least two times higher extraction efficiencies with potential for further improvements. The current liquid-liquid extraction method alone can efficiently extract cocaine and pre-concentrate the analytes in a new solvent. Our fully integrated optofluidic system successfully detected cocaine in real saliva samples spiked with the drug (500 µg/mL) and allowed real time measurements, which makes this approach suitable for point-of-care applications.

Phase anomalies in Talbot light carpets of self-images.

An interesting feature of light fields is a phase anomaly, which occurs on the optical axis when light is converging as in a focal spot. Since in Talbot images the light is periodically confined in both transverse and axial directions, it remains an open question whether at all and to which extent the phase in the Talbot images sustains an analogous phase anomaly. Here, we investigate experimentally and theoretically the anomalous phase behavior of Talbot images that emerge from a 1D amplitude grating with a period only slightly larger than the illumination wavelength. Talbot light carpets are observed close to the grating. We concisely show that the phase in each of the Talbot images possesses an anomalous axial shift. We show that this phase shift is analogous to a Gouy phase of a converging wave and occurs due to the periodic light confinement caused by the interference of various diffraction orders. Longitudinal-differential interferometry is used to directly demonstrate the axial phase shifts by comparing Talbot images phase maps to a plane wave. Supporting simulations based on rigorous diffraction theory are used to explore the effect numerically. Numerical and experimental results are in excellent agreement. We discover that the phase anomaly, i.e., the difference of the phase of the field behind the grating to the phase of a referential plane wave, is an increasing function with respect to the propagation distance. We also observe within one Talbot length an irregular wavefront spacing that causes a deviation from the linear slope of the phase anomaly. We complement our work by providing an analytical model that explains these features of the axial phase shift.

Resonant absorption of a chemically sensitive layer based on waveguide gratings.

A colorimetric sensor providing a direct visual indication of chemical contamination was developed. The sensor is a combination of a chemically sensitive dye layer and a resonant waveguide grating. Enhancement of the light absorption by the photonic structure can be clearly seen. The detection is based on the color change of the reflected light after exposure to a gas or a liquid. Low-cost fabrication and compatibility with environments where electricity cannot be used make this device very attractive for applications in hospitals, industries, with explosives, and in traffic.