Asymmetric (freeform) optical components have gathered a large deal of attention triggered by the substantial performance gains demonstrated in imaging and non-imaging optical systems. In the growing trend for device miniaturization and cost reduction through optical components with relevant dimensions well below the millimetre scale, freeform microlens arrays (FMLAs) are very promising albeit considerably more challenging to design, manufacture and characterize that their macroscopic counterparts. Here, we present some innovative designs, fabrication approaches and characterization strategies developed at CSEM to overcome some of the existing limitations.
An electrically tunable filter based on a plasmonic phase retarder and liquid crystal cells is reported. The plasmonic phase retarder consists of a periodic array of deep-subwavelength metallic nanostructures. A first entrance polarizer prepares the incident light in a polarization state oriented at 45° from the nanowires orientation. A strong phase retardation between TM and TE polarizations is induced by the plasmon resonances. A polarization analyzer based on liquid crystal cells allows to project the transmitted light onto a polarization state whose orientation depends on the applied voltage. Using this approach, a range of 8V is enough to span more than 70% of the area covered by standard RGB filters in CIE color coordinates with a single filter, including yellow, orange, red, magenta, purple, blue, cyan and green as well as different tones of white. In order to ensure the applicability to large area production, UV nanoimprint lithography (UV-NIL) and thin film coatings have been used to fabricate the plasmonic phase retarder. The evaporation is performed with an angle, so that a self-shadowing effects prevents full coverage of the surface. The resulting structure consists in a periodic array of silver nanowires. Multiple interfering resonances are observed so that the nominal transmission can reach levels above 70%. The construction of the colors transmitted by the tunable filter is modeled and validated through a series of optical characterization of the individual elements.
An electrically tunable filter based on a plasmonic phase retarder and liquid crystal cells is reported. The plasmonic phase retarder consists of a periodic array of deep-subwavelength metallic nanostructures. A first entrance polarizer prepares the incident light in a polarization state oriented at 45° from the nanowires orientation. A strong phase retardation between TM and TE polarizations is induced by the plasmon resonances. A polarization analyzer based on liquid crystal cells allows to project the transmitted light onto a polarization state whose orientation depends on the applied voltage. Using this approach, a range of 8V is enough to span more than 50% of the area covered by standard RGB filters in CIE color coordinates with a single filter.
In order to ensure the applicability to large area production, UV nanoimprint lithography (UV-NIL) and thin film coatings have been used to fabricate the plasmonic phase retarder. The nanoimprint master consists in a periodic binary grating with a sub-wavelength period below 200nm in order to avoid diffraction effects in the visible range and maximize the angular stability. The grating master is imprinted and coated with a silver thin film and encapsulated. The evaporation is performed with an angle, so that a self-shadowing effects prevents full coverage of the surface. The resulting structure consists in a periodic array of silver nanowires of total width 50nm, with a cross section forming an inverted U-shape. This particular shape shows a high degree of tunability of the plasmon resonance position given the constraints of a sub-wavelength periodicity. Multiple interfering resonances are observed so that the nominal transmission can reach >70%.
Placed between a polarizer oriented at 45° from the nanowires orientation and a liquid crystal cell, the transmission spectrum of the plasmonic phase retarder can be tuned with the applied voltage. For a low voltage, the polarization transmitted through the liquid crystal cell is oriented along the gratings lines. For higher voltage, the light transmitted through the liquid crystal cell is oriented across the grating lines and the resulting spectrum has a dip in transmission, which is the signature of a plasmon resonance. At a voltage of 8V, a full rotation of the polarization by 180° has been applied. Different colors can be obtained within this range, including orange, magenta, purple, blue, turquoise, green and yellow with the same tunable filter. Other designs have been investigated in order to obtain more saturated blue, green or red using this approach.
Some of the latest developments at CSEM Center Muttenz using diffraction gratings applied for a high resolution spectroscopy, light management in solar cells and LED, color filters, optical security, nano-membrane fabrication etc. will be presented. CSEM Center Muttenz has built a full chain for the grating fabrication, beginning with the design and ending with the optical device. The simulations are made to fulfill the appropriate properties of reflected/diffracted light for the planned application. This permits to fully specify the grating geometry: period, profile, duty cycle and depth. Gratings (linear or crossed) are initially fabricated in a photoresist using a Laser Interference Lithography (LIL) with periods going from 220nm up to 2000nm this on a surface of 5”. If needed the gratings can be transferred into the substrate (glass, Quartz, steel or dielectric) using a dry etching processes and a Cr masking. With this technology flat substrates can be structured with gratings, but also concave or convex surface can be processed. Different grating shapes are available like: sinusoidal or quasi-sinusoidal, triangle, rectangular, “U” type or blazed. This technology allows adjusting the duty cycle of the rectangular gratings, 20/80 is the minimum we can achieve but, this will depend on the depth to period ratio requested. A typical value we can achieve as “Depth to Period” ratio is 1.6. Most of the applications involving gratings will need a hard copy, the later can be made into glass, or a SolGel type of material like, Ormocer or also with a Ni shim. The Ni shims are useful for replication like hot embossing, including the Roll to Roll processes, UV casting. It allows transferring the grating structures into different flexible polymers, like PMMA, PC, PET etc. Specific metals or dielectric materials deposited on the gratings will permit to create different requested color effects.
Single-photon avalanche diodes (SPADs) are direct photon-to-digital detectors that enable scalable arrays with Poisson-limited signal-to-noise ratio and picosecond timing resolution. However, SPAD detectors require a guard-ring structure to prevent lateral edge breakdown. The guard ring, in addition to pixel electronics, reduces the sensitive area within the pixel, often below 50%. We present the simulation, design and characterization of microlens structures to increase the effective fill factor and SPAD photon detection efficiency. The main challenges in designing microlenses for SPADs are a relatively large SPAD pitch and a low native fill factor, requiring high microlens efficiency over a wide angular distribution of light. In addition, we addressed the requirements of several designs in the same technology, featuring native fill factors which range from 10.5% to 28%, by carrying out the microlens fabrication at wafer reticle level. The fabrication process starts with creating a photoresist microlens master, used to fabricate a mould for microlens imprints. After dispensing a UV curable hybrid polymer on top of the SPAD array, the mould is used to imprint the microlens array shape, and then cured with UV exposure. By using microlenses, we were able to increase the initial fill factor to more than 84% effective fill factor for a 28.5 μm pixel pitch. We also explore the influence of the passivation layer on the SPAD photon detection efficiency.
Chronic wounds represent a significant burden to patients, health care professionals, and health care systems, affecting over 40 million patients and creating costs of approximately 40 billion € annually. We will present a medical device for photo-stimulated wound care based on a wearable large area flexible and disposable light management system consisting of a waveguide with incorporated micro- and nanometer scale optical structures for efficient light in-coupling, waveguiding and homogeneous illumination of large area wounds. The working principle of this innovative device is based on the therapeutic effects of visible light to facilitate the self-healing process of chronic wounds. On the one hand, light exposure in the red (656nm) induces growth of keratinocytes and fibroblasts in deeper layers of the skin. On the other hand, blue light (453nm) is known to have antibacterial effects predominately at the surface layers of the skin. In order to be compliant with medical requirements the system will consist of two elements: a disposable wound dressing with embedded flexible optical waveguides for the light management and illumination of the wound area, and a non-disposable compact module containing the light sources, a controller, a rechargeable battery, and a data transmission unit. In particular, we will report on the developed light management system. Finally, as a proof-of-concept, a demonstrator will be presented and its performances will be reported to demonstrate the potential of this innovative device.
We present an innovative disposable endoscope based on extra flat flexible polymer slabs used as multimode
waveguides. The waveguides are compatible with low-cost roll-to-roll production technologies and can be easily
customized by patterning, coating and printing techniques according to the specifications of the target application. In
order to couple the light (i.e. the illumination beam and the imaging beam) in and out of the waveguide, diffractive
subwavelength gratings are used. These nano-scale optical structures enable an efficient and controlled light trapping by
total internal reflection, thus minimizing the distortion effects generated by the rough edges. Nano-patterning is obtained
using established techniques (i.e. hot embossing and/or UV casting) that are compatible with industrial roll-to-roll
production lines or plastic injection molding.
Unique features of these innovative endoscopes are i) the achievable very thin form that can be reduced to thicknesses
below 200 μm, ii) the ability to record lateral images with respect to the endoscope direction, iii) the ability to image
samples (e.g. tissues, tiny objects) in direct contact with the polymer slab, with a minimum imaging distance equal to
zero, and iv) the access to high volume fabrication techniques that can enable the production of low-cost disposable
A possible device implementation is demonstrated and tested, which consists of a flat line-scanning endoscope enabling
the acquisition of 1D images in monochromatic illumination and the reconstruction of 2D images by scanning. Images
taken with such a disposable endoscope are discussed and the related technological constraints such as manufacturing
tolerances, image distortion, scattered light and signal to noise ratio are further described. Finally, advantages and
disadvantages with respect to other endoscopic techniques will be discussed, thus demonstrating the potential of this
innovative approach for endoscopic applications in very confined volumes.
This work addresses feature size effects (the lag-effect and roughness development) in chemically assisted ion beam etching (CAIBE) etching of InP based photonic crystals. Photonic crystal fields with varying hole size and periods were etched with different etching times. The slope of the etch depth versus diameter curves (lag-curves) reveals a hole size dependence, with a critical aspect ratio higher than 25. A model for the etch rate specific to Ar/Cl2 CAIBE is proposed. We calculate the etch rate using a physico-chemical model which takes in to account the effect of Ar-ion sputtering and surface chemical reactions. In addition, it combines the aspect ratio dependence of the gas conductance of the etched holes. The origin and evolution of the bottom roughness of the etched holes is examined. The impact of the feature size dependence of the etching on the photonic crystal optical properties is then assessed by measuring the quality-factor of one dimensional Fabry Perot cavities using the Internal Light Source method, and discussed in terms of hole shape and depth. A systematic trend between the determined quality factor (Q) and the lag-effect is evidenced: Q decreases from about 250 to 60 when the hole depth drops from 5 μm to 2 μm.
Recently there has been a growing amount of attention devoted to tuneable photonic crystals (PhCs) where the optical response of PhC structures can be dynamically modified. We will show how infiltrating planar PhCs with a synthetic organic material allows the trimming and tuning of their optical properties. The potential of PhC infiltration
will be demonstrated for InP-based planar PhCs consisting of a hexagonal array of air holes (hole diameter = 200 − 400 nm; air filling factor = 0.40-0.50) etched through a planar waveguide in which light emitters (i.e. quantum wells) were embedded to enable optical measurements. The PhC pores were infiltrated with LC-K15 (5CB) nematic liquid crystals (LCs) in a specifically designed vacuum chamber, thereby changing the refractive index contrast between the holes and the semiconductor (trimming). Moreover, the possibility of tuning the optical response of PhCs by an external perturbation (i.e. temperature) was demonstrated. The change of the PhC optical properties due to infiltration and temperature tuning was studied both experimentally and theoretically. Experimental measurements were compared to theoretical calculations in order to obtain information on the in-filling efficiency, the LC refractive index, and the molecule orientation inside the holes. In the first case, optical measurements were performed as a function of
temperature, whilst the average LC director configuration was determined by comparing transmission spectra in the transverse electric and magnetic polarization directions.
Electromagnetic Bloch waves are the standard representation of the
optical field in two-dimensional photonic crystals (2D-PhCs). We
present an intuitive description of Bloch waves based on their
Fourier transform into series of electromagnetic plane waves. The
contribution of each plane wave to the global energy and group
velocity is detailed and the valid domain of this decomposition is
discussed. This approach enables a continuous description of light
propagation from the homogeneous medium to the strongly modulated
PhC case and resolves inconsistencies that result from band
folding. Finally this model provides a clear physical
understanding of the negative refraction effects observed in
We report on the temperature tuning of the optical properties of planar Photonic Crystal (PhC) microcavities. Studies were made on one and two dimensional PhCs that were etched in InP and GaAs vertical waveguides. Two dimensional (hexagonal) and one-dimensional (Fabry-Perot) cavities were optically investigated by an internal light source technique. The samples were mounted on a Peltier-stage which allowed temperature variation from T = 20 °C up to T = 76 °C. A linear dependence of the resonance wavelengths with respect to temperature is observed. A gradient of dλ/dT = 0.09 nm/°C and 0.1 nm/°C for the GaAs and InP based cavities was observed, respectively. These results are in agreement with the theoretical calculations based on the thermal dependence of the refractive index of the PhC semiconductor component.
Photonic crystals have seen major advances in the past few years in the optical range. The association of in-plane waveguiding and two-dimensional (2D) photonic crystals (PCs) in thin-slab or waveguide structures leads to good 3D confinement with easy fabrication. Such structures, much easier to fabricate than 3D PCs, open many exciting opportunities in optoelectronic devices and integrated optics. We review the basics of these structures, with emphasis on basic properties and loss performance, as well as modeling tools, which show that 2D PCs etched through waveguides supported by substrates are a viable route to high-performance PC-based photonic integrated circuits (PICs). A companion paper by Benisty et al. in these proceedings illustrates further high performance building blocks and integrated devices.
Practical realizations of 2D (planar) photonics crystal (PhC) are either on a membrane or etched through a conventional heterostructure. While fascinating objects can emerge from the first approach, only the latter approach lends itself to a progressive integration of more compact PhC's towards monolithic PICs based on InP. We describe in this talk the various aspects from technology to functions and devices, as emerged from the European collaboration "PCIC." The main technology tour de force is deep-etching with aspect ratio of about 10 and vertical sidewall, achieved by three techniques (CAIBE, ICP-RIE, ECR-RIE). The basic functions explored are bends, splitters/combiners, mirrors, tapers, and the devices are filters and lasers. At the end of the talk, I will emphasize some positive aspects of "broad" multimode PhC waveguides, in view of compact add-drop filtering action, notably.