Multispectral imagers collect a hypercube of data, where the spatial image is along two-dimensions and the spectral information is in the third. Two main technologies are used for multispectral imaging: sweeping, where the hypercube is built by scanning through different wavelengths or spatial positions and snapshot multispectral spectral imaging, where the 3D cube of images is taken in one shot. Sweeping imaging systems tend to have more lines and better spectral resolutions whilst snapshot cameras are often used for dynamic analysis of scenes. A common method to obtain the hypercube in snapshot imagers is by pixel level filtering on the sensor chip. Pixel level filtering, where the filter is placed directly on the pixels are intergrated into the wafer-level making processing making them difficult to customize. Therefore, these sensors tend to aim for equally spaced spectral lines in order to cover many applications. This results in an often in an unnecessarily large data cube when only a few spectral lines are needed, moreover the spectral lines are not adapted to the specific application. In this work we propose a multispectral camera based on plenoptic imaging, where the filtering is done in a front-end optics module. Our camera has the usual advantages of a snapshot imager, and the added advantage that the spectral lines can be both reduced and tailored to the specific application by customizing the filter. This procedure reduces the hypercube whilst keeping performance by selecting the relevant data. Moreover, the filter is interchangeable for different applications The camera presented here is built with off-the-shelf components, shows >40 spectral channels, image sizes are 260x260 pixels, with pixel limited spatial resolution. We demonstrate this technology by fruit quality control using machine learning algorithms.
We have designed, fabricated and tested narrow-band Fabry-Perot filters in the infrared using gold porous mirrors and a silicon spacer layer. The filter peaks at 10 μm and 15 μm have approximately 10% transmission and a 1.5% linewidth. A Fabry-Perot structure with plane metal layers having a similar linewidth would have a transmission of only 0.2%. Thus, for the same linewidth we have improved the transmission by a factor of 50. Apart from the optical enhancements, these filters also have the advantage that they can be made inexpensively in a standard silicon MEMS technology and that their resonances can be finely tuned through post processing.
Diffractive MEMS are interesting for a wide range of applications, including displays, scanners or switching elements. Their advantages are compactness, potentially high actuation speed and in the ability to deflect light at large angles.
We have designed and fabricated deformable diffractive MEMS grating to be used as tuning elements for external cavity lasers. The resulting device is compact, has wide tunability and a high operating speed.
The initial design is a planar grating where the beams are free-standing and attached to each other using leaf springs. Actuation is achieved through two electrostatic comb drives at either end of the grating. To prevent deformation of the free-standing grating, the device is 10 μm thick made from a Silicon on Insulator (SOI) wafer in a single mask process.
At 100V a periodicity tuning of 3% has been measured. The first resonant mode of the grating is measured at 13.8 kHz, allowing high speed actuation. This combination of wide tunability and high operating speed represents state of the art in the domain of tunable MEMS filters.
In order to improve diffraction efficiency and to expand the usable wavelength range, a blazed version of the deformable MEMS grating has been designed. A key issue is maintaining the mechanical properties of the original device while providing optically smooth blazed beams. Using a process based on anisotropic KOH etching, blazed gratings have been obtained and preliminary characterization is promising.
Hyperspectral imaging allows the collection of both spectral and spatial information. This modality is naturally fitted for object and material identification or detection processes, and has encountered a large success in the agriculture and food industries to name a few.
In snapshot spectral imaging, the 3D cube of images is taken in one shot, with the advantage that dynamic scenes can be analyzed. The simplest way to make a hyperspectral camera is to put an array of wavelength filters on the detector and then integrate this detector with standard camera objectives. The technical challenge is to make arrays of N wavelength filters and repeat this sequence up to 100‘000 times across the detector array, where each individual filter is matched to the pixel size and can be as small as a few microns.
In this work, we generate the same effect with just one N wavelength filter array which is then multiplied and imaged optically onto the detector to achieve the same effective filter array. This was first outlined by Levoy and Hoystmeyer using microlens arrays in a light field camera (plenoptics 1.0). Instead of building our own light field camera we used an existing commercial camera, Lytro™ and used it as the engine for our telecentric hyperspectral camera. In addition, the tools to extract and rebuild the raw data from the Lytro™ camera were developed.
We demonstrate reconstructed hyperspectral images with 9 spectral channels and show how this can be increased to achieve 81 spectral channels in a single snapshot.
The EUSO (Extreme Universe Space Observatory) project is developing a new mission concept for the scientific
research of Ultra High Energy Cosmic Rays (UHECRs) from space. The EUSO wide-field telescope will look down
from space onto the Earth night sky to detect UV photons emitted from air showers generated by UHECRs in our
atmosphere. In this article we concentrate on the mitigation strategies agreed so far, and in particular on the
implementation of a careful early selection and testing of subsystem materials (including optics), design and interfaces of
the subsystem and an optimization of the instrument operational concept.
We present an optical system which integrates a plasmonic sensing surface and an angular tracking system to enable a compact refractive index measurement. A refractive index change at the surface of the sensing membrane causes a change in the angle at which monochromatic light is transmitted through the membrane. This transmission angle is measured by the angular tracking system. We show good theoretical and experimental agreement of the transmission of the plasmonic sensing surface at different angular illumination of the membranes. Using this compact optical setup the embedded angular tracking system has an accuracy of <10-4 deg. This corresponds to a sensitivity <10-5 refractive index units. Finally we demonstrate this measurement technique using different concentrations of saline solution.
In this paper we show that using optical photolithography it’s possible to obtain submicron features for periodic structures using the Talbot effect. To use the Talbot effect without the need of an absolute distance measurement between the mask and the wafer we integrate over several exposures for varying wafer mask distances. Here we discuss the salient features of ‘integrated Talbot lithography’. Particularly, we show that to obtain good contrasts an excellent control of the illumination light is essential; for this we use the MO Exposure Optics (MOEO) developed by SUSS MicroOptics (SMO). Finally we show that 1μm and 0.55μm diameter holes can be made using this technique.
Programmable MEMS diffraction gratings are used for spectroscopic applications because of their potential in tailoring visible and infrared spectra. A fully programmable MEMS diffraction grating (FPMDG), where every micro-mirror can move independently in a range 0 - λ/2, where λ is the wavelength of light, leads to a better control of the intensity for each wavelength in the synthetized spectrum – the intensity can take any value from 0 (micro-mirror λ/4-condition) to the maximum (no micro-mirror displacement). The FPMDG chip contains 64 micro-mirrors which are actuated electrostatically. Rigid Si micro-mirrors are connected to side electrodes via linkage arms, permitting the micro-mirror to follow a pure vertical displacement, reducing the micro-mirror bending throughout actuation. Microfabrication is based on a 4 mask photolithography process, using SOI and Pyrex wafers. Each micro-mirror of the FPMDG chip can move by 1.25μm at voltages below 100 V. Two families of micro-mirrors, 50μm or 80μm wide, show negligible cross-talk during actuation. The micro-mirror bowing is as small as 0.14 μm over 700 μm and remains unchanged throughout actuation. Extinction ratios of up to 100 have been achieved by actuating only 3 adjacent micro-mirrors. The measurements have shown high stability and good reproducibility over time. Finally, FPMDGs are used to demonstrate shaping of the input spectrum: the intensity in a particular wavelength region is controlled through independent actuation of a set of adjacent micro-mirrors. The result is attenuation or cancellation of the corresponding wavelengths.
A review of three different systems based on the MEMS tunable blazed grating technology is presented. A MEMS tunable blazed grating is a versatile optical element providing a compact tunable mechanism for optical systems. The MEMS chip measures 5x5 mm2, including a position encoder, and is shock resistant up to 3000 g. The grating can operate in different spectral regions (Visible to Mid-IR) and high optical throughput is guaranteed at all wavelengths by operating it in Littrow condition. The first system shown uses the MEMS grating in a compact wavemeter. It is tested as Fiber Bragg Grating interrogating system. At 1.5 μm wavelength, it detects lines as narrow as 0.2 nm, resolves lines 2 nm apart and retrieves the central wavelength with accuracy better than 20 pm. By using the position encoder the expected accuracy can be on the order of 1pm. The second system shown demonstrates a compact (<10 cm3) tunable external cavity Quantum Cascade Laser using the MEMS grating. The resulting laser operates at a center wavelength of 9.5 μm and is tunable over a range of 150 nm. Finally a double stage monochromator is presented. Two MEMS chips with different grating periods are cascaded, in order to cancel out undesired grating orders, and to improve the filter linewidth (~1nm) and the extinction ratio (26 dB). The cascaded filter can be combined with a broadband source to select an arbitrary wavelength in the 400-800 nm range or the 800-1600 nm range.
In this paper we show that it is possible using optical photolithography to obtain micron and submicron features for
periodic structures in non-contact using the Talbot effect. In order for this effect to work it is important to have good
control of the illumination light and here we show that the MO Exposure Optics (MOEO) developed by SUSS
MicroOptics provides uniform and well collimated illumination light suitable for Talbot lithography. The MOEO can
easily be incorporated into a standard mask aligner. Here we show 1μm and 0.65μm diameter holes in a hexagonal array
in photoresist made in large-gap proximity printing.
Enhanced transmissions at infra-red wavelengths are measured through hole arrays made in gold-covered silicon nitride
free-standing membranes. The membranes are made by a standard photolithography batch process. They are cheap to
fabricate, reproducible and robust. The optical transmission of the membranes are investigated with varying hole size
(down to 1μm), period, and thickness. The membranes show enhanced optical transmission. The spectra show good
agreement with a very simple mode matching model which can be used for design. Calculations are also shown giving
absorption enhancements of 5.7 normalized to the same material on a silicon membrane. Finite difference time domain
calculations are also presented to show the spatial distribution of the enhanced field. Field enhancements of 3.3 are
calculated. The field enhancements are concentrated in the hole which makes the membranes ideally suited for a
microfluidic setup. Hence, this paper shows that through enhanced transmission cheap, disposable membranes in a
simplified transmission can be used for measurements for molecular absorption.
This work describes a method for tracking the dynamics of electrostatic discharge (ESD) sensitive MEMS structures
during ESD events, as well as a model for determining the reduced combdrive snap-in voltage under vibration and shock.
We describe our ESD test setup, based on the human body model, and optimized for high impedance devices. A brief
description of the MEMS tunable grating, the test structure used here, and its operation is followed by results of the
measured complex device dynamics during ESD events. The device fails at a voltage up to four times higher than that
required to bring the parts into contact. We then present a model for the snap-in of combfingers under shock and
vibration. We combine the results of the analytical model for combdrive snap-in developed here with a shock response
model to compute the critical shock acceleration conditions that can result in combdrive snap-in as a function of the
operating voltage. We discuss the validity regimes for the combdrive snap-in model and show how restricting the
operation voltage below the snap-in voltage is not a sufficient criterion to ensure reliable operation especially in
environments with large disturbances.
Enhanced optical transmission (EOT) through subwavelength apertures is usually obtained for p-polarized light. The
present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by
periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized
dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. Transmission
measurements are compared with a coupled mode model and show good qualitative agreement. Adding a waveguide can
improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.
The miniaturization of photodetectors often comes at the expense of a smaller photosensitive area. This can reduce the
signal and thus limit the image quality. One way to overcome this limitation is to reduce the photosensitive area but with
no reduction of signal i.e. harvest the light. Here we investigate, theoretically and experimentally, light harvesting with
nanostructured metals. Nanostructured metals can also give additional functionality such as polarization filtering which
is also investigated. After defining the figure of merits used when characterizing light harvesting and polarization
filtering structures, we detail the fabrication and measurement process. Structures were made on glass substrate, as a post
process step on CMOS fabricated detectors and directly in the CMOS fabrication of the detectors. The optical
characterization results are presented and compared with theory. Finally, we discuss the challenges and advantages of
integrating metallic nanostructures within the CMOS process.
In this paper, we show that a two-dimensional random system can display strong structural colors in transmission.
Polystyrene microspheres with a diameter between 0.5 and 1μm have been randomly adsorbed onto a glass substrate. In
this size range, light is mainly scattered in the forward direction. Consequently, in-plane multiple scattering can be
neglected while spheres are not too close to each others. This allows one to use a single scattering approximation to
reproduce transmission spectra of the system. Under appropriate conditions, destructive interferences between incident
and scattered light can cause a full extinction in the transmission. In our case, transmission can be as low as 5% at some
frequency ranges, generating strong color effects. Additionally, the film color changes with the angle of observation.
This angular dependant color is reproduced theoretically taking into account multiple scattering between spheres.
Devices based on SOI technology are subject to bow due to residual stress induced by the buried oxide. We have
designed and fabricated a compact tunable piston tip-tilt mirror device in which the shape and the arrangement of the
suspension beams result in both a reduced stress in the suspension beams and an optically flat mirror. The piston tip-tilt
mirror is characterized by an accurate vertical displacement of up to 18 &mgr;m @ 80 V with good repeatability, and a tip-tilt
of up to 2 mrad @ 50 V.
Devices based on SOI technology are subject to bow due to residual stress induced by the buried oxide. We have designed and fabricated a compact tunable piston tip-tilt mirror device in which the shape and the arrangement of the suspension beams result in both a reduced stress in the suspension beams and an optically flat mirror. The piston tip-tilt mirror is characterized by an accurate vertical displacement of up to 18 μm @ 80 V with good repeatability, and a tip-tilt of up to 2 mrad @ 50 V.
Diffractive MEMS are interesting for a wide range of applications, including displays, scanners or switching elements. Their advantages are compactness, potentially high actuation speed and in the ability to deflect light at large angles.
We have designed and fabricated deformable diffractive MEMS grating to be used as tuning elements for external cavity lasers. The resulting device is compact, has wide tunability and a high operating speed.
The initial design is a planar grating where the beams are free-standing and attached to each other using leaf springs. Actuation is achieved through two electrostatic comb drives at either end of the grating. To prevent deformation of the free-standing grating, the device is 10 μm thick made from a Silicon on Insulator (SOI) wafer in a single mask process.
At 100V a periodicity tuning of 3% has been measured. The first resonant mode of the grating is measured at 13.8 kHz, allowing high speed actuation. This combination of wide tunability and high operating speed represents state of the art in the domain of tunable MEMS filters.
In order to improve diffraction efficiency and to expand the usable wavelength range, a blazed version of the deformable MEMS grating has been designed. A key issue is maintaining the mechanical properties of the original device while providing optically smooth blazed beams. Using a process based on anisotropic KOH etching, blazed gratings have been obtained and preliminary characterization is promising.
With increasing demand for flat panel displays, which usually incorporate indium tin oxide (ITO) thin films, the price of
indium will rise dramatically in the future. For simple and cheap applications (such as LogoLED™, see
www.logoled.com) alternative anode materials have to be used. We will show that polymer-only anodes and wires are
sufficient to fabricate patterned polymer light-emitting devices (PLEDs), such as seven segmented displays. As another
approach to replace ITO we will present results from aluminum / PEDOT anodes devices with better stability and
bottom & top emission.
High brightness AlGaInP thin-film resonant cavity LEDs with an emission wavelength around 650 nm are presented. The combination of a thin-film waveguide structure and a resonant cavity with an omnidirectional reflector (ODR) leads to significantly higher efficiencies compared to standard resonant cavity LED (RCLED) structures. Preliminary devices based on this configuration show external quantum efficiencies of 23% and 18% with and without encapsulation, respectively, despite a non-ideal detuning. These devices exhibit a narrow far-field pattern and are therefore adapted for applications requiring high brightness emitters such as for example plastic optical fiber communications. By opting for a negative detuning, i.e. a cavity resonance that is red-shifted compared to the intrinsic emission spectrum, even higher efficiencies should be achievable.
Detailed study of external quantum efficiency (eta) QE is reported for AlGaInP-based Microcavity Light-Emitting Diodes (MCLEDs). Unlike conventional LED's the extraction efficiency (gamma) ext and far field profile depend on the linewidth of the intrinsic spontaneous emission and wavelength detuning between cavity mode and peak electroluminescence. This dependence makes it difficult to estimate the intrinsic spectrum, hence the performances of MCLED's. By using a non- destructive deconvolution technique, the intrinsic spectra of a MCLED and a reference LED (with the same active regions) could be determined at different current densities. This allowed precise calculation of (gamma) ext for both devices (values close to 11% were found for the MCLED), hence of their apparent internal quantum efficiencies (eta) int. At 55 A/cm2, values of 90% and 40% were determined for the LED and MCLED respectively. In order to explain this difference, we measured (eta) QE for devices with different sizes. From a fitting procedure based on a simple model taking into account the device size, we found out that the radiative efficiencies of LEDs and MCLEDs were close to 90%. We concluded that the low (eta) int of MCLED was due to a bad current injection, and especially to electron leakage current, as confirmed by numerical simulations.
Microcavity light emitting diodes (MCLEDs) are planar emitting devices that can achieve large brightness increase compare to conventional LEDs. We designed and fabricated a GaAs/AlxGa1-xAs surface-emitting MCLED emitting at 880 nm. Two InGaAs quantum wells are included in a (lambda) -Al0.3Ga0.7As cavity between two Al0.1Ga0.9As/Al0.8Ga0.2As Bragg mirrors. The top n-doped Bragg mirror has 4 pairs, the bottom one is p-doped like the substrate and has 20 pairs. The detuning between the source emission wavelength and the Fabry Perot wavelength is -20 nm. It is optimum for an extraction into air. By inserting the bonded MCLED device into an integration sphere we measured a maximum external quantum efficiency of 14% at 10 mA. An epoxy lens is placed on top of the device and the external quantum efficiency is increased up to 20.5% at 10 mA. These values are in good agreement with theoretical calculations if the internal quantum efficiency of the structure is equal to 85%. Additional calculations and measurements are performed and lead to a good physical understanding of the MCLED.
We present a monolithically integrated coupled cavity vertical cavity surface emitting laser or BiVCSEL: this two- terminal electrically injected device exhibits stable laser emission at two design wavelengths simultaneously. The coupling between the two monolithically grown cavities leads to two distinct Fabry-Perot modes whose separation and localization are designed in such a way that the interaction between the two modes leads to dual laser emission. Simultaneous lasing at 925 nm and 955 nm is achieved experimentally with a threshold for dual lasing of 4 kA/cm2 and dual lasing is stable over 6 times threshold.
Efficient, cheap, and simple, LEDs are used in many applications and make up the bulk of the opto-electronic component market. Due to the small critical angle at the semiconductor-air interface, relatively little light escapes per facet. The conventional route is to collect light from all six facets and redirect it, using external reflectors into a useful direction. While this increases external quantum efficiency it does little to increase brightness. In the last few years the microcavity approach has been used to persuade the light to leave by just one facet, thus increasing the brightness considerably. Although remarkable efficiencies have been achieved, microcavity LEDs (MCLEDs) have yet to surpass conventional LEDs. We present here a single mirror LED, grown by MBE, which falls between the conventional LED and the planar MCLED.
Microcavity light emitting diodes (MCLEDs) present several interesting features compared to conventional LEDs such as narrow linewidth, improved directionality and high efficiency. We report here on MCLEDs with a top emitting geometry. The MCLED layers were grown using molecular beam epitaxy on GaAs substrates. They consist of a 3-period Be- doped distributed Bragg reflector (DBR) centered at 950 nm wavelength, a cavity containing three InGaAs quantum wells and a 15-periods Si-doped DBR. Different values for the wavelength detuning between spontaneous emission line and Fabry-Perot cavity mode were explored, between -40 nm and +10 nm. Devices sizes ranged from 420 X 420 micrometers 2 to 22 X 22 micrometers 2. As expected from simulations, the higher efficiencies are obtained when the detuning is in the -20 to 0 nm range. The devices exhibit then up to 10% external quantum efficiency, measured for a 62 degree(s) collection half-angle. After correction for the surface shadowing due to the grid p-contact, the efficiency increases to 14% and is practically independent of device size.
The excitonic transition in II-VI quantum well materials has recently been used as the basis for optical modulators and also as the lasing transition at low temperatures. The central aspect in the use of the exciton resonance in optical devices is the understanding of the exciton linewidth. We present a detailed study of the interactions that affect the linewidth in II-VI semiconductor quantum well materials. The broadening of the resonance with increasing temperature can be controlled by altering the material parameters of the II-VI structures. In so doing the exciton binding energy can exceed the LO-phonon energy and thereby reduce the homogeneous contribution to the measured linewidth. Efforts to reduce well-width fluctuations in the growth of the II-VI quantum well structures, which are responsible for the inhomogeneous linewidth, must also be made to a limit where the room temperature Iinewidth is narrow and homogeneously broadened.
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