The proper functionality of microelectromechanical systems (MEMS) is governed by characterisation methods reliant on optical metrology. Static characterisation methods like interferometry are well established. The standard dynamic characterisation method is Laser Doppler Vibrometry (LDV), which can be used for areal measurements by merging data from pointwise measurements. However, the increasing complexity of MEMS structures is raising the demand for more efficient spatial dynamic analyses. Phase-shifting interferometry (PSI) is state of the art in high-resolution topography acquisition techniques. By equipping interferometers with a stroboscopic illumination source, it is possible to perform dynamic analyses of periodically oscillating samples by synchronising the illumination flashes to the sample motion. Although this is a known approach, interferometers with this technology embedded are scarce and restricted to lower bandwidths. This paper introduces an external standalone stroboscopic illumination module, which can be installed onto the interferometer objective through optical fibres. Firstly, fast-switching power-LEDs were selected as illumination sources. Optical pulse length as short as 15 ns was realized, leading to a bandwidth of some tens of MHz. Micromirror arrays (MMAs) developed at the Fraunhofer IPMS were used as test samples to prove the concept. For MMAs switched at 2 kHz, overshoot and oscillation frequency were determined. The main advantage of the stroboscopic interferometric method is in recording thousands of points simultaneously, in contrast to the LDV. This is equivalent to a performance increase in the order of tens or even hundreds of times.
Micromirror arrays (MMA) are spatial light modulators (SLM) used in a wide variety of applications for structured light manipulation i.e. structured illumination microscopy.
In our setup, we use a combination of two micromirror arrays, which allow not only to spatially structure the light in the field of view, but also to control the direction and angle of the incident light. In order to achieve this, a first MMA is imaged in the focal plane and used as a black and white (or even greyscale) mask. With a fully illuminated objective, this image would normally be formed from the complete light cone. By imaging the second MMA onto the backfocal plane of the objective only a portion of the light cone is used to form the image. This enables avoiding the unwanted illumination of out of focus objects. The MMAs in our setup consist of an array of 256x256 micromirrors, that can each be individually and continuously tilted up to 450nm, allowing the creation of greyscale images in real time in the illumination pattern. The mirrors themselves can be tilted for times as short as 10μs up to several seconds. This gives unprecedented control over the illumination times and intensities in the sample. Furthermore, our enhanced coating technology yields a high reflectivity over a broad optical spectrum (240- 1000nm).
Overall, the setup allows targetted illumination of subcellular regions enabling the precise, localized activation of optogenetic probes or the activation and deactivation of signaling cascades using photo-activated ion-channels.
Diffractive micromirror arrays (MMA) are a special class of optical MEMS, serving as spatial light modulators (SLM)
that control the phase of reflected light. Since the surface profile is the determining factor for an accurate phase
modulation, high-precision topographic characterization techniques are essential to reach highest optical performance.
While optical profiling techniques such as white-light interferometry are still considered to be most suitable to this task,
the practical limits of interferometric techniques start to become apparent with the current state of optical MEMS
technology. Light scatter from structured surfaces carries information about their topography, making scatter techniques
a promising alternative. Therefore, a spatially resolved scatter measurement technique, which takes advantage of the
MMA’s diffractive principle, has been implemented experimentally. Spectral measurements show very high contrast
ratios (up to 10 000 in selected samples), which are consistent with calculations from micromirror roughness parameters
obtained by white-light interferometry, and demonstrate a high sensitivity to changes in the surface topography. The
technique thus seems promising for the fast and highly sensitive characterization of diffractive MMAs.
Photoactivation and “optogenetics” require the precise control of the illumination path in optical microscopes. It is equally important to shape the illumination spatially as well as to have control over the intensity and the duration of the illumination. In order to achieve these goals we use programmable, diffractive Micro Mirror Arrays (MMA) as fast spatial light modulators for beam steering. By combining two MMAs with 256×256 mirrors each, our illumination setup allows for fast angular and spatial control at a wide spectral range (260-1000 nm). Illumination pulses can be as short as 50 μs, or can also extend to several seconds. The specific illumination modes of the individual areas results in a precise control over the light dose to the sample, giving significant advantage when investigating dosage dependent activation inasmuch as both the duration and the intensity can be controlled distinctly. The setup is integrated to a microscope and allows selective illumination of regions in the sample, enabling the precise, localized activation of fluorescent probes and the activation and deactivation of cellular and subcellular signaling cascades using photo activated ion channels. The high reflectivity in the UV range (up to 260nm) further allows gene silencing using UV activated constructs (e.g. caged morpholinos).
We report on our investigation to precisely actuate diffractive micromirror arrays (MMA) with an accuracy of λ/100. The
test samples consist of analog, torsional MEMS arrays with 65 536 (256x256) mirror elements. These light modulators
were developed for structured illumination purposes to be applied as programmable mask for life science and
semiconductor microscopy application. Main part of the work relies on the well known characterization of MEMS
mirrors with profilometry to automatically measure and approximate the MMA actuation state with high resolution.
Examples illustrate the potential of this strategy to control the tilt state of many thousand micromirrors within the
accuracy range of the characterization tool. In a dynamic range between 0 and >250 nm the MMA deflection has been
precisely adjusted for final MMA application in the deep-UV - VIS - NIR spectral range. The optical properties of
calibrated MMAs are tested in a laser measurement setup. After MMA calibration an increased homogeneity and
improved image contrast are demonstrated for various illumination patterns.
The present article discusses an optical concept for the characterization of diffractive micromirror arrays (MMAs) within
an extended wavelength range from the deep ultra-violet up to near-infrared. The task derives from the development of a
novel class of MMAs that will support programmable diffractive properties between 240 nm and 800 nm. The article
illustrates aspects of the achromatic system design that comprises the reflective beam homogenization with divergence
control and coherence management for an appropriate MMA illumination as well as the transfer of phase modulating
MMA patterns into intensity profiles for contrast imaging. Contrast measurements and grey scale imaging demonstrate
the operation of the characterization system and reflect the encouraging start of technology development for
multispectral, diffractive MMAs.
A new generation of micromirror arrays (MMAs) with torsional actuators is being developed within the European
research project MEMI in order to extend the usable spectral range of diffractive MMAs from deep ultraviolet into the
visible and near infrared. The MMAs have 256 x 256 pixels reaching deflections above 350 nm at a frame rate of 1 kHz,
which enables an operation in the target wavelength range between 240 nm and 800 nm. Customized driver electronics
facilitates computer controlled operation and simple integration of the MMA into various optical setups. Tests in the
visible wavelength range demonstrate the functionality and the high application potential of first MMA test samples.