Membrane Mirror Research Articles

Ultrahigh reflectivity photonic crystal membranes with optimal geometry

Photonic crystal (PhC) structures with subwavelength periods are widely used for diffractive optics, including high reflectivity membranes with nanoscale thickness. Here, we report on a design procedure for 2D PhC silicon nitride membrane mirrors providing optimal crystal geometry using simulation results obtained with rigorous coupled-wave analysis. The Downhill Simplex algorithm, a robust numerical approach to finding local extrema of a function of multiple variables, is used to optimize the period and hole radius of PhCs with both hexagonal and square lattices, as the membrane thickness is varied. Following these design principles, nanofabricated PhC membranes made from silicon nitride have been used as input couplers for an optical cavity, resulting in a maximum cavity finesse of 33 000, corresponding to a reflectivity of 0.999 82. The role played by the spot size of the cavity mode on the PhC was investigated, demonstrating the existence of an optimal spot size that agrees well with predictions. We find that, compared to the square lattice, the hexagonal lattice exhibits a spectrally wider reflective range, less sensitivity to fabrication tolerances, and higher reflectivity for membranes thinner than 200 nm, which may be advantageous in cavity optomechanical experiments. Finally, we find that all of the cavities that we have constructed exhibit well-resolved polarization mode splitting, which we expect is due primarily to a small amount of anisotropic stress in the silicon nitride and PhC asymmetry arising during fabrication.

Phononically shielded photonic-crystal mirror membranes for cavity quantum optomechanics.

We present a highly reflective, sub-wavelength-thick membrane resonator featuring high mechanical quality factor and discuss its applicability for cavity optomechanics. The 88.5 nm thin stoichiometric silicon-nitride membrane, designed and fabricated to combine 2D-photonic and phononic crystal patterns, reaches reflectivities up to 99.89 % and a mechanical quality factor of 2.9 × 107 at room temperature. We construct a Fabry-Perot-type optical cavity, with the membrane forming one terminating mirror. The optical beam shape in cavity transmission shows a stark deviation from a simple Gaussian mode-shape, consistent with theoretical predictions. We demonstrate optomechanical sideband cooling to mK-mode temperatures, starting from room temperature. At higher intracavity powers we observe an optomechanically induced optical bistability. The demonstrated device has potential to reach high cooperativities at low light levels desirable, for example, for optomechanical sensing and squeezing applications or fundamental studies in cavity quantum optomechanics; and meets the requirements for cooling to the quantum ground state of mechanical motion from room temperature.

Adaptive parabolic membrane mirrors for large deployable space telescopes.

A key element for the development of extremely large telescopes in space or balloon-borne observatories will be a reduction in the areal weight of the primary mirror. Large membrane mirrors offer a very low areal weight but are difficult to manufacture with the optical quality needed for astronomical telescopes. This paper demonstrates a practical method to overcome this limitation. In a test chamber we have successfully grown optical quality parabolic membrane mirrors on a rotating liquid in a test chamber. These polymer mirror prototypes of up to 30cm in diameter show a sufficiently low surface roughness and can be coated with reflective layers. By manipulating the parabolic shape locally using radiative adaptive optics methods, it is shown that imperfections or changes in the shape can be corrected. With only tiny local temperature changes induced by the radiation, many micrometers of stroke have been achieved. Scaling the method investigated to produce mirrors with diameters of many meters is possible using available technology. This approach opens the possibility to produce affordable extremely large primary mirrors for space telescopes. With the flexibility of the membrane material, this type of mirror can be compactly rolled up when stored in the launch vehicle, and then be deployed in space.

Site selection, thermodynamics, environment and life support analysis for the PneumoPlanet inflatable lunar habitat concept

Human habitats on the lunar surface have been planned and designed already since the beginning of the space age in the early 1960's, long before the successful Apollo missions, which for the first time brought humans to the Moon and safely back to Earth. While existing plans for manned lunar stations were postponed for a long time for various reasons, the interest in building permanent research stations on the surface of the Moon re-awoke in the late 1990s and early 2000s. Hereby, the focus turned more and more towards the lunar poles, where strong indications for the existence of water ice inside permanently dark craters were found. In this study we present a design for a lunar habitat, which makes maximal use of the natural resources available on the Moon, in particular in the vicinity of the poles, while at the same time the necessary material transfer from Earth to the lunar surface is minimized. We identify and study the sites around the lunar poles (both north and south pole) and present a design for a lunar habitat consisting of up to 16 greenhouses suitable for permanently housing and nourishing up to 32 humans. The greenhouses have a toroidal shape and are connected by tunnels. The horizontally incident sunlight is re-directed by a rotating mirror towards an artificial crater and from there through a transparent foil to the interior of the greenhouses. The main feature of our design is that the walls of the greenhouses as well as the attached rooms and tunnels are made of extremely light-weight materials, which are inflated to their final size simply by applying air pressure. Effective protection against harmful cosmic radiation is reached by covering greenhouses, living areas and connecting tunnels with a several metres thick layer of loose regolith. This cover ensures also a high level of thermal isolation, which helps to avoid too fast cooling of the interior during phases of total darkness. Finite element models of the thermodynamic behaviour of the suggested design confirm that life-friendly conditions can be created and kept over long time spans by using only the available solar illumination as power source and installing a state-of-art cooling system for the interior. In contrast to other concepts, the basic building blocks of this habitat design could be transported to the lunar surface by existing rocket systems offering a medium transport capacity, like Ariane 64 , in combination with the planned European Large Logistics Lunar Lander Finally, various safety issues that might occur during setup and operation of such a lunar habitat, are discussed. • Low weight lunar habitats. • Thermo-physical modelling. • Inflatable membranes as building elements. • Mirror membranes for use of natural sunlight. • Loose regolith for protection against cosmic radiation and micro-meteorites.

Membrane-Fresnel Diffractive Lenses with High-Optical Quality and High-Thermal Stability.

The membrane–Fresnel diffractive lens (M-FDL) has great potential in the field of high-resolution and lightweight imaging in orbit. However, the M-FDL with high-optical quality and high-thermal stability cannot be fabricated to a standard by the existing processing methods. In this paper, we propose a method for fabricating an M-FDL composed of three steps: the improved repeated spin-coating of the polyimide (PI) membrane, the secondary mucosal method of silica-framed membrane mirror, and the high-precision fabrication of a multi-level microstructure on a flexible, ultrathin membrane substrate. The results show that the root mean square (RMS) of the wave-front error for M-FDL obtained by the above method is 1/28λ (F# = 8.7 at 632.8 nm) with an 80 mm clear aperture, the average diffraction efficiency is more than 70%, the silica-framed membrane mirror possesses approximately 40 times the overall thermal stability of the traditional metal-framed mirror, and the weight is less than 40 g. The measurement results indicate that the M-FDL has high-optical quality and high-thermal stability and can satisfy the imaging requirements.

Distortion Correction In Particle Image Velocimetry For Measurements Through Fluctuating Interfaces Using A Deep Neural Network And Wavefront Sensing

Imaging-based flow measurements like Particle Image or Particle Tracking Velocimetry re-quire an undistorted optical access to the measurement location. Changes of the refractive index in the light path can cause image deterioration and lead to severe measurement errors. While static aberration as e.g. from curved walls can be corrected by of means static correction optics or dedicated light ray models, this is hardly possible if the aberration change with time at high bandwidth. Many cases of this kind exists in fluid mechanics as e.g. free liquid jets, bubbles, droplets or film flows. These interfaces are related to several open research questions that concern the interaction between the media on both sides of the interface. Hardware-based corrections adapted from astronomy that employs a deformable membrane mirror, a wavefront sensor and a fast control electronics are a viable approach for solving this measurement problem. However, these closed-loop control schemes require extensive and dedicated hardware and are limited by the technical specifications of the adaptive optical mirror in terms of correction bandwidth, stroke and spatial resolution. Here we present an approach based on Multiple-Input deep convolutional neural network that works with a wavefront sensor only and spares the adaptive optical component. Based on a model experiment and a PIV measurement, we demonstrate that this system is able to correct for the errors that are induced by an oscillating water surface by 82 %. The technique can be an alternative to classical closed-loop adaptive optical systems where the performance of the actuators is not sufficient. This can open a perspective towards measurement of interface-related flows that were not accessible before.

Silicon based photo-controlled deformable mirror

The fabrication and characterization of a Silicon based photo-controlled deformable mirror (PCDM) for spatial light modulation and wavefront control is reported. The device is a membrane mirror based on a 1-inch monolithic non-pixelated Silicon photoconductive substrate with a transparent ITO electrode on one side coupled with a 5 µm thick nitrocellulose membrane. The device is driven applying an AC voltage and it is characterized by measuring the generated wavefront using a Shack-Hartmann wavefront sensor. Upon uniform illumination with a NIR LED, the deformation increases with the voltage applied, its frequency and the light intensity. The maximum deformation achieved is approximately 2 µm PtV (wavefront). The repeatability of the induced deformation is also evaluated together with its time stability, proving that the device is reliable in the working conditions. The response time is evaluated and it is in the range of 10 ms. The device is suitable for an efficient adaptable focusing element with an important focus control.

Theoretical calculation of fiber cavity coupling silicon carbide membrance

Single spin color centers in solid materials are one of the promising candidates for quantum information processing, and attract a great deal of interest. Nowadays, single spin color centers in silicon carbide, such as divacancies and silicon vacancies have been developed rapidly, because they not only have similar properties of the NV centers in diamond, but also possess infrared fluorescence that is more favorable for transmission in optical fiber. However, these centers possess week fluorescence with broad spectrum, which prevents some key technologies from being put into practical application, such as quantum key distribution, photon-spin entanglement, spin-spin entanglement and quantum sensing. Therefore, optical resonator is very suitable for coupling centers to filter their spectrum and enhance the fluorescence by Purcell effect. It is very advantageous to use the fiber end face as cavity mirrors, thereby the fiber can provide small cavity volume corresponding to a large enhancement in spin color centers, and collect the fluorescence in cavity simultaneously, which has no extra loss in comparison with other collection methods. In this work, the properties and performance of fiber Fabry-Perot cavity coupling silicon carbide membrane are mainly studied through theoretical calculation. Firstly, some parameters are optimized such as membrane roughness and mirror reflection by calculating the mode of the fiber cavity and enhancing the color centers coupling into the cavity, then analyzing the properties of different modes in cavity, the enhancement effect on cavity coupling color centers, and other relevant factors affecting the cavity coupling color centers. Next, the influences of dominated factor and vibration on the properties of the cavity, the enhancement and outcoupling of centers coupled into the cavity are investigated, and finally the optimal outcoupling efficiency corresponding to different vibration intensities is obtained. These results give direct guidance for the further experimental design and direction for optimization of the fiber cavity coupling color centers.

Surface micromachining multilayer porous silicon for spectral filtering applications

Novel micromachining processes were developed to produce optical filters operating in the long-wave infrared region (8–12 μm). The filters were composed of a multilayer porous silicon top mirror and silicon bottom mirror separated by air cavity. Release of the top mirror membranes (with sizes ranging between 300 × 300 μm2 and 600 × 600 μm2) was achieved using electropolishing. Inclusion of stress-relief notches in the top mirror (the suspended structure) improved both the flatness and the yield. Due to the high porosity of the films, a key finding was the need to reduce the plasma power during the etching process, to reduce damage to the film. The optical response of the device was evaluated by comparing the measured transmittance to the model, with favourable results confirming the future potential application of this technology for long wave infrared spectral filtering.

Simulative and Experimental Characterization of an Adaptive Astigmatic Membrane Mirror.

Adaptive optical (AO) components play an important role in numerous optical applications, from astronomical telescopes to microscope imaging systems. For most of these AO components, the induced wavefront correction, respectively added optical power, is based on a rotationally symmetric or segmented design of the AO component. In this work, we report on the design, fabrication, and characterization of a micro-electronic-mechanical system (MEMS) adaptive membrane mirror in the shape of a parabolic cylinder. In order to interpret the experimental characterization results correctly and provide a tool for future application development, this is accompanied by the setup of an optical simulation model. The characterization results showed a parabolically deformable membrane mirror with an aperture of 8 × 2 mm2 and an adaptive range for the optical power from 0.3 to 6.1 m−1 (dpt). The optical simulation model, using the Gaussian beamlet propagation method, was successfully validated by laser beam profile measurements taken in the optical characterization setup. This MEMS-based adaptive astigmatic membrane mirror, together with the accompanying simulation model, could be a key component for the rapid development of new optical systems, e.g., adaptive laser line generators.

Numerical analysis of an electrostatically formed membrane mirror

The paper presents a numerical algorithm developed to determine the deformed shape of a membrane mirror controlled by electrostatic forces. Deformable mirrors are key components that are used in combination with wavefront sensors and real-time control systems in adaptive optics. The electrostatic membrane mirror concept implies using a thin conductive reflective membrane stretched over a solid flat frame and deformed electrostatically by applying control voltages to electrostatic actuators positioned behind the membrane. The proposed algorithm implies solving a coupled structural-electrostatic problem by using finite element and boundary element methods. Small deflections of a membrane are described by Poisson’s equation. The electric charge distribution over the membrane surface having a prescribed potential is governed by a Fredholm integral equation of the first kind. The coupled problem is solved iteratively, and a criterion for terminating iterations when searching for a steady-state solution is presented. The distinctive feature of this approach is that it allows us to take into account electrical edge effects typical for conducting thin-walled structures of very small thickness. Illustrative examples are provided to show the applicability and validity of the proposed method as well as its advantage over some existing techniques.

Design routine and characterisation of a biconic deformable metal mirror for focus shifting.

This paper describes an opto-mechanical concept of a deformable metal mirror membrane, which can shift the focus position over a large range by use of a single actuator. The core element of the mirror is a diamond turned tulip-shaped membrane, the design is optimized to correct astigmatic aberrations which arise from the use of a curved mirror under a deflection angle. For this purpose, the target mirror surface is biconic. The manufactured mirror was tested with a maximum central deflection of 28 µm and, when used in combination with a 200 mm focal lens, is capable of producing a focus shift of up to 17.9 mm with a resulting wavefront aberration of 238.7 nm RMSerror.

Tubular optical microcavities based on rolled-up photonic crystals

The self-rolling of micro-structured membranes via the stress-engineering method opens new ways to create 3D photonic micro-objects with original designs and optical properties. This article validates this approach by producing 3D hollow micro-resonators based on rolled-up 2D photonic crystal membrane mirrors, capable of trapping light in 3D and in air. We fabricated the 3D tubular microresonators with 10 μm–20 μm diameters by rolling photonic crystal membranes using stress-engineering technique on the prestressed InGaP/InP bilayer. We also added a design feature to lift the microtubes vertically and facilitate optical measurements, but also to attach the structures to the substrate. The dispersion of the planar 2D photonic crystal membrane was optimized to exhibit high reflectivity (>95%) at normal incidence over a large spectral band (100 nm) in the near-infrared domain (1.5 μm–1.6 μm). The cylindrical cavity model and numerical simulations predicted the presence of quasi-pure radial cavity modes with a strong concentration of light over nearly 3% of the photonic microtubes’ cross section. We demonstrated experimentally the presence of those modes through scanning near-field optical microscopy measurements. Using a bowtie nanoantenna, we selectively detected and mapped transverse electric modes in the hollow core of photonic microtubes. Spatially resolved cartographies allowed for the identification of the modes in good agreement with theoretical predictions. This work brings theoretical and experimental proof of concept of light cages based on rolled-up photonic crystal membranes. It also opens the path to the realization of original photonic microstructures as combinations of a specific photonic crystal design and a targeted 3D form.

Stable membrane candidate for deployable membrane space telescopes

Larger mirrors are needed to satisfy the requirements of the next generation of UV–Vis space telescopes. Our study attempts to meet this requirement by demonstrating a technology that would deploy a large, continuous, high figure accuracy membrane mirror. The figure of the membrane mirror is corrected after deployment using a contiguous coating of a magnetic smart material (MSM) and a magnetic field. The MSM is a magnetostrictive material that is operable by magnetic write head(s), locally imposed on the nonreflective side of the membrane mirror. We report preparation, figure accuracy, stress analysis, and stability of the MSM coated CP1 polyimide substrate membrane mirror. The figure accuracy and magnetostrictive performance of the MSM coated membrane mirror are measured; furthermore, stability of the CP1 membrane for 48 h is observed and the results are found to be promising. In addition to membrane coating and the experimental procedure, the results of the surface profiling experiments are introduced and discussed.

Aberration correction based on wavefront sensorless adaptive optics in membrane diffractive optical telescope

The membrane diffractive optical telescopes are one of areas of active research of large aperture optical telescopes. However, due to the processing technology of thin membrane mirror, the changes of temperature and humidity, etc., the imaging performance of membrane diffractive optical telescope is degraded sharply and it is difficult to reach the design target. Adaptive optics (AO) technology is a potential solution to improve the imaging performance of this telescope. Our work is the first time to correct system aberration of hundred millimeter-level aperture membrane diffractive optical telescope based on AO technology. The point target and extended target imaging experiment of the telescope with WFSless AO system in the laboratory are accomplished. The experimental results demonstrate the effectiveness of AO correction in improving the imaging performance of membrane diffractive optical telescope. Especially when there are unexpected aberrations introduced in the telescope, such as disturbances caused by assembly and operation, AO correction will be an important guarantee for the telescope to maintain good imaging performance.

MEMS-in-the-lens 3D beam scanner for in vivo microscopy.

The "MEMS-in-the-lens" active lens for a laser scanning microscope comprises a high numerical aperture front element, a 3D+ MOEMS beam scanner and a collimating back lens. The scanner utilizes a silicon gimbal with SU-8 polymer flexures and deformable membrane mirror. The mirror aperture is 4 mm in diameter, and is capable of 9 μm deflection for focus control, with four annular electrodes to allow tuning of primary and secondary spherical aberration. The gimbal supports tip/tilt actuation up to ±3° for lateral beam scanning. We show confocal imaging using a benchtop mockup of the active lens, illustrating the potential for this approach to support 3D microscopy for optical biopsy applications.

Analytical decomposition of Zernike and hexagonal modes over a hexagonal segmented optical aperture

Zernike polynomials are widely used to describe common optical aberrations of a wavefront as they are well suited to the circular geometry of various optical apertures. Non-conventional optical systems, such as future large optical telescopes with highly segmented primary mirrors or advanced wavefront control devices using segmented mirror membrane facesheets, exhibit a hexagonal geometry, making the hexagonal orthogonal polynomials a valued basis. A cost-benefit trade-off study for deriving practical upper limits in, e.g., polishing, phasing, alignment, and stability of hexagons imposes analytical calculation to avoid time-consuming end-to-end simulations, for the sake of exactness. It is important to include global modes over the pupil for Zernike decomposition over a hexagonal segmented optical aperture into the error budget. However, numerically calculated Zernike decomposition is not optimal due to the discontinuities at the segment boundaries that result in imperfect hexagon sampling. In this paper, we present a novel approach for a rigorous Zernike and hexagonal mode decomposition adapted to hexagonal segmented pupils by means of analytical calculations. By contrast to numerical approaches that are dependent on the sampling of the segment, the decomposition expressed analytically only relies on the number and positions of segments comprising the pupil. Our analytical method allows extremely quick results minimizing computational and memory costs. Further, the proposed formulae can be applied independently from the geometrical architecture of segmented optical apertures. Consequently, the method is universal and versatile per se. For instance, this work finds applications in optical metrology and active correction of phase aberrations. In modern astronomy with extremely large telescopes, it can contribute to sophisticated analytical specification of the contrast in the focal plane in presence of aberrations.

MEMS Fabry–Pérot Interferometers With Double Membrane Mirrors for Improved Mirror Parallelism

This paper presents surface micromachined MEMS Fabry-Pérot interferometers (FPIs) with two released membrane mirrors. Fabrication of such FPIs with large-area membrane reflectors of up to 5 mm in diameter for first-order operation in the near infrared is shown. The mirrors are distributed Bragg reflectors based on silicon and silicon carbonitride. Single mirror membrane FPIs with the same mirror design are used as a benchmark in order to demonstrate the improvements achievable by the double membrane approach. To that end, the FPIs are characterized with respect to their top surface flatness by white light interferometry as well as to the variations of the optical gap width over the aperture area by spatially resolved transmittance measurements. It is shown that the upper mirror membranes are flat within the nanometer range for both approaches. Nevertheless, single membrane FPIs exhibit strongly increasing gap variations for increasing membrane diameters due to a bow in the substrate wafer, thus decreasing the filter resolution over the full aperture. On the other hand, for double-membrane FPIs, the mirrors are highly parallel with a standard deviation of the gap as low as 2 nm.

A new bi-directional giant magnetostrictive-driven compliant tensioning stage oriented for maintenance of the surface shape precision

Membrane mirror, known for its perspicuous feature of light weight, high packaging efficiency, low manufacturing cost and etc., has been widely used in the precision optical instruments, such as the primary mirror applied on a space telescope. However, how to maintain the membrane mirror's surface shape precision at a high level is still a challenging problem. Therefore, a bi-directional giant magnetostrictive-driven compliant tensioning stage is proposed in this research, and the basic performance of the stage is studied through theoretical analysis, numerical simulation and experimental investigations. First, the performance requirements and conceptual design are presented and illustrated in detail. Second, the Pseudo Rigid Body Model method is selected as the basis of further investigating the stage and the dynamic model is established through Lagrange's equation. Meanwhile, numerical simulations are performed to analyze the basic performances. Finally, a prototype is manufactured and based on this device, some experiments are conducted. The results indicate that the developed tensioning stage is able to achieve the desired capability and could effectively maintain the surface shape precision.

Adaptive Shape Control for Thermal Deformation of Membrane Mirror with In-plane PVDF Actuators

Optical membrane mirrors are promising key components for future space telescopes. Due to their ultra-thin and high flexible properties, the surfaces of these membrane mirrors are susceptible to temperature variations. Therefore adaptive shape control of the mirror is essential to maintain the surface precision and to ensure its working performance. However, researches on modeling and control of membrane mirrors under thermal loads are sparse in open literatures. A 0.2 m diameter scale model of a polyimide membrane mirror is developed in this study. Three Polyvinylidene fluoride (PVDF) patches are laminated on the non-reflective side of the membrane mirror to serve as in-plane actuators. A new mathematical model of the piezoelectric actuated membrane mirror in multiple fields, (i.e., thermal, mechanical, and electrical field) is established, with which dynamic and static behaviors of the mirror can be analyzed. A closed-loop membrane mirror shape control system is set up and a surface shape control method based on an influence function matrix of the mirror is then investigated. Several experiments including surface displacement tracking and thermal deformation alleviation are performed. The deviations range from 15 μm to 20 μm are eliminated within 0.1 s and the residual deformation is controlled to micron level, which demonstrates the effectiveness of the proposed membrane shape control strategy and shows a satisfactory real-time performance. The proposed research provides a technological support and instruction for shape control of optical membrane mirrors.