MEMS spectrometers operating in the shortwave infrared (SWIR) range are achieving significant progress in agriculture by offering portable solutions for soil and crop analysis. Recent innovations focus on extending the actuation range of MEMS tunable optical filters and addressing issues like snap-down with a new floating electrode design. This approach improves stability and expands the wavelength range from 1.25 mu m to 1.76 mu m. These enhancements ensure the devices perform reliably under various conditions, helping in precise agricultural assessments and better crop management.

MEMS spectrometers operating in the SWIR range are achieving significant progress in agriculture by offering portable solutions for soil and crop analysis. Recent innovations focus on extending the actuation range of MEMS tunable optical filters and addressing issues like snap-down with a new floating electrode design. This approach improves stability and expands the wavelength range from 1.25 mu m to 1.76 mu m. These enhancements ensure the devices perform reliably under various conditions, helping in precise agricultural assessments and better crop management.

Over the past three decades, silicon photonic devices have been core to the realization of large‐scale photonic‐integrated circuits. However, silicon nitride is another key complementary metal oxide semiconductor‐compatible material for high‐density photonic‐integrated circuits, having low manufacturing costs, low optical losses, and excellent mechanical properties, that can provide enhanced performance over silicon in an integrated photonic platform. This article presents the design, fabrication, and testing of a proof‐of‐concept switchable silicon nitride photonic coupler that leverages these properties combined with microelectromechanical systems actuation. The photonic platform uses a moveable suspended waveguide to enable efficient out‐of‐plane switching and is built using conventional lithographic techniques to demonstrate the high compatibility with existing microelectronic fabrication techniques. The photonic switch is measured to have an insertion loss of 2.6 dB and an ON/OFF extinction ratio of 34 dB at the output of the suspended waveguide, at a wavelength of 1470 nm. Detailed simulations demonstrate broadband operation over a 600 nm wavelength range from 1.25 to 1.85 μm which is experimentally validated over the range from 1.25 to 1.61 μm. To the best of knowledge, this is the broadest operation range ever demonstrated by a photonic switch in simulation.

Preliminary data obtained with PS-OCT through the eyes of hypertensives suggests a mechanistic connection between arterial health and hypertension. In this study we want to determine whether arterial health may be more predictive of negative clinical outcomes than hypertension and blood glucose. Patients with various stages of hypertension, diabetes and coronary artery disease were recruited from Fiona Stanley Hospital (Perth). They were subsequently imaged with PS-OCT. The data were analyzed for retinal vessel wall thickness and vessel wall birefringence. We demonstrated that the combination of blood vessel wall tissue structure and wall thickness, a recognized clinical biomarker (Afsharan et al, BOE, 2021), could diagnose hypertension and diabetes with high sensitivity and specificity. PS-OCT measurements can detect the smallest changes related to cardiovascular disease in the retina before the disease manifests itself clinically. The method is cheap, noninvasive and easy to apply, which makes it highly suitable for screening, especially in underserved communities.

This paper presents the design, modeling, fabrication and optical characterization of electrostatically-actuated silicon-based thin film Fabry-Pérot filters for spectroscopic sensing applications at mid-wave infrared (MWIR: 3–5 $\mu \text{m}$ ) wavelengths. The distributed Bragg reflectors of the FP filters consist of silicon and air-gap layers in order to enhance the refractive index contrast and performance of the filter. A peak-to-peak surface variation of less than 30 nm in the fabricated micromachined structures was achieved across a large spatial area of 1 $\text{m}\text{m}$ × 1 $\text{m}\text{m}$ . Although, spectral measurements on released Fabry-Pérot actuated filters show good agreement with optical simulations the filter performance indicates there is a significant peak-to-peak surface variation within the main air optical cavity. The fabricated Fabry-Pérot filters demonstrate peak transmittance values between 38% and 50%, with measured full width at half maximum values in the range of 70 $\mathrm{n}\text{m}$ making them promising devices for use in spectral sensing and imaging in the MWIR wavelength range.

While agile multispectral imaging solutions presently exist, their size, weight and power (SWaP) specifications prevents deployment on small portable platforms such as drones. As much of the size and weight of existing solutions is attributed to the wavelength-selective optical subsystem, realizing low-SWaP hinges on miniaturization of this subsystem. The ultimate multispectral imaging implementation would integrate the wavelength-selective component at the imaging focal plane array. This paper presents a solution which aims to achieve such integration. Recent developments in microelectromechanical systems (MEMS) have realized a surface-micromachined optical tunable filter, operating in the shortwave infrared wavelength band (SWIR: 1 mu m - 2.6 mu m) for applications in miniature optical spectrometers. The tunable filter is a Fabry-Perot (FP) structure, composed of a fixed dielectric mirror on a silicon substrate, and a movable dielectric mirror suspended above. The separation (air gap) between these two mirrors defines the optical transmission centre-wavelength of this Fabry Perot structure. Consequently, electrostatic actuation of the top mirror towards the bottom mirror allows the gap, and thus the transmission centre-wavelength, to be controlled. This paper presents work towards integration of such a MEMS tunable filter technology directly on an infrared focal plan array. Realizing this integration relies on: (1) expanding the optical area of the MEMS Fabry Perot structure to cover a significant portion of the two-dimensional focal plan array, which is generally multi-millimetre in each of its two dimensions; and (2) devising a structure that will allow actuation of the MEMS filter with under 20 V.

The Microelectronics Research Group (MRG) at The University of Western Australia is a key partner of the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems. In this presentation, an overview of ongoing research will be given with an emphasis on the flagship research activities of MCT- based imaging arrays and Microelectromechanical Systems (MEMS). The MCT research and development utilise a vertically integrated capability from semiconductor material growth, through device modelling and design, to focal-plane-array fabrication and packaging. In support of the detector array capability, fully integrated MEMS technology can be used to further enhance the sensor device performance through the focal plane integration of tunable filters for spectral classification and infrared spectroscopy. The combination of high-performance detector designs and tunable spectral filters provides a major differentiator for military imaging systems, particularly for those operating in complex and degraded environments. This talk will highlight several research activities that are highly relevant to defence applications including metamaterial enhanced infra-red detectors, and the fabrication of infra-red focal plane arrays on flexible substrates. For the MEMS technology, both wideband and narrowband tunable spectral filters will be discussed for multispectral imaging in the SWIR, MWIR and LWIR bands, and for hyperspectral imaging and spectroscopy. Considerations on future research activities and technology trends will be presented including opportunities for the rapid development of high- performance and spectrally adaptive low SWaP sensing systems for enhanced detection and discrimination of partially concealed or camouflaged targets in cluttered backgrounds.

Abstract Blood vessel walls are made of organized fibrous tissue with intrinsic birefringence. Even in its very early stages, hypertension can change the structure of a blood vessel wall. We showed that this structural change can be quantitatively measured non-invasively in the human retina using polarization-sensitive optical coherence tomography (PS-OCT). Organizational loss in vessel wall tissue was quantified in hypertensive patients and compared to data obtained from age-matched healthy subjects. The wall tissue in patients with hypertension was shown to be thicker, and exhibited lower birefringence, presumably due to a loss of tissue organization. The blood vessel wall birefringence index (BBI) combines vessel wall birefringence and thickness into one number and is introduced here to readily distinguish between blood vessel walls of hypertensive and healthy subjects. Based on receiver operating characteristic (ROC) curves, BBI showed 99% sensitivity and 100% specificity when discriminating normotensive (N = 11) and hypertensive (N = 11) subjects. Accurately determining the thickness of the blood vessel wall is not possible without polarization-sensitivity. Moreover, just blood vessel wall thickness or wall birefringence were not sufficient to achieve this high classification performance. Retinal vessel wall measurements with PS-OCT cannot be affected by a patient being anxious, by hormones or other blood values, since the measurement is intrinsic to the optical and mechanical properties of the vessel wall. Furthermore, this relatively low-cost system combines a very short imaging and analysis time with high sensitivity and specificity, making it highly suitable for low-cost screening.

Future remote imaging systems promise spectroscopic functionalities extending well beyond the visible wavelengths. This allows real-time spectral information to be gathered from multiple wavelength bands which is highly attractive for numerous remote sensing spectroscopy/imaging applications and aids target recognition. This paper briefly presents a micro-electromechanical systems (MEMS) based electrically tuneable adaptive filter technology developed for the technologically important infrared (IR) bands of the electromagnetic spectrum and reports on the progress towards extension to the significantly longer wavelength THz band. The demonstrated concepts focus on merging MEMSenabled dynamic modulation with the spectral sensitivity and selectivity of metamaterials, as well as on the possibility of adopting the rapidly evolving 3D printing technologies.

Background Modern field pea breeding faces a significant challenge in selecting lines with strong stems that resist lodging. Traditional methods of assessing stem strength involve destructive mechanical tests on mature stems after natural senescence, such as measuring stem flexion, stem buckling or the thickness of dry stems when compressed, but these measurements may not correspond to the strength of stems in the living plant. Optical coherence tomography (OCT) can be used as a noncontact and nondestructive method to measure stem wall thickness in living plants by acquiring two- or three-dimensional images of living plant tissue. Results In this proof-of-principle study, we demonstrated in vivo characterisation of stem wall thickness using OCT, with the measurement corrected for the refractive index of the stem tissue. This in vivo characterisation was achieved through real-time imaging of stems, with an acquisition rate of 13 milliseconds per two-dimensional, cross-sectional OCT image. We also acquired OCT images of excised stems and compared the accuracy of in vivo OCT measurements of stem wall thickness with ex vivo results for 10 plants each of two field pea cultivars, Dunwa and Kaspa. In vivo OCT measurements of stem wall thickness have an average percent error of − 3.1% when compared with ex vivo measurements. Additionally, we performed in vivo measurements of both stem wall thickness and stem width at various internode positions on the two cultivars. The results revealed that Dunwa had a uniform stem wall thickness across different internode positions, while Kaspa had a significantly negative slope of − 0.0198 mm/node. Both cultivars exhibited an increase in stem width along the internode positions; however, Dunwa had a rate of increase of 0.1844 mm/node, which is three times higher than that of Kaspa. Conclusions Our study has demonstrated the efficacy of OCT for accurate measurement of the stem wall thickness of live field pea. Moreover, OCT shows that the trends of stem wall thickness and stem width along the internode positions are different for the two cultivars, Dunwa and Kaspa, potentially hinting at differences in their stem strength. This rapid, in vivo imaging method provides a useful tool for characterising physical traits critical in breeding cultivars that are resistant to lodging.

Significance:Post-burn scars and scar contractures present significant challenges in burn injury management, necessitating accurate evaluation of the wound healing process to prevent or minimize complications. Non-invasive and accurate assessment of burn scar vascularity can offer valuable insights for evaluations of wound healing. Optical coherence tomography (OCT) and OCT angiography (OCTA) are promising imaging techniques that may enhance patient-centered care and satisfaction by providing detailed analyses of the healing process.Aim:Our study investigates the capabilities of OCT and OCTA for acquiring information on blood vessels in burn scars and evaluates the feasibility of utilizing this information to assess burn scars.Approach:Healthy skin and neighboring scar data from nine burn patients were obtained using OCT and processed with speckle decorrelation, Doppler OCT, and an enhanced technique based on joint spectral and time domain OCT. These methods facilitated the assessment of vascular structure and blood flow velocity in both healthy skin and scar tissues. Analyzing these parameters allowed for objective comparisons between normal skin and burn scars.Results:Our study found that blood vessel distribution in burn scars significantly differs from that in healthy skin. Burn scars exhibit increased vascularization, featuring less uniformity and lacking the intricate branching network found in healthy tissue. Specifically, the density of the vessels in burn scars is 67% higher than in healthy tissue, while axial flow velocity in burn scar vessels is 25% faster than in healthy tissue.Conclusions:Our research demonstrates the feasibility of OCT and OCTA as burn scar assessment tools. By implementing these technologies, we can distinguish between scar and healthy tissue based on its vascular structure, providing evidence of their practicality in evaluating burn scar severity and progression.

We examine the use of foundry process optical waveguide couplers in making 1 x 2 optical switches. The concept involves a post-foundry process to provide a moveable dielectric load over one of the waveguides in the coupler, such that the dielectric load changes the propagation constant of the affected waveguide depending on its proximity to the waveguide. Coupled mode theory is employed to explain the operation of the switch and identify key requirements of the dielectric load. Finite difference time domain simulations are employed to verify that the concept is viable for two standard photonic integrated circuit platforms. The concept ensures that the optical signals are always constrained within the high-quality foundry process waveguides while also allowing the material and lithography requirements for the layer in which the movable load is realized to be relaxed. Results show that a contrast between the switch ports of > 20 dB is possible with relaxed tolerances for the dielectric load layer, at an operating wavelength of 1550 nm. We envision that the dielectric load would be moved by a micro-electromechanical systems actuator. Having the optical signals always within the foundry waveguides will permit fabrication of highperformance mechanically switched optical systems by a wide range of facilities. (c) 2023 Society of Photo- Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.62.8.087103]

We report here the first demonstration of a cryogenic mid-wave infrared (MWIR) hyperspectral fixed-cavity Fabry-Perot filter based on a suspended tensile-strained single-layer 2-D subwavelength grating (SWG) mirror. Optical design optimization of the 2-D SWG mirror and parameter tolerance study are performed. For the first time, process control of grating air-hole sidewall angle and the grating air-hole fill-factor fabrication error caused by e-beam lithography electron-scattering effect is reported. At 80 K, namely the operating temperature of MWIR photodetectors, the as-fabricated suspended 2-D SWG mirror has achieved excellent surface flatness with a slight center-to-edge bowing of 15 nm over a 1-mm2 large mirror area and a high average reflectivity of 0.97 across a wavelength range of 3.72-5 µm, which represents an unprecedentedly wide fractional bandwidth Δλ/λc of 30%. The cryogenically cooled Fabry-Perot filter exhibits an unrivaled high spectral resolution of 10 nm that far exceeds the optical requirement for MWIR hyperspectral imaging applications.

We report the first study of the effects of grating fill-factor variation and sidewall angle on 2-D subwavelength grating shortwave infrared mirrors, and the first development of a geometry compensation approach to correct for the grating fill-factor patterning error caused by EBL proximity effect and a plasma etching process based on CHF 3 passivation to control grating sidewall angle. Mirrors with a large grating air-hole diameter-to-pitch ratio of 0.954 and vertical sidewall angle of 89.8° are demonstrated with an average reflectivity of 99% over an ultrabroad wavelength range of 560 nm (1.92-2.48 µm), which represents an unprecedented fractional bandwidth of 26%.

Evolving from past black-and-white images, through present red-green-blue spectral colors, future remote imaging systems promise spectroscopic functionalities extending well beyond the visible wavelengths. This allows real-time spectral information to be gathered from multiple wavelength bands that is applicable to numerous remote sensing spectroscopy/imaging applications and aids target recognition. This paper reviews the wavelength tunable microelectromechanical systems (MEMS) optical filter technologies developed for the important infrared and the emerging terahertz wavelength bands of the electromagnetic spectrum with the fabrication effort being enabled by the Western Australian Node of the Australian National Fabrication Facility. A low size, weight and power (SWaP) platform solution is demonstrated delivering mechanically robust, field-portable, spectroscopic, chem/bio sensing suitable for deployment in remote sensing and imaging applications.

This paper reports the development of a broadband highly reflective and structurally flat large-area 2-D subwavelength grating (SWG) reflector, based on a tensioned single-layer silicon metamaterial membrane. Rigorous coupled-wave analysis is adopted to design the metamaterial reflector, resulting in a wavelength range of 600 nm (1.9- $2.5~\mu \text{m}$ ) with >99% reflectivity, which represents the largest reported fractional bandwidth $\Delta \lambda /\lambda _{\mathrm {c}}$ of 27%. Effects of design parameter deviations on the SWG reflector reflectivity are studied, showing that the reflector design has good fabrication tolerances. A freestanding 2-mm dimension 2-D SWG reflector has been fabricated with high precision, and optical measurements indicate polarization-independent average reflectivity of 99% over the wavelength range of 1.91- $2.49~\mu \text{m}$ , which agrees well with the modeled result. This paper also, for the first time, examines the surface flatness characteristics of a suspended single-layer 2-D SWG reflector. With a tensile stress of 10 ± 5 MPa in the silicon grating membrane, the freestanding reflector achieves nanometer-scale surface flatness, which provides a significant advantage over multilayer DBRs requiring stress-balancing. Furthermore, a hyperspectral Fabry-Perot shortwave infrared filter based on a top suspended 2-D SWG reflector and a bottom 4-pair Si/SiO 2 distributed Bragg reflector (DBR) has been experimentally demonstrated for the first time. The filter is measured to have a peak transmission of 80% with a narrow full-width at half-maximum (FWHM) of about 4 nm. This spectral resolution is one order of magnitude higher than that of other reported MEMS/DBR-based Fabry-Perot filters and is well-suited to highly demanding hyperspectral shortwave infrared imaging applications. [2022-0060]

Elastic modulus and hardness of thin-films are critically important in determining the behaviour of free-standing actuatable microstructures. In this study, nanoindentation has been used to investigate the mechanical properties of thermally evaporated Ge and BaF2 thin-films. Nanoindentation experiments indicate that Ge and BaF2 thin-films are characterised by a reduced modulus of 95 ± 3 GPa and 33 ± 9 GPa, respectively, and hardness of 4.6 ± 0.4 GPa and 0.75 ± 0.4 GPa, respectively. The elastoplastic response of both thin-films was predominantly elastic for low indentation loads, but exhibited plasticity ≥ 60% for indentation loads approaching 8 mN. Indentation-induced creep deformation was found to be limited to ≤ 5%.

The anticipated feature of future generation remote infrared (IR) sensing and imaging technologies includes adding so called multi-colour capabilities. Such enhancement of the current state-of-the-art IR detector and imaging focal plane array (FPA) technologies allows real-time spectral information to be gathered from multiple wavelength bands. Multi/hyper-spectral imaging results in improved target recognition and is applicable to numerous remote sensing spectroscopy/imaging applications. In order to provide a reduced size, weight and power (SWaP) solution, a micro electromechanical systems (MEMS) based electrically tuneable adaptive filter technology has been developed for important IR bands of the electromagnetic spectrum. The adopted approach is capable of delivering on-chip remote hyper/multi-spectral sensing by obtaining narrow-band spectral sensitivity utilising a tuneable MEMS optical filter fabricated directly on a detector. This paper summarizes the performance demonstrated within the most technologically relevant bands of short-wave IR (SWIR, 1.4-2.5 µm), mid-wave IR (MWIR, 3-5 µm), and long-wave IR (LWIR, 8-12 µm). In SWIR, the demonstrated nanometer-scale uniformity in the flatness of suspended MEMS allows for spatial uniformity of the filtered peak centre wavelength and the achieved 30-35 nm spectral width to remain within single nanometers over 500µm x 500µm optical apertures. In LWIR, the spatial peak wavelength selectivity variation is achieved to be less than 1.2% across 200μm × 200μm optical imaging areas, exceeding the requirements for passive multispectral thermal imaging and validating the suitability for mechanically robust multi/hyper-spectral remote sensing and imaging applications deployable on low-SWaP field-portable platforms.