Cantilever-Based Sensor Utilizing a Diffractive Optical Element with High Sensitivity to Relative Humidity

Scalar far-field diffraction modelling using nonuniform fast Fourier transform for diffractive optical phase elements design

Due to their advantages of compact geometries and lightweight, diffractive optical elements (DOEs) are attractive in various applications such as sensing, imaging and holographic display. When designing DOEs based on algorithms, a diffraction model is required to trace the diffracted light propagation and to predict the performance. To have more precise diffraction field tracing and optical performance simulation, different diffraction models have been proposed and developed. However, they are limited in diffraction angles or still suffer from serious aberrations within the nonparaxial region in the far-field, which are not desired for the aforementioned applications. In this work, we developed an optimized diffraction modelling method using a nonuniform fast Fourier transform (NUFFT) to minimize the aberrations in the nonparaxial diffraction area in the far field for DOE design. The simulation result shows that the imaging distortion of DOE designed using iterative Fourier transform algorithm (IFTA) with integration of our proposed diffraction modelling method was effectively optimized. Moreover, the designed DOE has a diffraction efficiency of 90.73% and a root mean square error (RMSE) of 0.4817. It exhibits 7.17% higher in diffraction efficiency and 8.59% smaller in RMSE (0.0453), respectively, compared to DOE designed with a diffraction modelling method by directly taking nonuniform diffraction sampling points that are mapped from the diffracted wavefronts surface on the output plane, which has a diffraction efficiency of 83.56% and a RMSE of 0.5270. Furthermore, a compensation matrix was introduced into the developed diffraction model to further improve the imaging quality of designed DOE. A further increase of diffraction efficiency by 0.18% and a decrease of RMSE by 12.43% (0.0599) were achieved. In addition, we also utilized the proposed approach for DOE design in the case of off-axis diffraction, and diffraction fields with an incident illumination angle up to 30° can be reconstructed and simulated.

The Development of Optomechanical Sensors-Integrating Diffractive Optical Structures for Enhanced Sensitivity

The term optomechanical sensors describes devices based on coupling the optical and mechanical sensing principles. The presence of a target analyte leads to a mechanical change, which, in turn, determines an alteration in the light propagation. Having higher sensitivity in comparison with the individual technologies upon which they are based, the optomechanical devices are used in biosensing, humidity, temperature, and gases detection. This perspective focuses on a particular class, namely on devices based on diffractive optical structures (DOS). Many configurations have been developed, including cantilever- and MEMS-type devices, fiber Bragg grating sensors, and cavity optomechanical sensing devices. These state-of-the-art sensors operate on the principle of a mechanical transducer coupled with a diffractive element resulting in a variation in the intensity or wavelength of the diffracted light in the presence of the target analyte. Therefore, as DOS can further enhance the sensitivity and selectivity, we present the individual mechanical and optical transducing methods and demonstrate how the DOS introduction can lead to an enhanced sensitivity and selectivity. Their (low-) cost manufacturing and their integration in new sensing platforms with great adaptability across many sensing areas are discussed, being foreseen that their implementation on wider application areas will further increase.

Beyond biology: alternative uses of cantilever-based technologies

Micromechanical cantilever sensors are attracting a lot of attention because of the need for characterizing, detecting, and monitoring chemical and physical properties, as well as compounds at the nanoscale. The fields of application of micro-cantilever sensors span from biological and point-of-care, to military or industrial sectors. The purpose of this work focuses on thermal and mechanical characterization, environmental monitoring, and chemical detection, in order to provide a technical review of the most recent technical advances and applications, as well as the future prospective of micro-cantilever sensor research.

A Magnetic Nanoparticle-Doped Photopolymer for Holographic Recording

Functionalised holograms are important for applications utilising smart diffractive optical elements for light redirection, shaping and in the development of sensors/indicators. This paper reports on holographic recording in novel magnetic nanocomposites and the observed temperature change in dry layers and liquid samples exposed to alternating magnetic field (AMF). The nanocomposite consists of N-isopropylacrylamide (NIPA)-based polymer doped with magnetic nanoparticles (MNPs), and local heating is achieved through magnetic induction. Here, volume transmission holographic gratings (VTHGs) are recorded with up to 24% diffraction efficiency (DE) in the dry layers of magnetic nanocomposites. The dry layers and liquid samples are then exposed to AMF. Efficient heating was observed in the liquid samples doped with Fe3O4 MNPs of 20 nm average size where the temperature increased from 27 °C to 64 °C after 300 s exposure to 111 mT AMF. The temperature increase in the dry layers doped with the same nanoparticles after exposure to 4.4 mT AMF was observed to be 6 °C. No temperature change was observed in the undoped layers. Additionally, we have successfully recorded Denisyuk holograms in the magnetic nanocomposite materials. The results reveal that the magnetic nanocomposite layers are suitable for recording holograms and need further optimisation in developing holographic indicators for mapping AMFs.

Review of Optical Humidity Sensors

Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.