The scattering of surface acoustic waves by electrical effects in two-dimensional metal film structures

Recent advances in nanobiosensors for sustainable healthcare applications: A systematic literature review

The need for novel healthcare treatments and drugs has increased due to the expanding human population, detection of newer diseases, and looming pandemics. The development of nanotechnology offers a platform for cutting-edge in vivo non-invasive monitoring and point-of-care-testing (POCT) for rehabilitative disease detection and management. The advancement and uses of nanobiosensors are currently becoming more common in a variety of scientific fields, such as environmental monitoring, food safety, biomedical, clinical, and sustainable healthcare sciences, since the advent of nanotechnology. The identification and detection of biological patterns connected to any type of disease (communicable or not) have been made possible in recent years by several sensing techniques utilizing nanotechnology concerning biosensors and nanobiosensors. In this work, 2218 articles are drawn and screened from six digital databases out of which 17 were shortlisted for this review by using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) technique. As a result, this study uses a systematic methodology to review some recently developed extremely sensitive nanobiosensors, along with their biomedical, point-of-care diagnostics (POCD), or healthcare applications and their capabilities, particularly for the prediction of some fatal diseases based on a few of the most recent publications. The potential of nanobiosensors for medicinal, therapeutic, or other sustainable healthcare applications, notably for ailments diagnostics, is also recognized as a way forward in the manifestation of future trends.

A HISTORY OF SURFACE ACOUSTIC WAVE DEVICES

This paper gives a historical account of the development of Rayleigh-wave, or surface-acoustic-wave (SAW), devices for applications in electronics. The subject was spurred on initially by the requirements of pulse compression radar, and became a practical reality with the planar interdigital transducer, dating from 1965. The accessibility of the propagation path gave rise to substantial versatility, and a huge variety of devices were developed. Passive SAW devices are now ubiquitous, with applications ranging from professional radar and communications systems to consumer areas such as TV, pagers and mobile phones.

The paper describes the extensive work, particularly in the 1970s, to investigate SAW propagation in crystalline media, including piezoelectric coupling, diffraction and temperature effects. This led to identification of many suitable materials. Concurrently, many devices began development, including pulse compression filters, bandpass filters, resonators, oscillators, convolvers and matched filters for spread spectrum communications. In the 1970s, many of these became established in professional systems, and the SAW bandpass filter became a standard component for domestic TV.

In the 1980s and 90s, SAW responded to the new call for low-loss filters, particularly for mobile phones. With losses as low as 2 dB required (and subsequently achieved) at RF frequencies around 900 MHz, a raft of new technologies was developed. Additionaly, for IF filters special techniques were evolved to reduce the physical size needed for narrow bandwidths. Such devices are now manufactured in very large quantities. In order to satisfy these needs, new types of surface wave, particularly transverse leaky waves, were investigated, and materials using such waves now have their place alongside more traditional materials.

Evaluation of a novel surface acoustic wave gyroscope

A novel type of gyroscope sensor using metal dot arrays on the surface of a surface acoustic wave (SAW) device has recently been independently proposed. In this paper, we report experimental trials of several devices fabricated to evaluate the effects described and also an order-of-magnitude estimate of the sensitivity to be expected. The conclusions are that this device is extremely insensitive as currently proposed.

SAW diffraction using the thin-element decomposition method

We adapt the angular spectrum of plane waves (ASPW) decomposition to numerical simulations of the diffraction of surface-acoustic waves (SAWs) on anisotropic model substrate, such as YZ lithium niobate. We utilize the thin-element decomposition (TED) method, appropriately modified for an anisotropic substrate; we also introduce a novel "average-wavenumber" variation of this scheme; these are numerically found to be mutually consistent. We apply the TED method to simulate wave propagation both in infinite periodic structures of metallized gratings and also in finite gratings. We demonstrate that the ASPW provides a convenient and numerically fast tool for precise diffraction calculations in practical SAW devices which also display structural variations in the lateral direction, perpendicular to the principal direction of wave propagation. Additionally, we compare the present TED method to waveguide theory in an analysis of the role of SAW reflections from the electrodes.

Modular two-dimensional analysis of SAW filters. I. Theory

An accurate simulation tool to meet the stringent requirements for broadband surface-acoustic-wave (SAW) filters is described. The primary goal was to develop a modular software tool in which first- and second-order effects can be easily modeled as separate modules providing an overall high-precision model for SAW devices. The model takes into account surface wave diffraction and refraction, quasi-two-dimensional static charge distribution, metal resistance and external load impedances. The angular spectrum of waves formalism is interpreted in such a way as to render possible the simultaneous description of all effects mentioned. A sample of the results is given for the potential distribution.

Medium-loss SAW filters with synthesized characteristics

Medium-loss surface acoustic wave (SAW) filters based on the in-line dot array structure are reported. This configuration is very amenable to device synthesis, potentially to specifications comparable to conventional high-loss designs. The design procedure fully allows for multiple reflections within the reflecting arrays and can handle chirps. The devices have achieved up to 0.2-dB RMS passband ripple and 32-dB rejection after direct coupling and multistrip coupler (MSC) reflection are removed. Passband loss (6-8 dB) could be reduced if transducer and MSC loss (3 dB) can be improved. The devices also have sharp cutoffs, and fairly uniform input and output impedances in the pass and transition bands. The reflectors chosen for the reflecting arrays are thin metal dots. The results of computation and experimental verification of dot reflectivity and velocity change for the particular size of dots and dot pattern used are given. The device design, and the measured results are discussed.

The focusing of surface acoustic waves launched from a slanted chirped transducer. II. Anisotropic substrate

For pt.I see ibid., vol.UFFC-36, p.565-73 (1989). A characteristic feature of a slanted chirp transducer (SCT) with fingers only a few wavelengths long is that waves launched from it tend to a focus. Analytical expressions for key features of the radiation field are obtained for a linear chirp SCT on an anisotropic substrate and validated by comparison with experimental results obtained on Y-cut lithium niobate. It is found that anisotropy of velocity introduces cusps into the radiation field, giving the focal region properties comparable to those at the focus of a simple lens. Selection of the cuspoidal region combined with choice of finger length is shown to form the basis of a method of apodization.

Phase correction of SAW ILRACs and RACs using Langmuir-Blodgett films

A novel method of adjusting surface-acoustic-wave (SAW) velocity in reflective array pulse compressors (RACs), including in-line devices (ILRACs) and hence correcting fabrication errors, is described. The velocity change is effected by depositing Langmuir-Blodgett films (LBFs) on the surface of the device. An accurate, stepped thickness profile can be created, enabling position-dependent velocity errors to be corrected. Experimental results for the velocity perturbation per LBF layer are first given together with data on temperature, humidity and age dependence. This is followed by the theory required to calculate the necessary thickness profile, including retrofitting to existing devices and allowing for the limitation to positive integer numbers of layers. Finally, experimental results are presented. In one device, simulated compressed pulse sidelobe levels are reduced by 19 dB.

The focusing of surface-acoustic-waves launched from a slanted chirped transducer. I. Isotropic substrate

The focusing behavior of surface acoustic waves launched from a slanted chirped transducer (SCT) is explored using both a continuum model and a discrete model for sources associated with a linear FM chirp transducer on a substrate with isotropic properties. The continuum model leads to the prediction of an understanding of effects that would arise in the radiation field of an SCT on an isotropic substrate. It is based on a representation of the transducer as a continuum of sources, with the radiation field determined by applying the stationary phase method, and is presented for infinitesimally short fingers. It is compared with a more direct and exact but less revealing method of determining the field based on a discrete array of sources. The effects of increasing finger lengths are considered. The results are related to the focusing of radiation by a lens.