The low propagation loss of electromagnetic radiation below 1 MHz offers significant opportunities for low power, long range communication systems to meet growing demand for Internet of Things applications. However, the fundamental reduction in efficiency as antenna size decreases below a wavelength (30 m at 1 MHz) has made portable communication systems in the very low frequency (VLF: 3–30 kHz) and low frequency (30–300 kHz) ranges impractical for decades. A paradigm shift to piezoelectric antennas utilizing strain-driven currents at resonant wavelengths up to five orders of magnitude smaller than electrical antennas offers the promise for orders of magnitude efficiency improvement over the electrical state-of-the-art. This work demonstrates a lead zirconate titanate transmitter > 6000 times more efficient than a comparably sized electrical antenna and capable of bit rates up to 60 bit/s. Detailed analysis of design parameters offers a roadmap for significant future improvement in both radiation efficiency and data rate.

This paper presents a short review of the microwave acoustics area, where exciting material innovations and performance advancements have been made in the past decade. The ever-growing demand for more sophisticated passive signal processing functions on-chip has fueled these developments. As a result, microwave acoustic devices have maintained performance leadership in mobile applications. By evaluating three fundamental parameters, namely electromechanical coupling (k 2 ), quality factors, and frequency scalability, of microwave acoustics, this paper aims to, extensively but not exhaustively, capture the rationales behind approaches achieving higher performance microsystems. Outlooks for different material systems and addressing their underlying challenges are also offered in hopes of establishing a balanced roadmap for future microwave acoustics development.

Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/millimeter-wave systems. Conventional microwave-to-photonic converters, based on resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal-processing applications. In this paper, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photoelastic effect. This approach is power-efficient, with phase shifts up to 0.0166    rad / √ mW over a 45 μm modulation length and with a bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9 × more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

In this paper, we designed, implemented, and characterized compact Mach-Zehnder interferometer-based electro-optic modulators. The modulator utilizes spiral-shaped optical waveguides on Z-cut lithium niobate and the preeminent electro-optic effect which is applied using top and bottom electrodes. Optical waveguides are made of rib etched lithium niobate waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, we achieved compact modulators that fit 0.286 cm and 2 cm long optical waveguides in 110 µm × 110 µm and 300 µm × 300 µm areas, respectively. For single arm modulation, the modulators achieved a VπL of 7.4 V.cm and 6.4 V.cm and 3-dB bandwidths of 9.3 GHz and 2.05 GHz, respectively. Push-pull modulation is expected to cut these VπL in half. The proposed configuration avoids traveling wave modulation complexities and represents a key development towards miniature and highly integrated photonic circuits.

This letter presents the first piezoelectric micromachined ultrasonic transducer (PMUT) based on thin-film lithium niobate (LiNbO 3). The figures of merit (FoMs) of LiNbO 3 as ultrasound sensors and transducers are first studied, showing great prospective as a balanced transceiver platform. Efficient flexural mode excitation is achieved using a proposed lateral-field-excitation (LFE) structure. The implemented device shows a flexural mode at 7.6 MHz, with a high electromechanical coupling (k 2) of 4.2%. Measured quality factor (Q) in vacuum is 2605, indicating the low structural loss, while measured Q in air is 264, suggesting the ultrasound radiation. A dynamic displacement sensitivity of 20.2 nm/V is measured. Upon further optimizations, LiNbO 3based PMUTs are promising candidates for miniature ultrasound applications.

Next-generation photonic integrated circuits require compact electro-optic modulators (EOMs) that achieve high performance and efficient use of the on-chip area simultaneously. We demonstrate a compact EOM based on a spiral-shaped waveguide Bragg grating on thin-film lithium niobate. The modulator utilizes the grating’s optical filter characteristics along with electro-optic tuning of the central Bragg wavelength to achieve simple and efficient intensity modulation. The spiral Bragg gratings were realized on Z-cut lithium niobate and modulated using top and bottom electrodes. The integrated design wrapped a 2.2 mm long grating into a 120×120µm2 area. The modulator bandgap, with an extinction ratio of over 35 dB at 1550 nm, could be efficiently tuned with a sensitivity of 8.36 pm/V and a 3 dB operating bandwidth of 25 GHz.

Acoustically driven antennas operating at resonant wavelengths up to 105 times smaller than electrical antennas offer great potential for portable, low power communication systems in the very low frequency and low frequency range. Acoustic antennas with real resonant impedances have been demonstrated to offer orders of magnitude better total efficiency compared to similar sized, subwavelength electrically small antennas exhibiting large reactances. While most acoustic antennas share favorable impedance characteristics offering significant matching efficiency advantages over electrically small antennas, radiation efficiency varies greatly based on the implementation of the acoustically driven antenna. This paper presents a theoretical analysis of the three primary methods for implementing acoustically driven radiating elements, investigating both radiation and matching efficiencies comprising the total antenna efficiency. Radiation from the linear movement of unipolar charge driven both piezoelectrically and capacitively, the piezoelectrically actuated rotation of fixed dipole charges, and from flipping dipoles inside strain driven piezoelectrics are all presented and analyzed in terms of their design parameters and fundamental challenges. The efficiency of each type of acoustic antenna is referenced to an equivalent electrical antenna to benchmark the performance to a more familiar framework. Of the analyzed antenna types, piezoelectric alternating dipole antennas exhibit the most promise, with efficiencies more than a million times greater than electrically small antennas expected as piezoelectric materials, and resonator designs are optimized for acoustic radiation.

In this work, we present the first group of gigahertz low-loss, wideband, and high power handling delay lines (ADLs) using a thin-film lithium niobate (LiNbO 3 )-on-sapphire platform. The ADLs leverage a single-phase unidirectional transducer (SPUDT) to efficiently excite the shear horizontal surface acoustic wave (SH-SAW) in the film stack. The fabricated miniature SH-SAW ADL at 1.1 GHz shows a low insertion loss (IL) of 2.8 dB, a wide fractional bandwidth (FBW) of 6.14%, and a fast phase velocity of 5127 m/s. The device also features a high 1-dB compression point (P1dB) of 30.4 dBm. The temperature coefficient of frequency is −45 ppm/K. ADLs with delays between 12 and 172 ns have been implemented, with IL between 2.8 and 8.3 dB. SH-SAW propagation characteristics are extracted, showing a group velocity of 4747 m/s and a propagation loss of 6.73 dB/mm or 31.9 dB/µs. The simultaneous low-loss and high power handling illustrate the great potential of LiNbO 3 -on-sapphire for RF and cross domain applications at gigahertz.