Gigahertz Surface Acoustic Waves Research Articles

Time-resolved imaging of GHz acoustic vibrations at arbitrary frequencies using asynchronous periodic probe laser pulses

We present a technique for imaging gigahertz surface acoustic waves in the time domain at arbitrary frequencies, in which acoustic waves are excited asynchronously to the optical probe pulse train used to detect them. We apply this approach to the two-dimensional imaging of electrically excited single-frequency surface acoustic waves on a GaAs substrate with a deposited interdigital transducer. This technique significantly increases the possible fields of application for gigahertz acoustic wave device imaging.

Velocimetry of GHz elastic surface waves in quartz and fused silica based on full-field imaging of pump–probe reflectometry

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump–probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6.0nsto11.5ns, image stacks of wave field propagation were created.Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2% to 5%.These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Nonreciprocity of Gigahertz Surface Acoustic Wave Based on Mode Conversion in an Inclined Phononic Crystal Heterojunction

In this report, a rectifying surface acoustic wave (SAW) device is proposed and simulated based on a simple inclined phononic crystal (PnC) heterojunction, consisting of monolithic pillars on $\mathrm{Si}$ substrate. The designed nonreciprocal operation principle is initially based on the frequency alignment of the surface-coupled guiding bands in the first half of the $\mathrm{Pn}\mathrm{C}$ with the local-surface-resonance (LSR) band gap in the second half of the $\mathrm{Pn}\mathrm{C}$, along two different equivalent incident directions. Benefiting from flexible LSR band-gap engineering, we tune the band-gap central frequency by optimizing the structural dimensions of the pillars in a small chip area, which is not achievable in conventional Bragg band gaps without varying the lattice constant. The other physical principle that dominantly affects the broken reciprocity in our proposed structure is the induced SAW shear-to-sagittal mode conversion at a limited frequency range in the trapezoidal $\mathrm{Pn}\mathrm{C}$ half, which depends on the incident direction with respect to the inclined cut line of the $\mathrm{Pn}\mathrm{C}$. Moreover, we optimize the spacing gap between the PnCs to modulate the elastic coupling strength between the half $\mathrm{Pn}\mathrm{Cs}$, and prove a significant SAW nonreciprocity of 34 dB at a frequency of 6.9 GHz, in addition to an acceptable rectified transmission of about \ensuremath{-}10.68 dB, by the proposed $\mathrm{Pn}\mathrm{C}$-based operation principles. The presented design benefits from a simple $\mathrm{Si}$-based structure and a CMOS-compatible fabrication process, without the need for any external excitation, and it is a promising miniature and efficient SAW rectifying candidate for wireless-communication applications.

Beyond GHz optical frequency up-converted modulation of LEDs with integrated acousto-optic transducer.

Traditional visible light communication (VLC) via light-emitting diodes (LEDs) employs the on-off keying (OOK) modulation scheme. Even though optical frequency modulation has many advantages, it is hardly used for LED VLC because a high carrier frequency cannot be applied to the LED cavity due to the resistance-capacitance limit. Here, by monolithically integrating an LED with an integrated digital transducer, we experimentally demonstrate the intermixing of gigahertz surface acoustic waves and electrical data signals in the LED cavity at room temperature. An optical transmitter was realized by in situ frequency up-conversion of the data signals from an LED, which has the advantages of improving transmission performance by up-shifting the data spectrum away from low-frequency noise. Our proposed integrated acousto-optic transducer opens a new developing scheme on the frequency up-mixed data encoding of an LED beyond its inherent modulation bandwidth for future VLC.

Hypersound waves at the surface of CdZnTe crystals: The role of surface orientation and scattering at twin boundaries

The patterns of propagation of gigahertz surface acoustic waves (SAWs) are obtained for cuts of CdZnTe single crystals with different crystallographic orientations. By comparing the experimental patterns with the calculated ones, the existence of at least two SAW modes, one of which is a Rayleigh mode, is demonstrated. It is shown that the anisotropy of propagation of different SAW modes makes it possible to determine the local crystallographic orientation of the CdZnTe surface and detect local imperfections of the crystal structure. Strong anisotropic scattering of the Rayleigh wave by coherent twin boundaries is found.

Extraordinary transmission of gigahertz surface acoustic waves.

Extraordinary transmission of waves, i.e. a transmission superior to the amount predicted by geometrical considerations of the aperture alone, has to date only been studied in the bulk. Here we present a new class of extraordinary transmission for waves confined in two dimensions to a flat surface. By means of acoustic numerical simulations in the gigahertz range, corresponding to acoustic wavelengths λ ~ 3–50 μm, we track the transmission of plane surface acoustic wave fronts between two silicon blocks joined by a deeply subwavelength bridge of variable length with or without an attached cavity. Several resonant modes of the structure, both one- and two-dimensional in nature, lead to extraordinary acoustic transmission, in this case with transmission efficiencies, i.e. intensity enhancements, up to ~23 and ~8 in the two respective cases. We show how the cavity shape and bridge size influence the extraordinary transmission efficiency. Applications include new metamaterials and subwavelength imaging.

Level repulsion of GHz phononic surface waves in quartz substrate with finite-depth holes

This paper presents numerical and experimental results on the level repulsion of gigahertz surface acoustic waves in an air/ST-cut quartz phononic structure with finite-depth holes. The colorful dispersion with the parameter of the in-plane (sagittal plane) ratio of polarization was adopted to determine the Rayleigh wave bandgap induced by the level repulsion. The results of numerical analyses showed that the frequency and width of the bandgap induced by the level repulsion strongly depend on the geometry of the air holes in the phononic structure. In the experiment, a pair of slanted interdigital transducers with frequency in the gigahertz range was designed and fabricated to generate and receive broadband Rayleigh waves, whereas the reactive ion etching process with electron-beam lithography was used to fabricate submicrometer phononic structures. The measured results of the bandgap induced by the level repulsion agreed favorably with the numerical prediction.

Watching surface waves in phononic crystals.

In this paper, we review results obtained by ultrafast imaging of gigahertz surface acoustic waves in surface phononic crystals with one- and two-dimensional periodicities. By use of quasi-point-source optical excitation, we show how, from a series of images that form a movie of the travelling waves, the dispersion relation of the acoustic modes, their corresponding mode patterns and the position and widths of phonon stop bands can be obtained by temporal and spatio-temporal Fourier analysis. We further demonstrate how one can follow the temporal evolution of phononic eigenstates in k-space using data from phononic-crystal waveguides as an example.

2E3-1 Extraordinary transmission of gigahertz surface acoustic waves

2E3-1 Extraordinary transmission of gigahertz surface acoustic waves

Ring waveguides for gigahertz acoustic waves on silicon

We demonstrate a ring resonator for gigahertz surface acoustic waves (SAWs) consisting of a Ge waveguide on a silicon chip. SAWs generated by interdigital transducers on a section of the waveguide are guided over a curved path and detected by a second interdigital transducers. The structure of the GHz waveguide modes mapped using high resolution interferometry compares well with elastic calculations. The acoustic propagation properties as well as the potential applications of these semiconductor-based resonators are discussed.

A method for the frequency control in time-resolved two-dimensional gigahertz surface acoustic wave imaging

We describe an extension of the time-resolved two-dimensional gigahertz surface acoustic wave imaging based on the optical pump-probe technique with periodic light source at a fixed repetition frequency. Usually such imaging measurement may generate and detect acoustic waves with their frequencies only at or near the integer multiples of the repetition frequency. Here we propose a method which utilizes the amplitude modulation of the excitation pulse train to modify the generation frequency free from the mentioned limitation, and allows for the first time the discrimination of the resulted upper- and lower-side-band frequency components in the detection. The validity of the method is demonstrated in a simple measurement on an isotropic glass plate covered by a metal thin film to extract the dispersion curves of the surface acoustic waves.

Generation and detection of gigahertz surface acoustic waves using an elastomeric phase-shift mask

We describe a convenient approach for measuring the velocity vSAW of surface acoustic waves (SAWs) of the near-surface layer of a material through optical pump-probe measurements. The method has a lateral spatial resolution of <10 μm and is sensitive to the elastic constants of the material within ≈300 nm of the surface. SAWs with a wavelength of 700 nm and 500 nm are generated and detected using an elastomeric polydimethylsiloxane phase-shift mask which is fabricated using a commercially available Si grating as a mold. Time-domain electromagnetics calculations show, in agreement with experiment, that the efficiency of the phase-shift mask for generating and detecting SAWs decreases rapidly as the periodicity of the mask decreases below the optical wavelength. We validate the experimental approach using bulk and thin film samples with known elastic constants.

Acousto-electric transport in epitaxial monolayer graphene on SiC

We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

Real-time imaging of acoustic rectification

We image gigahertz surface acoustic waves normally incident on a microscopic linear array of triangular holes—a generic “acoustic diode” geometry—with a real-time ultrafast optical technique. Spatiotemporal Fourier transforms reveal wave diffraction orders in k-space. Squared amplitude reflection and transmission coefficients for incidence on both sides of the array are evaluated and compared with numerical simulations. We thereby directly demonstrate acoustic rectification with an asymmetric structure.

Beam distortion detection and deflectometry measurements of gigahertz surface acoustic waves

Gigahertz acoustic waves propagating on the surface of a metal halfspace are detected using different all-optical detection schemes, namely, deflectometry and beam distortion detection techniques. Both techniques are implemented by slightly modifying a conventional reflectometric setup. They are then based on the measurement of the reflectivity change but unlike reflectometric measurements, they give access to the sample surface displacement. A semi-analytical model, taking into account optical, thermal, and mechanical processes responsible for acoustic waves generation, allows analyzing the physical content of the detected waveforms.

Imaging gigahertz surface acoustic waves through the photoelastic effect

This paper presents experiments and in-depth analysis of the imaging of surface acoustic waves by means of the photoelastic effect. Gigahertz surface acoustic waves, generated by optical pump pulses in a thin gold film on a glass substrate, are imaged in the time domain by monitoring ultrafast changes in optical reflectivity. We demonstrate how images of the in-plane acoustic shear strain component can be obtained by measurements with two different optical probe pulse polarizations incident from the substrate side.

Superheterodyne techniques in laser ultrasonics.

Laser-based ultrasonics is a powerful tool for the mechanical characterization of thin films and nanostructures. In this paper, superheterodyne approaches to single-point and full-field detection of laser generated ultrasonic signals will be discussed. In these techniques, an amplitude modulated laser source is used to excite a narrow-bandwidth signal. The detection laser, incorporated into an interferometric detection scheme, is also amplitude or phase modulated at frequency that is offset from the generation laser modulation frequency by a fixed amount, serving as the local oscillator for superheterodyne detection. Introduction of the local oscillator allows for the optical down-conversion of the high-frequency intensity modulation associated with sample motion to a low and fixed intermediate frequency given by the difference between excitation and detection laser modulation frequencies. The primary benefit of using this approach is that the upper frequency bound is not dictated by the speed of the detection photodiode and electronic circuitry or, in the case of full-field detection, the frame rate of the CCD. Results are presented demonstrating single-point superheterodyne detection of gigahertz frequency bulk and surface acoustic waves using a low-frequency photodiode and full-field superheterodyne detection of megahertz vibrations of nanostructure arrays using a low frame rate camera.

Gigahertz surface acoustic wave generation on ZnO thin films deposited by radio frequency magnetron sputtering on III-V semiconductor substrates

The authors demonstrate 1.6GHz surface acoustic wave (SAW) generation using interdigital transducers patterned by e-beam lithography on a thin ZnO piezoelectric film deposited on an InP substrate. The highly oriented, dense, and fine-grain ZnO film with high resistivity was deposited by radio frequency magnetron sputtering and was characterized by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and a four-point probe station. The acoustic wavelength of the 1.6GHz SAW generated by exciting the interdigital transducer on ZnO∕InP with a microwave signal is 1.6μm. This SAW filter device could be monolithically integrated with optoelectronic devices, opening new opportunities to use SAWs for applications such as gigahertz-frequency filters on optoelectronic devices and novel widely tunable quantum cascade lasers.

Multimode and multifrequency gigahertz surface acoustic wave sensors

A simple surface acoustic wave sensor system is presented that combines the advantages of multifrequency/multimode operation and GHz frequencies in a single acoustic device structure. Analyzing multiple frequencies and modes is the basis for the multicomponent analysis of gases and liquids. The sensor system is based on floating electrode unidirectional transducers that allow efficient excitation of two modes and up to 48 harmonics (or 5 GHz). It can be fabricated without the need of costly nanofabrication techniques. Higher sensor frequencies are advantageous as the sensitivity increases with the frequency. We tested the basic operation of our sensor system by applying it to humidity sensing without a sensitive layer.

Gigahertz surface acoustic wave probe for chemical analysis

This paper considers the propagation of high frequency 0.1–2.6 GHz surface acoustic wave pulses in aqueous solutions of pure water, glycerol and protein. The GHz frequency components of the pulse are used to provide the highest operating frequencies so far reported and also to construct the first acoustic absorption spectrum associated with the evanescent field. Acoustic generation is sourced from a single non-linear SAW device that provides a series of harmonic frequencies, simultaneously. The received power level is determined from digital samples of the received pulse waveform. The power leaked into glycerol solutions at the fundamental frequency was found to be 50% smaller for pulses, than for continuous acoustic waves, an effect that could be related to the equilibration of the evanescent field. Increasing the concentration of the glycerol solutions or time exposed to the protein (IgG) solution, showed that the power losses from the surface acoustic wave pulse were broadly consistent with the behaviour of transverse shear mode sensors. Atomic force microscope measurements of the bare device revealed that the morphology of the silica overlayer was uniformly granular, whereas adsorbed protein films formed non-contiguous islands. Confirmation of the presence of the IgG film was obtained from quantitative X-ray photoelectron spectroscopy. An 8 gigasample per second digitising oscilloscope running a fast Fourier transform routine captured the acoustic absorption spectrum, and revealed a smooth characteristic for the glycerol and IgG, although for the latter, frequencies beyond 500 MHz were associated with an irregular spectrum. These multiple frequency measurements of the solid–liquid interface provide evidence that when the penetration depth and film thickness are similar, disruption of the predicted exponential form of the evanescent wave occurs, as indicated by the fluctuations seen in the absorption spectrum recorded. These preliminary results have shown that multiple frequency operation of single non-linear SH-SAW devices is possible, and an evanescent interfacial absorption spectrum can be obtained. By extending the measurement technique it may be possible to obtain additional information about the structure and composition of the solid–liquid interface.