Photonic modulators are one of the most important elements of integrated photonics. We have designed, fabricated, and characterized a tunable photonic modulator consisting of two 180 • -dephased output waveguide channels, driven by a surface acoustic wave in the GHz frequency range built on (Al,Ga)As. Odd multiples of the fundamental driven frequency are enabled by adjusting the applied acoustic power. A good agreement between theory and experimental results is achieved. The device can be used as a building block for more complex integrated functionalities and can be implemented in several material platforms.

Abstract:We demonstrate compact tunable phased-array wavelengthdivision multiplexers driven by surface acoustic waves (SAWs) in the low GHz range. The devices comprise two couplers, which respectively split and combine the optical signal, linked by an array of single-mode waveguides (WGs). Two different layouts are presented, in which multi-mode interference couplers or free propagating regions were separately employed as couplers. The multiplexers operate on five equally distributed wavelength channels, with a spectral separation of 2 nm. A standing SAW modulates the refractive index of the arrayed WGs. Each wavelength component periodically switches paths between the output channel previously asigned by the design and the adjacent channels, at a fixed applied acoustic power. The devices were monolithically fabricated on (Al,Ga)As. A good agreement between theory and experiment is achieved.

This paper reviews the application of coherent acoustic phonons in the form of surface acoustic waves to control the response of semiconductor optical waveguide devices. We lay special emphasis on devices built upon three-dimensional rectangular waveguides, which offer excellent possibilities for integration due to the stronger confinement of the optical fields. We address the spatial distribution of the acoustic fields, as well as the excitation of surface acoustic waves in piezoelectric materials using interdigital transducers. The mechanisms responsible for the interaction between light and the acoustic modes in the optical waveguides, as well as the influence of waveguide parameters in the performance of the devices, are also discussed. Finally, we review the most important advances on the modulation of semiconductor optical waveguide devices built upon three-dimensional waveguides, and explore several exciting future technological possibilities. These include, among others, the generation of slow light in photonic crystal waveguides to enhance the sound-light interaction in the reviewed devices.

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Coherent super-high-frequency (SHF) vibrations provide an excellent tool for the modulation and control of excitations in semiconductors. Here, we investigate the piezoelectric generation and propagation of longitudinal bulk acoustic waves (LBAWs) with frequencies up to 20 GHz in GaAs crystals using bulk acoustic wave resonators (BAWRs) based on piezoelectric thin ZnO films. We show that the electro-acoustic conversion efficiency of the BAWRs depends sensitively on the sputtering conditions of the ZnO films. The BAWRs were then used for the study of the propagation properties of the LBAWs in GaAs in the frequency and temperature ranges from 1 to 20 GHz and 10 and 300 K, respectively, which have so far not been experimentally accessed. We found that the acoustic absorption of GaAs in the temperature range from 80 K to 300 K is dominated by scattering with thermal phonons. At lower temperatures, in contrast, the acoustic absorption saturates at a frequency-dependent value. Experiments carried out with different propagation lengths indicate that the saturation is associated with losses during reflections at the sample boundaries. We also demonstrate devices with high quality factor fabricated on top of acoustic Bragg-reflectors. The results presented here prove the feasibility of high-quality acoustic resonators embedding GaAsbased nanostructures, thus opening the way for the modulation and control of their properties by electrically excited SHF LBAWs. I. arXiv:1907.09787v2 [cond-mat.mtrl-sci] 4 Sep 2019 FIG. 2. (a) Calculated dependence of the |s11| 2 scattering parameter (corresponding to the rf-reflection coefficient) for a BAWR on a GaAs (001) wafer consisting of a dZnO = 440 nm-thick ZnO layer sandwiched between bottom and top metal contacts formed by a 40 nm Au film and a 10 nm/30 nm/10 nm Ti/Al/Ti layer stack, respectively. The active area of the resonator is equal to 1960 µm 2 . (b) Dependence of the resonance frequency on dZnO: the joined full square symbols are finite element method calculations, and the open circle symbols are experimental values. The dashed line reproduces the approximation for the resonance frequency given by Eq. (1). The blue open square at fR ∼ = 5 GHz corresponds to the device simulated in (a) and (c). (c) Cross-section map of the amplitude of the displacement field of the BAWR in (a) at the resonance frequency. The calculation domain was surrounded by perfectly matched layers (PMLs) to reduce acoustic reflections from the sample boundaries.