Integrated Acousto-optics: Steering light with sound waves on a chip

Abstract

When an acoustic wave propagate inside an optical transparent material, it periodicallymodulates the permittivity medium due to the elasto-optical effect. This can generate a moving phase grating that can diffract the incident light into one or more orders. Such phenomenon is known as acouto-optic (AO) diffraction which leads to a various of applications such as temporal modulators, spatial modulators, spectral modulators and more. Historically, the AO diffraction (so called, Brillouin scattering (BS)) was first predicted by Brillouin in 1922 [1]. Ten years later, the phenomenon was experimentally observed by Debye and Sears [2], and Lucas and Biquard [3] successively. Other than than Brillouin’s predictions, instead of only one order of diffracted beam, there are many more orders observed. which was later theoretical analyzed by Raman and Nath [4]. Therefore, in terms of the Brillouin scattering, there are two diffraction regimes, the Raman-Nath regime, characterized by the multiple of diffraction orders, and the Bragg regime, characterized by a single diffraction order [5, 6]. The exploitation of acouto-optics has led to demonstrations of a variety of applications and novel physical phenomena in a lot of optical systems and devices [1,6–12]. The elastic wave can be spontaneously produced by thermal agitation of the environment (i.e., spontaneous Brillouin scattering), or stimulated (narrowly defined to be optically excited) by a stronglight source (i.e., stimulated Brillouin scattering (SBS)). The elastic wave can also be excited by external stimuli, such as optical pulses, thermal shocks, and electrical and magnetic fields. The acoustic wave involved in the acousto-optical scattering process can be launched in different ways. For example, the elastic wave can be spontaneously produced by thermal agitation of the environment (i.e., spontaneous Brillouin scattering), or thermal excitation and the optomechanical stimulation by radiation pressure and electrostriction. While the former method is the original Brillouin’s description, the later one is more explored by recent researchers, especially for the stimulated Brillouin scattering (SBS), [13–15] [15–21] as well as in cavity optomechanical systems [22–25], which feature many intriguing photon-phonon interactions [26]. Meanwhile, the acoustic waves can be also electromechanically excited by the interdigital transducers (IDT) on the piezoelectric material. The IDTs convert radio frequency (RF) and microwave (MW) electromagnetic waves to propagating elastic waves, or in some cases, localized mechanical modes. The advanced electromechanical transducer can achieve near-unity transfer efficiency with compact footprint [27–29]. Such strong acoustic waves have the high scattering efficiency [30], which is essential for the practical applications, especially for the nowadays quantum transduction [31]. The high transfer efficiency from RF power to acoustic power, generated from the electromechanical excitation, leading to a high acoustic wave intensity, is unparalleled with other methods forementioned. In the contrast, in the SBS process, each pump photo can generate at most one phonon. Due to the large frequency difference, (in more than 3 orders), the excitation efficiency is no larger than 10-3. Recently, there are several works using gigahertz electromechanically generated acoustic wave to modulate the guide optical waves inside the waveguides and the cavities [32–38]. Novel physical phenomena has been explored in these works, such as induced transparency [35] and nonreciprocal mode conversion [36], and other advanced optical functionalities [30, 32–35, 37, 38]. Thanks to the development of nanofabrication technology, the state-of-art integrated guided wave acousto-optical device succeedthe conventional acousto-optical devices with significant advances in terms of the power consumption and physical footprint [10, 39–42]. The frequency of the nowadays device can also exceed 10 GHz easily, compared to the previous acousto-optic devices. More interestingly, the newly emerging integrated phononic circuit is anticipated to complement the functionalities of the photonic and electronic circuits, leading to integrated nano-opto-electro-mechanical systems (NOEMS). Such exciting prospect of integration of the three ”x-ons” (photons, phonons and electrons), that implement sophisticated sensing and information processing functionalities through Brillouin scattering in the classical and quantum regimes is attracting increasing research efforts. In this dissertation, a brief overview of BS processes is introduced first in Chapter 1, including the electromechanical excitation of acoustic waves based on piezoelectric IDTs, configurations of such devices, and some prospective applications of the BS devices in reviewed. What follows the introduction and the theory review is the revised compilation of the author’s selected research work, including the acousto-optic beamsteering (AOBS) and the scaling integrated photonic computing in the synthetic frequency dimension. AOBS device has been successfully simulated and fabricated for the first time in 2021. When light interacts with guided acoustic wave inside the acousto-optic waveguide, the scattered beam can be deflected into the designated directions by controlling the acoustic wave frequency. The introduced frequency upshift of the deflected light beam obeys the BS process, mapping the deflection angle to the controlled acoustic wave frequency. Since the angular position of the object is “labeled” by the frequency of the reflected light, the receiver can determine the object’s position without a priori knowledge of the outgoing beam angle. Based on this property, a prototype frequency angular resolving (FAR) light-detection-andranging (LiDAR) system based on chip-scale AOBS devices has been demonstrated. This work is presented in Chapter 2.The scaling integrated photonic computing in the synthetic frequency dimension has been successfully demonstrated in 2020. [43] The optomechanical coupling is improved by one order in our heterogeneous AlN-on-SOI platform. With such a strong AO modulation, the large scale vector multiplier in frequency domain is achieved. This work is presented in Chapter 3.