Integrated optomechanical cavity-waveguide system for coherent photonic-microwave signal processing

In an integrated silicon optomechanical cavity-waveguide system, we demonstrated on-chip coherent photonic-microwave signal processing which shows clear advantages over purely photonic methods. Specifically, we obtain an optical delay of 13.3 μs for a phonon waveguide length of 43 μm. In addition, we demonstrated microwave filter with reconfigurable pass/rejection bands in the gigahertz band due to the radiation-pressure force induced boundary condition change. During the last decade, the field of optomechanics has undergone tremendous development [1]. However, most lithographically-defined optomechanical devices demonstrated so far consist of only a single cavity, which limits the applicability of the technology. Scaling up from one cavity to a multiple cavities and waveguides coupled system is anticipated to bring new functionalities inaccessible in single-cavity devices especially for classical and quantum information processing using phonons [2, 3]. In this work, we fabricated coupled optomechanical cavity-waveguide devices on a silicon chip for photonic-microwave signal processing. While traditional photonic delay lines are generally bulky and have limited delay times due to the fast speed and large bandwidth of optical carrier even with state-of-the-art integrated engineering, slow phonons are able to render unprecedented delays for photonic and microwave signals. Moreover, mechanical filters are crucial for modern RF microwave technologies. For example, they are widely being leveraged for both RF and IF filters in cellular transceivers. Here, we also demonstrated a tunable and reconfigurable microwave filter using the optomechanical cavity-waveguide coupled system due to a novel contribution from optomechanical damping induced boundary condition change. We first design and fabricate an optomechanical single-cavity and waveguide coupled device to study the property of phonon waveguide and show frequency tuning of cavity-waveguide hybridized mode. A scanning electron micrograph of the device is shown in Fig. 1a. The optomechanical cavity is realized in a nanobeam which supports co-localized optical and mechanical resonant modes. The nanobeam is connected to a phonon waveguide which is terminated by an air gap from the bulk silicon. A typical mechanical spectrum of the fabricated device is shown in Fig. 1b, where a series of Fabry-Perot (FP) modes is observed. From the free spectral range of FP modes, we can infer the group velocity of the travelling phonon in the waveguide. By engineering the waveguide dispersion, phonon group velocity as low as 150 m/s is observed. Remarkably, we observed a large frequency shift of the cavity-waveguide hybridized modes when varying the detuning of pump laser. The physical mechanism can be understood in a schematic representing the fabricated device (Fig. 1c), where the mechanical cavity serves as an optically-tunable mirror for the waveguide, whose reflection coefficient depends on the optomechanical damping rate. Consequently, the waveguide FP resonance condition can be modified via a change in the phonon reflection phase, which induces a frequency shift as much as half of the free spectral range of the waveguide. Fitting based on this model is shown in Fig. 1d which well explains the experimental data. Next, we demonstrate the capability of the highly tunable optomechanical cavity-waveguide system in coherent microwave signal processing. The integrated device is shown in Fig. 2a with two nanobeams connected to a phonon waveguide. Procedure for signal transmitting through the device includes microwave signals encoded in an optical carrier that is fed into cavity L and converted to phonons and picked up by another optical carrier fed into cavity R. We first studied the continuous wave operation of the device. The S−matrix element SRL is measured by a network analyzer and is shown in Fig. 2b. Signal delay is measured versus optical pump power and a long optical delay of 13.3 μs is observed (Fig. 2c), induced by a highly excited narrow band mechanical resonance (Fig. 2d). This narrow band mechanical mode naturally realizes a microwave filter with 3-dB bandwidth of 17 kHz which is optically tunable. Based on the boundary condition principle demonstrated before, the pass/rejection band of such a microwave filter can be switched when the FP modes are Figure 1: Tunable optomechanical cavity-waveguide device. a Scanning electron micrograph of an optomechanical cavity-waveguide coupled device. b Mechanical spectrum of a typical device. c A schematic representing the optomechanical cavity-waveguide coupled system. a and b represent the optical and mechanical cavity modes. d Model fit of the frequency and decay rate of the three modes M1−3 of b versus laser detuning. Figure 2: Photonic-microwave signal processing using optomechanical cavity-waveguide system. a Scanning electron micrograph of the fabricated silicon optomechanical device. b Continuous microwave characterization using S−matrix as measured with a network analyzer. b Peak signal delay at 5.991 GHz vs. pump power PpR. c Transmission spectrum |SRL| at 5.991 GHz corresponding to peak delay of 13.3 μs. d A short phonon pulse is detected in the two optomechanical cavities as it bounces between them. shifted by half of the free spectral range. We then studied propagation of short microwave pulses. The measured RF signal corresponding to the evolution of coherent phonon number in the device excited by a 20 ns microwave pulse of 5.952 GHz is shown in Fig. 2e. Subsequent observation of the pulse in one cavity is separated by 58 ns, which is consistent with the free spectral range (17 MHz) measured from the spectroscopy. We note the travelling phonon induced signal delay is a true time delay, since both delay time as well as the delay-bandwidth product can be increased by using slower phonon or longer waveguide. In conclusion, we demonstrated coherent microwave signal processing on chip using an optomechanical complex system. Our approach promises a much more powerful and versatile microwave signal processing comparing to tradition methods, with the radiation-pressure force controlled mechanical degrees of freedom in addition to the advanced silicon photonic integration.