Piezoelectric Optomechanical Approaches for Efficient Quantum Microwave‐to‐Optical Signal Transduction: The Need for Co‐Design

Abstract

Piezoelectric optomechanical platforms represent one of the most promising routes toward achieving quantum transduction of photons between the microwave and optical frequency domains. However, there are significant challenges to achieving near‐unity transduction efficiency. The authors discuss such factors in the context of the two main approaches being pursued for high efficiency transduction. The first approach uses 1D nanobeam optomechanical crystals excited by interdigitated transducers and is characterized by large single‐photon optomechanical coupling strength, limited intracavity pump photon population to avoid absorption‐induced heating (at cryogenic temperature), and low phonon injection efficiency from the transducer to the optomechanical cavity. The second approach uses (quasi) bulk acoustic wave resonators integrated into photonic Fabry–Perot cavity geometries and is characterized by low single‐photon optomechanical coupling strength, high intracavity pump photon population without significant heating, and high phonon injection efficiency. After reviewing the current status of both approaches, the need for co‐designing the electromechanical and optomechanical sub‐systems is discussed in order to achieve high transduction efficiencies, taking the GaAs piezo‐optomechanical platform as an example.

Piezo-optomechanical signal transduction using Lamb wave supermodes in a suspended Gallium Arsenide photonic integrated circuits platform

“…In contrast to 1D nanobeam optomechanical crystals which have a higher g 0 , the rib waveguide geometry studied in this work provides two key advantages: straightforward integration into an integrated photonics platform [24] because of relaxed dimensional tolerances and side-on coupling; and access to higher (> 5 GHz) frequency mechanical modes with moderate g 0 . This becomes critical for engineering quantum transduction [8] in a material with strong acoustooptic interactions, but relatively low speed of sound like GaAs (v SAW < 3000 m/s). Traditional 1D nanobeam optomechanical crystals, with g 0 ≈ 1 MHz have breathing mode frequencies < 3 GHz, with minimum feature sizes < 100 nm [23].…”

“…Traditional 1D nanobeam optomechanical crystals, with g 0 ≈ 1 MHz have breathing mode frequencies < 3 GHz, with minimum feature sizes < 100 nm [23]. Pushing these designs to mechanical frequencies > 5 GHz, in order to achieve the requisite spectral separation from the strong optical pump [8], places prohibitive constraints on nanofabrication.…”

“…The key challenge with building efficient microwave to optical signal transducers lies in overcoming the massive disparity in frequencies (GHz -100s THz) and associated wavelengths (cm -µm), which reduces the interaction strength between the fields considerably. By converting the microwave signal to an acoustic wave which has ≈ µm wavelength at GHz frequencies, piezoelectric optomechanical transducers overcome this wavelength mismatch problem and present one of the most promising routes towards building microwave to optical signal transducers [7,8], by engineering strong acousto-optic interactions in * ankur.khurana@bristol.ac.uk † krishna.coimbatorebalram@bristol.ac.uk nanoscale optomechanical cavities [9]. Such devices have been explored across a wide range of material platforms ranging from monolithic implementations in aluminum nitride (AlN) [10][11][12][13][14] , gallium arsenide (GaAs) [15,16], gallium phosphide (GaP) [17,18] and lithium niobate (LN) [19,20] to hybrid material platforms, AlN-on-silicon (Si) [7] and Si-on-LN [21].…”

“…The overall transduction efficiency (η peak ) [22] in such piezo-optomechanical devices scales as η peak ∝ η PIE C om , where η PIE represents the phonon injection efficiency, defined as the fraction of the input microwave energy that is transmitted into the desired mechanical mode of the optomechanical cavity and C om ∝ g 2 0 represents the optomechanical cooperativity, which is dependent on the optomechanical coupling rate g 0 1 . To achieve high η peak , it is therefore critical to design piezoelectric optomechanical platforms that simultaneously enhance η PIE and C om , and this balance is not easy to achieve [8]. In particular, it is challenging to achieve high η PIE with high g 0 optomechanical cavities like 1D photonic crystals [23] that exploit strong elastooptic interactions.…”

“…In particular, it is challenging to achieve high η PIE with high g 0 optomechanical cavities like 1D photonic crystals [23] that exploit strong elastooptic interactions. This occurs [8] because the piezoelectric transducer needs to be simultaneously impedance matched to 50 Ω and mode-matched to ≈ µm-scale dimensions. It was recently proposed [22] that mechanical supermodes can in principle overcome this challenge.…”