Plasmonic Controlled Nonreciprocal Transmission of Ultrasonic Waves in an Acoustic Meta-Atom

Controlling the direction of energy flow in a matter-wave or vibrational mode of oscillation is challenging compared to electrons or electromagnetic waves. Vibrational or acoustic waves are dominantly longitudinal in nature and are not directly influenced by electric, magnetic, or electromagnetic fields. Effective control of the directionality of sound wave propagation and reversing the flow of energy can yield acoustic isolators, diodes, or rectifiers. Current nonreciprocal techniques use complex metamaterial systems or require large mechanical pumps for physical actuation of the medium to break the time-symmetry or require asymmetric structures that induce dissipation. The mechanical actuation process is also slow and cannot be precisely controlled. A new class of acoustic meta-atom-based laser-driven nonreciprocal acoustic devices is demonstrated. Novel light-controlled techniques are used to control the nonreciprocal transmission of sound waves. A photoacoustic or magneto-optical pump based on nanophotonic principles has been used to control fluid flow within meta-atom in the form of a ring resonator or linear Bragg cavity. Instead of mechanical actuation and flow pumps, a non-contact laser and or magnetic force field are applied for remote control of the acoustic energy to modulate the direction of the acoustic wave propagation transmitted through the fluid medium.<br/>In the first case, a fluid-filled acoustic meta-atom cavity in the form of a 2cm diameter ring resonator was designed using 3D printing to confine acoustic modes excited by an external transducer as a probe wave. A photo-elastic process induced by lasers resonant to the plasmon frequency of Au-implanted quart windows can induce pump ultrasonic waves within this meta-atom cavity. The plasmonic window was fabricated using metal-ion implantation or photo-thermal evaporation of metal to form plasmonic nanoparticles resonant to a 532nm laser source. The photo-elastic effect also acts as a fluid pump and induces an active motion of the fluidic medium within the cavity. The ponderomotive forces can drive the fluid within the meta-atom cavity that can either co-propagate or counter-propagate with acoustic waves in the cavity. The fluid motion within the cavity and the wave mixing between the laser-induced pump waves and the external probe waves can result in a nonreciprocal or unidirectional transmission of sound waves within the acoustic cavity. A fluid flow as fast as 6 cm/s was generated that can induce a nonlinear wave mixing and active motion for T-symmetry violation resulting in nonreciprocal wave transmission within the meta-atom resonator. A large nonreciprocity exceeding 25dB was observed. An optically controlled acoustic diode operation was realized and the transmission of the acoustic wave can be biased using the laser intensity. In the second case, a DBR cavity-like device design was used to demonstrate magneto-optically controlled acoustic nonreciprocity. In this case, a magneto-caloric force based on the temperature-dependent magnetization of the ferrofluid was used to drive the fluid in the presence of a magnetic field. Acoustic isolation due to nonreciprocity as large as 20dB was realized which can be controlled by the magnetic field strength and the laser intensity.<br/>We used theoretical analysis, and numerical simulation for the design of additively manufactured ultrasonic devices which were then characterized using ultrasonic spectroscopy for the study of nonreciprocal acoustic wave transmission. Quantum optical effects such as Zeeman-like and Optical Stark-like effects on acoustic states have been observed. Ultrasonic spectroscopy characterized the optically driven diode and the isolator. This work is the first report on the realization of laser and magneto-optical control of ultrasonic acoustic diodes and isolators in a compact fluidic medium.